Heat release analysis brings out the various phenomena that cause the development of the combustion chamber pressure. A three-zone heat release model computes the rate of heat release and other properties of all the three zones (representing vaporized fuel, unburnt and burnt gas zones) from measured pressure-crank angle data obtained from an instrumented engine. It is a useful diagnostic tool giving a great deal of information about an existing engine. Heat release analyses for five different operating conditions have been conducted for a DI diesel engine. Also, ignition delay, wall heat losses and mass, volume and temperature in each of the three zones have been computed. It is found that the overall quality of the computed heat release rates are strongly dependent on the accuracy of the measured pressure data. The model can be integrated to a high speed data acquisition system for concurrent analysis of the heat release patterns.