The present work investigates the effect of piston bowl geometry and initial swirl ratio on the engine performance and emission. The numerical simulation is carried out on a single cylinder diesel engine. Various configurations of piston with a rectangular cavity and a wide range of initial swirl ratios are considered; variation in bowl geometry is effected through a corresponding change in ratio of bowl to piston diameter ratio while maintaining the bowl volume, compression ratio, engine speed and. the mass of fuel injected constant. The predictions of this work reveal that high turbulent kinetic energy with a large swirl is crucial to enhance the quality of combustion. It is also found that variation in initial swirl affects in-cylinder pressure, temperature and the emission parameters more significantly for piston geometries with high bowl to piston diameter ratio than with low diameter ratios. Further, an optimization is carried out on the large number of cases results obtained for various initial swirl and diameter ratios. The two cases with 70% diameter ratio and 0.5 initial swirl ratio and 55% diameter ratio and 2.5 initial swirl ratio are found to have optimum emission and performance characteristics. (C) 2015 Elsevier Ltd. All rights reserved.