Secondary Circuit Operation

The secondary circuit carries high voltage to the spark plugs. The exact manner in which the secondary circuit delivers these high voltage surges depends on the system design. Until 1984 all ignition systems used some type of distributor to accomplish this job. However, in an effort to reduce emissions, improve fuel economy, and boost component reliability, most auto manufacturers are now using distributorless or Electronic Ignition (EI) systems. In a Distributor Ignition (DI) system, high-voltage from the secondary winding passes through an ignition cable running from the coil to the distributor. The distributor then distributes the high voltage to the individual spark plugs through a set of ignition cables. The cables are arranged in the distributor cap according to the firing order of the engine. A rotor which is driven by the distributor shaft rotates and completes the electrical path from the secondary winding of the coil to the individual spark plugs. The distributor delivers the spark to match the compression stroke of the piston. The distributor assembly may also have the capability of advancing or retarding ignition timing. The distributor cap is mounted on top of the distributor assembly and an alignment notch in the cap fits over a matching lug on the housing. Therefore the cap can only be installed in one position, which assures the correct firing sequence. The rotor is positioned on top of the distributor shaft, and a projection inside the rotor fits into a slot in the shaft. This allows the rotor to be installed in only one position. A metal strip on the top of the rotor makes contact with the center distributor cap terminal, and the outer end of the strip rotates past the cap terminals as it rotates .This action completes the circuit between the ignition coil and the individual spark plugs according to the firing order. EI systems have no distributor; spark distribution is controlled by an electronic control unit and/or the Vehicle’s computer. Instead of a single ignition coil for all cylinders, each cylinder may have its own ignition coil, or two cylinders may share one coil. The coils are wired directly to the spark plug they control. An ignition control module, tied into the vehicle’s computer control system, controls the firing order and the spark timing and advance. This module is typically located under the coil assembly. A specific amount of energy is available in a secondary ignition circuit. In a secondary ignition circuit, the energy is normally produced in the form of voltage required to start firing the spark plug and then a certain amount of current flow across the spark plug electrodes to maintain the spark. Distributor less ignition systems are capable of producing much higher energy than conventional ignition systems. Since DI and EI systems are both firing spark plugs with approximately the same air gap across the electrodes, the voltage required to start firing the spark plugs in both systems is similar. If the additional energy in the EI systems is not released in the form of voltage, it will be released in the form of current flow. This results in longer spark plug firing times. The average firing time across the sparkplug electrodes in an EI system is 1.5 milliseconds compared to approximately 1 millisecond in a DI system. This extra time may seem insignificant, but it is very important. Current emission standards demand leaner air-fuel ratios, and this additional spark duration on EI system help to prevent cylinder misfiring with leaner air-fuel ratios. This is why most car manufactures have equipped their engines with EI systems.

Metal Clad or Can Type Coil

In this, the secondary is first wound over the core and then the primary over this. The inner end of the secondary winding is connected to the core, which further leads to H.T. terminal of the coil, while the other end of the secondary is connected to the primary winding. The ends of the primary winding are connected to the L.T. terminals of the coil, one of which is connected further to the contact breaker and the other to the ignition switch. Between the primary and the case iron strips are sometimes placed. These increase the inductance and prevent losses that would occur otherwise if the flux is linked with the metal case.

Comparison of Core and Can types

1. The hear losses in the primary winding in the core type have to pass through the secondary, whereas in the run type, the primary being situated immediately under the metal case, can readily part with its heat without affecting secondary.

2. In the core type, the primary being inside, its mean turn is relatively short, Therefore, it is necessary to add external resistance called “ballast resistance”, in order to limit the battery current in the coil. In the can type, however, no ballast resistance is necessary.

BALLAST RESISTOR

When the engine is running at slow speed, the contact breaker points are closed for a comparatively longer period of time. This causes overheating of the induction coil. To avoid this ballast resistance is sometimes inserted in the primary circuit of the coil. This increased resistance decreases the amount of current in the primary circuit and thus avoids overheating of the coil at low engine speeds.

At higher engine speeds, however, due to smaller closing time of the contact breaker points, the current in the primary circuit and thus its temperature decreases, due to which the resistance value of the ballast resistor decreases. Thus it guards against any drop of secondary voltage at higher speeds, which if occured would be quite opposite to what is desirable. Further during starting of the engine, the resistance is bypassed from the ignition circuit to provide correct voltage to the coil when the starting motor is drawing a very heavy current, thereby drawing a very heavy current, there by draining the battery voltage. This is done by the solenoid switch which short circuits the resistance, thus putting it out of the primary circuit completely. The value of the ballast resistance varies generally from 1to 4 ohms. The ignition coil in Maruti 800 employs such a resistance.

CONTACT BREAKER

The function of a contact breaker is to make and break the primary ignition circuit. This is probably the weakest member of an ignition system. This is clear from the fact that a four-cylinder engine operating at 4000 r.p.m. must make and break the circuit 8,000 times a minute. The essential requirements of a good contact breaker are :

1. The contacts must open and close at the correct time.

2. The contacts must close without bounce.

3. The contacts must open without "Fling". Flinging occurs when the spring force is too low. In this, heel is flung clear of the cam surface.

4. Excessive oxidation of the contacts should be avoided. The oxide film that may form on the contact faces is particularly detrimental in a low-resistance circuit of the primary winding.

5. The corrosion of the points should be minimum. The contact breaker is placed in the distributor of housing itself. The cam is fitted to the distributor spindle. A hardened steel cam attached to the end of the driving spindle actuates the lever through the heel. The speed of cam is always one half of the engine speed. However, the number of cam faces is always equal to the number of cylinders except in case of the double lever type contact breakers. The main components of a contact breaker are the lever, heel, bush and the contacts. One contact point is mounted on a steel pressing fixed by a screw or screws to the distributor base plate, thus making it adjustable. The other contact is mounted on the pivoted lever which is made of steel. The heel and the bush are generally of plastic. For the contact tungsten or alloy of platinum and iridium (with iridium content varying from 10 to 25 per cent, depending upon the degree of hardness required) is used. The tungsten is quite hard and has high melting point, but it has the disadvantage that it is liable to oxidation. Platinum- iridium contacts, however, are costlier and therefore not used so frequently. Generally their use is restricted to aircraft magnetos. Compared with tungsten they are relatively free from oxidation, but do not resist impact deformation so well. The double arm type has certain advantages over the single arm type. In this for the same contact force, the force and wear on the heel are smaller, and because of smaller moment of inertia, contact bounce is less. This is why the double arm contact breaker is widely used in magnetos when operating speeds are normally higher than with the coil ignition.