Cylinder Compression Ratios

AdamZ

AdamZ

Touchdown! Greaser!
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Adam Zucker
I am revealing my ignorance as although I fly ( I'm a renter)I don't know a whole lot about various mechanical and electrical components of the plane and I am trying to learn.
One thing that I read quite often is about cylinder compression ratios. Would someone kindly explain what they are, why they are important, how they are measured, what is a good vs. bad ratio and what the cause and fix for the problem is. Thanks.

:confused:
 
AdamZ said:
One thing that I read quite often is about cylinder compression ratios. Would someone kindly explain what they are, why they are important, how they are measured, what is a good vs. bad ratio and what the cause and fix for the problem is. Thanks.
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What they are? The expression of air pressure measured within the cylinder versus air pressure applied to the cylinder.

Why are they important? 1. They are somewhat an indication of leakage from the combustion chamber and therefore somewhat an indication of cylinder wear/health. 2. For those owning Continental engines, Continental has published minimum allowable compression values.

How are they measured? With the piston held at top-dead-center on the compression stroke, 80 psi air is fed into the cylinder via a fitting installed into the spark plug hole. The air enters the cylinder after passing through a supply orifice (a pin hole) which allows a fixed amount of air to flow/unit time. If they cylinder had zero leaks the cylinder pressure would equalize at the inlet pressure (80 psi), and in that case the compression would be reported as 80/80. However, cylinders always have leaks (rings, valves, etc.), so the air escapes from the cylinder. The pressure in the cylinder equalizes such that the pressure drop across the inlet orifice (80 psi supply vs cylinder pressure) creates a flow through the supply orifice that exactly matches the flow out the cylinder leaks as determined by the pressure drop between the cylinder and ambient. This balancing pressure is then reported as compression (i.e. 72/80). IOW, the cylinder holds 72 psi when fed at 80 psi through the supply orifice.

What is good versus bad reading? 80/80 is perfect. Progressively less is progressively worse. IIRC, Continental sets an absolute minimum of 40/80 (check that number if it matters to you). Most folks get excited around 60/80. Personally, I'd get nervous around 70/80.

What is the cause and the cost to fix? It depends. Some leakage is normal (ring gaps). Some leakage is somewhat bad (valves) and may require removing the cylinder and grinding the valve seats. Some leakage is very bad (i.e. cracked cylinder) and may require replacing the entire cylinder.
 
AdamZ said:
I am revealing my ignorance as although I fly ( I'm a renter)I don't know a whole lot about various mechanical and electrical components of the plane and I am trying to learn.
One thing that I read quite often is about cylinder compression ratios. Would someone kindly explain what they are, why they are important, how they are measured, what is a good vs. bad ratio and what the cause and fix for the problem is. Thanks.

:confused:
Click to expand...

The compression ratio is the ratio between the volume of the cylinder with the piston at Bottom Dead Center (BDC) and the size of the combustion chamber.

IOW it is how much the gases in the cylinder get compressed each time the cylinder comes up on compression stroke. the higher this ratio is the higher the octane rating of the fuel must be.

usually read in this format 6.5:1 (Six point five to one) is a low ratio, 8.0:1 is about as high as you'll see. except in diesel which start at 14:1


Compression checks done at annual are a totally different theory, the "Compression check" achieves one thing, it tell us if the combustion chamber is leaking. and where, and how much. Normally read as ?/80 as in 60/80. This means that an input of 80PSI was introduced thru a calibrated orifice to the combustion chamber and all but 60 psi leaked out. Not good that is the reading that the FAA guidance is said to be the lowest reading considered airworthy. 80/80 is the best you can get.

A good read on this is "Firewall forward" by Bingles
 
So compression checks are what Ed described which is different from compression ratios?
 
AdamZ said:
So compression checks are what Ed described which is different from compression ratios?
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Yes.

