The purpose of this study is to investigate the influence of re-entrant bowl geometry on both engine performance and combustion efficiency in a direct injection (DI), turbocharged diesel engine for heavy-duty applications. The piston bowl design is one of the most important factors that affect the air–fuel mixing and the subsequent combustion and pollutant formation processes in a DI diesel engine. The bowl geometry and dimensions, such as the pip region, bowl lip area, and toroidal radius, are all known to have an effect on the in-cylinder mixing and combustion processes. Based on the idea of enhancing diffusion combustion at the later stage of the combustion period, three different bowl geometries, namely, bowl 1 (baseline), bowl 2, and bowl 3 were selected and investigated. All the other relevant parameters, namely, compression ratio, maximum diameter of the bowl, squish clearance and injection rate were kept constant. A commercial CFD code STAR-CD was used to model the in-cylinder flows and combustion process, and experimental results of the baseline bowl were used to validate the numerical model. The simulation results show that, bowl 3 enhance the turbulence and hence results in better air-fuel mixing among all three bowls in a DI diesel engine. As a result, the indicated specific fuel consumption and soot emission reduced although the $NOx$ emission is increased owing to better mixing and a faster combustion process. Globally, since the reduction in soot is larger ($−46%$ as regards baseline) than the increase in $NOx$ ($+15%$ as regards baseline), it can be concluded that bowl 3 is the best trade-off between performance and emissions.

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