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CHAPTER 11 IGNITION SYSTEMS Including Digiplex and Microplex purpose of the ignition system being to initiate combustion, it is vital that the system an electrical discharge (spark) of the necessary intensity at right place in the firing cycle. If the spark is too particularly the is rich or the cylinder pressure is the engine will not run. If the spark occurs in the wrong place, at power will lost, and at worst, damage may occur. To create a spark (which is the means of initiation of the explosion of the fuel/air mixture) requires a potential (voltage) difference to exist between the positive electrode of the spark and negative, earth electrode, ie the body of the plug. The and more advanced modern systems all employ a transformer coil to generate this discharge at the plug. The primary and secondary coils are wound around an iron core, and when to the primary is switched off, collaps- ing magnetic field a current to be in secondary windings. The means of switching can be either by a simple contact-breaker, which is actuated at a speed to match the cycle (half engine speed for the four-stroke TC), or by a system closes to charge up the primary windings of the coil; and this is known as the period (measured in degrees or percent). When it opens, arcing tends to occur across the contacts as the current the primary winding opposes this change of condition and a capacitor is across the points to absorb this current. When the points again, capacitor discharges the coil, so to build up the primary current. It is a fact that a greater current is by collapsing the primary field than creating it, thus it should be remembered, when timing a TC, that the spark occurs when the contact-breaker opens. The problems with a contact-breaker system are that the system is operated at full battery voltage, using a large current drawn from main starting system; that the performance of the points at engine speed is by the inability of the spring to control the system accurately. The points them- selves are subject to wear, and since some minor arcing occurs across them, on a well-maintained system, the points to closing up in the process and affecting period/timing. systems overcame these pitfalls; a very low switching current is required, almost mainte- and the intensity and duration of the spark is greater. The earlier used a system; ignition (transistorized) was introduced on the 132 131 versions, plus certain Lancia models in the The ignitions were basically of two In the Bosch system, a four-blade reluctor rotates around an inner coil to the switching current; as each of the reluctor arms passes the pickup in the distributor the current is fed to the amplifier unit. With system, the reluctor is a steel four-lobe rotor, but it the current a similar way. Dwell variations can occur on a distributor due to worn bearings, if it is of the type with the centrifugal weights mounted above the CB points baseplate. Similarly, the perfor- mance of the points can cause to alter at high It is therefore, to examine the dwell with a and ensure that at both and low rpm it falls within the stated data, than to set them with a feeler gauge and for the of electronic ignition dwell variation. Useful data to CB Dwell angle CB points gap Coil resistance primary secondary 52-58 Marelli Bosch Marelli ±400 Diagrammatic layout of ignition coil and contact breaker system. 137

Transcript of fiat lancia tuning - lakebox.dnsalias.comlakebox.dnsalias.com/photos/1988 4runner/megasquirt... ·...

CHAPTER 11

IGNITION SYSTEMSIncluding Digiplex and Microplex

purpose of the ignition system beingto initiate combustion, it is vital that thesystem an electrical discharge(spark) of the necessary intensity atright place in the firing cycle. If the sparkis too particularly theis rich or the cylinder pressure is theengine will not run. If the spark occurs inthe wrong place, at power will lost,and at worst, damage may occur.

To create a spark (which is the means ofinitiation of the explosion of the fuel/airmixture) requires a potential (voltage)difference to exist between the positive

electrode of the spark and negative, earth electrode, ie the body

of the plug. The andmore advanced modern systems allemploy a transformer coil to generate thisdischarge at the plug. The primary andsecondary coils are woundaround an iron core, and whento the primary is switched off, collaps-ing magnetic field a current to be

in secondary windings. Themeans of switching can be either by asimple contact-breaker, which is actuatedat a speed to match the cycle (halfengine speed for the four-stroke TC), orby a system

closes to charge upthe primary windings of the coil; and this

is known as the period(measured in degrees or percent). When itopens, arcing tends to occur across thecontacts as the current the primarywinding opposes this change of conditionand a capacitor isacross the points to absorb this current.When the points again, capacitordischarges the coil, so tobuild up the primary current. It is a factthat a greater current is bycollapsing the primary field thancreating it, thus it should be remembered,when timing a TC, that thespark occurs when the contact-breakeropens.