Compression checks are one way to help determine the health of your engine. Specifically a way to measure how much air/fuel mixture is leaking out of a cylinder. The cylinder is not perfectly sealed and so some leakage from the perfect 80/80 is to be expected. Some people look at 70/80 as a good barometer of cylinder health. Another is all cylinders relatively equal where if they are all 74/80 except one which is 68/80 you might have the start of an issue.

Note that compression checks are part art part science as is finding out where the leak is coming from so that you repair the issue without spending more money than is required. For example, the sound of air escaping via the exhaust system would tend to indicate an exhaust valve that has not closed. There are a few ways to fix that which can be described as really expensive, expensive and not so expensive.

Of course, low compression can be from worn cylinders or issues with cylinder rings. Course of action with an engine with low total time might be to replace the affected cylinder. If the engine is close to recommended TBO then you get to consider engine overhaul options as well.

Compression ratio is as Tom D described. Typically higher ratios for the same cylinder displacement size will produce higher horsepower output. They also require higher octane fuel to prevent predetination. Octane measurement is really the measurement of how hard it is to ignite the fuel. With high compression ratios lower octane fuel could be ignited early before compression of the air fuel mixture is complete...that's a bad thing.

Len
 
If you get a low one after a properly done check, make sure the mags are gounded, shutoff the fuel and mix and throttle, and move the prop through TDC. Listed at the exhaust and at the breather tube. Breather tube sounds = rings or cylinder wall problem. Exhaust sound = exhaust valve/seat problem. Best done by two individuals.
 
NC19143 said:
This means that an input of 80PSI was introduced thru a calibrated orifice to the combustion chamber and all but 60 psi leaked out. Not good that is the reading that the FAA guidance is said to be the lowest reading considered airworthy. 80/80 is the best you can get.
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This sounds more like a leak down test. Are you turning the engine during this test, or leaving it stationary at TDC, introducing 80psi, and waiting some set time to read the pressure?
 
Bill Jennings said:
This sounds more like a leak down test. Are you turning the engine during this test, or leaving it stationary at TDC, introducing 80psi, and waiting some set time to read the pressure?
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Engine stays stationary (at TDC) during the test. When you introduce the 80psi air, it equalizes almost immeadiately and then take a reading of the pressure downstream of the orifice.

Gary
 
Please remember a compression check will not find a cylinder crack in the barrel in most cases. Some cracks you can see in the head area of cylinders others require a soap test, which means you soap the cylinder and look for bubbles. A cylinder may have 80/70 but still have a crack. With the piston at TDC you would not find a crack, unless you soaped the cylinder and move the piston to bottom dead center with the cylinder pressurized.

Stache
 
Stache said:
With the piston at TDC you would not find a crack, unless you soaped the cylinder and move the piston to bottom dead center with the cylinder pressurized.

Stache
Click to expand...

That's a recipe for getting some one hit with a prop.

DO NOT DO THAT
 
NC19143 said:
That's a recipe for getting some one hit with a prop.

DO NOT DO THAT
Click to expand...

Agreed. I suspect that Stache meant to say that you can check for leaks better by applying pressure when the piston is at BDC. That would be safe, but IMO that's not likely to help finding a crack in the head or a leak in the head/barrel joint. I would work to find a crack in a cylinder barrel itself, but I'v never seen such a crack. Also it may be difficult to get the piston at BDC with both valves completely closed depending on the valve timing.

I'd also like to comment on Ed's statement that he'd be concerned about a differential cylinder pressure reading below 70/80. Many people seem to think that the DCP reading represents the engines ability to produce rated horsepower. IOW an engine that reads 60/80 will produce less power than one that reads 70/80 but this is rarely true. From what I've read, most engines that barely meet Continental's lower limit still produce more than 95% of their rated power, and some brand new engines don't do any better. Keep in mind that a perfectly accurate tester reading of 40/80 means that the cylinder has a leak equal to a hole that's 40 thousandths of an inch in diameter (the same size as the orfice in the tester). Also many DCP's are woefully inaccurate. Continental attemtped to clear up that problem by issuing instructions to use a special orifice to calibrate the gauge, but many mechanics don't seem to do this.

