Twin Plug Combustion
Some engines, since the earliest petrol engines, have used twin spark systems. These have twin spark plugs, supplied by separate sources. There may be several reasons for this: reliability (typically aircraft), starting (e.g. Model T), or better combustion performance by initiating the flamefront at opposing points simultaneously (e.g. Alfa Romeo). None of these are counted as wasted spark systems, as their sparks all take place after the useful compression stroke, rather than "wasted" in the exhaust stroke.
Read more about this topic: Wasted Spark
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