Mathematical models of large diesel engines traditionally include detailed models of the internal processes in the engine. Though such constructs may be very useful for detailed laboratory analysis of engine performance and may be very accurate, they require mainframe computers for their implementation, long computing times and large numbers of fitting parameters. This article shows that the results of a detailed large engine simulation program may be compressed with equations of basic engine physics to obtain a compact dynamic mean value model which can run on a microcomputer. The model is physically based and includes the main engine nonlinearities. Besides being small enough to run on-line this model uses very few fitting parameters but is nevertheless very accurate, even for large and rapid changes in the engine operating point.