Filtration in Fahrzeugen Engl[1][1]

VERLAG MODERNE INDUSTRIE

AutomotiveFiltration

Basics and examples of air, oil and fuel filtration

MANN+HUMMEL

Au

tom

otiv

e F

iltra

tion

930576

verlag moderne industrie

Automotive Filtration

Basics and examples of air, oil and fuel filtration

Michael Durst, Gunnar-Marcel Klein, Nikolaus Moser

+Umbruch_M+H_englisch 18.09.2002 7:59 Uhr Seite 1

Contents

Introduction 4

Basics of filtration 9

Mechanisms of particle separation.......................................................... 10

Performance characteristics of filters ...................................................... 12

Engine intake air filters 20

Parameters for evaluating air filter media .............................................. 21

Methods of examining air filter media .................................................... 26

Requirements of modern air filter systems ............................................ 29

Design criteria for engine air filter elements .......................................... 33

Filter housings ......................................................................................... 37

Oil filters for crankcase breathers ........................................................... 39

Lube oil filtration 46

Wear and filtration................................................................................... 46

Full-flow oil filtration.............................................................................. 50

Bypass oil filtration ................................................................................ 61

Fuel filters 68

Functions of a modern fuel filter............................................................. 68

Gasoline filters ........................................................................................ 69

Diesel fuel filters ..................................................................................... 75

Methods of testing fuel filters ................................................................. 85

Performance data of fuel filters ............................................................... 87

Summary 91

Literature 93

The company behind this book 95

This book was produced with the technical collaboration of FILTERWERK MANN+HUMMEL GMBH.

The authors would like to thank Günter Görg, Dr. Michael Harenbrock,Markus Kolczyk, Jochen Reyinger, Dr. Pius Trautmann and Uwe Weipp-recht for their assistance in compiling the diagrams and manuscripts.

Translation: Sprachendienst Dr. Herrlinger, Kirchentellinsfurt

© 2002 All rights reserved withverlag moderne industrie, D-86895 Landsberg/Lechhttp://www.mi-verlag.de

First published in Germany in the series Die Bibliothek der TechnikOriginal title: Filtration in Fahrzeugen© 2002 by verlag moderne industrie

Illustrations: No. 5b Institute for Mechanical Process Engineering, University of Karlsruhe; No. 13 Freudenberg Vliesstoffe KG, Weinheim; No. 33 Kolbenschmidt Pierburg AG, Neckarsulm; No. 59 Robert BoschGmbH, Stuttgart; Nos. 67, 68 Institute for Mechanical Process Engineering,University of Stuttgart; all others FILTERWERK MANN+HUMMELGMBH, LudwigsburgTypesetting: abc.Mediaservice GmbH, BuchloePrinting: Himmer, AugsburgBinding: Thomas, AugsburgPrinted in Germany 930576


Introduction 54

IntroductionFiltration functions in motor vehicles have be-come extremely diverse and complex as a re-sult of the exacting requirements of modernengines, increasingly stringent environmentallegislation and, not least, rising demands forgreater comfort and convenience on the partof customers. Whereas the first cars were ini-tially fitted with the most rudimentary oil fil-ters, followed by intake air and fuel filters, anumber of other filtration functions are per-formed today. Only these can ensure thetroublefree operation of the engine and indi-vidual components throughout the entire life ofthe vehicle.Figure 1 shows the various locations of a carwhere filtration, separation and cleaning arecarried out. At first glance, it is clear that therespective requirements vary widely in natureand that all the filters must be specifically de-signed for their intended purpose, not least be-cause of the wide range of mounting locations.Today, many filtration functions can no longerbe considered separately; to some extent, theyare inter-related, forming an integral part of asystem and performing complex processes andfunctions within it, which go well beyond theactual task of separation.This can be seen, for example, with air filtra-tion (Fig. 2). In addition to the important, fa-miliar task of filtering dust from the engine in-take air, the air in the passenger compartmentsof over 95% of new vehicles produced in Eur-ope is now filtered to exclude particles andgaseous pollutants. Fuel tanks are increasinglybeing fitted with filter elements which retainparticles and, by a process of absorption, ex-tract fuel fumes from air which escape fromthe tank during refueling.

Fig. 1 (facing page):Filtration and separ-ation in vehicles

Complex filtra-tion demands

Consideration ofthe entire system

E-b

oxfil

ter

Wat

er s

epar

ator

Air

clea

ner

(incl

. se

rvic

e in

dica

tor)

Air

filte

rel

emen

tC

oolin

gw

ater

filte

rIn

-line

fuel

filte

rW

ashe

r sy

stem

filte

r

Fue

l filt

erm

odul

e

Met

al-f

ree

fuel

filte

rel

emen

t

Pla

stic

roc

ker

cove

r

Oil

mis

t sep

arat

or

In-t

ank

fuel

filte

r

Tank

ven

tilat

ion

filte

r

Die

sel p

artic

ulat

efil

ter

Ure

a fil

ter

for

SC

R-c

atal

ysts

Cab

in a

ir fil

ter

el

emen

tG

earb

oxoi

l filt

erF

ilter

for

pow

erst

eerin

g

Filt

er fo

rbr

akin

gsy

stem

Sus

pens

ion

hydr

aulic

filte

rA

ir dr

yer

box

Spi

n-on

filte

r

Oil

filte

r m

odul

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etal

-fre

eoi

l filt

erel

emen

t

Cra

nkca

se v

entil

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Introduction 7

ticles, continuous and/or regular regenerationof the (ceramic) filter presents the greatestchallenge.Figure 3 gives an overview of the various typesof liquid filtration in vehicles, with engine oil filtration occupying a leading role. In add-ition, long-lived filter media, extremely resist-ant to chemicals, are used to filter gearbox oil and to protect hydraulic systems whichmust meet ever-increasing standards of effi-ciency as a consequence of continuous techno-logical progress. The same applies to fuel fil-ters, another focus of this book, and for themedia used in them. In recent years, fuel fil-ters for diesel and gasoline engines with directinjection have undergone considerable devel-opment in order to keep pace with the require-ments of advanced injection systems.In addition to the filtration of coolant, whichhas proved particularly effective with commer-cial vehicles, it may well become necessary inthe future to filter aqueous solutions of urea.When the new, even more stringent exhaustregulations on nitrogen oxide emissions comeinto force (e.g. EURO-4 standards in 2004 or

6 Introduction

Ultrafine droplets of oil in the blow-by gas (acontinuous, heavily pulsating flow which leakspast the clearance gaps between pistons, piston rings and cylinder liners into the crank-case and which must be returned from the en-gine into the induction system) are separatedby suitable coalescing elements (fiber or diffu-sion separators), cyclones or centrifuges, inorder to meet the new exhaust emission limits.At the same time, these filters prevent depositsfrom forming on the blades of diesel engineturbochargers, or possible failure of entire in-jection systems through precipitation of car-bon from cracked oils onto the injectors.In the case of commercial vehicles with com-pressed-air braking systems, the air must bedried by adsorbent dehumidifiers (so-called airdryer boxes) to ensure that the brakes functionreliably even at very low temperatures. Des-pite all the technical and design measures andadvances achieved in recent times in the devel-opment of engines, the filtration of diesel ex-haust gases will continue to be a major subjectof research and development in the immediatefuture. Apart from the filtration of soot par-

Fig. 2:Objectives and mech-anisms of air filtrationin vehicles

Fig. 3:Objectives and mech-anisms of liquid fil-tration in vehicles

Examples of filtration of air …

… and liquids

Air filtration

App

licat

ion:

Tankventing

Crankcaseventilation

Engineintake air

Engineexhaust gas

Brakingsystem air

Cabin air

Obj

ectiv

e:

Oil dropletseparation

Exhaust gascleaning

Airdrying

Particleseparation

Gas cleaning

Mec

hani

sm:

Centrifugalforces, coales-cence, static

electricity

Filtration,adsorption

AdsorptionFiltrationFiltration,

adsorption

Particleseparation,

gas cleaning

Diesel sootfiltration,

regeneration,SCR

Filtration of liquids

Flu

id:

App

licat

ion:

Water Fuel Oil

CoolantGasoline

engineDiesel engine

Gearboxoil,

hydraulic oil

Engine oil(full-flow andbypass flow)

Obj

ectiv

e:

Particleseparation

Particleseparation

Particle andwater

dropletseparation

Particleseparation

Particleseparation

Mec

hani

sm:

Filtration Filtration FiltrationFiltration,centrifugalseparation

Filtration,coalescence


9

Basics of filtrationTo facilitate a better understanding of the pro-cesses of filtration, a number of importantbasics which underlie the subject are outlinedin the following sections. A comprehensivedescription can be found in the bibliography,e.g. under [2, 3].Most vehicles are fitted with depth filters.Depth filtration is always the most economicalmethod when there is a low concentration ofparticles to be separated. The purpose of thefilter elements used is to separate particles (thesolid phase) from fluids (the continuousphase), i.e. gases and liquids, to the utmost ex-tent. For example, solid particles are physic-ally removed from the engine intake air, thefuel and the lube oil. Impurities come from nu-merous sources and consist of, e.g. organicand mineral dusts, particles of abraded metal,and soot from incomplete combustion. Theydo not, however, appear only as solid particles,but may also be of liquid form, thus neces-sitating, for example, the filtration of drop-lets of oil from the blow-by gas in crankcaseventilation or droplets of water out of dieselfuel.Particles may be round or angular, flat or cu-boid in shape, and rough or smooth. As a rule,they can be described in accordance with ascale of sizes. Depending on origin, these mayextend over a wide range. Mineral particles,principally grains of quartz (SiO2), occur as“dust” in the intake air. Their average diameteris between 0.1 and 2000 µm (Fig. 4, illustrat-ing the distribution density of dust particles by quantity q0 and corresponding distributiondensity by mass q3). The diameters of primarysoot particles range from a few nanometers tothose of agglomerates (between 0.1 and 2 µm),

8 Introduction

EURO-5 standards in 2008), some form ofSCR (selective catalytic reduction) technologywill be needed to ensure reliable observance ofthe exceptionally low NOx limits.Of course, not all the filtration functions incars mentioned above or shown in Figure 1 canbe described within the framework of this book.In addition to certain important principles,essential to an understanding of filtrationprocesses, the first part contains informationabout the construction and characteristics offilter materials or media. It then turns to appli-cations. The filtration of engine intake air (in-cluding crankcase ventilation), lube oil andfuels is considered in detail and the considerableadvances achieved in the last ten years are de-scribed [1].The book focuses particularly on filtration andseparation, filter media and the filter elementsproduced from these materials. At the sametime, it is intended to illustrate how the processas a whole and interaction with other compon-ents must be taken into account when design-ing filter elements. Filter elements incorpor-ated in the engine, the cylinder head cover, oiland fuel filter modules and air intake systemsare only capable of functioning economicallyand efficiently if the requirements of all the in-dividual components involved are regarded asa complete system and are matched to theoverall process in the optimum way.

Depth filtration

Sources of impurities

Characteristicsof contaminantparticles


Interception

Fiber

Diffusioneffect

Inertia effect

a)

Flow direction

Mechanisms of particle separation 11

ticles (with diameters less than about 0.5 µm)also move irregularly through the fluid (Brown-ian movement). If these strike the fibers “bychance”, they are likewise trapped (diffusioneffect).The process of separation can be greatly affect-ed by other interactions, such as electrostaticforces generated by electric charges on thesurface of so-called “electret fibers” or forcesimposed from outside, e.g. the centrifugalforces generated in cyclones and centrifuges.On striking the fibers, the particles are retained

10 Basics of filtration

a size range which is considered critical withregard to wear.

Mechanisms of particle separationWidely differing methods are employed to sep-arate particles, depending on the size of theparticles and the properties of the fluid. Figure5 shows the interactions between particles andthe filter medium underlying the filtration pro-cess. The filter medium is depicted as a singlefiber whose axis runs perpendicular to theplane of the image.The continuous phase (e.g. the oil or fuel) ex-hibits a laminar flow over the fibers, which isindicated by the stream lines, i.e. the curvedpaths of the fluid (in which the flow velocity vis not constant). When large particles of con-siderable mass approach an obstacle, e.g. thefibers, their inertia causes them to leave thestream line and to collide with the fibers (iner-tia effect). Somewhat smaller particles are ableto follow the stream lines. If they are just largeenough in diameter to touch the fibers, theycling to them (interception). Still smaller par-

Fig. 4:Particle distributiondensities of originaldust taken from an airfilter element. Par-ticles with a diameterof between 5 and 100 µm represent thegreatest proportion(> 75% by mass).(For a detailed ex-planation of the dia-gram, see Figs. 8 and9, pp. 15 and 16.)

Fig. 5:a) Schematic descrip-

tion of separatingmechanisms at asingle fiber

b) SEM (scanningelectron micro-scope) picture of asingle fiber ladenwith particles (gasfilter)

on the surface through adhesion forces, pre-dominantly “van der Waals” forces.Interception is the essential separating mech-anism in the filtration of viscous fluids, i.e.under the general conditions prevailing in oiland fuel filters, the fluids predominantly flowthrough the filter media in laminar flow. Sinceair is considerably less viscous, inertia effectsand the unobstructed diffusion of the smallestparticles in the turbulent air flow constitute es-sential separating mechanisms in addition tointerception. As a result, it is highly probablethat the fine particles will strike the fibers of adepth filter medium, thus achieving a high de-gree of separation efficiency. Sieving effects

Inertia …

… interceptionand diffusioneffects

Mechanisms for liquid …

Particle diameter [µm]

0.1 0.7 1.6 3.6 8.1 19 43 98 224 514 1178 2046

1

0.8

0.6

0.4

0.2

0

0.05

0.04

0.03

0.02

0.01

0

Par

ticle

dis

trib

utio

n de

nsity

by n

umbe

r q 0

[1/µ

m]

Par

ticle

dis

trib

utio

n de

nsity

by

mas

s q 3

[1/µ

m]

Particledistributiondensity bynumber q0Particle distribution density bymass q3

b)


Performance characteristics of filters 13

gradual initial rise in differential pressure istypical of the behavior of depth filters. It isonly after a certain period of time, when ahigh proportion of the pores are clogged withparticles, that the differential pressure risessteeply. To prevent the engine from losingpower, the filter element should be changedat the latest when the predefined maximumdifferential pressure, e.g. as laid down in thecar manufacturer’s specification, is reached.

