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Combustion of a lean air-fuel mixture diluted with a large amount of air or Exhaust Gas Recirculation (EGR) gas is one of the important technologies that can reduce thermal NOx and improve gasoline engine fuel economy by reducing cooling loss. On the other hand, lean combustion increases unburned Hydro Carbon (HC) and unburned loss compared to stoichiometric combustion. This is because lean combustion reduces the burning rate of the air-fuel mixture and forms a thick quenching layer near the wall surface. In this study, the relationship between the thickness of the unburned HC and the excess air ratio is analyzed using Laser Induced Fluorescence (LIF) method and Computational Fluid Dynamic (CFD) of combustion. The HC distribution near the engine liner when the excess air ratio is increased is investigated by LIF. As a result, it is found that the quenching distance of the flame in the cylinder is larger for lean conditions than the general single-wall quenching relationship.

The Raman scattering method has been developed for the simultaneous, cycle by cycle measurement of HC, O2, H2O, and N2 in a direct injection gasoline engine with EGR. By using the Raman scattering method, the effect of EGR on stratified charge combustion can be investigated in a direct injection SI gasoline engine. The results show that (1) at the compression stroke homogeneous EGR gas exists, (2) variation of component mass fraction of EGR (qualitative fluctuation) introduced in the previous combustion cycle is the primary reason for EGR fluctuation, (3) under normal operating conditions, EGR fluctuation (component mass fraction and quantitative fluctuation) doesn't influence on the combustion fluctuation at the stratified charge operation.