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Pressurised magnetos - when required?

So, yes probably possible to fly high altitude with no misfires as stated by Peter_G, but not sure if that’s LoP or not.

I have always flown LoP and not had problems.
I do, though, have GAMIjectors: I don’t know whether because these create a very exact and equal mixture in each cylinder whether this helps.

Rochester, UK, United Kingdom

The reason why LOP is supposedly a possible extra stress on magnetos is that the flashover voltage of a spark gap depends on the gas pressure.

The type of gas also affects it greatly, which is made use of in high voltage switchgear, but in an engine you get to choose that

However I don’t understand why the fuel % affects it because the mixture is around 14:1 air:fuel so going from no fuel at all to fully rich should change the gas pressure at the spark gap by only 7%. It would to be an extremely marginal system for leaning the fuel to expose the problem.

Administrator
Shoreham EGKA, United Kingdom

The distribution of fuel through the air is not uniform. A simple model says that to avoid misfire, the spark has to be big enough to reach an area of slightly rich mixture, which is probably further from the spark plug with leaner average mixture. The spark might exist regardless of cylinder pressure, but with weak mags it’s just not big enough to do the job.

The same model can explain why the development of lean misfire looks like a cliff relative to air to fuel ratio, while rich misfire looks like a gradual slope.

Last Edited by Silvaire at 18 Mar 15:30
13 Posts
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