IGNITION
SYSTEM
INTRODUCTION:
The system which ignites the compressed Air fuel mixture at the end of compression stroke is called Ignition System. It is an electrical system which carries electrical current to the spark plug which gies electric spark to ignite the air fuel mixture. The Ignition system supplies high voltage upto a maximum of 24000 volts to the spark plug. The voltage produce electric spark at the spark plug gap between central and ground electrodes for the purpose of Ignition.
TYPES OF
IGNITION SYSTEM:
There
are two types of Ignition systems used in petrol engines
1.
Battery Coil Ignition system
2.
Magneto Ignition system
3.
Electronic Ignition System
1. Battery coil Ignition system: In Battery Coil Ignition system the current in the primary winding is supplied from the battery.
This system consists of Battery, Ammeter, Ignition switch, Ignition coil, Condenser, contact breaker points, Distributor and spark plugs.
The primary Ignition circuit starts at the battery and passes through the switch, Ammeter, primary winding, contract Breaker points tot the ground. A condenser is also connected in parallel to the contact breaker points. One end of condenser is connected to the contact breaker arm and the other and is grounded.
The secondary Ignition Circuit starts from the ground and passes through the secondary winding, distributor, spark plug to the ground.
The Ignition coil steps up the voltage of 6 or 12 Volts received from the battery to the high tension voltage of about 20000 to 24000 volts required to jump the spark at the spark plug gap which ignites the combustible charge in the cylinder.
The rotor of distributor revolves and distributes the current to the segments which in trun send it to the spark plugs as per the number of cylinders.
The purpose of condenser is to reduce arcing and pitting of contact breaker points.
When the ignition switch is on, the current will flow from the battery through the primary winding . It produces magnetic field in the coil. When the contact points open, the magnetic field collapses and the movement of magnetic field induces current in the secondary winding coil in which many more turns of fine wire will increase the voltage up to a maximum valve which is required for jumping of spark at the spark plug gap. The distributor then directs this high voltage to the proper spark plug as per the firing order arranged in the distributor rotor. This gives spark to the requiredspark plug.
2 Magneto Ignition System:
In this system the Magneto produces and supplies current in the primary winding. The magneto consists of a fixed armature having primary and secondary winding and rotating magnetic assembly which is driven by the engine . When the magnets rotate current flows in the primary winding. The Secondary winding gives high voltage current to the distributor which distributes it to the respective spark plugs according to the firing order.
In a magneto, the magnetic field is produced by means of permanent magnets. The magneto may be either rotating armature type or rotating magnet type.
Ignition Coil: It is simply a transformer to step up the Voltage in the ignition system. It consists of a soft iron core, primary winding and secondary winding. The primary winding consists of 200 to 300 turns of thick wire and the secondary winding 15000 to 20000 turns of fine wire. The Core is formed by lamination of soft iron.
When the ignition switch is turned on, the current follows from the battery through the primary winding. When the Contact Breaker Points open the primary circuit breaks and magnetic filed collapses and induces high voltage in both primary and secondary winding.
Contact Breaker Points: The contact breaker is a mechanism to make and break the primary circuit of the Ignition system for obtaining high voltage current in the secondary circuit.
The main parts of a contact breaker are cam, contact points, leavers, rubbing block, spring and bush. The heel and bush are generally made of plastic.
DISTRIBUTOR: In Multi cylinder engine, the high voltage current from ignition coil is distributed to the required spark plug according to the firing order. The distributor consists of a housing, driving shaft with breaker cam, breaker plate with contact points, governor, condenser, rotor, advance mechanism and cap. A rotor is mounted on the breaker can which is carried by the drive shaft.
The distributor opens and closes the circuit between the battery and the ignition coil. During closing of circuit the current flows in the ignition coil and builds up a magnetic filed. While opening the magnetic field collapses and high voltage current is produced by the coil. It distributes this high tension current to the proper spark plug at correct time as per the firing older.
SPARK PLUG: Spark plug is a device to produce electric spark to ignite the compressed air fuel mixture inside the cylinder. The spark plug is screwed in the top of the cylinder so that its electrodes project in the combustion chamber. It consists of mainly three Parts.
1.
Center electrode (or) Insulated electrode
2.
Ground electrode (or) Outer electrode
3.
Insulation separating the two electrodes
The High tension lead from Ignition coil is connected to the center electrode terminal cap. The high voltage current then passes through the center electrode and jumps in the gap between the central and ground electrodes. The ground electrode is attached to the shell of the spark plugso that there is a gap of 0.4 mm to 1.0 mm. The Center electrode is surrounded by porcelain insulator. The lower half portion of the insulator is fastened with metallic shell in which screw thread of hexagonal shape is given to fit the spark plug into the cylinder head.
FIRING
ORDER:
The order or sequence is which the firing takes places in different cylinders of a multi cylinder engine is called firing order. The following are the most common firing orders in different engines.
- 3cylinder engine : 1-3-2 ( Inline)
- 4Cylinder Engine: 1-3-4-2 (Inline)
- 6Cylinder Engine: 1-5-3-6-2-4(Inline)
- 8Cylinder Engine: 1-6-2-5-8-3-7-4(Inline)
- 8Cylinder V-type Engine: 1-5-4-8-6-3-7-2(Inline)
ELECTRONIC
IGNITION SYSTEM:
In this system an interrupter is provided in the distributor by an inductive impulse generator. A toothed rotor made from soft magnetic steel produces magnetic flux changes in a permanent management during its rotation. The flux changes effect voltage impulses in the ignition coil surrounding the management. These control impulses which are modified and amplified electronically actuate the switching transistor and thus produce the ignition voltage.