Compression testing hand start engines

[slippedthebonds]

On hand started engines like the VW on my RF4, how do you do a compression test? I understand how to on other engines but how do you turn the engine over fast enough to get an accurate reading on the test gauge?

[ferrydude]

Volkswagen powered RF4? They told us they were J-79 powered:eek:

I'd be doing a differential compression test rather than a direct one.

No need to turn over and you'll have more usable data.

[SNS3Guppy]

Are you talking about performing a compression test on a car, or an airplane?

In aircraft, we generally perform a differential compression test using an external pressure source and a calibrated tester. This is generally done by a certificated aircraft mechanic (or for some, "engineer.").

[Karl Bamforth]

Although you say you know how to do a compression test, you clearly have no idea how it is done on aircraft.

It can be dangerous, find an engineer to do it for you.

[FullWings]

On a car, you'd do it by putting a compression tester in place of a spark plug, taking the fuse out of the fuel pump circuit, then cranking the engine to get a reading. Repeat for however many cylinders and compare with the manual.

With an aeroplane engine, I'd have thought it would have been possible to use a similar procedure, not having done it myself? The speed the engine is turned over by the starter should be adequate for a fairly accurate reading? If not, then there's a big problem somewhere in that cylinder! Maybe everyone uses differential testing - don't know.

[Mark 1]

As was said above. The standard test is to use a differential pressure tester.
You also need a compressor capable of maintaining 80psi in the cylinder being tested.

It works by detecting the leakage rate from the cylinder. The air passes through a 0.040" diameter orifice before going to the cylinder being tested.
If the pressure downstream of the orifice is the same as the upstream pressure it means that the leakage effective area is less than the orifice size.
If the downstream pressure is lower, then the orifice is choked and the pressure difference is related to the leakage flow rate.

If you get a low reading, it may be possible to diagnose the leak as the air is likely to be flowing either past the rings to the crankcase breather or past the valves to the exhaust or intake manifolds.

Precautions apply like removing all other spark plugs and not standing in the prop arc. Somebody has to hold the prop at the TDC position while under pressure, which means it will want to turn if moved slightly in either direction.

The engine should ideally have just been run so that it is near working temperature.

Generally, pressures dropping down near 60/80 over a couple of test cycles may be cause for further investigation.

[SNS3Guppy]

With an aeroplane engine, I'd have thought it would have been possible to use a similar procedure, not having done it myself? The speed the engine is turned over by the starter should be adequate for a fairly accurate reading? If not, then there's a big problem somewhere in that cylinder! Maybe everyone uses differential testing - don't know.

Differential compression testing is often viewed by the layman as meaningful information, much the same way many people think they can tell something by viewing one or two blood pressure readings.

Compression readings will change with repeat testing, with engine temperature variations, with the test unit, and with the person performing the test. Differential values can be artificially increased, or decreased by the person performing the test.

The chief value of performing a compression test is not the compression reading that's generated, but using the test to determine leakage sources. Usually simply by rotating the propeller a few degrees each way from top dead center while the cylinder is under pressure, and listening at the exhuast and induction inlet for airflow.

Differential compression testing involves putting pressure into a cylinder, traditionally 80 psi for no particular reason, and determining how much pressure the cylinder will maintain based on a calibrated rate of flow through the test unit. Typical values for most light piston manufacturers are 75% of input value being the recommended limit prior to considering overhaul of the cylinder. It's not uncommon when reviewing a history of compression tests to see values rise and fall and rise again. The specific numbers don't mean much.

Performing a compression test is a maintenance function. Failure to control the propeller can result in serious personal injury. With a cylinder under pressure, the propeller can easily move rapidly and hurt the person performing the test, or an assistant. Speed of the engine is irrelevant, because the test isn't performed with the engine being cranked; it's performed with the propeller being moved by hand, and then only back and forth through a few degrees of arc at top dead center on the compression stroke.

[Pilot DAR]

Guppy has given an excellent description of the process, and presented an excellent reminder of why aircraft maintenance is best performed either by qualified maintainers, or at least under their direct supervision. Nobody discourages learning, but this particular test can be very dangerous, as has been pointed out. Though as I think about it, working close enough to perform a compression test with a propeller spinning right there does not sound so safe either!

Guppy and the others have presented enough information that you can be an informed observer. Observe away....

Pilot DAR

[slippedthebonds]

Thanks to everyone who answered. Maybe I was expecting something different on this particular subforum.

Yes I do know how to do one.

Yes I can maintain my own aircraft under LAA rules and I do so and have done so for several years.

Yes I do understand the differences between the various types of test.

Yes I do operate under the full guidance of a licenced aircraft engineer. He's now told me how to do it anyway and it's really straightforward and not dangerous at all so if anyone is interested in the engineering rather than the philosophy of it then happy to share info if you drop me a line.

Cheers

STB

[Capot]

STB

Pity the "experts" who leapt in to patronise you, with your perfectly sensible question, from their high ground of pomposity didn't know what an RF4 with a VW engine is, isn't it?!

Bit of egg being wiped off some faces.....

[SNS3Guppy]

Who leapt in to patronize? The man didn't offer much of an explaination. Clarity was sought, and good counsel was provided all around. For one who has maintained his airplane for three years, but has "just been told" how to do this check...it still leaves questions. As for the attitude of "if anyone is interested in the engineering rather than the philosophy of it then happy to share info if you drop me a line" from one who has just been told how to do it...that's arrogance.

Maintenance is a serious profession. Normally one is grateful for counsel provided by serious professionals. Counsel has been provided. Enough said.