EGT and CHT Interpretation

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This article appeared in Light Plane Maintenance, Oct. 2008.

Maintenance

The most important reason to have a multi-probe engine-diagnostic system is the in-flight diagnostic capability that such a system brings. If the pilot knows and understands the system, a multi-probe cylinder head temperature/exhaust gas temperature (CHT/EGT) system can serve as an unparalleled "early warning" device, pinpointing the location and nature of various types of engine problems (sometimes) long before they show up in other ways. Of course it also helps visually quantify precise leaning by the pilot. With the cost of gas so high, we want every drop of liquid dinosaur that we can get to perform useful work without frying cylinders. These multi-probe systems go a long way toward that end. The key is to know and understand the system. It takes a good deal of experience with a particular system, and a thorough grounding in the principles of exhaust analysis, to use a CHT/EGT system to maximum advantage. All the manufacturers have published detailed brochures on understanding their products. There are also courses. In this article, we'll explore some of the more common "mechanical gremlins" that can be diagnosed on such a system. For illustration purposes, we have chosen to use a generic, bar-graph-type display (similar to either Insight, EI or JP Instruments) in which the top stack of lit bars represents the EGT for a particular cylinder, while the non-illuminated bar within each stack represents separation between the EGT stack above and the corresponding CHT stack below. The principles described below apply to all multi-probe systems, however. (It's much easier to see certain trends on the bar-stack type display.)

Multi-Cylinder Model Basics

JP Instruments EDM 800

The EGT probe is located in each exhaust pipe, typically four to six inches away from the cylinder head. It measures the temperature of the exhaust gases exiting the cylinder. The actual temperature of the exhaust varies with a number of elements such as the power setting, altitude, ambient air temperature, and cylinder compression. It is also influenced by engine mechanical conditions such as ignition timing and cylinder leakage (compression loss), so you can see it's a very dynamic instrument with lots of diagnostic potential. EGT even varies from combustion event to combustion event as the engine is running. (The thermocouple actually registers a kind of moving average: Exhaust gases are jetting past the probe in a pulsing manner, as the exhaust valve opens and closes.) The EGT offers a peephole into this combustion process. Because it is so dynamic, the absolute numeric value of the EGT at any given moment is not as important compared to the moving average. Also, it's the relative value of EGT (relative to other cylinders and relative to "normal" day-over-day performance) and the way that EGT responds to changes in the mixture control (i.e., changes in fuel-air ratio) that are of primary interest. That said, absolute numbers can provide additional important information, but are somewhat secondary to the big picture provided by the bar stacks. Moreover, a barrage of varying digits can lead to sensory overload for the pilot. In the very simplest sense, you can think of EGT as being a "rectal thermometer" for the engine. The single most fundamental piece of information any EGT system gives the pilot is the knowledge that combustion is occurring. This is actually a very useful function, such as during an engine-out occurrence in a twin. An engine that's not producing power can still show rpm (and therefore oil pressure), manifold pressure (MP), fuel pressure, and oil temp. In fact, the MP of an engine that isn't running is the same as one that is at takeoff power (give or take an inch). Just looking at the power gauges when one engine is surging (in a twin-engined plane) won't necessarily tell you which engine you're losing. Looking at the EGT will, because faltering combustion always shows up as dwindling EGT. One of the best-known uses of EGT is to track fuel/air ratio (or mixture strength). When the pilot pulls the mixture control back, the F/A ratio decreases, and EGT begins to increase. As the F/A ratio continues to decrease, a point is reached when the EGT peaks and then starts back down. This point of peak EGT is the reference point by which most pilots lean. It's the point at which the ratio of fuel to oxygen is chemically optimum for complete combustion (neither any leftover fuel, nor any leftover oxygen). The chemical name for this is stoichiometry.

CHT Indication

CHT is the residual heat from combustion and therefore ought (in theory) to be able to provide much of the same diagnostic information that EGT can. The main problem with CHT is that it is slow to respond. It also measures the temperature of one spot on the cylinder head, and nowhere else. CHT can typically be taken from a probe in a threaded boss in each cylinder or from a special, wired gasket under a spark plug. The threaded boss is the more desirable option, and any mixing of temperature probe types can lead to pilot confusion, since the two probe types do not track each other closer than 30-60 degrees F or so. Cylinder-barrel temperature can be and often is quite different from probe-indicated CHT. In contrast, EGT is much quicker to change, and the changes are larger in magnitude. This makes EGT a much more valuable tool for looking at irregularities in the combustion process. The two elements of EGT/CHT also have a symbiotic relationship in the sense that they help to confirm each other in a diagnostic sense. But CHT can be helpful alone for some diagnostic issues. Individual CHTs or banks can be dramatically affected by engine baffle issues/problems as well.

