Comparison is made between the transient response of a medium-speed, turbocharged diesel engine subjected to sudden load changes on a test bed and the response of a computer simulation model of the engine. Brief details are given of the simulation techniques involved and the data required to set up the model. Despite close agreement of model and engine steady-state results over the whole normal operating range, the transient responses of the model were initially found to be much faster than the test-bed responses. It is shown that the most important factor causing this difference is the lack of knowledge of the combustion at low air-fuel ratios and hence prediction of engine exhaust temperature during transient operation. Good agreement was obtained when this was modified.