Report No. NA-68-27,DS-68-8) FINAL REPORT

Project No. 520-0G3-0iX

RECIPROCATING ENGINE AND EXHAUST VIBRATION

"-AND TEMPERATURE LEVELS IN GENERAL AVIATION AIRCRAFT

JUNE 1968

DEPARTMENT OF TRANSPORTATIONFEDERAL AVIATION ADMINISTRATION

National Aviation Facilities Experimental CenterAtlantic City, New Jersey 08405

Reproducod by 'heCLEARINGHOUSE

for Federal Scienific & TochnicalInformaion Springfiuld Va. 22151 S4"

"The Federal Aviation Administration is responsible for the promotion, regulationand safety of civil aviation and for the development and operation of a commonsystem of airnavigation and air traffic control facilities which provides for the

safe and efficient use of airspace by both civil and military aircraft."

"The National Aviation Facilities Experimental Center maintains laboratories,facilities, skills and services to support FAA research, development and imple-mentation programs through analysis, experimentation and evaluation of aviationconcepts, procedures, systems and equipment."

FINAL REPORT

RECIPROCATING ENGINE AND EX!AUST VIBRATICN

AND TD2IPERATURL LEVELS IN GENERAL AVIATION AIRCRAFT

PROJECT NO. 520-003-OIX

REPORT NO. NA-68-27(DS-68-8)

Prepared by:GERALD R. SLUSH ER

for

AIRCRAFT DEVELOPMENT SERVICE

June 1968

This report is approved for unlimited availability. It does notniecessarily reflect Federal Aviation Administration policy inall respects, and it does not, in itself, constitute a standard,

specification, or regulation.

DEPARTMENT OF TRANSPORTATION* Federal Aviation Administration

National Aviation Facilities Experimental Center

Atlantic City, New Jersey 08405t

ABSTRACT

The engine and exhaust system vibration and exhaust gae and metaltemperature levels were determined for flight and ground conditionson several single-engine aircraft for purposes of establishing exhaustsystem and heat exchanger design and test criteria. The temperaturedata were presented as a function of engine compression ratio and thevibration data were plotted against engine horsepower to foster thegeneral utilization of the information.

Method of data presentation permits the estimation of exhaust gastemperatures for horizontally-opposed, reciprocating engines.Temperature measurements indicated uneven heating of the muffler outerwall (heat exchanger surface) reflecting uneven flow of the exhaustgases through and around the baffles and diffusers probably producingthermal stresses and contributing to failures. Baffles and diffuserswithin the mufflers of engines with compression ratios of 8.5:1 orhigher are exposed to exhaust gas temperature levels under whichstandard construction materials (AISI 321 and AISI 347 stainless steels)become marginal with respect to high-temperature oxidation, carburizatioll,and attack by lead compounds.

Vibration of general aviation aircraft engines was noted to increase withincreased power rating and reached maximum intensities under takeoffconditions. The acceleration level of mufflers on engines of high powercompared favorably with the HIL-STD-810A Vibration Test Specificationfor equipment mounted directly on aircraft engines. Recommended pro-cedure for development of new exhaust system designs involved randomvibration testing under operating thermal conditions.

iii

TABLE Of CONTENTS

Page

ABSTRACT ii

INTrRODUCTION

Purpose LBackground 1

DISCUSSION 2

Description of Equipment and Procedures 2General 2In-Flisht Tests 3Ground Tests 3

Results and Analysis 6Exhaust Gas Temperatures 6Exhaust System Metal Temperatures 8fngine Vibration Intensity 8Muffler Vibration Intensity 14Power and Vibratiou 20Vibration Specification 20

CONCLUSIONS 27

RECOMMENDATIONS 28

REFUENCES 29

APPENDIX I Description of Instrumentation (4 pages) 1-1

APPENDIX 2 Basic Otto Engine Cycle Theory as Related to ExhaustGas Temperature (6 pages) 2-1

