Further guidance
Technical Information
The primary ignition is so called as it forms the first part of the ignition circuit. Through the ignition coil, it drives the secondary High Tension (HT) output. The primary circuit has evolved from the basic contact breaker points and condenser to the distributorless and coil-per-cylinder systems in common use today. All of these ignition systems rely on the magnetic induction principle.
Magnetic Induction
This principle starts with a magnetic field being produced, as the coil's earth circuit is completed by either the contacts or the amplifier providing the coil negative terminal with a path to earth. When this circuit is complete, a magnetic field is produced and builds until the coil becomes magnetically saturated. At the predetermined point of ignition the coil earth is removed and the magnetic field collapses. As the field inside the coil 250 to 350 primary windings collapses it induces a voltage of 150 to 350 volts.
The induced voltage is determined by,
- The number of turns in the primary winding.
- The strength of the magnetic flux, which is proportional to the current in the primary circuit.
- The rate of collapse, which is determined by the speed of switching of the earth path.
Dwell Period
Dwell is measured as an angle, with contact ignition, this is determined by the points gap. The definition of contact ignition dwell is the number of degrees of distributor rotation with the contacts closed.
As an example, a 4 cylinder engine has a dwell of approximately 45 degrees, which is 50% of one cylinder's complete primary cycle. The dwell period on an engine with electronic ignition is controlled by the current limiting circuit within the amplifier or control unit.
The dwell angle on a constant energy system expands as the engine speed increases, to compensate for a shorter period of rotation and maximise the strength of the magnetic field. The term constant energy refers to the available voltage produced by the coil. This remains constant regardless of engine speed, unlike contact ignition where an increase in engine speed means the contacts are closed for a shorter time and gives the coil less time to saturate .
The induced voltage on a variable dwell system remains constant regardless of engine speed, while it reduces on contact systems. This induced voltage can be seen on a primary waveform.
Both dwell periods will expand as the engine revs are increased. This is to maintain a constant coil saturation time, hence the term constant energy. If time rulers are placed at the beginning of the dwell period and on the induced voltage line the coil saturation time can be measured. This will remain exactly the same regardless of engine speed.
