MooneyDriver78
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Tom
Engines are certified by running them at redline for 100 hours. That’s 475° for my IO-360.
High CHT on number 1
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Engines are certified by running them at redline for 100 hours. That’s 475° for my IO-360.
1RTK1
Pattern Altitude
Do they report how much damage is done after that test??
Kenny Phillips
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Kenny Phillips
They write a detailed report, but after the endurance tests (which are 150 hours for jets; I do certification work on turbofans, the rules are similar for the recips) the FAA has the manufacturer lay out every single piece for them to inspect, whether it's a four-cylinder piston engine, or a 100K lbf turbofan. All parts besides standard nuts and bolts are described in the report as to condition. They also have to document every deviation from the test criteria (and are sometimes forced to re-run part of the test if they miss something). There are several certification tests that are witnessed by the FAA or a representative thereof. (For turbofan ETOPS cert, there is a three thousand cycle test. Yikes.)
NordicDave
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Talk to anyone with any experience in metallurgy, you do not want to run your aluminum cylinders at 400º or more. It will dramatically affect top-end longevity.
MooneyDriver78
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Yes, so it would pass an annual inspection, but they are not saying it’s good for the engine.
Dan Thomas
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There's aluminum, and there's aluminum alloys. There are hundreds of different aluminum alloys, and depending on the alloying elements and their percentages, the cold and hot strengths and melting points are all over the map. There are extrusion and casting alloys. An engine designer will specifiy a head alloy that will take millions of repeated pressure cycles and high temperatures and corrosive combustion byproducts for thousands of hours without failing. For a piston, it also has to resist wear, so a different (high-silicon, typically) alloy is used, and that piston is designed to be cooled from under the head by oil flung off the crank and sometimes by oil squirt holes in the big end of the connecting rod, and a lot of the heat is also transferred to the cooler cylinder wall as well as into the connecting rod though the wrist pin.
Some of use have welded aluminum. It does not act like steel at all. Steel has a wide "plastic" temperature range, where you can work it easily as it's soft, and easy to weld. Aluminum is solid right up until it's not, and it doesn't give many clues as to when that is. It doesn't start glowing red, for instance. It just suddenly slumps or drips. Tricky stuff. It does lose considerable strength as it gets hot, though, but the numbers I see while Googling the properties of typical casting alloys tell me that a 500°F redline probably leaves a decent safety margin. Heads don't melt that easily. As others pointed out, the elevated temperature promotes detonation as the complex, detonation-resistant fuel molecules break down during compression and turn into auto-ignitable compounds. Detonation produces big pressure spikes that blow the head off the cylinder. Or pull the cylinder off the case.
The makers of a lot of cheaper aluminum products often use an aluminum-zinc alloy. The presence of the zinc adds hardness and strength but also lowers the melting point dramatically, even below the melting point of either zinc or aluminum by themselves. Lots of door handles and other complex shapes are cast from such alloys.
7075, a really strong aircraft aluminum alloy (but NOT cheap) uses zinc, but not in critical engine parts. Airliner airframes have a lot of 7075 and other 7XXX alloys in them. The typical spam can uses plenty of 2024, a copper-alloyed aluminum. It's strength is good (less than 7075) but it corrodes very easily, which is why the skins are coated with a layer of pure aluminum, which protects the alloy from corrosive compounds, and it's why you don't want to go sanding your airplane. That thin skin has only 2% of the skin's thickness in that layer of pure aluminum on each side. Oure aluminum rapidly forms an oxide that helps shield the metal from corrosion. It's that oxide that the guys polish off when they want their airplane shining like a mirror. Do that enough and the layer is gone.
Everything in airplanes involves compromise. We could have cast-iron cylinder heads, but they'd be harder to air-cool and they'd be heavy, and the fins would crack easily.
