Inductive Multi-spark Ignition [601293]

Inductive Multi-spark Ignition
Multi-spark Ignition Systems- Inductive

Article Con text:

Multi sparki ng is the fir ing of the sp ark plug mul tiple times in rapid clo se succession t o create an ‘ overall’
spark of lon ger duration and energy.

The discu ssion centres o n inductive ignition syste ms, not Capacitor Disch arge Ignition systems (CDI). Multi-
sparking ha s been incor porated into CDI systems in product ion vehicles but is most well known in the
performance industry (e.g. MSD ignitions). Due t o the very s hort spark d uration inher ent in CDI they benefit
greatly from multi-sparking, particula rly at low rpm where this function re mains possible.

Many produ ction vehicle s with ‘stand ard’ induct ive ignition s now perform a multi-spar k function a nd it is
available in some aftermarket ECU’s (eg. Motec). European (cold climate) applicat ions go back several
decades.

The energy available fro m an ignitio n coil is gen erally enough to achieve the minimum spark duration
required an d still have enough in reserve for 1 or 2 more firings. This can happen when cylinder t urbulence
pushes the spark away from the plug and 'blow’s it out' , the coil voltage goes so hig h it 're-strike s'. This is no t
multi-sparking, it just re lates to a nor mal ignition characteristic.

This article will discus t he merits of multi sparki ng with an i nductive system and ma y explain wh y not all
manufacturers adopt the strategy. After all, it’s ju st implemented in softwa re, no extra hardware required!

Please read the article o n dwell mapping at www.dtec.net.au for backgrou nd information.

Production Vehicles that Multi-spa rk:

There are many exa mples of producti on vehicles that implement multi-sp arking, curre nt large volume
examples that come to mind are the Ford and BMW applicat ions.

Below is a F ord BA Falcon 6 cyl (cer tain V8 Falcons also) showing a multi spark (3 t imes when <1500 rpm)!
Top trace is coil curre nt, lower trace is coil pr imary voltage. It can be seen how the first spark is on ly allowed
to sustain f or approximat ely 0.5ms (shown in the primary patt ern) and the n the coil is turned back on again to
‘top up’ for a bout 0.75ms. After the third and final coil ch arge up the result ant spark is allowed to e xtinguish
naturally (duration > 1ms as shown)
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Inductive Multi-spark Ignition
The late model BMW ignition scope patterns below s how varied numbers of firings, th ey altered with
temperature, rpm and running mode (i.e. crankin g), abov e approximately 1500 rpm it reverts back to a single
spark.

BMW coil current during cranking, 9 sparks!

BMW coil current at idle, 6 sparks

BMW coil current and primary voltage at idle, 4 sparks

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Inductive Multi-spark Ignition

Multi-spark ing Benefits:

Multi-sparking allows su fficient ener gy over eno ugh time to ensure complete and co nsistent combustion.

It is often e mployed to improve cycl e to cycle variation in cylinder pressu re (often expressed as coefficient o f
variation COV = standard deviation/mean pressure*100 [%]). In other words, to keep the engine smooth due
to consi stent cylinder pr essure. Improving COV will allo w lower emissions, allow lo wer idle spe eds (therefo re
less fue l consumption, CO2) and also improve customer perception of th e vehicles n oise/vibratio n/harshness.

Instability comes about when mixtures are leane r than that f or smoothest operation, t here is poor mixture
formation (rich and lean 'pockets' a s not fully homogenous) or low ignitio n energy. At higher rpm it’s not
needed due to cylinder t urbulence cr eating better mixture formation and COV isn't an issue a s engine loads
get higher.

The combustion stabilising needed is a large fun ction of temperature. Cold start mixture can be very
inconsistent and the plug may not ha ve access to an optimal ignitable mixture initia lly. There are so many
variables th at it’s simply not possib le to determine ex actly what ignition angle would be the best to fire the
plug. Multi spark can help overcome this problem .

Manufacturers that use multi-sparking often pla ce an emphasis on it for these cold start/run con ditions! Some
even phase it out at high er temperatures (or red uce number of sparks at least). An example of this is in the
old Holden V6 Calibra's with Bosch Motronic, they used to multi spark only when cold and stop ed completely
when warm.

Multi-sparking has seve ral positive attributes bu t also has some constraints that de signers must carefully
consider.

Time Constraints on Multi-sparking:

There are time issues a nd processo r loading co nsideratio ns when implementing multi-spark (tim e to calcu late
it and actua lly effectively do it).

