ABSTRACTLegislations worldwide have started imposing stringentemission standards for particulate matter (PM) emitted bydiesel engines. The main reason for these actions is theadverse effects on human health caused by particle emissions.Conventional ceramic Diesel Particulate Filters (DPF) haveproven exceptionally effective in reducing particulateemissions with efficiencies of 90% or more. However, thesefilters require regular active regenerations as well asperiodical ash removal in order to avoid a blockage of theexhaust line. These procedures are both costly and complexand as a result alternative aftertreatment solutions have beendeveloped. One of these solutions is the Particle OxidationCatalyst, POC-X. The main aim of the POC-X is not to equalthe high efficiencies of the DPF, but to achieve the bestpossible particle reduction without creating the risk ofblocking or the need for complex filter regenerationprocedures.

The substrate used in the POC-X is a fine mesh screen madeof metal, which is rolled into a cylinder and placed into theexhaust line. The unique construction forms tortuouschannels which run through the filter. This means that theexhaust gas can either flow through the substrate cells, whichact as trapping agents for soot particles, or along the tortuouschannels should the filter become overloaded. Additionally, aspecially developed washcoat is applied to the substrate in

order to facilitate the production of Nitrogen dioxide (NO2),which aids the regeneration process of the filter.

In an experimental study, the performance of the POC-X hasbeen investigated using a 1.6 liter, Euro 4 Diesel Engine on adynamic test bench. Sophisticated exhaust gas measurementequipment supplied by Horiba was used to evaluate soot,soluble organic fraction (SOF), particle number (PN) as wellas gaseous emissions in real time (1 Hz) during stationary anddynamic measurements. For the dynamic tests, the newEuropean driving cycle (NEDC) was used. The combinationof these measurements provided an accurate performancepicture of the POC-X. By evaluating a variety of POC-Xsizes, the optimum filter dimensions and key parameters weredetermined. Furthermore, by conducting a series of particlesize distribution measurements using a scanning mobilityparticle sizer (SMPS), the relationship between particle sizeand filter efficiency was investigated. Based on these results,a calculation model is being developed, which will supportthe design and application of the POC-X based on engineoperating parameters and filter dimensions. This will allowfor efficiently designed solutions to specific applications.

As particulate emission limitations are also beingimplemented for gasoline direct injection (GDI) technologies,a new POC prototype has been tested in a GDI vehicle with ashort, on-road, durability run.

Particle Oxidation Catalyst (POC®) - From Diesel ToGDI - Studies on Particulate Number and MassEfficiency

2012-01-0845Published

04/16/2012

Toni Kinnunen and Pekka MatilainenEcocat Oy

Daniel Scheder and Werner CzikaHoriba Europe GmbH

David Waters and Gerald RussUniversity of Applied Sciences Darmstadt

Copyright © 2012 SAE International

doi:10.4271/2012-01-0845

INTRODUCTIONIn several studies [1, 2, 3, 4, 5] the health effect of fineparticulate air pollution has been investigated and hasrevealed that prolonged exposure to these types of emissionsis likely to cause serious negative effects on human health.Although the detailed mechanisms are unclear, generalconsensus is that particle size and composition play a vitalrole. Considering the composition, research has shown thatthe SOF emissions, those which attach themselves to theparticulate surface, are the most likely to cause negativehealth effects. Particles are classified as very fine particles(<2.5μm), which covers a wide range of typical particulateemissions generated by internal combustion engines, andlarger particulates (>10μm). Large particles tend to bedeposited in the nose and throat, whereas fine particles canreach the alveolar region in the lungs. As particle sizebecomes smaller, the total surface area increases whichresults in a higher possible mass of SOF's. It was also foundthat at these small sizes, a solid kernel is not always formedand if that should be the case, the particle is considered to bepart of the SOF. Thus some scientists have proposed thatsmaller particulates, i.e. nanoparticulates, are more toxic.With the introduction of Euro 5+ in 2013 and Euro 6 in 2015,a particle number limit of 6.0E+11 [#/km] will be placed onautomotive manufacturers in European countries (Directive88/77/EEC, amendment 2001/27/EC). For most cases, inorder to meet these limits, reliable and cost effectiveaftertreatment systems are required.

Aftertreatment devices are widely employed in modern dieselvehicles. Traditional diesel oxidation catalysts (DOC) areused to remove carbon monoxide (CO) and hydrocarbons(HC) components. Nitrogen oxide (NOx) emissions arecommonly reduced by selective catalytic reduction (SCR), atechnique which will be utilized more and more in the future.Conventional ceramic diesel particulate filters have provenexceptionally effective in reducing particulate emissions,with filter efficiencies of 90% or more. However, these filtersrequire regular active regenerations as well as periodical ashremoval in order to avoid a blockage of the exhaust line.These procedures are both costly and complex and as a resultalternative aftertreatment solutions have been developed. Oneof these solutions is the POC-X developed by EcoCat Oy.The aim of the POC-X is to achieve a moderate degree ofparticle reduction without the risk of blocking or the need forcomplex filter regeneration procedures.

