See Tune-up for electronic ignition information.
There are two basic functions the automotive ignition system must perform: (1) it must control the spark and the timing of the firing to match varying engine requirements; (2) it must increase battery voltage to a point where it will overcome the resistance offered by the spark plug gap and fire the plug.
To accomplish this, an automotive ignition system is divided into two electrical circuits. One circuit, called the primary circuit, is the low voltage circuit. This circuit operates only on battery current and is controlled by the breaker points and the ignition switch. The second circuit is the high voltage circuit, and is called the secondary circuit. This circuit consists of the secondary windings in the coil, the high tension lead between the distributor and the coil (commonly called the coil wire), the distributor cap and rotor, the spark plug leads and the spark plugs.
The coil is the heart of the ignition system. Essentially, a coil is nothing more than a transformer which takes the relatively low voltage available from the battery and increases it to a point where it will fire the spark plug. This increase is quite large, since modern coils produce on the order of about 40,000 volts. The term ``coil'' is perhaps a misnomer since a coil consists of [cf2]two coils of wire wound about an iron core. These coils are insulated from each other and the whole assembly is enclosed in an oil-filled case. The primary coil is connected to the two primary terminals located on top of the coil and consists of relatively few turns of heavy wire. The secondary coil consists of many turns of fine wire and is connected to the high tension connection on top of the coil. This secondary connection is simply the tower into which the coil wire from the distributor is plugged.
Energizing the coil primary with battery voltage produces current flow through the primary windings. This in turn produces a very large, intense magnetic field. Interrupting the flow of primary current causes the field to collapse. Just as current moving through a wire produces a magnetic field, moving a field across a wire will produce a current. As the magnetic field collapses, its lines of force cross the secondary windings, inducing a current in them. The force of the induced current is concentrated because of the relative shortness of the secondary coil of wire.
The distributor is the controlling element of the system, switching the primary current on and off and distributing the current to the proper spark plug each time a spark is produced. It is basically a stationary housing surrounding a rotating shaft. The shaft is driven at one-half engine speed by the engine's camshaft through the distributor drive gears. A cam which is situated near the top of the shaft has one lobe for each cylinder of the engine. The cam operates the ignition contact points, which are mounted on a plate located on bearings within the distributor housing. A rotor is attached to the top of the distributor shaft. When the bakelite distributor cap is in place, on top of the unit's metal housing, a spring-loaded contact connects the portion of the rotor directly above the center of the shaft to the center connection on top of the distributor. The outer end of the rotor passes very close to the contacts connected to the four high-tension connections around the outside of the distributor cap.
Under normal operating conditions, power from the battery is fed through a resistor or resistance wire to the primary circuit of the coil and is then grounded through the ignition points in the distributor. During cranking, the full voltage of the battery is supplied through an auxiliary circuit routed through the solenoid switch. Current will begin flowing through the primary wiring to the positive connection on the coil, through the primary winding of the coil, through the ground wire between the negative connection on the coil and the distributor, and to ground through the contact points. Shortly after the engine is ready to fire, the current flow through the coil primary will have reached a near maximum value, and an intense magnetic field will have formed around the primary windings. The distributor cam will separate the contact points at the proper time for ignition and the primary field will collapse, causing current to flow in the secondary circuit. A capacitor, known as the ``condenser,'' is installed in the circuit in parallel with the contact points in order to absorb some of the force of the electrical surge that occurs during collapse of the magnetic field. The condenser consists of several layers of aluminum foil separated by insulation. These layers of foil, upon an increase in voltage, are capable of storing electricity, making the condenser a sort of electrical surge tank. Voltages just after the points open may reach 250 V because of the vast amount of energy stored in the primary windings and their magnetic field. A condenser which is defective or improperly grounded will not absorb the shock from the fast-moving stream of electrons when the points open and these electrons will force their way across the point gap, causing burning and pitting.
The very high voltage induced in the secondary windings will cause a surge of current to flow from the coil tower to the center of the distributor, where it will travel along the connecting strip along the top of the rotor. The surge will arc its way across the short gap between the contact on the outer end of the rotor and the connection in the cap for the high-tension lead of the cylinder to be fired. After passing along the high-tension lead, it will travel down the center electrode of the spark plug, which is surrounded by ceramic insulation, and arc its way over to the side electrode, which is grounded through threads which hold the plug in the cylinder head. The heat generated by the passage of the spark will ignite the contents of the cylinder.
Most distributors employ both centrifugal and vacuum advance mechanisms to advance the point at which ignition occurs for optimum performance and economy. Spark generally occurs a few degrees before the piston reaches top dead center (TDC) in order that very high pressures will exist in the cylinder as soon as the piston is capable of using the energy just a few degrees after TDC. Centrifugal advance mechanisms employ hinged flyweights working in opposition to springs to turn the top portion of the distributor shaft, including the cam and rotor, ahead of the lower shaft. This advances the point at which the cam causes the points to open. A more advanced spark is required at higher engine speeds because the speed of combustion does not increase in direct proportion to increases in engine speed, but tends to lag behind at high revolutions. If peak cylinder pressures are to exist at the same point, advance must be used to start combustion earlier.
Vacuum advance is used to accomplish the same thing when part-throttle operation reduces the speed of combustion because of less turbulence and compression, and poorer scavenging of exhaust gases. Carburetor vacuum below the throttle plate is channeled to a vacuum diaphragm mounted on the distributor. The higher the manifold vacuum, the greater the motion of the diaphragm against spring pressure. A rod between the diaphragm and the plate on which the contact points are mounted rotates the plate on its bearings causing the cam to open the points earlier in relation to the position of the crankshaft.