Points/Condenser Ignition Explained

The ignition is one of the four vital systems in your engine's operation. Along with fueling, cooling and lubrication, it would simply not be possible to run your engine without it. Its purpose is to ignite the air/fuel mixture in the engine's cylinders and begin the power stroke.
Almost every car built in Britain before 1980 used the traditional Points/Condenser ignition system shown below. Although prone to unreliability due to the number of components needed, in operating principle it is a relatively simple affair.

The ignition system is comprised of two separate circuits:
1. The low-tension (primary) circuit, which generates the spark.
2. The high-tension (secondary) circuit, which delivers the spark to the relevant spark plug.

The Ignition Coil

The heart of the ignition system is the ignition coil. It is in this sealed unit that the high-voltage electrical pulse necessary to create an ignition spark is generated. The ignition coil contains two separate coils of wire arranged concentrically (one inside the other). The outer, primary coil (1) is made up of around 200 turns of thick wire, whilst the inner, secondary coil (2) comprises of around 20,000 turns of much thinner wire. Using the transformer effect, charge can be transferred from the primary to the secondary coil, converting 12v of electricity from the car's battery into the 10,000v that creates the spark.


The Primary Circuit

With the contact breaker points closed, a 12v current flowing from the car's battery induces an electro-magnetic charge in the primary coil (1). At the same time, a small capacitor, or 'condenser', placed in parallel to the circuit is also charged for an auxiliary purpose. 
As the engine-driven distributor drive shaft rotates, a corner lobe of its cam pushes the contact breaker points open, breaking the primary circuit and preventing the primary coil from charging any further. The condenser acts as dampener at this point, preventing current from arcing (jumping) across the open gap in the breaker points, ensuring that the circuit is broken cleanly.


The Secondary Circuit
Once the primary circuit is broken, the electro-magnetic field in the primary coil (1) becomes unstable and begins to collapse. This charge is then transferred to the inner, secondary coil (2), at which point the transformer effect drastically increases the voltage from 12v to around 10,000v, whilst proportionally decreasing the amperage. This pulse of high-voltage electricity is then taken away from the secondary coil through a high-tension lead to the rotor arm, which is mounted on top of the rotating distributor drive shaft. At this point, the rotor arm is aligned with one of its contacts. The pulse passes through this contact and along a second high-tension lead to the spark plug. At the end of the spark plug is a small gap, the other side of which is earthed to the negative side of the battery through the vehicle's engine block. As the pulse jumps across this gap, it effectively short-circuits and a large spark results, igniting the air/fuel mixture in the cylinder and beginning the engine's power stroke.

The Cycle
As the distributor drive shaft continues to rotate, and the corner lobe of the cam moves away, the contact breaker points will spring shut, completing the primary circuit and beginning the process of charging the primary coil once again.
As the rotor arm is connected to the same drive shaft as the cam, the two rotate at the same rate. Therefore, at the point that the next corner lobe of the cam is opening the contact breaker points, the rotor arm will be aligned with the next contact ready to distribute the impending pulse to the next spark plug in the engine's cycle.
The cycle continues in this manner of charging and discharging the coils indefinitely. In a 4-stroke, 4-cylinder engine (as shown in the diagram), for every 1000 revolutions of the engine, the ignition cycle will be completed 2000 times.


For more information on the individual components in a points/condenser ignition system, see the separate articles in the Components Explained section.

To see how to dismantle, test and rebuild a points/condenser ignition system, check out the Tutorial article in the Step-by-Step Guides section.