<P> The amount of energy in the spark required to ignite the air - fuel mixture varies depending on the pressure and composition of the mixture, and on the speed of the engine . Under laboratory conditions as little as 1 millijoule is required in each spark, but practical coils must deliver much more energy than this to allow for higher pressure, rich or lean mixtures, losses in ignition wiring, and plug fouling and leakage . When gas velocity is high in the spark gap, the arc between the terminals is blown away from the terminals, making the arc longer and requiring more energy in each spark . Between 30 and 70 milli - joules are delivered in each spark . </P> <P> Formerly, ignition coils were made with varnish and paper insulated high - voltage windings, inserted into a drawn - steel can and filled with oil or asphalt for insulation and moisture protection . Coils on modern automobiles are cast in filled epoxy resins which penetrate any voids within the winding . </P> <P> A single - spark system has one coil per spark plug . To prevent premature sparking at the start of the primary pulse, a diode or secondary spark gap is installed in the coil to block the reverse pulse that would otherwise form . </P> <P> In a coil meant for a dual - spark system, the secondary winding has two terminals isolated from the primary, and each terminal connects to a spark plug . With this system, no extra diode is needed since there would be no fuel - air mixture present at the inactive spark plug . </P>

How does a 12 volt ignition coil work