The POC-X has been in the development stage for the pastyears and has been presented by Lylykangas et al. [6],Vakkilainen et al. [7], Vaaraslahti et al. [8] and Lehtoranta etal. [9]. The substrate used in the POC-X is a fine mesh screenmade of metal, which is rolled into a cylinder and placed intothe exhaust line. The unique construction forms tortuous, ‘X’shaped channels which run through the filter (figure 1). Thismeans that the exhaust gas can either flow through the

substrate cells, which act as trapping agents for soot particles,or along the tortuous channels should the substrate becomeoverloaded or blocked. By applying a specially designedwashcoat, the generation of NO2 is made possible whichallows for the filter regeneration at exhaust gas temperaturesduring normal operating conditions. In addition, the DOC isgenerally placed upstream of the POC-X in order to furtherenhance NO2 generation. The major advantage of the POC-Xover the traditional DPF is that there is no blocking risk, dueto the tortuous channels inherent in the structure.

Figure 1. - POC-X Filter. The two layers form the ‘X’shaped structure

To ensure that POC-X technology can be applied effectivelyto the wide range of engines available, a mathematical modelis required which calculates the performance of the filterbased on engine parameters. Key parameters include exhaustgas massflowrate and temperature, as well as the particle sizedistribution. Using these values, the optimum dimension forthe POC-X filter can be determined for a specific enginemodel. Therefore a basic understanding of the particulatetrapping process within the metal screen structure is required.

In addition to diesel engine particle emissions, the comingEuro 6 regulations will set a limit for PN emissions whichwill require further development of GDI engines [10, 11].One of the routes to achieve the emission targets in a feasibleway could be to apply an after treatment solution. The POChas the advantages of lower backpressures and cost whencompared to conventional ceramic filters. Therefore, POCdevelopment has been continued and further prototypedesigns for optimized filtering performances are underevaluation. One of these potential solutions, in this paperlabeled as “POC prototype”, was tested in combination with aGDI vehicle on a roller dynamo NEDC test after a short onroad durability test.

EXPERIMENTAL SET-UPTEST BENCH AND SAMPLESDiesel Engine - POC-XThe experiments with the POC-X and the diesel engine wereall conducted on a dynamic engine test bench. A turbocharged, 1.6 liter, common rail diesel engine with a variableturbine geometry (VTG) turbocharger was used with slightadjustments made to the engine control software in order tobe able to run both stationary and dynamic tests (figure 2).The aim was to achieve operating conditions as close aspossible to those experienced in an actual vehicle. NOxemissions were controlled solely through the addition ofrecirculated exhaust gas (EGR), which was conditioned usinga water cooled heat exchanger. To monitor the percentage ofEGR, an air-fuel (A/F) sensor is placed downstream of theexhaust gas turbine of the turbo charger.

In its serial setup the engine is operated with a semi close-coupled DOC, DPF combination, which is locatedapproximately 600 mm downstream of the turbine exit (figure2). The DPF is responsible for the soot and particleemissions, while the DOC reduces the gaseous elements inthe exhaust gas. The combined aftertreatment system iscontrolled using two temperature sensors, one before theturbocharger and one between DOC and DPF. To preventsoot overloading of the DPF and hence potential blocking ofthe exhaust line, the pressure difference across the filtersubstrate is monitored by engine control unit (ECU) usingtwo pressure sensors before and after the filter.

Figure 2. - Engine test cell

In order to incorporate the test samples into the exhaust line,the DOC and POC-X were assembled in series, analogous tothe original DOC, DPF combination. The DOC dimensionsremained unchanged (diameter = 133 mm, length = 74.5 mm,

cells per square inch (cpsi) = 350) but was additionally coatedwith an Ecocat wash coat at a platinum group metal (PGM)

load of 70 g/ft3.

The samples tested are shown in table 1. Different sizedPOC-X's were welded to three separate DOC's, each with thesame dimensions and washcoat. The size variation of thePOC-X was based on the diameter to length ratio (D/L ratio).An initial size was chosen and the resulting D/L ratio wasused as a reference value. The sizes for the small diameterand large diameter samples were calculated using D/L - 30%and D/L + 30% respectively, thus keeping filter volumeconstant. By choosing the total included diffuser angle aslower than 20° for the transition from the DOC to the POC-Xfor the larger diameter sample it has been assured, that theuniformity index of the flow in front of this POC-X is above0.885.

Table 1. - Sample dimensions and specifications fortesting with the Diesel engine.