The problems with a contact-breaker

system are that the system is operated atfull battery voltage, using a large currentdrawn from main starting system; thatthe performance of the points atengine speed is by the inabilityof the spring to controlthe system accurately. The points them-selves are subject to wear, and since

some minor arcing occursacross them, on a well-maintainedsystem, the points to closingup in the process and affectingperiod/timing.

systems overcame thesepitfalls; a very low switchingcurrent is required, almost mainte-

and the intensity and duration ofthe spark is greater.

The earlier used asystem; ignition (transistorized)was introduced on the 132 131versions, plus certain Lancia models inthe The ignitionswere basically of two In the Boschsystem, a four-blade reluctorrotates around an inner coil tothe switching current; as each of the

reluctor arms passes the pickup in thedistributor the current is fed to theamplifier unit. With system,the reluctor is a steel four-lobe rotor, butit the current a similar way.

Dwell variations can occur on adistributor due to worn bearings,

if it is of the type with thecentrifugal weights mounted above theCB points baseplate. Similarly, the perfor-mance of the points can cause

to alter at high It istherefore, to examine the dwell witha and ensure that at both andlow rpm it falls within the stated data,than to set them with a feeler gauge and

for the of electronicignition dwell variation.

Useful data to CBDwell angleCB points gapCoil resistance

primary

secondary

52-58

MarelliBoschMarelli ±400

Diagrammatic layout of ignition coil and contact breaker system.

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IGNITION SYSTEMS

200 A Bosch 0.227.100.014 ignitionFiring order

DISTRIBUTORMake Marclli Bosch

SM 801 AX 0.237.001.002 rotor and 0.40mm

Ignition coil winding ofelectromagnetic impulse at 730 ± 7 fi ± 10

RESISTOR Bosch 0.227.900.002

Resistance ± 0.05 Q

IGNITION COIL Marclli Bosch

Type BAE 207 A 0.221.122.012Ohmic resistance of primary at 20°C 0.75 ± 0.81 Q 1.2 ±

resistance of secondary at 9500 500 Q 6000 10 000

11/2: Electronic ignition technical for Fiat

electronic systems 11/4)

control unit

Electronic ignitionsought if the is intended to run

7500rpm, where willlead to loss (as much as 5% at

over an electronic This, the availability of sys-

tems is virtually nil for 'mappable1

systems), will invariably mean aMarclli from another model.

Ignition timing/advance principlesThe combustion phase tends to take placeover a fixed whereas as theengine rotates the erank moves afurther distance in this time

to ensure maximum onthe piston TDC, the enginesincorporate systems orelectronic) to advance the ignition timingas the Prior to the intro-duction of systems this wasachieved by the use of spring-loadedcentrifugal weights in the distributors.Centrifugal them to flyoutwards as the speed thus

(orbaseplate to the distributor shaftand advancing the ignition. Their move-ment is controlled by springs and stops.Thus by matching weight of the

spring rate and position of stops to the ignition

of the engine, a characteristic curvereasonably to the needscan be produced. the TCs (withcentrifugal distributors) useadvance, and the characteristic curves forend-drive (non-mapped) and block-mounted distributors are similar.

11/3: Diagram showing inductive discharge ignition with electronic module. (Fiat Auto SpA- copyright reserved)

11/5: Diagram showing automatic centrifugaladvance of magnetic impulse distributor in 20002ACT engine. Auto SpA - copyrightreserved)

Interestingly, the 124 was designed to maximum

advance at lower[Author's note: For competition

purposes, may assume that all distributors have the same

11/4: Diagram showing Bosch breakerless ignition system with electronic control unit. (Fiat AutoSpA - copyright reserved)

138

IGNITION SYSTEMS

INITIAL FAULT FINDING WITH SYSTEMS ON PREVIOUS PAGE

POSSIBLE CAUSE

POINTS SYSTEM

Engine hard to start,misfirecold, engine idleserratically

spark Check with heavy discharge testerCheck volt drop across points; when closed should heCheck condition points/coil; they should be inspected on a compe-tition engine. On a road car replace points every miles or at least every 12months. If points OK, suspect/replace condenser

Check/replace and reset

ELECTRONIC SYSTEMS

hard to start,misfires

Faulty amplifier Replace (Bosch units are only from manufacturer, amp units areavailable from

Always check the connections on electronic before replacing components.

