What is the purpose of glow plugs on a diesel engine and when are they used?

• A. Preheating to aid cold starts; used before cranking in cold conditions
• B. They are for lubrication
• C. They regulate idle speed
• D. They are for cooling

Answer: (A)

Glow plugs preheat the combustion chamber to aid cold starts, especially in cold conditions. Diesel engines rely on compression to ignite fuel, but when temperatures are low the air in the cylinder isn’t hot enough for reliable ignition. The glow plugs are heating elements in the cylinder head that are energized before cranking (and sometimes during the initial start) to raise the chamber temperature so the fuel will ignite when injected. Once the engine starts and warms up, the plugs stop heating. They’re used when starting a cold engine or in cold weather; in warm conditions they may not be needed. They’re not for lubrication, do not regulate idle speed, and do not cool the engine.