Engines convert chemical energy into mechanical energy primarily through the process of combustion. In internal combustion engines, for example, fuel (such as gasoline or diesel) is mixed with air and ignited in a confined space (the combustion chamber). This reaction produces high-temperature and high-pressure gases that expand and exert force on the engine's components, typically pushing a piston. This linear motion of the piston is then converted into rotational motion, which ultimately powers the vehicle or performs mechanical work.
In addition to combustion engines, other types of engines may utilize different chemical processes. For instance, fuel cells convert chemical energy directly into electrical energy through electrochemical reactions, which can then be used to drive electric motors, producing mechanical energy without combustion.
Overall, the fundamental process involves the release of energy stored in chemical bonds through reactions (typically combustion), which is then harnessed to produce mechanical work.