Why does the GEM use a bar graph for EGT and CHT?

Ergonomics are an important part of instrument design. Insight pioneered the use of the gas-plasma bar graph for displaying EGT and CHT because of it's ideal suitability for this application. The graphical format displays all the data simultaneously in a way that's easy for the pilot to read and understand. Remember that Insight's 25 degree per bar resolution acts as a data filter to display only significant changes in temperature.

Why a digital display for TIT?

Many turbochargers are limited to a maximum inlet temperature of 1650 degrees Fahrenheit. Exceeding that temperature may result in catastrophic failure of the turbocharger and other parts of your airplane. The GEM prominently displays this critical temperature in it's most logical position, at the top of the bar-graph.

When I lean my engine the leanest cylinder is not the hottest. Sometimes the leanest is the coolest cylinder. Is there something wrong?

There is no correlation between the cylinder with the highest temperature and the one with the leanest mixture. In fact, a static temperature display conveys no mixture information whatsoever. Many pilots confuse the true meaning of the term PEAK EGT. PEAK EGT is the highest temperature that a cylinder will reach when leaned under normal conditions. The first cylinder to reach its own peak during the leaning procedure is the leanest. Other cylinders may be hotter or cooler at that time, but none is leaner.

The Graphic Engine Monitor's microprocessor identifies the leanest cylinder by simultaneously analyzing the changes in temperature and the rates of change in temperature in all the cylinders during leaning. It is not confused by the absolute temperature of any cylinders. This dynamic analysis is the only practical means to derive correct mixture information.

My EGT indications are not very uniform at low power settings. Is this normal?

At idle and taxi power settings, poor EGT uniformity is characteristic of most engines. The low manifold pressure and low fuel flows at idle result in disparities in fuel mixture among cylinders that are not generally considered significant or troublesome.

My EGT indications are not very uniform at cruise power settings when the engine is leaned for normal cruise. Is the instrument at fault or is it my engine?

The Graphic Engine Monitor depends on a multiplexed measurement technique that assures identical calibration for all channels of the instrument. Since the same circuit measures each cylinder, temperature differences among cylinders cannot be related to instrument calibration. They may be related to temperature probe placement. It is important that each EGT probe be installed a uniform distance from the cylinder. The exhaust gases cool as they expand and travel down the exhaust stack, so the farther down the temperature probes are mounted, the cooler the indication. We recommend a mounting position tolerance of .062 inches. For some aircraft the positioning must be compromised because of bends, obstructions or baffles.

The most likely causes of poor EGT uniformity are engine related. If your engine has good compression, reasonable oil consumption in all cylinders, and no ignition faults, then uniformity problems are generally mixture related. In fuel injected engines, a one or two bar difference is considered reasonable. The most likely problem with an injected engine is dirty fuel nozzles. All injectors need routine cleaning with 100 hours being a typical interval. Nozzle clogging is not necessarily related to fuel contamination nor can it be stopped by fuel filters. It is a gradual accumulation of deposits that are left when fuel evaporates after the engine is shut down. These deposits are not very soluble in fuel so they accumulate over a period of time. In many engines this gradual constriction of fuel flow happens at a different rate for each cylinder resulting in mixture imbalances. Routine nozzle cleaning should be in the maintenance schedule of all fuel injected engines. This procedure generally takes an hour or less with most normally aspirated engines and sometimes a little longer for turbos.

Another problem is a mismatch of the nozzle flow ratings. Generally each nozzle has a letter or number that identifies its flow rating. Some times during overhaul or maintenance these nozzles may get interchanged with ones that look identical and may even have identical part numbers, but have different flow characteristics. While most engines should be equipped with injector nozzles of the same flow rating, some might benefit from nozzle mismatch to tune the flow to match the induction system, but that's another story.

Carbureted engines generally do not achieve as uniform a fuel distribution as an injected engine. The fuel-air mixture, after leaving the carb, must travel down various lengths of induction system tubing to the cylinder. Some of the fuel will have an easier path than the rest. Fuel flow in the induction system is also affected by the throttle position and carb heat controls, which create turbulence that may even be desirable to even out the fuel distribution to some extent.

Uniform fuel distribution is important to the smooth operation of any engine, but a perfectly uniform EGT display is no indication of perfection. Some degree of mixture imbalance is inherent in any type of engine.

