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Home> Maintaining
your Continental and Lycoming engine
Aircraft Cylinder Barrel Temperature Requirements
There are no cockpit temperature instruments that tell you if your cylinder
barrel cooling system is operating properly. The CHT gauge measures cylinder
head temperature and is not in the heat path from the piston; to rings; to
barrel; to atmosphere.
It is possible to diagnose
cooling system problems by analyzing cylinder barrel wear patterns. Although
this is after the damage and may appear too late, it will prevent
repetitive repairs by fixing the damage and correcting the problem.
Cylinder Barrel cooling system must meet two requirements:
1. Conduct enough heat to the atmosphere so that piston temperatures are
below the oxidation point of the oil film and not so high that the piston:
- looses strength
- oxidize oil in the piston ring groove causing stuck and
broken piston rings
- suffers excess thermal expansion and seizes in the
cylinder barrel
- or develops hot spots that cause
preignition.
2. Temperature is evenly distributed around barrel circumference so that
barrel retains shape and does not suffer from uneven circumferential stress (Hoop
Stress)
and poor piston ring contact. See Circumferential cylinder
head cracks for what can happen if the barrel is cooled unevenly. Achieving
an even temperature distribution around the barrel is the function of the
cylinder baffles and is not an easy task. Using a simple baffle
arrangement, cylinder barrel temperature increases from the baffle
entrance to the baffle exit as the air going thru the baffle heats up.
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Chrome cylinder
barrel surface |
Piston Dome is a large heat absorbing surface
Heat Path - how the piston cools itself
Piston Dome > Piston Rings > Cylinder
oil film > Cylinder barrel > atmosphere
Since combustion temperatures exceed the piston's melting point,
the heat energy in the piston is absorbed by the rings. The rings then
transfer this heat into the cylinder walls and eventually into the
surrounding air.
Blowby of combustion gas past the ring belt ("ring
leakage") prevents heat flow from rings to barrel. Piston
temperature increases causing oil in ring lands to oxidize, thicken
and eventually causing ring sticking. Marvel Mystery Oil may un-stick the
rings but it does not correct the underlying problem of blow-by. |
The Lycoming O-235L2C engine installed in the Cessna 152 offers some
excellent examples of problems caused by high cylinder barrel temperatures and
uneven temperatures. Shortly after its introduction we started receiving
cylinders for repair due to piston ring leakage and piston ring seating. A few strokes of the cylinder
hone showed a depression in the cylinder barrel about the size of a quarter.
Before honing this depression was harder to see but was visible as a patch of
light colored glaze where the oil oxidized on the cylinder barrel.
If you honed the cylinder the depression remains and the problem quickly
returns as the barrel is the victim. We were sometimes told that the barrel
dimensions measured OK and the surface just needed honing. This is not correct.
One quarter size depression in an otherwise good barrel is enough to cause
problems. The barrel must not only measure within dimensions but it must also be
free from ripples, depressions, waves, and distortions. Such a barrel will
usually have to be bored to an oversize dimension to remove the
distortion.
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steel cylinder barrel surface
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Cylinder barrel distortion
abnormal common
Shinny - high spots that have worn
Scratched - low spots showing no wear. Scratched area is
original hone pattern that remains untouched by piston and rings.
Cylinder barrel distortion prevents piston rings from
making good contact with barrel. This creates combustion gas leakage past
rings, and prevents the piston from conducting heat to the cylinder
barrel. Distortion can be caused by uneven temperature
distribution around the barrel circumference or by a slightly bent or
warped cylinder hold-down flange. Sometimes it is necessary to stroke the cylinder barrel one or two times with
a hone to expose the high and low spots in the barrel such as we have done below
with the picture of eyebrow wear. If piston ring gaps are lined
up this could be caused by barrel distortion. The compression ring ends
exert a greater force against the cylinder at the area on both sides of
the gap than other portions of the ring. If the cylinder bore is
distorted the gap of the ring will seek out and stop in the largest
part of the bore.
A. Wear mark from piston pin plug
B. Area of no piston ring contact
C. Area of piston ring contact
large area of barrel where piston rings did not touch
indicates barrel is not round. Possible causes:
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steel cylinder barrel surface |
Eyebrow wear at combustion end of barrel
common
Rather normal wear at top of cylinder barrel. This type of
wear occurs when the piston stops at the top of its travel. The ring
looses its hydrodynamic lubrication as the oil wedge beneath the piston
ring squeezes out allowing ring to barrel contact. This type of wear is
not caused by barrel distortion.
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Cylinder head failure possibly caused by Hoop
Stress
abnormal rare
Missing or damaged cylinder baffling causing uneven air
flow around cylinder barrel expands barrel unevenly resulting in cylinder head stress at the attachment point
of the barrel to head. Reference Cessna Service Letter 64-32,
Beech Service Letter 6 4-20, or Continental Service Bulletin M65-13.
