The ignition in a petrol engine takes place by means of a spark plug at the end of the compression stroke. The voltage required to produce a spark across the gap between the sparking points of a plug is 6000 to 10000 volts. The following two ignition systems of petrol engines are important:

1. Coil ignition system (also known as battery ignition system); and

2. Magneto ignition system.

The coil ignition system has an induction coil, which consists of two coils known as primary and secondary coils wound on a soft iron core, as shown in figure above. One end of the primary coil is connected to the ignition switch, ammeter and battery generally of 6 volts. The other end of the primary coil is connected to a condenser and a contact breaker. A condenser is connected across the contact breaker for the following two reasons :

(a) It prevents sparking across the gap between the points,

(b) It causes a more rapid break of the primary current, giving a higher voltage in the secondary circuit.

The secondary coil is connected to a distributor (in a multi-cylinder engine) with the central terminal of the sparking plugs. The outer terminals of the sparking plugs are earthed together and connected to the body of the engine.

The coil ignition system is employed in medium and heavy spark ignition engines such as in cars.

The magneto ignition system has the same principle of working as that of coil ignition system, except that no battery is required, as the magneto acts as its own generator. This type of ignition system is generally employed in small spark ignition engines such as scooters, motor cycles and small motor boat engines.