A coil-on-plug ignition system gives each spark plug its own ignition coil mounted directly above it. This design replaces long spark plug wires with short, direct high-voltage paths, which improves reliability and spark strength. It matters because a strong, well-timed spark helps the engine start easily, burn fuel efficiently, and reduce harmful exhaust emissions.
Modern gasoline engines use coil-on-plug systems because they allow precise electronic control of every cylinder.
Key Facts
- Each cylinder has one ignition coil mounted directly on or above its spark plug.
- The engine control unit switches the coil primary circuit on and off to control spark timing.
- A typical ignition coil steps about 12 V up to 20,000 V to 40,000 V or more.
- Transformer relation: Vs / Vp = Ns / Np, where voltage rises when the secondary coil has more turns.
- Dwell time is the time the coil primary current is allowed to build before the spark fires.
- Spark energy must be high enough to jump the plug gap and ignite the compressed air-fuel mixture.
Vocabulary
- Coil-on-plug
- An ignition design in which a separate ignition coil is mounted directly on each spark plug.
- Ignition coil
- A transformer that converts low battery voltage into the high voltage needed to create a spark.
- Spark plug
- A device that creates an electric spark across a small gap to ignite the air-fuel mixture in a cylinder.
- Dwell time
- The time interval during which current flows through the ignition coil primary winding before it is switched off.
- Engine control unit
- The computer that uses sensor data to control fuel injection, ignition timing, and other engine functions.
Common Mistakes to Avoid
- Thinking the coil creates energy from nothing, which is wrong because it stores energy in a magnetic field and then releases it as a high-voltage pulse.
- Assuming higher voltage always means a better spark, which is wrong because the spark only uses the voltage needed to jump the plug gap under cylinder pressure.
- Ignoring dwell time, which is wrong because too little dwell weakens the spark and too much dwell can overheat the coil.
- Swapping coils without tracking the misfire cylinder, which is wrong because moving a suspected coil and checking whether the misfire follows it is a better diagnostic method.
Practice Questions
- 1 An ignition coil has 200 turns in its primary winding and 30,000 turns in its secondary winding. If the primary voltage is 12 V, what ideal secondary voltage does the transformer ratio predict?
- 2 A 4-cylinder engine uses a coil-on-plug system. If each cylinder has one spark plug and one coil, how many ignition coils are needed? How many coils would be needed for a V6 engine with one spark plug per cylinder?
- 3 Explain why mounting the ignition coil directly above the spark plug can improve reliability compared with using long spark plug wires.