Filtration efficiencyA variety of methods are used to evaluate theefficiency of filters. The filtration efficiency η(often also referred to as the degree of separa-tion or filter fineness) represents the proportionof particles trapped by the filtration process.Within this context, a distinction is made be-tween grade efficiency and overall filtration ef-ficiency. Whereas the grade efficiency relatesto individual particle diameters or categories ofparticle sizes (fractions), the overall efficiencydescribes the filter effect for all particles by thefilter. For example, the statement η (3–5 µm) =87% means that 87% of the particles with adiameter between 3 and 5 µm are separated bythe filter under consideration while the state-ment η = 95% means that 95% of all particlesin the fluid will be trapped by the filter.The initial filtration efficiency is another im-portant characteristic. This gives the efficiencyof a new filter medium or element. In the caseof depth filters without electrostatic charges, itis generally lower than that of a laden filtermedium in that, when fibers are alreadycovered with particles, the probability of itstrapping oncoming particles is higher than that of new “unladen” fibers with a “real”smaller diameter (cf. Fig. 5b).Figure 7 shows the graph of the grade effi-ciency as a function of particle size xj based


of the type achieved by surface filters playonly a subordinate role in particle filtration incars up to now.

Performance characteristics of filtersThe filter media used in vehicles are confinedalmost exclusively to depth filters. In otherwords, the separation of particles takes placeon the surface of the individual fibers deepinside the texture of the medium. At the be-ginning of the filtration process, individualparticles initially settle on the surface of thefibers. Over time, the density of the accumu-lation increases, allowing dendrite-like for-mations to develop (cf. Fig. 5b, p. 11). These,in turn, cause the volume of pores available to accommodate the filtered particles in themedium (which, depending on application,may be as high as to 95% porosity), to di-minish. By contrast, the differential pressure at the same flow rate increases. After a certain period of service, the capacity of thefilter is exhausted, necessitating its replace-ment.The increase in the differential pressure in afilter as a function of operating time is shownschematically in the diagram in Figure 6. The

Fig. 6:Schematic descriptionof the increase ofdifferential pressureof a depth filter as afunction of dust loador time, respectively.The filter elementshould be changed att1 at the latest.

… and airfiltration

Differentialpressure increase

Fractions ofseparated particles

Dust load or time

t1

∆pmax

∆po

Diff

eren

tial p

ress

ure

∆p



to which the filtration efficiency relates. Al-though conventional reference values arebased on the number of particles (characterizedby the index i = 0), in most cases the data relateto an equivalent volume of particles (e.g. asphere of the same volume, i = 3) or to the massof the particles (by multiplying the volumevalues with the mean density of the particles).Results of the measurements of particle sizedistribution can be presented in the form of acumulative distribution curve Qi(xj) (Fig. 8a).This derives from the sum total of the propor-tions of particles by quantity, from the smal-lest in size xj,min to the largest xj,max. The valueQi(xj) represents the non-dimensional propor-tion by quantity relative to the overall quantity


on the example of an air filter element consist-ing of synthetic fibers. The trough in the curveat approx. xj = 0.5 µm indicates that the fil-tration mechanisms which predominate in airfiltration (i.e. diffusion and inertia effects)have either not fully developed in this area

Fig. 7:Grade efficiencycurve as a functionof particle size

(left end of the curve, where diffusion predom-inates), or are no longer fully developed (rightend of the curve, where inertia effects pre-dominate). Virtually 100% of larger particles(> 6 µm) are separated as a result of inter-ception and inertia effects.

Particle size distributionUsually, the distribution of particles in thevarious fluids under operating conditions is anunknown quantity. To compare the efficiencyof individual filters, they are tested with stand-ardized test dusts. Official standards also ap-ply to methods of testing (e.g. single-pass ormulti-pass arrangements) as well as samplingand evaluation (on-line or off-line tests, optic-al evaluation with particle counters or gravi-metric evaluation by weight analysis).For comparisons of filter element performance,it is also necessary to know the reference value

Fig. 8:a) Cumulative distri-

bution curveb) Particle distribu-

tion density

Reference valuefor filtrationefficiency

Cumulativecurve

Particle size xj [µm]

100

50

00.5

98%

1.0 4 5 10 20

Gra

de e

ffici

ency

η(x

j) [%

]

Particle size xj

Cum

ulat

ive

dist

ribut

ion

curv

e Q

i(xj)

Par

ticle

dis

trib

utio

nde

nsity

qi(x

j)

0

0.5

a)

b)

1

xj

Qi(xj)

qi(xj)

∆Qi(xj)qi(xj,m)

∆xj xjxj,mxj-1

xj,maxxj,min

Initial dustdistribution

xj,maxxj,min

Particle size xj



vals ∆xj are not equidistant; the dimension ofqi(xj) is that of a length [1/µm].The effectiveness of a filter can be clearly pre-sented in graphic form in this way. Figure 9ashows the initial particle distribution densityof the dust qA(xj), the distribution density g · qG(xj), which represents the distribution ofall separated particles (often referred to ascoarse material or coarse dust), and the distribu-tion density f · qF(xj), which corresponds to thedistribution of particles having passed throughthe filter (often referred to as fine fraction orfine material). The values f and g represent theintegral fine or coarse proportion of the dust (f + g = 1); qF(xj) and qG(xj) are not normed.So, the sum total of the individual areas ofcoarse and fine material distribution densitiesrepresents the initial particle distribution dens-ity curve qA(xj).

Grade efficiency curveFrom these results, it is possible to derive thegrade efficiency curve TG(xj) which representsthe result of the filtration process in graphicform (Fig. 9b). The curve begins at a particlesize of xj,0, up to which 100% of particles stillpermeate the filter. It extends to particle sizexj,100, from which point 100% of the particlesare separated by the filter.A characteristic value is often singled out as ameans of describing the overall effectivenessof filters. For example, the particle size xj,50 isfrequently given, at which the filtration effi-ciency η(xj) or T(xj) equates to 50%. Thus,50% of particles size xj are separated by thefilter while 50% pass through it. In manycases, these reference values are also definedas “filtration efficiency”.In keeping with the above system, the designa-tion q3(xj) therefore represents a particle distri-bution density by mass or volume, whilst the


up to the respective particle size xj (frequentlyalso designated as “fraction”). It can assumevalues of between 0 and 1 (corresponding toproportions of 0 and 100%).The cumulative distribution ∆Qi(xj) is fre-quently standardized by dividing it by the re-spective interval width ∆xj (Fig. 8b). If thesevalues are superimposed on the average par-ticle size of interval xj,m, the result is the so-called particle distribution density curveqi(xj,m). The area below the distribution densitycurve corresponds to the sum total of indivi-dual products qi(xj,m) multiplied by ∆xj andagain equals 1. The histogram, obtained as theresult of including discrete measured values,e.g. with the aid of a particle counter, is like-wise recorded (cf. Fig. 4). Normally the inter-

Fig. 9:a) Graphs showing

the initial distribu-tion density ofparticles and dis-tribution densitiesof coarse dust(particles retainedby the filter) andfine dust (particlespassing the filter)

b) Grade efficiencycurve TG(xj) of fil-tered particles

Particle distri-bution densitycurve

Coarse and fineproportion

Grade efficiency …

0

50

100

Particle size xj

xj,0 xj,50 xj,100

xj,maxxj,min

g · qG(xj)Coarse dust (G)

Fine dust (F)Initial particle distri-bution density qA(xj)

Proportions byquantityg + f = 1

f · qF(xj)

Gra

de e

ffici

ency

cur

ve T

G(x

j) [%

]P

artic

le d

istr

ibut

ion

dens

ity q

i(xj)

TG(xj) = g · ∆Qcoarse/∆Qinitial

xj,0 xj,50 xj,100

Particle size xjb)

a)

… or filtrationefficiency



In general, the pleated filter medium, i.e. thefilter element, should form the basis for acomparison of performance. In this way, it ispossible to detect damage to the medium dur-ing the production process or even locationswhere glued fastenings have parted company.

Filter service intervalA further characteristic indicating the perform-ance of filter media or elements is the filterservice interval or service life. This is definedby the specific dust holding capacity (definedin [g/m2]) or the dust holding capacity G (de-fined in [g]) of the element. In this way, it ispossible to designate the amount of dust or themass of trapped particles which the medium orelement is capable to hold or to store before apredetermined maximum differential pressureis reached. This material-related value pro-vides the basis for the design of the filter. Atthis point it should be noted that particles ofdifferent sizes and sources have entirely dif-ferent effects on filter media. In general, ahigh number of small particles (e.g. soot) willclog an air filter element appreciably morequickly (given less mass striking the element)than coarse particles of sand and dust.


designation q0(xj) represents a particle distri-bution density by number. Usually, both curvesand the values derived from them vary consider-ably for one and the same dust (cf. Fig. 4).In some cases, the particle distribution dens-ity by mass q3 is used as a standard, so it is con-venient to omit the index i = 3.

β-valueSince the differences between filters offeringhigh filtration efficiencies are often not ob-vious at first glance, the β-value is increas-ingly being used to describe the performanceof a fluid filter element. This is defined as thenumber N1 of particles up to a certain size xjupstream of the filter, divided by the measurednumber N2 of particles of the same size inter-val or the same particle size downstream ofthe filter, i.e.

β(xj) = (N1 ≥ xj)upstream /(N2 ≥ xj)downstream

This value, which clearly explains the differ-ences in performance between individual fil-ter elements, particularly in the case of highefficiencies, can be derived from the filtrationefficiency η(xj), too, i.e.

β(xj) = 100/(100 – η(xj))

A filter with a β-value of 200 for particle sizexj = 10 µm (corresponding to a filtration effi-ciency of 99.5%) is four times more effectivethan a filter with a β-value of 50 for particlesof the same diameter (here, the filtration effi-ciency is 98%). In other words, a filter withβ(10 µm) = 1000 or η(10 µm) = 99.9% allowsonly half as much dust and particles to passthrough it as a filter with β(10 µm) = 500, i.e.η(10 µm) = 99.8% (Table 1). Although thepercentage difference in efficiency is only0.1%, filtration performance is twice as effi-cient.

Table 1:Filtration efficiencyand correspondingβ-values

Dust holdingcapacity

η(xj) β-value[%]

0 1

50 2

80 5

90 10

95 20

98 50

99 100

99.5 200

99.8 500

99.9 1000

99.98 5000


Parameters for evaluating air filter media 21

lie within the 5 to 100 µm size range. In thisregard, the overall distribution and concen-tration factors depend on the surroundingenvironment. Figure 10 represents the averagemass concentration of dust under differentconditions of use.If the air is poorly or only inadequately filter-ed, these particles of dust will penetrate intothe engine and, to some extent, into the oil. Inthis way, they then penetrate into critical areassuch as the clearance gaps between the cylin-der liners and pistons, piston rings and con-rods, where they cause wear on the compon-ents [5].Particles suspended in the air not only con-tribute to wear in the engine but may alsoform deposits on the sensitive air mass sen-sor (HFM), which is located on the clean airside downstream of the intake filter. Thissensor is responsible for metering the de-livered volume of fuel. If the signal deviatesfrom the desired value, losses of power willresult, and thus increased fuel consumptionand pollutant emissions. Modern air filtersachieve filtration efficiency of up to 99.8%(cars) and 99.95% (commercial vehicles). Asa result, inducted volumes of dust and theconsequent risk of wear are substantially re-duced.

Parameters for evaluating air filter media

Modern air filter media are expected to com-ply with the filtration values laid down in therelevant specifications for, e.g. dust holdingcapacity and the overall degree of filtration ef-ficiency under all operating conditions. Thefilter media must exhibit high stability underpulsating forces and not allow any dust to per-

20

Engine intake airfiltersWhen running, modern car engines of mediumsize (with a power output of around 60 kW)induct about 6 m3 of air per minute. Depend-ing on the type and place of use, the particlecontent in the air may range from less than 0.2to 50 mg per m3 [4]. The total volume of con-tamination or particles directed into the ve-hicle in the course of its life can be estimatedas follows. For the complete combustion of 1 kg of fuel, the engine requires 14 kg or 10.8 m3 of air. Assuming an annual mileage of12,500 miles and fuel consumption of approxi-mately 30 mpg, the engine inducts 12,400 m3

of air a year. This means that between 24 gand 6.2 kg of dust are directed into the vehicleengine over a period of ten years. The dust par-ticles contained in the air inducted by the en-gine are of between 0.01 and 2000 µm in diam-eter. Some 75% of the mass-related particles

Fig. 10:Concentrations ofdust under differentconditions of use,measured 1 m fromthe ground

Amount of particles

Consequences of inadequatefiltration

Requirements ofair filter media

Dus

t con

cent

ratio

n [m

g/m

3 ]

200

1006040

20

1064

2

10.60.4

0.2

0.1

Other European roads

Scandinavian and Southern European roads

Construction site

Agricultural land

Convoy of vehicles

Crawler trucks



(DIN 53 887) or size of pore [6,7]. Other char-acteristics are resistance to tearing and burst-ing (DIN 53 113), bending (DIN 53 864),flame-retarding category (DIN 53 438), andfiber diameter and length. The diameters ofcellulose and staple synthetic fibers lie be-tween 10 and 50 µm. Melt-blown (synthetic)fibers are finer. In the case of paper thicknesses

22 Engine intake air filters

meate even under dynamic conditions, i.e. en-gine pulsation. In addition, the typical struc-ture of pleats (high area density occupying theminimum of space – see also Fig. 11) of thefilter element must not change if water pene-trates into the filter, as may occur when the ve-hicle is driven through drizzle or rain. Further-more, a good quality air filter medium must beresistant to engine oils, fuel fumes and crank-case gases which reach the medium from theintake air or as a result of diffusion (when theengine is stopped). Finally, materials must ex-hibit high thermal stability, since the tempera-tures at the filter element can rise to 90°Cwhen driving the vehicle.

Material-related parametersAir filter media in motor vehicles consist ofrandomisations of natural (cellulose) or syn-thetic (e.g. polyester) fibers (Fig. 12). The char-acteristics of media are defined by data relat-ing to grammage, thickness, air permeability

Fig. 11:A filter element madeof high-efficiencynon-woven material(left) takes up 35%less space than apaper filter element(right) of the samedust holding capacityand fineness.

Fig. 12:a) Standard filter

paperb) Multi-layer filter

paper(top: plan view;below: cross-section)

Table 2:Characteristic datafor different filtermedia

of 0.45 mm, cellulose-based air filter mediahave a grammage of around 100 g per m2. Theparameters for various media are summarizedin Table 2.

