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GEM Pilot's Guide


Insight's Graphic Engine Monitors are the most advanced engine instruments available to the pilot. Traditional multicylinder exhaust gas and cylinder head temperature systems that force the pilot to switch or scan an indicator from cylinder to cylinder in search of critical engine data, are long obsolete. Using the latest computer technology, the Graphic Engine Monitor presents a clear, concise, graphic picture of all cylinder temperatures simultaneously for interpretation at a glance. Never before has so much engine diagnostic information been available to the pilot and never before, has the pilot been able to control mixture with such ease and precision for peak fuel efficiency.

Insight's latest Graphic Engine Monitors automatically record flight temperature data in non-volatile memory. The data-log files can be easily retrieved by the pilot, in-flight or post-flight, for instant viewing or permanent record-keeping.

Fundamentals of EGT

The basic ingredients of combustion are fuel, air (oxygen), compression, ignition, and timing. The measurement of exhaust gas temperature (EGT) is really an indication of the harmony of interaction of these ingredients. A slight change in any of these five factors will result in noticeable changes in EGT.

The measurement and dynamic analysis of these changes is a very valuable tool for engine management. The use of exhaust gas temperature for mixture control depends on certain characteristics of combustion that are common to all engines. It is generally known that the exhaust gases get hotter as the mixture is leaned. This temperature rise is a sign of increased combustion efficiency as the optimum mixture setting is approached. If the leaning progresses past a certain point, the temperature will begin to drop. This temperature drop is the result of reduced energy output from the diminished fuel flow. Figure 1 is an actual temperature curve from an aircraft engine operating in flight at 75% power. Notice that the curve is quite round on top.

This is true for all engines. For a variety of reasons, the best operating mixture for aircraft engines is in the vicinity of this peak. Some high performance engines require slightly more fuel for cooling and run best on the rich side of peak while others are designed for operation on the lean side of peak, but for most, peak EGT is optimum. The shape and character of this curve is typical for all normally aspirated engines; it is, however, slightly affected by some turbocharger installations (See Special Considerations for Turbos for details).

The Principles of EGT Measurement

Exhaust gas temperature is measured with a temperature sensing probe that penetrates the exhaust stack a few inches away from the cylinder. The sensing probe is made from a special alloy designed to provide long term protection for the temperature sensing elements inside. The temperature measurement is actually made with a thermocouple sensor. A thermocouple is a welded junction of two alloys that generates a tiny voltage when heated. Only 22 millionths of a volt are generated per degree Fahrenheit. The GEM measures these tiny signals and translates them into temperature. The EGT probes are designed to have a small thermal mass for fastest possible response, and the manufacturing procedures are tightly controlled to maintain probe calibration to within one degree.

Thermocouple sensors, an old and proven technology, are the heart of many industrial and aerospace measurement applications. In the Graphic Engine Monitor they are used for much more than just measuring temperature. That's why we say the GEM is more than just a thermometer.

In fact, the GEM will help you monitor mixture, timing, fuel distribution, compression, oil consumption, and many other subtle engine phenomena. The GEM can actually resolve engine phenomena that occur in millionths of a second.

Principles of CHT

Like EGT measurement, cylinder head temperature (CHT) is monitored by means of a thermocouple which generates a voltage proportional to its temperature. The GEM is designed to work with three different kinds of probes. The gasket probe replaces one of the spark plug gaskets on a cylinder and is held in contact with the cylinder by the spark plug. The spring-loaded probe screws into the temperature well in the cylinder and its tip is pressed against the cylinder by spring pressure. The third kind of CHT probe is called an adapter probe. It too screws into the temperature well, but unlike the spring-loaded type, it allows the factory installed bayonet probe to remain in place. While the basic principles of CHT measurement are similar to that of EGT measurement, the range of temperatures is much lower; typically 500°F or less.

Understanding the Display

The Graphic Engine Monitor is designed to display the exhaust gas and cylinder head temperatures of all cylinders simultaneously. The cylinders are numbered across the bottom of the display (1, 2, 3, 4, 5, 6). Four cylinder installations will have only 1, 2, 3 and 4 illuminated.

