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In order to allow for expansion of the piston
in a two cylinder air cooled engine the fit between the piston and the cylinder
wall is such that it allows for this expansion. As the
piston and cylinder heat under
normal operating conditions they expand, the piston
becomes "larger" while at the same time the cylinder walls expands
making the bore smaller. Engine manufactures allow for this
"expansion" and "contraction" when they produce the engine
Normally a Rotax air cooled
engine should read between
350F to 375F on a cylinder head temperature gauge (CHT) . When an engine overheats the amount
of expansion between these parts is greater than allowed
- the
piston becomes too tight in the cylinder and the piston "seizes' creating permanent damage to the piston
and or cylinder wall.
A properly installed CHT gauge
can alert the pilot to potential problems that may lead to
engine seizure.
CHT
temperatures in excess of 40OF to 425F are a result of
1. Lack of oil in fuel or NO oil in the fuel.
2.
Poor piston cylinder lubrication, due to poor quality of
oil. For example using 10W30 motor oil instead of 2 stroke
oil.
4. The use of a fan belt
other than that supplied by rotax, a loose, or damaged
fan belt.
5. A blocked fan intake
duct, or blocked cylinder head fins. (for example from a
mouse nest).
6. Insufficient cooling due to
air intake design. For example the fan being mounted
right up against the firewall.
7. Pre-ignition
or detonation, caused by old fuel, improper octane,
ignition timing.
How
does a CHT work ? -
A CHT gauge uses a
Type K thermocouple, which requires no power.
The
thermocouple produces a small voltage which
to work properly requires good continuity to reach
the
instrument
The thermocouple, which is
available in several different sizes depending on the spark
plug used - is mounted under the spark
plug, and replaces the spark plug washer. All of
the Rotax two cylinder aircraft
engines use a 14mm probe.
On all two cylinder Rotax
aircraft engines I would suggest a CHT for EACH cylinder to
properly monitor the engine temperature.
On water cooled engines -
532/582 the cylinder head temperature will read about 200
degrees F., during normal operation. This is because the
cylinder head is liquid cooled, and the probe is being
effected by the additional cooling supplied by the liquid
coolant.
You
might want to check out gauging your engine!
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