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Compression Test Compressor Minimum

cfiidon

cfiidon

Well Known Member
I am in a hangar without electricity and use a Honda generator for juice. The limitation is about 1600 watts continuous (13amps or less).

I would like to purchase an electric compressor solely for compression tests. A small two gallon pancake compressor fits the power requirements, and logic says that compression check volumes are low with the piston at TDC.

Do you think that 2 gals with a 90psi rating would keep up with the required 80psi input? That one draws about 5 amps. The next step up would be 2 gallons or so at 125psi requiring 10 amps.

I do not wish to purchase a gasoline powered compressor.

Don
 
I think you'd probably be okay. There is some leakage as you move the prop around a bit but I'd guess that would be accommodated. And if not, you can buy an external tank, put it in series, and let it fill up, too.

Dave
 
Differential compression test use a small volume of air. If you need gobs of air something is major wrong with the cylinder.
 
zilik said:
Differential compression test use a small volume of air. If you need gobs of air something is major wrong with the cylinder.
Click to expand...

Absolutely correct. A "perfect" cylinder at 80/80 will have zero flow. I'm not sure what the flow is at 80/0, but as long as the inlet gauge stays at 80, you'll get a good reading.
 
zilik said:
Differential compression test use a small volume of air. If you need gobs of air something is major wrong with the cylinder.
Click to expand...

I agree completely. If you can get 80 PSI you should be fine. Even if that doesn't work, you could put 70 psi in an get a reading of how much it holds. 80 is the standard number, but there's no reason 70 wouldn't work, just not as good for standard comparison.
 
Get one with a big tank and small motor. You want the storage because you want it quiet when you do compression.
 
I've just used a portable tank for the last five years. I always bring a spare, just in case, but haven't needed it so far. Hope my new cylinders are as tight as the old ones!
 
longranger said:
Absolutely correct. A "perfect" cylinder at 80/80 will have zero flow.
Click to expand...

I thought as long as the flow past valves/rings etc in the cylinder was less than, or equal to, flow through the tester orifice (which is a calibrated hole of a specific design and certain size) it would always read 80/80 (as long as your compressor didn't drop lower than that of course).

So there could be some leakage and still read 80. Is this not correct?
 
krw5927 said:
I thought as long as the flow past valves/rings etc in the cylinder was less than, or equal to, flow through the tester orifice (which is a calibrated hole of a specific design and certain size) it would always read 80/80 (as long as your compressor didn't drop lower than that of course).

So there could be some leakage and still read 80. Is this not correct?
Click to expand...

Correct

80/80 on a leak down test does not mean zero leakage.
 
Leakage Vs. Flow

rvbuilder2002 said:
Correct

80/80 on a leak down test does not mean zero leakage.
Click to expand...

Really? If you have flow across the control orifice (i.e. cylinder leakage) , you have to have pressure drop across it. Maybe it's not measurable, but the pressure drop has to be there!

Skylor
 
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another option

fl-mike said:
I've just used a portable tank for the last five years. I always bring a spare, just in case, but haven't needed it so far. Hope my new cylinders are as tight as the old ones!
Click to expand...

portable air, or scuba/propane ( filled with C02)/welding Nitrogen tank....LOTSA pressure = little volume needed ( sorry if my gas physics are wrong, but I think this is correct!)
 
skylor said:
Really? I you have flow across the control orifice (i.e. cylinder leakage) , you have to have pressure drop across it. Maybe it's not measurable, but the pressure drop has to be there!

Skylor
Click to expand...

Yes.

The gauge is graduated in one-pound increments. The pressure drop across the orifice is much less. We are not able to read the pressure drop across the orifice with the gauge that is on it. On our engines, we are unable to get a perfect seal on the rings and there will always be some leakage even on a perfect reading of 80/80 that we cannot measure.
 
We have inadequate power in our row hangar; my oil-less compressor will start only when the tank is at very low pressure. Thankfully it's a big tank. I run the compressor up to its rated 125psi, then shut it off and wheel the compressor over to the hangar where I'm doing a compression test. I've done more than a few compression tests, including diagnosing leaky cylinders where there clearly is air flowing. I've only had to re-charge the compressor tank once in over a decade of this type of use.

I'm a fan of the "small motor, big tank" philosophy!
 