GEM 602, GEM 603

At the top of the display are two annunciators, EGT and CHT that identify the temperatures displayed by the bar-graph. The GEM-603 model for turbocharged engines also displays Turbine Inlet Temperature (TIT) in digital format. At the top of the GEM-603 display are four annunciators EGT, CHT, TIT and °F.

GEM 610, GEMINI 1200

The GEM-610 and GEMINI 1200 utilize an advanced feature set including EGT trend indicators, Cylinder-number Highlight boxes, and a high-resolution digital numeric display. See the 610 and 1200 chapter for detailed description and operation information.

The Bar-Graph Display

The exhaust gas temperature is displayed in bar graph form and is interpreted much like a conventional mercury thermometer. The higher the bar, the higher the temperature. Reference marks for EGT are provided on the left side of the scale. These marks are not numbered because absolute temperatures are of no value for mixture control. The cylinder head temperature is displayed in negative single bar format. During normal operation it shows as an unilluminated bar in the lower half of the bar column. Calibrated reference marks (2,3,4,5) represent hundreds of degrees Fahrenheit and each column bar represents 25 degrees. The missing bar method of displaying two parameters on a single bar graph concentrated the information on the same display for easy comparison of each cylinder's EGT and CHT. Since EGT is normally higher than CHT, the dark bar which represents CHT is surrounded by illuminated bars and stands out clearly. However, when the engine is shutdown, the EGT quickly drops to zero and the cylinders remain warm for sometime. In this case, the CHT indicator reverts to an illuminated bar surrounded by a dark field. The GEM provides a reliable indication of cylinder head temperature even with the engine shut down. Should an EGT probe fail, the entire EGT column for that cylinder will go blank, but the CHT bar, instead of remaining black, will revert to a bright orange bar. The failure of one probe will not affect the display of any other probe.

The turbine inlet temperature on GEM-603 models is displayed numerically in tens of degrees Fahrenheit. For example, a display reading of 159 indicates TIT of 1590°F.

Modes of Operation

The Graphic Engine Monitor has three modes of operation:

  • · Lean Mode
  • · Monitor Mode
  • · Test Mode

Lean Mode is used during cruise to identify the leanest cylinder. Lean Mode can be entered at any time by depressing the Reset Button for about two seconds, until the EGT annunciator in the upper left corner of the display begins to blink. In Lean Mode the GEM's microprocessor analyzes the EGT outputs of each cylinder's thermocouple probe to arrive at a determination of the leanest cylinder. The lean cylinder is then annunciated to the pilot by blinking the corresponding EGT display column. As explained further below, Lean Mode is used during final fine tuning of the mixture.

The GEM is in Lean Mode only when the EGT annunciator is blinking. Monitor Mode, indicated by a steadily illuminated EGT annunciator, may be entered at any time by simply pushing the Reset Button momentarily. If the Reset Button is held too long, Lean Mode will be activated. Each time Monitor Mode is entered, any blinking columns will stop blinking, and current EGT readings (as registered with one degree accuracy by the GEM microprocessor) are automatically stored in computer memory for future reference. Should any cylinder's exhaust gas temperature subsequently rise 50°F or more, the corresponding column will begin blinking, to signify a change in combustion.

Test Mode on 602 and 603 only initiates a hardware diagnostic routine intended to be used before engine start-up. To activate Test Mode, depress the Reset Button while the power to the instrument is turned off then switch on power to the instrument. As soon as the GEM starts its precisely programmed test pattern the button may be released. Starting with the display column for cylinder number one, orange bars will stack up until the full column is illuminated, then blank out; next column two will light up, and each column will proceed in succession. The test will terminate with column six. The GEM-603 TIT indicator will display 000 during the test. At the end of the test the bars will flash in a random pattern and on the GEM-603 the TIT indicator will display a random number while the microprocessor resets itself. If necessary, the test may be repeated by switching off instrument power and initiating the procedure again. It is not necessary, or even recommended, that the pilot invoke Test Mode at the outset of every flight. Test Mode is designed to be used if trouble with the GEM system is suspected.

