Back to basics
By Joe Escobar
Compression testing can be an effective tool for monitoring engine condition. Despite the apparent simplicity of the test, it can be a valuable resource to determine engine wear and internal damage at an early stage, thus avoiding potentially catastrophic failures. But care must be taken to ensure the test is performed properly in order to avoid unnecessary maintenance actions to a perfectly good cylinder. In this article, we will go back to the basics of engine compression testing.
Compression testing is basically a test to determine the amount of leakage past the piston rings and valves in an engine cylinder. The type of compression test used for aircraft engines is a differential compression test. This is somewhat different from a direct compression test commonly used in testing automotive engines.
Direct compression testing. In a direct compression test, a pressure gauge is installed into the spark plug boss of the engine. The engine is then turned over and the peak pressure buildup in the cylinder is determined. This type of compression test can yield varying results and depends on many variables to be reliable.
Differential compression testing. Differential compression testing is a more reliable method of testing engine compression. The test uses two gauges to measure pressure. Compressed air is applied to the tester through the regulator gauge. This gauge indicates the air pressure being applied to the tester. The air then goes through a calibrated restrictor to the cylinder pressure gauge. This gauge indicates the actual pressure in the cylinder. Any compression loss in the cylinder would be indicated by a pressure indication in the cylinder pressure gauge lower than that of the regulator gauge.
Testing the tester
In differential compression testing, the tester is a critical element to ensure accurate test results are obtained. Needless to say, there are a few things we should keep in mind in regards to the tester to ensure it will give us the precise measurements we need.
Dead-end check. A good way to test your differential tester for proper operation is to "dead-end" it. You basically cap off the end of the tester that would normally go into the cylinder and apply regulated air pressure to the tester. You want to ensure that both the pressure regulator gauge and the cylinder pressure gauge stabilize out at the same pressure reading. Any difference in pressure indication could mean leaks in the tester or defective gauges. The tester can then be repaired as necessary.
Your tester could have a valve installed between the cylinder pressure gauge and the cylinder. You should close this valve in order to perform the check. Keep in mind, this is another area that can leak causing a difference in gauge readings during the unit test. The valve should be eliminated as a leak source before looking at the gauges for leaks or inaccuracy.
Another factor that can affect the accuracy of your tester is debris. Any contaminants such as dirt and oil can adversely affect the readings. The tester should be treated as a precision measuring device, not as just a hand tool. It should be kept clean at all times to help ensure accuracy of tests.
An important thing to keep in mind is to ensure you are using the proper orifice in your tester for the engine being inspected. As a general guideline, AC 41.13-1B states that the restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:
- Engines up to 1,000-cubic-inch displacement: 0.040-inch orifice diameter, 0.250 inch long, 60-degree approach angle.
- Engines in excess of 1,000-cubic-inch displacement: 0.060-inch orifice diameter, 0.250 inch long, 60-degree approach angle.
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