Wasted spark

The wasted spark system is an ignition system used with some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs even though one is on its compression stroke and one is on its exhaust stroke. The extra spark on the exhaust stroke has no effect and is thus "wasted". This design halves the number of components in a typical ignition system, while the extra spark, against much reduced dielectric resistance, barely impacts on the lifespan of modern ignition components. In a typical engine, it requires only about 2–3 kV to fire the cylinder on the exhaust strokes. The remaining coil energy is available to fire the spark plug under compression (typically about 8 to 12 kV).

Advantages

Perhaps the most significant advantage of the system, compared to a single coil and distributor systems, is that it eliminated the high-tension distributor. This significantly improves reliability, since many problems with a conventional system are caused by the distributor being affected by dampness from rain or condensation, dirt accumulation, and degradation of insulating materials with time. Although plug-top coil systems would later offer this same advantage, they were not available for another 30 years. Plug-top systems increase the number of coils required, the heat that these coils must survive, and so their demands on materials science for insulators.