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© 1997 Alcor, Inc. All rights reserved. EGT and Combustion Analysis in a Nutshell
© 1997 Alcor, Inc. All rights reserved.
EGT and Combustion Analysis in a Nutshell
www.alcorinc.com [email protected]
Foreword
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Since 1962 when Alcor ® introduced theExhaust Gas Temperature (EGT) method ofmixture control* over 150,000 ** piston-powered aircraft engines have had their"diets" controlled with Alcor ® EGT systems.Impressive as these statistics may be,Alcor ® has found that a great deal of misunderstanding exists in the proper utilization of the EGT method of mixturecontrol and particularly the more sophisti-cated Combustion Analyzer. Alcor ® has alsofound that practically every pilot is desirousof obtaining the necessary know-how onEGT systems to make possible minimumfuel consumption, maximum TBO, etc., to reduce the high cost of flying ...so this text.
So that terminology doesn't confuse you,the single probe EGT system is referred toin this text as an EGT Mixture ControlIndicator and the multi-probe EGT Indicator(exhaust probe for each cylinder) is called aCombustion Analyzer. The term EngineAnalyzer has been used in previous Alcor ®
literature and the change to CombustionAnalyzer has been made to be moredescriptive.
The problem of where to set the mixture isgreatly compounded by the fact that, evenin an engine in good mechanical condition,all cylinders do not receive the same fuel-air mixture . . . there can be as much as50% differential to contend with. (See "TheProblem of Mixture Distribution . . .'' pg 8.
Although the goal is to control the mixtureof the leanest cylinder, a pilot is faced with the dilemma of knowing for surewhich cylinder is the leanest. Therefore,excess fuel is used to compensate for thisuncertainty. The Combustion Analyzersolves this dilemma, permitting optimummixture control by being able to ''see" themixture distribution so that the leanestcylinder can always be selected for mixturecontrol.
A much greater advantage of theCombustion Analyzer is its ability to act as atrouble detector. Proper understanding anduse of the multi- probe EGT unit permits a
pilot to know when trouble is brewing inhis engine, long before he otherwisewould, so that corrective action can betaken to prevent emergencies and to minimize engine damage. At Alcor ® wehave quite a lot of letters of thanks fromcustomers who relate specific incidentswhere their Combustion Analyzer haswarned of a situation that would otherwisehave been certain engine failure. These pilots feel that the Combustion Analyzer isa must, particularly for the single enginepilot, from the standpoint of safety, and forthe twin engine pilot from the standpointof savings. Most agree that their unit has paid for itself within 500 hours of operation.
It is our goal in this booklet to present theEGT method of mixture control and com-bustion analysis in such a way that pilotscan derive maximum benefits from theirEGT systems. These benefits include notonly economy and safety, but the greaterpeace of mind that comes from increasedengine reliability.
If we fail to accomplish this goal in yourcase, please let us know so that we maywork directly with you in answering anyquestions you may have.
Al Hundere
www.alcorinc.com [email protected]
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A piston aircraft engine can be consideredthe same as a team of horses because eachcylinder is an engine in itself and the total“pull’’ is dependent on the ‘care and feeding’ of each ‘’horse,’’ or cylinder. Here we will first look at a single cylinderbefore introducing the complexity of multi-cylinders. Figure 1 presents the fourstrokes of the four-cycle piston engine. It is the quantity of fuel and air inducted in the first stroke, or intake stroke, thatdetermines: the combustion temperatureafter the second stroke; the power generated in the third stroke; and theexhaust temperature for the fourth stroke.
Figure 2 shows how the throttle controlsthe flow of air and the mixture controllever adjusts the flow of the fuel. It is theratio of fuel to air, which we call ‘’mixture,’’that determines the combustion temperature which, in turn, is directly related to the exhaust temperature. It must be kept in mind that an engine’s fuelrequirements are dependent on the mixture and not the fuelflow; therefore, a fuel flowindicator, no matter howaccurate, does not define the fuel requirements asaccurately as the EGT
method of mixture control. Fuel is addedto the air flowing through the engine carburetor or induction system for the purpose of generating heat. A pistonengine, like any other aircraft engine, is aheat engine. If fuel were added only toobtain heat, then mixture control would begreatly simplified, but a secondary functionof the fuel to a piston engine is to providecooling when needed.
The common method of measuring EGT is shown in Figure 2: a very small probe,inserted into the exhaust, is wired to a special millivolt meter in the panel; so thatvery small changes in EGT can be easilyobserved, the meter pointer doesn’t startmoving until the temperature is about1200°F, and is full scale about 1700°F.Figure 3 shows a typical meter and probe.
It is important to understand the hardware involved.
