Effect of piston crown shape on in-cylinder flow characteristics in a direct injection engine - A CFD study

In modern direct injection gasoline engines, air-fuel mixing has a strong influence on combustion and emission characteristics, which in turn largely depends on in-cylinder fluid motion. However, in-cylinder fluid motion dependent on many engine parameters viz., piston shape, engine speed, intake manifold orientation, compression ratio, fuel injection timing, duration, etc. Among them, piston shape has significant influence on the in-cylinder fluid motion. Therefore, this study aims on evaluating the effect of piston shape on in-cylinder flows in a direct injection engine using CFD. In this study, a single-cylinder, two-valve, four-stroke direct injection engine designed for two-wheeler application in India is considered for the analysis. 'STAR-CD' and és-ice' are used for CFD analysis. Pressure boundary values obtained from measurements in the actual engine are employed. Two piston-shapes viz., flat and bowl types at wide-open-throttle under non-firing conditions are considered. Mainly analysis has been done to obtain in-cylinder velocity vector fields and in-cylinder flows, which are characterized by tumble ratio and turbulent kinetic energy. In addition, motoring experiments were conducted on an actual engine to measure in-cylinder pressure variation in order to compare it with CFD results. From the analysis of results, it is found that bowl shaped piston generates higher TR and TKE than those of flat piston by about 15 and 12% respectively. © 2013 SAE International.