Ignition System: Description and Operation

GENERAL DESCRIPTION
The Electronic Ignition (EI) system controls fuel combustion by providing a spark to ignite the compressed air / fuel mixture at the correct time. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the Powertrain Control Module (PCM) controls spark advance of the ignition system. EI has several advantages over a mechanical system:

^ No moving parts.
^ Less maintenance.
^ Remote mounting capability.
^ No mechanical load on the engine.
^ More coil cool down time between firing events.
^ Elimination of mechanical timing adjustments.
^ Increased available ignition coil saturation time.

OPERATION
The Electronic Ignition system does not use the conventional distributor and coil. This ignition system consists of three ignition coils, an Ignition Control Module (ICM), a Camshaft Position Sensor, two Hall-effect crankshaft position sensors, related connecting wires, and the ignition control and fuel metering portion of the PCM.

Conventional ignition coils have one end of the secondary winding connected to the engine ground. In this ignition system, neither end of the secondary winding is grounded. Instead, both ends of each coil secondary winding is attached to a spark plug. Each cylinder is paired with the cylinder that is opposite it (1 - 4, 2 - 5, 3 - 6).

These two plugs are on "companion" cylinders, i.e., on top dead center at the same time. When the coil discharges, both plugs fire at the same time to complete the series circuit. The cylinder on compression is said to be the "event" cylinder and the one on exhaust is the "waste" cylinder.

The cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug. The remaining energy will be used as required by the cylinder on the compression stroke. The same process is repeated when the cylinders reverse roles. This method of ignition is called a "waste spark" ignition system.

Since the polarity of the ignition coil primary and secondary windings is fixed, one spark plug always fires with a forward current flow and its "companion" plug fires with a reverse current flow. This is different from a conventional ignition system that fires all the plugs with the same direction of current flow. Since it requires approximately 30% more voltage to fire a spark plug backwards, the ignition coil design is improved, with saturation time and primary current flow increased. This allows higher secondary voltage to be available from the ignition coils - greater than 40 kilovolts (40,000 volts) at any engine RPM. The voltage required by each spark plug is determined by the polarity and the cylinder pressure. The cylinder on compression requires more voltage to fire the spark plug than the one on exhaust.

It is possible for one spark plug to fire even though a plug wire from the same coil may be disconnected from its "companion" plug. the disconnected plug wire acts as one plate of a capacitor, with the engine being the other plate. These two "capacitor plates" are charged as a spark jumps across the gap of the connected spark plug. The "plates" are then discharged as the secondary energy is dissipated in an oscillating current across the gap of the still-connected spark plug.

Secondary voltage requirements are very high with an "open" spark plug or wire. The ignition coil has enough reserve energy to fire the still-connected plug at idle, but possibly not under high engine load. A more noticeable misfire may be evident under load; both spark plugs may then be misfiring.