Plug Design

[glenn654]

Can someone explain to me the reasoning for the design of a/c sparkplugs?

Why not something like auto plugs?


Glenn Wilkinson

[DakotaHawk]

I think the easy answer to your question is that aircraft plugs are designed differently, because of the altitudes that aircraft fly at.

As altitude increases, air (which acts as an electrical insulator) gets thinner, which could allow the spark to track from the plug to ground, bypassing the electrode. The porcelin interior, plug boot, and size of the plug are all designed to minimize the poosibility of tracking.

All that being said, many of the electronic ignitions (such as Light Speed Ignition) use standard automotive plugs...

[terrye]

Two reasons for the external metal casing of aircraft spark plugs:
1. To fully shield the plug and spark plug wires from propagating electromagnetic interference which would affect the navigation and communication equipment in an aircraft.
2. To provide a secure (threaded) attachment between the spark plug and spark plug wires.
The normal automotive push on spark plug to spark plug wire connection has been known to become disconnected in high force/high vibration environments (racing cars).

[Mike S]

Terrye got it correct.

Magnetos put out a really "dirty" radio spectrum radiation, IIRC, and the old time radios did not have very good filtering abilities.

[LarryT]

Originally Posted by terrye

Two reasons for the external metal casing of aircraft spark plugs:
1. To fully shield the plug and spark plug wires from propagating electromagnetic interference which would affect the navigation and communication equipment in an aircraft.
2. To provide a secure (threaded) attachment between the spark plug and spark plug wires.
The normal automotive push on spark plug to spark plug wire connection has been known to become disconnected in high force/high vibration environments (racing cars).

Never had a plug wire come off in over 250 races including crashes. SCCA Formula Ford, ARCA super late model, NASCAR Grand American (600HP, 8000 RPM redline). It is possible I suppose if wire routing and cable support is not well thought out. There is no reason it can't be well thought out.

LarryT

[Andy_RR]

Mostly because they were design in the 30's and haven't moved on much since then.

Automotive spark plugs have had loads of development dollars thrown at them to reduce cost, failure rate, EMC/RFI issues and increase service life.

Automotive spark plugs have to run to 10000' AMSL and all the ones I've tested (in cars!) are still working well up there and beyond.

Aircraft engine designers and automotive engine designers rarely meet. There's a lot of not-invented-here in both camps!

[N941WR]

Originally Posted by Andy_RR

Mostly because they were design in the 30's and haven't moved on much since then...

I find it funny that aviation plugs use the same threads as (old) tractor plugs and that the mags are just about identical to old tractor mags.

Could it be that some guy figured out that his tractor ignition was more reliable than the single ignition he had on his OX-5?

Early aircraft plugs look a lot like the lawnmower plugs I grew up with; a simple brass clip to to hold the wire on the end of the plug, no EMF shielding of any kind.

[David-aviator]

Having gone from aircraft plugs to auto plugs back to aircraft plugs and from mags to EI back to mags, my perspective is either works quite well. Whether or not the performance differences are all that spectacular is debatable. After all, the Lycoming is a slow turning engine and old plugs and mags suit it quite well. I do believe the CAFE flight testing does confirm up to 8000', mags and old plugs may be better.

I like the REM37BY's because they seem to last forever. In fact, the original Jeff Rose EI system recommended them. Other EI systems go with various auto plugs, none of them last as long as the 37BY's.

No question, auto plugs and EI in modern engines is the way to go and maybe for engines highly modified such as Klaus Saviors. But the jury really is out on whether they work better in these old technology engines for the average guy. The plugs are cheaper, yes, but they also have to be changed more often. The jury also is out on whether or not an EI system affects Lycoming engine life. We know for sure a pampered engine with mags will easily run 2500 hours but we do not know if it will make it with EI due to much different temperature patterns within the combustion chamber.

...just my 2 cents worth on the subject and it may not be worth that.