A schematic diagram of a typical ignition system for 4-cylinder engine is shown in Figure which is usually supplied from a 6 V or 12 V battery.



There are two circuits: Primary circuit and Secondary circuit..

The primary circuit consists of a battery one side of which is connected to earth. (usually the engine itself). The other side of battery is connected via an ammeter and ignition switch in series, to the primary winding of a high-tension coil which, is a step-up auto transformer. The other end of primary winding is connected to one side of the contact breaker which is a spring-closed switch operated by a cam. The other side of contact breaker is earthed. A capacitor is connected across contact breaker points to earth. This completes the primary circuit.

The lower end of secondary winding is connected to lower end of primary winding. The upper end of secondary winding is connected to the centre of the distributor rotor. This distributor rotor is a rotating contact which sweeps past and makes contact with distributor contacts which are, in turn, connected to sparking plugs fitted in the engine cylinders. The other ends of sparking plugs are earthed. This completes the secondary circuit.

Working principle

When the ignition switch is closed, electric current will flow through primary. winding. As soon as the contact breaker is closed thereby setting up primary magnetic flux. Then, the contact breaker cam, which is revolving continuously, opens the contact breaker. Immediately, the primary magnetic field begins to collapse thereby inducing a very high voltage in secondary winding. This high voltage is applied across the air-gap of that spark plug. This causes a high intensity spark or are to spread across the gap thereby igniting the air-fuel mixture in the cylinder.

The capacitor increases the rate of collapse of primary field thereby helping to induce higher voltage in secondary and also prevents excessive arcing across contact breaker points when they are opened by the cam and the primary magnetic field starts collapsing thereby producing self-induced e.m.f. in primary coil.

The ignition circuit (Figure) is meant for a 4-cylinder petrol engine. The cam is square having four lobes and is capable of affecting four opening of the contact breaker per revolution of the cam. Similarly, the distributor has four contacts. In fact, the number of cam lobes and the number of distributor contacts is the same as the number of engine cylinders. Both the cam and distributor are motored in-phase and are mounted on the same shaft. In the four-stroke cycle engine, they are motored at half the engine speed.

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