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 > > > _______________________________________________ > Search the KRnet Archives at http://tugantek.com/archmailv2-kr/search. > To UNsubscribe from KRnet, send a message to KRnet-leave at list.krnet.org > please see other KRnet info at http://www.krnet.org/info.html > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change > options
