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COIL IGNITION SYSTEM
1. Introduction To Ignition System:- In case of internal combustion engines (IC Engines), combustion of air and fuel take place inside the engine cylinder and product of combustion expand to produce reciprocating motion of piston. This reciprocating motion of piston is in turn converted into rotary motion of crankshaft through connecting rod and crank.
This rotary motion of crank shaft is in turn used to drive generators for generating power.
We know that there are 4-cycles of operations viz: suction, compression, power generation and exhaust.
These operation are preformed either during 2-stroke of piston or during 4-stroke of piston.
In case of petrol engine during suction operation, charge of air and petrol fuel will be taken in. During compression this charge is compressed by upward moving piston and just before end of compression, the charge of air and petrol fuel will be ignited by means of for spark plug. And the ignition system does the function of producing spark in case of spark Ignition engines.
Whereas in case of diesel engines (compression ignition engine) 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 expands, i.e. they force the piston to move downwards i.e. they produce power and after producing the power gases are exhausted during exhaust operation.
An Ignition process obeys the law of conservation of energy. Hence, it can be treated as a balance of energy betn: (1) that provided by an external source (2) that released by chemical reaction (3) that dissipated to surrounding by means of thermal conduction, convection and radiation.
In principle a conventional Ignition system should provide sufficiently large voltage across spark plug electrodes under all operating conditions. Further, it should supply the regular energy for spark to ignite the combustible mixture adjacent to plug electrodes under all operating conditions. Air is poor conductor of electricity, an air gap in electric circuit act as a high resistance, but high voltage is applied across the electrodes of spark plug it produces a spark across the gap. When such a spark produced to ignite a homogeneous air fuel mixture in combustion chamber of an engine it is called the spark ignition system.
1.1 Requirement Of An Ignition System:-
A smooth and reliable functioning of an ignition system is essential for reliable working of an engine. The requirements of such an ignition system are:
(1) It should provide a good spark between electrodes of plugs at correct timing. (2) It should function efficiently over entire range of engine speed. (3) It should be light, compact and easy to maintain. (4) It should be effective and reliable in service. (5) It should be cheap and convenient to handle. (6) The interference from high voltage source should not affect the functioning of radio and T.V. receivers inside an automobile. 1.2 Types of Ignition System:- The following basic ignition systems are used: (1) Battery Ignition System (2) Magneto Ignition System 2. Battery Ignition System:-
In battery ignition system, the energy required for producing spark is obtained from 6 or 12 volt battery. With the universal adaption of electrical starting for automobiles, a battery is provided as a constant source of electricity.
The construction of a battery ignition system depends on type of ignition energy storage as well as on ignition performance which is required by particular engine.
Passenger cars, light trucks, some motorcycles and large stationary engines are fitted with battery ignition systems.
“Fig 1: Battery Ignition System for a Six Cylinder Engine” 2.1 Components of Battery Ignition Systems:- Essential components of Battery Ignition Systems:- 2.1.1 Battery 2.1.2 Ignition Switch 2.1.3 Ballast Resistor 2.1.4 Ignition Coil 2.1.5 Contact Breaker 2.1.6 Capacitor 2.1.7 Distributor 2.1.8 Spark Plug 2.1.1 Battery:- To provide electrical energy for ignition, a storage battery is used it is charged by a driven dynamo by engine. Owing to electrochemical reactions. It is able to convert the chemical energy into electrical energy. The battery must be mechanically strong to withstand the strain to which it is constantly subjected to. A lead acid battery consists of a number of cells connected together in series and each having a nominal potential of 2 volts when fully charged. A 6 volt battery has 3 cells and a 12 volt battery has 6. Figure shows how 6 cells are couple together to form a 12-volt battery.
“Fig 2:Cell Connections for 12 Volt Battery” 2.1.2 Ignition Switch:-
An ignition switch is a switch in control system of an internal combustion engine vehicle that activates the main electric systems for vehicle. Battery is connected to primary winding of ignition coil through an ignition switch and ballast resistor. With the help of ignition switch ignition system can be turned on or off.
2.1.3 Ballast Resistor:-
A ballast resistor is provided in series with primary winding to regulate primary current. The object of this to prevent injury to spark coil by overheating if the engine should be operated for a long time at low speed. Ballast resistor increases the life of contact breaker points. For starting from cold this resistor is bypassed to allow more current to flow in primary circuit.
2.1.4 Ignition Coil:-
Ignition coil is the source of ignition energy in conventional ignition system.
The purpose of ignition coil is to setup 6 or 12 volts of battery to a high tension voltage sufficient to promote an electric spark across the electrodes of sparking plug. The ignition coil consists of two insulated conducting coil, called primary and secondary windings. The primary winding is connected to battery through an ignition switch and contact breaker. The secondary winding is connected to sparkplug through the distributor. In order to boost voltage primary winding has a few hundred turns of relatively thick wire, while secondary winding consists of several thousand turns of very thin wire. The two windings are also interconnected allowing the secondary winding to be earthed through primary circuit. The primary winding surrounds secondary winding. This arrangement permits use of a reduced length of expensive fine gauge secondary wire and also allows good heat flow from high current carrying primary winding to case of coil. Coils of insulated wire are wound on a laminated soft iron core.
