In the present research a numerical analysis is carried out to study the fluid and gas flow phenomenon and to determine the performance of the engine with various piston shapes. Diesel engines are widely used in heavy duty transport applications due to its superior fuel consumption. However, diesel engines are known to produce relatively high emissions of NOx and soot. Due to this fact, stringent emission standards have been imposed on diesel engine emissions. The diesel emission targets have been met by the industry by techniques such as increasing injection pressure and boost pressure using injection timing retard and recently through the use of electronically controlled injection systems. Inspite of these efforts, with proposed further reduction in the Environmental Protection Agency’s (EPA) emission levels, the viability of diesel engine is being challenged. Extensive research continues today to find better methods to reduce both soot and NOx emissions from diesel engine. In the present study the emission analysis is carried for different shapes of the piston. In recent years, increasingly stringent legislation has been enacted to control NOx, CO, soot and particulate emission from diesel engine. Traditional approaches to control these pollutants are providing diminishing returns. Also, there is the continuing effort to increase the specific power output of such engines while maintaining high efficiency and low emissions. These pressures have created a need to understand the combustion processes better and to control them through modification in engine with respect to its geometry and injection strategies. In this regard, multidimensional model should be useful. The fuel spray model, turbulence model and combustion models are used and made as elaborate as possible so as to capture the real processes. The three dimensional model with variable piston geometry, valve motion simulation and non axis-symmetric spray would require at least a mini-super computer for analysis.