Configuration ofair filter media

a) b)

Filter medium Specific dust holding Grammage [g/m2]capacity [g/m2]

Paper 190 – 220 100 – 120

Multi-layer medium 230 – 250 100 – 120(composite)

Non-woven1) 900 – 1100 230 – 250

1) High-efficiency non-woven, structure as Fig. 13



graded structure and the same filtration effi-ciency η achieve a specific dust holdingcapacity of 900 to 1100 g per m2 (Tables 2 and3). The area of the filter medium which can becontained in a filter element of a fixed size isless in the case of a synthetic non-woven thanthat of a cellulose medium. According to ISO 5011, the dust holding cap-acity of a filter element based on a syntheticnon-woven is up to 50% greater than cellu-lose. Initial practical trials, however, indicatethat increases of up to 150% can eventually beexpected.

Pleat patterns and impregnationPleat patterns are particularly important for thefunctional efficiency of the filter element.Only if the arrangement of pleats remains un-changed is it possible for the specific dust hold-ing capacity measured in the laboratory to beachieved throughout the lifetime of the filter.In the case of filter papers, this stability is par-tially achieved by impregnation, which signifi-cantly improves the bending resistance of thecellulose medium and protects the fibers fromenvironmental influences. In addition, air filtermedia are embossed, with the result that thepleats support each other and thus stabilize thefilter element. In the case of wholly syntheticmedia which are not impregnated, the emboss-ing plays a particularly important role. Corru-


Dust holding capacityIn the development of new filter materials, therequirement for prolonged service intervals,i.e. high dust holding capacities, has prior-ity. For example, efforts are being made toextend the service intervals for car filters to75,000 miles without any increase in size. Inthe case of commercial vehicles, these valueshave already been reached or significantly im-proved upon, as a result of generously propor-tioned filter elements.The texture of modern filter media which meetthese exacting demands is distinguished by apronounced graded structure. Figure 13 shows

Fig. 13:Section through ahigh-efficiency non-woven material withnoticeably gradedstructure, for air fil-tration

a non-woven material which is considerablymore dense on the clean air side than on theincoming air side. As a result of this controlleddensity progression, a graded and, in turn, se-lective degree of efficiency is achieved withinthe filter medium. This leads to excellent dustholding capacities which are significantly higher than the values for standard media. Standard materials based on cellulose achievespecific dust holding capacities up to 220 gper m2. By comparison, non-wovens with a

Table 3:Characteristic datafor different filtermedia

Objective: longer serviceintervals

Comparison ofdifferent media

Stability andprotection fromenvironmentalinfluences

Filter medium vcrit Filtration efficiency [%][cm/s]

Car (gasoline) Car (diesel) Lorry

Paper 10 > 99.5 > 99.8 > 99.9

Multi-layer medium 17 > 99.5 > 99.8 > 99.9(composite)

Non-woven1) 33 > 99.8 > 99.8 > 99.9

1) High-efficiency non-woven, structure as Fig. 13

Direction of incoming air


Methods of examining air filter media 27

in ISO 5011, i.e. at a temperature of 23±5°Cand relative atmospheric humidity of 55±15%(Fig. 15). The filtration efficiency can be de-termined by two complementary methods.First, the filtration efficiency η is calculatedfrom the relationship between the increase inweight of the filter medium and the weight ofthe dust directed into it. This method usesstandardized dust (PTI coarse/fine) in order to obtain consistent comparable informationon the retaining capacity of different filter


gated structures, for example, have provedsuccessful here.In order to investigate how the distortion ofpleats is affected by water, and how the conse-quent change in the pressure drop affects thefilter element, an experiment was carried out inwhich water was applied to the media to simu-late rain (Fig. 14). The diagram represents acomparison between the progressive losses ofpressure in an impregnated paper air filter anda synthetic air filter. Although a high pressuredrop rapidly builds up in the paper filter, thesynthetic medium remains uncritical.Since cellulose media age or become brittledespite their protective impregnation, caremust be taken to restrict their use to not morethan five years, even in low-mileage vehicles.Damage cannot be ruled out, due to high ther-mal and mechanical stresses.

Methods of examining airfilter mediaThe effectiveness of air filter media is deter-mined under standardized conditions laid down

Fig. 14:The addition of waterincreases the resist-ance to volume flow(measured at thenominal flow rate) of non-woven filtermedia to a lesserextent than that ofpaper filter media.

Fig. 15:Layout of a test rigfor the investigationof filter media

Fig. 16:Distribution ofcoarse and fine PTIdust by particle size.The proportion ofparticles with a diam-eter of 5 µm is only1% of coarse dust,3% of fine dust. Par-ticles with a diameterof 50 µm account for 5% of coarsedust and around 2% of fine dust.

Standardizedconditions

0

30

25

20

15

10

5

00.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Water input [l]

Pre

ssur

e dr

op [m

bar]

Paper filter elementNon-woven filter element

Conveyor belt dust dispenser

Dust

Testspecimen

Absolutefilter

Compressed air

Expansion tank

Fan

RI

PI

FIC


6

5

4

3

2

1

00.5 5 50P

ropo

rtio

n by

vol

ume

[%]

PTI finePTI coarse


Requirements of modern air filter systems 29

filter positioned in the intake duct does notaffect engine output to any significant extent.The dust holding capacity G measured in thelaboratory is correlated with the results of on-road trials to calculate the permitted period ofuse of the filter in the vehicle. The laboratoryinvestigations are reinforced by regular prac-tical tests using fleets of vehicles and station-ary ambient air test rigs to determine this im-portant information. These tests eventuallyshow the progression of differential pressureat a constant air flow rate as a function of time(Fig. 18).

Requirements of modern air filtersystemsA modern air filter system consists of an incom-ing air duct with air intake and a filter housingwith filter element. Under certain circum-stances, these are supplemented with dampers(resonators) to reduce noise at the mouth. Theclean air duct, positioned downstream of thefilter, contains the air mass sensor and termin-ates at the engine with the engine inlet mani-fold. The exhaust gas return system and crank-


media (Fig. 16). Detailed information on thefiltration efficiency of filter media is obtainedfrom measuring the filtration efficiency as afunction of particle size, i.e. the grade effi-ciency η(x) (Fig. 17).The proportion of contamination directly rele-vant to the engine can be deduced from thecomplementary factor to the filtration effi-ciency η, namely the degree of permeabilityD. If two filter media are considered, with afiltration efficiency of 99.5 and 99.9% respect-

Fig. 17:Grade efficiency ofa standard filter me-dium. 97.5% of par-ticles of 0.4 µm indiameter are filteredout after a pressuredrop increase of 1 mbar; when thedifferential pressurerises to 5 mbar, thegrade efficiency is100%.

ively, the resulting degrees of permeability are 0.5 and 0.1%. This result immediately re-veals that the volume of particles affecting theengine is four-fifths lower in the case of thesecond filter. Although its overall degree offiltration η is only 0.4% higher, the secondfilter is five times more effective than thefirst.To determine dust holding capacity G, dust isdirected onto the filter element until its resist-ance to permeability increases by a predefinedvalue ∆p (e.g. by 20 mbar in cars and 40 mbarin commercial vehicles at the nominal flowrates in each case). This ensures that the air

Fig. 18:The investigationinto air filter media(filters 1 and 2) onthe ambient air testrig realistically re-flects the perform-ance of the filterelements. The pres-sure drop at the filterelement as a functionof time is depicted.

Measurement of grade effi-ciency …

… and dust holding capacity

Air filter systemcomponents

Particle size [µm]

100

98

96

94

92

900.1 1 10

Gra

de e

ffici

ency

[%]

Filtration efficiency after ∆p = 1 mbarFiltration efficiency after ∆p > 5 mbar

Running time [h]

50

40

30

20

10

00 500 1000 1500 2000 2500 3000 3500

Flo

w r

esis

tanc

e [m

bar]

Filter 1Filter 2

∆ = +140%


Requirements of modern air filter systems 31

straints in terms of the space available for fil-ters. A typical outcome of the desire for in-creasing passenger comfort is that an air con-ditioning system has become standard equip-ment, even in Europe. This, together with thepower steering pump, the intercooler, the heatexchanger for recycled exhaust gas and thesecondary turbocharger for reducing exhaustemissions reduce the space available for filters(Fig. 20). This has, amongst other things, ledto the development of new filter media whichachieve the same performance as traditionalfilter papers yet occupy up to 35% less space.In this way, the high degree of integrationdensity required can be achieved for air filtersystems (Fig. 21).


case gas feeder pipe are located between these(Fig. 19). The individual components areadapted in function and design to fit into thelimited space under the hood.Modern high-performance engines, particu-larly car engines, repeatedly impose new con-

Fig. 19:A complete air intakesystem (modulardesign) including fil-ter housing, air masssensor, bypass reson-ator and engine inletmanifold

Fig. 20:Available space inthe engine compart-ment of vehicles ofdifferent years ofmanufacture Fig. 21:

By the use of new fil-ter media, optimizedflow control in thefilter housing and anappropriately adapt-ed acoustic layout,high integration dens-ities can be achievedfor filter systems(left: starting condi-tion; right: optimizedfilter).

The position of the air intakes should bechosen to minimize exposure to dust andwater. They should preferably be located inareas of the vehicle which are unaffected bythe airstream, e.g. in the wheel arch enclosuresor similarly unaffected locations in the enginecompartment. In the case of trucks, the intakeis usually positioned above the driver’s cabroof or at the side of the cab to minimize thevolumes of inducted dust [8] and to achievelonger service intervals (Fig. 22). With filter housings optimized for the incom-ing airflow, it is possible to utilize the poten-tial of the filter media as regards dust holding

Reduced spacefor filters

1.6-l engineYear of manufacture 1988

1.6-l engineYear of manufacture 1999


Design criteria for engine air filter elements 33

is that each element can be optimized for itsparticular purpose. In such cases, the filterhousing, which must be accessible from abovefor inspections, can be made to the optimumdegree of compactness.Another very recent method of noise insula-tion is offered by the Active Noise Control Sys-tem. This absorbs the characteristic noises(frequency spectrum) of the engine and invertsthem electronically, i.e. the phase angle is dis-placed from the original signal by half a wave-length. At an appropriate engine speed, the in-verted signal is re-emitted through a loud-speaker and the initial noise actively silenced.Significantly improved silencing can beachieved with these systems, compared withpassive systems.

Design criteria for engine air filter elementsIn the design of engine air filter elements, a dis-tinction is made in respect of filtration effi-ciency between filters for cars with gasoline or diesel engines and filters for commercialvehicles (cf. Table 3). The area of the filtermedium is calculated from the volume of air


capacity and filtration efficiency to the utmostextent. The uniform airflow (Fig. 23) affords aconsistently high degree of filtration from thevery start (see also Fig. 24). The filter housingis frequently larger than would be necessaryfor filtration as its acoustic characteristics canbe significantly improved by this (passive sys-tems). This type is referred to as a silencing air filter because acoustic insulation and thefiltration process are combined in a singlecomponent. Alternatively, the enlarged filterhousing can be replaced by separate elementsto insulate noises at the mouth of the intake.The advantage of separating the two functions

Fig. 22:The position of theair intake is decisivefor the volume ofinducted dust andwater (measured asper [8]).

Fig. 23:Simulations of theairflow through afilter housinga) Non-uniform flowb) Uniform flow

through the filterelement

Fig. 24:Diagram depictingpenetrating dust orfiltration efficiency asa function of specificload. At values below1500 cm2/ (m3/min) orspeeds higher than 11 cm/s, the degree of filtration declinesdrastically. If the flowonto the surface of thefilter element is non-uniform, localizedoverloading may oc-cur, leading to a re-duction in filtrationefficiency.

Distinction between filtersfor cars …

1

4

3

2

Intakepoint

1 13

2 78

3 60

4 22

Dust in filter[mg per 60 miles]

a) b)

Specific load [cm2/(m3/min)]

1000

– 0.2

– 0.4

– 0.6

– 0.8

– 10 1000 2000 3000 4000 5000

99.8

99.6

99.4

99

99.2Per

mea

ting

dust

[%]

Filt

ratio

n ef

ficie

ncy

[%]


Design criteria for engine air filter elements 35

vcrit. Table 3 shows the critical speeds for dif-ferent filter media with provision for the filtra-tion efficiencies required by gasoline and diesel engines.To give an example of filter design: Given aspecific volumetric airflow (V̇ = 5 m3 per min)and type of vehicle (car with diesel engine),the filter area required to maintain the filtra-tion efficiency η is calculated. In this exampleη equals 99.8%, with the result that a filterarea of A = 1.25 m2 is required. The choice offilter medium is determined by the requireddust holding capacity (e.g. 200 g) or the targetservice interval (37,500 miles). According toTable 2, the designated requirements are metby a square meter of standard filter paper. Theuse of a paper filter of this size, however,would exceed the critical filtration rate of vcrit = 10 cm per s. To achieve the required fil-tration efficiency, the larger area of 1.25 m2

would be needed.Single and two-stage air filters are installed incommercial vehicles. The choice of system de-pends on whether the vehicles are used onlong-distance highways in Central Europe,over dust-laden terrain (e.g. construction sites,agricultural land), or on unmade tracks inSouth America.Single-stage air filters usually contain roundstar-pleated filter elements (Fig. 26). In excep-tional cases, square filters are also used. Thefiltration efficiency of 99.9% laid down in thespecification for the filter media is higher thanthat for cars, reducing dust permeability by50% (from 0.2 to 0.1%). The underlyingreason for this is the essentially longer oper-ating hours of truck engines, equivalent toover 60,000 miles a year.Fail-safe filter elements are frequently inte-grated into air filters. When the main filter ischanged, these prevent the clean air side from


required by the engine. The mean volumetricflow rate for a four-stroke gasoline engine is0.07 m3 per minute per kW and for a four-stroke diesel engine 0.08 m3 per minute per kW,based on the power output of the engine. Forobvious reasons, the airflow rate is corres-pondingly higher in the case of turbochargedengines.To remove particles from the incoming air, afilter of sufficient area is installed so that theairflow speed does not exceed a certain criticalvalue vcrit (cf. Fig. 24). The filtration effi-ciency of the filter is dependent on the airflowspeed. Excessive speeds dramatically reducefiltration performance and, in turn, the filtra-tion efficiency of the medium. If the airflowspeed is too high, particles no longer cling tothe fibers but bounce off. Moreover, trappedparticles are re-released (re-entrainment ef-fect). The filtration efficiency diminishes ac-cordingly and the engine is subject to morewear and stress. These effects can be avoidedif the correct filtration velocity is selected, be-cause the particles are then able to form de-posits on the fibers (Fig. 25). Each filter me-dium has a different characteristic airflow speed

Fig. 25:Photograph (takenwith a scanning elec-tron microscope) ofparticles of between1 and 25 µm in size,trapped by the filtermedium

… and commer-cial vehicles

Relationshipbetween filterarea …

… airflow speed …

… and filtrationefficiency

Typical layout


Filter housings 3736 Engine intake air filters

becoming contaminated. Consisting usually ofcylindrical non-woven elements, they are onlyreplaced after around three main filter replace-ments, depending on dust levels.In the case of two-stage air filters, the air isprefiltered by the tangential inlet before itreaches the filter element. Coarse particles, inparticular, are separated by centrifugal force.Depending on design, a prefiltration efficiencyof up to 85% can be achieved. Significantlyless dust reaches the downstream filter elem-ent, resulting in a marked increase in durabil-ity. At the same time, however, detailed opti-mization and adaptation are required, sincefine dust clogs on the filter element muchmore rapidly than dust consisting of particlesof all size categories. In the case of commer-cial vehicles, the filter element should bechanged as soon as the pressure drop has risenby 40 mbar. The prefilter is similarly affectedby a reduction in pressure, which must betaken into account to determine the nett lengthof service life.

