Dual ignition

Dual Ignition is a system for spark-ignition engines, whereby critical ignition components, such as spark plugs and magnetos, are duplicated. Dual ignition is most commonly employed on aero engines, and is sometimes found on cars and motorcycles.

Dual ignition provides two advantages: redundancy in the event of in-flight failure of one ignition system; and more efficient burning of the fuel-air mixture within the combustion chamber. In aircraft, redundancy is the prime consideration, but in other vehicles combustive efficiency is the target.

A dual ignition system will typically provide that each cylinder has twin spark plugs, and that the engine will have at least two ignition circuits, such as duplicate magnetos or ignition coils.

Dual ignition in aero-engines can allow the aircraft to continue flying and land safely after an ignition system failure. Operation of aero engines on one magneto (rather than both) typically results in an rpm drop of around 75 rpm. Its existence on aviation powerplants dates back to the World War I years, when such engines as the Hispano-Suiza 8 and Mercedes D.III, and even rotary engines as the later Gnome Monosoupape model 9N 160 hp (119 kW) versions featured twin spark plugs per cylinder.

The Hewland AE75, an inline three cylinder aero-engine created for the ARV Super2, had three ignition circuits, each circuit serving a plug in two different cylinders. If just one of the three circuits failed, all three cylinders still received sparks, and even if two circuits were to fail, the remaining circuit would keep the engine running on two cylinders.

While true dual ignition uses completely separate and redundant systems, some certified engines, such as the Lycoming O-320-H2AD use a single engine magneto drive-shaft turning two separate magnetos. Whilst saving weight, this creates a single point of failure in mechanical terms, that could cause both ignition systems to cease working.

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