“Fig 3:Ignition Coil” The entire unit when assembled is enclosed in a metal container and focus a neat and compact unit. On the top of coil assembly is the heavily insulated terminal block, which supports three terminals. To the two smaller terminals usually marked SW and CB, the two ends of primary are connected.
One end of the secondary winding is connected to central high tension terminal in moulded cover of distributor. The other end is connected to primary. An external high tension wire connects this central terminal of distribution.
2.1.5 Contact Breaker:-
This is a mechanical device for making and breaking the primary circuit of ignition coil.
Purpose:-The purpose of contact breaker is to interrupt current flowing in primary circuit of ignition coil. When this occurs, the collapsing current induces a high voltage in secondary winding of coil, which have many more windings. This causes a very high voltage to appear at coil output for a short period-enough to arc across the electrodes of sparkplug. Operation:- The contact breaker is operated by an engine driver cam, the position of contact breaker is set so that they open and hence generate a spark at the exactly correct moment needed to ignite the fuel at top of piston’s compression stroke. The contact breaker is usually mounted on a plate is able to rotate relative to camshaft operating it. The plate is rotated by centrifugal mechanism, thus advancing the ignition timing making the spark occur earlier at higher revolutions. This gives the fuel time to burn so that the resulting gases reach their maximum pressure at same time as piston reaches the top of cylinder. The plate’s position can also be moved a small distance using a small manifold vacuum operated servo mechanism,
“Fig 4:Contact Breaker”
“Fig 5:Contact Breaker”
Providing advanced tuning when engine is required to speed up on demand. This helps to prevent pre ignition (pinging). 2.1.6 Capacitor:- A capacitor (originally known as condenser) is a passive too terminal electrical component used to store energy in an electric field. The principle of construction of ignition capacitor is same as that of every electrical capacitor. Two metal plates separated by an insulating material are placed face to face. The insulation is often only air in case of air capacitors, but in most cases it consists of some high quality insulating material suitable for particular technical requirements. The condenser absorbs and stores this indication flow of current and causes the current in the coil to die away rapidly which increases the voltage in secondary coil.
“Fig 6:The Condenser”
A condenser is constructed strip of sheets of thin foil insulated by thin sheets of paraffined paper or mica. The alternate layers of thin foil are connected in parallel. Forming two groups, each group provided with a terminal for external connections. Rolled or cylindrical condensers are made by winding alternate layer of thin foil and insulation into a tight roll.
2.1.7 Distributor:-
The distributor may be considered as a revolving switch located in secondary circuit, which connects the tension wire from coil to proper spark plug at proper time.
The distributor consists of a distributor housing contact breaker, rotating shaft with a breaker cam, condenser, rotor cap and an ignition advance mechanism to vary ignition timing in accordance with the engine load and speed.
Main functions of distributors are: (1) To interrupt the flow of current through the primary winding so that a high voltage is produced in secondary winding. (2) To distribute the ignition surges to the individual spark plugs in correct sequence and at correct instants in time.
The distributor may be of gap type or contact type; classified according to the method used in completing circuit betn rotor and distributor head terminals.
2.1.8 Spark Plug:- A spark plug is a device for delivering electric current from an ignition system to combustion of a spark ignition engine to ignite compressed fuel air mixture by an electric spark.
The function of spark plug is to provide a gap in combustion chamber for the discharge of a high voltage electric pulse that well ignite air fuel mixture at desired point in the cycle. The spark plug must satisfy the no. of requirements:
(a) The spark plug must provide a gap spacing and be positioned in the combustion chamber at such a location that allows smooth engine operation over the fuel operating range from idling to wide open throttle running with maximum fuel economy. (b) No part of spark plug must ever become hot enough to cause pre-ignition. (c) The size, composition and temperature of electrodes should be such that erosion from electric discharge betn electrodes and due to chemical action of combustion products is minimum. (d) The firing end of spark plug should have adequate resistance to fouling. (e) The spark plug has to withstand widely fluctuating temperatures and pressure in engine cylinder. (f) The insulator material must process desirable properties of high electric resistance, good thermal conductivity and sufficient mechanical strength. (g) Spark plug when fitted must be gas tight.
“Fig 7:Schematic of a Typical Spark Plug”
A spark plug consists of a steel shell an insulator of steel shell, an insulator and two electrodes to which the high tension supply from the ignition coil is connected is well insulated with porcelain or other ceramic materials. The other electrode is welded to steel shell of plug and thereby is automatically grounded when plug is installed on cylinder head of engine. The electrodes are usually made of high nickel alloy to serve erosion and corrosion to which they are subjected in use. The tip of central electrode and insulator are exposed to combustion gasses.
