Oldeee is it a io 360 tcm. if you like some help pm as mush info as you have and I'll tell you the fix for it.
Cheers |
But, if you are suggesting that a 5/80 cylinder can run happily then how do you reconcile that with your statement about installation problems causing the cylinder to not make TBO? The alternative is to get approval to run the aircraft in the experimental category. My understanding is Mr Atkinson did that with his 5/80 cylinder. :ok: |
That is correct. We were trying to prove (and did prove) that a static compression which would cause one to change the cylinder was not necessarily an indication that the valve was leaking while running. It clearly was not leaking while running. That was all there was to the issue. If one realizes that a static leak does not equal a dynamic leak then this is useful knowledge. If one is not willing to think about this in these terms and insists that ANY leak is a leak and do not care whether or not the problem is real or imagined, then this exercise was useless.
We use compression testing ONLY because for decades it was all we had. It is a suboptimal test and not nearly as accurate as engine monitor data. Eventually the FAA and CASA will catch up and change the rules for the poor mechanic who can't make the call on his own. I have not, and do not recommend running an exhaust valve AT ALL which is leaking dynamically, it will soon fail. But I do know, intellectually, that just because it is leaking during a very poor test, that it does not mean that it is leaking while running. "Those who insist that something is impossible, should not get in the way of those who are already doing it." That is all. You may now return to your regularly scheduled programming. :D |
Thanks Old Akro and others for trying to keep this thread on topic John this is not an APS thread Yet again you blame the installers for problems with valves etc The machining that takes place regarding valves is one of the most exacting and difficult tasks in the engine. The valve guide must be straight and true and precisely aligned with the seat, everything must be concentric, and the valve must end up precisely centered in the seat with the contact area exactly equal all the way around. The tiniest mistake in any of that machining will result in a valve destined to fail at 400 to 600 hours. Take the matter of a valve where the machining process puts the valve microscopically off-center. The valve-to-seat interface will have a contact surface a bit narrower on one side. We used to think that's ok, because it does make contact, it passes a compression check fine, the Kerosene check is fine. A couple hundred hours, it works well. But all the time, the valve is NOT EQUALLY COOLED AROUND THE CIRCUMFERENCE, because most of the heat passes through the valve seat. Eventually, the unequal cooling causes the valve to actually begin to warp, and at 400 to 600 hours (approx) a tiny bit of the valve no longer seals properly, and now a bit of the 3500 degree combustion event can partially leak out during the time the valve should be sealed. At that point, the poor valve hasn't long to go. It takes about 75 hours of operation from the first sign of this to failure. We have borescope pictures, EMS data, pilot descriptions, and the remaining parts from these failures. All the way from first indications and prophylactic removal to failure and engine destruction. This is just one of the failures, there are others. We don't require your email address Are you a sysop or forum administrator? John Deakin |
I have a suspicion that we have one cylinder location (TSIO 360) that causes a problem. (Its a recent hypothesis, the logs are 100km away and I haven't been back to check). I have assumed that this is a cylinder whose mixture is out of step with the others (no, the aeroplane doesn't have an analyser - yet). If this is the case, it could play straight into your "red box" area. I have looked at a number of removed cylinders and most have sunken valve seats. This gets us back to a point you made on the previous closed thread about an engine running happily with a 5/80 compression. Clearly you understand that the valve springs are there to make the valves follow the camshaft and not seal the valve. The cylinder pressure does that. When I was young and building engines, I'd grind valves until my hands were sore. Now the shop next door does all the head work (including porting & valve seats) on a 5-axis computer controlled mill. He says its better to leave the inlets a bit rough and that grinding paste does more harm than good. sigh. But, if you are suggesting that a 5/80 cylinder can run happily then how do you reconcile that with your statement about installation problems causing the cylinder to not make TBO? The other reason we have had cylinders replaced is that they "barrel" ie, the bore gets bigger in the middle. I had assumed that this condition could be improved by more attention to cooling management. Are you suggesting that it might be just a material issue and the luck of the draw? John Deakin |
John D