I was surprised to learn at Lycomign school that you actually CAN check compression with a standard "direct reading" compression tester liek you'd use on a car. This is allowed becasue there are places in the world that simply don't have electricity to power any type of compressor, so maintenance has to be done without compressed air. The trick, however, is that you have to do a baseline test on the cylinders when they are healthy, and then compare subsequent diagnostic checks to that value. I forget the SI number that references this.

Paul
 
krw5927 said:
I thought as long as the flow past valves/rings etc in the cylinder was less than, or equal to, flow through the tester orifice (which is a calibrated hole of a specific design and certain size) it would always read 80/80 (as long as your compressor didn't drop lower than that of course)...
Click to expand...

In a properly functioning tester, the flow out of the cylinder IS the flow through the tester. Any flow out of the cylinder has to have come through the calibrated orifice. The difference in the readings of the two gauges is representative of the flow. IF there is zero flow, there is zero pressure drop across the orifice, and (properly calibrated) gauges read the same.
 
longranger said:
In a properly functioning tester, the flow out of the cylinder IS the flow through the tester. Any flow out of the cylinder has to have come through the calibrated orifice. The difference in the readings of the two gauges is representative of the flow. IF there is zero flow, there is zero pressure drop across the orifice, and (properly calibrated) gauges read the same.
Click to expand...

Everything you said is correct except the last part.

The principle of a differential pressure test is that a cyl is expected to have some leakage, but the test is to confirm that the level of leakage is within a predetermined limit. A perfect 100% seal is pretty much impossible.

While the test is being conducted and pressure is leaking from the cycl, as long as the leakage rate is slower than what the flow is through the orifice, the gauges will both read 80. As the leakage of a cyl gets higher, the flow rate through the orifice is not high enough to keep the cyl fully up to the reference pressure so the gauge will show a slight reduction in pressure.

I have done lots of differential pressure checks that showed 80/80 while always being able to hear air leakage into the crankcase (listen in the oil filler tube).
 
rvbuilder2002 said:
Everything you said is correct except the last part.
longranger said:
IF there is zero flow, there is zero pressure drop across the orifice, and (properly calibrated) gauges read the same.
Click to expand...
Click to expand...

My last statement IS correct. IF THERE IS NO FLOW (tester isolated from the cylinder), there is NO pressure drop across the orifice.

rvbuilder2002 said:
... a cyl is expected to have some leakage, but the test is to confirm that the level of leakage is within a predetermined limit. A perfect 100% seal is pretty much impossible.
Click to expand...

Agreed

rvbuilder2002 said:
While the test is being conducted and pressure is leaking from the cycl, as long as the leakage rate is slower than what the flow is through the orifice, the gauges will both read 80. As the leakage of a cyl gets higher, the flow rate through the orifice is not high enough to keep the cyl fully up to the reference pressure so the gauge will show a slight reduction in pressure.
Click to expand...

You seem to have agreed that "In a properly functioning tester, the flow out of the cylinder IS the flow through the tester. Any flow out of the cylinder has to have come through the calibrated orifice." are correct statements. Then how can the leak rate through the cylinder be lower than the flow through the orifice?


rvbuilder2002 said:
I have done lots of differential pressure checks that showed 80/80 while always being able to hear air leakage into the crankcase (listen in the oil filler tube).
Click to expand...

Only because the human ear is a much more sensitive instrument than a bourdon tube pressure gauge. If there is flow through the orifice, there is a pressure drop across the orifice, just not necessarily measurable by the gauge.
 
cfiidon said:
I am in a hangar without electricity and use a Honda generator for juice. The limitation is about 1600 watts continuous (13amps or less).

I would like to purchase an electric compressor solely for compression tests. A small two gallon pancake compressor fits the power requirements, and logic says that compression check volumes are low with the piston at TDC.

Do you think that 2 gals with a 90psi rating would keep up with the required 80psi input? That one draws about 5 amps. The next step up would be 2 gallons or so at 125psi requiring 10 amps.

I do not wish to purchase a gasoline powered compressor.