How does the instrument handle a probe failure? Will it give erroneous indications?

The Graphic Engine Monitor continuously checks for faults in the entire measurement system and will instantly detect a faulty or marginal temperature probe or lead wire. Should a CHT probe fail, the black bar will disappear from the display and the EGT column will remain unaffected. When an EGT probe fails, the corresponding EGT column will read full scale and then go blank. The CHT display will revert to a bright bar and remain fully functional. The failure of one probe will not affect the readings from the other cylinders. However, the failure of any EGT probe will affect Lean Mode and reliable leaning indications will be unlikely.

Can any engine be operated at peak at normal cruise power settings?

No. Engine manufacturers differ in their approval of operation at peak EGT. In general, Lycoming recommends operation at peak for power settings of 75% and less while Continental recommends operation at peak for power settings of 65% and less. Other restrictions apply to some special engines and to some special conditions. Consult your Pilot's Operating Handbook or engine handbook for specific details.

Is it important to use Test Mode before every flight?

No. Test mode is intended as a diagnostic aid when a problem with the GEM system is suspected. It is unnecessary to use it routinely. If Test Mode is used, be sure to power down the instrument to exit Test Mode before starting the engine.

Does the GEM require any routine maintenance or calibration?

The Graphic Engine Monitor continuously checks and maintains its own calibration as part of the routine tasks performed by the microprocessor. Unlike other instruments, the GEM will remain in perfect calibration year after year without any adjustment.

What should I do when I detect a rise in EGT for one cylinder and the corresponding column blinks in flight?

The most common cause of a temperature rise is an ignition fault such as a fouled or internally cracked plug or an ignition harness wire breakdown. Switching mags in the air will pinpoint ignition problems, but is is not without some risk. If the fault happened to be a total mag failure then switching mags would cause total engine failure! Not a good way to impress your passengers and the sudden power stoppage might harm your engine. Some might say that even the rough running with one bad plug will harm your engine, or promote crankcase cracks. Some engine manufacturers have recommended that the mags be left alone while in flight. Switching mags in the air to diagnose problems should be done with extreme caution, but it is a fine technique for ground run-up tests. When you switch to the magneto connected to the bad plug, you will halt combustion to that cylinder and the EGT indication will drop rapidly and the engine will run rough. When that happens you have found the problem.

Why does EGT rise when a spark plug, ignition wire or magneto fails? With a lack of ignition, shouldn't it drop?

The combustion in a cylinder is designed to be initiated by both spark plugs. The flame front travels from each plug and the combustion process is largely complete by the time the exhaust valve opens. From the instant of combustion, the temperature in the cylinder rises rapidly to about 4000°F. The expanding combustion gases push the piston toward the bottom of its stroke and the combustion gases cool in the process. By the time the exhaust valve has opened and the piston has risen to expel the combustion gases, the temperature read by the EGT probe is about 1500°F. Since the EGT probe samples the temperature at the completion of the combustion cycle when the exhaust valve opens, any phenomenon that affects the timing relationship between the initiation of combustion and the opening of the exhaust valve will show as a change in exhaust gas temperature. Combustion initiated by a single plug is not as complete or as cool when the exhaust valve opens, so the EGT indicates 75-100°F higher.

Why does EGT drop during detonation. Shouldn't it rise?

Detonation is an abnormally rapid form of combustion that follows ignition induced combustion. It occurs under conditions of high compression and temperature and overly lean mixture. By the time the exhaust valve opens, the rapid combustion of detonation is significantly more advanced than normal, resulting in lower EGT and higher CHT indications.

I was taught to not lean until reaching cruise altitude, and never before 5000 ft. Is this correct?

Many pilots who routinely lean for high altitude takeoffs don't lean in the climb phase of flight. The fact remains that for normally aspirated engines higher altitudes effectively enrich the mixture whether you are in the air or on the ground. Leaning during climb is a recommended procedure for normally aspirated engines; it will improve performance and save fuel. Failing to lean in climb will foul plugs, create carbon deposits in the cylinders and make it harder to lean accurately at cruise altitude. A rule of thumb like this prohibition against leaning at low altitudes may make sense from an instructor's point of view on training flights, but it shouldn't apply to the experienced pilot seeking peak performance.

Why does EGT drop on the lean side of peak?

