How the ignition system works
The ignition system has the task of generating a very high voltage from the car’s 12-volt battery and sending it to each spark plug in turn, igniting the fuel-air mixture in the engine’s combustion chambers.
The coil is the component that generates this high voltage. It is an electromagnetic device that converts low voltage (LT) battery current to high voltage (HT) current when the distributor contact switches are opened.
The distributor unit consists of a metal container with a central shaft normally driven directly by the camshaft or sometimes by the crankshaft.
The bowl houses the breaking points, the rotor arm and a device for changing the ignition timing. It also contains the distributor cap.
Power distribution
The distributor cap is made of non-conductive plastic and the current is conducted through the HT cable from the center of the coil to its central electrode.
Inside the cover there are several electrodes, often called segments, to which the spark plug wires are connected, one for each cylinder.
The rotor arm is mounted on top of the central shaft and connected to the central electrode by means of a metal spring or spring brush in the distributor cap.
The current enters the cap through the central electrode, flows through the brush to the centre of the rotor arm and is distributed to the individual outlets as the rotor arm rotates.
As the rotor arm approaches a segment, the contact switch opens and the HT current flows through the rotor arm to the appropriate spark plug wire.
The contact points are mounted inside the distributor. They act as a switch that interrupts the 12 volt low voltage (LT) circuit synchronously with the engine and reconnects it to the coil.
The points are opened by cams on the central shaft and closed again by a spring arm on the moving contact.
When the points close, the LT current flows from the battery to the primary windings of the coil and then through the points to the ground.
When the switches are opened, the magnetic field of the primary winding is broken and a high voltage (HT) current is induced in the secondary windings.
This current is transmitted through the distributor cap to the spark plugs.
There are four camshafts in a four-cylinder engine. With each full revolution of the shaft, the tips open four times. Six-cylinder engines have six cams and six electrodes in the cap.
The position of the points and the distributor body in relation to the central shaft can be adjusted manually.
This changes the ignition timing for precise adjustment (see Engine timing operation).
Other changes are automatic, as the engine speed varies according to the opening of the throttle.
In some modern ignition systems, microelectronics ensure optimum ignition timing for all engine speeds and engine load conditions (see How the engine timing system works).
Closing the circuit
The whole ignition system.
The spark plugs are screwed into the combustion chambers in paint.
The HT current flows from each segment of the distributor cap downwards from the plug wires to the socket covers.
It then flows through the central electrode, which is insulated over its entire length, to the tip of the plug.
A side electrode connected to the connector housing protrudes just below the center electrode, with the distance between the two usually set at 0.025 inches. (0.6 mm) to 0.035 inches. (0.9 mm).