Twin magneto musings

You are correct. The spark plug's center electrode will erode more rapidly when negative in relation to ground, and the ground electrode will erode more rapidly when polarity is reversed. We rotate plugs in aircraft engines to equalize the erosion. For standard (massive electrode) plugs, the difference is quite noticeable after 100 hours of operation. The difference isn't as noticeable with platinum or iridium electrodes.

Dale Scroggins

You know I've never thought of that but I'm sure you are right, the primary current will be flowing in opposite directions when the points open for each successive firing, won't it? As for any effect on the engine, that performance icon the 2CV seems to manage quite well with one 'backwards' spark from its double ended coil and I believe many modern distributerless car engines share a coil between pairs of cylinders, though I guess these may be double wound rather than double ended.

And also, one plug wears the centre electrode. and t'other wears the earth electrode. I used to swap them over from time to time to equalise the wear.

Regards

Philip T-E

The same applies on 2CV's - swapping the plugs over DOES make 'em last longer!

Brian L Dominic

NB Rumpus

Web Sites: NB Rumpus:

you are right - some modern cars share coils. For example my renault has 4 coil packs. Each is wired up as a pair, and fire simultaneously. They achieve this by wiring the coils in series - each coil is infact rated at

adrenalin

No (not today). There's a possible difference in electrode erosion rate, but modern plugs don't do much of that anyway.

What really used to cause trouble was with aircraft in the '30s, back when lead deposition on insulators was a major problem (mainly fixed by chemistry). Deposition was much more of a problem than erosion, and polarity dependency certainly did show up. .

Reading the 1916-1920 Abridged Patents book, there was a Patent issued for a magneto that reversed the spark polarity every so often to equalise wear on the plug electrodes.

Peter

-- Peter & Rita Forbes snipped-for-privacy@easynet.co.uk Engine pages for preservation info:

I've been putting the finishing touches to an industrial ST P55, ready to sell on E-bay. Whilst doing so, it occurs to me that the magneto fires two vertical cylinders disposed at 180 degrees. Being a two stroke, they do so at the top of every stroke.

Before my brain turns itself inside out and the cells leak out of my ears trying to visualise the effect of turning the two cylinders outwards, am I not correct in thinking that such a magneto - running at half engine speed - will have the correct firing intervals for a four stroke flat twin?

Regards,

Kim Siddorn,

Save your brain - the answer is yes!

Arrrgh, my brain hurts! but I'm glad to have it confirmed as my search is over - I've got one of they lurking in the warm and dry under the stairs.

Regards,

Kim Siddorn,