Compression testing is a way to determine engine health. We use differential compression testing in aviation because it is a more reliable method of testing engine compression than direct compression testing used in automotive applications. We will take a look at the basics of differential compression testing and share some tips on getting the most out of your tests.
Differential testing 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. 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.
Engine manufacturers may have other requirements for their engines. Be sure that you consult the manufacturer’s maintenance manuals and service bulletins for specific testing requirements.
You want to make sure you have a source of dry compressed air capable of providing a minimum line pressure of 125 psi with a minimum flow capability of 15 cubic feet per minute. In addition, you need to have a differential tester. Refer to the engine manufacturer for approved testers. In addition, for Teledyne Continental engines, depending on the tester you are using, you may also need a master orifice tool P/N 646953A to determine the Minimum Acceptable Leak Limit for your engine.
Warm Up That Engine
It is recommended that the compression test be performed on a hot engine. This ensures that the piston rings, cylinder walls, and other engine parts are well-lubricated and at operating clearance. “Before doing a compression check, make sure to run up the engine so that it gets up to normal operating temperatures,” says David Noe, airframe shop manager for Poplar Grove, IL-based Poplar Grove Airmotive.
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