Ignition Systems

IGNITION SYSTEMS

In the ignition system, the fuel taken inside the engine cylinder is ignited. According to the method of ignition, internal combustion engines are classified as:

1. Spark Ignition (S.I.) Engines 2. Compression Ignition (C.I.) Engines

1. SPARK IGNITION FOR PETROL ENGINE

In the petrol engine, ignition takes place by means of an electric spark by a spark plug at the end of the compression stroke. There are two systems of ignition for the petrol engine, namely, Coil or Battery Ignition System and Magneto Ignition System.

1. Coil or Battery Ignition System or High Tension System for S.I. Engine

 Fig. 14 shows a typical coil ignition circuit for a four cylinder petrol engine.

Description

1. Battery: It provides 12 Volts supply. It is charged by a dynamo driven by engine.

2. Ammeter: It indicates the primarycurrent in the circuit.

3. Switch: It is used for putting on and off the primary circuit. It is usually

placed within the easy reach of the driver.

4. Induction Coil: It consists of primary winding and secondary winding. The ratio between the secondary to primary turns is 50 to 1.

5. Contact Breaker: It is a mechanical device for breaking and making the primary circuit. . The gap between the two metal points may vary from 0.25 mm to 0.5 mm. 6. Condenser: It is connected across the contact breaker (i) to avoid sparking at contact breaker points and (ii) to induce a high voltage in the secondary circuit by causing a more rapid break of the primary current.

Working Principle

 When current flows through the primary coil, it sets up a magnetic field, which surrounds both the primary and secondary coil. As soon as the primary circuit opens by the contact breaker points, current from the battery starts charging the condenser.

When the condenser is fully charged, the flow of current stops and the magnetic field of the induction coil collapses. This reverses the flow of current, i.e., current starts flowing from condenser to battery. Consequently, magnetic field in the coil is also reversed. This rapid collapse and hence the reversal of magnetic field in the coil causes very high voltage in the secondary. The high voltage current is distributed to the spark plug by the metallic arm of the distributor.

SPARK PLUG (Fig. 15)

In the petrol engine, temperature of the air-fuel mixture at the end of compression stroke is not sufficient to ignite the air-petrol mixture. So, a spark is provided at the right moment by means of a spark plug.

The spark plug consists of a metal casing, which is screwed in the combustion chamber on the cylinder head. A central electrode is insulated with porcelain or any other ceramic material. It is sealed within the casing as shown. A ground electrode is fastened to the grounded part of the casing.

At the end of the compression stroke, the charge is ready for ignition. The central electrode is provided with high tension supply from the ignition system. The spark plug transforms the required voltage, generated by the ignition system into a spark within the combustion chamber.

The electric spark that is formed, jumps the gap between the ends of two electrodes. It ignites the air-fuel mixture in the combustion chamber. The space between the points of the two electrodes is known as Air Gap or Spark Gap. The gap offers so much resistance to the flow of current that a very high voltage of 10,000 to 15,000 Volts is required to cause the current to burst through the gap and produce a spark. The spark gap varies from 0.6 to 1 mm. The gap should be maintained correctly.

2. Magneto Ignition System

In this, no battery is required and the magneto acts as its own generator. The source of electrical energy is the magneto to produce high voltage current. This current flows to the distributor which connects the spark plug. This is used for stationary engines.

2 . COMPRESSION IGNITION SYSTEM

In the diesel engine, no special arrangement is necessary for ignition, since the temperature of the compressed air is higher than the ignition temperature of diesel. Diesel is injected into the cylinder shortly before the end of the compression stroke. As diesel particles come in contact with the hot compressed air, they vapourise and ignite.