Fig. 26: a) Single-stage air

filter with cylind-rical filter elementfor commercialvehicles. Incomingair enters the inletsocket on the right,passes through thefilter element andexits by way of thecentral pipe andair outlet socket atthe top.

b) Two-stage air filterfor commercialvehicles. Incomingair is set in rotationby the tangential in-let. Dust is drivenonto the outerwalls, from whereit falls into a col-lecting receptacle(bottom). The pre-filtered air passesthrough the filterelement.

Fig. 27:Optimizing an air fil-ter housing with theaid of computationalfluid dynamics(CFD) computationsand models of thecorresponding distri-bution of the airflowon the unfiltered airside (right). The fil-ter medium performsat its best when theairflow is uniformlydistributed.

a) b) Filter housings

In the design of the filter housing, two factorsare of major importance, namely the availablespace and the ease of servicing, i.e. it shouldbe possible to replace the filter element with-out difficulty. This aspect is directly linked tothe requirements for the filter housing to be ef-fectively sealed. Most air filter elements havea polyurethane (PUR) seal. The maximum per-missible air leakage rates at a pressure dropof 20 mbar are 50 cm3 per min for trucks and200 cm3 per min for cars. These levels are tobe maintained under all operating conditions,i.e. at temperatures between –40 and +100°Cand under typical vehicle-related vibrations be-


Oil filters for crankcase breathers 39

baffle plates or, if the associated reduction inpressure is permitted, cyclones or anti-snowsystems. At this stage, it should again be notedthat the filter element does not retain water; ifwater gets that far, it will continue on throughto the clean air side and into the engine.

Oil filters for crankcase breathers

In addition to filtration of the engine intakeair, all internal combustion engines necessitatefiltration of the air vented from the crankcase.When an engine is running, so-called “blow-by gases” flow through the unavoidable clear-ance gaps between the pistons and cylinderliners, and in the valve guides and turbochar-ger bearings, into the crankcase. In addition toremnants of fuel and the intermediate and end-products of the combustion process and soot,these gases sometimes contain significantamounts of engine oil. Droplets of oil, suspend-ed in the blow-by gases, break away from thefilm of lubricant adhering to the pistons andcylinder liners, drip from rotating componentsand the piston crown cooling system, and formultrafine condensation aerosols consisting ofvaporized engine oil and water vapor.In the past, it was customary to extract the oilfrom this gas in a straightforward way and torelease it into the atmosphere through an“open breather”. Today, this process is unac-ceptable to customers and legislators alike.The problem has been solved by the “closedbreather”, whereby the blow-by gas includingits impurities, is returned to the engine induc-tion area. At the same time, however, the oilcontent and its associated soot can contami-nate the turbocharger, air mass sensor, inter-cooler, valves and catalytic converters. Conta-


tween 20 and 250 Hz at which accelerationrates up to 15 g take effect (g = 9.81 m per s2).In addition, the design of the filter housing isessentially determined by the requirement fora constant airflow onto the element. Usingcomputational fluid dynamics (CFD) calcula-tions, the housing can be systematically opti-mized in terms of shape and technically con-figured to accommodate the expected flowrate (Fig. 27).Due to differences in application, air filterhousings are subdivided into designs for flatand cylindrical elements (Fig. 28). In the caseof rectangular filters, of the type predomin-antly used in cars, the PUR profiled seal isplaced in a groove in the housing and clampedin the axial direction by the filter cover. Cylin-drical filter elements dominate the commercialvehicle field by virtue of their simple, radialsealing properties and greater strength.When the vehicle is running in bad weather,e.g. heavy rain or snow, and even when beingdriven through drizzle, water may penetratethe filter housing by way of the inducted air.To minimize its volume, water separators areinstalled in the intake duct in front of the filterhousing. These consist of collar separators,

Fig. 28: Types of air filter elem-ents for use in filterhousings optimized forspace (left: trapezoid-al, tapered oval cylin-der, orthodox cylinderand rectangular filterelements; right: rect-angular filter elementwith step)

Requirementsfor filterhousings

Devices forwater separation

Removal of oilfrom blow-bygases …

… by closedbreathers


Oil filters for crankcase breathers 4140 Engine intake air filters

mination of this nature can affect both thecombustion process and exhaust emissions, aswell as impair the operation and durability ofthe specified components. To ensure the relia-ble operation of engines over increasingly pro-longed periods of service, it is essential forthese elements in blow-by gases to be extract-ed [9, 10].The average diameter of the oil droplets (re-presenting all the other aerosols and the ultra-fine particles and soot they contain) in the gas is only around 0.9 µm (Fig. 29). As a re-

Fig. 29:Typical distributionof the droplet size of oil mist aerosols in the crankcasebreather [9]

Fig. 30:Methods of filteringoil mist in the crank-case breather [9]

users, separators should also be designed aslong-life components. Suitable filtration methods are selected for theintended task on the basis of physical charac-teristics and customer requirements. The sel-ection is made with the aid of an evaluationmatrix (Table 4). This, however, reveals thatthe “ideal” filter does not exist. In order toharmonize customers’ requirements with tech-nical and economic specifications, it thereforebecomes necessary to work out a specific solu-tion in each individual case. The simplest and most cost-effective separ-ators are cyclones, i.e. centrifugal separators inwhich the blow-by gas rotates inside a taperedcylinder. Due to their mass inertia, the dropletsof oil are drawn from the gas flow and precipi-tated onto the wall of the housing. The cleangas leaves the cyclone through a central pipe.Provided the cyclone separator is properly de-

Droplet size [µm]

Vol

umet

ric

cont

ent [

%]

100

80

60

40

20

00.01 0.1 1 10 100

Methods of oil mist separation

Diffusion separators(coalescent)

Inertia or impactseparators

Electric separators

Wire mesh and fiber filters

Centrifugal separators(cyclones)

Electric filters

Labyrinth andimpact separators

Centrifuges andseparators

sult, only a few methods can be considered foreffective filtration (Fig. 30) [9]. These are sub-divided into diffusion or coalescent separators,inertia or impact separators, and electrostaticseparators, depending on their basic physicalcharacteristics. In addition to the principal re-quirements for use in motor vehicles, otherparameters include the least possible reductionin pressure (due to the partial vacuums re-quired in the crankcase) and compactness. Inorder to keep down service costs for vehicle

Cyclone separators



components are designed as service parts andare configured to provide adequate degrees ofreliability.Centrifuges are inertia separators. The forcesavailable for the separation process can be sig-nificantly increased in the centrifugal zone,enabling even the most minute droplets to beseparated. The separation capacity can be ad-justed by regulating the speed of the centrifugeindependently of the operating conditions ofthe engine. The pressure drop from a separatorof this type depends on its design; in extremecases, it is even possible to increase the pres-


signed and an adequate drop in pressure isavailable, excellent degrees of separation canbe achieved.By utilizing the opportunity to direct the gasthrough a number of cyclones arranged in par-allel, exceptionally compact dimensions canbe achieved, allowing them to be integratedinto small spaces. Moreover, the fact that cyc-lone separators are designed as lifetime com-ponents is an important decision-making criter-ion. Figure 31 shows two parallel cyclonesintegrated into an oil filter module.With fiber or diffusion filters (coalescent elem-ents), the most minute droplets can also beextracted with a high degree of filtration effi-ciency. At the same time, however, there is arisk that the soot particles in the blow-by gaswill clog the fiber layer, causing the reductionin pressure to increase with advancing operat-ing hours. In most cases, therefore, these

Table 4:Criteria for evaluat-ing different separat-ing processes

Fig. 31:Oil filter module withintegral duplex cyc-lone for the separ-ation of oil dropletsfrom blow-by gas

Fiber or dif-fusion filters

Cyclone Fiber Centrifuges Electrostaticdemister separator

Filtration efficiency + to ++ 0 to ++ ++ ++

Pressure drop 0 + to – ++ ++

Space required ++ + + 0

Flow rate – to 0 0 + +dependency

Lifetime component yes no yes yes

Costs ++ + to – – – –

Auxiliary energy no no yes yesrequired

Mounting + + + +restrictions

Modular integration ++ ++ ++ +feasible

Centrifuges



the engine. Electrostatic separators incur add-itional costs, as a high-voltage supply and el-ectrical insulation and shielding are required.If the blow-by gases contain a high proportionof soot, deposits can form on the electrodes.These must be regularly removed, otherwise acomplete failure of the system may result. Theessential challenge lies in designing oil separ-ators which meet technical requirements, i.e.which are functionally reliable and optimizedto fit their allocated space, e.g. in the housingof an oil filter module or on a valve cover (cf.Fig. 31).


sure. The use of centrifuges, however, incursadditional costs because it is a rotating compon-ent which must be driven and appropriatelymounted and sealed. Centrifuges are also life-time components. Figure 32 shows a plate-type separator of a design suitable for oil separ-ation.The highest degree of separation with minimalpressure drop is achieved with electrostatic sep-arators [11]. After being electrically charged,the droplets of oil migrate towards separatingelectrodes under the effect of an electricalfield. Here again, the exceptional efficiency isachieved by an additional external force, inthis case electrical. This ensures reliable separ-ation, independent of the operating state of

Fig. 32:Cross-section of adisk stack centrifugefor the separation of oil mist aerosols in the crankcasebreather

Electrostaticseparators

Objective: integrated solutionsCleaned blow-by gas

Rotatingplate stack

Blow-bygas inlet

Oil drain

Drivesystem


Wear and filtration 47

can lead to increased oil and fuel consump-tion, reduced engine power and a markedincrease in environmental pollution from ex-haust emissions. Finally, if the wear limits areexceeded, a risk of severe damage may arise tothe engine. Figure 33 shows a piston exhibit-ing severe wear patterns on the running sur-faces as a consequence of the friction causedby particles.The relevance of individual particles or accu-mulations of particles to wear also depends onthe engine itself, e.g. the tolerances and size ofthe clearance gaps in bearings etc. In recentyears, significant improvements have beenachieved in this field. Production methods havebeen further refined while, at the same time,machining tolerances have been reduced and,with them, the size of lubricating clearances.As a result, even particles with diameters ofaround 1 µm and more have now becomecritical, particularly when they occur in high

46

Lube oil filtrationWear and filtrationThe lubricating oil in internal combustion en-gines fulfils a number of important tasks. First,it reduces friction in bearings and clearancegaps, and between moving parts, thus reducingwear of metal components. At the same time,it serves to dissipate heat and to provide pro-tection against corrosion. Thin films of oil sealoff the combustion chamber and transmit forces. Finally, the lubricating oil plays a con-tributory role in keeping the engine clean byloosening and dispersing impurities, i.e. hold-ing them in suspension so that they are unableto form deposits.Nevertheless, impurities of all kinds do collectin the engine oil and penetrate into the enginefrom a wide variety of sources. They includeorganic and inorganic particles of dust fromthe surrounding air which are even capable ofpermeating high-efficiency air filters. The oilalso contains residual contaminants from theproduction and assembly of the engine and itscomponents, as well as abraded metal fines(particles from wear) and soot from the incom-plete combustion of the fuel. To this water(condensates), acids from the combustion pro-cess and unburnt fuel (which dilutes the oil)must be added. These combine with the de-composition products of the oil, e.g. the oxida-tion and reaction products of the additives, toform a complex, multi-phase fluid mixture.If the engine oil is insufficiently filtered or notchanged at the appropriate time, wear-relevantparticles penetrate into the narrow gaps in bear-ings etc. and may cause damage. Since the oilis in a continuous state of circulation, the wear-ing processes occur repeatedly. In time, this

Fig. 33:Traces of wear on a piston caused byparticle abrasion

Demands forlube oil

Sources andcomposition of impurities

Consequences of …

Wear relevance

… inadequatefiltration


Wear and filtration 49

[12]. With high concentrations in the oil, smallparticles can cause wear and abrasion to thesame extent as their larger counterparts.The above comments have made clear the im-portance of ensuring a high standard of filtra-tion for engine oil. The oil filter which, likethe air filter, is of the depth-filter type, repre-sents an effective trap for solid particles. Fig-ure 36 illustrates a selection of full-flow en-gine oil filter elements in current use. It mustbe said, however, that even a high-efficiency

48 Lube oil filtration

concentrations. Individual particles of between8 and 60 µm, in particular, cause severe wear(Fig. 34). Abraded metal fines in the enginehave been measured with the aid of tracer-marked metals to reveal variations in the rele-vance of individual particles to wear. Largeparticles (over 60 µm) also pose a seriousthreat – when they disintegrate, they fall precisely into the critical categories of grain size.The relationship between friction or wear andconcentrations of particles is demonstrated inFigure 35 in respect of particles of three sizes

Fig. 34:Graph showing theeffect of wear rele-vance as a functionof particle size

Fig. 35:Graphs showing theeffect of wear (as perISO 4402) for threedifferent particlesizes as a function of particle concen-trations

Fig. 36:Selection of oil filters(metal-free elementsand spin-ons withpaper, composite orsynthetic filtermedia)

oil filter element cannot prolong the intervalsbetween oil changes or delay chemical reac-tions. Particle separation reduces wear anddeposits to a significant extent. A problemmay arise if an excessive amount of soot getsinto the oil, as can happen in the case ofmodern diesel engines (cf. “Bypass oil filtra-tion” p. 61).

