In this article we will discuss about:- 1. Introduction to Ignition Systems 2. Ignition System Types 3. Electronic Ignition System.
Introduction to Ignition Systems:
We know that in case of Internal Combustion (IC) engines, combustion of air and fuel takes place inside the engine cylinder and the products of combustion expand to produce reciprocating motion of the piston. This reciprocating motion of the piston is in turn converted into rotary motion of the crank shaft through connecting rod and crank.
This rotary motion of the crank shaft is in turn used to drive the generators for generating power.
We also know that there are 4-cycles of operations viz., suction; compression; power generation and exhaust.
These operations are performed either during the 2-strokes of piston or during 4-strokes of the piston and accordingly they are called as 2-stroke cycle engines and 4-stroke cycle engines.
In case of petrol engines during suction operation, charge of air and petrol fuel will be taken in. During compression this charge is compressed by the upward moving piston. And just before the end of compression, the charge of air and petrol fuel will be ignited by means of the spark produced by means of for spark plug. And the ignition system does the function of producing the spark in case of spark ignition engines.
Spark plug used with petrol engines mainly consists of a central electrode and metal tongue. Central electrode is covered by means of porcelain insulating material. Through the metal screw the spark plug is fitted in the cylinder head plug. When the high tension voltage of the order of 30000 volts is applied across the spark electrodes, current jumps from one electrode to another producing a spark.
Whereas in case of diesel (Compression Ignition—CI) engines only air is taken in during suction operation and in compressed during compression operation and just before the end of compression, when diesel fuel is injected it gets ignited due to heat of compression of air.
Once the charge is ignited, combustion starts and products of combustion expand i.e., they force the piston to move downwards i.e. they produce power and after producing the power the gases are exhausted during exhaust operation.
Basically Convectional Ignition systems are of 2 types:
1. Battery or Coil Ignition System
2. Magneto Ignition System
Both these conventional, ignition systems work on mutual electromagnetic induction principle. Battery ignition system was generally used in 4-wheelers, but nowadays it is more commonly used in 2-wheelers also (i.e., Button start, 2-wheelers like Pulsar. Kinetic Honda; Honda-Activa, Scooty, Fiero etc.). In this case 6 V or 12 V battery will supply necessary current in the primary winding.
Magneto ignition system is mainly used in 2-wheelers, kick start engines. (Example- Bajaj Scooters, Boxer, Victor, Splendor, Passion etc.).
In this case magneto will produce and supply current to the primary winding. So in magneto ignition system magneto replaces the battery.
I. Battery or Coil Ignition System:
Figure 29.2 shows line diagram of battery ignition system for a 4-cylinder petrol engine. It mainly consists of a 6 or 12 volt battery, ammeter, ignition switch, auto-transformer (step up transformer), contact breaker, capacitor, distributor rotor, distributor contact points, spark plugs, etc.
Note that the Fig. 29.2 shows the ignition system for 4-cylinder petrol engine, here there are 4-spark plugs and contact breaker cam has 4-corners. (If it is for 6-cylinder engine it will have 6-spark plugs and contact breaker cam will be a perfect hexagon).
The ignition system is divided into 2-circuits:
(i) Primary Circuit:
It consists of 6 or 12 V battery, ammeter, ignition switch, primary winding—it has 200-300 turns of 20 SWG (Sharps Wire Gauge) gauge wire, contact breaker, capacitor.
(ii) Secondary Circuit:
It consists of secondary winding. Secondary winding consists of about 21000 turns of 40 (SWG) gauge wire. Bottom end of which is connected to bottom end of primary and top end of secondary winding is connected to centre of distributor rotor. Distributor rotors rotate and make contacts with contact points and are connected to spark plugs which are fitted in cylinder heads (engine earth).
When the ignition switch is closed and engine in cranked, as soon as the contact breaker closes, a low voltage current will flow through the primary winding. It is also to be noted that the contact beaker cam opens and closes the circuit 4-times (for 4 cylinders) in one revolution. When the contact breaker opens the contact, the magnetic field begins to collapse. Because of this collapsing magnetic field, current will be induced in the secondary winding. And because of more turns (@ 21000 turns) of secondary, voltage goes upto 28000-30000 volts.
This high voltage current is brought to centre of the distributor rotor. Distributor rotor rotates and supplies this high voltage current to proper stark plug depending upon the engine firing order. When the high voltage current jumps the spark plug gap, it produces the spark and the charge is ignited-combustion starts-products of combustion expand and produce power.
(1) The Function of the capacitor is to reduce arcing at the contact breaker (CB) points. Also when the CB opens the magnetic field in the primary winding begins to collapse. When the magnetic field is collapsing capacitor gets fully charged and then it starts discharging and helps in building up of voltage in secondary winding.
(2) Contact breaker cam and distributor rotor are mounted on the same shaft.
In 2-stroke cycle engines these are motored at the same engine speed. And in 4-stroke cycle engines they are motored at half the engine speed.
II. Magneto Ignition System:
In this case magneto will produce and supply the required current to the primary winding. In this case as shown, we can have rotating magneto with fixed coil or rotating coil with fixed magneto for producing and supplying current to primary, remaining arrangement is same as that of a battery ignition system.
Electronic Ignition System:
Electronic Ignition System are as follows:
1. Capacitance Discharge Ignition System:
It mainly consists of 6 – 12 V battery, ignition switch, DC to DC convertor, charging resistance, tank capacitor, Silicon Controlled Rectifier (SCR), SCR-triggering device, step up transformer, spark plugs.
A 6 – 12 volt battery is connected to DC to DC converter i.e., power circuit through the ignition switch, which is designed to give or increase the voltage to 250-350 volts. This high voltage is used to charge the tank capacitor (or condenser) to this voltage through the charging resistance. The charging resistance is also so designed that it controls the required current in the SCR.
Depending upon the engine firing order, whenever the SCR triggering device, sends a pulse, then the current flowing through the primary winding is stopped. And the magnetic field begins to collapse. This collapsing magnetic field will induce or step up high voltage current in the secondary, which while jumping the spark plug gap produces the spark, and the charge of air fuel mixture is ignited.
2. Transistorised Assisted Contact (TAC) Ignition System:
1. The low breaker-current ensures longer life.
2. The smaller gap and lighter point assembly increase dwell time, minimise contact bouncing and improve repeatability of secondary voltage.
3. The low primary inductance reduces primary inductance reduces primary current drop-off at high speeds.
1. As in the conventional system, mechanical breaker points are necessary for timing the spark.
2. The cost of the ignition system is increased.
3. The voltage rise-time at the spark plug is about the same as before.
3. Piezo-Electric Ignition System:
The development of synthetic piezo-electric materials producing about 22 kV by mechanical loading of a small crystal resulted in some ignition systems for single cylinder engines. But due to difficulties of high mechanical loading need of the order of 500 kg timely control and ability to produce sufficient voltage, these systems have not been able to come up.
4. The Texaco Ignition System:
Due to the increased emphasis on exhaust emission control, there has been a sudden interest in exhaust gas recirculation systems and lean fuel-air mixtures.
To avoid the problems of burning of lean mixtures, the Texaco Ignition system has been developed. It provides a spark of controlled duration which means that the spark duration in crank angle degrees can be made constant at all engine speeds. It is a AC system. This system consists of three basic units – a power unit, a control unit and a distributor sensor.
This system can give stable ignition up to A/F ratios as high as 24 : 1.