In these engines, the combustion of a fuel releases a high-temperature gas, which, as it expands, applies a force to a piston, moving it. Thus, combustion engines convert chemical energy into mechanical work. Decades of engineering by many scientists went in to designing the internal combustion engine, which took its (essentially) modern form in the latter half of the 19th century. The engine ushered in the Industrial Age, as well as enabling the invention of a huge variety of machines, including modern cars and aircraft.
Pictured are the operating steps of a four-stroke internal combustion engine. The strokes are as follows:
1) Intake stroke - air and vaporised fuel are drawn in.
2) Compression stroke - fuel vapor and air are compressed and ignited.
3) Power stroke - fuel combusts and piston is pushed downwards, powering the machine.
4) Exhaust stroke - exhaust is driven out.