This phenomenon is commonly and incorrectly attributed to the cooling effect of excess air. In actual fact, the lower temperatures are the result of less fuel being admitted to the cylinder at leaner mixture settings. Less fuel means that less heat will be produced, resulting in lower temperatures.

I have read that EGT probe response time affects measurement accuracy. Is this true?

Several misstatements, based on a confused understanding of the thermodynamics involved, have been published on this subject. The accuracy of temperature measurement is unrelated to probe response time. However, probe response time severely limits the utility of unsophisticated gauge systems in finding peak EGT. All temperature probes, regardless of type or design respond to temperature changes exponentially. This means that a probe responds quickly at first and then more slowly as it approaches equilibrium with the heat source. For example, a probe exposed to a hundred degree temperature differential would indicate a 63 degree temperature change in a certain length of time, but would take five times as long to indicate the next 36 degrees. As might be expected, the nature of this response curve has serious implications when thermocouple probes are used to identify peak EGT. for leaning. The Graphic Engine Monitor was designed with this in mind. Instead of merely measuring temperatures, the GEM's microprocessor calculates the first and second derivatives of the temperature data and thus entirely eliminates probe response characteristics from the peak EGT equation. The main factors in probe response time are thermal mass and exposed area. Most non-microprocessor based systems use small thin walled EGT probes sacrificing durability for faster probe response, while others have compromised on response time with thicker more durable probes. The design of the GEM yields both instantaneous response and probe durability.

The 610 and 1200 have some advanced features not found in the 602 and 603. Following are frequently asked questions about the 610 and 1200.

When should I use NORMALIZE MODE?

NORMALIZE MODE temporarily equilizes all EGT indications on the GEM's bar graph. A subsequent change in any EGT will be immediately apparent against the "normalized" background. Be advised that NORMALIZE MODE can mask a developing problem by hiding the actual differences between cylinders. Press and hold the RESET and SELECT buttons for two seconds on the 610 to toggle between MONITOR and NORMALIZE MODES. Press and hold both SELECT buttons for two seconds on the 1200.

What are the TREND INDICATORS used for?

The up/down arrows give the pilot an indication of the most recent trend in each cylinder's EGT. Watch the trend arrows while leaning, or any time you want EGT trend information.

Why would I want to log temperature data?

Aviation training faculities find the GEM's advanced features valuble for educating pilots and mechanics in the complex science and art of engine management. A data-logging GEM can help you better understand your engine and keep it running efficiently and reliably. Researchers in engine and airframe development have used the GEM 602 and GEM 603 for over a decade. Now they are using the GEM 610 and GEMINI 1200 to data-log test-flights as part of the development and approval process.

Modern aircraft engines should run smoothly to TBO or beyond. When they don't, the operating history of the engine is often in question. The GEM's data-log can provide the type of information needed to resolve warranty disputes.

In the unfortunate event of a mishap, the GEM's datalog can serve the purpose of a "flight data recorder" for accident investigation.

Many pilots have found the GEM display indispensible for perfecting their engine management technique in flight. Data-logging allows the operator to review pilot technique on the ground post-flight. The permanent record of the engine's temperature performance can be analysed to identify inefficient or dangerous practices and adjust procedures to correct any problem areas.

The GEM's data-log is the perfect addition to a comprehensive maintenance program. While the GEM display gives the pilot the short-term trend information needed to monitor and manage the engine in real-time, the data-log contains the long-term trend data to track and predict failure modes.

A favourite issue of aircraft buyers and sellers is powerplant condition. A well maintained and properly run engine documented with a GEM data-log could only enhance resale value of an aircraft.

I don't have much computer experience. Will I need a Computer Science Degree to understand the GEM's data-log system?

Absolutely not! The GEM automaticaly records every flight and lets you transfer the data to your HP palmtop at your convenience. The data can be viewed immediately on the palmtop, or copied to your desktop personal computer, or just stored for future reference.

How much data can the 610 or 1200 store?

Hourly capacity of the GEM data-log varies depending on several factors including system configuration, condition of the aircrafts ignition systems, and pilot engine management technique. The data-log always contains the most recent flights. The GEMs data-compression system favours long flights with few power/mixture/altitude changes. GEM 610 user's typically see 20 to 30 hours, GEMINI 1200 user's see 10 to 20 hours.

How do I transfer data from the GEM to the palmtop computer?

