Components of Battery Ignition System:
(i) battery
(ii) ignition switch
(iii) ballast resistor
(iv) ignition coil
(v) contact breaker
(vi) capacitor
(vii) distributor
(viii) spark plug
(1.) Battery:
To provide electrical energy for ignition, a storage battery is used. It is charged by a dynamo driven by the engine. Owing to the electro-chemical reactions,it is able to convert the chemical energy into electrical energy.
Two years or more trouble-free life may be obtained from a battery.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 six volt battery has three such cells and a 12 volt battery has six.
how six cells are coupled together to form a 12 volt battery and shows that for this coupling in series the positive of one cell is connected to the negative of the next in Figure.
Two types of batteries are used for spark-ignition engines, the lead acid battery and the alkaline battery.
Ignition Switch:
Battery is connected to the primary winding of the ignition coil through an ignition switch and ballast resistor. With the help of the ignition switch the ignition system can be turned on or off.
Ballast Resistor:
A ballast resistor is provided in series with the primary winding to regulate the primary current. The object of this is to prevent injury to the spark coil by overheating if the engine should be operated for a long time at low speed,or should be stalled with the breaker in the closed position.
This coil is made of iron wire, and iron has the property that its electrical resistance increases very rapidly if a certain temperature is exceeded.
For starting from cold this resistor is by-passed to allow more current to flow in the primary circuit.
Ignition Coil:
Ignition coil is the source of ignition energy in the conventional ignition system. This coil stores the energy in its magnetic field and delivers it at the appropriate time in the form of a ignition pulse through the high-tension ignition cables to the respective spark plug.
The purpose of the ignition coil is to step up the 6 or 12 volts of the battery to a high voltage, sufficient to induce an electric spark across the electrodes of the spark plug.
The ignition coil consists of a magnetic core of soft iron wire or sheet and two insulated con-ducting coils, called primary and the secondary windings. The secondary coil consists of about 21,000 turns of 38-40 gauge enameled copper wire sufficiently insulated to withstand the high voltage.and The primary winding, located outside the secondary coil is generally formed of 200-300 turns of 20 gauge wire to produce a resistance of about 1.15Ω.
The ends are connected to exterior terminals. More heat is generated in the primary than in the secondary coil. On the top of the coil assembly is the heavily insulated terminal block, which sup-ports three terminals. To the two smaller terminals usually marked SW (switch wire) and CB (contact breaker) the two ends of the primary are connected.
one end of the secondary winding is connected to the central high-tension terminal in the moulded cover of the distributor.The other end is connected to the primary. An external high tension wire connects this central terminal to the central terminal of the distributor.
Contact Breaker:
This is a mechanical device for making and breaking the primary circuit of the ignition coil.
The metal used is invariably one of the hardest metals,usually tungsten and each point has a circular flat face of about 3 mm diameter. The fixed contact point is earthed by mounting it on the base of the contact breaker assembly whereas the arm to which the movable contact point is attached, is electrically insulated.
When the points are closed the current flows and when they are open, the circuit is broken and the flow of current stops. The pivoted arm has, generally, a heel or a rounded part of some hard plastic material attached in the middle and this heel bears on the cam which is driven by the engine. Consequently, every time the cam passes under the heel, the points are forced apart and the circuit is broken. The pivoted arm is spring loaded, so that when the points are not separated by the action of the cam, they are held together by the pressure of the spring thereby closing the primary circuit.
An eight cylinder engine running at 3000 rpm requires 12000 sparks per minute, i.e. 200 sparks per second. If the breaker is to operate satisfactorily at this speed, the travel of the breaker arm must be held down to the minimum to ensure a positive spark and the breaker arm must be made very light.
Capacitor:
The principle of construction of the ignition capacitor is the same as that of every electrical capacitor, which is very simple: two metal plates – separated by an insulating material – are placed face to face. The insulation is often only air, but in most cases it consists of some high-quality insulating material suitable for the particular technical requirements.
