lml@lancaironline.net Mailing List Archive

You make a persuasive case for the advantages of electronic ignition when cruising LOP at reduced manifold pressures.

I guess my reservations are what the electronic ignition does in other regimes of flight.

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Gary Casey
Sent: Tuesday, November 16, 2010 9:25 AM
To: lml@lancaironline.net
Subject: [LML] Re: Electronic Ignition

I certainly agree with the statements below from Mr. Brunner. A few reinforcing comments:

Back in the old days when everyone flew at 8,000 feet or less, the mag with its fixed timing worked quite well. At takeoff the mixture was so rich that combustion slowed and the peak cylinder pressure could end up at 16 degrees ATC, as suggested below. When at a cruising altitude the mixture was leaned, probably to about peak EGT (carbureted systems) and that sped up the combustion so that even with the lower manifold pressure the peak pressure was still near 16 ATC. Then we started to fly above 10,000 feet and wanted to go LOP. Lower manifold pressures slow the combustion as does LOP mixtures. Now the spark was definitely retarded from optimum. The solution? An ignition system that advances the timing as the manifold pressure drops. But that still doesn't compensate for LOP operation - the Prism system will do that.

And, yes, the electronic system only increases power when it advances timing over the fixed mag AND the peak cylinder pressure occurs after 16 ATC. When is that? Probably at any mixture when flying at significantly over 10,000 feet (manifold pressures less than 20). And especially when at low manifold pressures AND running LOP. So, in my opinion the advantage is real when typical cruising is done over 10,000 and LOP. Otherwise the benefits are not all that persuasive. What about turbocharged engines? How often are they operated at less than 20 inches? The only problem I see with mags (on turbocharged engines) is potential for cross-firing within the distributor cap, eliminated by most electronic systems. TBO? Certainly any time the peak cylinder pressure is increased TBO is reduced. But I think a 10:1 engine operated like it was turbocharged (avoid peak EGT like the plague) could last as long as a standard engine operated more casually. And the lower EGT produced by the higher compression is likely to increase exhaust valve life.

Gary Casey

ES N224SG, IO-540, 10:1, single Lightspeed, always cruising at 12,000+, LOP

At the risk of starting a “theological” controversy, I would like to raise some reservations about electronic ignition systems (other than reliability and loss of electrical power)

Peak intracylinder pressure (the point in the combustion cycle at which pressure in the cylinder is highest = effective timing) is related to 3 things; spark timing, rpm and MIXTURE.

Why mixture? Because air and fuel mixtures burn at different rates depending on how rich or lean they are. So at a given RPM, MAP and spark advance you can vary your effective timing by changing the mixture.

· Want to advance your timing? Change your mixture to a 50 deg ROP mixture – which is the fastest burning.

· Want to retard your timing? Make your mixture richer or leaner from 50 deg ROP.

And since optimal power is achieved at an effective timing of 16 deg ATDC, advancing the spark timing may or MAY NOT increase horsepower depending on the mixture. But advancing the timing will likely increase the magnitude of peak intracylinder pressure leading to higher intracylinder pressures, higher cylinder head temperatures and greater stress = shorter enging life.

For those of you who are using electronic ignition (I am not):

· Are you sure you are getting more horsepower? How do you know?

· If you are getting more horsepower, are you getting it during all modes of engine operation? Rich of peak, lean of peak, high MAP, low MAP, etc

· What is happening to your TBO?

D. Brunner