View Full Version : What is EGT?
01-01-2002, 04:53 PM
Could someone please explain to me what the EGT gauge shows, and why is it important in life?
01-01-2002, 05:05 PM
Exhaust Gas Temperature. One use is to set mixture with a constant speed prop, although not the best way. Set mixture for maximum EGT.
01-01-2002, 05:21 PM
Thanks, but why would one want to achieve the highest temperature of the exaust gas?
01-01-2002, 05:40 PM
It's the point of highest thermal efficiency. The fuel flow is lowest so you're going to get good hours out of your tank of gas. Watch the fuel flow gauge as you lean it out to raise the EGT...
Don't go too far leaning it out though. If you go too lean, the engine will start sputtering and losing power.
01-01-2002, 06:00 PM
EGT stands for Exhaust Gas Temperature. It is one of the most critical engine parameters.
In turbine engines (Turbojets, Turbofans, Turboprops for airplanes and Turboshafts for helicopters) it is a temperature taken aft of or just before the last turbine stage. Similar ITT or Inter Turbine Temperature is taken between the high pressure and low pressure turbine stages. Sometimes you can also find TIT or Turbine Inlet Temperature.(They all serve the same purpose. Only the location from where the temperature is taken are slightly different.)
Any of these are a limiting factor in operating a turbine engine. A specific temperature limit is set for each engine type by its manufacturer to prevent the engine from burning itself up. If you look at a typical EGT gauge you will find a green, a yellow and a red arc or red line. Obviousely the green arc is the safe range, the yellow arc the caution range and the red line is the limit.
Temperature limits must be observed by the pilot(s) during starting, acceleration, take-off power,climb thrust and maximum continuous thrust settings. Starting and acceleration limits are only momentary, while max. take-off EGT may be reached for up to five minutes. Climb and Max. continuous are for extended time periods.
In piston type engines EGT is used to lean the fuel/air mixture (usually set to slightly below the max. EGT)to obtain the best fuel to air ratio for maximum performance and best range (least fuel consumption).
Hope this helps.
01-01-2002, 06:23 PM
To add a point or two to the above posts, the correct ratio of fuel to air is very important to get the most power and lowest fuel burn from a piston engine. The temperature of the exhaust directly reflects the mixture settings, because the more efficient your mixture (therefore the closer to the proper ratio), the higher the temperature.
If you watch the EGT while slowly leaning the mixture from full rich, you'll see the EGT rise, peak out, and then, just before the engine starts to stumble from insufficient fuel, the temp will start to go down again.
Different carburetion and/or fuel injection systems will have some limited effects on exactly how the engine operates so Continental and Lycoming, for example, have slightly different recommendations on precisely where to set the EGT for best power or for best economy.
So while peak EGT is probably the closest to the proper ratio, 25º to 50º on the rich side of peak will give you best power (depending on the engine) and peak to 50º on the lean side of peak will give you best economy.
Since aircraft engines are typically air cooled, the fuel going into the engine and even the oil in the crankcase play a part in cooling, too. I mention this because full throttle typically injects a tad extra fuel for cooling purposes (I'm speaking of real aircraft now -- don't know what MS has done about modeling this), so this must be taken into consideration.
I hope the above helps you understand something about the real world after which FS is supposed to be modeled. In FS, I'd probably use peak EGT, unless I needed best power, then I'd go 50º rich of peak (if MS made that effective). In the real world, possible long-term (or maybe even short term) engine damage would make me go with the manufacturer's recommendations for that specific engine, generally found in the pilot's handbook for the aircraft.
01-02-2002, 02:56 AM
so what is the story on CHT then? My gauage does both and I spend all my time (so far) on EGT.
01-02-2002, 10:37 AM
CHT is cylinder head temperature. During different phases of flight (takeoff, climb, cruise, etc...)try opening and closing the cowl flaps (if equipped) and you'll see what effect they have on CHT.
I'm not a real pilot but I think the cowl flaps are there to help control CHT. I keep them at least partially, if not fully, open during climb. Closing the cowl flaps at cruise actually gives you a few extra knots. I'm assuming that's due to the lowered drag of having the cowl flaps closed and thus reducing drag.
Hope this helps.
Jeff S KDTW
01-02-2002, 10:57 AM
That was a great post--very clear and succinct. Thanks.
01-02-2002, 01:57 PM
Fatty, spot on as usual.
Cylinder Heat Temperature is normally only found in piston-engined aircraft (it can be found in rotaries too, but that's another story...) and is yet another `engine health indicator` to be monitored during all phases of flight.
Too hot - engine might explode, too cold - engine might explode.
At high power settings and lower airspeeds the CHT needs to be monitored as the airflow through and around the engine is not at its peak, so the engine can get hot. Very hot.
During cruising flight the CHT will generally not alter much unless the mixture is very badly out, and the EGT will give a clearer indication of that. But it still needs to be watched
However, altitude is another factor that can affect CHT. As aircraft climb higher and air density reduces, there comes a point where the CHT beomes more important than power or EGT settings (it varies, but around 18-20,000 feet is typical). Even if the engine is turbocharged and capable of reaching much greater power levels, that may not be permissible because to do so will exceed maximum CHT. Or you may need to open the cooling gills a little to improve cooling... Which sometimes robs you of more speed than a lower power setting with gills fully closed. Tricky stuff, this simming!
In descent, one may not simply wang the throttles back to idle and drop out of the sky making brick impressions, as this might cause the CHT to dive very suddenly, sufficient to cause what is know as `cold shock` - the engine seizes as the metal parts contract at different rates.
So CHT is usually monitored as part of the descent profile from high altitude, as you may not want to reduce the throttle too much in the early stages of descent. So you jiggle and juggle throttle, mixture and prop settings to find a happy medium.
Of course, none of this faffing about applies to gliders. Clean and simple gliders, fly like a bird etc. etc (cont'd p52)...
Hope this helps.
01-02-2002, 02:14 PM
Thanks all for the very informative posts. I can now more or less work out the gauges, but one very basic thing slips my mind, and sorry if this seems like a silly question: Why is it that high temperature indicates efficient fuel flow? I mean, why would the engine be less efficient if its exhaust gases were colder?
01-02-2002, 03:48 PM
Here's how I learned it and remember it. Fuel cools the engine (partially) because it is liquid and therefore absorbed and retains heat better than air or gasses. If it is not all burned in the combustion chamber (too rich a mixture), then some will escape into the exhaust and therefore cool the exhaust.
This will result in lower EGT. Therefore, you want all that fuel burned. Otherwise, you're just wasting the fuel to cool the exhaust. You need to adjust the mixture so that the proper ratio of fuel:air is obtained.
01-02-2002, 03:50 PM
Seems logical enough :) Thanks.
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