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An engine with dual ignition is set up to have the flame front within the cylinder coming from two points in the cylinder, typically top and bottom.? It really doesn't have anything to do with the cylinder volume itself, but with efficiency and redundency.? For instance, if one wanted to run a typical aircraft engine designed for dual ignition on a single ignition, you would advance the timing several degrees to compensate for the time it takes for the flame front to propagate from a single plug vs dual plugs.? ? Most aircraft engines have each magneto fire 2 top plugs and 2 bottom plugs.? However, some of the small Continentals have the magnetos split so one mag fires all the top plugs and the other mag fires all the bottom plugs.? Engines configured in this manner typically have the mag that is firing the bottom plugs advanced 3? ahead of the mag firing the top plugs.? The bottom seems to run a bit richer, so the flame front tends to propagate slower. ? As Larry described, most aircraft engines use an impulse coupler to retard the timing back to near TDC for starting.? This is to avoid kickback, whether turning on the starter or hand propping.? Most Lycomings only have an impulse coupler on the left mag, so the mag switch grounds out right mag when starting.? Most Continentals use an impulse coupler on both mags.? Additionally, the impulse Coupler stops the magnet inside the magneto.? The snap you hear is the magnet and shaft releasing, which will also cause the magneto to generate a hot spark for starting.? The impulse coupler has a set of counterweights on it that will cause the catch pawl on the coupler to retract once the engine is turning more than 400 rpm. ? ? Most of the after market ignition systems for aircraft also use an electronic scheme to sense that the engine isn't running and retard the ignition timing back to TDC for starting.? Retarding the timing for starting, while desirable, isn't really necessary on the smaller Continentals like the A series engines.? Same is true for the VW and Corvairs in that the kickback is light enough that it isn't going to be tearing up a starter drive or hurting someone propping the engine.? Many of the 1940s vintage aircraft with A-65 Powerplants didn't have impulse couplers when they were new.? ? -Jeff Scott Los Alamos, NM ?
