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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.
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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.
Design
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.
Safety
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.
Partial dual ignition
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.
Another form of partial dual ignition that has been used on amateur-built aircraft uses a single spark plug, but duplicates the coil and pick-up for better redundancy than traditional single ignition.
A further form of partial dual ignition (such as on the Honda VT500) is for each cylinder to have a single HT coil which sends the current to one plug and completes the circuit via the second plug, rather than via the earth, the current necessarily jumping two plug gaps.
Efficiency
Dual ignition promotes engine efficiency by initiating twin flame fronts, giving faster and more complete burning and thereby increasing power. Although a dual ignition system is a method of achieving optimum combustion and better fuel consumption, it remains rare in cars and motorcycles because of difficulties in siting the second plug within the cylinder head. The Nash Ambassador for 1932-1948 used twin sparkplugs on the straight eight engine, while later Alfa Romeo Twin Spark cars use dual ignition, as do Honda cars with the i-DSI series engines, and Chrysler's Modern Hemi engine. In 1980 Nissan installed twin sparkplugs on the Nissan NAPS-Z engine, with Ford introducing it on the 1989 Ford Ranger and 1991 Ford Mustang four-cylinder models. Several modern Mercedes-Benz engines also have two spark plugs per cylinder, such as the M112 and M113 engines. Some motorcycles, such as the Honda VT500 and the Ducati Multistrada, also have dual ignition. The 2012 Ducati Multistrada was upgraded with "twin-plug cylinder heads for smoother, more efficient combustion", the change contributing to a 5% increase in torque and a 10% improvement in fuel consumption.
Wankel engines
Wankel engines have such an elongated combustion chamber that even non-aero wankel engines may adopt dual ignition to promote better combustion, in some cases like the Mazda 787B racing car even triple ignition. The MidWest AE series Wankel aero-engine has twin plugs per chamber, but these are placed side-by-side, not sequentially, so their main purpose is to give redundancy rather than improved combustion.