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The Nissan NAPZ 4 cylinder single cam engines had dual plugs as well as Mercedes. I think the purpose is to initiate two flame fronts for more complete combustion. Hondas current ISDI 4 cylinder, used in the Insight and I think the CRZ (not positive on that one) is a 1.3 liter with only two valves per cylinder which uses the swirl turbulence for better fuel distribution and two plugs per cylinder to optimize combustion. The Honda 1.3 is used in other countries without a hybrid system and is supposed to come very close to the old VX 4 cylinder as far as fuel mileage without the NOX issues of the VX engine when in lean burn. The ISDI engine does not use lean burn. I think the engine in my Ranger may have some of the same combustion characteristics as the Honda engine and the Nissan NAPZs were capable of excellent mileage, but I think when they went to port fuel injection they stopped using the twin plug setup. Later they went to twin cam and that was the engine used in the first Altimas and pickup trucks, without twin plugs.
regards
mech
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