FIGURE 3
P/N 86255
P/N 46150
FIGURE 2
FIGURE 1
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What is so special about EGT for mixturecontrol? Why not use CHT (cylinder headtemperature? More than thirty years ago,flight engineers for some large transportaircraft used CHT as a method of mixturecontrol by a procedure requiring a fuel flowindicator and the plotting of a curve to findpeak CHT. The novelty of the EGT methodof mixture control is the observing of peaktemperature simultaneously with adjustingthe mixture, eliminating the need for a fuelflow indicator and the plotting of a curve.The method requires a fast-response EGTsensor and an indicating means as shown inFigure 3, sensitive enough so that verysmall changes in EGT can readily beobserved while they are occurring, substantially instantaneous indication of EGT.
Figure 4 shows how EGT changes with mixture leaning. As the mixture is leanedfrom the full rich position, the EGT increases as shown because the amount ofexcess fuel is being decreased. The lesserthe excess fuel, the greater the EGT. As
long as excess fuel is present, the EGTincreases with mixture leaning until noexcess fuel exists, at which point peak EGToccurs, as shown in Figure 4. Further leaning decreases the EGT as shownbecause of the cooling from the excess air.“Peak EGT” is the key to the EGT method ofmixture control and it is the mixture settingwhere there is no excess fuel nor excess air,both of which cause cooling of theexhaust. The black band shown in Figure 4for the lean side of peak EGT is to indicatehow engines vary greatly in their ability toburn lean mixtures; this affects the degreeof leaning beyond peak EGT before leanmisfire rough running engine occurs. Withsufficiently poor lean mixture combustionthe peak can disappear completely orbecome very flat. Also a secondary peakcan develop with poor lean mixture combustion, which should be ignored insetting the mixture.
REMEMBER-When using the EGT method ofmixture control, all mixture settings aremade relative to peak EGT.
“Peak EGT” is the key to the EGTmethod of mixture control.
FULL
RIC
H
EXHAUST GAS TEMPERATURE
* PEAK EGT
FUELCOOLING
AIRCOOLING
LEA
N M
ISFI
RE
FIGURE 4
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At cruise power settings, peak EGT is normally considered optimum because it is the mixture setting that gives maximumefficiency of fuel utilization, that is, maximum energy output per unit of fuel.As will be discussed later, a mixture leanerthan peak increases aircraft range only as aresult of a decrease in power and not fromthe efficiency of fuel utilization.
Figure 5 shows the effect of mixture leaning on aircraft airspeed holding rpmand manifold pressure constant. As shown,the maximum airspeed or ‘’best power’’mixture is obtained when the mixture is setto give an EGT approximately 100°F belowpeak EGT on the rich side. This is at theexpense of a 15% increase in fuel consumption. Note also that as the mixtureis leaned from best power mixture to peakEGT, the air speed decreases two miles perhour and then decreases rapidly with further leaning. Since aircraft range is controlled by both mixture setting andpower setting, the effect of mixture setting on range is best illustrated underconditions where the power is held constant (constant airspeed) by varying themanifold pressure as the mixture ischanged, as shown in Figure 6. Note that
going leaner than peak EGT does notincrease the miles per gallon significantlybut simply results in the requirement of ahigher manifold pressure to maintain constant airspeed. For this reason peak EGTis considered the optimum mixture formaximum range.
Adding excess fuel for a slight gain inpower or airspeed is only one reason foroperating richer than peak EGT. If the cruisepower setting is sufficiently high, thenexcess fuel is needed for cooling; at lowcruise power settings there is no benefitfrom the use of excess fuel unless it is tocompensate for the mixture distributionproblem of a multi-cylinder engine. Boththese will be discussed in the next section.
Alcor ® has seen no indication that the TBO(time between overhauls) of any modernaircraft engine is shortened by operating atpeak EGT mixture setting up to 65% power,provided that the leanest cylinder is trulyselected and used for mixture control.Lycoming, on the other hand*, hasapproved operating at peak EGT for cruise up to 75% power except for gearsupercharged engines, where they set the limit at 65% power.
The mixture setting for maximum range at cruise powers up to 65%... is “Peak EGT”.
FIGURE 5 FIGURE 6
*For the Engine Manufacturers’ specific recommendations, see Continental’s Service Bulletin M7316, Rev 1 and Lycoming’s Bulletin No 1 094C The recommendations given in this text are intendedonly to supplement the leaning instructions given in the Airplane Owner’s Manual and the EngineOperator’s Manual and not as replacements there of.
