THE HEALTH OF YOUR ENGINE
The engine compression check or differential pressure test is integral to determining the health of an engine. Most people in the aviation community are familiar with the test but not many really know how or why it’s done. Here is a quick look at how a basic engine compression inspection is carried out and some follow up actions that maybe necessary as a result of the test.
The engine compression inspection checks the condition of the working parts in the combustion chamber of a cylinder. This is accomplished by measuring the static leak rate of the cylinder as compared to the leak rate through an orifice of specified size. This is done by attaching a differential compression measuring device, which incorporates the orifice, two pressure gauges, with a regulator to one spark plug hole of the cylinder under test while the piston is at top dead center of the compression stroke.
Lycoming recommends a compression check of the cylinders any time you encounter loss of power, increasing oil consumption, hard-starting, or evidence of unexplained abnormal operation, and to check the health of the engine at 100 hour or annual inspection intervals.
Before the inspection, the engine must be run until the cylinder head and oil temperatures are in the normal range. The test should be completed as soon as practical after shut down. A differential compression measuring device is attached to the cylinder via the spark plug hole after the piston is positioned at top dead center of the compression stroke. Shop air is supplied to the tester and the regulator is adjusted to 80 PSI as air is sent is to the cylinder through the tester. To assure that the piston rings are seated, the propeller is moved slightly back and forth with a rocking motion. The cylinder compression reading is taken from the gauge on the downstream side of the orifice and recorded as “X” psi over 80. Pressure readings for all cylinders should be nearly equal. Each engine manufacturer sets the limits for differential pressure and the appropriate service information should be referenced for this criteria.
Lycoming offers the following guidance. A difference of 5 psi between cylinders is satisfactory; a difference of 10 to 15 psi indicates an investigation should be made. Unless the pressure difference exceeds 15 psi the investigation does not necessarily mean removal of the cylinder. A follow-on compression check should be made within the next 10 hours of operation to determine the wear rate. If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by an appreciable decrease in cylinder pressure, removal and overhaul or replacement of the cylinder(s) should be considered.
YOU CAN SAVE COSTS
Failing to warm up the engine or to do the 10-hour recheck can be expensive. The cold engine shortcut might save time before the test but it may fail to properly identify a poorly operating cylinder. Conversely, often during the 10-hour operating period, valves reseat themselves and compression improves to acceptable levels, thereby saving the cost of a cylinder.
Questions, please contact:
Cessna Customer Care
316-517-5800 or 1-800-423-7762