The ignition circuit supplies high voltage surges (some as high as 50,000 volts in electronic ignition circuits) to the spark plugs in the engine cylinders. These surges produce electric sparks across the spark plug gaps. The heat from the spark ignites the compressed air-fuel mixture in the combustion chambers. When the engine is idling, the spark appears at the spark plug gap just as the piston nears top dead center (TDC) on the compression stroke. When the engine is operating at higher speeds, the spark is advanced. It is moved ahead and occurs earlier in the compression stroke. This design gives the compressed mixture more time to bum and deliver its energy to the pistons.

The functions of an ignition circuit are as follows:
Provide a method of turning the ignition circuit ON and OFF.

Be capable of operating on various supply voltages (battery or alternator voltage).

Produce a high voltage arc at the spark plug electrodes to start combustion.

Distribute high voltage pulses to each spark plug in the correct sequence.

Time the spark so that it occurs as the piston nears TDC on the compression stroke.

Vary spark timing with engine speed, load, and other conditions.

PRIMARY AND SECONDARY CIRCUITS
The ignition circuit is actually made of two separate circuits which work together to cause the electric spark at the spark plugs. These two circuits are the PRIMARY and SECONDARY.

The primary circuit of the ignition circuit includes all of the components and wiring operating on low voltage (battery or alternator voltage). Wiring in the primary circuit uses conventional wire, similar to the wire used in other electrical circuits on the vehicle.

The secondary circuit of the ignition circuit is the high voltage section. It consists of the wire and components between the coil output and the spark plug ground. Wiring in the secondary circuit must have a thicker insulation than that of the primary circuit to prevent leaking (arcing) of the high voltage.

IGNITION CIRCUIT COMPONENTS
Various ignition circuit components are designed to achieve the functions of the ignition circuit. Basic ignition circuit components are as follows:

BATTERY- provides power for the circuit. (This was discussed earlier in this chapter.)

IGNITION SWITCH- allows the operator to turn the circuit and engine ON and OFF.

IGNITION COIL- changes battery voltage to high ignition voltage (30,000 volts and greater).

IGNITION DISTRIBUTOR- distributesignition voltage to the spark plug. Contains either mechanical contact points or an electronic switching circuit.

SPARK PLUG- device that provides an air gap in the combustion chamber for an electric arc.

Ignition Switch
The ignition switch (fig. 2-43) enables the operator to turn the ignition on for starting and running the engine and to turn it off to stop the engine. Most automotive ignition switches incorporate four positions, which are as follows:

OFF.- The OFF position shuts off the electrical system. Systems, such as the headlights, are usually not wired through the ignition switch and will continue to operate.

ACCESSORY. -The ACCESSORY position turns on power to the entire vehicle electrical system with the exception of the ignition circuit.

IGNITION ON.- The IGNITION-ON position turns on the entire electrical system including the ignition circuit.

START.- The START position will energize the starter solenoid circuit to-crank the engine. The START position is spring-loaded to return to the IGNITION-ON position when the key is released automatically.