Background
It was during the 1970's that solid-state or transistorized ignition systems were introduced. These systems revolutionized the modern engine performance. This was achieved in part by deleting contact breaker points thus eliminating the frictional contacts between breaker points and distributor cams. This reduced the need for constant maintenance of wearing contact breaker points. A revolving magnetic pulse generator incorporates alternating-current pulses to trigger the high voltage needed for ignition by means of an amplifier electronic circuit replaced the breaker point. Changes in engine ignition timing are made by vacuum or electronic control unit (microprocessor) connections to the distributor.
By the 1990’s the primary objective of the ignition system was to provide a constant secondary voltage available to maximum engine speed by maintaining the dwell time at a fixed value. Many of these systems use a magnetic pulse generator distributor (or crankshaft) using a gear shaped iron rotor with as many teeth as cylinders. The rotor teeth move past a stationary pickup which comprises a small coil wound around a permanent magnet core. As the tooth passes the pickup it induces a voltage signal which has amplitude in proportion to the tooth speed.
Functionality
The signal voltage rises to a maximum as the tooth and pole piece comes into alignment, falling abruptly as they move apart. When it drops below the 0V threshold, the control module switches off the ignition coil primary current to generate a spark. Since the coil switch off point has a reference value of 0V, the spark timing remains fixed with increasing engine speed, however the start point of the “dwell period” occurs earlier due to the higher signal voltage.
Overview
In electronic ignition systems the contact breaker is eliminated and the switching or triggering of the primary circuit is carried out electronically.
In induction type systems, the pulse generator has a stator mounted on the distributor body and a rotor unit, called a reluctor, attached to the distributor shaft. The stator has a circular permanent magnet with a number of projections or teeth corresponding to the number of engine cylinders, and a stationary coil of fine enameled copper wire wound on a plastic reel and positioned inside the magnet. The reluctor has the same number of teeth as the stator and, as it rotates, these teeth approach and leave the stator teeth, changing the air gap between them. As this occurs, the strength of the magnetic field changes, increasing as the teeth approach, reaching a maximum when they are in alignment, and decreasing as they move away.
As the stationary winding is influenced by the magnetic field, then, in accordance with Faraday's Law, a voltage is induced across the ends of the winding, each time the magnetic field changes. And if the winding forms part of a complete circuit, the voltage will cause a current to flow.
As the teeth approach, the strength of the magnetic field is increasing. This induces a voltage and current flow in the winding. The polarity of the voltage is said to be positive as it produces a current flow in a certain direction. When the teeth are in alignment, the magnetic field is at its strongest but, at that point, it is not changing. Voltage and current now fall to zero.
As the teeth move away, the strength of the magnetic field changes again, and once again voltage and current flow is induced in the winding. This time, current flow is in the opposite direction, and the polarity is now said to be negative. Since polarity changes every time the teeth approach and leave the stator teeth, the voltage produced is an A.C. voltage, and current flow is an alternating current.
Source: CDX Global & Wikipedia - en.wikipedia.org
