A glow plug is a small electric heater used in many diesel engines to help start the engine when it is cold. Diesel engines do not use spark plugs because fuel ignites from heat created by compressing air. In cold weather, the cylinder head, cylinder walls, and incoming air can absorb heat, making ignition difficult.
A glow plug warms the combustion chamber so the first fuel spray can ignite more easily.
Key Facts
- A glow plug converts electrical energy into heat using resistance heating: P = VI.
- Higher current usually means faster heating, with electrical power also given by P = I^2R.
- Modern glow plug tips can reach about 800°C to 1100°C in a few seconds.
- Diesel ignition depends on hot compressed air, not an electric spark.
- The engine control unit may keep glow plugs on briefly after startup to reduce smoke and rough running.
- Ohm's law relates glow plug voltage, current, and resistance: V = IR.
Vocabulary
- Glow plug
- A glow plug is an electric heating device that warms the air and metal surfaces in a diesel engine combustion chamber before and during cold starting.
- Compression ignition
- Compression ignition is the process where diesel fuel ignites because compressed air becomes hot enough to start combustion.
- Combustion chamber
- The combustion chamber is the space in the cylinder where air and fuel mix, ignite, and release energy to push the piston.
- Resistance heating
- Resistance heating is the production of heat when electric current flows through a material that resists the flow of charge.
- Engine control unit
- The engine control unit is the vehicle computer that controls systems such as glow plug timing, fuel injection, and sensors.
Common Mistakes to Avoid
- Thinking a glow plug makes a spark, which is wrong because it heats continuously instead of creating a brief electrical arc.
- Ignoring battery voltage, which is wrong because a weak battery may spin the starter and still fail to supply enough current for rapid glow plug heating.
- Assuming glow plugs only work before cranking, which is wrong because many engines use afterglow to smooth idle and reduce white smoke after startup.
- Replacing all cold-start problems with glow plugs, which is wrong because poor compression, weak injectors, bad fuel, and sensor faults can cause similar symptoms.
Practice Questions
- 1 A glow plug operates at 12 V and draws 15 A during preheating. What electrical power does it use in watts?
- 2 A diesel engine uses four glow plugs, each rated at 120 W. If they preheat for 6 s, how much total electrical energy is used?
- 3 Explain why a cold diesel engine may need glow plugs even though diesel fuel normally ignites from compression rather than a spark.