Don
Click to expand...
Don,
Since the discussion has seemed to drift off into the weeds, I wanted to add a suggestion (it was mentioned in a previous post) to your original plan. Purchase a portable bottle, and install it in series to your compressor. As has been previously stated, you should already have plenty of air from the 2 gallon tank, but this will ensure you have plenty of air for an extended test. As has also already been mentioned, the ability to "listen" to where air may be escaping is important, and having the compressor running will make that difficult. Having the added capacity of the tank will preclude that from happening.
Good luck!
 
Ironflight said:
I was surprised to learn at Lycomign school that you actually CAN check compression with a standard "direct reading" compression tester liek you'd use on a car. This is allowed becasue there are places in the world that simply don't have electricity to power any type of compressor, so maintenance has to be done without compressed air. The trick, however, is that you have to do a baseline test on the cylinders when they are healthy, and then compare subsequent diagnostic checks to that value. I forget the SI number that references this.

Paul
Click to expand...
Of course when I read this my ADHD kicked in, and I had to try to find the SI. However, I was not successful and since I have a cup of Poly Tone gelling, I needed to move on. However, I did find the following excerpt in a 2004 AOPA article:
Direct compression tests

Ask any mechanic from the piston-airliner era and you'll find that direct compression tests were used extensively at that time. Thirty years ago, surplus catalogs sold matched sets of nine direct compression gauges in fitted metal boxes. But these tests have fallen out of favor as a diagnostic tool.

Direct compression tests are done with the engine at operating temperature. A spark plug is removed from each cylinder (remove the plug that permits the best access) and a pressure gauge adapter is screwed into each spark plug hole. Flexible hoses connect the fittings to a set of pressure gauges reading in pounds per square inch (a 300-psi gauge has sufficient capacity).

After gauges are installed on each cylinder, the starter is used to spin the engine until each cylinder has completed at least three compression strokes. The magnetos must be grounded during these tests to prevent magneto damage and inadvertent engine operation. Ground power should be used when doing this test to ensure maximum cranking power and accurate readings.

The maximum values will vary considerably according to the compression ratio of the engine.


Just for FYI, this is the link to Lycoming SI 1191A which details checking cylinder compression:
http://www.lycoming.com/Portals/0/t...I 1191A (09-28-1998)/Cylinder Compression.pdf
 
longranger said:
Absolutely correct. A "perfect" cylinder at 80/80 will have zero flow. I'm not sure what the flow is at 80/0, but as long as the inlet gauge stays at 80, you'll get a good reading.
Click to expand...

I see this conversation is pointless because it has fully drifted from the original correction that I and many others were trying to make (though for some reason only I am the one who is wrong:rolleyes:)

So, for the masses that are interested in learning about differential compression tests, the above post is not correct. If you see 80/80 on the tester, it does not mean you have a perfectly sealed cyl with zero flow (but it is better than most).
 
As you perform a differential compression leak-down check, if you initially set the supply pressure at 80 psi with the flow at Zero, (Output valve shut, and before hooking the hose up to the spark plug adapter), you should notice that once you start the leak-down test, the supply pressure will be slightly less than 80 lbs, DUE TO THE FLOW, AND LEAKAGE.

So, to get an accurate and repeatable number, you must adjust the regulator a bit to get 80 psi INPUT, as you do each cylinder. If a cylinder has a large leak, you will have to adjust the input psi regulator more turns to get the input psi up to 80 lbs. Each cylinder will be slightly different.

What you are adjusting for is the flow (volume of air moving).

It is not a sealed system, and 80/80 doesn't mean zero flow.
 
PCHunt said:
What you are adjusting for is the flow (volume of air moving).

It is not a sealed system, and 80/80 doesn't mean zero flow.
Click to expand...

Actually, what you are adjusting for is the inability of simple pneumatic pressure regulators to control pressure with zero "droop" under flowing conditions. Miles'(Longranger) previous post has the concept exactly right. The cylinder pressure test setup is a system that obeys conservation of mass principles. i.e. Mdot regulator = Mdot orifice = Mdot into cylinder = Mdot out of cylinder (assuming a leak free compression test or). In other words, the air leakage through the cylinder equals the air flow through the pressure tester orifice.

Please note that I am not implying that our cylinders can ever be absolutely leak free. I'm only saying that any cylinder leakage MUST result in pressure drop across the orifice...it's just too small to be measured by our small diameter 100psi gauges at low leakage rates.

Skylor
 
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