Ignition Systems


Technical guru Mark Haycock delves into the mysterious world of ignition systems, and reveals why getting it back to front is sometimes quite OK.

I had a letter recently from one of our readers, Mr A Birch, raising questions about the
workings of an ignition coil so I thought it might be an idea to take a closer look, in particular at
the principle of the ‘wasted spark’.

What we call an ignition coil is actually an example of a so-called induction coil. It is
merely a specialised electrical transformer, the purpose being to greatly increase the voltage
provided by the battery. Despite its name, like any other simple transformer it actually
contains two coils of wire, or windings, one for the low-voltage side (the primary) and the other for the high-voltage (the secondary).

The windings are concentric, and wrapped around a central core made from laminations
(thin sheets) of iron. The core is usually not visible, but in old-style Honda coils it extends
out of the enclosing plastic case (Picture 1). Incidentally, the word ‘voltage’ is sometimes
replaced by ‘tension’ when dealing with ignition coils.

There are considerably more turns in the secondary windings than the primary, and it is
this that leads to the increase in voltage. What causes the output of high tension is a
change in the flow of current through the low-tension windings, with that change traditionally
being brought about by the contact breaker. The aim is to reduce the current to zero as
rapidly as possible. This ‘induces’ a high voltage in the secondary windings, which is fed to the spark plug.
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