0

0.1

0.2

0.3

0.4

10 20 30 40 50 60


Abr

asio

n [m

g]

Operating period until damage incident [h]

20

0102 103 104 105 106

4

8

12

16

“Wea

r pa

ram

eter

”

> 3 µm> 5 µm> 15 µm


Full-flow oil filtration 51

generally proportional to the differential pres-sure) and the size of the filter.The full-flow filter may be circumvented by abypass valve mounted in a bypass line. Thevalve opens depending on the differentialpressure of the filter element, thereby assuringthe supply of oil to the engine – which takesprecedence over the filtration of the oil. By-passing the full-flow filter may be necessary,e.g. in conditions where very low temperaturesprevail and oil viscosity is high. For this rea-son, it is particularly important to ensure thatthe filter element is in sound condition, other-wise the bypass valve will open too early, e.g.under normal operating conditions, allowing acontinuous flow of unfiltered oil to bypass thefilter. The appropriate design and layout of the by-pass valve is an essential quality feature of aspin-on filter, though generally, this isbeyond the control of the motorist. The oilfilter must be changed at the very latest assoon as the maximum differential pressure isreached, which, dependent on the car manu-facturer’s specifications, ranges between 1.5


By meeting the increasing requirements im-posed by advances in engine development, theindustry is able to offer the motorist the ad-vantage of extended filter replacement inter-vals. Figure 37 illustrates this trend, taking asan example the oil filters for gasoline-enginedcars in Europe.

Full-flow oil filtrationMotor vehicles are fitted with full-flow oil fil-ters which reliably separate fine and coarseparticles. Their efficiency is satisfactory if theoil does not become contaminated with a highproportion of ultrafine particles in the intervalsbetween services. Figure 38 shows the func-tion of a full-flow filter in the oil circuit. Theoil pump draws oil from the sump which, ifnecessary, is cooled in an oil cooler, then dir-ected to the full-flow filter. A pressure regulat-ing valve returns any surplus oil to the sump.Since all the oil must pass through the full-flow filter, a compromise is generally reachedbetween filtration efficiency (which, in thecase of comparably configured filter media, is

Fig. 37:Changes of oil filterservice intervals forEuropean cars (withgasoline engines)with time

Fig. 38:Diagram depictingthe position of thefull-flow filter in theoil circuit

Full-flow oil circuit layout

Oil supply has priority

Ser

vice

inte

rval

[103

km

]

Averages

Year

Forecast

50

40

30

20

10

0<1940 1950 1960 1972 1982 1992 1995 2000 2005

Oil pan

Oil cooler

Full-flow filter

Oil pump

Pressureregulating valve

Gasoline/dieselengine

Bypass valve

T

Conditions for oil filterreplacement



monitor the oil pressure and temperature and(in the future) the quality of the oil, enablingits condition to be determined at any giventime. Another advantage is that it ensures re-liable control of the liquid flows (oil, water)within the module, thus eliminating the time-consuming connection of hoses during as-sembly. What is more, the integration of theducts into the module means that less space isrequired than in the case of a configurationconsisting of individual components. Withthe incorporation of other elements, e.g. aheating system, a bypass oil filter, multi-cyc-lones for crankcase ventilation (see “Designcriteria for engine air filter elements”, p. 33),or a mounting fixture for other attachments tothe engine, these multifunctional oil filtermodules are increasingly superseding exist-ing spin-on filters.


and 2.5 bar. Even under normal operatingconditions, high forces act on the filter me-dium. To prevent the pleats from clinging to-gether, the filter medium undergoes an appro-priate preliminary pleating and embossingprocess. Figure 39 shows a cutaway drawingof a typical spin-on filter. This reveals the fil-ter medium and its star-pleated pattern whichoffer large filtration areas while occupyingthe minimum of space. Additional zigzag

Fig. 39:Cutaway illustrationof a modern spin-onoil filter; the inno-vative medium shownhere is fully resistantto synthetic oils

pleats do not achieve any greater area with thesame volume. Instead, they have sharp-edgedcreases, increasing the risk of the paper being damaged. In the illustration, the bypassvalve (below) can also be seen, together withthe one-piece reverse-flow valve which pre-vents the filter from being drained when theengine is not running, thereby ensuring thatthe oil can achieve its full lubricating effectas soon as the engine is started.An oil filter module (Fig. 40) represents amore expensive option. In addition to thenon-metallic filter element, which can be dis-posed of without polluting the environment,the oil cooler is also incorporated in themodule. In addition, it contains sensors which

Fig. 40:Oil filter moduledesigned for a four-cylinder diesel enginewith metal-free filterelement and addition-al integrated func-tions (oil cooler, pres-sure and temperaturesensors, oil heater,valves, mountingfixture for the alter-nator, flow channelsfor oil and water)

Oil filter module

Integration of additional functions

One-piecesilicone reverse

flow valve

Long-lifeelastomer

gasket

Teflon-coatedbypass valve

Long-lifefilter medium



filter media and the design of full-flow oilfilters, particular attention must be paid to en-suring that initial pressures are kept to a min-imum. At the same time, there is a clear coun-ter-trend towards so-called “lifetime oil filterdesigns”. Sacrificing filtration efficiency, thefilters used in these cases only guarantee thereliable retention of relatively large particles(> 20 µm). They exhibit excellent chemical re-sistance to all the substances of which oil iscomposed. Compared with standard oil filterelements of the same dimension, some offerover 200% more particle holding capacity andpermit service intervals to be extended accord-ingly.Compared with a standard oil filter medium,the outstanding feature of the new filter mediabased on composites or wholly synthetic fibersis their excellent filtration performance, asdemonstrated in Figure 42 which shows theirefficiency in filtering particles of 10 µm indiameter. The filter media undergo a multi-pass test to ISO 1548-12 under constant condi-tions. In the test, the oil is circulated and thefiltration efficiency of the test filter deter-mined with the continuous introduction of con-taminants using particle counters. Comparedwith the 9-µm standard filter medium, the 4-µm composite medium is not just twice as


Grade efficiency and filtration efficiencyIn contrast to the filtration of engine intakeair and fuel, no generally binding minimumfiltration efficiency is specified for engineoil. Manufacturers stipulate widely varyingoil filtration efficiencies for their engines intheir specifications; the grade efficiencycurves for the individual media also vary to asimilar extent (Fig. 41). The standard valuex3,50 serves as the yardstick for average fil-

Fig. 41:Graphs showing thefiltration efficienciesof different filtermedia

tration efficiency (see “Basics of filtration”,p. 9).In many cases, an average filtration efficiencyx3,50 of 9 or 12 µm is specified for full-flow oilfilters. These figures, however, should alwaysbe considered in conjunction with the stand-ards and regulations underlying the measure-ments. As in the case of fuel filtration, newmeasuring methods very often produce differ-ent results. In order to permit the data obtain-ed by different methods to be compared, theymust be reciprocally converted using standard-izing graphs.The requirements for the purity of oil are be-coming increasingly severe. For this reason, fil-ter media of extreme fineness, e.g. x3,50 = 4 µmare being developed. In the development of

Fig. 42:Filtration efficiencyof two filter media incomparison (multi-pass test as per ISO4548-12; particlereference diameter:10 µm)

Increasingrequirements

Improved solutions

Development ofinnovative filtermedia


0 5 10 15 20 25 30 35 40 45 50

100

80

60

40

20

0

Filt

ratio

n ef

ficie

ncy

[%]

Composite I

Composite IILong-life mediumStandard celluloseLifetime medium

100% synthetic medium

Time [min]0

1200

800

400

010 20 30 40 50 60 70 80 90 100N

umbe

r of

par

ticle

s [1

/ml]

x3,50 = 9-µm standard medium

x3,50 = 4-µm composite medium



2 bar is used. The test rig can be seen in Fig-ure 44.

Packing densityThe size is an important aspect in the develop-ment of oil filters. Due to the restricted spaceavailable in the engine compartment, filtersare optimized to comply with far more strin-gent dimensions than was the case in the past.This is leading to higher specific filtration vel-ocities in both design and practice. Provisionmust be made for these at the design stage(e.g. flow distribution or support for thepleats) to the same extent as in the develop-ment of new filter media.To accommodate these filtration velocitieswhich differ from the earlier standard, synthet-ic fibers are increasingly being used. Insteadof pure cellulose fibers (homogenous texture)or filter media made of paper reinforced withsynthetic fibers, composites consisting of mul-tiple cellulose and non-woven or melt-blown


efficient, but rather thanks to its multi-layertexture, it separates particles of 10 µm in diam-eter (in this case the selected reference diam-eter) from the lube oil more effectively by afactor of 20. The potential of the filtered oil toinduce wear is correspondingly reduced.Figure 43 shows the results with a completerange of particles typical of a test run on themulti-pass test rig. The values for a total of 16particle categories (from 3 to 50 µm) weremeasured and recorded, from which the pro-gression over time of the filtration efficiencyof the respective filter was derived. The differ-ential pressure initially rises slowly but de-velops rapidly at the end. The diagram repre-sents a good quality 12-µm standard filter (x3,50value).The most minute leaks can be identified fromthe multi-pass test. These reveal themselvesby, for example, a sudden drop in the filtrationefficiency of a certain category of particlesize. At the component itself, these leaks,which are very difficult to verify gravimetric-ally, only become noticeable at high differen-tial pressures. As a criterion for discontinuingthe test, a near-practical final pressure loss of

Fig. 43:Original efficiencycurves of a multi-pass test (as per ISO4548-12)

Fig. 44:Multi-pass test rigfor the on-line meas-urement of particlefiltration as per ISO4548-12

Measurement of filtration efficiency …

… and verifi-cation of leaks

Optimization of filter size …

… using syn-thetic filtermedia

Time [min]

Particle fractions:15 µm20 µm25 µm30 µm35 µm40 µm45 µm50 µm

3 µm 4 µm 5 µm 6 µm 7 µm 8 µm10 µm12 µm

0 10 20 30 40 50 60 70

100

80

60

40

20

0

Filt

ratio

n ef

ficie

ncy

[%]



and, in turn service intervals can be prolonged,even in the case of considerably improved fil-tration efficiency.

Chemical resistance of filter mediaWholly synthetic low-viscosity oils, with newly developed additives which are intendedto prolong their service life or intervals be-tween changes, act much more aggressively onfilter media than the mineral and semi-syntheticoils previously used. This is demonstrated inFigure 47 with examples of two oil filtermedia.


layers, and wholly synthetic media (non-wovens) are used. Figure 45 shows a compositefilter medium consisting of two layers, one ofwhich is a layer of melt-blown fibers with ahigh particle holding capacity while the otheris a paper layer of high filtration efficiency (ahomogenous mixture of cellulose and synthe-tic fibers) which also adds to the mechanicalstrength of the composite. The know-how tocombine individual layers essentially influ-ences the performance data of the filter media.Fiberglass media (with very small fiber diam-eters), which could potentially be used as a re-sult of their outstanding filtration characteris-tics, too, have so far been rejected by mostvehicle manufacturers, due to their potentialfor inducing wear and problems of fiber mi-gration, for which solutions have so far notbeen found.Figure 46 depicts the particle holding capacityof the filter media shown in Figure 41. A com-parison reveals that increased filtration effi-ciency does not necessarily equate to a re-duced particle holding capacity. On the con-trary, thanks to the special design of compositeand wholly synthetic filter media, service life

Fig. 46:Specific particle ordust holding capaci-ties of different filtermedia

Fig. 47:The aging of filtermedia in lube oils;diagram showsstrength vs. time(laboratory data; T = 140°C)

Fig. 45:Design and texture ofa typical compositefilter medium

Properties of filter media

Aggressive synthetic oils …

Composite medium

Mixed fiber medium (paper)

Layer 1 (melt-blown)

Layer 2 (paper)

Synthetic high-tech fiber layer(melt-blown)

Spe

cific

par

ticle

hol

ding

cap

acity

[g/m

2 ] 200

160

120

80

40

0Standardcellulose

Long-lifemedium

Com-posite I

Com-posite II

100%syntheticmedium

Lifetimemedium

0

6

5

4

3

2

1

0200 400 600 800 1000

Time immersed [h]

Bur

stin

g st

reng

th [b

ar]

Long-life filter medium

Standardfilter medium

Full synthetic oil

Semi-synthetic oil


Bypass oil filtration 61

from the multiplicity of engines covered bythe trials and the conditions under which thevehicles were used. As a case in point, policevehicles are in constant use, but only overshort distances, so that the engine is oftenstarted from cold and is usually run at higherspeeds. The oils and oil filters used in high-mileage vehicles were also investigated andthe results are shown in the diagram, too. Oneparticular, non-quantifiable influence is themotorist’s personal driving style. The diagramillustrates the bursting strength of the filtermedia (to DIN 53113 or ISO 2758) obtainedfrom the used filter elements, as a function ofthe completed mileage.The results can be summed up in two generalstatements. First, media strength declines withincreasingly prolonged use, as confirmed bythe results of the laboratory tests (cf. Fig. 47).Secondly, the measured values reveal that thefilter media in the engines of the latest gener-ations of vehicles are more heavily stressed than was previously the case. Although nofunctional failures occurred in any of the oilfilter elements investigated, and residualstrengths continued to be adequate even in extreme cases (> 0.8 bar), the use of filter media capable of withstanding high mech-anical stresses is recommended.

Bypass oil filtrationAs previously mentioned, a high concentrationof minute particles can cause just as muchwear as a high proportion of large particles (cf.Fig. 34). So-called “bore polishing” is particu-larly serious. When this occurs, areas of thecylinder liners become exceptionally highlypolished in the course of time, with the resultthat films of oil no longer cling to the surfaceand the lubricating film breaks away.


