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  #11  
Old 08-08-2011, 03:51 PM
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Neal@F14 Neal@F14 is offline
 
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I also think its probably fairly difficult to get an 8.5:1 or lower compression Lyc to detonate while running 100LL avgas... unless you had all of the worst circumstances happening simultaneously of extremely high CHT, leaned the mixture into the very peak of the "red box", and pulled the CS prop all the way back to low rpm while pitching the plane into a hard climb to lug it and wide open throttle and had your magnetos or EI spark advance way too high. Not impossible to get such an engine to detonate, mind you, but you'd probably have to work at it.

The RV-8 I fly has 9.2:1 pistons in it, and that's probably the highest you'd want to run with 100LL fuel. I think that it would be quite a bit easier to induce detonation in this engine, that extra 0.7 points is probably enough to get it near the brink, and I think perhaps it does detonate a wee bit during the few seconds that you pull the mixture back to kill the engine when its on the ramp, and still quite hot after a flight. The reason why I suspect this is... if you're old enough to remember the high compression V-8 car engines from the late 1960's thru 1971, and remember how some of them would "diesel" when you shut the engine off while running gasoline with octane rating too low for the engine, the exhaust while dieseling would have a very characteristic pungent, eye-burning odor to it -- the odor of nitrogen oxides that are only created under detonation circumstances in most piston engines... well this 9.2:1 ECI Lyclone engine makes that very *exact*, very unique, unmistakable same smell in its exhaust when you pull the mixture to kill it under the above circumstances. The very first time I smelled that odor coming from this plane's exhaust, it instantly brought back car memories from my childhood and teenage years. I have no doubt that this engine would "diesel" forever when hot if you simply switched off the mags and didn't cut off the fuel supply. And dieseling *is* detonation keeping the engine running
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  #12  
Old 08-08-2011, 04:07 PM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by Toobuilder View Post
I guess my point is, if it was possible to hurt an engine by mismanaging the power settings, 172ís and their student pilots would be falling out of the skies left and right. The fact that they arenít leads me to believe the risk is far less than most of us are willing to accept.
I'm not terribly confused about it - I think that with a STOCK (not high compression, not hopped up, N/A) Lycoming, it is pretty tough to ruin it unless you are wide open at sea level and lean it untiul it squawks....with this summer's temperatures!

While I agree with almost everything that Deakins has written, it is important to remember that he was writing to the big engined (often turbo-charged) crowd...a diferent audience. We simple folks with simple engines aren't ruining engines on a regular basis, best I can tell.

Paul
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  #13  
Old 08-08-2011, 04:11 PM
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Originally Posted by Neal@F14 View Post
well this 9.2:1 ECI Lyclone engine makes that very *exact*, very unique, unmistakable same smell in its exhaust when you pull the mixture to kill it under the above circumstances. The very first time I smelled that odor coming from this plane's exhaust, it instantly brought back car memories from my childhood and teenage years. I have no doubt that this engine would "diesel" forever when hot if you simply switched off the mags and didn't cut off the fuel supply. And dieseling *is* detonation keeping the engine running
Curious how you can smell the exhaust that's underneath you blown by a propeller? I've shut down my RV thousands of times and never smelled anything.

Michael is spot-on about the inability to forcefully cause detonation on a normally-aspirated Lyc. I've tried, with 87 octane mogas that I ran for over 1000 hours and couldn't make it do it.
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  #14  
Old 08-08-2011, 05:09 PM
sailvi767 sailvi767 is offline
 
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Originally Posted by Neal@F14 View Post
I also think its probably fairly difficult to get an 8.5:1 or lower compression Lyc to detonate while running 100LL avgas... unless you had all of the worst circumstances happening simultaneously of extremely high CHT, leaned the mixture into the very peak of the "red box", and pulled the CS prop all the way back to low rpm while pitching the plane into a hard climb to lug it and wide open throttle and had your magnetos or EI spark advance way too high. Not impossible to get such an engine to detonate, mind you, but you'd probably have to work at it.

The RV-8 I fly has 9.2:1 pistons in it, and that's probably the highest you'd want to run with 100LL fuel. I think that it would be quite a bit easier to induce detonation in this engine, that extra 0.7 points is probably enough to get it near the brink, and I think perhaps it does detonate a wee bit during the few seconds that you pull the mixture back to kill the engine when its on the ramp, and still quite hot after a flight. The reason why I suspect this is... if you're old enough to remember the high compression V-8 car engines from the late 1960's thru 1971, and remember how some of them would "diesel" when you shut the engine off while running gasoline with octane rating too low for the engine, the exhaust while dieseling would have a very characteristic pungent, eye-burning odor to it -- the odor of nitrogen oxides that are only created under detonation circumstances in most piston engines... well this 9.2:1 ECI Lyclone engine makes that very *exact*, very unique, unmistakable same smell in its exhaust when you pull the mixture to kill it under the above circumstances. The very first time I smelled that odor coming from this plane's exhaust, it instantly brought back car memories from my childhood and teenage years. I have no doubt that this engine would "diesel" forever when hot if you simply switched off the mags and didn't cut off the fuel supply. And dieseling *is* detonation keeping the engine running

I think you might have some issue with the engine you might want to look at. I have 10 to 1 pistons and the engine shuts down instantly with either the mixture or the ignition. It was recommended to me that if I intended to restart the engine within a few minute on very hot days to shut it down with the ignition for a easy restart. This seems to work. Never any smells different or otherwise. I do have a FI system with a fuel return. Even very hot starts have never been a problem.

