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Heat release data have been obtained from a 5 in bore direct injection diesel engine operating over a range of conditions including off-design conditions, such as running at low air to fuel ratios, using six-hole injector nozzles in place of the standard three-hole nozzles, and reducing compression ratio and thereby swirl ratio at tdc. The paper shows that when operating with adequate utilized air fuel ratios (A/Fs) the experimental heat release data could be correlated with a single zone combustion model and that changing the number of nozzle holes or compression/swirl ratio had little effect on the correlation after the initial period of heat release. At inadequate A/Fs a modification to the model to allow for partial combustion was introduced. This improved correlation and enabled reasonable heat release prediction in the cycle synthesis program. Examples are given of heat release and cycle pressure diagrams computed using these models.