These were subjected to prolonged immersionin different engine oils at 140°C to determinetheir resistance to ageing. The graphs relate tobursting strength, a yardstick for the mechanic-al strength of a filter medium over time. Theyshow that synthetic oil has a much more dam-aging effect on filter media than its semi-synthetic counterpart. The use of a long-lifemedium is recommended for these applica-tions, given that it performs significantly betterthan a pure cellulose medium, despite a deteri-oration in strength, too.The increased stress on oil filter media im-posed by modern high-performance engines canalso be verified by the results of field trials. Atthis stage, it should be noted that the data havebeen influenced by many parameters – thequality of the (different) oils, their reactionproducts and temperature during service andthe intervals between changes being worthy ofmention. Figure 48 shows the ageing of filter media incars with gasoline engines; the results are fromfield trials carried out between 1980 and 2000.The considerable spread of the data results

Fig. 48:Results of field testsshowing filter mediaaging in vehicleswith gasoline en-gines

… affect mech-anical strength

Field trials onfilter mediaageing

Conclusions

Relevance ofultrafine par-ticles to wear

Mileage [miles]

0 6,000 12,000 18,000 24,000 30,000 36,000

5

4

3

2

1

Bur

stin

g st

reng

th [b

ar]

1997 – 2000116 Vehicles

1980 – 199559 Vehicles

Upper limit Averages Lower limit



ticles of soot is only in the nanometer range,but these form agglomerates and clusterswhich than can be physically efficiently separ-ated). With the increasing contamination of the bypass filter element, the volume of oilflowing through this filter diminishes, whilefiltration efficiency continues to increase. Fig-ure 50 depicts new and contaminated bypassfilter elements for comparison. Depending onsize, a filter element of this type in, e.g. a HDvehicle, should trap around 500 g of soot andultrafine particles between oil changes. In thisway, ultrafine particles are removed in a by-pass filter which, because of their small size,otherwise consistently permeate the full-flowoil filter and cause increased wear at the lubri-cating points or increase the viscosity of theoil [14].Figure 51 indicates the progressive increase inviscosity of a semi-synthetic oil as a functionof soot concentration at a constant tempera-ture. In general, a substantial rise in viscositystarts with a mass concentration of around 3%.This can lead to a delay in the full supply oflubricating oil to the engine, particularly when


The smallest particles act as “polishing paste”in the fine lubricating clearances and on thecylinder liners. Problems may also arise in thecase of ageing oils with a high proportion ofultrafine particles and at low temperatures.These increase the viscosity of the oil and de-tract from its lubricating properties [13].To overcome these risks, high-mileage diesel-engine vehicles with prolonged intervals be-tween changes (applicable to heavy-duty orHD vehicles, for example) and a high inci-dence of soot can be fitted with an additionalseparation step for ultrafine particles by install-ing, e.g. a bypass oil filter in the oil circuit(Fig. 49). A small proportion of the oil flow isdiverted from the circuit upstream of the full-flow oil filter, at a location where the max-imum oil pressure is available, and directedthrough a bypass oil filter. The diverted bypassflow corresponds to 5 to 10% of the total oilflux. To achieve the desired filtration of sootparticles with a diameter of less than 1 µm, thefilter medium must be correspondingly finerand the filtration efficiency below that of thefull-flow filter (the size of the primary par-

Fig. 49:Full-flow and bypassflow oil filters in theoil circuit of a dieselengine

Fig. 50:Bypass filter ele-ments in new andend-of-life condition(laden mainly withsoot particles)

Increasing oil viscosity

Bypass flow filtration

Filtered particleamount betweenoil changes

Oil pan

Oil cooler

Full-flow filter

Oil pump Bypass filter

Pressureregulatingvalve

Diesel engine

Flow throttleBypass valve

T

Soot, fine particles

New filter element End-of-life filterelement



of the centrifuge used depends on the serviceintervals between oil changes or replacementsof the full-flow oil filter. Figure 52 showscross-sections of a plastic rotor before andafter use. The compact filter cakes in the indi-vidual compartments can be seen clearly. Acompact arrangement for full-flow and bypassoil filtration is shown in Figure 53. The full-flow oil filter and the centrifuge (or a bypassoil filter element, depending on the customer’sdemands) are mounted in a single housing.Besides HD applications the potential forthese small centrifuges extends even to dieselengines for cars. A conflict has arisen with theincreasingly severe directives on exhaustemissions (EURO 4 and EURO 5). The com-bustion process can only be optimized to bringabout a reduction in the emissions of eithernitrogen oxides or soot particles. At present,the tendency favors keeping down NOx con-centrations. In this way, attempts are beingmade to circumvent the use of SCR technolo-gies (known from power station emissions) or


started from cold, and cause increased wear asa result of deficient lubrication [15]. As inves-tigations, e.g. into urban buses, have shown,volumes of soot increase in practice by up to15% between routine oil service intervals.Another very effective method of removingultrafine soot particles from the bypass oilflow is offered by using centrifuges. Instead ofa bypass oil filter element, an free jet centri-fuge made of metal or plastic is used. The plas-tic type is distinguished by light weight andenvironmentally friendly disposal. Dispensingwith an external source of energy, centrifugesare driven by the oil pressure alone. With theaid of small, open jet nozzles, they are accel-erated up to speeds of 10,000 revolutions perminute (rpm). After the oil has passed throughthe separator, it returns unpressurized to thesump.Due to the high centrifugal forces generated,centrifuges achieve a high degree of separ-ation efficiency for ultrafine particles. At thesame time, the filter cake which forms on theinside of the rotor wall is very compact, so alot of soot can be stored. After service, therotor, packed with fine particles, is removedand simply replaced by a new one.Centrifuges thus constitute an effective alter-native to bypass flow filter elements. The size

Fig. 52:Plastic centrifugerotors in new condi-tion (left) and afterseparation of ultra-fine particles (right)for bypass flow oilfiltration

Fig. 51:Lube oil viscosity asa function of sootparticle concentra-tion in the oil (resultsof measurementswith different newand used oils)

Centrifuges asan alternative tobypass oil filters

Advantages ofcentrifuges

Conflict – reduction in NOxemissions …

Potential appli-cation: dieselengines for cars

Soot content [% by mass]

300

200

100

05 10 150

Rel

ativ

e vi

scos

ity [%

]

550 h

New rotor 500 g of soot and dust



they have no effect on the ageing of the oil.Nevertheless, the use of bypass oil filtersrelieves the burden on the full-flow oil filterto some extent, with the result that the dif-ferential pressure in that filter progressesmore slowly.

• A bypass oil filter has a further positiveeffect. As a result of the additional volumeof the second circuit, more oil is filled fromthe very outset. As a result, specific unitloadings are reduced, allowing intervals be-tween oil services to be slightly prolonged.

It remains to be said that lube oil should con-tinue to be changed at the intervals recommend-ed by the vehicle manufacturer. Omitting anoil change because a bypass oil filter elementor centrifuge has been installed carries an in-herent risk and may cause serious damage tothe engine.


accumulator-type catalytic NOx converters.This, however, increases the production ofsoot with the consequence that the concentra-tions of particles also increase in the oil and,in turn, the blow-by gas.The question frequently arises in connectionwith bypass oil filtration as to whether oilchange intervals can be extended in this wayor whether oil changing can be dispensed withaltogether. The response can be summarizedby three clear statements:

• Even lube oil is subject to ageing. If, e.g.additives, acid buffers and other constituentswhich effect lubricating properties are used,the oil must be changed.

• Filters are mechanical separators which, bytrapping particles, ensure that the oil is ableto fulfill its intended functions. However,

Fig. 53:Combined full-flowfilter element andbypass flow centri-fuge; the design isoptimized to min-imize the necessaryspace

… or generationof soot

Evaluation of by-pass oil filtration

Oil changing not superfluous


Gasoline filters 69

by size. Expressed in absolute values, a liter ofdiesel fuel contains more than 5 · 104 particlesover 15 µm in size (coarse fraction), and over5 · 105 particles over 5 µm in size (fine frac-tion). To protect even modern diesel injectionsystems, the coarse fraction must be almostentirely removed by the fuel filter, i.e. by afactor significantly higher than 100. In investi-gations into wear, the fine fraction has alsorevealed itself to be critical. In recent years,the filtration efficiency of the finest particlefraction, which can now be determined bystandardized test methods (3 to 5 µm) has ac-cordingly been established as the characteristicparameter for filter fineness.Injection systems in modern gasoline and diesel engines react sensitively to even minuteimpurities in the fuel. Damage occurs princip-ally as a result of particle erosion and, in thecase of diesel engines, by corrosion attributableto excessive amounts of water in the fuel.

Gasoline filters

The arrangement and function of the fuelfilter in the fuel supply system of a gasolineengineModern gasoline engines are equipped withsolenoid-actuated injectors which inject fueleither into the inlet manifold upstream of eachinlet valve (inlet manifold injection) or directlyinto the cylinder (direct injection). The func-tion of the fuel filter is to protect the injectioncomponents (particularly the electric injector)from wear and the penetration of particles intothe engine combustion chamber which couldcause wear.Engines with inlet manifold injection and elec-tronically controlled unit injection systemscurrently operate at injection pressures of 3 to

68

Fuel filtersFunctions of a modernfuel filterLike every other operating fluid, fuel is sub-ject to a certain contamination by particleswhich get into it in the course of the produc-tion process, during transportation and storageand, finally, whenever the tank is refilled.After the refueling process, further contamin-ation occurs as a result of particles and waterentering by way of the fuel tank vent pipe.This access route becomes a matter of particu-lar importance if the surrounding air is dust-laden or excessively humid or if severe tem-perature fluctuations occur during the day.Other sources of impurities are to be found inthe residual contaminants from the manufac-ture of all the fuel supply components and the penetration of soot by way of the engineoil in the case of oil-lubricated fuel pumps(although today, only commercial vehicles arestill fitted with in-line pumps). The contamination itself is composed of bothextremely hard mineral particles and organicparticles such as soot and tar. To comply withDIN EN 590, the particle content of diesel fuelsmust not exceed 24 mg per liter. The inter-national automotive industry associations re-commend values below 24 mg per kg [16]. In Germany, the particle content of diesel fuels is usually below 10 mg per liter. In thefuel sold worldwide, however, high concen-trations of contaminants which considerablyexceed the above limits are increasingly beingfound.The risk of wear, nevertheless, is not deter-mined by the overall particle content alone, butessentially by the distribution of the particles

Sources of contamination

Composition

Relevance of sizedistribution

Protection ofsensitive injec-tion systems


Gasoline filters 71

The high-pressure pump pumps fuel into apressurized accumulator, which is directly con-nected to the injectors, at pressures up to 120bar. Fuel pressure is controlled by means of a pressure sensor and pressure regulatingvalve. By comparison with inlet manifold in-jection systems, direct injection engines mustbe protected against wear by considerably finerfuel filters. Firstly, the pressures at the injectorare higher by a factor of 30 while, secondly,other components such as the pressurized ac-cumulator and pressure regulating valve of the injection system must be protected frompenetrating particles.

Required filtration efficienciesThe required filtration efficiency (initial par-ticle retention efficiency to ISO/TR 13353,part 1: 1994, see “Methods of testing fuel fil-ters”, p. 85) is determined as the result of testrig and field trials conducted by the manufac-turers of engines and injection systems in con-junction with filter manufacturers. Figure 55depicts recommendations on the minimuminitial particle retention efficiency for gasoline

70 Fuel filters

4 bar. In these systems, fuel is directed by theelectric fuel pump to the injectors by way ofthe fuel filter and the distributor manifold(Fig. 54). The injectors are protected by add-itional filters (small screening filters with avery large mesh of at least 200 µm). Fuel pres-sure is maintained at a constant level by apressure regulating valve. Surplus fuel is di-verted and returned to the tank by way of a re-turn line. The volume of fuel delivered by thefuel pump and, in turn, the nominal rate offlow through the fuel filter, is significantlyhigher than the actual fuel consumption.For direct injection, considerably higher in-jection pressures are required to achieve therequisite mixture in the cylinder. The fuelsupply system is divided into a low-pressurecircuit with the fuel pump and a high-pressurecircuit. In direct injection engines, the fuelpump serves merely to maintain a constantadmission pressure of around 3.5 bar in thehigh-pressure circuit. The fuel filter, on theother hand, is located downstream of the pump.

Fig. 54: Fuel supply system toa gasoline enginewith inlet manifoldinjection

Fig. 55: Recommendations forthe minimum initialparticle retention ef-ficiencies of gasolinefilters

Fuel supply circuit

Finer filtersrequired

Fuelfilter

Electric fuelpump (EFP)

Return line

Pressureregulator

Sensors

Control unit

Injectors

Fuel tank

Fuel distributor (fuel throughway)

M

Initi

al p

artic

le r

eten

tion

effic

ienc

yη(

3-5

µm)

[%]

1009585

67

50

25

Extreme conditions

Minimum requirements

ISO/TR 13353: 1994

Carburetor

Indirect injection

up to4 bar

Directinjection

up to120 bar


Gasoline filters 73

duction in overall emissions of hydrocarbons,moreover, necessitates all the external com-ponents of the low-pressure circuit such as the fuel pump, fine filter and pressure regulat-ing valve being integrated to form an in-tankmodule. Other components such as the fuel gauge, surge pot and optional prefilter,which protects the pump, can likewise be inte-grated to form an in-tank unit. Figure 57 de-picts a modern long-life filter for a unit of thistype.

Construction of the filter element and mediumCurrent requirements for filtration efficiencyand particle storage capacity call for innova-tive filter designs. The service life (particlestorage capacity) of a simple screen-type fil-ter (surface filter) is only about one-tenth ofthat of modern deep-bed filter media of thesame degree of fineness. These media,

72 Fuel filters

injection systems. The data represent the cur-rent situation and will have a tendency to risein future years. If an engine is operated with high concentrations of particles in thefuel or intake air, finer fuel filters will beneeded to provide effective protection againstwear. These “critical conditions” occur dur-ing journeys over corresponding terrain orwhen refueling elsewhere than in Europe,Japan or the North American Free Trade Area(NAFTA).

