A simulation and design tool applicable to hydrogen powered spark ignition engine systems is introduced in this paper. This software is applicable to single and multi-cylinder engines under steady state or transient operating conditions, and is capable of simulating one-dimensional unsteady chemical species transport through intake and exhaust engine ducting, the induction and combustion of those chemical species, and the engine performance characteristics and emissions which are produced. Results are presented from validation studies carried out on a 1.61 spark ignition engine converted to operate with manifold-fuelled gaseous hydrogen. These experimental results validate the ability of the simulation to accurately describe the transport of gaseous hydrogen through engine intake ducting, and the displacement of intake air due to hydrogen introduction. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All fights reserved.