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The economic aspect of mixture leaning is illustrated in Figure 7, which presents actualflight test data from a typical single engine aircraft at 10,000 feet. The engine is an 0-470with a power setting of 65%. Note that $2,600 is spent on excess fuel in 1000 hours whenbest power mixture is used rather than peak EGT. This is considered with fuel at $2.00/gal.*
*Cost per gallon may vary depending on regional FBO, location and availability.
The economical considerations of mixturecontrol over 1000 hours of operation.
FIGURE 7
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As an example, assume an average unsupercharged engine that has a take-offmanifold pressure of 29.0” and that themixture is maintained at peak EGT as thethrottle is slowly moved from cruise (22.0”)to full throttle (29.0” manifold pressure).The resultant increased temperature of theexhaust valve and/or spark plug electrodewould cause preignition, the onset ofwhich would be hastened by detonation.Preignition of this type is destructive andengine failure can occur in a matter ofseconds. Figure 8 illustrates how such a
failure would be reflected in the EGT; a typical failure from preignition is shown inthe photograph above.
Excess fuel must be used for cooling at the higher power settings to keep from exceeding maximum allowable temperatures. The exhaust valve is usuallythe most critical with respect to excessive
temperatures. When the mixture isenriched at powers above 65% to maintainconstant exhaust valve temperature, theEGT versus power curve is as shown inFigure 9. As illustrated, the EGT at 100%power needs to be 100°F lower than peakEGT at 65% if the exhaust valve is to bemaintained at the same temperature as it is at peak EGT for 65% power.
What happens when the mixture ismaintained at “Peak EGT” and the poweris increased from 65°% to 100%?
Alcor ® recommends that the peak EGT at 65% power beconsidered the maximum allowable EGT NEVER TO EXCEED.
WARNING! *Don’t lean the mixture to “PeakEGT” above normal cruise power settings.
FIGURE 8 FIGURE 9
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The dial presentation shown in Figure 10 is intended to make all the precedinginformation easier and simpler forthe pilot. The reference mark(*) is peak EGT at 65% powerand, with such calibration,all the pilot has to remem-ber is to keep the EGT inthe ‘’green’’ at all times—takeoff, climb and cruise,and descent.
EGT dials are provided withrelative scales rather than colored range markings as shownin Figure 10, which makes it necessaryto keep a mental picture of these markingsand remember that the reference point (*)is peak EGT at 65% power under fixed oper-ating conditions such as 2300 rpm and thealtitude where full throttle produces 65%power. Peak EGT will occur above or belowthe reference mark for cruise power settings which vary from that used for calibration. The blue area marked on thedial in Figure 10 indicates the normal EGTduring ground runup, say 1700 rpm. Thiscan be checked at sea-level, full-rich,ground runup, and can be used for leaning
during ground runup prior to take off fromhigh altitude airports. Why not provide
a specific mark for the correctmixture during takeoff and
climb, like the center of thegreen arc? The reason isthat such a mark would bevalid only on a day whenthe outside air tempera-ture is average, say 70°F.During takeoff and climb
the cylinder head tempera-ture must be considered in
mixture leaning and on a veryhot day, more than the normal
amount of fuel must be used forengine cooling so that the EGT readingneeds to be decreased to control CHT.Similarly, on a very cold day, the EGT can behigher because less fuel is needed for cooling, as illustrated in Figure ll.
For cruise, using the Figure 10 dial presentation, all one needs to remember isto lean to peak EGT for best fuel economyunless this puts the EGT into the yellow arc,in which case the mixture should beenriched sufficiently to keep the EGT in the green.
All the pilot has to remember is to keepthe EGT in the “green” at all times.
FIGURE 10
FIGURE 11
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The term ‘’mixture distribution’’ means theuniformity by which the fuel distributesitself with the air being inducted into theengine through the carburetor or the airintake system of a fuel injection engine
Perfect mixture distribution is where thequantity of fuel to each cylinder is thesame percentage of air ingested into each.In the preceding sections, an engine withperfect mixture distribution is assumed,which is practically non-existent. The leanest cylinder (one receiving the lowestpercentage of fuel) should be selected for mixture control, as the engine manufacturers have recognized by recommending the location of the EGTprobe in the exhaust of the leanest cylinder. Excess fuel provides a margin ofsafety so one can say that the leanest cylinder has the lowest margin of safety!The mixture distribution problem takes onastounding significance because there is nosuch thing as THE leanest cylinder under alloperating conditions, as is shown by examining some typical engine mixture distribution patterns. Even with an engine that approaches perfect mixturedistribution, any one cylinder can suddenlybecome much leaner through a simplemalfunction.
