What is the function of spark plugs in a piston engine?

• A. They regulate fuel flow to the cylinders.
• B. They deliver the electric spark to ignite the fuel-air mixture in the cylinders.
• C. They convert mechanical energy to electrical energy.
• D. They serve as a pressure relief valve.

Answer: (B)

Spark plugs are part of the ignition system and their job is to deliver a high-energy electric spark inside the cylinder to ignite the compressed fuel‑air mixture at the right moment. When the mixture is close to being fully compressed, the ignition system sends a pulse to the spark plug, and a spark jumps across the gap, starting combustion. The timing of that spark is critical because it ensures peak pressure occurs at the proper point in the piston’s motion to produce efficient power. The spark plug itself doesn’t regulate fuel flow, isn’t a device that converts mechanical energy to electrical energy (that’s the generator), and isn’t a pressure relief valve.