Exhaust Gas Temperature
The rest of the story

Our special thanks to Mr. Roger Zerkle, author and Ultra-Flight Magazine for allowing us to reprint this article from the April 2003 issue of Ultra-Flight Magazine.

We are very pleased to present the following article. Although there are differing viewpoints about the effects of engine/prop loading on the EGT readings on various engines, Mr. Zerkle's article gives the reader an excellent understanding of the overall combustion and exhaust effects on temperatures in two cycle engines.

Exhaust gas temperature gauges are one of the most important two-cycle engine instruments. It is hard to imagine flying a two-cycle without one.

Of course, during the earlier years of light sport aviation, an aircraft with an EGT was as rare as an aircraft without one is now. Even though there is no question that the EGT has saved many an engine from in-flight failure, sometimes I think that the fixation with the EGT may actually be detracting from safe operation.

The saying “Ignorance is bliss” has some merit. I am certainly not trying to make an argument for ignorance. In fact, to the contrary.

As a very young, and not very brave, student pilot during my college days, I became fixated on the artificial horizon. I thought that as long as I kept the wings of the little airplane symbol relatively aligned with the horizontal line on the dial, the airplane would not tumble off the pedestal it seemed so precariously balanced on, and I would not fall out of the sky.

Just like EGTs, artificial horizons are really nice instruments and can certainly enhance safety, but in my case, I was relying too much on one instrument for safe operation. And as I later learned, flying safely requires much more than keeping one instrument lined up. Having confidence, however misplaced, in the artificial horizon made me feel more secure. I was blissful in my ignorance. And so it can be with the EGT.

Outside the study of geometry, not many things in this world can be stated as simply as “if this is so, then that must be so.” The popular conception of the EGT, is that high temperature indicates a lean mixture, and that a low temperature indicates a richer mixture. Therefore, if your EGT is running a little higher than the engine manufacturer's recommendation, the engine must be running lean. This is not necessarily true.

The EGT measures the temperature of the exhaust gases in the immediate area of the exhaust system where the sensing probe is installed. The temperature that is critical to the engine is the temperature of the burning gasses inside the combustion chamber. Since it is not practical to physically measure the temperature there, a representative measurement is taken at a point in the exhaust system.

Because the exhaust gas temperature is being measured in a location removed from the actual point of combustion, the readings can be affected by the dynamics of the exhaust system, after-burning and engine loading. Therefore, a higher or lower than normal EGT reading is not always mixture related.

When gasoline and air are mixed in such proportion to have complete combustion, in other words all the fuel and all the air are completely used, the resulting temperature will be in the 1500 F range. This temperature is far too hot for an internal combustion engine to sustain for any length of time without incurring catastrophic failure.

If the chemically perfect fuel-air mixture is altered, either by adding more air, or by adding more fuel, the combustion temperature will cool down. However, the amount of fuel available to the engine is directly related to the power that the engine produces. Thus, the fuel-air mixture is made richer to lower the combustion temperature, rather than leaner.

The maximum EGT considered to be safe is in the 1200 F to 1300 F range, depending upon the engine manufacturer, where the EGT probe is located, and the type of cooling system employed. A fuel-air mixture that produces an EGT above the recommended maximum is considered too lean, even though it is still too rich for complete combustion.

Because of this fuel-air mixture relationship to EGT, most pilots have come to believe that the solution for a high EGT is a richer mixture. Unfortunately, this is only true when the cause of the high EGT is due to a lean mixture. There are other things going on in an operating two-stroke aircraft engine which can cause high EGTs, totally unrelated to carburetion and fuel-air mixture.

The location of the exhaust gas sensing probe within the exhaust system can cause the EGT reading to run consistently high or low. This variance may be due, in multi-cylinder engines, to the additive nature of exhaust gasses from more than one cylinder being imposed upon the sensing probe.

Also, the location of the probe may be in an area of the exhaust system in which the fuel rich, hot gas, is exposed to additional oxygen and “after burning” is occurring. Some engine manufacturers make allowances for these anomalies in their maximum EGT limits. For this reason, the maximum EGT recommendation for a 582 Rotax, for instance, should not be imposed as a standard upon another make and model engine.

In every engine installation, to be a meaningful source for comparison, the temperature sensing probe must be installed at the location in the exhaust system specified by the engine manufacturer. Remember, it is the temperature in the combustion chamber that is critical, not the temperature where the sensing probe is located. If you have 1200 F wired to a red warning light in your brain, and the manufacturer of the new engine you just installed is telling you that 1350 F is OK, you might be having some trouble accepting that. But remember, what the manufacturer is really saying is that when you have 1350 F at the probe, the exhaust gas temperature in the combustion chamber is within limits.

Two-stroke engines depend upon the design of their exhaust system for their usable power band and maximum power output. Waves of positive and negative pressure travel back and forth in the exhaust system. These positive and negative pulses are created by various components designed into the system and travel at near the speed of sound, depending upon the gas temperature. The engineers use these negative and positive waves as valves for the exhaust port.

Negative waves, timed appropriately, can help evacuate the combustion chamber of burned gasses and pull in a fresh fuel air charge, while positive pressure waves can hold the fresh charge in the combustion chamber until the piston covers the exhaust port.