Flanges were then welded to the entrance and exit of theDOC/POC-X structure and provided the means of fixing thesample into the exhaust line. Emission probes andthermocouples were placed up- and downstream of the testsample and remained fixed when the samples wereexchanged. The probes placed between DOC and POC-Xwere exchanged when samples were changed. A schematic ofthe measurement scheme is shown in figure 3.

GDI Engine TestsThe tests with the GDI POC prototype were conducted solelyin a Euro 4 GDI vehicle, which in its serial version isequipped with a close coupled Three-Way-Catalyst (TWC).The serial TWC has been left unchanged and the POC

prototype was installed 400 mm downstream of the TWC.The vehicle is powered by a 1.4l turbocharged GDI enginewith direct injection. It is certified to meet Euro 4 limits andif no particulate aftertreatment is applied exceeds theparticulate number limit, which has been introduced forgasoline engines as a part of the Euro 5+ legislation. It doeshowever meet the particulate mass emission limit of 5mg/km, which is part of the Euro 5 legislation for GDIvehicles.

MEASUREMENT EQUIPMENTThe investigation and interpretation of the filtrationperformance of the POC-X requires that several parametersare measured and recorded. An especially detailed analysis isrequired of soot levels in order to better understand theresults. A number of different analyzers were used to attainmore information concerning PN, soot mass and the SOFmass emissions. It was possible to measure this data on amodal base which also helped to understand the transientbehavior of the POC-X. The measurement equipment usedwas supplied by Horiba and is listed in more detail in thefollowing chapters.

Particle Number Measurement SystemMEXA-1000SPCSThe MEXA-1000 SPCS provides a real time count of theparticles contained in the engine exhaust gas for a specificrange of particle sizes. It conforms to the ECE RegulationR83 (ECE/TRANS/WP.29/GRPE/2008) which states thatonly particles with a solid kernel need to be measured andthat volatile particles e.g. SOF and Sulphates, can be removed

as they interfere with the dilution of the exhaust gas and thusmake reproducible measurements difficult.

A schematic layout of the MEXA-1000 SPCS measurementsystem is shown in figure 4. The raw exhaust gas sample ispassed through highly efficient particulate air (HEPA) andCarbon filters and then led through a continuous volumesampling (CVS) dilution tunnel which dilutes the sample.The aerosol is then passed over a cyclone which removesparticles larger than 2.5 μm. The remaining sample gas is ledthrough the volatile particle remover (VPR). In the primarydiluter (PND1) the dilution air is heated to 150 °C to preventthe formation of further volatile particles. Following thePND1, the volatile particles are evaporated in the evaporationtunnel (ET) at 300 °C to 400 °C. To avoid particle formationdue to condensation, the concentration and temperature of theevaporated substances is reduced in the cooling diluter(PND2).

The particles are then counted in the particle number counter(PNC) using a condensation particle counter (CPC) detector.With the use of a heated saturator, which provides anenvironment which is saturated with butane gas, the particlesform solid kernels and increase in size, making them easier todetect. Finally, the particles are counted with a laser diode.The size of the particles that are counted is limited by theCPC detector which only detects particles larger than 23 nmand by the cyclone, which removes particles larger than 2,5μm. A detailed layout of the CPC unit is shown in figure 5.

The dilution system of the MEXA - 1000 SPCS primarilyconsists of the diluters PND1 and PND2. An optional predilution unit (PDU) can be installed (figure 6), which makes

Figure 3. - Measurement Scheme

it possible to take the sample probe directly out of the exhaustgas stream of the engine. A PDU was used for the tests madewith the POC-X.

These three elements are part of the wide range continuousdilution (WRCD) system which allows a broad range ofdilution factors to be set based on the individual dilutionfactors set at PND1, PND2 and the PDU. A detailedschematic of the dilution system is given in figure 7.

In this configuration, the sample flow rate (Qsample) of thesample gas taken into the diluter is given by equation 1.

(1)

(2)

Figure 4. - Particle number measurement setup as defined by PMP.

Figure 5. - Schematic setup of CPC (© by TSI inc.).

(3)

The dilution flow rate (Qd.air), the flow rate in the exhaustline (Qvacuum) and the flow rate used in the measuringinstruments (Qinst.) are regulated externally and in generalremain constant. The sample flow rate is measured across aflow orifice (FO) while the makeup air (Qmake-up) is closeloop regulated, so that Qsample can be set. Using thismethod, Qd.air can be controlled accurately and deviations inQvacuum and Qinst. are regulated via closed loop control. Thisway, a very stable dilution factor (DF) can be maintained.Then equation for the DF is given by equation 3.