The metal plates themselves are usually replaced by metal foil or by metallic layers deposited by evaporation on the insulating material itself. In order to save space, these thin strips, for example, consisting of two strips of aluminium foil and several layers of special capacitor paper,are rolled up in a solid roll.
Distributor:
The function of the distributor is to distribute the ignition surges to the individual spark plugs in the correct sequence and at the correct instants in time.Depending on whether a particular engine has 4, 6 or 8 cylinders, there are 4,6 or 8 ignition pulses (surges) generated for every rotation of the distributor shaft.
The use of a distributor represents a considerable simplification in a battery ignition system because in most cases we want to use only a single ignition circuit. The contact breaker and the spark advance mechanism are combined with the distributor in a single unit because of the absolute necessity that the distributor operate in synchronism with the crankshaft.
There are two types of distributors, the brush type and the gap type.In the former, carbon brush carried by the rotor arm slides over metallic segments embedded in the distributor cap of molded insulating material, thus establishing electrical connection between the secondary winding of the coil and the spark plug, while in the latter the electrode of the rotor arm pass close to, but does not actually contact the segments in the distributor cap.With the latter type of distributor, there will not be any appreciable wear of the electrodes.The distributor unit also consists of several other auxiliary units. In the lower part of the housing there is a speed sensitive device or governor, whose function is to advance the spark with increase in engine speed. Above this unit is the contact breaker assembly which can be rotated to adjust the timing of spark. In the upper part of the housing is located the high tension distributor.It also carries the vacuum ignition governor, which serves to retard the spark as the load on the engine increases. Each of the segments of the distributor is connected to a sparking plug and as the rotor presses it, the contact breaker opens, the high tension current is passed through the rotor and brass segment through the high tension wiring to the appropriate spark plug. Obviously, the order in which the sparking plugs are connected to the distributor head will depend on the firing order of the engine.
Spark Plug:
The spark plug provides the two electrodes with a proper gap across which the high potential discharges to generate a spark and ignite the combustible mixture within the combustion chamber.
A spark plug consists essentially of a steel shell, an insulator and two electrodes. The central electrode 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 the steel shell of the plug and thereby is automatically grounded when the plug is installed on the cylinder head of the engine. The electrodes are usually made of high nickel alloy to withstand the severe erosion and corrosion to which they are subjected in use.The tips of the central electrode and the insulator are exposed to the combustion gases. This results in the insulators having a tendency to crack from the high thermal and mechanical stresses. Some insulators are also seriously affected by moisture and by abnormal surface deposits. Since, the central electrode and the insulator are subjected to the high temperature of the combustion gases, the heat must flow from the insulator to the steel shell which is in contact with the relatively cool cylinder head in order to cool the electrodes and thereby prevent pre-ignition.
Spark plugs are usually classified as hot plugs or cold plugs depending upon the relative operating temperature range of the tip of the high tension electrode. The operating temperature is governed by the amount of heat transferred which in turn depends on the length of the heat transfer path from the tip to the cylinder head and on the amount of surface area exposed to the combustion gases. A cold plug has a short heat transfer path and a small area exposed to the combustion gases as compared to a hot plug.
The type of spark plug used in an engine depends on the particular engine requirements. Every engine manufacturer determines the type of plug, cold or hot, that is best suited to his engine. A spark plug which runs at a satisfactory temperature at cruising speeds may run so cool at idling speed that abnormal deposits are likely to foul the electrodes. These deposits may be of soft dull carbon from incomplete combustion or of hard shiny carbon from excess lubricating oil that passes the piston rings and enters the combustion chamber.The carbon deposits from incomplete combustion will burn off at temperatures above 340◦C while the excess oil carbon deposits from lubricating oil require a temperature above 540◦C to burn. If a spark plug runs hot enough at idling speeds to prevent carbon deposits, it may run too hot at high speeds and cause pre-ignition. If a spark plug runs at a temperature above 800◦C pre-ignition usually results. A compromise must be made in order to obtain a proper spark plug which would operate satisfactorily throughout the entire engine operating range. An improper spark plug is always a major source of engine trouble such as misfiring and pre-ignition.
Comments