No not doing mods job, you are making valued contributions here and would be a pity to lose you from the thread. Reference valve and seat installation, are you suggesting that it is the overhaul shops not using the correct technique or the OEM? CASA has given advice to the industry through ADs and AWBs about boroscope evaluation etc to assess cylinder condition when compression is below the 60/80 differential. I have seen comp as low as 20/80 improved to an acceptable level by running the engine and rechecking. Low compression from blow by can be simply a matter of the ring gaps aligning, well that was one theory. |
OLD AKRO
While a touch of thread drift is involved with this I would add the following story to your compression comments. While carrying out diff comp checks on a Lycoming 0-320 installed on an R22 helicopter that had just flown in with no reported defects I was confronted with 0/80, 0/80, 0/80, 0/80. Obviously the compression tester was u/s so I used a second one. 0/80, 0/80 e.t.c. Obviously I was having a brain storm so I had a second engineer check the engine. Same result. We dropped the exhaust and watched the exhaust valves closing and it was apparent that at slow rotation the valves were hanging in their guides due to excessive guide wear and with no inertia were not seating properly. That this amount of wear had no apparent effect on the engine performance when running was startling, a helicopter hovering at high gross weight will normally reveal any engine abnormalities. |
Reference valve and seat installation, are you suggesting that it is the overhaul shops not using the correct technique or the OEM? CASA has given advice to the industry through ADs and AWBs about boroscope evaluation etc to assess cylinder condition when compression is below the 60/80 differential. I have seen comp as low as 20/80 improved to an acceptable level by running the engine and rechecking. Low compression from blow by can be simply a matter of the ring gaps aligning, well that was one theory. John Deakin |
CASA has given advice to the industry through ADs and AWBs about boroscope evaluation etc to assess cylinder condition when compression is below the 60/80 differential. In my opinion it requires a very high degree of skill & proficiency to diagnose developing issues with a borescope. If you have part of a valve missing, its pretty easy - but you don't really need a borescope for that. I'm prepared to bet that the guy at CASA who wrote this has never done it. I'm interested to hear from Walter or John, but at 60/80 I don't believe you are likely to get any useful information from a borescope. And back on the compression test, the ones I have witnessed rarely seem to get the same number twice. LAME's do it by feel a bit, rocking the prop until they get a number they like. Any test that is this inconsistent is questionable. Its also highly dependant on piston ring orientation. Like yr right's valve seats, piston rings rotate also. Every now and then all the piston ring gaps line up and the compression goes to hell. Run the engine or just leave it till next time and you get a completely different number. |
AIRCRAFT ENGINES????? CMI AND LYCOMING???? Please, tell me more! You've got my full attention! I've read old reports of that, but those are due to either "soft seats" or timing issues related to below standard octane fuel. We should never see that today! At the time, I had quite a lengthy discussion about valve seat design and we compared the valve seat geometry with a number of modern car engines. It was quite interesting. The view was that the CMI design is quite old fashioned and not as effective at heat dissipation as modern designs. The CMI design was as I remember valve seats when I used to hand lap them. Angular and geometric, whereas now they tend to be more like compound curves. This aeroplane has had the same owners since 1985 and (from memory it has done 2 engine rebuilds in that time). No-one remembers what brand these cylinders were. It may be that they were a bad batch. I have a recollection that CMI cylinders were not available and that they were another brand. We replaced them with CMI cylinders. BTW, the 5 axis CNC machine is made by Centronic in the USA. Its spooky to watch it doing a cylinder head. It will change the cylinder head shape, do a port job, skim the head and cut valve seats all in one go. One of the advantages of race engines is that they get torn down a lot. So its easy to try a change, then see how it performs. I spent some time with a drag race guy a year or so ago. The drag guys use no data logging at all. They don't need to because the pull the engine down after every race and they can look directly at the parts. They also use the almost forgotten art of "plug cuts" and reading insulator colour to set mixture. |
Old Akro
The engine is "pumped up" with cylinder pressurized to ensure the rings are seated. Also ensures piston is at TDC as this affects comp as well. Boroscope is an art but not that difficult on piston engines, more skill required for turbine engine inspections though. Do you know the chap who wrote the AWB? |
I ran the low compression cylinder for over 250 hours. The only reason I stopped was that the engine was overhauled (not for that reason).
