Effect of piston shape on in-cylinder flows and air-fuel interaction in a direct injection spark ignition engine - A CFD analysis

DISI (Direct injection spark ignition engines) are very popular today because of their low fuel consumption and exhaust emissions due to their operation at lean stratified mixture. Their performance depends on the level of mixture stratification, which in turn depends on in-cylinder flows and air-fuel interaction at the time of fuel injection. However, these are very much affected by combustion chamber configuration, which in turn is mainly dependent upon the piston shape. In this study, a CFD (computational fluid dynamics) analysis is carried out to analyze in-cylinder air flows and air-fuel interaction created by different piston shapes viz., flat, flat-with-center-bowl, inclined and inclined-with-center-bowl pistons in a four-stroke engine at an engine speed of 1500 rev/min. Here, a start of fuel injection is considered at end of the suction stroke with a multi-hole injector operating at an injection pressure of 500 bar. Engine specifications are selected from the literature for which experimental data is available for comparison. Finally, it is found that flat-with-center-bowl piston results in about 51% higher tumble ratio and about 21% higher turbulent kinetic energy of in-cylinder flows, better mixture stratification along with 33% higher evaporation rate and 33% higher percentage of fuel evaporation as compared to those of flat piston.