How Automobile Ignition Systems Work



The automobile ignition system of internal combustion engine is an important part of the whole engine system that provides for the timely burning of the fuel mixture within the engine. All petrol (gasoline) engines require an ignition system. The ignition system is usually switched on and off through a lock switch and is operated with a key or even a code patch.

Most four stroke engines have used a mechanically timed electrical ignition system. The heart of this system is the distributor which contains:

  • A rotating cam which runs off the engine’s drive
  • A set of breaker points
  • A condenser
  • A rotor
  • A distributor cap.

External to the distributor is the:

  • Ignition coil
  • Spark plugs and then the wires that link the spark plugs and the ignition coil to the distributor.

The power source is a lead-acid battery that is kept charged by the vehicle’s electrical system. This generates electricity using a dynamo or alternator. The engine operates contact breaker points, which interrupt the current flow to the ignition coil.

The ignition coil consists of two transformer windings that share a common magnetic core – primary and secondary windings. For an ignition coil, one end of the windings of both the primary and secondary are connected together. This point is connected to the battery. The other end of the primary is connected to the points within the distributor. Then the other end of the secondary is connected, via the distributor cap and rotor, to the spark plugs.

The sequence of the ignition firing begins with the points (or the contact breaker) closed. A current will flow from the battery, through the current limiting resistor, then through the coil primary and across the closed breaker points and finally back to the battery. This current produces a magnetic field within the coil’s core. This magnetic field will form the energy reservoir that will be used to drive the ignition spark.

As the engine turns, so does the cam inside the distributor. The points will ride on the cam so that as the engine turns and reaches the top of the engine’s compression cycle, a high point in the cam will cause the breaker points to open. This will break the primary winding’s circuit and it will stop the current flow through the breaker points.

If there is no steady current flow through the points, the magnetic field that is generated in the coil will immediately begin to collapse. This rapid decay of magnetic field induces a high voltage in the coil’s secondary windings.

The ignition coil’s secondary windings are connected to the distributor cap. A turning rotor that is located on top of the breaker cam within the distributor cap, connects the coil’s secondary windings to one of several wires that lead to each spark plug. The, often over 1000 volts from the coil’s secondary will cause a spark to form across the gap of the spark plug. This will then ignite the compressed air fuel mixture in the engine.