Insight's software for interfacing with the GEM puts you in charge of the whole data retrieval process. Here is a quick run-through:

Make sure the GEM is turned on.

Run the GEMCOM program on your palmtop computer.

Select DATA TRANSFER from the main menu.

Get a flight index from the GEM by selecting LIST ALL FLTS, then press ENTER and hold the palmtop up so the Infrared communication port is about six inches in front of the GEM. Hold it there long enough for the computer to get the flight index (fanfare indicates successful completion, buzz means that contact was lost).

Decide which files you would like to transfer and select a TRANSFER function (such as TRANSFER ALL). Hold the computer before the GEM and let the file transfer complete (fanfare on completion). The transfer should take no longer than four minutes even if the entire data-log is copied.

Are the data files removed from the GEM by the transfer operation?

The DATA TRANSFER function actually makes a copy of the GEM's data files and stores the new copy in the palmtop computer's file system. The original data files in the GEMs memory are retained until the GEM needs the storage space to record a new data file.

Why must the data-log files be EXPANDED to view the data?

The GEM 610 and GEMINI 1200 store data in a proprietary compressed- file format. FILE EXPANSION is Insight's terminology for converting the GEM's compressed IDF or ADF files into standard ASC (ASCII) format. The compressed files are extremely small in size for the amount of data they contain and therefore require little storage space in the GEM or on your computer. The small size also means they take little time to transfer. The ASC files require much more storage space but are ASCII format (ASCII is the American Standard Code for Information Interchange) and are compatible with nearly every type of computer.

How can I look at the data once it's transferred to the palmtop?

The palmtop computers (HP95, HP100, and HP200) come with built-in applications that help you to manage and view your data in a variety of ways.

The FILER is a file management utility that lets you see what files are stored in the computer. It has the capability of copying and deleting files, and allows you to view the contents of a file. Use the FILER for a quick look at your GEM data files. Remember that data-log files with IDF or ADF filenames are compressed data format, files with ASC filenames can be viewed directly with the FILER.

The spreadsheet program (LOTUS 123) is designed to view and manipulate numerical data in a rows-and-columns format. Import data-log files with ASC filename into a spreadsheet (use the NUMBERS option) to take advantage of math functions and graphing capabilities.

Spend a few minutes reading the manual that came with your palmtop computer. Most operations require just a few button pushes.

How about copying the data from the palmtop to my desktop or laptop computer?

There are several different ways that data can be transferred from the HP palmtop computers; wired serial interface, wireless Infrared interface, and the PCMCIA card slot.

The wired serial interface is the most common method of transferring files, however Infrared communications and PCMCIA products are rapidly growing in popularity.

To use the wired interface, a serial connection cable and data- communications software are required. Complete packages are available from several manufacturers. Consult your favourite supplier of computer products to determine the best method of interfacing the HP palmtop with your other computers.

I want to graph and analyze my data-files on my desk-top PC. Are there any tools available for this?

The ASCII format data-files produced by Insight's GEM95 software are compatible with a wide variety of PC software. Spreadsheet programs are a standard application installed on most computers and lend themselves well to manipulating and viewing numerical data. Most spreadsheets now offer powerful graphing capabilities.

Glossary

Best economy mixture

The mixture or fuel/air ratio producing the optimum ratio of horsepower to fuel consumption. This mixture is generally found at an EGT around 50°F on the lean side of peak.

Best power mixture

The mixture or fuel/air ratio producing the most power from a given amount of fuel. This mixture is generally found at an EGT around 125°F on the rich side of peak EGT.

Booststrapping

A condition which can occur in turbocharged aircrraft in high altitude cruise (with the wastegate closed; See Wastegate below), in which large, unexpected manifold pressure excursions take place spontaneously. The manifold pressure instability is due to feedback loop effects in the turbo system. The usual "cures" are to increase engine rpm and/or open the wastegate.

Camshaft

A shaft that runs at half of normal engine (crankshaft) speed, with metal lobes on it to trip open the engine's intake and exhaust valves at the right time and in the right order. Camshaft action determines valve action.

Charge

Fuel-air charge; the incoming of fuel and air as it arrived at the combustion chamber.

CNC

Computer Numerical Control

Critical altitude

In a turgbocharged aircraft, the altitude above which sea level rated manifold pressure can no longer be maintained; or the altitude above which the wastegate is closed all the way, all the time.

