Jeff, for smaller volume cylinders, it's a Ford vs Chevy argument about
whether to use one or two plugs per cylinder and the debate will rage on
long after we've passed. I prefer single plug where possible due to 1.
flame front calculations, 2. fewer holes in the head for stronger
structural integrity, and 3. lower overall cost and maintenance.
Yes, beyond a certain volume, two plugs are absolutely required.
Don't even get me started on the mag versus electronic ignition debate...
David M.
On 12/14/2015 09:24 AM, Jeff Scott via KRnet wrote:
> Sorry for the blank emails. My email keeps changing itself to html
> formatting...
> -
>
> 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
>
>
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