A multi-zone combustion model has been developed to predict the rate of heat-release in a turbocharged direct-injection diesel engine. The model considers the complete air-fuel jet mixing process and temperature-dependent reaction rates in each zone; this allows the distribution of the burning rate, local temperature and equivalence ratio to be determined both spatially and temporally. The predicted combustion pressures in the closed engine cycle are validated satisfactorily by experimental results obtained from a computer-controlled engine test-bed. The extension of the model to prediction of exhaust emissions during transient engine operation is also discussed.