Scanning Mobility Particle Sizer (SMPS)A SMPS from TSI was used to measure the particle sizedistribution and is a commonly used technique to measuresize distributions [12, 13, 14]. The principle of this device isbased on the electrical mobility of differently sized particles.The SMPS sample line is connected to an auxiliary output ofthe Mexa-1000SPCS. At this point the particles have alreadypassed through the cyclone and the VRP in the form of apolydisperse aerosol. By passing the gas through an aerosol

neutraliser the particles are given a known charge. The flowis then directed across two plates which are positively andnegatively charged called the Differential Mobility Analyzer(DMA). The positively charged particles will then beattracted to the negative plate and the negatively chargedparticles to the positively charged plate. By measuring thedistance it takes for a particle to reach the plate a relationshipto the particle size can be established based on the mobilitydiameter of the particle. Finally, the now monodisperseaerosol reaches the CPC detector where the system scansthrough a range of particle sizes and records the counts withineach size classification. The CPC detector functionsaccording to the same principles explained earlier and shownin figure 5.

Continuous Particulate Matter AnalyzerMEXA-1230PMThe MEXA-1230PM measurement system continuouslyanalyzes soot, SOF, and total PM in engine exhaust gas. Sootis measured by the diffusion charging method while SOF ismeasured using the dual-FID method which uitilises twoheated flame ionization detectors (HFIDs) (figure 8). Thetotal PM is obtained by the sum of soot and SOF.

Figure 6. - Sampling Configuration using pre-dilution.

Figure 7. - Outline of WRCD (Continuous Wide Range Diluter).

The system consists of:

• control PC

• main cabinet incorporating soot analyzer, SOF analyzer,and sampling unit

• pre-sampling unit incorporating soot diluter operating onejector principles

The system provides separate measurements of soot and SOF.The total PM is calculated from the measured soot and SOFquantities. Measurement values are displayed on the controlPC screen and they can be externally transmitted to a recorderthrough an analog output. A Measurement data loggingcapability is included. The concentration output can be savedin an ASCII format.

TEST PROGRAMSA particle size distribution measurement was made using aSMPS. A operating point was chosen in the EGR regionwhere changes in the EGR percentage was applied, whilekeeping other parameters constant, in order to achievechanges in particle size distributions. When the EGR amountis increased, predominantly large particles are generated andas the EGR is decreased, the percentage of small particlesincreases. The stabilization times and test procedure wereadapted from procedures used by Desantes et al [15]. TheEGR content was regulated by defining the amount of freshintake air. Since the total amount of fresh and recirculated airis constant at a single operating point, the amount of EGRwill drop if the fresh air portion is increased. The test wasconducted only for the small diameter sample.

For the stationary measurements the sample was tested withregard to its filtration and oxidation performance. 65stationary operation points creating an engine map coveringspeeds from 1000rpm to 4000rpm. Measurements were takenat engine out and before and after the POC-X. The tests wereconducted for the small diameter sample, the medium

diameter sample and the large diameter sample. As themeasurements could not be conducted in parallel across theFilter, SOF, Soot and PN emissions across the engine mapwere recorded several times before and after the POC-X inorder to obtain a reliable representation of the filteringperformance. Test to test variation was calculated as less than10%. By applying frequent and systematic regenerations itwas ensured that the POC-X remained free of soot.

To understand the behavior under dynamic operationconditions, a simulation of the NEDC was used. To do so,vehicle data of a B-class vehicle was used to set up a roadload model, which was then operated by the engine benchcontrol software to calculate the transient speed and torquedata. This test was again completed for all three of thedifferent sized POC-X samples.

The testing of the POC prototype and the vehicle with theGDI engine was performed on a roller dynamo. The NEDCwas used as the dynamic test. To measure the PN emissionsthe test procedures were conducted according to PMPprotocol. In addition to the particulate number emissions, thegaseous emissions were measured, but no modal soot andSOF mass concentration were recorded during the NEDCtest. The soot mass emissions were measured following thestandard procedure by taking an exhaust probe from thediluted exhaust and using standard filter plates. Tests weretaken after the vehicle covered distances of 2300 km and14400 km.

RESULTSPARTICLE SIZE DISTRIBUTIONMEASUREMENTSThe shifts in particle size distribution as a result of EGRchanges are shown in figure 9. A clear shift is seen to smallerparticles as EGR rates decrease. Also, the number of particlesreduces dramatically with reduced EGR.

Figure 8. - Setup of MEXA-1230PM.

