In combustion, heat release parameter (or gas expansion parameter) is a dimensionless parameter which measures the amount of heat released by the combustion process. It is defined as
where
* is the burnt gas temperature
* is the unburnt mixture temperature.
In typical combustion process, . For isobaric combustion, using ideal gas law, the parameter can be expressed in terms of density, i.e.,
The ratio of burnt gas to unburnt gas temperature is