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How does a spark plug ignite the fuel-air mixture in a gasoline engine?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

In a gasoline engine, the spark plug serves as the critical component that initiates combustion within the cylinder. It functions as part of the ignition system, which converts low-voltage current from a battery into high-voltage pulses—often reaching 40,000 volts—to ensure a reliable discharge.1

The process works as follows:

Delivery: The ignition system routes high-voltage current to the spark plug via a central electrode, which is insulated from the plug's metal body.12
Discharge: The current travels to the tip of the center electrode and jumps across a precisely calibrated air gap to a grounded electrode welded to the plug's body.32
Ignition: This electrical discharge creates a high-intensity spark. The heat from this spark ignites the compressed fuel-air mixture within the combustion chamber, triggering the burning process that generates the engine's motive force.31

Proper operation depends on maintaining the correct spark gap; if the gap is too wide, the engine may misfire, while a gap that is too small may fail to produce a spark intense enough to ignite the mixture effectively.2