What ignites the compressed air-fuel mixture in a gasoline engine?

In a gasoline engine, the compressed air-fuel mixture is ignited by the spark plug. The spark plug generates a small electrical spark at the optimal moment in the engine cycle, which is crucial for combustion. When the spark occurs, it ignites the compressed mixture of air and gasoline, causing a rapid expansion of gases that pushes the piston down, producing the power needed to turn the engine's crankshaft.

The other components mentioned play different roles in the engine's operation. The combustion intake refers to the process of bringing the air-fuel mixture into the combustion chamber but does not ignite it. The piston moves up and down in response to the explosion of the ignited mixture, but it doesn’t provide the ignition itself. The starter is responsible for initiating the engine's operation by turning the crankshaft until it can run on its own, but it does not directly ignite the air-fuel mixture either. Thus, the spark plug is the essential component that directly causes ignition in a gasoline engine.