Multidimensional computations were carried out on a spark-ignition natural gas engine with a bowl-in-piston combustion chamber. The engine in-cylinder flow distributions and their effects on combustion are examined. The fact that the engine swirl would speed up the combustion process is confirmed due to the enhancement of turbulent diffusion process. The engine squish increases both the mean velocity and the turbulence intensity of the gas flow and, therefore, quickens the combustion process. The computations indicate further that the engine swirl impacts the engine in-cylinder flow fields more profoundly than the engine squish does. When the piston bowl is offset, the in-cylinder gas motion can be enhanced considerably. Computations were also made to study the sensitivity of the computed cylinder pressure history to initial values of the selected thermodynamic parameters and chosen initial turbulence conditions. Two approaches to initialize the turbulence length scale at the beginning of computation were examined and compared. It is found that the variations of the initial turbulent kinetic energy could lead to significant changes of the pressure history when turbulence length scale is initialized to be uniform throughout the cylinder. However, the pressure history is much less sensitive to the variations of the initial turbulent kinetic energy when turbulence length scale is initialized to be proportional to the distance to the nearest solid wall.