Reluctor and pick-up assembly.
Each time a lobe on the rotor (reluctor) passes the pick-up probe a pulse of electrical energy is induced in the pick-up winding. The pick-up winding is connected to the electronic ignition module and when the pulse generator voltage has reached a certain level (approximately 1 V) the electronic circuit of the module will switch on the current to the ignition coil primary winding.
As the reluctor continues to rotate, the voltage in the pick-up winding begins to drop and this causes the ignition module to ‘switch of’ the ignition coil primary current; the high voltage for the ignition spark is then induced in the ignition coil secondary winding. The period between switching on and switching off the ignition coil primary current is called the dwell period. The effective increase in dwell angle as the speed increases means that the coil current can build up to its optimum value at all engine speeds. Figure 1.2 shows how the pulse generator voltage varies due to the passage of one lobe of the reluctor past the pick-up probe. From the graphs in this figure may be seen that the ignition coil primary current is switched on when the pulse generator voltage is approximately 1 V and is switched off again when the voltage falls back to the same level. At higher engine speeds the pulse generator produces a higher voltage and the switching-on voltage (approximately 1 V) is reached earlier, in terms of crank position. However, the ‘switching-of ’ point is not affected by speed and this means that the angle (dwell) between switching the coil primary current on and off increases as the engine speed increases. This means that the build-up time for the current in the coil primary winding, which is the important factor affecting the spark energy, remains virtually constant at all speeds. It is for this reason that ignition systems of this type are known as ‘constant energy systems’. It should be noted that this ‘early’ type of electronic ignition still incorporates the centrifugal and vacuum devices for automatic variation of the ignition timing.
Low speed input/Output wave forms.
High speed input/Output wave forms