Figure 9. - Particle size distribution measured in front ofthe small diameter POC-X with the Diesel Engine. Shifts

in the EGR quantity resulted in distribution shifts

The calculated efficiency as a function of the particle size isgiven in figure 10 at a speed of 2000 rpm and 30% throttle.As the particles become smaller (<60nm), a clear increase inthe performance of the filter is noted. As the particle sizebecomes larger (>60nm), the efficiency evens out toapproximately 30% for this operating point. The quantitativevalues of the efficiencies at certain particulate sizes arestrongly dependent on the condition of the exhaust gas(temperature, massflowrate, etc). However the tendency thatefficiency increases as particle size decreases remains. Thelink between particle size and filter efficiency can best beexplained according to diffusion and interceptionmechanisms. Particle trapping as a result of diffusion forcesare most prominent when particle size and mass is small,while interception efficiency, based on the mass inertia of theparticles, is largely responsible as particle size increases.

Figure 10. - Particle size versus filter efficiencycalculated from size distributions measured in front of

and after the small diameter POC-X with the DieselEngine and generated through EGR variations

STATIONARY MEASUREMENTSFor these tests, measurements at operating points above 2700rpm with a brake mean effective pressure (BMEP) higherthan 4 bar showed more repeatable results. To a large extent,this was traced back to known EGR influences on particulateformation and characteristics [16] which negatively influencethe reproducibility of the measurements. Boost pressureregulation was also less accurate at low loads and speeds,especially at 1000 rpm. The results shown refer tomeasurements made with the small diameter sample. Similardiagrams were made for the large and medium diametersample.

The POC-X was connected to a DOC in series as was shownin Figure 3 earlier. The DOC showed efficiencies of over95% for CO and more than 85% for HC. While PN and Sootefficiencies across the DOC remained low (less than 15%),SOF emissions were reduced drastically, with efficiencies inthe high nineties (figure 11). The expectedly low efficiencyfor PN and Soot over the DOC can be related to a trappingeffect, rather than an oxidation effect.

Figure 11. - SOF efficiencies calculated frommeasurements before and after the DOC with the Diesel

Engine

Despite the low concentrations of SOF in front of the POC-X,large reductions of up to 100% are still achieved across largeparts of the engine map (figure 12). This is due to thecatalytic wash coat which is applied to the mesh structure ofthe POC-X, giving it a significant oxidizing ability. Thecatalytic efficiency is influenced by the temperature, spacevelocity and the concentrations of the educts. Thus, at pointswhere large efficiencies are measured across the DOC, it isexpected that low efficiencies will be measured across thePOC. The combination of the DOC and POC efficienciesresults in an SOF reduction of 95% across the engine map.The local low points in the diagram coincide with those areasin the DOC efficiency map (figure 11) where efficienciesabove 95% are measured. This means that the concentration

of SOF's at these points are exceptionally low when theyarrive at the POC. This results in problems when calculatingthe arithmetic efficiency and can result in local low points.

Figure 12. - SOF efficiencies calculated frommeasurements before and after the small diameter POC-

X with the Diesel Engine

As mentioned, reduction of the particulates is relatively lowacross the DOC. POC-X efficiencies with respect to Soot areshown in figure 13. Values between 25% and 45% are shownacross the engine map. In general, the efficiency increases asspeed and load increases. Shifts in particle size distributionsat high speeds and loads are suspected to be one of reasonsbehind the increasing efficiencies in those areas. As wasshown in the particle distribution measurements the filterefficiency increases drastically as the particles becomesmaller. Furthermore, particle size distribution shifts towardssmaller particles occur due to either increased rail pressure ora reduction in the EGR percentage. EGR is only active up toa speed of 2500 rpm and the rail pressure increasespredominantly with higher loads. Thus, one can expect ahigher percentage of smaller particles with higher speeds andloads and hence a higher efficiency.

The results for the PN measurements show a relatively strongcorrelation to the Soot results with efficiencies for PNshowing similar tendencies with higher efficiencies beingcalculated at high speed and loads (figure 14). As Soot andPN emissions are measured with two separate systems, thisgives an indication of the repeatability of the test setup.Similarly to the Soot results, particle size distributions seemto have a large effect on the performance of the filter withrespect to PN. Considering the areas where efficiencies arebetween 20% and 30% at speeds between 1500 rpm and 2500rpm. This area has a high EGR rates and as a result, particledistributions with predominantly large particles. Whencomparing this to the particle size distribution measurements,it was shown that as the particle size increases, so theefficiency decreases. Of the 4 samples tested, the small

diameter sample showed the highest overall efficiencies forSoot, SOF and PN emissions.

Figure 13. - Soot efficiencies calculated frommeasurements before and after the small diameter POC-

X with the Diesel Engine

Figure 14. - PN efficiencies calculated frommeasurements before and after the small diameter POC-

X with the Diesel Engine

NEDC TEST RESULTS- DIESELENGINEBoth the Soot and PN efficiency curves show the smalldiameter sample as being the most effective choice in termsof Filtering efficiency, with measured efficiencies of 37.6%and 43.2% respectively (figures 15 and 16). Next, themedium diameter sample shows efficiencies of 32% for Sootand 37.5% for PN. The Large sample yielded the lowestefficiencies in Soot and PN tests, with 18% efficiency forSoot and 26.4% for PN. These results correlate with thestationary measurements which also showed the smalldiameter sample as being the most suitable choice.