Gasoline filter designsThe preferred design for gasoline filters is thein-line filter (Fig. 56). In some cases, the pres-sure regulating valve is incorporated in the filter head. Depending on its position in theengine compartment (crash safety) and the spe-

Fig. 56: Gasoline filter, formounting in the fuelline; the filter elementfeatures outwardradiating pleats

Fig. 57: Filter elements forlifetime use, of irregu-lar shape to make theoptimum use of thespace in in-tank units

Recommenda-tions for min-imum filtrationefficiency

Star pleatingpermits highpacking density

Direction of flow

cifications of the vehicle manufacturer, the filter housing may be made of plastic, alu-minum or even of sheet steel.Maintenance-free lifetime fuel filters are in-creasingly being specified for gasoline enginesnow going into production in Europe. The re-


Diesel fuel filters 75

Diesel fuel filters

The arrangement and function of the fuelfilter in the fuel supply system of a dieselengineThe rapid development of diesel engine tech-nology has now made it possible for dieselfuel to be efficiently used in both car and com-mercial vehicle engines. All modern diesel en-gines employ direct injection systems. The ef-ficiency of the combustion process can beimproved by the increasingly fine atomizationof the fuel and the corresponding modulationof the injection process using electronicallycontrolled valves.At the same time, the new technology necessi-tates higher and higher injection pressures. So-called “pump/injector” systems now reachmaximum pressures in excess of 2000 bar.Each cylinder features a separate unit injector(or UI) which is driven through rockers by anoverhead camshaft and controlled (in the caseof car and commercial vehicle engines) by asolenoid valve. In the case of offset camshafts,the pump and solenoid valve are connected tothe injector by a short pipe. These systems,referred to as unit pumps (or UP) are used ex-clusively for commercial vehicle engines.In common rail (CR) systems, the functions ofpressure generation (performed by a high-pres-sure axial or radial piston pump) and fuel injec-tion are separated from each other. All the in-jectors are supplied from a common high-pres-sure accumulator – the rail. While the max-imum injection pressures do not yet reach thoseachieved by pump/injector systems, the separ-ate control of the injection process by elec-tronically controlled valves has its advantages.Direct injection systems with distributor injec-tion pumps controlled by solenoid valves also

74 Fuel filters

moreover, are capable of very high packingdensities by virtue of their outward radiatingpleats (cf. Fig. 56). The star-pleated element,which offers a considerable area for filtra-tion, is placed on a pressure-resistant centraltube. The fuel flows through the filter radi-ally from outside to inside. As an alternative,so-called “spiral-wound” filters are usedwhich consist of concentrically arranged pa-per filter cells.Todays fuel-filter media consist predominantlyof ultrafine cellular fibers or blends of cellu-lose and polyester fibers with a special fuel-re-sistant impregnating coating. The duroplasticresin is interlaced in a thermosetting oven afterthe production of the pleat packs. Modern fil-ter media are constructed of multiple layers(composites), consisting in some cases ofultrafine melt-blown synthetic fibers (cf.“Performance data of fuel filters”, p. 87). Anultrafine filter layer is positioned downstreamof a prefilter layer, enabling particle stor-age capacity to be increased by over 100% bycomparison with standard filter media. Com-posite filter media are made with outwardradiating pleats exclusively.

Filter configurationFirst, the type of filter medium is selected,the decisive factor being the initial filtrationefficiency required, i.e. that of a new filterbefore exposure to particles (cf. Fig. 55). In asecond stage, the requisite filter area is deter-mined by one of two methods. In the first,data from the field relating to pollution levelsand rises in differential pressure are used asthe basis for the filter configuration. In thesecond, the filter area is calculated by deter-mining particle storage capacity from spe-cially standardized filter tests (cf. “Methodsof testing fuel filters”, p. 85).

Materials used

Methods fordetermining filter area

Injection pres-sures of UI,UP …

… and commonrail systems



side. Pressure-side layouts are accordinglyused in commercial vehicle engines and in-creasingly also for car engines.The fuel flow rate in the low-pressure circuitis higher than in the high-pressure circuit.Surplus fuel is directed to the tank by a pres-sure regulating valve, which can even be inte-grated into the filter head. In the high-pressurecircuit, it is diverted by a pressure limitingvalve located on the rail. Some of this hot re-turning fuel, having a temperature in excess of70°C, is returned directly to the low-pressurecircuit in order to prevent paraffin from separ-ating from the cold fuel. In this case, the re-turn flow is regulated by a thermostat whichcan also be integrated into the fuel filter hous-ing.

Required degrees of filtration efficiencyWith the introduction of modern diesel injec-tion systems, controlled by solenoid valves, ithas become necessary to increase filtration ef-ficiency to a marked extent [17, 18]. Figure 59depicts the valve seat of a solenoid valve(commercial vehicle pump/injector system)after a period of service with a filter having an initial particle retention efficiency of η(3–5 µm) = 45% complying with ISO/TR13353 of 1994 (until 1997, this was the finest

76 Fuel filters

reach high pressures (over 1600 bar). The im-portance of these systems, however, is declin-ing, given the advantages of the UI and CRsystems mentioned above, as are also in-linepumps and prechamber injectors, which are nolonger used in new engines. Today, car en-gines are dominated by CR systems andcommercial vehicle engines by UI/UP sys-tems, with the CR proportion gaining ground,even in the commercial vehicle sector.Here again, the function of the fuel filter is toprotect all the components forming a high-pressure injection system. To this end, the fil-ter may be arranged in the low-pressure cir-cuit, either on the pressure side in the feedlineto the high-pressure pump or on the suctionside in the feedline to the fuel pump. Figure 58depicts the layout of a CR fuel supply systemin diagrammatic form. With an arrangement on the pressure side, adifferential pressure of up to 6 bar is availablefor fuel filtration (depending on the layout ofthe system), which is thus considerably higherthan that of an arrangement on the suction

Fig. 58:Fuel supply to adiesel engine withcommon rail injectionand fuel filter locatedon the pressure side

Fig. 59:Solenoid valve seat(UI system for com-mercial vehicles)after use with a fuelfilter of insufficientfiltration efficiency[19]

Arrangement of filter in fuelcircuit: …

… pressure orsuction side

Fuel filter

High-pressurepump

Electricfuel pump(EFP)

Return line

High-pressure line

Fuel tank

Pressureregulator

Rail pressure sensor

Common rail

Sensors

Control unit

Injectors

M



Water separationIf water reaches the high-pressure side of thediesel injection system, damage may becaused by localized lubrication deficienciesand, in particular, corrosion [19]. In manycases, the fuel filter is given the additionaltask of separating water from the fuel. In thisprocess, the droplets of water are precipitatedonto the fibers of the filter medium wherethey form increasingly large drops. Beingdenser than diesel fuel, they eventually mi-grate outwards and, to some extent, also in-wards inside the filter, into a water collectingchamber. From there, the water is led offthrough a drain.In the case of distributor injection pumps andcommon rail systems, some form of protectionis generally required against the penetration ofimpermissibly high volumes of water. Due toshorter contact times, unit injector systems arerelatively immune; if an exceptionally high in-cidence of water is expected, however, theyalso require a water separating device. At present, comprehensive data on the watercontained in diesel fuels sold worldwide, andin vehicle tanks, are not available. The spreadranges from a customary maximum of 200 ppm(parts per million) [16] to values exceeding2%, which occur in situations such as re-fueling from barrels, badly maintained fill-ing stations or operational use in countriessubject to high atmospheric humidity andtemperatures which fluctuate severely in thecourse of a day.To determine the water separation capacity ofa fuel filter, a 2%-emulsion is passed throughthe test specimen and the concentration re-maining on the clean side of the filter is meas-ured (ISO 4020). The emulsification processis carried out with a diaphragm pump, al-though this does not actually comply with the

78 Fuel filters

standard for diesel engine filters in Europe).Longitudinal grooves, caused by particle ero-sion, can be seen clearly. These cause internalleaks and, as a result, reduce the volume offuel injected. In practice, this pattern of dam-age makes itself evident by diminishing en-gine power, uneven running (caused by vary-ing degrees of wear in the individual cylin-ders) and the increasing development of soot. Both in field trials and test rig experiments,the resulting wear indicators correlate to amarked extent with the initial particle reten-tion efficiency of particles in the 3 to 5 µmrange as laid down in ISO/TR 13353, 1994.

Fig. 60:Recommendationsfor the minimuminitial particle reten-tion efficiencies ofdiesel fuel filters

Investigations into unit pump engines (Ameri-can commercial vehicles) confirm the corres-ponding degree of wear for the finest fractionat around 5 µm [20]. Figure 60 depicts recom-mendations for the minimum initial particleretention efficiency of filters for diesel injec-tion systems. A distinction is made betweennormal and extreme conditions, similar to thatshown in Figure 55.

Consequences ofparticle erosion

Water damage

Water content ofdiesel fuels

Determinationof water separ-ation efficiency

Initi

al p

artic

le r

eten

tion

effic

ienc

yη(

3-5

µm)

[%]

100

80

60

40

20

0In-linepump

Distributorpump

Time-con-trolled dis-

tributor pump

Unit pump/unit injector

system

Commonrail

system

Extreme conditions

Minimum requirements

ISO/TR 13353: 1994



Another type of filter in widespread use,which cannot be opened, is the spin-on fuelfilter (Fig. 62). This is screw-mounted to a fil-ter head with a male thread, which is sealedwith an external elastomer gasket.The requirement for maximum filtration effi-ciency and efficient water separation, accom-panied by simultaneous demands for increas-ingly prolonged intervals between changes(particularly in the case of commercial ve-

80 Fuel filters

current standard. Motor vehicle manufac-turers’ specifications, therefore, often relate tobench tests in which a standard electric fuelpump (e.g. a roller-cellular pump) is used foremulsification. In tests as laid down in ISO4020, vehicle manufacturers require degreesof water separation of at least 90% for criticalrelationships.

Diesel fuel filter designsThere are two types of diesel fuel filter, name-ly those which can be opened and those which must be replaced, complete with the fil-ter housing, during services. The latter cat-egory includes in-line filters made of steel,aluminum or plastic. The increased require-ments for collision safety have actually led to

Fig. 61:Diesel fuel filter withstar-pleated filter elem-ent and additionalfunctions, designed to be mounted in thefuel line

Fig. 62:Replaceable spin-on diesel fuel filter with filter head and electrical fuel preheating

hicles) can only be met with multi-stage fil-ters. In this case, a prefilter is installed on thepressure or suction side, which performs thefunctions of water separation and particle pre-filtering. Next, the fuel flows through a finefilter, located on the pressure side, in which

Water drain with pull-rod

Water drain

Water sensor

Filter element(opened out)

a revival of steel filters. Additional featuressuch as a water drain, a water sensor (in theform of an electrically conductive sensor), athermostat (to return hot fuel), and heating sys-tems can also be integrated into in-line filters(Fig. 61).



Construction of the filter element and mediumToday, filter elements for diesel fuels are pre-dominantly made with outward radiatingpleats (see Fig. 56). Here again, the require-ment for extreme filter fineness and prolongedintervals between services necessitates the useof new types of composite filter media.Thanks to the hydrophobic properties of thebasic material and the small diameter of thefibers, the melt-blown fine fiber layer of thesefilter materials exhibits a high level of watercoalescence. It is positioned on the incomingside, so that water separation also takes placeon that side.Similarly satisfactory performance data can beobtained with so-called “fiberglass compositepapers”. This filter medium contains 5 to 20%of microglass fibers of around 1 µm in diam-eter. Not used in Europe, these filter media arecontroversial in that the ultrafine, brittle glassfibers can break away, migrate to the cleanside of the filter and cause damage in the in-jection system. Older designs consisting of,

82 Fuel filters

the ultrafine particles are trapped. Figure 63depicts a spin-on fuel prefilter for commercialvehicles.Modular fuel filters fall into the category offilters which can be opened for servicing pur-poses. To this end, the lid of the housing isscrew-mounted and only the filter element isreplaced. To simplify servicing, the housingshould preferably be placed in such a way thatthe lid is uppermost. In modern versions, thefilter element is made entirely of non-metallicmaterials (metal-free filter elements) whichcan be thermally recycled without difficulty.Recent designs stand out by virtue of a num-ber of additional integral functions. These in-clude sensors and control valves for pressureand temperature, electrical heating systems,heat exchangers, water sensors and water ex-tractors. Figure 64 depicts a modern, modularfuel filter for cars. The arrangement in parallelof two metal-free, star-pleated filter elementsensures the optimum use of the availablespace. The only parts which require servicingare the filter elements.

Fig. 63:Replaceable dieselfuel prefilter forcommercial vehicleswith integral waterseparator and elec-trical fuel preheating

Fig. 64:Modular diesel fuelfilter for cars

Designs forease of service Good perform-

ance by com-posite media

Filter headwith 4-hole flange

Electric fuelpreheating system

Priming pump

Water drain screwWater collectingchamber

Replaceablefilter with specialwater separatingfilter medium

Metal housing

Fuel heat exchanger

Electric heatingsystem with integral preheating valve

Two metal-free recyclablefilter elements arrangedin parallel


Methods of testing fuel filters 85

tween changes corresponding to half thatrecommended for diesel fuel complying withDIN EN 590: 2000.

Methods of testing fuel filtersThe methods of testing fuel filters can be sub-divided into filter function tests and com-ponent tests. Component tests (cleanliness ofnew filters, freedom from leaks, differentialpressure, bursting pressure of filter elementsand housings, pulsation and vibration resist-ance) and filter function tests (durability, fil-tration efficiency and separation of emulsi-fied water from diesel fuel) are described inISO 4020.Other tests are laid down in which the filtra-tion efficiency is measured by automatic par-ticle counters. Due to the rapid developmentof diesel injection technology, these test speci-fications are currently being completely re-vised and adapted to the new requirements by the ISO/TC22/SC7/WGI committee of ex-perts. Particle storage capacity and filtration effi-ciency tests to ISO 4020 take place on thegravimetric principle and in a single pass withthe addition of a mixture of mineral particles(ISO 12103-M2, a near-monodispersion with anaverage grain size of 6.9 µm) and submicronparticles of soot. Filter capacity (correspond-ing to the added volume of mineral particles ingrams at which the differential pressure risesto 70 kPa) correlates well with the servicelives measured in practice.In the medium term, modern multi-passmethods will be adopted for particle storagecapacity and filtration efficiency testing, simi-lar to the methods used for other fluid filters(hydraulic fluid filters, lubricating oil filters).The new multi-pass test ISO/DIS 19438: 2000,

84 Fuel filters

e.g. uniform cylindrical felt rings (dry poly-ester non-wovens) are now no longer used.The importance of spiral-wound filters is alsodeclining.

