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Reciprocating Engine Differential Pressure Compression Check
The differential pressure test is designed to check the compression of reciprocating engines by measuring the leakage through the cylinders caused by worn or damaged components. The operation of the compression tester is based on the principle that, for any given airflow through a fixed orifice, a constant pressure drop across that orifice will result. The restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:
The orifice size is selected on the following:
- 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.
- As the regulated air pressure is applied
to one side of the restrictor orifice with
the air valve closed, there will be no leakage
on the other side of the orifice and both pressure
gauges will read the same. However,
when the air valve is opened and leakage
through the cylinder increases, the cylinder
pressure gauge will record a proportionally
- While performing the check the
following procedures are listed to outline the
principles involved, and are intended to supplement
the manufacturer’s instructions for the
particular tester being used.
- Perform the compression test as
soon as possible after the engine is shut down
to ensure that the piston rings, cylinder walls,
and other engine parts are well-lubricated.
- Remove the most accessible
spark plug from each cylinder.
- With the air valve closed, apply
an external source of clean air (approximately
100 to 120 psi) to the tester.
- Install an adapter in the spark
plug bushing and connect the compression
tester to the cylinder.
- Adjust the pressure regulator to
obtain a reading of 20 psi on the regulator
pressure gauge. At this time, the cylinder
pressure gauge should also register 20 psi.
- Turn the crankshaft, by hand, in
the direction of rotation until the piston (in the
cylinder being checked) is coming up on its
compression stroke. Slowly open the air valve
and pressurize the cylinder to 80 psi.
- CAUTION: Care must be exercised
in opening the air valve since sufficient
air pressure will have built up in
the cylinder to cause it to rapidly rotate
the propeller if the piston is not at
top dead center (TDC).
- Continue rotating the engine
against this pressure until the piston reaches
TDC. Reaching TDC is indicated by a flat
spot or sudden decrease in force required to
turn the crankshaft. If the crankshaft is rotated
too far, back up at least one-half revolution
and start over again to eliminate the effect of
backlash in the valve operating mechanism and
to keep piston rings seated on the lower ring
- Open the air valve completely.
Check the regulated pressure and readjust, if
necessary, to read 80 psi.
- Observe the pressure indication of
the cylinder pressure gauge. The difference
between this pressure and the pressure shown
by the regulator pressure gauge is the amount
of leakage through the cylinder. A loss in excess
of 25 percent of the input air pressure is
cause to suspect the cylinder of being defective;
however, recheck the readings after operating
the engine for at least 3 minutes to allow
for sealing of the rings with oil.
- If leakage is still occurring after a
recheck, it may be possible to correct a low
reading. This is accomplished by placing a fiber
drift on the rocker arm directly over the
valve stem and tapping the drift several times
with a hammer to dislodge any foreign material
between the valve face and seat.
- When correcting a low reading
in this manner, rotate the propeller
so the piston will not be at TDC.
This is necessary to prevent the valve
from striking the top of the piston in
some engines. Rotate the engine before
rechecking compression to reseat
the valves in the normal manner.
A typical schematic diagram of the
differential pressure tester