One of the core differences between the small, the mediumand the large samples is the change in the exhaust gas speedthrough the filter. As the sample diameter becomes smaller,the velocity increases and higher efficiencies are measured.The other parameter is the filter length which determines theamount of passes through the POC-X wire mesh. To whatextent each of these parameters is responsible for the higherefficiencies requires further investigation.

Figure 15. - Soot efficiencies over NEDC calculatedfrom measurements before and after the POC-X's with

the Diesel Engine during the NEDC test cycle

Figure 16. - PN efficiencies calculated frommeasurements before and after the POC-X's with the

Diesel Engine during the NEDC test cycle

NEDC TEST RESULTS- GASOLINEENGINEAfter the installation of the POC in the exhaust system, thevehicle was operated for 2300km on the road to stabilize theperformance of the exhaust system. To better understand thedurability of the POC in this application, the vehicle wasoperated for another 12100km. After a total of 14400km withthe POC the emissions of the vehicle were measured again.

The test results with regard to the particulate mass andnumber emissions are depicted in figure 17.

Figure 17. - Particle number and mass efficiency of thePOC sample applied to a GDI in the NEDC.

These results show, that during the short durability run theperformance of the POC structure did not suffer, even a slightefficiency increase for both, particulate number and massreduction has been measured. A paper by Lepperhoff [17]suggests that the particle size distribution of a direct injectiongasoline vehicle tends to have a higher distribution of muchsmaller particles when compared to diesel engines. Thiscould also be the reason for the higher efficiencies calculatedas POC efficiency is inclined to increase with smallerparticles.

MODEL RESULTSA model was developed which predicts the efficiency of agiven POC-X filter across the engine map. The modelessentially consists of two parts which in combinationcalculate the expected efficiencies at a single operating point.Measured particle size distribution variations based on EGRand Rail variations will be used to determine sizedistributions at each point while the efficiency modelcalculates the efficiency with respect to particle sizeaccording to diffusion and mass inertia forces.

Characteristic parameters which govern the deposition ofparticles in the POC-X structure are the size of theparticulates, the structure of the wire mesh, the flow fieldaround the mesh and the temperature. For very small particlesin the nanometer range, motion is governed by Browniandiffusion. As the particle size increases, interception andimpaction need to be considered. Since the particle sizedistribution as well as exhaust gas flow rate and temperaturevaries throughout the entire engine map, it is critical to takeall of these factors into account when constructing the model.Empirical relations were chosen which calculate efficiencieswithin the natural boundaries, one or zero, should anyparameter become dominant or negligible.

In figure 18 a picture of the simulated flow field within thePOC-X structure is depicted, which indicates, that part of theflow remains within the channels formed by the metal screenstructure and another part flows through the wire screens witha relatively low flow speed. Quantities of the flow rate aredependent on the specific engine operation point.

Figure 18. - Simulation of the flow field through thePOC-X structure

Of the available empirically determined flow fields to beconsidered for the flow through a filter with low Reynoldsnumbers and thus negligible inertia, the Kuwabara-Happel[18, 19, 20] flow field, which is a so-called cell modelapproach for creeping flow in a system of spheres, is onepossible choice. Happel's flow and the Kuwabara's flow fieldare essentially the same, with the exception, that Kuwabaraapplied the condition of vanishing shearing stress at theimaginary concentric spherical boundary around thecollecting sphere. For a filter constructed of a very fine wiremesh it is assumed that the approach of vanishing inertia andcreeping flow still holds since the orders of the calculatedReynolds Numbers for most operating conditions is lowerthan 10. In contrast to a packed bed reactor the geometry ofthe multiple layers of very fine wire mesh in the POC-X cannot be modeled as spheres. However, as shown in [21] and[22] for flow around multiple cylinders, the Kuwabaracondition of vanishing shearing stress at the outer boundaryseems to be a suitable approach and was thus applied for thePOC-X structure.

The solution of the diffusion rate of small particles towardsthe collector surface was found in [23] with the assumption ofKuwabara's flow field and applying the method of boundarylayer theory:

(4)

where:

(5)

and the Peclet Number:

(6)

In [24] the collection efficiency for single wire screen wasexperimentally investigated and the same exponent of thePeclet Number as given in equation (4) resulted in a good fitof the empirical function to the measured data. However,since only a single mesh layer has been investigated adifferent pre-exponential factor has been found. Also inseveral studies of flow through fibrous filters the Kuwabarasolution has been found as applicable [25, 26].

