Study of Mixture Inhomogeneities and Combustion Development in a S.I. Engine Using a New Approach of Laser Induced Fluorescence (FARLIF)

A new tool for quantitative measurements of fuel-air ratio in the combustion chamber of a spark ignition engine (FARLIF) has been achieved. This optical method is based on a new approach of PLIF, taking advantage of the quenching phenomenon. Toluene is present in a small proportion (5 % in iso-octane) and used as a tracer to visualise the fuel distribution. In a four stroke monocylinder engine equipped with optical access, instantaneous imaging of the air-fuel ratio is performed inside the cylinder during the compression phase. We study in the present paper the effect of different injection configurations on the charge homogeneity and on the cycle-by-cycle variations of the average equivalence ratio. Four injection timings, two positions of the injector and three rates of inlet manifold heating are tested. When the injector is far from the inlet valve, or with a cooled inlet manifold, the mixture inhomogeneity and the variations of average equivalence ratio from cycle to cycle increase. Influence of the average fuel-air ratio on the homogeneity of the charge is discussed. The relationship between the equivalence ratio distribution and heat released 10° after TDC shows the benefits on combustion of fuel distribution inhomogeneities far from the spark plug.