Compression & Power
Sub Judice Angel Lovegod
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Compression & Power
I have just been told that the left engine of my Aztec has zero compression on two pots.
Immediately prior to the CofA on which this was found, I did the test flight, and with the right engine feathered the SE climb was on the reasonable side of adequate.
So, to my questions: what proportion of the power of a 6-pot engine would you expect to be lost if there is no compression on two of them?
Does zero compression = zero power, or is that too simplistic?
TIA
W
Immediately prior to the CofA on which this was found, I did the test flight, and with the right engine feathered the SE climb was on the reasonable side of adequate.
So, to my questions: what proportion of the power of a 6-pot engine would you expect to be lost if there is no compression on two of them?
Does zero compression = zero power, or is that too simplistic?
TIA
W
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HHHMMMMM zero compression on 2 pots with a 6 pot engine... well you would have lost approx 40% (probably even more ) of the total max power at least. That is if the other pots are working at their max efficency, which i guess they wont be. The u/s pots will drag down the good ones you see.
Oh and yes, zero compression = zero power
Oh and yes, zero compression = zero power
Sub Judice Angel Lovegod
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faulty
Thank you.
So I now have three bits of information, all of which I have good reason to believe:
1) Me: My aircraft felt fine and performed fine, both on two engines and with the right feathered. There was no tendency for the aircraft to fly in circles and the needles all sat on top of one another.
2) Engineers: During a compression test, 10 pots were fine, but two, both on the left engine, had zero compression. I have worked with the same engineers for years and I know them to be totally truthful and reliable.
3) faultygoods: The left engine must have been operating at 60% power or less.
I must say that when I made the posting I already suspected that this was the case, hence the original question.
Can you, or anyone else, reconcile these bits of apparently irreconcilable data?
The only logical suggestions are that the two pots lost compression during the taxi in, or that the tests that showed zero compression were not done properly, but both seem vanishingly unlikely!
W
Thank you.
So I now have three bits of information, all of which I have good reason to believe:
1) Me: My aircraft felt fine and performed fine, both on two engines and with the right feathered. There was no tendency for the aircraft to fly in circles and the needles all sat on top of one another.
2) Engineers: During a compression test, 10 pots were fine, but two, both on the left engine, had zero compression. I have worked with the same engineers for years and I know them to be totally truthful and reliable.
3) faultygoods: The left engine must have been operating at 60% power or less.
I must say that when I made the posting I already suspected that this was the case, hence the original question.
Can you, or anyone else, reconcile these bits of apparently irreconcilable data?
The only logical suggestions are that the two pots lost compression during the taxi in, or that the tests that showed zero compression were not done properly, but both seem vanishingly unlikely!
W
Last edited by Timothy; 12th Jan 2003 at 10:49.
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I disagree that zero compression, as recorded on a compression test, equals zero power, unless of course there is catastrophic failure to your two offending cylinders. (Broken pistons). If this was the case, you would definitely have other indications of poor engine condition. Your engine wouldn't run properly.
The compression test that your engineers would have carried out, involves attaching pressure gauges to the cylinders by means of an adapter screwed into a spark plug hole and turning the engine over on the starter. At no time is the engine actually started.
Your problems are likely to be caused by worn/burned valve seats and/or worn piston rings/cylinder bore. The wear is allowing the pressure to escape and nothing is being registered on the gauge.
This does not mean that there won't be some compression there when the engine is actually running. The increased speed of engine rotation and therefore linear piston speed will build up the pressure faster than it can escape. The resulting burning and expansion of the fuel/air charge will provide power.
The compression test that your engineers would have carried out, involves attaching pressure gauges to the cylinders by means of an adapter screwed into a spark plug hole and turning the engine over on the starter. At no time is the engine actually started.
Your problems are likely to be caused by worn/burned valve seats and/or worn piston rings/cylinder bore. The wear is allowing the pressure to escape and nothing is being registered on the gauge.
This does not mean that there won't be some compression there when the engine is actually running. The increased speed of engine rotation and therefore linear piston speed will build up the pressure faster than it can escape. The resulting burning and expansion of the fuel/air charge will provide power.
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Joe i tend to agree with what you have said, however, once the gauge adaptor is fitted and the engine turned over to obtain a pressure reading you will still have a reading even with sloppy valve guides and or rings,....... but the man said 'zero reading' which to me means a big hole somewhere. Again i agree with you though its highly unlikely to have zero compression with no other symtems ie rough running, high exhaust temp etc
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I also would have thought that there should be other symptoms. High oil consumption, smokey exhaust and fouled plugs to name another three. I would be interested to learn the result of further investigations by the engineers.
Sub Judice Angel Lovegod
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One cylinder cracked beyond repair - needs replacement, a second cylinder needs repair, I suspect to the valve guides or seat.
There were none of the symptoms mentioned.
W
There were none of the symptoms mentioned.
W
