What is the primary function of glow plugs during starting?

• A. Spark igniter
• B. Inject fuel
• C. Regulate air intake
• D. Heat the combustion space to assist ignition during starting

Answer: (D)

Glow plugs are heating devices used to raise the temperature inside the combustion chamber to help ignition when starting, especially in cold conditions. By warming the air (and any residual fuel-air mixture) just before and during the start, they bring the temperature to where combustion can occur reliably as the engine turns over. Once the engine starts and warms up, the glow plugs reduce or stop heating.

This isn’t about creating a spark, injecting fuel, or regulating the air intake. Spark igniters would be used in spark-ignition engines, fuel injectors supply fuel, and air intake is controlled by the throttle and intake system. The glow plugs’ job is specifically to heat the combustion space to aid ignition during starting.