Wasted spark

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

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.

Wasted spark systems still require a timing signal from the crankshaft. In a conventional four-stroke engine, this signal must also observe the phase of the camshaft relative to the crankshaft, so contact breakers are normally driven from the camshaft and distributor drive. With a wasted spark, the crankshaft can be used instead, as the system fires on both revolutions. It simplifies the mechanical arrangements since there is no distributor drive and the contact breaker cam can be fixed to the crankshaft. Although a 2:1 reduction gear is still required to operate the camshaft and valves, the precision of this drive is now less critical (ignition timing is more critical for engine performance than valve timing): engines continue to run adequately even with worn camshaft drives and imprecise timing.

This system has been widely used, including such engines as the MG MG6 1.8T engine; Mitsubishi Evolution 4G63 engine, Mercedes-Benz M104.94x, M104.98x, M104.99x I6 engines; Buick V6 engine 3800 LN3 and newer, Harley-Davidson V-Twin, air-cooled BMW Motorcycles, 1948 Citroën 2CV, Mazda B engine, Chrysler V10, GY6 engine, Volkswagen Mark 3 2.8 VR6 (other than 2.0 ), Saturn Corporation 4 cylinders, Toyota VZ engine 5VZ-FE V6, and Chrysler 1.8, 2.0 & 2.4 engines. Some Ford engines also do. Many Honda and Kawasaki motorcycle and PWC engines also follow a similar design, to allow for a smaller number of more powerful coils to replace a larger number of smaller coils in the same limited space.

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