Advantages of Electronic Ignition


Advantages of Electronic Ignition

Electronic ignition systems have many advantages over breaker point ignition systems.

High Secondary Voltages

Electronic ignition systems can carry the increased primary current needed to produce the higher secondary system voltages needed to ignite leaner air/fuel mixtures.

The primary circuit in most breaker point systems carried 3.5 to 4.0 amperes. When the breaker points opened, this current tried to arc across the points. Point arcing caused electrolysis and corrosion on the metal surface of the contacts. When primary current increases above 4 amperes, point life begins to decrease at an increased rate, resulting in every limited point life. Faster wearing points require extra maintenance and result in an ever-decreasing dwell period, which in turn decreases the potential voltage induced in the secondary system. Dwell is the period of time that current flows through the primary circuit.

Better High-Speed Performance

Another handicap of the old breaker point system was that as engine speed increased, the dwell time decreased. This, in turn, decreases the output of the coil. From the ignition coil to generate maximum secondary voltage, maximum primary current flow must be flowing through the primary winding before the field is collapsed. In a breaker point system, the length of time the primary circuit is closed is controlled by the speed of the breaker cam. This period of time is called dwell angle and is expressed in a number of degrees of distributor shaft rotation. For example, many V-8 engines have a dwell angle of 30 degrees during which time the points are closed and current builds in the primary winding. This dwell angle remains constant regardless of engine speed; but as engine rpm increases, the actual time, in seconds, the points are closed decreases. Any increase in engine speed above a specific rpm reduces the saturation time of the ignition coil, causing the available voltage to decrease.

This phenomenon is due to the fact that the current in the coil does not instantaneously reach its maximum value when the contact points close. Current in the coil must build for several milliseconds for this value to be reached. At 1,000 rpm, the distributor shaft rotates once every 0.12 second. Of this time, the points are closed for 0.10 second, or 10 milliseconds, for every cylinder of an 8-cylinder engine. This is sufficient time for saturation of the primary winding.

When the engine speed increases to 2,000 rpm, the time that the points are closed for each plug firing is reduced to 5 milliseconds. A dwell period of 5 milliseconds allows the primary current to build to 3.8 amperes. At 3,000rpm, the dwell period drops to 3.3 milliseconds and the current drops to 3.2 amperes. The reduced saturation time lowers the available secondary voltage. This can result in a misfire as there may be less voltage available than is needed to fire the plug. This increases exhaust emissions and decreases fuel economy and engine performance.

An electronic ignition system, however, is not limited by a fixed dwell angle. The system's control unit can vary the on-time of the primary circuit based on engine speed, load, and temperature. Because coil primary current levels are not limited by breaker points, low resistant coils are used in the electronic ignition system. By decreasing the resistance in the primary circuit, the required saturation time of the coil is greatly reduced. It takes 10 milliseconds for the current to reach maximum saturation in a coil with a resistance of 2.6 ohms. In a coil used in electronic ignition systems, the primary winding can have a resistance as low as 0.5 ohm. This allows full current to be reached in about 3.4 milliseconds. Because it takes less time to reach full current, coil saturation can be obtained at much higher engine speeds. For example, the HEI system developed by General Motors in 1974 is able to generate 35,000 volts at engine speeds above 3,000 rpm. A typical breaker point system, on the other hand, developed a maximum of 20,000 volts at 1,000 rpm. Above this speed, the voltage dropped off.

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