11/6: Advance curve for Fiat Rally.(Fiat Auto SpA - copyright reserved)

It is vital that highsetting is checked. Set the distributor togive the max stated setting at the relevantspeed, and a amount(perhaps +3-5°) excess advance at idle ona distributor with worn springs. Replace-ment parts for some of the distributorsarc no longer available

so if the advance isdeemed to totallysearch would be well and truly on for abetter - or acompany able to remodel the unit tospecification.

The advance for anyengine mirrors the torque characteristic.For when torque is low (at low

or part-throttle), the advance is high. When torque is

good, the requirement lower.Giving an engine

more than at full-throttle on any TC) will not necessarilylead to a power increase; indeed, iffigures stated need to be to givea good torque, the problem withengine is probably not atall, but more likely to be incorrectfuelling, cam timing wrong, low CR, orpoor volumetric efficiency.

reason for the coincidentalrelationship (see Case History No 5)

between low advance and good torque issimple: good torque is generated

is minimal contamination of the on phase by

residuals. This mixtureburns well and docs not to be

early. Minimal contaminationoccurs good or

of the takes place -due to a good match of engine speed,cam/inlet tract characteristics and highvolumetric efficiency. with forcedinduction behave like their

cousins until the manifold reaches absolute (atmos-

pheric — if scavenging process is so effective that

high advance is no longer needed (andindeed will lead to engine damage) and asthe charge density increases (at higherboost) the advance requirement dropsprogressively - that advance

still needed at high than at lowrpm of the crank angle/combus-tion

Remodelling a distrib-utor to suit an engine is tedious in

engine has to run on a from lowest speed at

which will hold full-throttle under loadto the highest speed/full-throttle position.At various speeds, the distributor has tobe swung a few degrees each way to

optimum position for besttorque. Notes are taken of the ignitiontiming, and the distributor is then

on a distributor testing to the curve required.

To increase the advance, the springslightened or made heavier bywelding. To increase the maximumadvance, the stops arc

GCT are often asked: the standardcentrifugal distributor The shortanswer is that provided itcriteria of @ (124

or (others) and producesat (possibly as much as

with worn springs ) then it shouldbe alone. To properly modify it couldrequire as much as three hours of bench

(on a rig, forrepeatability at the rightand at least the again on adistributor-testing machine. If theworking power band of the incompetition, is forexample, this work would be awaste of time distributor wouldbe producing 34-37° all the time anyway.For an rally engine required to goas low as modification ofthe distributor produce a torqueimprovement mid-range, but offsetagainst this is the fact that there areprobably ways of optimizing

- eg valves, cam swap, etc.Over-advanced ignition leads to

ignition and if air temperature are OK)

combustion is fighting the piston theway up), and over-retarded ignition

combustion, leading to ofthe exhaust valve and manifold.

Some incorporate vacuumadvance. Vacuum exists in the inletmanifold until full-throttle operation.At part-throttle the ignition is usefullyadvanced tocurve. A vacuum diaphragm oftenold ones are ruptured, or too badly wornto work distributorbaseplate to perform a similar tothe centrifugal weights. Vacuum advancecan be highly effective.

