A comprehensive computer simulation has been developed to predict the performance of a vehicle's hydrogen engine. The effects of various coefficients, such as compression-ratio, excessive air parameter and ignition advancing, to the engine's performance were calculated and then the optimal parameters of the engine structure were determined. The simulation and analysis show several meaningful results. Hydrogen engine may achieve a lean-combustion. While the portion of hydrogen in the mixture is large, the cylinder pressure will increase quickly and the thermal efficiency will decline. It has a higher thermal efficiency in the range of 35-50%. Its cylinder diameter has got an optimal value between 0.07 and 0.09 m. Its spark advance angle has no considerable effect on the engine's performance. The thermal efficiency grows with the increase of compression ratio. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.