A New Approach to Engine Management

A great deal of misinformation exists concerning EGT measurement and its application to engine management. While the underlying principles have long been a matter of indisputable fact, many EGT myths persist. Part of the problem is, that until the advent of the GEM, the technology did not exist to exploit the physics of exhaust gas temperature for the practical purposes of the pilot. The old, inadequate technology, it seems, fostered an inadequate and inaccurate understanding of the subject.

The Graphic Engine Monitor is a sophisticated tool for engine management. Its microprocessor performs many tasks that used to be handled by the pilot. One of the basic functions performed by the GEM is monitoring exhaust gas temperatures for all cylinders with one degree accuracy. Some pilots ask why the GEM does not display EGTs on an absolute scale. The GEM does not display absolute temperatures because they are of no value for mixture management or engine troubleshooting. What is important is the exhaust gas temperature of a particular cylinder in relation to its peak. But peak EGT is not a constant; it changes with atmospheric conditions, altitude, power setting and engine condition and for this reason absolute exhaust gas temperatures in degrees Fahrenheit are quite meaningless.

The real objective of mixture management is finding a mixture setting which represents the correct position on the EGT/Fuel Flow Curve (see Figure 1). As we will see later, this abstract task is easily accomplished by the GEM's microprocessor. The EGT curve is often depicted as rising to a sharp peak and then falling precipitously on the lean side (see Figure 2). But any pilot with experience using EGT for leaning can tell you this isn't true. In actual fact, as the engine approaches peak, it takes a proportionally larger change in fuel flow to effect a change in EGT. Finding peak EGT is not as easy as some would have us believe. In order to find peak reliably, one must monitor and compare EGTs for all cylinders throughout the leaning procedure. In fact, manual leaning to any degree of accuracy with a needle or digital readout is always problematical and often impossible. However, the GEM's microprocessor, which samples EGTs for all cylinders many times a second and subjects this data to a complex mathematical analysis can identify peak much more accurately and reliably than even the most skilled user of a traditional EGT gauge. This capability allows the pilot to operate his or her aircraft engine at the most economical mixture settings without fear of the damage caused by over-leaning.

It is generally known that EGT can be a valuable source of information for engine diagnosis and troubleshooting, but there is a great deal of confusion when it comes to interpreting this data. One of the basic principles of EGT engine analysis is that engine temperatures (EGT and CHT) achieve equilibrium in an engine operating at a constant power and mixture setting. What is often overlooked is that this equilibrium cannot be defined as a single point but rather a range of temperatures.

In the real world, engine temperatures are affected by a number of variables (such as atmospheric conditions) and fluctuate constantly over a small range with little significance. It follows that one of the subsidiary tasks of engine analysis and troubleshooting is to separate the significant excursions from variations that fall within the normal temperature range. Three key design features of the GEM have made this job easier.

The first was the optimization of display resolution. The 25 degree increments of the GEM display serve as a data filter. Unlike numerical representations which display engine temperatures with one degree resolution and fluctuate constantly, the GEM display changes only when there is a significant alteration in engine equilibrium. Though some pilots insist that one degree display resolution is important to them, it serves no useful purpose in engine diagnostics, leaning or engine operation.

A second important design choice was the bar graph display. With a little experience, GEM users learn to relate the pattern of bars with what is normal for their engine. Any change in this pattern is perceived with a glance, and because of the display resolution, is indicative of a significant change in engine conditions.

The third key feature of the GEM addresses the problem of detecting intermittent engine problems. Often temporary excursions in EGTs are indicative of a serious impending problem. With traditional EGT systems it is impractical to detect this class of problems since it would require the constant attention of the pilot. The GEM, unlike passive temperature indicators, monitors EGTs for you and annunciates any rise of 50 degrees or more by blinking the appropriate column.

The Graphic Engine Monitor is a new approach to engine management made possible by the utilization of microprocessor technology. The GEM makes sophisticated in-flight engine analysis practical for the first time, and leaning accurate and reliable. Its carefully designed ergonomics make it the easiest to use of all engine monitoring instruments.

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