Variable ignition timing (100LL no longer available at RHV).

[agent4573]

Originally Posted by Toobuilder

We also need to understand that knock sensors in cars are required because they are trying to optimize timing and mixture to a very, very tight standard to meet fleet emissions standards. Our standard is much easier - do not knock. I have shown that the typical Lycoming running typical take off power mixture is very insensitive to timing retard. Put another way, you can back timing off far enough that you will never have to worry about detonation and retain essentially all of your take off performance - and you can do this open loop with a simple lookup table. This capability is available right now, today. Just give SDS a call.

OTOH, if you are really looking for that absolute corner case where you want to ride right on the edge of detonation reliably with a closed loop system, then please let us know what you find out. We will all benefit.

My main goal is to find out how 94UL affects detonation margin on an engine with 9:1 CR. Lycoming has only approved 8.5 or 8.7 CR engines for use on the new unleaded. I don't want to push the limits and run it on the edge. If I run back to back tests with 100LL and 94UL and keep advancing the timing until I hit initial detonation on each, I'll know how much extra timing to pull when running 94UL to have the same amount of margin. If 100LL starts to knock during the takeoff roll at 28 degrees advance, and 94UL starts to knock at 26, then I know I have to pull 2 degrees of timing out whenever I fill up with 94UL.

Larry, as far as getting parameters correct, I don't see it being that difficult. Fill one tank with 94 and the other with 100LL. Spend 30 minutes in the pattern to get everything at operating temps and do back to back takeoffs with the 2 fuels. I'm not trying to determine absolute knock margin, which would require having all the temps be super high. I just want to know the relative difference between the 2, which means the parameters only have to be the similar.

[Toobuilder]

Detonation on takeoff is a "thing" when you have fixed timing with magnetos. Remember that with fixed timing you are trying to satisfy two masters - a large enough advance to give reasonable high altitude and lean performance, but not so much advance that you detonate at take off power. It just so happens that those two requirements are in almost perfect opposition. So in the certified world Lycoming has to play a very careful dance. With variable timing we can completely eliminate this sometimes razor edge compromise.

You dont need to find out what the margin is at high advance at takeoff because you should not be at high advance at take off. Thats a problem for the magneto guys.

[lr172]

Originally Posted by agent4573

Larry, as far as getting parameters correct, I don't see it being that difficult. Fill one tank with 94 and the other with 100LL. Spend 30 minutes in the pattern to get everything at operating temps and do back to back takeoffs with the 2 fuels. I'm not trying to determine absolute knock margin, which would require having all the temps be super high. I just want to know the relative difference between the 2, which means the parameters only have to be the similar.

That assumes that heat increases detonation at a comparable rate for both fuels. Not a combustion engineer, so cant answer that. I, however, would not assume they are the same.

I agree with Mike. Just get a variable ignition system and drop the high MAP timing levels. I only run 21* at 29". Don't believe I am losing anything over 25*, as it is not optimum for ROP WOT; It is a compromise for fixed ignition timing. I run close to 33* at LOP cruise in the 540.

Larry

[Toobuilder]

Originally Posted by agent4573

My main goal is to find out how 94UL affects detonation margin on an engine....

....If 100LL starts to knock during the takeoff roll at 28 degrees advance, and 94UL starts to knock at 26, then I know I have to pull 2 degrees of timing out whenever I fill up with 94UL....

Just re read this part. Why on earth would you want timing set anywhere near this on takeoff? My 8.5 engine is set to 17 degrees on TO. It's a set and forget issue - 100LL or car gas, the engine is not going to detonate.