Final answer:
The spark plug ignites the air-fuel mixture in the compression and ignition step of the four-stroke engine cycle, leading to combustion and expansion.
Step-by-step explanation:
The spark plug initiates the combustion reaction in a four-stroke internal combustion engine during the compression and ignition step. After the intake stroke and the compression stroke, in which the air-fuel mixture is compressed, the combustion and expansion step occurs. During this step, the spark plug ignites the compressed air-fuel mixture, leading to a rapid conversion of chemical potential energy into thermal energy, resulting in a significant increase in pressure. This pressure pushes the piston down in the power stroke, which is the primary phase of the cycle where work is done to move the vehicle. It is only after the power stroke that the exhaust stroke follows, releasing the spent gases and preparing the engine for the next cycle with a new intake stroke.