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A simplified combustion and emission formation model for diesel engines has been developed in a project where the long term objective is to predict emissions during transient operation. The intended application implies that the final model must be both computationally inexpensive and comprehensive so that it can be used for optimization of engine control variables when coupled to full-engine simulation software. As starting point, the proposed model uses diesel spray correlations established in combustion vessels regarding spray penetration, dispersion, gas entrainment, ignition and flame lift-off. It has been found that with minor adaption, these correlations are valid also for combustion in an engine. By assuming a fully mixing controlled combustion after ignition and by use of simplified emission models, the correlations have been found useful for predicting trends in engine-out emission with low computational cost.