Cylinder Head Temperature (CHT)

The temperature of the head portion of the cylinder, as measured by a thermocouple placed either in the spark plug area or a threaded boss in the head. The head portion of the cylinder contains the combustion chamber dome, the valves, valve springs, rockers, intake port, and exhaust port. Overheating of the head can occur easily in an air cooled engine, hence the need for CHT instrumentation.

Detonation

Combustion knock; the premature, spontaneous auto-ignition of the unburned fuel/air charge ahead of the flame front, in a combustion chamber in which discharge of the spark plug(s) has already occured. In other words, the a spark event has taken place at its time, fuel and air are present, and a portion of the fuel and air mixture has begun burning. However, the unburned portion (compressed by the expansion of the burning charge) reaches a pressure and temperature sufficient to cause a sudden explosion of the entire charge. This produces the familiar knocking sound in an automobile engine. It also produces mechanical stresses which can eventually fail rings, pistons, connecting rods, or cylinder heads or valves. In an airplane engine, the ping or knock sound cannot usually be heard from the cockpit. Hence it is especially important for a pilot to avoid letting an engine detonate in the first place. Detonation can be caused by fuel of insufficient octane rating, or (with proper octane fuel) operation of a very high power output with a very lean mixture. Improperly advanced magneto timing will also hasten the onset of detonation, as will heating of the incoming fuel-air charge (by carburetor heat or by compression via turbocharger).

DSP

Digital Signal Processing

IFR

Instrument Flight Rules

Induction system

Generally, the entire air intake system of the engine, from the air filter (or airscoop) to the intake ports.

Leanest cylinder

The cylinder that operated at the lowest over all fuel/air ratio compared to the other cylinders in the engine. By the EGT method, leanest cylinder is defined as the cylinder that reaches peak EGT first during progressive leanout.

LSI

Large Scale Integration.

Manifold pressure

The pressure, in inches of mercury, of air within the engine induction system downstream of the throttle butterfly. This pressure is directly related to air flow and hence (under most circumstances) engine power output.

NPT

National Pipe Thread.

OEM

Original Equipment Manufacturer.

Preignition

Ignition of the fuel-air charge in a cylinder before normal discharge of the spark plug. In a broad sense, anything that causes combustion to occur before the specified ignition timing (as set forth on the engine data plate) could be said to be causing preignition. Thus, even the aircraft ignition system -if maladjusted- could cause preignition to occur. Because, of the abnormal stresses induced, preignition can be extremely damaging to an engine, and at high power settings the damage can occur very quickly (often in less than 30 seconds). Operation with detergent-type automotive oils can cause preignition due to ash deposits (from the Barium and Calcium containing detergents) which leaves hot spots in the combustion chamber. Hence, the use of "ashless dispersant" oils in aviation. If piston or rod failiure does not occur first, preignition will make itself evident by a very high CHT indications.

Pressure ratio

In turbocharging, the ratio of outlet to inlet pressure at the turbo compressor.

Supercharger

Generally, any device that forcibly increases the amount of air flow by an engine's induction system. Although technically speaking a turbocharger is a type of supercharger. Today the word is usually used to connote a mechanical supercharger (that is a fan or pump driven mechanically off an engine's crankshaft.) Mechanical superchargers can be found on many radial engines, and on certain 480 and 540 cubic inch Lycoming engines.

TCA

Terminal Control Area (Class B Airspace).

Thermocouple

A junction between two dissimilar metal alloys in which a small voltage is produced which varies with temperature. The voltage is produced because different metals have different tendencies to "donate" their electrons. Different alloys are used in thermocouples for different heat range applications.

TBO

Time Between Overhaul.

Turbocharger

Also sometimes called a "turbosupercharger." A turbocharger is an exhaust driven supercharger, used for increasing the manifold pressure, and therefore the total power output of an engine.

Wastegate

A control valve (which may be of either the butterfly or poppet type) in the exhaust system which governs the flow of exhaust to a turbocharger turbine. Usually there is a 'Y' in the exhaust system, with the wastegate on the one arm of the 'Y' and the turbocharger itself on the other arm. Since the exhaust must pass through one of the arms before leaving the engine, closing the wastegate increases the flow through the turbocharger/wastegates may be manually controlled or automatically actuated via a bellows-type aneroid controller, depending on the installation.