APPENDIX 3 Engine Exhaust Cas and Metal Temperatures (8 pages) 3-1

v

LIST OF ILLUSTRATIONS

Figure Page

1 Typical Temperature and Vibration Instrumentation,Engine and Exhaust System 4

2 Engine Stands, Ground Test 5

3 Engine Exhaust Gas Temperatures Within the Stackor Manifold Under Maximum Power Conditions 7

4 Engine Exhaust Gas Temperatures Within theTailpipe Under Maximum Power Conditions 7

5 Stack Metal Temperatures Under Maximu PowerConditions 9

S6 Manifold Metal Temperatures Under Maximum PowerConditions 9

7 Muffler Outer Wall Metal Temperatures UnderMaximum Power Conditions 10

8 Tailpipe Metal Temperatures Under Maximum PowerConditions 10

9 History of Engine Broadband Vibration Intensity,Aircraft Code Model "B" 11

10 Vibration Wave Identification, Engine CrankshaftSpeed 1306 RPM 13

11 Spectral Characteristics, Engine VerticalVibration Intensity 15

12 Spectral Characteristics, Engine Lateral VibrationIntensity i,

13 Spectral Characteristics, Engine LongitudinalVibration Intensity 17

14 Spectral Characteristics, Muffler VerticalVibration Intensity, Separate Type ExhaustSystems IC

15 Spectral Characteristics, Muffler VerticalVibration Intensity, Cantilevered CrossoverType Exhaust Systems 19

vii

LIST OF ILLUSTRATIONS (continued)

Figure Page

16 Engine Vertical Acceleration Level Versus 21Horsepower

17 Engine Lateral Acceleration Level Versus 22Horsepower

18 Engine Longitudinal Acceleration Level Versus 23Horsepower

19 Muffler Vertical Acceleration Level Versus 24Engine Horsepower

20 MIL-STD-810A, Vibration Test Specification for 25Equipment Mounted Directly on Aircraft Engines

1.1 Instrumentation Location, Cross-Over Type 1-2Exhaust System

1.2 Instrumentation Location, Separate Type 1-3

Exhaust System

2.1 Presgure-Volme Diagram, Otto Engine Cycle 2-2

3.1 Exhaust Gas and Metal Temperatures, Aircraft 3-2Code Model "A"

3,2 Exhaust Gas and Metal Temperatures, Aircreft 3-3Code todel "3"

3.3 Ec.haust Gas and Metal Tesperatures, Aircraft 3-4Lode Model "C"

3.4 Exhaust Gas and Metal Tmiperatiares, Aircraft 3-5Code Model "D"

3.5 Exhaust Gas and Metal Tenperatuires, Aircraft 3-6Code Model "r'

3.6 Exhaust Gas and Metal Tenpe-a.ures, Aircraft 3-7Code Model "r"

3.7 Exhaust Gas aid Metal Teimperatures, Aircraft 3-8Code Hodipl "C'

viii

INTRODUCTIONi

Purpose

The purpose of this project yes to measure and anaLyse the engineaen exhaust system vibration and exhaust temperature levels in generalaviation aircraft for use in cstablishing exhaust and heat exchangerdesign and test criteria in aircraft certification.

Background

The Federal Aviation Administration wAs engaged in a progivAconcerned vith the safety and reliability of engine oxhaust systemsin light aircraft. The objectives of this program were: (a) toidentify deficiencies in design and construction of engine exhaustsystems which compromise safety through possible -erbon mocoxidepoisoning, in-flight fire and power loss; (b) to develcp an exhaustsystem qualification test and procedure suitable tor use bymanufacturers as a requirement for certification to enhance thereliability and integrity of these exhaust systems and t~o reducethe hazards associated with failures; (c) to investigate end developcabin heaters designed to eliminate or miniAize the carbon monoxidehazard; and (d) to evaluate low-cost carbon monoxide indicators todetermine their performance and suitability for use in generalaviation aircraft.

Published results of the program include technical reports listedunder References 1, 2, and 3. The results reported here were directedtoward identification of the operating environment.

The severe conditions under which engine exhaust systems andexhaust heat exchangers operate have been responsible for a lsrgenumber of failures, some of which have resulted in fatal accid'ins.Malfunctions an(' defects in the exhaust system can create threesepai'ate hazards to flilht safety: (a) fracture* in the heatexchanger surface (muffler outer wall) my result in contamination ofthe cabin with exhaust gases containing carbon mo-noxide; (b) failelmuffler baffles may restrict the exhaust gas path and effect enginekwr loss by creating excessive exhaust beck pressure; and (c) whenruptured, the exhaust manifold or stacks mey induce a fire hazard byfailure to contain the exhaust flaes.

The continual vibratio, 1&der corrosive and high-thermwloperating conditions most likely -tllI cause fatigue fractures,irticularly following deteriorati•i• r' the etal by csrburipitiom,high-temperature oxidattio, attack by ,a comiounds, and metallurgicalphase changes. Accurate information coarua'i exhaust system vibrationintensities and s:haust temperature levels can N utilized as criteriaby the designer trtr seloctioa of the required mattiial (alloy) end&%;.erisl thickness for a particular appli.-ation. TnIs information is also

I

needed for realistic simulation of the operating conditions vhenevaluating exhaust assemblies on thermal-vibration test equipment.Accurate information concerning engine vibration intensities andexhaust temperature levels is also beneficial and valuable to theengine manufacturers for many purposes.