It is pretty easy and straightforward to see a valve that is leaking while the engine is running through a borescope (not using the borescope while it's running!!). A heathy valves will have a very even, donut appearance to it where the heat is evenly distributed as it leaves the valve for the head. A leaking valve will show a very definite change in the circular pattern. CMI (TCM) put out a SB on this with pictures, etc. ANYONE can see this change with even a cheap borescope. Any LAME who does not have a borescope should get one-- they are very inexpensive and an invaluable tool. There are many A&Ps in the US who now take a quick peak at the valves every time they gap the plugs. We are not removing cylinders nearly as often as we used to as a result. According to CMI's SB, compression below 60/80 is no reason to remove the valve unless the borescope shows the burning pattern. In addition, as the OEMs catch up to the A&P/pilot population's knowledge about how to use the engine monitor for this (it is even better) we will likely begin to see recommendations that we have been teaching for over 14 years adopted about how to determine whether or not a valve is leaking form inside the cockpit during flight. We've been doing this quite accurately and successfully for well over a decade. We will be happy to have the OEMs, engine builders and FAA join the party of knowledge. More and more are doing so. Our recommendation: 1) NEVER run a cylinder with a known leaking exhaust valve--it is soon to fail. 2) Verify this with a borescope or engine monitor data and, if it is NOT leaking, no need to stop running it. The above has been our recommendation for well over a decade, so those who have been misquoting my recommendation might want to read the above again. |
OK lets have a think here'
We do a leak rate test iaw manufactures procedure's lyc and tcm are different. They say no leak past the valves is excepted period. Regulators enforce this in LAW Now you have four places for a leak to occur Rings evident via the breather for a leak Exhaust valve via the exhaust Inlet valve via the inlet track Crack head via a leak in the head All give a different result. And there are techniques that we use when a valve is leaking to try and get it to reseat before we remove a cly. There are also limits via SB for the wear on a valve guild. A worn guild can lead to a dropped valve due to uneven loading on the valve during closing . Next and this is important. We do this test at the end of a m/r and before issue of a new m/r unless there is a problem the engine may develop a problem 1 hour into a m/r our 1 before the end of the m/r. Now they ran a 5/80 after finding it. So how long was it running before this was found and in the other thread they said how long they ran it for. Now it can be safely assumed that they where trending the engine before this had taken place and it wasn't pick up . So why not Instrument wasn't calibrated Instrument wasn't fitted They didn't know how to read the instrument The instrument didn't pick the defect up What did the instrument say when they knew of the defect or a small bit of carbon under the seat Did you do retesting during this period And the list can continue. Now this leads me into thinking why or why would you run the risk maybe to yourself but the risk to others. And unless you flying over nowhere like here you pose a risk to someone on the ground however small that risk is. So what was the wrong with the cly in the end did you change it or just leave it for Devine intervention or send it to the healing room. Now despite them saying a leak rate tester is not a really good tool its a very powerful tool and used correctly can and will lead to stop an engine failure. Boros are a great tool even the little cheap ebay one I had a selection of small mirrors now I don't, I ha at my disposal a $1000 one to a $75k one all singing all dancing But the leak rate is still a better tool to see if a valve is leaking. Cheers |
Sorry forgot to add
Only leaks passed the ring is allowed. Cheers |
Yr Right
Can't fault your what you just posted, except to add that with leaks I have also found it benificial to throw some soapy water at the cylinders, on two or three occasions have found cracks around the head as well as ring leaks |
Yeah soap in water to find bubbles just didn't put it all in sorry. I know tcm say to do that in there SB as we'll
Cheers |
Now they ran a 5/80 after finding it. So how long was it running before this was found and in the other thread they said how long they ran it for.
Watched it descend below 60/80 and kept an eye on it every 25-50 hours. Now it can be safely assumed that they where trending the engine before this had taken place and it wasn't pick up . So why not It was picked up. Instrument wasn't calibrated Yes it was. Instrument wasn't fitted Yes it was. They didn't know how to read the instrument Yes we do. The instrument didn't pick the defect up Yes it did. What did the instrument say when they knew of the defect The correct values. or a small bit of carbon under the seat No. Did you do retesting during this period Constantly. Every 25-50 hours. And the list can continue. No it can't. We were doing research. We were very careful. We learned a lot. Sorry you don't "get this." |
Oldee I was a drag racer. We even made our own fuel injection. Now whilst a lot dont use data loggers a lot do. That's why you can have a N/A 500 ci suspension car running faster than nitro TF cars in the earlier 80s
And btw I also can pretty much tell you how to fix your plane. Is it a turbo model ? Cheers |
So research then hey. We'll let's look at that again. You claim data I've already said it's not data it's obverations. Now 5/80 is not research at all. Un less it carried out on a calibrated dyno how can you ever say what power the cly is producing.
Compression = power A low cly will not make as much power as a cly running at 70/80. That's why we have limits. That's why tcm and lyc have different methods of performing a lake rate test. And different tesster one with a calabrated orifice and one with out. You can use a tcm on a lyc but you can't use one that has not got a cal with out useing the external orifice on a tcm. Why is that then. And whist a low comp generally will not be picked up by a pilot it will how ever be picked up at So now this low cly all thought making power isn't making what it should be making. The whole reason for doing this is to make the engine safe. So now what is your option of an un safe engine. And of course it's all in the name of research but I'm sorry I don't subscribe to this theory as I have people's life's in my hands and I take that responsabity extremly seriously. And now you are say I give miss quotes we'll no you given out dangerous information Cheers |
I also said at the start of this thread that the AD from what I read seam quite a normal response to a problem that has occurred. Then we going the Nstb report which supported the action of the FAA but did say we'll maybe not all the cly need to be recalled.
Everyone said what about Oem cly and I said compare apples to apples. It would appear what I said in the start to be true. Cheers |
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