gas | Blackstone Laboratories

EGT Myths Debunked

Reprinted with permission from the author, Mike Busch

Pilots still seem to have a lot of misconceptions about exhaust gas temperature (EGT). Let’s see if we can clear some of them up.

These days, pilots of piston-powered aircraft seem to have fixated upon EGT. Scarcely a day goes by that I don’t receive a phone call, email, or support ticket asking some EGT-related question.

Pilots will send me a list of EGT readings for each of their cylinders and ask me if think they look okay, whether I think their EGTs are too high, what maximum EGT limit I recommend, why their EGTs seem to be higher in the winter than in the summer, or why the EGTs on their 1972 Cessna 182 are so much higher than the ones on their friend’s 1977 model. They’ll voice concern that the individual cylinders on their engine have such diverse EGT readings, worry that the spread between the highest and lowest EGT is excessive, and ask for advice on how to bring them closer together. They’ll complain that they are unable to transition from rich-of-peak (ROP) to lean-of-peak (LOP) operation without producing EGTs that are unacceptably high.

Each of these questions reveals a fundamental misunderstanding of what EGT measures, what it means, and how it is interpreted. Let me attempt to clear up some of this confusion by asking you to forget everything you thought you knew about EGT and start at the beginning.

What EGT is not

The absolute values of EGT are not particularly interesting for a number of reasons. The most important is that indicated EGT is not a “real” temperature. To understand what I mean by this, I’d like you to conduct a thought experiment: Imagine that you’re an EGT probe, located in an exhaust riser between two and four inches from the exhaust port of a cylinders, and think about what you would see.

You’d see nothing much for two-thirds of the time¾during most of the intake, compression, and power strokes, because the exhaust valve is closed and so no exhaust gas is flowing out of the exhaust port and past the probe. During the one-third of the time that the exhaust valve is open, you’d see a constantly changing gas temperature that starts out very hot when the valve first opens but cools very rapidly as the hot compressed gas escapes and expands, and then ultimately is scavenged by cold induction air during the valve overlap period (at the end of the exhaust stroke and the beginning of the intake stroke) when both intake and exhaust valves are open simultaneously.

Now, all these gyrations are happening about 20 times per second, and you (the EGT probe) cannot possibly keep up with them. You wind up stabilizing at some temperature between the hottest and coolest gas temperature you see, and you dutifully report this rather arbitrary temperature to the panel-mounted instrument, where it is displayed to the pilot as a digital value accurate to one degree. The temperature you report to the pilot is not exhaust gas temperature (which is gyrating crazily 20 times a second) but rather exhaust probe temperature (which is stable but related to actual exhaust gas temperature in roughly the same fashion as mean sea level is to high tide).

To make matters worse, numerous factors can affect indicated EGT besides actual exhaust gas temperature. These include probe mass and construction (grounded or ungrounded), cam lobe profile, lifter leak-down rate, valve spring condition, and exhaust manifold topology, among others.

For example, the two front cylinders (numbers 5 and 6) on the left engine of my Cessna T310R always indicate lower EGTs than the other four cylinders. The exact same phenomenon also occurs on the right engine. This is not because those front cylinders produce cooler exhaust gas than their neighbors (they don’t), but because the exhaust risers for those cylinders curve aft while the other four risers go straight down. Thus, the gas flow past the EGT probe is different for the front cylinders than for the others, and their indicated EGT is lower. This temperature anomaly is quite obvious on my digital engine monitor¾and also quite meaningless.

What EGT means

Even if indicated EGT accurately reported actual exhaust gas temperature (which it doesn’t), it’s important to understand that exhaust gas temperature does not correlate with stress on the engine the way cylinder head temperature does. In fact, many things that increase engine stress (such as advanced ignition timing and high compression ratio) cause EGT to go down, while things that reduce engine stress (like retarded ignition timing and low compression ratio) cause EGT to go up.

Remember that CHT mainly reflects what’s going on in the cylinders during the power stroke when the cylinder is under maximum stress from high internal temperatures and pressures, while EGT mainly reflects what’s going on during the exhaust stroke after the exhaust valve opens and the cylinder is under relatively low stress.

High CHTs often indicate that the engine is under excessive stress, which is why it’s so important to limit CHTs to a tolerable value (no more than 400°, preferably 380° or less). By contrast, high EGTs do not indicate that the engine is under excessive stress, but simply that a lot of energy from the fuel is being wasted out the exhaust pipe rather than being extracted in the form of mechanical energy.

For instance, a 1972 Cessna 182 with an O-470-R engine will typically have indicated EGTs that are 100 degrees hotter than those seen in a 1977 Cessna 812 with an O-470-U engine. The -R has a relatively low 7.0–1 compression ratio because it was certificated for 80-octane avgas, while the -U engine has a much higher 8.6–1 compression ratio because it was certificated for 100-octane. Because the high-compression -U engine is significantly more efficient at extracting heat energy from the fuel, it wastes less energy out the exhaust and this its EGTs are cooler (despite the fact that the -U engine is much more highly stressed than the -R).

High EGTs do not represent a threat to cylinder longevity the way high CHTs do. Therefore, limiting EGTs in an attempt to be “kind to the engine” is simply misguided.

Diff versus Gami spread

Right behind the “high EGTs are bad” myth is the “identical EGTs are good” myth. Many pilots believe incorrectly that a flat-topped graphic engine monitor display (with all EGTs equal) is the mark of a well-balanced engine, and that unequal EGTs are a sign that something is wrong. This common misconception tends to be reinforced by digital engine monitors that display a digital “DIFF” showing the difference between the highest and lowest EGT indication.