Filter configurationThe filtration efficiency required to protectdifferent types of diesel injection systems canbe taken from Figure 60. The filter area isdetermined from the anticipated amount ofcontamination, which is calculated from fieldtrials, and standardized bench tests (cf.“Methods of testing fuel filters”, p. 85). In thecase of filters with integral water separationproperties, a maximum flow rate is specifiedfor the area of the filter medium which mustnot be exceeded, otherwise the water will notbe separated to a sufficient extent. The max-imum area loading is the critical configurationparameter, particularly in the case of com-pact filters for cars. Through the use of com-posite filter media with an outer layer of ultra-fine synthetic fibers, this limit has beenconsiderably raised by comparison with con-ventional cellulose filter media.With rising energy costs and tax subsidies, theuse of vegetable methyl esters, also knownunder the designation FAME (fatty acid me-thyl esters) is becoming increasingly wide-spread. These “biodiesel” fuels are now avail-able in a variety of grades ranging from re-cycled used fat to pure RME (rape methylesters). FAME fuels possess corrosive charac-teristics which must be taken into account inthe selection of materials. Due to their highcontent of organic particles, their use is alsoassociated with reduced intervals betweenchanges. These are established from the re-sults of field trials. As a rule of thumb, the useof high-grade vegetable methyl esters (to DIN 51 606: 1997) imposes an interval be-

Maximum arealoading – a critic-al factor

Different testingmethods

Single-pass …

… and multi-pass tests


Performance data of fuel filters 87

rily attributable to the new sensor calibrationstandard and are particularly apparent at thefinest stage which embraces particles of be-tween 3 and 5 µm.As soon as the specifications for the manufac-turers of cars and injection systems have allbeen adjusted to the new test method, it will benecessary for the initial particle retention effi-ciency at the finest level (see also Figs. 55 and60, p. 71 and 78) to be converted in accordancewith the new sensor calibration standard.Figure 66 depicts the method for convertingparticle sizes as described in ISO 11171.

Performance data of fuel filtersTo enable the requirements for intervals be-tween changes, dimensional specifications andfilter fineness to be met, it will be necessaryfor new types of filter media of improved per-formance to be available. Today, the develop-ment of filter media of this type from theground up is undertaken with the aid of ultra-modern test methods and the increasing use ofthe latest development tools such as compu-tational fluid dynamics (CFD) [21].

86 Fuel filters

in which particle counting takes place on adirect basis (i.e. on line), allows the filtrationefficiency of the filter to be recorded for 16 particle categories (between 3 and 50 µm)over its entire service life in a single test. Test dusts to ISO 12103-M2 (medium) are usedfor all filter tests with on-line particle counters.These mineral particles cover the spectrum of grain sizes (between 0.1 and 80 µm) cor-responding to practical conditions. Leadingfilter manufacturers have already converted tothis testing and development tool. Figure 65depicts the filtration efficiency of a moderndiesel fuel filter containing a composite filtermedium.Today, filtration efficiency is determined bymeasuring the initial particle retention effi-ciency in a single pass. While the initial filtra-tion efficiencies currently specified (see Figs.55 and 60) still relate to ISO/TR 13353: 1994,the new version, ISO/WD 13353: 2000 (equi-valent to the multi-pass test ISO/DIS 19438:2000) is based on the new reference particlecounts of the test dust ISO 12103-A2. The dif-ferences between the test methods are prima-

Fig. 65: Filtration efficiencyof modern diesel fuelfilters during particlecontamination caus-ing a rise in differ-ential pressure up to70 kPa (multi-pass,ISO/DIS 19438: 2000)

Fig. 66: Chart depicting thefactor for convertingfrom the old sensorcalibration standard(ISO/TR 13353: 1994)to the new standards(ISO/WD 13353:2000 and ISO/DIS19438: 2000)

Use of standard-ized test dusts

0 10 20 30 40 50 60 70 80 90 100

Time [min]

100

80

60

40

20

0

Filt

ratio

n ef

ficie

ncy

[%]

3 µm4 µm5 µm6 µm7 µm

ISO 12103 A2 (Calibration NIST ISO 11171)[µm]

0

40

35

30

25

20

15

10

5

05 10 15 20 25 30 35 40

AC

FT

D IS

O 4

402:

199

1 [µ

m]

Filter develop-ment using CFD


Performance data of fuel filters 89

sively increasing their density in the directionof fluid flow (graded structure). This finding is already being applied to modern filter media.The diagram in Figure 69 represents the con-struction of a medium of this type. By contrastwith conventional, mixed fiber media based on cellulose, performance has been markedlyimproved not only in terms of particle storagecapacity but also filter fineness.

88 Fuel filters

Figure 67 depicts the paths of particles througha fuel filter medium with 15 layers of fiber, ascalculated by means of CFD. The graphs in Fig-ure 68 represent the influence of the diameterof the fiber on the initial particle retention effi-ciency as determined by computer. Both theor-etical and experimental investigations lead tothe conclusion that an improvement in perform-ance can be primarily achieved by the use offibers of a smaller diameter and by progres-

Fig. 67:Flow lines (blue) andparticle paths (red)through a fuel filtermedium (depht filter)made of 15 fiber lay-ers, computed withthe aid of CFD

Fig. 68:Influence of the fiberdiameter on the ini-tial filtration effi-ciency (CFD com-putation)

Fig. 69:Diagram showingthe construction of amodern compositefuel filter medium

150

10

20

30

40

50

60

70

80

90

100

20 25 30 35 40 45 50

Fiber diameter [µm]

Impa

ct p

roba

bilit

y [%

]

Filter medium:Thickness: 750 µmPorosity: 86%Number of layers: 15

5 µm10 µm15 µm20 µm

Particle sizes:

Contaminated fuelMelt-blown prefilter layerwith high contaminantstorage capacity

Dense, cellulose-basedultrafine filter layer

Filtered fuel

Fig. 70:Capacity and filtra-tion efficiency ofdiesel fuel filters with a standard filtermedium (cellulose/PES) and compositefilter media withmaximized filtrationperformance

0 25 50 75 100 150125 175 200

Time [min]

100

80

60

40

20

0

Filt

ratio

n ef

ficie

ncy

[%]

Cellulose/PES mediumComposite IComposite IIComposite III

ISO 4020


91

SummaryAt present, filtration technology in motor ve-hicles is characterized by dynamic develop-ments. On one hand, almost all filters are sub-ject to the requirement for greater compact-ness, given that engines will become increas-ingly powerful and that space must be foundunder the hood for more and more componentsassociated with convenience and safety. Onthe other hand, cellulose-based filter media,which have so far proved satisfactory, are reach-ing their limits, because synthetic oils and new fuels are being used which necessitatenew, more resistant media.Radically improved injection technology meansthat the fineness of fuel filters must be consider-ably increased; even this is only possible withmedia based on finer fibers. There is also a trendtowards longer service intervals. Lifetime con-cepts, e.g. for in-tank gasoline filters or multi-cyclones for crankcase ventilation, are alsobeing developed and installed in vehicles. Ultra-fine filter elements are increasingly being usedas gearbox oil filters e.g. in continuous variabletransmission (CVT) systems. Cabin air filters,i.e. particle or pollen filters, and filters with additional adsorption capabilities for gas clean-ing, are increasingly being fitted as standard.Neither these nor diesel particulate filters aredealt with in this discourse, due to space limita-tions. Nevertheless, the installation of a dieselparticulate filter (DPF) in every diesel-enginedvehicle could soon become compulsory if thereare further reductions in exhaust emission limits.Due to their regular cleaning and reconditioningcycles as well as unanswered questions on per-formance after long distances, they would oc-cupy an entire section of their own in a dis-course on exhaust filtration technology.

90 Fuel filters

The performance data of composite filter me-dia of this type are shown in comparison withan earlier standard (mixed fiber paper, until1997 the finest grade of diesel filter mediumin Europe) in Figure 70. With these modernfilter media, it is possible to achieve not onlylifetime gasoline filtration in compact in-tankunits, but also ultrafine diesel fuel filtrationwith extended service interval at the sametime. Modern diesel injection systems requireimproved water separation (cf. [19]), neces-sitating the use of new types of filter mediawith greater water coalescence. This can beachieved with the use of finer, hydrophobicfibers. Figure 71 depicts the water separationefficiency of a composite filter medium withan outer layer of hydrophobic ultrafine fibers bycomparison with filters consisting of standardcellulose filter media with inner and outerwater separation layers.

Fig. 71:Water separationefficiency of differentfilter media

Requirements:more compact-ness …

… finerfilters …

… and longerservice intervals

Added water [ml]

0 250 500 750 1000

100

80

60

40

20

0

Wat

er s

epar

atio

n ef

ficie

ncy

[%]

Composite mediumStandard cellulose (external water separation)Cellulose/PES medium (internal water separation)

ISO 4020, 2%-emulsion


9392 Summary

In general, it can be said that filters in cars arenow being made to perform increasing num-bers of functions, for technical and environ-mental reasons. Many new applications arebeing added to their existing uses, whichthemselves are subject to continuous develop-ment and improvement. This will be instantlyapparent to the reader by a glance at the firstillustration in this brief review and a compari-son with the first edition of this volume issuedin 1989 [1].

Literature[1] Blumenstock, K.-U.: Engine Filters. Verlag Moderne Industrie,

Landsberg/Lech, 1991.

[2] Löffler, F.: Staubabscheiden. Georg Thieme Verlag, Stuttgart, NewYork, 1988.

[3] Sommer, K.: 40 Jahre Darstellung von Partikelgrößenverteilungen –und immer noch falsch? Chemie Ingenieur Technik (72) No. 8, P.809-812, 2000.

[4] Erdmannsdörfer, H.: Trockenluftfilter für Fahrzeugmotoren, Aus-legungs- und Leistungsdaten. MTZ Motortechnische Zeitschrift 43,1982.

[5] Affenzeller, J.; Gläser, H.: Lagerung und Schmierung von Verbren-nungsmotoren. Springer Wien, New York, 1996.

[6] Purchase, D.: Handbook of Filter Media. Elsevier Science Ltd., 1997.

[7] Manegold, E.: Kapillarsysteme. Straßenbau, Chemie und TechnikVerlag, Heidelberg, 1955.

[8] Sturm, H.; Richter, H.: Ein Beitrag zur Beurteilung von Luftfilter-anlagen an Fahrzeug-Dieselmotoren und zur Klärung von Zusammen-hängen zwischen Einflußgrößen und Verschleiß im Fahrbetrieb mitHilfe eines fahrbaren Isotopenlabors. KFT, No. 8, No. 10, 1981.

[9] Trautmann, P.; Sauter, H.: Messung und Abscheidung von Ölnebel-aerosolen aus der Kurbelgehäuseentlüftung von Verbrennungsmotoren.Part 1, MTZ (61), 12/2000, Part 2, MTZ Motortechnische Zeitschrift(62), 1/2001.

[10] Hill, S.; Systsma, S.: A Systems Approach to Oil Consumption. SAE-Paper 910743.

[11] Parker, K. (Ed.): Applied Electrostatic Precipitation. London, BlackieAcademic & Professional, 1997.

[12] Zoebl, H.: Filtrationstechnik. Expert Verlag, Renningen, 1996.

[13] Mach, W.; Trabandt, T.: Auswirkungen fester Fremdstoffe in Ge-brauchtölen auf das Verschleißverhalten von Dieselmotoren. Mine-ralöltechnik 10, 1998.

Outlook


The company behind this book

FILTERWERKMANN+HUMMEL GMBH71631 Ludwigsburg, GermanyE-mail: [email protected]: www.mann-hummel.com

The name MANN+HUMMEL stands for innovation, out-standing quality and reliability in the international automotiveindustry. Acting as partner and co-developer with motorvehicle manufacturers and as supplier of parts and completesystems, MANN+HUMMEL produces filter elements,modules and systems for the filtration of fuel, oil and air (in-cluding complete air intake systems). Its priority is to maxi-mize customer benefits – whether in terms of safety, technol-ogy and cost effectiveness, or service and convenience.

This is also reflected in the company mission “Successthrough Filtration”. The entire spectrum is covered, from thefiltration of the engine intake air to cabin air filters, as wellas the filtration and management of almost all fluids used ina vehicle.

Working closely with customers, our R&D teams develop ef-ficient, class-leading products and advanced systems to meetthe challenges of tomorrow and the day after tomorrow. InEurope alone, the company registers 150 patents a year.

For MANN+HUMMEL, focus to customers’ demands andwishes is a matter of course, in the supply of OEM parts andsystems as well as for OES and IAM parts alike. As a result,our aftermarket spare parts – the famous MANN-FILTERbrand – are made to original equipment standards of quality.

The MANN+HUMMEL group is made up of two divisions,Automotive Technology and Industrial Technology, and em-ploys some 9100 people at thirty-eight sites worldwide. Thecompany was founded in 1941 and since then has made asignificant contribution to the development of filter technol-ogy. It has at its disposal decades of know-how and experi-ence in a key area of automotive technology.

[14] Spanke, J.; Müller, P.: Neue Ölwechselkriterien durch Weiterentwick-lung von Motoren und Motorenölen. MTZ Motortechnische Zeit-schrift 58 (1997), 10.

[15] Dahm, W.; Daniel, K.: Entwicklung der Ölwechselintervalle und de-ren Beeinflußbarkeit durch Nebenstromfeinstölfilterung. MTZ Motor-technische Zeitschrift 57 (1996), 6.

[16] World-Wide Fuel Charter. ACEA, Alliance, EMA, JAMA, April2000.

[17] Klein, G.-M.: Kraftstofffilter. Kraftfahrtechnisches Taschenbuch/Bosch, H. Bauer (Ed.), 23rd Edition, Brunswick, Wiesbaden, Vieweg1999, P. 436-437.

[18] Klein, G.-M.: Changes in Diesel Fuel Filtration Concepts. Proceedings2nd Int. Conf. Filtration in Transportation, Stuttgart, L. Bergmann (Ed.),1999, P. 45-49.

[19] Projahn, U.; Krieger, K.: Diesel-Kraftstoffqualität – Erkenntnisse ausSicht des Einspritzlieferanten. Proceedings 9. Aachener KolloquiumFahrzeug- und Motorentechnik, S. Pischinger (Ed.), Aachen 2000, P.929-944.

[20] Bessee, G. B. et al.: High-Pressure Injection Fuel System Wear Study.SAE 980869.

[21] Klein, G.-M.; Banzhaf, H.; Durst, M.: Fuel Filter Solutions for Fu-ture Diesel Injection Systems. Proceedings World Filtration Congress8, Brighton, UK 2000, P. 887-890.

94 Literature


Filtration in Fahrzeugen Engl[1][1]

Documents

Transcript of Filtration in Fahrzeugen Engl[1][1]