Ignition Systems and Engine Electrical Management
Ignition Systems and Engine Electrical Management covers coil operation, distributor and distributorless systems, coil-on-plug, dwell and timing, and the sensor inputs the ECU uses to control spark and engine operation. You need to understand primary and secondary circuits and how management inputs affect them.
Core rules
- The ignition coil stores energy in the primary; when current is interrupted, secondary voltage rises and fires the plug.
- Dwell is the time primary current flows; too short gives a weak spark, too long overheats the coil.
- Ignition timing is measured in degrees before top dead centre; it must advance with speed and retard under load or knock.
- Electronic systems use CKP for speed and position, CMP for sequence, MAP or MAF for load, knock sensor for retard, and O2 for mixture feedback.
- Coil-on-plug may include an igniter; wasted spark fires two plugs in series.
Worked example
A four-stroke four-cylinder engine at 3000 rpm has 3000/2 = 1500 firing events per minute per cylinder, or 25 per second. The interval between sparks for one cylinder is 1/25 = 0.040 s = 40 ms. If dwell is 3 ms, dwell duty is 3/40 = 7.5% for that cylinder. At 6000 rpm the interval halves to 20 ms, so the same 3 ms dwell becomes 15%; the ECU must schedule dwell carefully as speed rises.
Traps
Setting timing with vacuum advance connected when the specification says disconnected. Assuming a no-spark fault is the coil when the crank sensor is open. Fitting incorrect heat-range plugs. Ignoring bad grounds on coil packs.