To predict the behavior of diesel engines with altered operating conditions by means of cycle simulation, a knowledge of the rate of heat release is necessary. On a medium-speed diesel engine, experimental investigations were carried out to determine the relationship between the heat release diagram and parameters such as equivalence ratio, charge air pressure, charge air temperature, engine speed, and injection timing. For a mathematical representation of the results, the actual heat release diagrams are replaced by simplified “Wiebe” heat release diagrams, which have the same beginning and duration of combustion; the shape, however, is simplified and chosen so that if they are used for cycle simulations, the calculated values of peak pressure, power output, and fuel consumption are in agreement with the measured data. Such a simplified Wiebe heat release diagram is characterized by four parameters: the beginning and duration of combustion, the Wiebe parameter m, and the equivalence ratio. Empirical correlations are established whereby it is possible to predict variations of these parameters with altered operating conditions. If, for an engine under consideration, the heat release diagram and the rate of injection are known from a measurement for one particular operating point, by means of these correlations it is possible to predict the heat release diagram for any altered operating conditions.