It's getting hot in here..
Highflight said:
George, I think you may have been the first to hit on the most relevant problem with high temps, and that is reaching the flash point of the oil. Different oils vary, but flash points of oils are generally a minimum of 400F up to about 485F.
I think you hit the nail on the head, if you operate with the normal temp range (even if it is hot) you can expect to not melt metal. I do have have a back ground in mechanical engineering and not chemical, but I have read The Sky Ranch Engineering Manual by John Schwaner. (
Skyranch engine book) It is excellent and its like staying at a Holiday Inn. Many of the additives in oil start to come out and you just plan break the oil down faster. It has been a while since I read the book, but it is well explained and anyone can understand it. Bottom line high CHT has a direct relation on exhaust valve temp and wear.
The point about lubrication is the exhaust valve needs dissipate heat and that is done thru the seat and stem to guide. The oil in the little space is under tremendous temps and pressure. The combo can produce deposits causing valve problems and the dreaded "top overall". What is the oil temp in that little space? I know there is data on it, but it is all relative to the CHT and it is much higher than what you read on the OT gauge.
I highly recommend Sky Ranch's Engineering book. It gets into everything: metallurgy, fatigue, residual stress, common operation, maintenance and assembly error and the affect, lubrication, engine performance (operation dos and dont's and why). You don't have to be an engineer to read it. This is a must read for any Lycoming/Continental pilot, especially if you are getting your engine overhauled. There is a bunch of info on what to ask and check for when selecting over-haulers and decisions you need to make.
Yes you can melt a hole in an aluminum piston, I have done it twice, once in a hot rod car with a Chevy V8 and once in a plane engine. The plane engine was a O-235 Lyc experimental engine with high compression pistons. I was flight testing the engine in a Piper Tomahawk to get a STC for a company. This engine is also common in the Grumman Yankee. The purpose STC was to increase to compression. The STC really just used stock Lycoming pistons from a certified verson of the same engine. Long story short, Lycoming once offered the O-235 in both low and high compression. The low compression has 118hp and the high about 125hp. Although the HC version of the O-235 was certified by Lycoming and flown on production planes, it was withdrawn from the market. It worked fine on the test stand, but in the field pilots where causing detonation, leading to the burning hole in piston syndrome! Ouch!
(When the piston blew the airplane shook fairly wildly and I made a partial power forced landing at a small grass strip. The engine was removed right there and overhauled, flown and STC was granted; since there was already another STC for the same mod; Lycoming still has the certified engine with the same compression on the books. Consider this before you buy your HC pistons for you Lycoming. We never figured out why this happened, but the engine stayed in the plane and flew it for years with no problem, last I heard.)
How does aluminum melt when detonation happens?
Normally combustion is in a "swirl" motion with a controlled steady flame front. With detonation the heat of combustion is held much longer in the chamber and there is no boundary layer between piston and the flame front of the combustion. In a manner it is an instantaneous blow torch but now directly against the metal. There are temps around 4000F happening in there instantaniously. If it happens too early (pre ignition) while the piston is on the way up it can be desastrious.
(Interestingly enough in jets the temps are so hot no metal on earth can take the heat, but with the use of internal "bleed air", the combustion is controlled in the "burner can" so not to directly contact metal. If it did the engine would melt and I am talking about exotic high temp steels, not aluminum. Starting jet engines is critical for that reason and why they are spooled up first with just the starter before adding the fuel. Once it is spinning fast enough to provide the internal protective layers of air, the fuel is added with a whooosh and an increase in RPM and the whine.)
High CHT's alone will not burn a hole in aluminum, but detonation will do the dirty deed. To avoid the long explanation the word "detonation" is self explanatory. A sudden spontaneous uncontrolled explosion. Bottom line the pressures and temps build quickly in the combustion chamber and the piston is the one that tends to get pounded. There is a laundry list of items that can be damaged but the piston hole is one of them.
When we are talking run of the mill high CHT's, it is different (usually) than detonation. Detonation can lead to high CHT, but detonation happens so fast the CHT is not sensitive enough to detect the onset of detonation. Detonation can cause damage in seconds. In cars the detonation detectors are sometimes microphones! Aircraft engines are too loud to use this technique. You know how you can hear the "ping" in a car; that is pre ignition. You will never hear that in a plane. Since the car is trying to keep the mixture as lean as possible it runs on the edge of detonation. Car engines with small pistons and water cooling are more forgiving to detonation. In air cooled engines we need that little extra fuel to cool the combustion and improve cooling overall, especially the exhaust valves.
As long as you have normal timing, mixture (rich above 75%) and no fault or malfunction that can lead to pre ignition, you are NOT going to melt metal. Malfunctions that can cause detonation, or more correctly pre ignition first than detonation, is a cracked spark plug ceramic insulator. A hot spot in the plug can cause the mixture to pre ignite well before top dead center. The preignition may lead to detonation. Again detonation is runaway combustion that is abrupt and at much greater pressures and temps. Once it starts damage can occur very quickly.
Again in the sky ranch book, John address lubrication in great detail. It really is a must read and well worth the $20. I have a copy and will not sell it. I needed to re read it, it is that good. I also recommend going to the Lycoming web site and reading the key reprints. The key reprints was published by some very knowledgeable old timers at Lycoming for years and there are pearls of wisdom in there.
FLYER- Key reprints from Lycoming on Operations and Maintenance
George
rv9aviator said:
I use to fly two stroke ultralights and one thing you don't ever do is pull the throttle back during climb out. In a two stroke the fuel has a lot to do with cooling the piston. Pulling the throttle back just a little is the best way to sieze an engine. Once the plane is leveled off and proper cooling is restored then you can back off the throttle.
Jim: Interesting and yes there is some correlation. The only comment I would make is the valves on the Lycoming are critical and the two-stroker has no valves as such, but the point is valid. As I mentioned in the Lycoming key reprints, one recommendation is to make slow changes in throttle and mixture changes.
George
