A mathematical simulation of a, four stroke six cylinder turbo-charged diesel engine vas used to study the changes in performance caused by the variation of engine parameters and operating conditions. The principle variables were turbocharger size combustion timing and turbocharer bearing friction. The effect of these and other factors are tabulated and discussed. The results were explained in terms of the mathematical model employed and by comparison with the physical system. The results suggest that the mathematical model behaves sensibly, in most cases, but that further development is required in order to incorporate the refinements which are necessary for an accurate simulation.