The combustion process and NOx emission of the world's largest medium speed diesel engine, the Wärtsilä W64L (W64L), were simulated with the modified Kiva2-CFD code. The following submodels were added to the standard Kiva2 code: The Magnussen EDC (Eddy Dissipation Concept) turbulent mixing combustion model, the extended Zeldovich NO model and a simple rebounding model for the droplet wall impingement. The Tesner&Magnussen soot formation and combustion model was implemented in the code, but it has not yet been used in this stage. The modified TAB model was used for the spray.The simulation results, such as the cylinder pressure, the heat release rate and the cumulative heat release were compared to the measured values of the W64L engine. The spray model correctness was tested by comparing the predicted spray tip penetration with Hiroyasu's correlation at room temperature conditions. It is quite safe to use Hiroyasu's correlation for estimating the spray tip penetration if measured values are not available. The predicted droplet sizes were compared with the corresponding measured values of the nozzle, which has a slightly smaller orifice.The predicted spray tip penetration and the SMD of droplet size are quite close to values mentioned above. The cylinder pressure and the heat release rate are in reasonable agreement with the measured values. At the moment it is not possible to compare the predicted NOx emission levels from cylinder with the measured values because they are not available.The simualtion results show that the modified Kiva2 combustion simulation tool works quite correctly for the W64L engine and further that this CFD simulation tool can apply quite safely to a 12-cylinder power plant V-engine, the W64V, which is now under design. The simulations will reduce design expenses and time by decreasing the required trial and test work using a different construction.