A multi-cylinder four stroke cycle spark ignition engine equipped with an exhaust gas recirculation (EGR) system to reduce nitric oxide emission has been comprehensively simulated in a computer program including intake and exhaust manifolds. The program was tested against experiments performed on a standard production four cylinder four stroke engine equipped with a simple laboratory made EGR system. A nitric oxide emission reduction of about 50% was obtained at the peak NO condition. In spite of simplified assumptions the comparison between prediction and measurement of some major engine variables was good. The simulation program holds promise as a tool for engine development work.An appendix is added giving the outline of the calculation procedure.