A gasoline engine needs a precisely timed spark to start and keep running. The ignition system turns low-voltage electrical energy from the battery into a high-voltage pulse that can jump across a spark plug gap. That spark ignites the compressed air-fuel mixture inside the combustion chamber.
Understanding this path from key to combustion helps explain why engines start smoothly, misfire, or fail to start.
Key Facts
- A 12 V car battery supplies low-voltage electrical energy to the ignition system.
- Turning the key or pressing Start closes a control circuit that powers the starter motor and engine electronics.
- An ignition coil acts like a step-up transformer, raising voltage from about 12 V to 20,000 V to 40,000 V.
- Spark plug firing must occur near the end of the compression stroke for efficient combustion.
- Transformer voltage ratio: Vs / Vp = Ns / Np, where V is voltage and N is coil turns.
- Spark energy depends on coil storage: E = 1/2 L I^2, where L is inductance and I is current.
Vocabulary
- Ignition switch
- The switch or start button circuit that allows battery power to activate the starting and ignition systems.
- Ignition coil
- A device that uses electromagnetic induction to convert low battery voltage into a high voltage needed for a spark.
- Spark plug
- A threaded engine part with two electrodes where a high-voltage spark jumps to ignite the air-fuel mixture.
- Combustion chamber
- The space above the piston where compressed fuel and air burn to produce pressure and mechanical work.
- Engine control unit
- The computer that uses sensor data to control spark timing, fuel injection, and other engine functions.
Common Mistakes to Avoid
- Thinking the battery voltage directly makes the spark, which is wrong because 12 V is usually not enough to jump the spark plug gap under compression.
- Ignoring spark timing, which is wrong because a strong spark at the wrong moment can reduce power, waste fuel, or cause knocking.
- Assuming the starter motor and ignition coil do the same job, which is wrong because the starter turns the crankshaft while the coil creates the high-voltage spark.
- Replacing spark plugs without checking wiring, coils, or sensors, which is wrong because misfires can come from several parts of the ignition and control system.
Practice Questions
- 1 An ignition coil has 150 turns on its primary winding and 30,000 turns on its secondary winding. If the primary voltage is 12 V, what ideal secondary voltage is produced?
- 2 A coil stores energy with inductance L = 0.006 H and current I = 8 A. Use E = 1/2 L I^2 to find the energy stored before the spark.
- 3 Explain why a gasoline engine needs the spark plug to fire near the end of the compression stroke rather than during the intake stroke.