Mercedes’ Multi-spark Ig­ni­tion

Wheels (Australia) - - Contents - DE­CEM­BER 2018

What is it?

The lat­est ver­sion of Mercedes-benz’s vari­a­blespark ig­ni­tion sys­tem for petrol en­gines. While many ig­ni­tion sys­tems rely on a sin­gle spark to ig­nite the mix­ture of fuel and air in the com­bus­tion cham­ber, the Multi-spark Ig­ni­tion (MSI) can fire the spark plug up to five times per power stroke.

Many of the Ger­man man­u­fac­turer’s en­gines are adopt­ing the so­phis­ti­cated sys­tem in­clud­ing its hero hand-built AMG 4.0-litre twin-turbo V8, which pow­ers a wide range of po­tent ma­chines across Mercedes’ pas­sen­ger and SUV line-up.

How does it work?

In all petrol en­gines, as the pis­ton rises to the top of the com­pres­sion stroke, the ig­ni­tion sys­tem fires a spark to ini­ti­ate a con­trolled burn­ing of the air/fuel mix­ture. This means the spark plug has just one chance to start a good com­bus­tion process, which, un­der nor­mal con­di­tions, is a rel­a­tively sim­ple process.

How­ever, un­der cer­tain con­di­tions, such as when the mix­ture is lean or the en­gine load is light, the va­por­ised fuel can be more dif­fi­cult to ig­nite, so the MSI has five shots at it. With each spark, the com­bus­tion ‘ker­nel’ grows, al­low­ing the flame front to spread to a greater area of the com­bus­tion cham­ber for more ef­fi­cient and con­sis­tent com­bus­tion.

If a sin­gle-spark sys­tem is like try­ing to start a damp camp fire with a flint, the multi-spark ig­ni­tion is the equiv­a­lent of us­ing a Zippo.

With more con­ven­tional sys­tems, the

Why does it mat­ter?

For op­ti­mum com­bus­tion and power, a petrol en­gine prefers to burn a mix­ture of fuel and air at a ra­tio of 14.7 parts of air to ev­ery one part of fuel – known as the sto­i­chio­met­ric ra­tio.

But in the quest for ever-more ef­fi­cient en­gines, man­u­fac­tur­ers have been ex­per­i­ment­ing with far leaner ra­tios, where there is less fuel for each part of air. Typ­i­cally, lean mix­tures are dif­fi­cult to ig­nite and can cause po­ten­tially dam­ag­ing ‘det­o­na­tion’ where the com­bus­tion is vi­o­lently fast, but the five sparks al­low mix­tures as lean as 23:1 – less fuel for the same power. ig­ni­tion coils have two en­tire en­gine rev­o­lu­tions to recharge for the next spark, but the MSI coils must be able to de­liver all five sparks in one mil­lisec­ond, which re­quires an in­cred­i­bly high­en­ergy sys­tem and spe­cialised com­po­nents.

At full-throt­tle and un­der high en­gine loads, sto­i­chio­met­ric is still king but the MSI sys­tem is al­low­ing Mercedes’ high-out­put en­gines to sip less fuel dur­ing low-load con­di­tions, boost­ing fuel econ­omy and low­er­ing emis­sions, with­out com­pro­mis­ing peak per­for­mance.

Fuel sav­ings of two per­cent might sound like small fish, but that fig­ure can be dou­bled in con­junc­tion with the com­pany’s strat­i­fied charge and di­rect fuel in­jec­tion, says Mercedes. As ever-tight­en­ing emis­sions reg­u­la­tions loom, ev­ery drop counts.

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