DISCUSSION

Description of Equipment and Procedures

Gegra 1

The project endeavor was concerned with single-engine, two throughsix-place, general aviation aircraft incorporating exhaust gas-to-airheat exchangers. Aircraft powerplants were four or six-cylinder, hori-zontally-opposed, reciprocating engines ranging from 100 to 260 hp.Engine compression ratios varied from 6.75:1 to 8.6:1. The aircraftand engines were models manufactured in relatively large quantitiesby two light-aircraft companies and tvo engine companies, respectively.Aircraft and engine specifications are listed in Table I.

TABLE I

AIRCRAFT AND ENGINE INFiM•IATION

Aircraft Number Number EngineCode of Engin, of Engine Compression

Model Places Ratin Cylinders Displacement Ratio---- (cu in)

A 4 250 6 540 8.5:1

B 6 260 6 470 8.6:1

C 4 180 4 360 8.5:1

0 4 145 6 300 7.0:1

g 2 4 200 7.0:1

F 2 108 4 235 6.75:1

C 4 160 4 320 8.5:1

2

The program consisted of two types of testing: (a) involvingoperating aircraft; and (b) testing on the ground involving aircraftengines and parts installed on engine stands.

The instrumentation for both types of testing was identical. Thephotograph of Figure 1 depicts typical engine and exhaust systeminstrumentation. A description of instrumentation is provided inAppendix 1.

Exhaust gas temperatures were measured within the stack ormanifold and within the tailpipe. Metal temperatures were monitoredat locations on the stack, manifold, tailpipe, and two places on themuffler outer wall (heat exchanger surface).

The engine vibration (required for testing exhaust systems) wasmeasuced (with accelerometers) in each of the three major axes ofthe engine at the exhaust flange locations or in proximity thereof.Vibration of the muffler or heat exchanger wos recorded in the motion-sensitive direction or in most instances the vertical axis with twoaccelerometers.

In-Flight Tests

Five aircraft were tested in flight to measure the vibrationand the temperature levels of the engine and the exhaust system.Measurements were recorded during the takeoff, beginning at thestart of the roll and ending at the altitude of 300 feet. Meas-urements were also recorded under stabilized engine speed conditionsat altitudes of 4000 to 7000 feet. Transient measurements wererecorded during changes of engine rpm under similar altitudeconditions.

Ground Tests

Testing on the ground was accomplished on each of six engineinstallations mounted on stands, incorporating aircraft parts andconfiguration forward of the firewall, as shown in Figure 2. Theengines were mounted on the arcraft vibration isolators and sup-ported in cantilevered fashion fru. the firewall with the standardaircraft engine mount assembly.

Engine power was absorbed by the identical two-blade propellersas utilized in flight. The cowling and baffles were standard with theexception that the top cowling was modified for incorporation of airscoops to provide additional engine-cooling air from the flc. inducedby the propeller. large oil coolers were placed in the propeller slipstream for cooling the engine oil as required for running the engineson the ground.

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Vibration measurements were recorded under both steady-stateand transient conditions of engine speed. Inspection of the datarevealed that the vibration waveform was composed of a large numberof superimposed frequencies and a spectral analysis was performedfor determination of vibration levels at discrete frequencies.

Infcrmation regarding the frequency and magnitude of accelerationwas observed visually utilizing a vibration wave analyzer. The exhaustgas and metal temperatures were recorded from visual observations on aprecision direct-reading potentiometer. A detailed description includ-ing the accuracy of the instrumentation systems is contained inAppendix 1.

Results and Analysis

Exhaust Gas Temperatures

Engine exhaust gas temperatures, measured within both the stackand tailpipe in various aircraft in flight and on the ground, werecorrected to Standard Day conditions, and the corrected measurementsat rated engine power were plotted against the engine compreasionratio as depicted in Figures 3 and 4. Exhaust gas temperature isknown to be a function of the fuel-to-air mixture and the enginecompression ratio. The theory and the mathematical relationshipsconcerning engine compression ratio as related to exhaust gastemperature are discussed in Appendix 2. Suffice to note here thatthe increase in exhaust gas temperature with compression ratio(Figures 3 and 4) reflects the heat of compression added when thegas was compressed to the higher pressures. The gas temperatureinformation was prepared as a function of the engine compressionratio for presentation of the data in a form for generalized use.Maximum exhaust gas temperatures may be estimated for four-cycle,horizontally opposed, air-cooled engines when designed and con-structed under present day state-of-the-art technology.

