A two zone model used for engine performance calculations has been developed into four zones by dividing the air fuel mixing, or burning zone into three parts, a fuel zone, a stoichiometric burning zone and a products plus air zone. This gives a high temperature zone for reaction kinetics calculations of NO and a colder quench zone. Equilibrium calculations are used for CO and CO2. The model was used to predict the effect of design and operating conditions on the formation of nitric oxide. Some predictions are compared with results from gas sampling in the cylinder with a timed valve.