Most of the modern spark-ignition engines use battery ignition systems. The essential components of a battery ignition system are a battery, ignition switch, ballast resistor, ignition coil, breaker points, condenser, capacitor distributor and spark plugs.
The breaker points, condenser, distributor rotor and spark advance mechanisms are usually housed in the ignition distributor. The breaker points are actuated by a shaft driven at half-engine speed for a four-stroke cycle engine. The distributor rotor is directly connected to the same shaft.
The system has a primary circuit of low-voltage current and a secondary circuit for the high-voltage circuit.
The primary circuit consists of the battery, ammeter, ignition switch, primary coil winding and breaker points. The primary coil winding usually has approximately 240 turns of relatively heavy copper wire wound around the soft iron core of the ignition coil.
The secondary circuit contains the secondary coil windings, distributor, spark plug leads and the spark plug. The secondary windings consist of about 21000 turns of small, well-insulated copper wire.
When the ignition switch and the breaker points are closed a low-voltage current flows from the battery through the primary circuit and builts up a magnetic field around the soft iron core of the ignition coil.
When the breaker points are opened by the action of the cam on the distributor shaft, the primary circuit is broken and the magnetic field begins to collapse, an induced current from the collapsing magnetic field flows in the same direction in the primary circuit as the battery current and charges the condenser which acts as a reservoir for the flowing current due to a rapidly collapsing magnetic field, high voltage is induced in the primary (it might be as high as 250 volts) and even higher in the secondary (10,000 to 20,000 volts).
The high voltage in the secondary passes through the distributor rotor to one of the spark plug leads and into the spark plug. As soon as sufficient voltage is built up in the secondary to overcome the resistance of a spark plug, the spark arcs across the gap and the ignition of the combustible charge in the cylinder takes place.
The induced current in the primary to overcome the resistance of a spark across the gap and the ignition of the combustible charge in the cylinder takes place. The induced current is the primary, as it was pointed out above flows in the same direction as it did before the breaker points opened up and charges the condenser.
The increasing potential of the condenser retards and finally stops the flow of current in the primary circuit and rapidly ‘backfires’ or discharges again through the primary, but in the direction opposite to the original flow of current. This rapid discharge of condense produces directional oscillation in the current flow in the primary circuit.
This oscillation is weekend with every succeeding reversal in the current flow until the original potentials and the direction of current flow the primary circuit is established. The discharge of the condenser by itself does not produce the spark, but only hastens the collapse of the magnetic field around the soft iron core.
The condenser, which has a capacitance range from 0.15 to 0.24 mf in the automotive system, not only assists in the collapse f the magnetic field but also prevents arcing at the breaker points by providing a place for the induced current to flow in the primary circuit.
If the condenser is too small or too large, the breaker points will lead to excessive pitting will result in the breaker points and the distributor must be carefully synchronized with the crankshaft of the engine to give the proper timing of the spark in each of the cylinders. The breaker is often referred to as the timer since the time or point in the cycle that the spark occurs depends upon the time of opening of the breaker points.
The spark plug leads are called the ignition harness. Since the lead carries a very high potential, special insulation is required to prevent a short circuit. Even with the special insulation, these leads are subjected to breakdowns which result in high-tension short circuits and to leakage that lower the voltage available at the work plug. Also, the leads should be shielded to aid in the prevention of radio interference.