The combustion process and fluid flow in a compression ignition natural gas engine with separated chamber are studied by coupling Multidimensional Computational Fluid Dynamic (CFD) code FIRE with detailed chemical kinetic mechanism. The calculated data are first validated by experimental data. Subsequently, Multidimensional simulations at a baseline condition are carried out to investigate combustion process, fluid flow, mixture formation and NO emission distribution in the chamber. Ultimately, parametric studies are conducted to illustrate the effect of some parameters on engine performance. The results show that calculated data are in good agreement with the experimental data. It is revealed that auto-ignition of natural gas occurs in separated chamber when the local mixture temperature reaches around 1300 K and engine performance has a strong dependency on some parameters, such as injection timing, composition of natural gas and initial temperature. (c) 2005 Elsevier Ltd. All rights reserved.