Also DCP readings are pretty variable depending on engine temperature, mechanics's techniques, and the presense of oil on the rings. One should never pull a cylinder because of one low reading unless there is an indication of valve leakage (even with 70/80).

Finally, within limits, the size of the leak is nowhere near as important as the location. Any perceptible leakage past the valves, especially the exhaust valve should be addressed, but if the leakage is only past the rings, the problem can probably be ignored for quite a while safely.
 
Re: Dpc?

Mike Schneider said:
I give up, what does DCP stand for? -- Mike
Click to expand...
differential cylinder pressure ... not that I know anything about it, but in reading Lance's 2nd paragraph, the definition follows the acronym:

"I'd also like to comment on Ed's statement that he'd be concerned about a differential cylinder pressure (GDK Note - tech docs would add the acronym DCP in parenthesis here) reading below 70/80. Many people seem to think that the DCP reading represents the engines ability to produce rated horsepower. "
 
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The soap check should only be accomplished by a A&P who knows and has the correct equipment to perform this test. This is not a test the owner should perform. I would not want to miss lead anyone in saying anyone can perform this test. A certificated shop or repair facility should only do this. And yes it does find cracks if they are there.

Stache
 
lancefisher said:
I'd also like to comment on Ed's statement that he'd be concerned about a differential cylinder pressure reading below 70/80.
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Lance, that comment was based on my personal maintenance experience. My personal experience has been that good, stable cylinders are 70/80 or better. IME, when cylinders slide below 70/80 they have been prepared for a nose dive. I agree with your subsequent statements that engines produce rated power with cylinders well below that compression reading point. However, IME cylinders below 70/80 haven't stayed stable below that point for very long at all.
 
Ed Guthrie said:
Lance, that comment was based on my personal maintenance experience. My personal experience has been that good, stable cylinders are 70/80 or better. IME, when cylinders slide below 70/80 they have been prepared for a nose dive. I agree with your subsequent statements that engines produce rated power with cylinders well below that compression reading point. However, IME cylinders below 70/80 haven't stayed stable below that point for very long at all.
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You've got a Lycoming, right? Continental cylinders are infamous for losing their choke about halfway to TBO, but if you can live with the oil consumption, cylinder work isn't really needed. IME if the low compresion is due to a leaking valve, it will indeed deteriorate quickly, but if it's just ring leakage, the engine will run until the oil buring gets so bad the plugs keep fouling, and further decreases due to ring wear will come slowly.
 
Ed, Occasionally on a Continental it can be tranisent alignment of the rings. When my mech told me to fly it another 25 hours I was incredulous. Next check- 76/80!
 
bbchien said:
Ed, Occasionally on a Continental it can be tranisent alignment of the rings. When my mech told me to fly it another 25 hours I was incredulous. Next check- 76/80!
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Understood. I was referring to a properly executed test, repeatable, etc., etc.

As for Lance's comments, IME when the reading dips below 70/80 there has been some maintenance on the immediate horizon. Yes, it could be just general ring leakage, but IME that is less common than some other afflictions. Yes, the cause is often valves which can be fixed relatively low cost, but it still is a hit to the pocket. Given my choices I'd rather watch my cylinders hold 74+/80 versus dive to ~64/80 and pray it holds. YMMV.
 
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Re: Dpc?

gkainz said:
differential cylinder pressure ... not that I know anything about it, but in reading Lance's 2nd paragraph, the definition follows the acronym:

"I'd also like to comment on Ed's statement that he'd be concerned about a differential cylinder pressure (GDK Note - tech docs would add the acronym DCP in parenthesis here) reading below 70/80. Many people seem to think that the DCP reading represents the engines ability to produce rated horsepower. "
Click to expand...

You've got it. And you're right that I should have put the acronym in parenthesis after the phrase.
 
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