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jtpitkin06
06-14-2011, 09:15 PM
EGT info

EGT is one of the largely misunderstood subjects in aviation. It is not limited to Rotax engines.

Exhaust gas temperature is just that. Exhaust temperature. It indicates how the fuel is burning but does not tell you anything about the temperatures or pressures inside the cylinder. It does not tell you if you are stressing the engine.

To understand what EGT is telling you, we start with combustion in the cylinder.

EGT is a relative reading only. Not an absolute. There is no red line for EGT. If the fuel is completely burned in the cylinder, EGT will be lower. If the fuel is partially burned in the cylinder and continues to burn in the exhaust, EGT will be higher. How do we know this? Do a mag check.

With two spark plugs per cylinder operating you have better combustion than with one. With the engine running on dual ignition, note the EGT. Now switch to single ignition. On a single plug the EGT goes up. Gasses are burning in the exhaust. Switch back to both and the EGT goes down. The dual ignition burns more fuel in the cylinder. With fewer burning gasses in the exhaust the EGT is lower.

If the load is increased on an engine, such as opening the throttle, entering a climb or increasing prop pitch, EGT will go down. That’s because increasing load makes the internal cylinder pressures go up. When pressures in the cylinder go up, the flame in the cylinder burns faster. (It doesn’t explode.) With more fuel burning in the cylinder, less is burned in the exhaust and you see a drop in EGT.

But what should I do about high EGT?

For a moment let’s take the EGT out of the airplane. You are flying along in a climb and you see cylinder head temp is higher than normal, indicating you may be getting detonation. What should be your reaction? Normally you would lower the nose, reduce manifold pressure, increase prop rpm if controllable and enrich the mixture. Would any pilot seeing high cylinder head temperature think they should increase the load on the engine and make it run hotter?

Yet, we see this all the time. A pilot sees high EGT and isn’t sure what it means. So the pilot diddles with the controls and finds by increase the load, adding more throttle, bumping in some more prop pitch or climbing steeper, it lowers the EGT. The unknowing pilot thinks the problem is solved but has accomplished just the opposite of what needs to be done. The engine is now under greater stress.

This is where many pilots get lost on EGT. They see a high EGT and dial in some more prop pitch. The EGT drops and they incorrectly think all is well. Instead, they have increased the load on the engine, increasing cylinder pressures and cylinder head temperatures. An engine like a Rotax with water cooled heads, you won’t immediately see an increase in cylinder head temperature. It takes time to warm up the coolant and the thermostat opens to attempt to cool the engine. You can enter the area of detonation without knowing it and long before it shows up on any gauge.

Is high EGT bad?

Not necessarily. It could mean you are approaching peak EGT when leaning the mixture for best power or economy. That’s good. On aircraft with automatic mixture or altitude compensating carburetors it could mean you are reaching peak efficiency. That’s good. When an engine is running way too rich and way below peak power the EGT will be low. That could be bad with low power and poor economy.

Again, let’s take the EGT gauge out of the cockpit and say you are flying a Cessna 150. How do you lean the mixture for cruise? You probably pull the red knob back until you see the engine RPM reach maximum (peak power) and then continue to lean until the RPM starts to fall off a bit (peak EGT to lean of peak) then you push the mixture back in a bit to restore maximum power. At this point you are probably operating near peak EGT. That’s good. But without a gauge you don’t know, and don’t care, what the EGT is. You are just trying to save fuel and get the most power.

Put the EGT gauge back in the airplane and you would probably use the same technique for leaning the engine. However, with the EGT you could get more control and save fuel.

So what should I do with high EGT?

If you have a mixture control, adjust it like you normally would. Use the EGT to assist your leaning. For maximum power, you should be on the rich side of peak EGT. For maximum economy you need to be at peak EGT or slightly lean of peak EGT.

If you do not have a mixture control, EGT can tell you if your carburetor is operating properly.

If EGT and cylinder head temperature are low or drop significantly during climb or cruise; you are probably running too rich.

If EGT remains high and you have good power, the altitude compensation is doing its job.

There is far more to EGT and what it means than I can post here. The main points about EGT are these:

Most pilots seeing a high EGT think something is wrong when the engine is operating normally.
EGT is an aid to leaning the engine for power or economy.
High EGT is not an indication of stress on the engine but increasing loads can lower
EGT indications.
There is no redline on EGT, it is a relative reading. In fact, many EGT gauges do not have any numbers or temperatures on them.
EGT Installations vary. Max readings on one airplane will not necessarily match a duplicate airplane with the same type engine.


I hope this helps people understand EGT and what it does and does not tell you.

Next time you fly, take note of this posting and see if you can duplicate the results. Watch what happens as you do a run up, add power for take-off, climb and cruise. See what happens when you enter a climb. Try to learn what is normal. Then when something abnormal occurs you will recognize it.

John Pitkin

ahazi
06-14-2011, 11:40 PM
Good explanation John and much better than what you will hear from many CFI's.
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