The purpose of the ignition system is twofold: first to create a voltage high enough (20,000+) to arc cross the gap of a spark plug, thus creating a spark strong enough to ignite the air/fuel mixture for combustion; second to control the timing so that the spark occurs at the right time and at the right cylinder.
The basic principle of the electrical spark ignition system has not changed for over 75 years. What has changed is the method by which the spark is created. The automobile has evolved from a mechanical system (distributor) to a solid state electronic system. Both systems control a low voltage primary circuit through an ignition coil which will induce a high voltage in the secondary circuit which is then directed to the right spark plug at the right time.
The primary circuit consists of an ignition switch, ballast resistor, some type of off/on switch , ( in the older cars a set of breaker points) , which is usually inside a distributor, an ignition coil and the connecting wires. The purpose of the primary circuit is to allow low voltage from the battery, to pass through the ignition coil where the voltage is stepped up from 12 volts to as much as 40,000 volts.
The ignition coil contains both the primary and secondary winding circuits. The coil primary winding contains 100 to 150 turns of heavy copper wire. The turns of this wire must be insulated from each other or they would short out and not create the primary magnetic field that is required. The primary circuit wire goes into the coil through the positive terminal and exits through the negative terminal. The coil secondary winding circuit contains 15,000 to 30,000 turns of fine copper wire, which also must be insulated from each other. To further increase the coils magnetic field both windings are install around a soft iron core. To withstand the heat of the current flow, the coil is filled with oil for cooling. The ignition coil is the heart of the ignition system. As current flows through the coil a strong magnetic field is built up. When the current is shut off, the collapse of this magnetic field induces a high voltage which is released through the large center terminal through the distributor to the spark plugs.
The secondary circuit consists of the secondary windings of the ignition coil, which produces the high voltage needed to arc across the spark plug gap. This voltage is sent out the center coil tower, through a high tension wire to the distributor cap where a rotor distributes the spark through the distributor cap, to the right spark plug at the right time.
Breaker points have not been used since the mid 70’s, but many older cars still have them. Because of the simplicity of breaker points, it is a good starting point in understanding the switching mechanism that controls the current flow through the coil. The points are made up of a fixed contact point and a movable contact point. The movable point is spring loaded and rides on a 4,6, or 8 lobe cam (depending on the number of cylinders). The points are located inside a distributor. As the engine rotates, the camshaft turns the distributor, which then opens and closes the breaker points as many as 15,000 to 25,000 times a minute. When the points are closed, current is allowed to flow through the ignition coil, thereby building a magnetic field around the windings. When the points are opened, they interrupt that current flow, thereby collapsing the magnetic field and releasing a high voltage surge. This high voltage enters the top of the distributor, where an ignition rotor distributes that voltage through a cap to the right spark plug at the right time. The distributor also contains a condenser that prevents arcing by absorbing excess current when the points open. The difficulty with the breaker point system is that the part that rubs against the cam wears. This wear causes a constant need for adjustment and eventual replacement. In the mid 70’s this problem was corrected through the use of solid state electronics and transistors as switching devices.