This paper describes the numerical and experimental research work carried out on a single-cylinder spark-ignition research engine with cryogenic port injection of gaseous hydrogen. A 1D thermo-fluid dynamic simulation code for the simulation of a hydrogen fuelled S.I. engine has been developed; in particular, a quasi-D multi-zone combustion model has been enhanced to predict the burning rate of a homogeneous mixture of hydrogen and air, on the basis of an extended database for laminar burning velocities. Moreover, a 1D simulation of the unsteady flows in the whole intake and exhaust systems coupled to the engine has been addressed, considering the transport of chemical species to account for the port injection of hydrogen at very low temperature (cryogenic conditions). The working fluid is treated as a mixture of ideal gases, including para-hydrogen, with specific heats depending on the gas temperature and the mole fractions. A validation of the simulation model is shown in the paper, comparing the computed results with the experimental data of in-cylinder pressures, cylinder No emissions and intake and exhaust instantaneous pressure pulses at different locations, for naturally aspirated engine conditions. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.