Cht

[ryan_vd]

Could someone please explain to me the importance of the cylinder head temperature gauge? Also is it a fundermental instrument to have, i mean to say, could you fly without it working?

Ryan

[Flyin'Dutch']

Dunno over there in Oz but most 'simple' piston GA aircraft fly over here without one.

FD

[FlyingForFun]

We are taught to lean the engine during the cruise. We are also taught that if we over-lean, the engine can overheat. The engine is specifically prone to overheating during the climb because of the high power and low airspeed (which is why we always select the mixture to full rich before climbing).

What we are actually talking about when we use the word "overheat" in this sense is the cylinder head temperature being too high. Therefore, it follows that if we have a CHT guage, we can use it to lean the engine far more accurately - which probably means leaning more aggressively (since we can be more certain that we haven't over-leaned) and therefore saving fuel.

Of course, each cylinder will be at a slightly different temperature because of the way the cooling air flows around inside the cowling. Some designers will decide which cylinder they think should be the hottest, and put the guage on that cylinder. Those designers who think their customers have lots of money will put a guage onto each cylinder so that the pilot can choose to look at the hottest cylinder himself when leaning.

That's a basic overview, but since no one has ever trusted me with an aircraft with such a guage, and I can't afford to pay to fly one myself, I'll leave the details for someone who understands them! Just one thing to add, though, which is to not confuse the CHT with the EGT. Both of them can be used to lean the engine, but the technique is different. And EGT guages are much cheaper (but, in my experience, rarely work).

FFF
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[ROB-x38]

In the aircraft I'm familiar with it isn't a procedure to lean based on CHT readings. Most flight manuals dictate mixture settings according to either fuel flow for a given power setting or a mixture setting which gives either peak EGT, 50 degrees rich, 125 degrees rich etc... (have a look at the J. Deakin articles to make up your own mind whether this is really the best method).

The EGT gauge gives a much more immediate indication of what is actually happening during combustion (heating a volume of air compared to a volume of metal).

During a sustained climb CHTs should be monitored, and if approaching the upper limits perhaps adopt a higher climb speed to get more air flowing through the engine. As well as adjusting airspeed cowl flaps are used to maintain acceptable CHTs and are typically open on the ground, during takeoff and climb. I usually close the cowl flaps approaching top of climb providing CHTs are at acceptable levels.

During cruise CHTs should also be monitored and power settings or cowl flaps adjusted if they start getting too high (typically around 500 degrees F).

[ACW 335]

There isnt a CHT gauge in my normal aircraft, but in the Grob 109B there is, and to shut down in flight; the temp. must be below 150degreesC. This used to be the case for engine shut down on the ground but has recently been ammended.

Have no idea why! Anyone care to expand?

[Mark 1]

The hottest lump of metal in your engine is probably around the exhaust port of one of the back cylinders. By having the CHT gauge(s) close to this point you can get a fair idea of how quickly you are heating or cooling the engine so avoiding distortion and cracking due to high temperatures and high rates of expansion/contraction.

By getting to 150C before shutting down you are limiting the risk of damaging the engine. Of course, its not the actual temperature that's important in this case but the difference between CHT and OAT.

[IO540]

ryan_vd

This is a subject which tends to result in a religious debate which runs on and on.

But in a nutshell, a CHT gauge is used to check that the engine is not getting too hot. This can occur during the following

In climb (high power but airflow around the engine too low),

During deliberate slow flight (high power because one is on the back of the power curve, but again airflow too low)

Engine mismanaged (detonation or even pre-ignition)

If the cylinder head gets too hot, the metal (aluminium usually) gets weak and will crack. Most likely, other bits will also be too hot.

One can also use a CHT gauge to lean the engine correctly but an EGT gauge is much more accurate and responsive for doing that. Especially if it is a multi-cylinder display e.g. EDM700. You can look at www.jpinstruments.com for some reading.

[ryan_vd]

thx guys for ya replies, appreciated!

[dirkdj]

The CHT is actually the most important engine instrument.

It is a direct reflection of the temperatures of the exhaust valves. At the highest CHT the exhaust valves will be at their hottest, not at the highest EGT however strange this may seem.

Advantage of the EGT is that it reacts in seconds, not minutes.

It is a pity most aircraft are not equipped with proper multi-cylinder instruments and properly trained pilots, .

Keeping CHTs below 400°F will go a long way to TBO.

[deing]

My plane has CHT & EGT probes for all cylinders. CHT's never go very high, max about 400°, however when leaning for the best fuelflow the EGT goes up quickly and its even possible to exceed the max 1650°F (IO360). Is it OK to cruise on that high EGTs?

Deing

[dirkdj]

Deing,

You should not worry about high EGT limits, there is no limit on EGT on normally aspirated engines. What counts is the CHT several minutes later, 400°F is a limit for long engine life, anything above 420°F needs immediate action.

The best use of EGT indication is to establish your location on the combustion curve in order to finetune your engine parameters (mixture being the most powerful and least understood engine control in the aircraft).

This being said, the only time I would want to see EGTs in the +1500 range is during very high power cruise, certainly not during take off and climb. During takeoff and full rich climb you want to see EGTs around 1200-1300 if not ask the shop to enrichen the full rich setting on the engine. Most TCMs come from the factory with too little fuel flow for proper full rich operation.