The burning mixture can be drawn away fro m the plug by turbulence and if fresh un-combusted mixture is
present it may be ignited with anoth er spark. R esearch has shown that the sparks n eeded to be less than a
couple of de grees apart to work effectively (you can see no w where time issue s start arising)

If you start to consider t he relationship betw een degrees, time and rpm you can understand the time
constraint s imposed.

At 1000 rpm the crank tr avels 6 degrees in ju st 1ms (this may well be the time required to charge t he coil up
again). 1 de gree pass’s in just 0.17 ms!

Its obvious multi-sparking can only be effective at very low engine spee ds.
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Inductive Multi-spark Ignition

Thermal Constraints on Multi-spa rking:

When extinguishing a spark early by turning on th e coil we ar e only 'topping up' the e nergy, as yo u can seen
in the curren t traces of th e seceding ignitions ab ove, they did n’t start from 0 Amps each time like t he first
dwell period (‘dwell’ i s the name give n to coil charge time ). This is a real problem as the amount of left over
energy is unknown and depends on how much energy was just used (this is effe cted by the firing voltage and
spark cond itions during t he combustion process). If we’r e not careful we e nd up with too little dwell and poor
spark energ y for the following spark or too much dwell and f ollowing dw ell period will cause the coil to
magnetically saturate, thi s will cau se a sharp rise in the curren t drawn in a given time period. As th e dwell is
pre programmed high currents could flow and this can lead to thermal issu es.

Manufacturers are often conservative with their dwell setting s when multi-sparking an d succe ssive coil
charges can sometimes deliberately be done at p rogressively lower curre nt levels. Th e fortunate f act is that
although the coil is being operated u nder difficu lt sustained conditions, it is only at low rpm and this leaves the
coil a coolin g period bef ore it is requ ired to fire th e cylinder a gain on the next cycle (2 crank rotat ions).

Single 'coil o n plug' applications are often alread y driven fairly hard to ext ract good sp ark energy (from often
compromised designs) a nd therefore are more likely to suffer from therma l issue s that arise if the d uty is too
great.

Thermal ove rload is a re al problem. Manufacturers sel dom u se multi-spa rking if not r eally require d on a
project, lot s of applicat ion measurements and co nfirmation tests are need ed to ensure their rigid d urability
requirements are met.

Many factory ECU’s also have a co mpensation table to adju st coil dwell based on predicted coil temperatures,
thus assistin g with keeping thermal issues in che ck.

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Inductive Multi-spark Ignition

Additional Constraints :

If a new spa rk is to o ccur soon enou gh then first spark must be terminated to allow th e coil to charge up
ready for a new spark. There will th erefore be a time period without a sp ark occurri ng (as the coi l charges
again) and a lso premature termination of first spa rk may lead to poor combustion! We also need to consider
that the low cranking voltage makes the coils cha rge time considerably lo nger as well. Whilst most of the
energy from a spark i s transferred in the initia l 'head' discharg e, the rest (spark duratio n basica lly) still
provides sig nificant ener gy and is therefore important.

Generally spark duratio n needs to b e at least 0. 5- 0.6m s or ignition suff ers. Hydrocarbon (HC) can rise in
engines with short duration sparks a nd lean burn ing design s may need greater than 1 .5ms or the negative
effects can be measured.

‘Duel fire’ co il ignit ion sy stems (two leads from a single coil g oing to oppo sing cylinder s) has it s own issue s in
that the dela yed sparks could be occurring durin g valve overlap and ther efore cause manifold backfiring in
the companion cylinder. Even single coil per plug systems have to consid er when last spark occur s relative to
any valve ti ming events, this will involve investigating the ful l range of valve timing events if varia ble cam
timing is use d.

Summary :

Aiding combustion by the additional ignition of u n-burnt fuel is limited to narrow range if it happe ns at all. Th e
flame front will spread t hrough the whole chamber in due course anyway!

Often the second coil re charge take s so long th at the ignitio n has alread y progressed so there is very little
benefit left. With rising temperatures and engin e speed multi-sparkin g has little effect. Most production
vehicle’s ab andon any multi sparking strategie s when the rpm goes mu ch above an idle (eg. Fo rd BA Falcon
at approximately 1500 rpm).

Manufacturers don’t gen erally waste their time/money on features the customer will never ever kn ow about,
yet alone understand. It most certain ly serves its purpose when required.

If multi-spar king can be done with due consider ation for any thermal issues (not as bigger concern for the
aftermarket) and the time involved in doing the a pplication is available, th en there’s certainly a positive benefit .

Darren Todd
DTec devices
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