To account for the interception, again the assumption ofKuwabara flow field was taken and thus the efficiency forinterception can be calculated as follows:

(7)

With the assumption that both mechanisms workindependently from each other the total efficiency fordiffusion and interception can be calculated as follows:

(8)

When increasing particle size and velocity the impaction ofparticles is considered as an additional interaction mechanismof the wire mesh structure with the particles in the exhaustflow. As in [23], it is assumed that the impaction mechanismis independent of other mechanisms. Also, it was assumedthat in contrast to the diffusion, which is especially ofimportance in case the flow passes multiple layers of wiremesh, the impaction dominates in areas of the POC-Xstructure where the flow is leaving the channel like structuresand enters into the wire mesh with a higher flow speed.

To model the deposition of the particles in the wire mesh ofthe POC-X structure by impaction, several models have been

investigated. Suneja and Lee [27] solved the completeNavier-Stokes equations in the intermediate range ofReynolds numbers (≤ 100) in order to obtain the flow fieldaround the cylindrical fibres. Based on the results of thissimulation, the particle trajectories and the collisionefficiencies were computed. The calculated results for avariety of Reynolds and Stokes numbers as well as differentparticle to fibre radii ratios have been plotted and anempirical correlation was derived for the collision efficiency.Furthermore, Muhr [28] developed an empirical equation forthe impaction efficiency based on the Reynolds and Stokesnumber. An approach, which can be seen as a moregeneralized form of the equations developed by Suneja et al.and Muhr, has been introduced by Kasper et al. [29].

The general equation of Kasper et al. for the impactionefficiency has been fitted to the experimental data which hasbeen measured for the POC-X to achieve the followingempirical formula:

(9)

Where the adhesion efficiency was left unchanged as reportedin [29]:

(10)

To achieve the efficiency for impaction, both terms aremultiplied:

(11)

Again assuming, that the collection mechanisms actindependently, a total efficiency can be calculated accordingto the following equation:

(12)

In this fashion it is possible to determine the efficiency at anyoperating point using only engine and filter data. Thestructure of the model is shown in figure 19.

Using data from the stationary tests conducted with the smallsample POC-X an efficiency map has been generated usingthe model. The simulated results are shown in figure 20 andthe corresponding calculated efficiency from the measureddata is given in figure 21.

The generated efficiency map shows a good correlation withthe measured results, as well as the same tendencies to higherefficiencies as speeds and loads increase.

Figure 20. - Simulated PN efficiencies for small diameterPOC-X

Figure 19. - Block diagram of Model

Figure 21. - PN efficiencies calculated frommeasurements before and after the small diameter POC-X with the Diesel Engine and limited to values between

2500 and 4000 rpm

In order to increase the accuracy of the model extensivetesting will still need to be conducted, especially with regardto particle size distribution and EGR and Rail effects.Furthermore, several researchers have suggested furtherfactors, especially fuels and engine design, which also affectthe size distribution [13, 30].

Nonetheless, the results achieved with the current modelshow the potential for modeling.

DISCUSSIONThere are a variety of diesel exhaust aftertreatment devicesand methods. One method such method is to use a DOC incombination with a particulate filtering apparatus. In thisstudy the particulates were collected and oxidized with aPOC-X, which becomes a feasible solution if complexregeneration processes want to be avoided.

The main aim of this paper was to create a betterunderstanding of the filtering performance of the POC-X withrespect to particle number, soot and the SOF under differentconditions. Changes in the D/L ratio made it possible toidentify some of the key performance parameters and incombination with stationary and dynamic tests (NEDC)established a comprehensive efficiency profile of the POC-X.Furthermore, a POC prototype was tested and evaluated forGDI applications.

The tests conducted for the particle size distribution studyshow a strong link between particle size and POC-Xefficiency. A constant point was chosen and the EGR wasvaried in order to shift the particle size distribution whilekeeping original conditions constant. At particle sizes smallerthan 60 nm a sharp increase in efficiency is noted, withvalues ranging up to 90%. As particle size increases past 60nm the efficiency curve evens out at an average of 30%. This

trend is noticed in the results of the stationary and dynamicmeasurements as well as the findings made for GDI.

The stationary studies incorporated a large selection ofconstant speed and load points which exposed the POC-X toa variety of different conditions and showed the importanceof selecting the right design for newly developed vehicles. Byadjusting the size of the POC-X, exhaust gas flow speed andtemperature were identified as factors which influenced thefiltration efficiency. It was found that by decreasing the D/Lratio, and thus increasing the flow rate through the structure,the performance of the POC-X improved with respect toparticle number and mass. This is reflected in the results withthe small diameter POC-X showing the highest efficiencies ofthe samples tested with values of 40% - 50% calculated inEGR free regions. For the areas where EGR is active, thefiltration efficiency was reduced to around 30%. The SOFreduction over the POC-X showed very high efficiencies forthe large, the medium and the small samples with averagevalues in the high nineties. Total reduction over DOC andPOC-X reduced the SOF content almost completely withaverage values over 95% across the entire engine map.

