CHT, cylinder head temperature gauges, or CHT's.


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Cylinder Head Temperature Gauges

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Experience has shown that CHT's on two stroke engines, can help prevent an engine out.  Twice now the use of CHT gauges has aided me in the prevention of an engine failure while flying.

In the first case while flying a Quicksilver MX powered by a 503 Rotax engine, I noticed my CHT temperatures climbing from the 350 degree F it normally sat at to over 400 degrees.  Using the gauge as a guide I was able bring the power back, let the engine cool down.  Then apply full power for short bursts until the engine temperature climbed again. Using this process I was able to get the plane back to the airfield, which was about 20 minutes away, without any damage to engine or plane. Inspection revealed that the fan belt had failed.

In the second instance, the CHT gauge which normally sat around 200 degrees F on my 582, started to increase, (on a liquid cooled engine the CHT reading is effected by the water circulating through the cylinder head, lowering the readings), and then suddenly decreased.  On landing inspection revealed I was low on coolant.

This shows up on the CHT because when you are loosing coolant, or are low on coolant the CHT temps will start to climb since the water temperature is increasing, not providing as much cooling effect to the cylinder heads.

This off course also should show up on a water temperature gauge, which all liquid cooled Rotax engines should have.

If you install a CHT you require one for each cylinder, and it is necessary to remove the spark plug ring when installing the CHT sending unit. Failure to remove the spark plug ring may result in spark plug leakage due to improper sealing, an improper temperature reading, because of two spacers between the plug and cylinder head instead of one, and can result in damage to the threads of the spark plug hole. 

With the spark plug sealing ring in place and the addition of the CHT coupler threads in lower part of the cylinder head are exposed, because the spark plug is spaced too far out of the cylinder head, this will allow carbon to build up on the spark plug hole threads. Resulting in damage to the threads if a new plug is put in place without a CHT coupler, since the spark plug can now go all the way down into these carboned threads, usually breaking them off and depositing them into the cylinder.

The CHT gauge's sensor takes the place of the washer under the spark plug, which is taking it's temperature reading from one of the hottest spots on the engine. This ring is a thermocouple which converts sensor heat into an electrical impulse which it sends to the gauge. Rotax engines should run between 350 and 400 degrees F  indicated on a CHT gauge.

While most gauges come with only a short lead, these can be extended using #20 or #22 copper wire.  (Never cut the thermocouple wire -  it is part of the circuit). CHT gauges are easy to mount requiring no modification to the engine, and standard gauges require no outside electrical power.

Problems -
When installing the CHT  sensor on the Rotax air-cooled engines it is difficult to get the sensor to fit into the cylinder head.
The CHT sensor can also become a problem when trying to remove or install the spark plugs.
The sensor has a tendency to crack or break right at the base of the unit. It is suggested that "heat shrink" be installed over the cable end up to the base of the sending unit.
The sensor cable wears due to vibration, against the engine. Running the sensor cable inside gas line with a series of stand offs helps prevent this wear.
For CHT gauge wiring information click here.

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