The model described here is a dynamic mean value model which is small enough to be realized on a microcomputer. Nevertheless it contains significant and very accurate information on the gross internal variables of an engine (the indicated efficiency, scavenge ratio, scavenge efficiency, etc.). This makes the model useful for control and expert system application on line and/or in parallel with an operating diesel engine. Moreover, because of its simplicity, it gives an overall picture of engine operation which is not possible with more complex single engine cycle models. Comparisons with time dependent experimental data show that the model is very accurate both for static and dynamic predictions of engine performance over a large operating range.