The combustion and flow results of experimental and numerical investigations conducted on a stationary direct injection engine are presented in this paper. Experiments are conducted at a speed of 1500 rpm with the standard hemispherical piston at full load condition. A three dimensional finite volume model of the baseline engine with hemispherical piston (HCC) was created and the reacting flow simulation for it provided good agreement with experimental data. Further simulations are conducted for the reacting flow with modified pistons of shallow combustion chamber (SCC) and torroidal combustion chamber (TCC) configurations. In the design perspective, the geometric dimensions of the pistons are chosen so as to maintain the same piston bowl volume. From the numerical study, tt was revealed that both modified pistons generate higher pressure, with TCC piston producing a highest pressure for the same quantity of fuel injected. Due to higher pressure, the modified piston is believed to give higher brake thermal efficiency at the given engine speed and load. Regarding NOx and Soot emissions, higher NOx is observed for modified piston geometries, while the soot emission is slightly lower than the base case. Finally, TCC was found to be the optimum design of combustion chamber in respect of improved combustion and reduced emissions.