This is called an internal combustion engine . The internal combustion engine is an engine in which the combustion of a fuel ( generally , fossil fuel ) occurs with an oxidizer ( usually air ) in a combustion chamber . In an internal combustion engine the expansion of the high temperature and high pressure gases , which are produced by the combustion , directly applies force to components of the engine , such as the pistons or turbine blades or a nozzle , and by moving it over a distance , generates useful mechanical energy .