Ignition System: Description and Operation

The High Energy Ignition (HEI) system uses one coil and two spark plugs per cylinder. Individual coils for each cylinder ensure consistent performance at high rpms and allow individual cylinder spark control. Dual spark plugs provide more complete combustion, particularly near the cylinder walls, thus decreasing emissions. Also, HEI enhances combustion efficiency when firing mixtures diluted by EGR (Exhaust Gas Recirculation), which in turn improves overall efficiency and lowers emissions.

The crankshaft position sensor and camshaft position sensor are hall effect devices. The camshaft position sensor and crankshaft position sensor generate pulses that are inputs to the PCM. The PCM determines engine position from these sensors. The PCM calculates injector sequence and ignition timing from crankshaft & camshaft position. For a description of both sensors Camshaft Position Sensor.

The two spark plugs per cylinder are fired slightly out of phase to prevent the cylinder pressures from rising too quickly, which could cause knocking. To prevent one spark plug from eroding more quickly than the other, they alternately lead each other. Under normal conditions, the timing is the same for all cylinders, but the timing can be delayed in individual cylinders if knocking is present in one or more. Highly sensitive anti-knock sensors,can distinguish knocking conditions in individual cylinders and retard the ignition timing on the cylinders that are knocking. This anti-knock control prevents damage to the engine and allows operation on lower grade fuel, but only in emergencies. Premium grade fuel is required under normal operating conditions to ensure full power and economy, and because the anti-knock control system may not be able to prevent knocking on low grade fuels under all operating conditions.