For a number of reasons, the trend of modern engine management technology is leaning toward using coil-on-plug (COP) ignition systems. One reason is that COP ignitions are very compact and are very adaptable to individual cylinder tuning. With the exception of a few designs that incorporate a driver or "module" into the COP assembly itself, most COP systems are two-wire systems with a 12-volt power source leading to the coil and a ground wire leading from the coil to the coil driver located in the Powertrain Control Module (PCM). By using various data inputs including engine speed, calculated engine load, throttle opening and coolant temperature, the PCM can calculate the spark advance needed for maximum power and fuel economy, and minimum exhaust emissions under various operating conditions.

THE PRIMARY CIRCUIT
To keep the concept of producing a high-voltage spark simple and to the point, let's say that we can create an electromagnet by wrapping an insulated wire around a large nail or similar soft iron core and "saturate" the windings by introducing a direct current into the wire. This wire is called the primary circuit. The strength of the resulting electromagnetic field depends upon the number of wire wraps around the core and the combined voltage and amperage (wattage) of the current flowing through the wire. If the flow of electricity is suddenly interrupted, the magnetic field collapses and oscillates throu...