Spark plugs are usually classified as hot or cold spark plugs, depending upon relative operating temperature range of tip of high tension electrodes. A cold running spark plug transfers heat rapidly from its firing end while a hot running plug has a much lower rate of heat transfer. A cold plug will have a short insulating tip, and short heat rejection path and a hot plug will have long insulating tip and long heat rejection path.
Spark plug in automobiles generally have a gap betn 0.9mm-0.8mm. 3. Working of Battery Ignition System:- Ignition system divided into two circuit:- (1) Primary Circuit:- It consists of 6or12v battery, ignition switch, primary winding. It has 200-300 turns, contact breaker, capacitor. (2) Secondary Circuit:-
It consists of about 21000 turns bottom end of which is connected to bottom end of primary and top end of secondary is connected to centre of distributor rotor. Distributor rotor rotate and make contact with contact point and are connected to spark plugs.
WORKING:-
“Fig 8:Circuit Diagram for a conventional ignition system”
When the ignition switch is closed and engine is cranked as soon as the contact breaker closes, a low voltage current will flow through primary winding. It is to be noted that the contact breaker can opens and closes the circuit 4 times (for 4-cylinders) in one revolution. When the contact breaker open the contact the magnetic field begins to collapse. In order to obtain highest voltage in secondary circuit a quick collapse of magnetic field is essential. Because of this collapsing magnetic field current will be induced in secondary winding. Due to rapidly collapsing magnetic field, high voltage is induced in the primary circuit and still higher voltage of order 11kv to 12kv in secondary circuit.
This high voltage current is brought to distributor rotor. Distributor rotor rotates and supplies this high voltage current to proper spark plug depending upon engine firing order. When the high voltage current jumps the spark plug gap, it produces spark and charge is ignited. Combustion starts-products of combustion expand and produce power.
Note:- (a) The function of capacitor is to reduce arching at contact breaker points. Also when CB opens the magnetic field in primary winding begins to collapse. When magnetic field is collapsing capacitor gets fully charged and then it start discharging and helps in building up of voltage in secondary winding. (b) Contact breaker cam and distributor rotor are mounted on same rotor. In 2-stroke cycle engines these are mounted at same engine speed. In 4-stroke cycle engine they are mounted at half engine speed.
3.1 Firing Order:-
“Fig 9:Firing Order” The order or sequence in which firing take place in different cylinders of multi-cylinder engine is called firing order. Every engine cylinder must fire once in every cycle. A 4-stroke 4 cylinder ignition system must fire for every 180o crank rotation, for 6-cylinder engine ignition system must fire for every 120o crank rotation. There are 3 factors which must be considered before deciding the firing order of engine: (1) Engine Vibration (2) Engine Cooling (3) Development of Back Pressure
Advantages :
(1) A proper firing order reduces engine vibrations.
(2) Maintain engine balancing.
(3) Secures an even flow of power.
Consider that 1st cylinder of 4-cylinder engine is fired first. A pressure F is generated in cylinder one giving rise to load equal to F*b and F*a on 2 bearings A and B. now the load on A is much less than that on bearing B. now if the next cylinder fixed is cylinder no. 2, this imbalance in load on 2 bearings would be further increased which would results in severe engine vibrations. If we fire 4th cylinder after first, the load is equally distributed.
Further considered effect of firing sequence of engine cooling, when first cylinder is fixed is temperature increases, When next cylinder no. 1 and 2 gets overheated. If 3rd cylinder fired is cylinder no. 3, this overheating is shifted to portion betn 2 & 3. The task of cooling system becomes very difficult because it is then required to cool more at a place than at other places and this place too changing position with time. If we fire the third cylinder after first the overheating problem can be mitigated.
Next consider the flow of exhaust gases in the exhaust pipe. After firing first cylinder exhaust gases flow out to exhaust pipe. If the next cylinder fired is cylinder no. 2, gases exhausted from 1st cylinder goes out of exhaust pipe the gases exhausted from second cylinder overtake them. This requires that exhaust pipe made bigger, otherwise back pressure in it would increase and danger of back flow will arise. If instead of firing cylinder no. 2, cylinder no. 4 is fired then by time the gases exhausted by cylinder 4 come into the exhaust pipe, the gases from cylinder 1 will have sufficient time to travel distance betn 1st cylinder and 4th cylinder and thus development of a high back pressure would be avoided.
Probable firing orders for different engines are- 3 cylinder 1-3-2 4 cylinder engine 1-3-4-2
(inline) 1-2-4-3
4 cylinder horizontal opposed engine1-4-3-2
(Volkswagen engine)
6 cylinder in line engine1-5-3-6-2-4
1-4-6-2-3-5
1-3-2-6-4-5
1-2-4-6-5-3
4. Advantages & Limitations of Battery Ignition System:-
Advantages:
(1) It offers better spark at low speeds, starting and for cranking purpose. (2) Initial cost of system is low. (3) It is reliable system and periodical maintenance required is negligible except for battery.
Limitation:
(1) With increasing speed, sparking voltage drops. (2) Because of battery, bulk of system is high. (3) In case of battery runs down, engine cannot be started as induction coil to operates. (4) Occupies more space.