A spark plug ignites the fuel-air mixture in a petrol engine using an electric spark. It is the key ignition component in a spark-ignition engine, where combustion begins inside the cylinder after the mixture has been compressed.
An ignition coil or magneto supplies high voltage to the plug. That voltage jumps across a small gap between the central and ground electrodes, creating a spark hot enough to start combustion. The resulting expanding gases push the piston down and produce useful power.
Spark plugs are not the same as fuel injectors, carburetors, or pistons. Injectors and carburetors prepare the mixture, while the piston compresses it and transfers combustion force to the crankshaft. Diesel engines are a common point of confusion: they normally ignite fuel through compression heat rather than a spark plug. Étienne Lenoir is often credited with using an early electric spark plug in an internal-combustion engine in 1860.