What is the function of spark plugs?

• A. To measure cylinder pressure.
• B. To provide the spark for ignition.
• C. To reduce engine vibration.
• D. Ignite the compressed fuel-air mixture.

Answer: (D)

The function of a spark plug is to ignite the compressed fuel-air mixture in the cylinder. When the high voltage from the ignition system makes the electrode gap arc, a spark forms and starts combustion at the precise moment the mixture is at peak pressure, driving the piston downward and producing power. This is why the option that describes igniting the compressed fuel-air mixture is the best choice—it communicates the actual combustion event inside the cylinder.

The other ideas don’t fit as the primary role: measuring cylinder pressure is something a sensor would do, not the spark plug; providing the spark is part of the process but the essential outcome is ignition of the mixture; reducing engine vibration is accomplished by other mechanisms like mounts and balancing, not by the spark plug.