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A new stratified charge system, which employs a thin fan–shaped fuel spray and a shell–shaped position cavity, has been developed for direct injection gasoline engines. In order to clarify the characteristics of stratified mixture formation and combustion, spray motion visualization, local fuel concentration in piston cavity, and frame behavior observation were performed. The newly developed system achieves stable stratified combustion in the high-speed region due to optimized piston cavity shape and fuel spray characteristics, especially suitable spray penetration with high injection pressure which ensures the mixture formation is “robust” against air flow and accelerating mixture diffusion in the cavity.

In order to study the trade-off between improved lean combustion and reduced NO x emissions under a locally stratified charge mixture, this paper examines the effects of the rich mixture volume on combustion duration and the NO x emissions under the same total equivalence ratio in a combustion vessel. The rich methane-air mixture in a soap bubble was ignited at the center of the bubble by a pulsed YAG laser. As a result, it was found that the magnitude of the NO x emissions does not increase as the rich mixture volume increases up to a critical volume, but the combustion duration at that time shortens remarkably. The same tendency was also obtained by means of computer simulation using a 4-cylinder lean burn engine.