Common Indications

Electronics International Reflect MVP50

Some indications of trouble are shown in the example displays below. Bear in mind, the actual instrument indications may look somewhat different in real life, depending on ambient conditions, the cylinders affected, the type of engine, etc. Also, one cannot always tell what exactly is wrong, because some malfunctions cause similar indications. Or there could be multiple malfunctions. But the key point is the simple fact that the display can show that something is wrong, and which cylinder is going astray places you far ahead of the minimally instrumented airplane. Moreover, with experience, you establish trends to help with diagnosing issues. Probes sometimes give trouble, but generally are quite reliable. Some self-test. A temporary switching of probes can usually rule out a probe problem. A common CHT differential is one or two bars among cylinders with proper baffles. Lycoming literature notes differentials of 100 degrees F or even 150 degrees F in carbureted engine, although we have not seen such divergence. We would do some serious baffle checking with such readings. For EGT, it's commonly three to four bars or up to a 75-100 degree F temperature differential (hottest to coolest) in a fuel-injected engine. The EGT spread is the difference in fuel flow between the richest and leanest cylinders. For a carbureted engine, the EGT differential can go to 150 degrees between the highest and lowest EGT -- possibly a bit higher for some poor induction designs such as the TCM O-470. Carbureted engine designs are simply less able to optimize the fuel flow to each cylinder. In the case where aftermarket GAMIjectors are used in a fuel-injected engine, narrower and more beneficial inter-cylinder EGT spreads -- or more importantly, fuel flows -- are obtained, sometimes nearly perfect. (GAMIjectors are custom fuel injection nozzles designed to optimize the fuel flow to each cylinder based on extensive testing of examples of the representative engines done by the folks at General Aviation Modifications.) Further note that this inter-cylinder EGT differential is not a fixed thing. It is dynamic in a given engine and will vary with the mixture and power settings (among other factors). At cruise power settings, generally the narrowest differentials for any given engine in good condition will be seen at wide-open throttle (most efficient), along with proper mixture management. Aviation fuel injection is a very basic mechanical design (fuel is constantly sprayed at the back of the valve, not pulsed into the cylinder based on demand and timing). There are no feedback electronics (notwithstanding FADEC), so the pilot has to be knowledgeable to get the best available mixture from his system by proper leaning.

Example 1

Example 1

The problem shown at right is a high EGT indication on cylinder #2 (numbering of cylinders is from left to right); CHT, however, is normal on all cylinders. The likely problem is that one spark plug has stopped working in #2 cylinder. If the EGT is intermittently high, the spark plug firing is intermittent as well. Note: A single, high EGT reading could also be due to a partially obstructed fuel injector nozzle (in an injected engine). Spark plug fouling is more common, however. You can see the single-plug high EGT effect when switching to one mag at run-up: Watch the EGTs jump on one mag.

Example 2

Example 2

In this instance, we are looking at a possible EGT bank-specific imbalance. The CHT indications are quite uniform. In a fuel-injected airplane, the staggered EGTs with the seven bar spread of around 175 degrees shown here could mean the need for injector nozzle cleaning. In a carbureted engine, this kind of spread could be normal, indicating differences in cylinder compression and/or poor induction tuning between cylinders at the given power setting. The bank-specific nature could be meaningful, however. More investigation should be made for the bank-specific EGT imbalance to see if there are any induction leaks/other causes.

Example 3

Example 3

In this case, we're looking at a high CHT on #3 cylinder. The CHT is grossly, abnormally high. Either there's some type of very abnormal combustion process going on in the cylinder (and certainly, pre-ignition should be considered with such a high CHT), or there is abnormally high friction in the cylinder, or the probe is being subjected to unusual heating. Notice the low EGT on cylinder #5. An exhaust leak at the #5 exhaust gasket may be blow-torching the CHT probe on the adjacent cylinder (#3). If such torching is happening, a precautionary airport landing in called for. It may be as simple as a bird's nest or other obstruction to cooling over #3 cylinder, but the cylinders 3 and 5 indications are abnormal enough to warrant a precautionary landing to see what's up.

Example 4

Example 4

Notice the staggered CHT pattern on this gauge. One bank of cylinders is running colder than the other bank. A check should be made of cowling and engine compartment for cooling irregularities affecting one side of the engine. That includes checking the cowl flaps on cowlings that have two separate cowl flaps. According to Insight Instrument Corporation, "On some aircraft, one inch of cowl flap misalignment will cause a 50-degree F difference in CHT." (See the Graphic Engine Monitor Pilot's Guide.) Additionally, if this was a fuel-injected engine, such an EGT spread is pretty excessive (and still bank specific). Further troubleshooting is in order, including a nozzle and spark-plug cleaning to see if the spread cannot be narrowed. The monitor can be used to easily identify which plug is the problem, saving maintenance time.

Example 5

Example 5

In this example, the EGT is low on #5 cylinder, as is CHT. The #5 cylinder is becoming a so-called "cold cylinder" through one cause or another. Either compression is low, or fuel flow to the cylinder is so restricted that "the fire is going out." (If the engine is fuel-injected, it's time to look for significant nozzle blockage.) So check the nozzles and check compression.

Example 6

Example 6

This time the #3 cylinder is indicating high EGT and CHT. The correlation of the two together can be taken as pointing to a combustion problem. (However, typically, preignition results in dropping EGT and rising CHT, and can be caused by something as basic as a faulty spark plug.) Upon seeing this indication, the whole stack may be maxed out by the excessive CHT reading and the stack could be flashing or other pilot-set alarm going off. Reduce power and enrichen the mixture if possible, and observe for immediate lowering of EGT followed by CHT dropping off. A precautionary landing and investigation should be made any time such divergent readings occur. In any event, upon landing, both spark plugs for #3 cylinder should be pulled and examined. Possibly a compression test and borescope examination should be done as well, but the cause must be found.

Example 7

Example 7

Finally, this example should cause a certain amount of worry for any Lycoming owner. Notice the low EGT (and CHT) indications for cylinders #1 and #2. This could just be a normal indication for this particular engine. But it could also spell problems in those two cylinders. One possibility is a bad intake lobe on the camshaft. The intake lobes (unlike the exhaust lobes) do "double duty" in that one lobe works the intake lifters for both opposing cylinders. Hence, cylinders #1 and #2 (on opposite sides of the engine) get intake-valve actuation from the same cam lobe. When this lobe -- which is quite often the first one to go bad on a Lycoming cam -- begins to wear flat, the reduced duration and lift on the intake valve will begin to show up as reduced power and lower EGT, indicating an overly rich mixture. (Compression may be fine.) We'll have more diagnostics with the engine monitor in upcoming articles in Light Plane Maintenance. If you would like to learn much more on mixture and general engine management, go to Advanced Pilot Seminars" for on-line and resident seminars.

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