Ignition Systems
Ignition Systems
This domain covers coil ignition, contact-breaker and electronic systems, dwell, timing, spark plugs and distributors, plus coil-on-plug and CDI arrangements.
Core rules
- The ignition coil is a step-up transformer: primary winding, secondary winding, soft-iron core. Primary current builds a magnetic field; when the circuit is broken, the field collapses and induces high secondary voltage.
- Typical secondary voltage is 15–30 kV; the coil must produce enough voltage to jump the plug gap under compression.
- Dwell is the time or angle the primary current flows. Too little dwell weakens the spark; too much overheats the coil.
- Ignition timing is measured in crank degrees before top dead centre (BTDC). Vacuum and centrifugal advance, or ECU mapping, adjust timing with load and speed.
- A four-stroke engine needs one spark per cylinder every 720° of crank rotation; the distributor turns at half crank speed.
- Spark plug heat range must suit the engine; gap and electrode condition affect firing voltage.
Worked example
A four-cylinder four-stroke engine runs at 3,000 rpm. Crank speed = 50 rev/s. Spark events = 50 × 2 = 100 per second. The distributor runs at 1,500 rpm = 25 rev/s; four spark events per distributor revolution also give 100 sparks/s. One spark interval = 10 ms. At 60% dwell, dwell time = 6 ms. At 6,000 rpm the interval halves to 5 ms, so dwell must be controlled electronically to avoid coil saturation problems.
Traps
Setting contact-breaker gap without checking dwell; connecting a coil with reversed polarity; mismatching resistive plugs and leads; assuming a spark at the plug means correct timing; testing the secondary with an ohmmeter while the ignition is on; ignoring suppressed HT leads when tracing no-start faults.