A conventional or Kettering ignition system used in an internal combustion engine makes use of mechanical breaker points which are opened and closed by a cam lobe driven by the engine to periodically interrupt the passage of electrical current through an ignition coil coupled thereto. The time when this current is interrupted must be synchronized with the optimum piston position within the cylinder for maximum mechanical torque and minimum exhaust emissions.
In operation, the spark plug firing time is initiated the moment the mechanical breaker points are separated by the cam lobe. In order to increase the life expectancy of the breaker points, the current which they must interrupt is typically reduced, thus decreasing the available energy in the system.
The time the coil is charged is defined in the conventional systems as the dwell angle. The dwell angle is a constant function of the configuration of the cam lobe and the mechanical breaker points. Because the dwell angle in a conventional system is independent of engine speed, at low engine speeds the coil may charge to a level higher than that required for optimum combustion thereby wasting energy and causing needless excessive wear of the breaker points. Conversely, at high engine speeds, the dwell angle in a conventional system is frequently insufficient for the coil to charge to the critical energy level required for optimum combustion.