The aim of the ignition system is to generate a very high voltage from the car’s 12 volt battery, and to send this across each spark plug in turn, igniting the fuel-air mixture in the engine’s combustion chambers.
The coil is the component of the ignition system that produces this high voltage. It is an electromagnetic device that converts the low-tension (LT) current from the battery to high-tension (HT) current each time the distributor contact-breaker points opens up.
The distributor unit consists of a metal bowl consisting a central shaft, which is generally driven directly by the camshaft or, sometimes, by the crankshaft.
The bowl houses the contact-breaker points, rotor arm, and a device for making changes to the ignition timing. It also carries with it the distributor cap.
Distributing the current
The distributor cap is made of nonconductive plastic, and the current is given to its central electrode by the HT lead from the centre of the coil.
Inside the cap there are more electrodes often known as segments to which the sparkplug leads are connected, one per cylinder.
The rotor arm is fitted on top of the central shaft, and it connects to the central electrode by through a metal spring or spring-loaded brush in the top of the distributor cap.
The current enters the cap by means of the central electrode, passes to the centre of the rotor arm through the brush, and is then distributed to each plug as the rotor arm revolves.
As the rotor arm approaches a segment, the contact-breaker points open up and HT current passes across the rotor arm to the suitable sparkplug lead.
The contact-breaker points are mounted in the distributor. They act like a switch, in synchronisation with the engine that cuts off and reconnects the 12 volt low-tension (LT) circuit to the coil.
The points are opened up using cams on the central shaft and are then closed once again by a spring arm on the moving contact.
With the points closed, LT current flows from the battery to the primary windings in the coil, and then towards the earth through the points.
When the points open, the magnetic field in the primary winding collapses and high-tension (HT) current is induced across the secondary windings.
This current is transferred to the sparkplugs with the help of distributor cap.
On a four-cylinder engine there are four cams. With full rotation happening each time , the shaft the points open four times. Six-cylinder engines consist of six cams and six electrodes in the cap.
The position of the points and the distributor’s body in relation with the central shaft can be adjusted manually.
This makes changes to the timing of the spark to obtain an exact setting.
Further changes occur automatically when the engine speed varies according to the throttle opening.
In some modern ignition systems, micro-electronics ensure the optimum ignition timing for all engine speeds and engine load conditions.
Completing the circuit
The sparkplugs are screwed within the combustion chambers in the cylinder head.
HT current passes from each segment on the distributor cap down the plug, leading to the plug caps.
It then passes down towards the central electrode, which is insulated along its length, to the nose of the plug.
A side electrode connected to the plug body protrudes just below the central one, with the gap between the two set from 0.025 usually. (0.6 mm) – 0.035 in. (0.9 mm).
The current sparks across this gap, then flows along the side electrode, through the plug body and the engine, and then back to the coil to complete the circuit.
Components of an Ignition System are –