Figure 12 represents a typical carburetor engine. Ateach altitude the leanestcylinder was leaned to peakEGT after setting off 65%power. At 9000 feet fullthrottle was reached andthen the power decreasedwith further increase in altitude. Note that cylinderNo. 1 was leanest part of thetime (highest EGT) and No. 3was leanest at full throttle;note further that the mix-ture distribution is best justbefore reaching full throttle.
Now look at Figure 13 for a fuel injection engine. A common misconception is
that fuel injection engines have a perfectmixture distribution. Any cylinder of a fuelinjection engine can suddenly become theleanest due to a restriction developing, likea piece of dirt getting into the fuel nozzleorifice, or from a residue build up.
How can one be sure that the cylinder with the highest EGT is truly the leanestcylinder?
Figure 14 shows the mixture distributionpattern of a four-cylinder engine with each cylinder in the same condition andreceiving the same airflow, all cylindersreaching peak at exactly the same EGT. Theleanest cylinder has the highest EGT at richmixtures and it peaks at highest fuel flow,as shown. This situation is not always thecase, as shown in Figure 15 where all cylinders peak at the same fuel flows as inFigure 14 but the leanest cylinder nolonger has the highest EGT. Anytime it issuspected that another cylinder other thanthe one with the highest EGT is the leanest,the simplest procedure for verification is tolean the cylinder with the highest EGT topeak, turn the selector switch to the othercylinder in question and then enrich themixture just enough to see some pointertravel. If the EGT increases, it is leaner, andwas already on the lean side of peak.
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FIGURE 12
FIGURE 14
FIGURE 13
FIGURE 15
The problem of mixture distributionresults from the fact there is no suchthing as THE leanest cylinder.
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One solution to the mixture distributionproblem is to enrich the mixture to dropthe EGT a given amount, say 50°F, to compensate. The amount of compensationwill depend on whether the exhaust probeis sensing one cylinder, so-called “leanestcylinder” or a cluster of cylinders such as 1,3, and 5. For example, Continental in theirService Bulletin M73-16 (Rev. 1 ) dated 29August '73 states, “If the unit (exhaustprobe) is located in the exhaust systemsensing multiple cylinders, operationfurther from peak is required to avoid any
one cylinder running too lean.” The obvious disadvantage of this method ofcompensation is loss of fuel economy. As was shown by Figure 7, a 100°F enrichment for compensation costs $4,400in 1000 hours for an aircraft with an 0-470engine, or half of this, $2,200, for the 50°Fenrichment from peak recommended byContinental. Additionally, the “over rich”cylinders are prone to develop combustionchamber deposits, plug fouling, and other detrimental effects from “over rich” operation
THE BEST SOLUTION is to monitor eachcylinder with a Combustion Analyzer, sothat the leanest cylinder can be pinpointedeven as it may change. In cases where poormixture distribution is noted, carburetorheat, alternate air, or throttle position canbe adjusted to improve distribution and''fine tune'' the mixture setting for maximum range and longer engine life.This can only be done with any degree of accuracy or safety. of course, by
monitoring each cylinder with an EGTCombustion Analyzer.
The equipment involved for the EGTCombustion Analyzer is the same as for theEGT Mixture Control Indicator with addedexhaust probes -one for each cylinder-anda selector switch which can be incorporatedwith the meter or the simultaneous readingEGT called the Multi-Channel CombustionAnalyzer (MCCA for short).
What is the solution to the mixturedistribution problem?
The EGT Combustion Analyzer.
P/N 46150with P/N 80825
P/N 47025 P/N 46354
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COMBUSTION ANALYSIS IN A NUTSHELL-Simply remember to select the leanestcylinder-normally the one with the highest EGT-for all mixture controlsettings, and then “scan” theother cylinders when convenient or when there isany concern for theirhealth, such as a “feel” of roughness. Any enginetrouble that needs immediate attention, suchas pre-ignition, will alert the pilot through engineroughness that a need existsto scan for engine trouble. TheAlcor ® Multi-Channel CombustionAnalyzer was designed for those who prefer the convenience and peace of mindthat is provided by monitoring all cylinderssimultaneously.
The first sign of trouble shows up as an
indication either higher or lower than normal. When the EGT increases as shown,enrich the mixture if possible to
compensate until you land and find amechanic. You may prefer,
however, from the standpointof safety and peace of mind(not to mention reducedmechanic's charges), to beyour own “engine doctor”to pinpoint the cause of thetrouble before landing.
EGT and Combustion Analysiscan be quite simple. Although
most engine problems are best diagnosed with EGT, there are certain
ones that can be detected most readily byan increase in CHT (cylinder head tempera-ture); for example, a cracked cylinder head,or engine baffling problems. For this reason, some pilots prefer EGT/CHTAnalyzers as shown on the previous page.
Trouble is reading in the yellow.
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