Contact Breaker points triggered ignition systems are the simplest of all the ignition systems. The current flowing through the ignition coil is switched on and off mechanically by a set of contact breaker points (points for short) in the ignition distributor. The points are opened and closed by the action of the distributor cam, which has as many lobes as the engine has cylinders. The period of time the points are closed is called the dwell angle. The dwell angle once set does not vary throughout the entire speed range. This results in less time for the coil to charge as engine rpm increases, which translates to less spark output. This is one reason why points type systems are not very well suited to high performance engines.
However, the dwell angle does change (increases) as the rubbing block on the points wears out. This causes the points to open later than normal, thus retarding the ignition timing and causing loss of power and economy.
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a- contacts closed b- large points gap, small dwell angle c- small points gap, large dwell angle |
This is one of the reasons points need to be renewed regularly and the dwell angle checked. A V8 engine operating at 5000rpm is switching the coil primary current on and off through the points 20,000 times per minute. Another reason why tuning is required, is because of points erosion (pitting) due to the primary voltage continually passing through the points. This causes inadequate charging of the coil due to the poor contact at the points.
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| A crater develops on the points lever contact while material accumulates on the fixed contact |
The single point system employs mechanical centrifugal ignition advance, comprising the main shaft, an ignition cam which pivots on the main shaft, advance weights which act on the cam and primary and secondary springs to control the rate of advance when the weights are acting on the cam. The role of this mechanical advance system is to vary the engine timing according to the engine rpm, to suit the engine's changing requirements throughout the entire rpm range.
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Further to this, a vacuum advance unit is used, which acts on the points base plate, to alter the timing dependant on the engine's manifold vacuum and therefore load. At light load, high manifold vacuum exists and this will activate the vacuum unit and give additional advance. High load and manifold vacuum is low, therefore very little if any extra advance other than centrifugal will occur.
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Ignition coils used by single point systems are particular to their fixed dwell operation. They usually use a resistor type coil, where the input voltage is limited by this resistor to 9-10 volts. This is necessary with points systems as a higher voltage will prematurely pitt the contacts on the points.