Beech solved the problem by modifying the cylinder barrel
baffling to improve heat distribution around the barrel. There may be many
installations where the Hoop Stress is not severe enough to cause the head
to blow off the cylinder but may cause repetitive piston ring problems. Nitride
hardening the cylinder barrel surface (along with providing a harder wear
surface) puts the barrel surface in compressive residual stress thereby
strengthening the barrel by counteracting tensile forces. Cannon
barrels (not too far different than a cylinder barrel) are similarly
strengthened by compressing the surface thru a process known as
Autofrettage.
Some cheap PMA'd cylinders are not nitride hardened. |
Stuck piston rings, low compression, and excessive oil consumption needs to
be fixed, but often these are symptoms of another underlying problem. Fix the
damage doesn't always fix the problem. A few minutes spent looking at the
cylinders may give some clues to what may have caused the damage.
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steel cylinder barrel surface
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Electronic Ignition Users
Piston temperatures increase rapidly with spark
advance. Increased cylinder barrel cooling may be required on some
engines. An examination of oil staining on the underside of the
piston can be used to determine if piston temperatures are normal
or not.
Normal oil staining on underside of piston. No
blow-by of combustion gas past ring belt, cylinder barrel cooling
system operating normally, no preignition. |
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Cylinder Bore Polishing
abnormal common
Something to be avoided. When you boroscope a cylinder
barrel and the surface looks like a mirror you also find oil past the
piston rings. Start budgeting for a cylinder repair.
Cylinder bore polishing is common but not normal
. Although cylinder barrel
dimensions are still well within limits, without the hone pattern the
cylinder barrel is not wetable and oil ends up in the combustion chamber.
Noticed as oil fouled spark plugs and greater than normal oil consumption.
causes:
Not enough oil metering to the cylinder wall for the cylinder
barrel temperature
Oil metering past the piston ring belt must be sufficient
to establish an oil film under the most severe conditions. Not all engine
installations or environments are the same. Aircraft that have higher
cylinder barrel temperatures require more oil on the cylinder barrel. This
requires that the engineer have a knowledge of the engine environment past
the test cell and reduce ring tension for those hotter running engines.
repetitive micro-rust films
When the aircraft is parked a very thin oxide layer forms
on the cylinder wall and then is wiped off by the piston rings when the
engine starts. Over many years this polishes out the cylinder barrel. It
may be detected as slightly elevated iron levels in an oil analysis.
extremely small wear particles
Wear particles whose diameter is less than the oil
film separating parts cannot cause wear. For this reason tiny wear
particles small enough to pass thru the oil filter are considered benign.
This is the popular notion.
However, on the cylinder wall under high temperature conditions the piston
rings contact the cylinder barrel. Small abrasive particles can combine to
form sludge and act as a rubbing compound to polish out the cylinder barrel.
Overuse of anti-adhesion additives
Anti-adhesive additives such as Lycoming's LW-16702 Tri-Cresyl Phosphate work
by creating a extremely thin, invisible oxide film (similar to what occurs
on aluminum or passivated stainless steel). This protective chemical film
is not a lubricant film. When the oil film
ruptures and metal to metal contact occurs this oxide film prevents the two
surfaces from adhering and galling. This is important for gear teeth and
camshaft lobes but the constant creation and rupture of oxide films on the
cylinder wall creates bore polishing. This additive
is also used in Shell Plus, Shell Multi-Grade 25W-50 and Exxon Elite oil Fuel washing
oil from the cylinder wall Overprime, rich running, and
using heavy weight oils in cold weather combine to wash oil from the
cylinder wall. |
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steel cylinder barrel surface. To get a good look at the
cylinder barrel place a white piece of paper as shown. |
Cylinder "bluing"
abnormal common on 520's
Cylinder "bluing" is an oxide film created by high cylinder
wall temperatures. Always associated with bore polishing as the oil film
provides insufficient lubrication at these high temperatures. Either lower
cylinder barrel temperature or increase oil metering to the cylinder wall. |
Is it possible to use differences in CHT change when
leaning to find engine baffling problems?
You lean the engine and he CHT changes as expected. But why does
the CHT on some cylinders change more than others?
One reason will be that each cylinder is operating on a
different point on the mixture ratio graph. Unless each cylinder is operating at
the same mixture ratio, to use our method described below the data needs to be
normalized so that each cylinder is at the same mixture ratio point.
Most publications show how the CHT changes as the fuel/air mixture is leaned
or richened. Also of interest is why some cylinders may experience a
greater or lesser temperature change than others. Extending the idea further, why
might one engine produce a greater change in CHT than the same engine in a
different airplane?
"A cylinder which is shielded from the air blast or improperly air
cooled will have a much greater rise in temperature due to leaning of the
mixture than one which is properly cooled." An old-old study N.A.C.A.
Technical Note No. 388
Take a rich running engine for example; fuel is used to cool the cylinder. We
also have the airflow (and some oil cooling) that also cools the cylinder. But,
as we lean the engine we take away fuel's role in cooling the cylinder. We are
left with air and oil cooling. Changes in air flow has a greater impact on CHT
temperatures on a lean engine.
Within some rich to lean range those cylinders (or engines) that are
not as well air-cooled will experience a greater CHT change. For the first time
we now have recording instruments in many cockpits that can capture this data.
Can we deduce cowling or baffling problems by comparing the amount of change?
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