Further guidance
As the illustration is a dual trace picture, the two traces will be explained one at a time.
Digital switching signal (red trace)
The low tension (LT) signal switches between 0 volts and 4 volts. When the trigger signal reaches 4 volts, the coil switches on and the 'saturation' or 'dwell' time begins. As the voltage returns to zero, the current in the coil's primary winding switches off, the magnetic flux in the iron core collapses, induces a voltage in the secondary, and the HT voltage is generated.
The coil switch-on and switch-off times are determined by the vehicle's Electronic Control Module (ECM). The dwell time on an engine with electronic ignition is controlled by the current limiting circuit in the amplifier or ECM.
In a constant energy system, the dwell time is fixed regardless of engine speed. This allows the coil to saturate fully and maximises the strength of the magnetic flux. The dwell angle, measured relative to a complete engine cycle of 360°, increases as the engine speed increases.
Secondary HT waveform (blue trace)
The modern engine management system with Coil-Per-Cylinder (CPC) ignition has all the advantages of a constant energy electronic ignition system, with the added bonus of eliminating the distributor cap, king lead, rotor arm and plug lead. Reliability problems from dampness and tracking are now almost eliminated.
Unlike a conventional Distributorless Ignition System (DIS) which fires the plugs with both a negative and a positive voltage, CPC only fires the plugs with a negative voltage, improving plug life and increasing the service life of the plugs.
Inside the coil's primary winding is the secondary winding. This is coiled around a multi-laminated iron core and has approximately 20,000 to 30,000 turns. One end is connected to the primary terminal and the other to the coil tower. The High Tension (HT) voltage is produced by mutual induction between the primary winding and the secondary winding, the central soft iron core serving to intensify the magnetic field between them.
The voltage measured at the spark plug is the voltage required to jump the plug gap under varying conditions. This voltage can be affected by any of the following:
- The size of the plug gaps: a large gap increases the kV
- Worn spark plugs: the loss of surface area will increase the plug kV
- The engine's compression: a low compression reduces the plug kV
- The engine's fuelling: a rich mixture reduces the plug kV
- Tracking to earth: reduces the plug kV
- Fouled plugs: reduces the plug firing voltage
