What is the function of the spark plug in a gasoline engine?

The spark plug ignites the compressed air-fuel mixture in the combustion chamber by creating an electric spark at precisely the right moment. This ignition causes the controlled explosion that drives the piston down during the power stroke. Spark plugs are part of the ignition system in gasoline engines (diesel engines instead rely on compression heat to ignite the fuel and don't use spark plugs). Fouled or worn spark plugs cause misfires and poor performance. Knowing the spark plug's role in igniting the mixture is essential ignition-system knowledge.