Ignition timing must with astrobe — with a variable-delay mechanism (Snap-On is one of thebest) so that a mark on the blockand front pulley is all that is required. Atany delay can be used toalign the two marks and the appropriate

IGNITION SYSTEMS

Schematic of the Digiplex electronic igni-tion system. Note that the drawing of the controlunit is wrong - viewed as shown the vacuumtapping should be on the right-hand side of theconnector. Auto SpA - copyright reserved)

battery2: electronic module3: mapping of spark advance ignition memo-

rized by module (2)4: sensor of crankshaft location5: sensor6: vacuum connection7: ignition distributor8: coil

degrees of advance recorded. If a not available the

front pulley must, at very least, have twomarks for 10° (at —

and (36° would besuitable enough) for higher rpm. Whenusing the strobe, vacuum advancemust be disconnected.

In the absence of hard data on thevacuum check that it advancesthe ignition a few degrees, ie 5 or so, attickover. (The vacuum unit addsa maximum of

It is worth noting at this that if points are incorrectly they will

upset the ignition timing; always set thedwell before the timing.

Mapped systems:These were first seen on the

A mapped contains a computercapable of selecting the optimum advancerequirement from a mass of stored data,for any given engine speed/load condi-tion. To monitor load, system requireseither a sensor

or asensor (Weber injection - see CaseNo 5), A conventional coil is andthe current to the coil is by asimilar switching device to

that used in the electronic systems previously.

The three-dimensional mapthe 130 TC of engine speed (GIRI/MIN),manifold vacuum (DP) and advance

demonstrates how the Digiplexdetermines which of stored valuesis appropriate to the engine's needs. Thecrank front and flywheel sensors aremerely types, and atransducer in the inlet manifold picks upthe vacuum condition. The position of

the distributor does not control thetiming of the system - this is establishedby relative position of the tworeference pegs on the front and thefront

The main advantages of Digiplex overthe earlier systems arc:1 No mechanical wear or moving parts2 No errors due to coupling of

distributor to crankshaft3 Superior accuracy — hence reduced

emissions and smoother torque curve

Digiplex dam

17

3

4

56

7

rpm sensor resistance

Gap: rpm sensor - flvwheel teeth

Gap: front

Coil data:(ignition oft)

Rotor arm res

Advance at

Max

ley TDC sensor

resistance

secondary

stancee vacuum)

without vacuum)

mm (the sensor should heopposite the peg at TDC)

(connect across 2xLT at (connect across

and one of the terminals)

10°

@ 5500 (no load)

HT

IGNITION SYSTEMS

0 . 2000 3000 4000 5000

Characteristic advance curves for different vacuum values in inlet manifold: Hg; right, Hg, (Fiat AutoSpA - copyright reserved)

systems:This is system used on

andtions in a similar fashion to the 130system except that it is programmed toreduce the advance as the manifold

11/10: Microplex advance curves versusat various manifold pressures.

Pressure valve (bar)0.5650.4980.4320.3650.2990.232

0.090.03

Vacuum (bar)0.030.090.1660.2320.2990.3650.432

Advance curve123456789

10111213141516

The ignition is designed to cut out at 0,6barpressure; the turbo is pre-set to a maximum of

pressure

Same as Digiplex maximumadvance (without vacuum, no load) shouldbe @ 5500rpm.

Safety precautions with Digiplex (andother mapped systems):a Do not start with

cablesb Do not use a to start the

enginec If vehicle is baked in a

oven where the tempremove the unit

d Do not undo the from thecontrol unit when the engine is running

e Always disconnect earthterminal welding on vehicle

Weber injection/ignitionThe later generation of TCs, egturbo 1.6 Delta

11/11: Diagram showing IAW injection/ignitionsystem - Delta 4WD. (Fiat Auto SpA - copyrightreserved)

tank2: electric fuel pump3: fuel filter4: fuel inlet5: fuel pressure regulator6: absolute air pressure sensor7: HT distributor with injection timing sensor8: butterfly valve position sensor9: inlet air temperature sensor

10: RPM and TDC sensor electronic control unit

12: injector 13: battery14: ignition switch15: injection/ignition relays16: additional air solenoid valve for automatic

adjustment of engine idling17: coolant temperature sensor18:19: ignition unit20: butterfly valve21: diagnostic socket22: system protective fuse23: vehicle electrical system connector24; overboost solenoid valve25: detonation sensor

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