Heat release data have been obtained by analysis of cylinder pressure diagrams from a variety of engines, two stroke and four stroke, small (3.4 in. bore) to medium size (12 in. bore), over ranges of power, speed, and air supply conditions. These data have been used to develop a method of calculating heat release rates suitable for performance calculations using a digital computer. Relatively simple equations are used, based on a single-zone model for conditions in the cylinder. The correlation is primarily empirical, based on a simple model for the rate of mixing between fuel and air, but the chemical kinetics of the burning process is also involved in the prediction of the initial burning rate. Some limitations of the simplified model used are made apparent by the experimental data, but these may be overcome to some extent by the use of empirical coefficients.