Building Your Own Ignition System

Understanding ignition basics and building your own ignition system.

| December/January 2001

Often all the mechanical work is complete on a fine little model gasoline engine, or a grand old restored engine, and then comes the question: 'What am I going to do for an ignition system?' If that is your dilemma, then here is an answer.  

To help understand the operation of an ignition system a little better, first we'll have a very basic course on electromagnetism. Whenever electrical current flows through a conductor such as a wire, circular magnetic lines of force from around that conductor. (See Fig. #1 in the Image Gallery). Now if you take that wire and wrap it into a coil, the magnetic flux lines complement each other. (See Fig. #2)

If an iron core is placed in the coil, the flux lines will travel through the iron. The flux lines like the iron core because they can travel through it much easier than through air. We have now created an electro magnet. (See Fig. #3).

There are some peculiar things about electricity and magnetism. If an electrical current passes through a conductor, magnetic flux lines form around the conductor as we have just seen, and inversely if magnetic lines cross a conductor they cause a current to flow in that conductor. The latter is how we generate electricity.

Suppose we wrap two coils around an iron core and induce an electrical current into one coil. We'll call it the "primary coil." This will cause a magnetic field or flux lines to build around the iron core. As the flux lines are building and cut across the wires in the "secondary" coil, electricity will flow in the secondary coil. This is called induction. (See Fig. #4)

Another peculiar thing about the above apparatus is that the voltage in each of the coils is proportional to the number of turns in the two windings. In an ignition coil, for instance, the primary coil energized by, say, a six volt battery would have relatively few windings, and the secondary coil would have thousands of times more windings of very small diameter wire in order to get the 15,000 to 25,000 volts needed to jump the gap in the spark plug.