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AIRCRAFT ENGINE
CLEARANCES AT LOW TEMPERATURES
Report on tests by Tanis
Aircraft Services
The coefficient of thermal
expansion of aluminum is approximately twice that
of steel or cast iron. Herein lies the source of a
problem for horizontally opposed piston engines.
The steel or cast components are supported in an
aluminum crankcase which "shrinks" at low
temperatures and "expands" at operating
temperatures. The cylinders are steel barrels with
tightly installed aluminum heads. These cylinders
"choke" at low temperatures and expand to a
straight bore at operating temperatures.
Aircraft engines turn over
with difficulty at low temperatures and most
popular thought explains this by saying the "oil is
stiff" --hence the difficulty with the engine. Some
failures have occurred in these engines which
showed signs of bearing failure and piston skirt
and top ring land scuffing.
We set out to find out what
actually happens in these engines at temperatures
in the -15 to -20 degree (Fahrenheit) range. Tests
were done in December of 1983 and January of 1984.
We checked the dimensions of several engine
components at room temperature and then again at
the low temperatures. We checked the following
parts:
Continental IO-520 crankcase
and bearings.
The temperatures of the
crankcases were determined by attaching a
thermocouple to the case "backbone" through-bolts,
which was connected to a digital instrument. Other
parts were allowed to cold-soak for several hours
alongside the crankcases and the crankcase readout
was used as their temperature. The parts were
allowed to soak at room temperature, and were then
remeasured. When comparing crankcase diameter
against crankshaft, note that different micrometers
were used. They were not calibrated against a
standard, but the same micrometer was used for both
temperature readings. The following results were
obtained:
The C-85 was assembled with a
piston and pin, with the rod-to-pin fit being
0.0014 inches loose. The combination was
cold-soaked at -15 degrees. The wristpin was found
to be locked firmly in the piston and the
pin-to-rod juncture was difficult to
move.
From this we conclude the
following:
With respect to crankcases,
the Continental and Lycoming showed the same
characteristics, although the Continental diameter
changed more. The crankshaft-to-main bearing
clearances may deteriorated to an unsafe condition
at these temperatures. The IO-520 lost 0.002 inches
bearing clearance, and the overhaul manual lists a
0.0018 fit as minimum for a new engine. This would
result in an interference fit. We would expect the
Lycoming engine to do the same thing, since
Lycoming lists a 0.0015 minimum fit for a new
engine.
It's ironic that this
indicates a brand new engine, assembled as tightly
as permitted, would suffer the most from the
effects of extreme cold, as compared to an engine
nearing TBO. And an overhauled engine assembled
with a wider bearing clearance would possibly
shrink to less than the "minimum new" clearances by
being exposed to these temperatures.
The Lycoming cylinder at room
temperature had a 0.003 choke, which increased to
0.013 at -15 degrees. When the cylinder was warmed
by use of a "preheater", the choke disappeared
completely at about 120 to 140 degrees. The
lycoming piston lost 0.004 diameter on its top ring
land at -20 degrees, while the skirt changed only
0.002. By comparing the piston and cylinder, one
can see that the choke increases more than the
piston diameter decreases, resulting in the piston
being forced into a smaller bore as the engine is
turned over while cold.
Another problem not well
known can be seen in the test of the C-85 rod,
piston and pin. The "small end" of the rod lost
0.0013 at -20 degrees, while the wristpin lost
less. The result was that the rod-piston juncture
was tight enough to cause scuffing of the piston
skirt and top ring land.
From these tests we conclude
that damage may result to an engine merely by
pulling it through to "free it up" at low
temperature.
Moreover, we believe there
should be some standards for temperatures on
preheated engines before starting. These should
include crankcase temperature and cylinder head
temperature, as well as oil temperature. We believe
these tests should be done on installed engine
propeller combinations, due to large heat losses
through the metal propeller.
Other areas not addressed,
but which we believe are significant:
Is congealed oil under piston
rings holding the ring out of the groove when the
piston is at the bottom of the stroke?
What is the amount of oil
pressure necessary to force congealed oil through
the passages of the crankshaft at low
temperatures?
What is the fit of lifter
bodies in the crankcase and what are the resulting
forces on the camshaft?
What are the internal
conditions of bearing fit and lubrication of
accessories, such as propeller
governors?
What is the proper
temperature of oil in the sump to allow flow
through the suction screen to the pump?
Picture Below: Crankcase
Measurements being taken after cold-soaking of case
and crankshaft.
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