This work presents an analysis of the operation of an indirect injection, naturally aspirated, diesel engine under transient conditions resulting from a rapid increase in load. For this purpose, a single-zone thermodynamic model, following the filling and emptying modelling technique, is developed, which accounts for all basic parts of the thermodynamic engine operation; it also covers the dynamic simulation of the engine with special submodels to model the conservation of energy in the crankshaft, the inertia forces, the governor dynamics and the fuel pump characteristics. Unlike most previous models, the transient operation is not regarded as a series of steady state operating points, but care has been taken for the special characteristics of this unsteady operation. To this aim, analytical expressions were derived for the better simulation of combustion, mechanical friction and governor movement, which during the transient operation behave in a different way than at steady state. The model has been tested favourably (at steady state conditions) against results derived from detailed experimental work conducted at the authors＇ laboratory. A detailed parametric study is made, which includes the influence of a variety of dynamic and thermodynamic parameters on the transient response of the particular engine, as well as of a four-cylinder one with the same technical characteristics. For the latter case, a special multi-cylinder engine model has been developed, accounting in detail for the solution of the governing differential equations for every cylinder separately. Analytical diagrams for the effect of the operating parameters on the transient operation of the two typical properties of speed droop and recovery period are given, which quantify each parameter＇s contribution. Finally, the effect of a faulty fuel injection system on one cylinder of the four-cylinder engine is studied, providing important information about the engine＇s non-optumum operation.