Wasted spark
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An ignition system in a four-stroke cycle internal combustion engine may fire spark plugs in pairs, on both the exhaust and compression strokes. The extra spark on the exhaust stroke has no effect and is thus "wasted". This design is simpler than a conventional ignition system, but the extra sparks reduce the lifespan of components (contact breaker, spark plugs).
Examples of engines using this design include the Buick V6 engine Series I and newer, Harley Davidson V-Twin, BMW Airheads, Citroën 2CV, Mazda B engine and Chrysler V10. Some Ford engines also do. Many Honda Motorcycle 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.
Many single cylinder 4-stroke engines also have a wasted spark. There is only one spark at a time, but at twice the rate that is needed, so half are wasted. The magneto on these engines is on the flywheel, so there is a spark on each revolution of the engine, but the cylinders on 4 stroke engines only fire every second revolution.
