Glow-plugs are used to help with cold starts by which action?

• A. Igniting the air-fuel mixture directly
• B. Cooling the chamber
• C. Preheating the air
• D. Increasing compression

Answer: (C)

Glow plugs raise the temperature inside the combustion chamber to warm the air for a diesel engine to start in cold conditions. Diesel engines rely on compression to ignite fuel, and cold air makes that ignition slower or less reliable. By heating the air in the chamber, glow plugs help reach the temperature needed for proper ignition when starting, which is why the correct idea is preheating the air. They don’t ignite the mixture directly, they don’t cool the chamber, and they don’t increase compression.