As illustrated by the earlier anecdote about the front cylinders on my Cessna 310R, difference between absolute EGT values are both normal and benign. It is not uncommon for well-balanced fuel-injected engines to exhibit EGT spreads of 100 degrees, and carbureted engines often have spreads of 150 degrees or more. In fact, as shown in Figure 4, EGT spreads are usually smallest near or just rich of peak EGT (the worst place to operate the engine), and often significantly greater at leaner or richer mixture (that are much kinder to the engine.

The mark of a well-balanced engine is not a small EGT spread (“DIFF”), but rather a small “GAMI spread”– defined as the difference in fuel flows at which the various cylinders reach peak EGT. Ideally, we would like to see this peak be no more than about 0.5 gph (or 3 pph). Experience shows that if the GAMI spread is much more than that, the engine is unlikely to run smoothly with LOP mixtures.

It’s all relative

The only important thing about EGT is its relative value: how far below peak EGT and in which direction (e.g., 100 degrees ROP or 50 degrees LOP). Absolute values of EGT (e.g., 1,475 degrees) are simply not meaningful and are best ignored. There is no such thing as a maximum EGT limit or redline, and trying to keep absolute EGTs below some particular value¾or even worse, leaning to a particular absolute EGT value¾is simply wrongheaded. Don’t do it. If you must fixate on those digital engine monitor readouts, fixate on something important, like CHT.

Fuel Contamination in Aircraft

Here in the northern latitudes autumn brings uncertainty about what to expect from the sky and wind each morning. Rain and overcast skies are frequent but counterbalanced by days when clear blue skies are accented with yellow sunlight that reflects the fall leaves and warms the spirit.

Those who fly in the winter months generally count the experience with mixed feelings. Cold toes and fingers are a certainty. So are hard-to-start engines and batteries that lack enthusiasm. But once the engine finally fires and the first BTUs of heat start filtering into the frosty cabin air, the whole experience can bring a smile to the face of the most determined pessimist.

The brakes may be stiff but they still work. Once you get to the run-up area, most of the breath-laden frost has cleared from the windscreen and you can stow the gloves. The sun streaming into the cabin does as much to warm things as the manifold heater. On lift-off, the rewards of winter flying come back to remind you why you do this in the first place. The prop bites the crisp air with authority. The dense air brings lift with a rush. Like the counter guy at the FBO said, “There’s a lot of lift going on out there today!” Indeed!

Understanding the gas

There are several reasons engines are hard to start in the cold. Parts are machined to operate easily in concert when they are at operating temperature. The further you get from that temperature—either hot or cold—the more interference there will be between interfacing parts. Poorer fitting parts increase internal engine friction.

Air-cooled aircraft engines typically run on SAE 50W oil when at operating temperature. Cold, the oil has the properties of molasses. The oil pump resists rotation. The oil resists being pushed around. The oil that starts out in the oil cooler may still be there when you land. If the surface air is cold, you know the air at altitude will be colder. It is not unusual to find the oil temperature at the bottom of the green arc, if it gets into the green at all. Pity the poor battery that has to coax all this reluctance into motion.

Mixture matters

Another reason cold engines are hard to start is the gas/air mixture is incorrect. The fuel system, whether carbureted or injected, is set up to operate at a given mixture for normal temperature operation, usually fifteen or sixteen parts air to one part gas. Cold cylinder walls condense gas from the mixture, causing the gas that’s left to become lean—far too lean to initiate normal combustion. Accordingly, engines of all types have an enrichment device to compensate when cold. For liquid-cooled land-based engines, a choke in the carb throat or an extra injector enriches the intake air. Both types of systems usually shut off automatically as the cylinder walls warm up, which usually doesn’t take long.

Air-cooled aircraft engines, on the other hand, have a primer. Even when carbureted, they won’t use chokes or have the acceleration pumps that are common for car and truck engines. Many aircraft engines that fire readily on zero or minimal prime on a sunny, warm day won’t even consider firing without prime in winter. If one squirt of prime works in July, it may take four to six in January.

If there is moisture in the first gasp of cold air that gets sucked into the cylinders, it can frost the plug electrodes. If this happens, no amount of priming or cranking (or swearing) will make any difference. The engine won’t fire until the frost is melted or otherwise eliminated.

Raw gas that condenses on the cold cylinder walls gets scraped down into the oil by the beveled oil control ring(s). It will mix perfectly with oil, so there is no good way to get it back out again unless you cook it out with heat and agitation (otherwise known as flying). If it is cold enough at the altitudes you fly, the gas from priming may still be in the oil when you land.

Problem or not?

But does gas in the oil really hurt anything? Hardly. It will cause a lower viscosity, but that may be an asset rather than a problem. There were WWII radial engines operating in the frozen north that were designed to inject gas into the oil before shutting down. The gas thinned the oil so that the engine could be cranked over in the morning with less resistance. After the engine got back to operating temperature, the oil returned to being an SAE 50 or 60W once the gas was distilled back out of the oil…sort of an automatic multi-weight oil before multi-weight oils were invented.

To say we find a lot of gas contamination in winter oil samples would be an understatement. We usually mention it because gas in the oil can show a fuel system problem. But that is rare in aircraft engines. We find a lot of moisture in winter oil samples too, and it goes into the same category as the fuel. Unless your aircraft engine is liquid-cooled, we don’t think the moisture is any more a problem than the gas.

When taking your sample, it’s ideal to have the oil warmed to operating temperature first, though if that’s not possible it’s best to just take the sample cold and not start the engine at all. Starting the engine but not flying it can introduce even more gas into the system. We realize an FBO mechanic won’t have the option of taking your airplane for a couple of turns around the pattern, even if he or she was qualified to do so, before draining the oil. Consequently, we turn up volatile gas and often moisture in your winter samples if you are flying in the cold. It only rarely points to a problem.