Ignition System: Description and Operation

OVERALL DESCRIPTION
The ignition system (timing advance and spark duration characteristics) is controlled by the ECCS control unit (ECU). Refer to Computers and Control Systems for further information on the control of the ignition system.
The automotive ignition system is designed to:
^ Increase battery voltage to 20,000 volts or more.
^ Distribute voltage to the spark plugs.
^ Ignite the air fuel mixture.
It accomplishes this in the following manner.
When the ignition key is turned on, battery voltage is supplied to the positive side of the primary and secondary windings of the ignition coil. As the engine begins to turn, the crank angle sensor determines the engine rpm and camshaft position and sends this information to the ECU. The ECU takes information from several sensors and based upon a map programmed into the ROM memory, it supplies a pulsed ground signal to the base of the power transistor. The power transistor cuts the voltage to the primary windings in the ignition coil. This causes the magnetic field surrounding the primary windings to collapse and a voltage surge is created in the secondary windings of the ignition coil. High voltage (approximately 20,000 volts) flows out of the coil, through the coil wire and into the center electrode of the distributor cap. Inside the distributor is a shaft that is spinning at 1/2 of the engine speed (same speed as the camshaft). Mounted to the top of this shaft is the distributor rotor. The voltage then passes to the distributor rotor through the spring loaded carbon point. The rotor then conducts the voltage to 1 of 4 electrodes in the distributor cap, depending upon rotational position. The voltage passes out of the distributor, into the ignition wire and into the corresponding spark plug. The voltage then jumps the gap at the spark plug electrode completing the path to ground, igniting the fuel mixture.

SYSTEM COMPONENTS
The ignition system consists of these components:
^ Battery
^ Coil
^ Ignition Wires
^ Distributor
^ Rotor
^ Distributor Cap
^ Spark Plugs
^ Power Transistor
^ Resistor
^ Condenser

Fig. 112 Ignition Timing Control System:

CONTROL FACTORS
The ignition timing is controlled by the ECU to help maintain the best air fuel mixture under all operating conditions.
The ECU uses several sensors to calculate the amount of timing based on a map stored in its memory. The ECU varies the timing during:
^ Starting
^ Warm up
^ Idle
^ Low battery voltage
^ Hot engine operation
^ Acceleration