BATTERY IGNITION SYSTEM

A battery of 12volts is generally employed. Battery ignition system for a 4-cylinder engine. As in clear from there are two basic circuits in the system viz., the primary and the secondary circuits. The battery, primary winding of the ignition coil, condenser and the contact breaker form the primary circuit, whereas the secondary winding of the ignition coil, the distributor and the spark plugs constitute the secondary circuit. when the ignition switch is in the ‘on’ position, the current flowing in the primary circuit will grow exponentially during the make period of the contact breaker. If V is the battery voltage and R & L the resistance and self-induction (reluctance) respectively of the primary winding, the instantaneous value of primary current I, at time t seconds after the close of the contact breaker points is given by, I = [1- ] The electromagnetic energy on account of this build up of current in the primary circuit is stored in the laminated iron core of the ignition coil. As the contact breaker points open, the magnetic field built up by the growth of the current collapses and the energy stored during the make period is projected in to the secondary circuit. if Ithe value of i when break just occurred, the energy stored at break = LIв. if It is this energy which is suddenly passed onto the secondary, inducing an e.m.f. there. As the break is made very sharp, the induced e.m.f. is very large being proportional to the rate of change of flux in the winding. This self-induction effect is combined with the transformer step-up action of the ignition coil. As a result a sudden voltage surge of very high amplitude is produced in the secondary, causing a spark to occur at the spark plug electrodes. Thus it is at points ‘S’ that the spark occurs. As an illustration, when the breaker points open, if a self-induced e.m.f. of 250 volts is produced in the primary winding and the ratio of turns in the secondary and the primary windings is 100, the secondary voltage produced will be 250 x 100 = 25,000 volts. From the secondary winding, the high voltage surge is led to each spark plug in turn with the help of h.t. cables and the distributor. The ignition circuit may be either positive-earth type or negative earth type. In the positive earth type, the ignition coil and the spark plug centre electrode are positive with respect to the ground. Where as in tire negative earth type, these are negative with respect to the ground. The advantage of negative earth type is that comparatively less voltage is required for the electrons to jump from the central electrode which is hotter than from the outer electrode which is comparatively colder. Further, larger erosion due to sparking occurs from the positive electrode. Therefore, if the centre electrode is made negative with respect to the earth, the plug life is increased. Moreover, since the polarity can be reversed by reversing the primary coil leads, care must be taken to connect the primary leads correctly. Otherwise, higher ignition voltage will be required for causing the spark to jump across the spark plugs.

6. IGNITION COIL

The ignition coil is simply a transformer with certain characteristics making it suitable for its special use but impairing to some extent its efficiency as a transformer. It serves to convert the relatively low battery voltage into high voltage.

The primary winding consists of 200--300 turns of thick wire (about 20 S.W.G.) of total resistance about 3 ohms while the secondary is made up of large number of turns (about 15,000-20,000) of fine wire (about 40 s.w.G.). As the voltage difference between adjacent turns is small, only thin insulation is required and enamelled Wire is used for this purpose. However, there is sufficient voltage drop between the layers and for this paper insulation is used. The whole assembly is then impregnated in wax under vacuum to remove any air pockets or submerged in thin insulating mineral oil and hermetically sealed and in an aluminium case. The oil acts as an insulator to prevent high voltage arcing within the coil. Hermetic sealing prevents the entry of moisture in the coil, which if not prevented, would cause coil failure. The core is formed of either laminations of silicon steel or annealed iron wire insulated by varnish from each other. The laminated core has lower hysteresis loss. However, it is not easily produced in circular cross-section and because of this, square section is often used, although when square for the same area it has a greater perimeter than that of a core formed of iron wires stacked in a circular tube. Only open cores are cause excessive damping.

When the contact breaker points are closed a magnetic field is built up In the ignition coil. The opening of the breaker points causes the magnetic field to collapse suddenly as a result of which high voltage surge is induced in the secondary winding and this voltage causes-a spark to jump across the spark plug gap. This collapse of magnetic field must be very rapid to produce the desired voltage. This is achieved with the help of condenser. Without the use of a condenser, the energy stored in the coil in the form of magnetic flux would be dissipated in an arc across the points and no high voltage surge would be induced in the secondary winding. The condenser momentarily absorbs the current and brings the flow to a quick stop causing the magnetic field to collapse rapidly

There are two types of ignition coil :

l. Core type.

2. Metal clad or can type.

IGNITION TIMING

Ignition timing refers to the precise time spark occurs and is specified by referring to the position of the #1 piston in relation to crankshaft rotation. Ignition timing reference marks can be located on engine parts and on a pulley or flywheel to indicate the position of the # 1 piston. Vehicle manufacturers specify initial or base ignition timing. When the marks are aligned at TDC, or 0, the piston in cylinder #1 is at TDC of its compression stroke. Additional numbers on a scale indicate the number of degrees of crankshaft rotation before TDC (BTDC) or after TDC (ATDC). In a majority of engines, the initial timing is specified at a point between TDC and20 degrees BTDC. If optimum engine performance is to be maintained, the ignition timing of the engine must change as the operating conditions of the engine change. Ignition systems allow for these necessary changes in many ways; these are covered in greater detail later in this chapter. All the different operating conditions affect the speed of the engine and the load on the engine. All ignition timing changes are made in response to these primary factors.

ENGINE RPM

At higher r.p.ms, the crankshaft turns through more degrees in a given period of time. If combustion is to be completed by 10 degrees ATDC, ignition timing must occur sooner or be advanced. However air/fuel mixture turbulence (swirling) increases with rpm. This causes the mixture inside the cylinder to turn faster. Increased turbulence requires that ignition must occur slightly later or be slightly retarded. These two factors must be balanced for best engine performance. Therefore, while the ignition timing must be advanced as engine speed increases, the amount of advance must be decreased some to compensate for the increased turbulence.

COMPONENTS OF BATTERY IGNITION SYSTEM

The main components of a battery ignition system are :

l. Battery.

2. Ignition Coil

3. Contact Breaker

4. Condenser

5. Distributor

6. Spark Plug