In the Otto and Diesel cycles the fuel is assumed to burn at rates which result in constant volume top dead center combustion, or constant pressure combustion, respectively. Actual engine pressure and temperature profile data do not match these simple models and more realistic modeling, such as finite heat release model, is required. A finite heat release model is a differential model of an engine power cycle in which the heat addition is specified as a function of the crank angle.
Heat release can answer questions that the simple gas cycle model cannot address. For example, if one wants to know about the effect of spark timing or heat transfer on engine work and efficiency, a heat release model is required. If heat transfer is included, then the state changes for compression and expansion are no longer isentropic, and cannot be expressed as simple algebraic equations.