Numerical simulation is currently applied in all engineering applications. The application considered in the present work is the simulation of the flow field and combustion phenomena in a diesel engine. The physics underlying the combustion and pollutant formation phenomena in a direct-injection diesel engine is quite complex due to the various processes such as the mixing of fuel with air, turbulence of the flow field, and combustion phenomena followed by the formation of new chemical species. All these processes are modeled using conservation principles and stochastic processes. They are finally expressed in the form of partial differential equations. These are then converted into algebraic equations after discretization by the finite-volume method. The final sets of equations are solved by a pressure implicit splitting of operators algorithm and the state-of-the-art solver is accelerated using the multi-grid technique.With regard to the physical aspects of the present work, two different combustion chambers are compared in this three-dimensional computational fluid dynamics (CFD) simulation for the whole cycle of a four-stroke direct-injection diesel engine. From the experimental engine setup, the crank angle and in-cylinder pressure data are observed using sensors and a data acquisition system. The results between the measurements and calculations are in close agreement for the whole cycle considered for one of the combustion chambers, which is the hump (re-entrant type). The hump in the toroidal combustion chamber is removed (flat type) to study its effect on the flow field and combustion using CFD while maintaining exactly the same compression ratio. The results also confirm that the turbulence in the flow field is enhanced in the case with hump (re-entrant type) compared with the flat type. With the flat type, the squish flow and combustion during the compression and expansion strokes is affected significantly. This results in a loss of 11% peak pressure when compared with the re-entrant type. The results also confirmed that the piston geometry significantly influences the in-cylinder flow during the intake stroke. The bowl shape plays a significant role at the end of compression and in the early stage of the expansion stroke. The NOx emissions are slightly higher while the CO and CO2 emissions are relatively less in the re-entrant type. © 2014 by Begell House, Inc.