Some of use have welded aluminum. It does not act like steel at all. Steel has a wide "plastic" temperature range, where you can work it easily as it's soft, and easy to weld. Aluminum is solid right up until it's not, and it doesn't give many clues as to when that is. It doesn't start glowing red, for instance. It just suddenly slumps or drips. Tricky stuff. It does lose considerable strength as it gets hot, though, but the numbers I see while Googling the properties of typical casting alloys tell me that a 500°F redline probably leaves a decent safety margin. Heads don't melt that easily. As others pointed out, the elevated temperature promotes detonation as the complex, detonation-resistant fuel molecules break down during compression and turn into auto-ignitable compounds. Detonation produces big pressure spikes that blow the head off the cylinder. Or pull the cylinder off the case.
The makers of a lot of cheaper aluminum products often use an aluminum-zinc alloy. The presence of the zinc adds hardness and strength but also lowers the melting point dramatically, even below the melting point of either zinc or aluminum by themselves. Lots of door handles and other complex shapes are cast from such alloys.
7075, a really strong aircraft aluminum alloy (but NOT cheap) uses zinc, but not in critical engine parts. Airliner airframes have a lot of 7075 and other 7XXX alloys in them. The typical spam can uses plenty of 2024, a copper-alloyed aluminum. It's strength is good (less than 7075) but it corrodes very easily, which is why the skins are coated with a layer of pure aluminum, which protects the alloy from corrosive compounds, and it's why you don't want to go sanding your airplane. That thin skin has only 2% of the skin's thickness in that layer of pure aluminum on each side. Oure aluminum rapidly forms an oxide that helps shield the metal from corrosion. It's that oxide that the guys polish off when they want their airplane shining like a mirror. Do that enough and the layer is gone.
Everything in airplanes involves compromise. We could have cast-iron cylinder heads, but they'd be harder to air-cool and they'd be heavy, and the fins would crack easily.
NordicDave
En-Route
A lot has been written on this topic. Cylinder temps should never exceed 400º. 400º is the temp point were the characteristics of the aluminum used in cylinders changes, resulting in much lower lifespans.
It actually has nothing to do with the annealing temp, melting point, or the minimum stress levels mandated by certification process. Generally top ends that make it to TBO are run below 400º over their life time. I'm not trying to pick a fight with anyone here, just running engines hot shortens longevity and that tipping point is about 400º for CHT's.
It actually has nothing to do with the annealing temp, melting point, or the minimum stress levels mandated by certification process. Generally top ends that make it to TBO are run below 400º over their life time. I'm not trying to pick a fight with anyone here, just running engines hot shortens longevity and that tipping point is about 400º for CHT's.
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bnt83
Final Approach
IMHO corrosion will eat up 90% of the cylinders out there long before the affects of operation over 400 (but under redline) could ever show up. Most of these engines just sit around.
NordicDave
En-Route
Quote from Bob Mose, former engineer at Continental:
"There are three things that affect how long your engine will last: (1) temperature; (2) temperature; and (3) temperature!
It's All About The Heat
Our piston aircraft engines are heat engines. They have moving parts -- notably exhaust valves and valve guides -- that are continually exposed to extremely high temperatures in the 1,200 °F to 1,600 °F range (and sometimes even hotter). Since engine oil cannot survive temperatures above about 400°F, these moving parts must function with no lubrication. They depend on extremely hard metals operating at extremely close tolerances at extremely high temperatures with no lubrication. It's nothing short of miraculous, and a testament to outstanding engineering, that they last as long as they do."
"The key to making these critical parts last is temperature control, and the most important temperature is cylinder head temperature (CHT). Mose has been monitoring and overhauling these engines for nearly four decades, and he claims that an engine that is operated at CHTs above 400 °F on a regular basis will show up to five times as much wear metal in oil analysis as an identical engine that is consistently limited to CHTs of 350 °F or less. "It's amazing how much a small increase in CHT can accelerate engine wear," says Mose.
As critical as CHT is, many owners don't have a clue whether their CHTs are above 400°F or below 350°F."