Exhaust gases wIthin the stack, or downstream of the engine,approached temperature levels of 1600°F when engines with a compressionratio of 8.5:1 were operated at maximum power and lean fuel-to-airmixtures. When rich fuel-to-eir mixtures were selected under similarconditions, the exh&ust gas temperatures within the stack were reducedto somewhat less than 15000F. Exhaust gases in the stacks or manifoidof engines with a compression ratio of 7:1 and operated with leanmixtures were measured at temperature levels of 1480 0 F, and with richmixtures, gas temperatures of 13800F were indicated. The temperatureof the exhaust gases in the tailpipe were 50 to 75'F less than thetemperature of gases in the stack.

On a corrected basis and at maximum engine power, the exhaustgas temperature levels recorded on the ground were similar to thegas temperature levels recorded in flight. Exhaust gas temperature

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exhaust System Metal Temperatures

The metal temperatures of the stacks, manifolds, muffler wall,and tailpipe were corrected to Standard Day Conditions and wereplotted as a function of the engine compression ratio in Figures 5through 8. The spread in the data is in part attributed to thetemperature extremes of rich and lean fuel-to-.dir mixtures. Thestack metal temperatures were also affected by local variations ofengine-cooling air existing between aircraft at the instrumentedlocation. The deviations in metal temperatures of the muffler outerwall (heat exchanger surface), Figure 7, reflect the uneven flow ofexhaust gases around and through the baffles and diffusers within themuffler, effecting uneven heating. Inspection of the thermal data indetail revealed general temperature variations of 300 to 400OF onspecific mufflers. The problem of heat exchanger distortion andcracking has been aggravated by uneven flow and local overheating(Reference 1). Metal temperatures of 1200OF (maximum hot spot) wereprevalent for exhaust systems of engines with a compression ratio of8.5 to 1 when operated under maximum engine power. Exhaust systemsof engines with a compression ratio of 7:1 operated at maximumtemperatures of 11000 F. Metal temperatures were only slightly lowerat downstream locations. Metal temperature meesurements plotted asa function of engine crankshaft speed are included in Appendix 3.

Although metal temperatures of 1200OF maximum were measured onthe outer surfaces of the exhaust system, the temperature of thebaffles and diffusers inside the muffler were probably approachingthe temperature of the gas. Since the baffles and diffusers are incontact with the exhaust gases and they are without the benefit ofcooling, it was believed that their operating metal temperaturesapproximate 1500 to 1600OF on engines with a compression ratio of8.5 to 1. Oxidation resistance of standard materials utilized inexhaust systems (AISI type 321 and type 347 stainless steels) becomemarginal for periods of extended operation at temperatures of 1500to 16000F. The standard material at these temperatures is subject toecAlintg and weight loss produced by severe oxidation and a combinationof high-temperature carburization and attack by the products of combus-tion, particularly lead compounds. Type 310 (25 percent chromium 20percent nickel) or Incoloy (21 percent chromium 34 percent nickel),with Incoloy perferred, was suggested for the parts that are exposedto high temperatures and the products of combustion (Reference 1).

Engine Vibration Intensity

Typical histories of broadband acceleration level and waveformfor each of the three major axes of the engine are presented inFigure 9. Vibration intensity and waveform at maximum engine poweris compared under conditions of: takeoff, in-flight, and ground test.A record of vibration under conditions of reduced engine power

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In operating aircraft, engine vibration levels were indicatedhighest during conditions of takeoff at maxim•n engine power. Vibre-tion intensity at maximin pover in flight between altitudes of 5000to 7000 feet was indicated to be significantly less than the levelsrecorded during takeoff although vibration waveform was similar.This result was attributed to the engi.nes developing less power underaltitude conditions. Although they were not tested, superchargedengines develop high power and may effect hiSh-vibration levels underaltitude conditions. During ground test at high engine power, vibra-tion levels were indicated to be somewhat higher titan the levelsrecorded during takeoff; waveform was generally similar.

