Fuel mixture of I.C. engine must be ignited in the engine cylinder at proper time for useful work. Arrangement of different components for providing such ignition at proper time in the engine cylinder is called Ignition system.
There are four different systems of igniting fuel:
(1) Ignition by electric spark i.e. spark ignition
(2) Ignition by heat of compression i.e. compression ignition
(3) Ignition by hot tub or hot bulb and
(4) Ignition by open flame. Only the first two are important methods for modern engines.
(1) Spark Ignition System in an I.C. Engine:
The purpose of spark ignition is to deliver a perfectly timed surge of electricity across an open spark plug gap in each cylinder at the exact moment so that the charge may start burning with maximum efficiency.
There are two methods in spark ignition:
(a) Battery ignition and
(b) Magneto ignition.
(a) Battery Ignition:
Principle of Working:
Battery ignition system includes two circuits:
i. Low voltage (Primary circuit) and
ii. High voltage (Secondary circuit)
The low-voltage circuit consists of:
(a) Battery
(b) Ignition switch
(c) A series register
(d) Primary winding and
(e) Contact breaker
All are connected in series.
The high voltage circuit consists of:
(a) Secondary winding
(b) Distributor rotor
(c) High voltage wiring and
(d) Spark plugs
They are also connected in series.
When the ignition switch is closed, current flows from the battery through the primary winding of the ignition coil, provided contact breaker points are closed. They produce magnetic field around the winding. When the piston is at the end of compression stroke, the contact breaker point opens.
Thus the flow of current in primary winding causes the magnetic field to collapse. As the field collapses, its lines of force cut the wire turnings of the secondary winding. This increases the voltage across the secondary winding terminals to a value of 20 to 24 thousand volts. The high-voltage surge is delivered to the centre terminal of the distributor cap where it is picked up by the rotor and directed to the proper spark plug. A spark jumps the plug gap and ignites the compressed air-fuel mixture.
Ignition Circuit:
Ignition circuit gets electric current from the battery. When the distributor points are closed, low voltage current flows through the primary winding of the ignition coil to the distributor terminal and through the breaker points to the ground. A strong magnetic field is built up during this period of operation. When the distributor points are opened, the magnetic field in the coil starts collapsing.
Thus a current is induced in the primary winding of the coil, which tends to prevent break down of the magnetic field. A very high voltage is produced in the secondary winding due to sudden collapsing of the magnetic field. This high voltage makes the spark to jump across the gap of the spark plug.
(b) Magneto Ignition System:
A magneto is used to generate an electric current for producing spark in I.C. engine. A high tension magneto generates very high voltage, needed for spark plug.
The electrical principle involved in magneto ignition is similar to battery ignition, the chief difference being that the primary current is produced by magneto and not by battery. Magneto may be considered as combination of a generator for producing low voltage current and an ignition coil for producing high voltage current.
Main Components of Magneto Ignition System:
The main components of magneto ignition system are:
(i) Frame
(ii) Permanent magnet
(iii) Armature
(iv) Soft iron field
(v) Rotor
(vi) Primary and secondary winding
(vii) Breaker points and
(viii) Condenser.
The armature consists of an iron core on which there are two sets of winding:
a. Primary, and
b. Secondary
The armature is driven by the engine. As the armature rotates, primary windings cut the lines of force of the magnetic field and an induced current flow in the primary circuit. As the primary current reaches its maximum value in each direction, the primary circuit is suddenly opened by a contact breaker and the current collapses. This action induces a very high voltage in the secondary winding which causes a momentary spark to jump at the spark plug gap. A distributor is provided which carries current to the spark plug through high tension wires.
The condenser is used to eliminate the arching at the breaker points and intensifying the current in the secondary circuit. For multi-cylinder engines, a distributor and a rotor are required to distribute the current to the different spark plugs. Some high speed engines have magneto built into the flywheel.
(2) Ignition by Heat of Compression in an I.C. Engine:
Process of igniting fuel by heat of compression is known as Compression ignition method. Diesel engine is a compression ignition engine. In this method, no electrical accessory is needed for ignition purpose. When air is compressed in an engine cylinder, the volume of air is reduced and heat is generated in the cylinder and consequently the fuel is ignited in the cylinder.
This causes a rapid rise of temperature in the cylinder. There is no external agency to produce a spark in the cylinder. The ignition temperature of the common internal combustion engine varies from 500° to 900°F. A compression pressure at 35 to 45 kg/cm2 easily ignites the fuel. Compression ignition engine has a compression ratio ranging from 14:1 to 22:1.
(3) Ignition by Hot Tube or Hot Bulb System in an I.C. Engine:
It consists of a tube like projection on the end of the side of the cylinder. The outer end of the tube is closed and the other end opened into the combustion space. It is heated by a flame or some other external source. As the charge is compressed, a certain portion enters the tube and is ignited by a contact with the hot inner surface.
In hot bulb method, there is a bulb in place of a tube. In some heavy duty stationary oil engines, a bolt or a pin is provided in the cylinder head that projects into the combustion space. When this bolt is heated by some external source, it causes ignition of fuel in the engine cylinder. It is not a common method used in modern days.
(4) Open Flame Method System in an I.C. Engine:
This method consists of two gas jets, one burning continuously on the outside of the engine and the other jet coming in contact with first jet and the combustion chamber alternately. The second jet is extinguished each time by the explosion and so it requires relighting. It is not a common method used for ignition purpose.