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The difference between a conventional and a long 4-2-1 exhaust system is shown here.

ignition timing, which increased the exhaust gas temperature, but this could only be done to a limited extent otherwise the combustion would become unstable due to a portion of the combustion now taking place in the exhaust manifold.

The resulting unstable combustion was countered by adopting a cavity in the piston and optimising the fuel injection so as to create a stratified air-fuel mixture around the spark plug.The cavity trapped the ini-

Stratified air-fuel mixture

Left: the SKYACTIV-G 2,0-litre petrol engine is much more efficient than the current petrol 2,0-litre engine and even more efficient than the current 2,2-l'itre diesel. as measured by the specific fuel consumption in grams per kilowatthour. Right an increase in compression ratio from 11,2:1 to 15:1 (on a particular test engine) causes the drop in torque shown here, unless steps are taken to eliminate knocking.

internal bonds of the fuel molecules to break and produce energy even before the spark occurred.This is known as low temperature oxidation and resulted in the fuel delivering extra energy that resulted in the engine developing more torque than which normal combustion alone could have supplied.

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