To identify the component partr of the engine vibration wave,the speed of the record was incrased and the engine crankshaftspeed was decreased. This expanded record is reproduced in Figure 10with wave identification noted. The maxim excitation pulse occurredduring combusticn of the power stroke. Minor excitation occurred inphase with the opening and closing of the valves. The engine vibra-tion wave was repeated in cycles of two rpm engine crankshaft speedin phase with the events of the four-cycle engine. The vibration waveof each cylinder was phased in relation to the engine firing order.Thus the vibration of the cylinders at the exhaust flange locationconsisted of identical periodic, complex vibration waveforms, eachout of phase with the other.

The complex wave with phase variance between cylinders effectsa severe requirement for realistic vibration testing of engineexhaust systems. The simulation problem involves the use nf a singlevibration exciter with a rigid test fixture fastened to the exhaustflanges to support the exhaust 7sytem for vibration test. Randomvibration testing may more closely simulate the actual conditions.

The following introductory paragraph on the desfinition orfrequency spectrum was excerpted from Reference 4. "All signalscan be thought of as existing in three dimensions. A sinewave isin reality an amplitude-time curve in the 'x-y' plane existing atsome fixed point on the frequency axis 'z.' An examination or pro-jection of the sinusoidal history on the mplitude-frequency 'x-z'plans would be a vertical liue at the frequency 'f.' This is thetrue spectr-m of a sinewavr, and can be viewed as the two-dimensionalprojection of a time function onto the amlitude-frequency plane.Such a projection of the frequency parametrs of a time-alitudewaveform is called a spectrum or frequency analysis."

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Characteristics of the engine vibration spectrum are illustrated inFigures 11, 12 and 13. These data were recorded from visual observationsusing a vibration wave analyzer incorporating tuneable one-third and one-tenth octave filters. The one-third octave band divides each 10-to-Ituning range into 10 bands. In each band, the ratio of the upper cutofffrequency to the loter cutoff frequency is 1.26 to I. The narrow band(one-tenth octave) in effect divides the range into about three times asmany bands. The bandwidths of the filters increase in cycles directlywith the mean frequency of the band. Thus, as frequency increases, thebandwidth increases and acceleration amplitudes are indicated higher.

Waveform characteristics of engine vibration in the vertical axiswere generally similar on each of five aircraft. Maximum accelerationamplitude, of the #sngaies in the vertical axis were measured between400 and 500 cps on each of four aircraft. The exact frequency isidentified by the symbols denoting the center frequency of one-tenthoctave filters, and is believed to be the natural frequency of theengine and mounting combination. Attention is called to the levelsof acceleration measured with filters as opposed to the broadbandlevels. The indicated broadband vibration amplitudes were reducedsignificantly when the vibration signals were filtered.

Maximum acceleration of the engines in the lateral axis occurredbetween frequencies of 1300 to 1600 cps on five aircraft. The vibrationwas sensed in a direction parallel to both the piston and valve motionand the amplitude of acceleration was indicated higher. Further thenaturai freq-yency of the engine was increased significantly. Maximumacceleration levels of the engines in the longitudinal axis occurredbetween 1400 and 2000 cps. The high frequencies reflect the relativestiffness of the engine and mount assemblies in the longitudinal axis.

Muffler Vibration Intensity

Spectral characteristics of the muffler vertical vibration aredepicted in Figure 14 for threa separate or dual-type exhaust systemsand in Figure 15 for two cantilevered crossover-type exhaust systems.Muffler vertical vibration of separate type exhaust systems reached amaximum acceleration level between 200 and 440 cps on each of threeaircraft, the exact value depending on the particular configuration.Maximum acceleration amplitude was measured at approximately 10 g'srus on mufflers cf enginas developing 62.6 to 77 hp. Fourteen g'srus were indicated on a muffler of an engine developing 228 hp. Amuffler of the cantilevered crossover-type exhaust system reachedresonant conditions at a relative low frequency of 100 cps with amaximum us acceleration of 15 &'a. The engine was developing 214 hpvwen the data were recorded. Muffler vertical vibration levels, asmeasured with narrowband pass filters, were higher than the

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corresponding engine vertical vibration levels. This increase wasattributed to amplification effected by exhaust system resonance andprobably excitation by the exhaust gas pressure pulsations.

Power and Vibration

Engine vibration intensity appeared to be a general function ofthe horsepower being developed at the time of observation as shownin Figures 16, 17 and 18. The illustrations show the broadbandacceleration level, the maximum one-third octave band level, and themaximum one-tenth octave band level, all plotted against the horse-power developed by the various engines at the instant of dataacquisition.