The pressure pulses arriving at the exhaust port at the right moment, result in a dramatic increase in the volumetric efficiency of the engine and a corresponding increase in power. The timing of these waves depends upon the design of the exhaust system, the temperature of the exhaust gasses, and the RPM of the engine.

Even though, exhaust systems for aircraft engines are designed to provide power over a wide RPM range, there will be operating ranges where the reflected pressure pulses will not be arriving at the exhaust port at the right time. The high and low pressure waves will shift one way or the other inside the exhaust. This shift will cause a relocation in the hot exhaust gas pulses, as well, and can cause spikes in EGT readings at certain, rather narrow, engine RPM ranges.

These spikes in EGT, either up or down, generally have nothing to do with fuel-air mixture. The temperature of the burning gasses in the combustion chamber may not have changed at all. But in the area of the exhaust system where the EGT sensing probe is located, the pressure pulses are moving the gasses in such a way as to create radical changes in EGT readings.

The most common, and misunderstood, condition affecting EGT is engine loading. If the engine is not loaded sufficiently, it will run high EGTs. If the mixture is richened, the EGTs will go up, rather than down. A lightly loaded engine can be enriched to the point that it will barely run, and still have high EGTs. The explanation is found in one of Newton's laws. Simply stated, it says that energy cannot be created, nor destroyed.

When we burn a fuel-air mixture inside our engine, we are converting chemical energy into heat energy. When applying the pressure created by the expansion of the burning gasses inside the combustion chamber, to a movable piston, we are converting a portion of the heat energy to mechanical energy. The heat energy not used in turning the crankshaft is given off through the exhaust and the engine cooling system. If we add all this up, it would equal the amount of energy present in the fuel before it was introduced into the engine.

So it would be correct to assume that, for the same amount of fuel-air mixture being burned in the combustion chamber, if less heat energy is being utilized to turn the propeller, then more heat energy will be going out the exhaust. In other words, a lightly loaded engine, will throw considerably more heat out of the exhaust than the same engine, with the same throttle setting, carrying a heavier load. So, not only will a heavier loaded two-stroke engine have lower EGTs, but the engine will produce more power for the same amount of fuel burned. If a lightly loaded engine, with a high EGT, has the mixture enriched, the extra fuel will go out the exhaust, increasing the EGT even more.

Hopefully, I have made the case that high EGT readings are not always mixture related. And, probably you are not real happy to hear this, since life was so simple before. This new information has ruffled your security blanket. Let's try to smooth it back out a little.

The very best indicator of what is going on inside the combustion chamber is the spark plug. It lives there, knows what is going on every minute that the engine is running, and will not lie. If you are concerned about having high or low EGT readings, examine the spark plugs. If the EGTs are really running high, the spark plug will show signs of overheating, there will be very little carbon buildup on the electrodes, and the carbon that is present will be light in color, tan, light grey, or even white. Overheated spark plugs will not have any remaining sharp edges on either electrode. There may be blisters on the center electrode ceramic insulator.

Low EGT, due to an excessively rich mixture, would cause the spark plugs to be dark in color. They may possibly even appear wet, with considerable carbon buildup, almost to the point of fouling.

If the spark plugs do not confirm the EGT readings, then the EGT is being affected by something other than fuel-air mixture.

If the EGT readings are consistently high (or low) throughout the RPM range, it may be an instrument error, or it may be the location of the temperature sensing probe.

On the other hand, if the EGT readings deviate from the normal engine EGTs dramatically, but only within very small RPM ranges, this deviation may be caused by the exhaust system's tuning. If this is the case, the engine CHT would not be affected. If the EGT was an actual indication of high combustion temperatures, then there would be a corresponding rise in the CHT, and the spark plugs would have signs of overheating, as well.

This can be tested during ground runs. Or, you can simply avoid these RPM ranges in flight. Trying to correct areas of dramatic deviation, with changes to carburetor jetting, without first confirming that these areas are caused by a lean mixture, is an exercise in futility.

Engine loading (or rather, lack of engine loading) is the most common cause of high EGTs not related to fuel-air mixture. High EGTs related to an under-loaded engine are easy to identify. The engine will be turning very high RPM, near the manufacturer's recommended red line, or above. While the EGTs are above the recommended range, the CHTs will be well below. Aircraft performance may be soft, and fuel consumption will be high. Additionally, if the mixture is enriched, there will be no corresponding decrease in EGT.

The solution, of course, is to load the engine with more pitch, a longer propeller, or a combination of the two, until the CHT comes down into the recommended range. The maximum RPM the engine will be able to turn will come down also, but there will actually be a gain in thrust. As the engine is loaded, the CHT will come up and the fuel burn will go down.

The limit to loading a two-stroke is the CHT. The engine can continued to be loaded until the CHT moves into the higher area of the manufacturer's range. It is critical to not exceed the CHT, since pre-ignition and detonation can be the result. A fifty degree buffer between the engine's normal, loaded CHT and the engine manufacturer's limit is recommended. Remember, EGT readings are only a guide. They are not always related to fuel-air mixture, and the spark plug is the most accurate indicator of combustion chamber temperature. n

Courtesy of Author:   Mr. Roger Zerkle
and Ultra-Flight Magazine