Furthermore:
"Engine instrumentation provided by most aircraft manufacturers is pathetically inadequate. The factory CHT gauge looks at only one cylinder, and it's not necessarily the hottest one. Further, the factory CHT gauge often isn't even calibrated, and its green arc extends up to a ridiculously hot 460 °F (for Continentals) or 500 °F (for Lycomings). Those numbers may be okay as an emergency red line, but they're abusive for continuous operation. If all you have is factory gauges, you could easily be cooking your cylinders to death while blissfully thinking that all is OK because the CHT gauge is well within the green arc."
"There are three things that affect how long your engine will last: (1) temperature; (2) temperature; and (3) temperature!
It's All About The Heat
Our piston aircraft engines are heat engines. They have moving parts -- notably exhaust valves and valve guides -- that are continually exposed to extremely high temperatures in the 1,200 °F to 1,600 °F range (and sometimes even hotter). Since engine oil cannot survive temperatures above about 400°F, these moving parts must function with no lubrication. They depend on extremely hard metals operating at extremely close tolerances at extremely high temperatures with no lubrication. It's nothing short of miraculous, and a testament to outstanding engineering, that they last as long as they do."
"The key to making these critical parts last is temperature control, and the most important temperature is cylinder head temperature (CHT). Mose has been monitoring and overhauling these engines for nearly four decades, and he claims that an engine that is operated at CHTs above 400 °F on a regular basis will show up to five times as much wear metal in oil analysis as an identical engine that is consistently limited to CHTs of 350 °F or less. "It's amazing how much a small increase in CHT can accelerate engine wear," says Mose.
As critical as CHT is, many owners don't have a clue whether their CHTs are above 400°F or below 350°F."
Furthermore:
"Engine instrumentation provided by most aircraft manufacturers is pathetically inadequate. The factory CHT gauge looks at only one cylinder, and it's not necessarily the hottest one. Further, the factory CHT gauge often isn't even calibrated, and its green arc extends up to a ridiculously hot 460 °F (for Continentals) or 500 °F (for Lycomings). Those numbers may be okay as an emergency red line, but they're abusive for continuous operation. If all you have is factory gauges, you could easily be cooking your cylinders to death while blissfully thinking that all is OK because the CHT gauge is well within the green arc."
donjohnston
Pattern Altitude
I'm a little unclear on your stance...
Do you think we should not run our engines over 400°F?
Do you think we should not run our engines over 400°F?
Tom-D
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Did we forget the spray nozzles in the case that sprays oil directly at the pistons?
The better known as the NEY nozzle.
I've only seen two catastrophic piston failures, both had broken rings during assembly and failed on the test stand.
Yes I know I'm a very small segment of the industry.
Raising this post from the dead for an update.
I got to looking at my probes for the CHT AND NOTICED THAT THE 3 cylinders reading normal on Temps have the bayonet style probes shown below.
The one cylinder that has been appearing hot does not have one of these because the suction line for the AC blocks this port. Instead it has the kind that attached to the spark plug and it appears broken and shoved close to the exhaust port. I believe this is why it is running hotter compared to the other cylinders. Pic below
Anyways what are yalls opinion? Seems like number one consistently runs warmer 30 degrees) on the jpi 700. I think it's just due to the probe being so close to the exhaust.
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I got to looking at my probes for the CHT AND NOTICED THAT THE 3 cylinders reading normal on Temps have the bayonet style probes shown below.
The one cylinder that has been appearing hot does not have one of these because the suction line for the AC blocks this port. Instead it has the kind that attached to the spark plug and it appears broken and shoved close to the exhaust port. I believe this is why it is running hotter compared to the other cylinders. Pic below
Anyways what are yalls opinion? Seems like number one consistently runs warmer 30 degrees) on the jpi 700. I think it's just due to the probe being so close to the exhaust.
Sent from my iPhone using Tapatalk
MooneyDriver78
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Normally ring probes run about 30° cooler.
That’s what I hear but this one is broken and just pushed to side of the exhaust. It’s hard to tell in the pic.
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1RTK1
Pattern Altitude
Always thought it was the other way around, plug sensor ran hotter
Stewartb
Final Approach
Spark plug gasket sensors run hotter than probes. Insight’s instructions say 50-100*f hotter. JPI says 25*.