A NEDC simulation was used to evaluate the dynamicperformance of the POC-X. These results correlate with thefindings made in the stationary tests as the small diametersample achieves the highest efficiencies for both PN and sootwith cumulative efficiencies of 43.2% and 37.6%respectively.

Higher efficiencies of up to 65% for both soot and PN weremeasured for the POC prototype used in the GDI tests. Thesehigh efficiencies seem to indicate that the particulate sizedistribution of the GDI engine tends towards a higherpercentage of smaller particles. This deduction is based onthe results made in the particulate size study.

Finally, first steps have been made to create a simulationmodel, which, when complete, will aid the design process ofthe POC-X. Promising results have been made in areas whereEGR is not active. However, more research is required toimprove the prediction of the particulate size distribution atpoints with high EGR percentages as well as predictingparticle size distributions.

CONCLUSIONSAs illustrated in this paper, there are valuable insights to begained by studying the properties and characteristics ofparticle emissions.

It was clearly shown that the engine load and the resultingparameters e.g. exhaust flow speed and particulate sizedistribution have a key effect on both particulate number andmass reductions of the DOC, POC-X combination.Collaboration between vehicle/engine manufacturers and

aftertreatment suppliers will prove vital to further improveand develop these systems.

With an optimized POC-X design, a NEDC test efficiency ofabout 40% for particulate number and mass efficiency wasmeasured for a diesel engine application and 65% with a POCprototype optimized for the combination with a GDI engine ispossible.

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CONTACT INFORMATIONToni KinnunenCTO, Ecocat OyTyppitie 1FI-90620 Oulu, Finlandtoni.kinnunen@ecocat.com

DEFINITIONS/ABBREVIATIONSA/F

Air Fuel Ratio

ASCIIAmerican Standard Code for Information Interchange

BMEPBrake Mean Effective Pressure

CFOCritical Flow Orifice

CFVCritical Flow Venturi

COCarbon Monoxide

CPCCondensation Particle Counter

CVSConstant Volume Sampling

dcCollector Diameter

dpParticle Diameter

dwWire Diameter

DpParticle Diffusion Coefficient

D/L RatioDiameter to Length Ratio

DC SensorDirect Current Sensor

DFDilution Factor

DMADifferential Mobility Analyzer

DOCDiesel Oxidation Catalyst

DPFDiesel Particulate Filter

ECEEconomic Commission for Europe

ECUElectronic Control Unit

EGRExhaust Gas Recirculation

ETEvaporation Tunnel

FOFlow Orifice

GDIGasoline Direct Injection

hAdhesion efficiency function

HCHydrocarbon

HEPAHighly Efficient Particulate Air

HFHydrogen Flame

HFIDHydrogen Flame Ionization Detector

g(ε)Kuwabara Function

MFCMass Flow Controller

ṁairIntake Air Mass flow rate

ṁExhExhaust gas Mass flow rate

ṁFuelFuel Mass flow rate

NEDCNew European Driving Cycle

NO2Nitrogen dioxide

NOxNitrogen Oxides

PPressure

PCFParticle Cyclone Filter

PDUPre Dilution Unit

PePeclet Number

PGPressure Gauge

PGMPlatinum Group Metals

PMParticle Matter

PMPParticle Measurement Program

PNParticle Number

PNCParticle Number Counter

PND1Primary Dilution Unit

PND2Secondary Dilution Unit

POC-XParticle oxidation catalyst with the “X” structure

PSPParticle Sample Probe

PTTParticle Transfer Tunnel

QinstInstrument Volume flow rate

Qmake-upMake-up air Volume flow rate

QsampleSample Volume flow rate

QvacuumVacuum Volume flow rate

ReReynolds Number

SCRSelective Catalytic Reduction

SMPSScanning Mobility Particle Sizer

SOFSoluble Organic Fraction

StStokes Number

TTemperature

TCTemperature Control

THCTotal Hydrocarbon

TWCThree Way Catalyst

UiFlow Velocity

VOFVolatile Organic Fraction

VPRVolatile Particle Remover

VTGVariable Turbine Geometry

WRCDWide Range Continuous Dilution

ΔPChange in Pressure

GREEK LETTERSε

Porosity

ηTotal Filtration Efficiency

ηDDiffusion Efficiency

ηIImpaction Efficiency

ηRInterception Efficiency

ηDRCombined Efficiency for Diffusion and Interception

Impaction Efficiency w/o Adhesion

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