George
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  #15  
Old 08-08-2011, 06:14 PM
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When I taxi up to our gas pumps with the canopy open I can smell the exhaust upon shutdown due to the way the prevailing wind blows in a circle at this spot and the nearby hangars which affect the propwash and winds. Also can smell the NOx when someone else is flying and I'm standing outside near the plane at shutdown.

I've never actually shut the engine off with the mags myself, only with the mixture cutoff. We've got the ECI constant flow return FI and it shuts down quickly. I only suspect that it would likely diesel if shut off with the mags. On an extremely hot day, even my old low compression Cherokee would try to diesel if I was running auto fuel when I shut it off with the mags, but it never made the characteristic NOx exhaust smell.
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Last edited by Neal@F14 : 08-08-2011 at 06:19 PM.
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  #16  
Old 08-08-2011, 09:27 PM
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Nucleus, you mentioned cruising WOT. What if cruising at 3500 feet above sea level? I normally operate LOP at most altitudes but I usually use 24MAP and 2400 RPM and about 50LOP. How does that sound? Reading lots of articles I had the idea that if I were to fly low and LOP I should limit power limiting MAP. Does it make sence? If I keep WOT low wouldn't I get more power than I should risking to damage the engine?

Is it ok to run at peak EGT when up high (9000 feet high or so)? What about run at peak down low but limiting MAP say 21 or 22?

Thank you

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  #17  
Old 08-08-2011, 09:49 PM
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Originally Posted by Moura View Post
Nucleus, you mentioned cruising WOT. What if cruising at 3500 feet above sea level? I normally operate LOP at most altitudes but I usually use 24MAP and 2400 RPM and about 50LOP...

Look at the graph in post #2 and #3... as long as you are leaner than 50, even at sea level and WOT, you are out of the "red box".

Also, this picture from another thread showing 3500 MSL, WOT and 2320 RPM where the engine was quite happy:

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Last edited by Toobuilder : 08-08-2011 at 09:55 PM.
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  #18  
Old 08-08-2011, 10:17 PM
nucleus nucleus is offline
 
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Talking WOT at 3500

Quote:
Originally Posted by Moura View Post
Nucleus, you mentioned cruising WOT. What if cruising at 3500 feet above sea level? I normally operate LOP at most altitudes but I usually use 24MAP and 2400 RPM and about 50LOP. How does that sound? Reading lots of articles I had the idea that if I were to fly low and LOP I should limit power limiting MAP. Does it make sence? If I keep WOT low wouldn't I get more power than I should risking to damage the engine?
Mauro, the meme about LOP being dangerous down low is part of what led me to start this thread.

One of the things I did as I was training myself to do LOP was to print out a copy of this chart

and keep it in the cockpit with me. Of course there are a lot of variables for each aircraft, but as long as you have have good instrumentation on each cylinder, it is safe. Safer than 50 ROP in my opinion.

This chart is likely over cautious by the way. Deakin's main focus is big turbocharges engines. I run 9:1 pistons in my 6 and I stay very cautious. I have killed (car) engines with detonation before. I don't want my engine to detonate at all. To me cautious means run even leaner at low altitudes, like 90-120 degrees LOP at 500 feet MSL.

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  #19  
Old 08-08-2011, 10:39 PM
Moura Moura is offline
 
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So it seems to me that as long as you have cool CHT you are safe. Most of the time I am running LOP with CHTs around 330, except cylinder 6 that is always around 360 due to oil radiator.
I do run LOP when low but always throttle backback to 24MAP before doing so. I want try WOT next time. What about prop. Flying low at high RPM wouldn't allow for a way too high power? Say 26MAP and 2600RPM LOP.
Sorry if my questions (or myself) sound stupid. I just have read so much about it and still it is not totally clear to me when going LOP low and at high RPM. BTW I Fly and RV10 with IO540D4A5.

Last edited by Moura : 08-08-2011 at 10:41 PM.
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  #20  
Old 08-09-2011, 01:12 AM
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Quote:
First, the scary "red box": The good shops run your brand new engine at full rated HP on a dyno, near sea level, well within the box (80-150 ROP). Why don't they detonate into oblivion?
Why not? Think about it for a minute. I run like this every flight for a few minutes some times..........they do not detonate into oblivion in a mater of seconds. The Dyno runs are for short periods of time not hundreds of hours.

What will kill your engine really quick is pre-ignition. The biggest problem with operating at near detonation is high CHT and high valve temps, combined with any poor manufacturing tolerences and thus short top end life.

My engine is the standard IO540, hard to kill, but I understand they have a much lessor detonation margin than say the IO320's and 360's. Its not much different, but if you look at which engines get MOGAS STC's from Petersen's

http://www.autofuelstc.com/autofuels...d_Engines.html

No IO540 D4A5 in that list

And that I understand is due to detonation margin.

Watch what happens when your mixture cable gets pulled out of sync with the stops because the bracket flexes with the engine mounts sagging.....and it pulls the mixture off the stop during the takeoff. When you see a couple of CHT's climbing better than the others just like your VSI, that is detonation. Did our engine die straight away, no, but we never let it go for very long until we found the problem. Had we flown that way with a single point CHT we would never know until one day a couple of hundred hours later we had a problem.
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