Narrowband vibration of engines in the vertical axis variedbetween 2 and 5 g's rms acceleration at 100 hp, while at 230 hpacceleration ranged from 5 to 9 g's rms. These data were maximumlevels recorded with the center frequencies of one-tenth octaveband filters tuned between 400 and 520 cps. The lateral accelerationlevel of engines developing 100 hp varied between 2 and 6 g's rms asmeasured with narrowband filters. Lateral vibration of enginesdeveloping 230 hp was indicated to be within limits of 5 to 12 g's rms.The data were recorded at maximum conditions observed with the filtercenter frequency tuned between 1300 and 1400 cps.

Considerable scatter of the data occurred when engine longitudinalacceleration was plotted as a function of horsepower. Factors suchas resonance of the accelerometer mounting may have affected the results.The accelerometer was mounted on the flat of a special intake stud capnut to sense longitudinal vibration.

Vibration Specification

Maximum acceleration amplitudes measured on the mufflers of sevenaircraft were plotted regardless of design configuration in Figure 19for purposes of forming a test specification. The top boundary of theenvelope (one-tenth octave band pass filter) represents an indicatedconventional vibration test specification wherein only one frequencyexists at a given time, but wherein the frequency is varied progres-sively from the minimum to the maximum specified frequency.

When the rms levels were converted to peak acceleration, theindicated test specification compared favorably with the militarystandard for equipment mounted directly on aircraft engines (Figure20). Test results indicated that the acceleration amplitudes andfrequencies of MIL-STD-810A, Vibration Test Specification (Reference5), were adequate for sinusoidal vibration testing of aircraft engineexhaust systems. Random vibration testing, however, is preferred,and broadband acceleration levels during vibration test should becomparable to those reported herein. It is suggested that the powerspectral density relationships be shaped with peaks at the frequencies

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. . . . ... . .. . .r - - ' - -+ . . - P- - ---

_______ BROADBAND SPECTRUM I30 _

- + : - . . . I -1 -- V 1

z28

0 40 80 120 16U 200 240 280

ENGINE HORSEPOWER

FIG. 16 ENGINE VERTICAL ACCELERATION LEVEL VERSTTS

HORSEPOWER

21

16 T ONE-TENTH OCTAVE BAND

z

MEW-

0 40 80 120 16u 200 240 280

ENGINE HORSEPOWER

20F[7 ~~ONE-THIRD OCTAVE BAND

CENTER FREQUENCY 1250 CPS TO 1600 CPS

Z 1220

040 80 120 160 200 240 280ENGINE HORSEPOWER

LEGEND

A~ AVERAGE ACCEL LRATION

co40 G I MAx:''UM ACCELERATION

30 BROADBAND SPECTRUMzI

~10

0 40 80 120 160 200 240 28UENGINE HORSEPOWER

FIG. 17 ENGINE LATERAL ACCELERATION LEVEL VERSUSHORSEPOWER

16 ONE-TENTH OCTAVE; BAND, 16CENTER FREQUENCY 1400 CPS TO 2100 CPS

12-4

20 40 80 _ 120-- 16f)---- 200 240 2-R-

ONE-THIRD OCTAVE BAND+ NTER FREQUENCY 15CU CPS TO 2100 CPS--'

16

z~_'

0~

0 40 80 120 102040280ENGINE HORSEPOWER LEGEND

A AVERAGE ACCELERATION

______ _______________( MAXIMUM ACCELERATION

~~~ 4E BROADBAND SPECTRUM

30

U 40 80 120 1 bu -IU40 Z80

ENGINE HORSEPOWER

FIG. 18 ENGINE LONGITUDINAL ACCELERATION LEVEL. VF1RSUSHORSEPOWER

23

20 - ,

ONE-TENTH OCTAVE BAND- CENTER FREQUENCY 108 CPS TO 1900 CPS--

Z 12

0

20 -, I,

8 iiU? ! - -1 7 .F 7.....o .... . .. -, - -I

8t

00 120 160 200 240 280ENGINE HORSEPOWER

SONE-THIRD 0CTAVBEN NDCENTER FREQUENCY 100 CPS TO A000 CPSI

.•T r r T _____-_.__

= BODBNSETRMI ' MAXMU A ..... TLO

0.12.wo 8

0 40 80 120 160 200 240 280ENGINE HORSEPOWER

FM AVERAGE ACCELERATION

40! , ! T ! I• r. I • 0 L--

S0

0 40 80 120 160 200 74n 28()

ENGINE HORSEPOWER

FIG. 19 MUFFLER VERTICAL ACCELERATION LEVEL VERSUSENGINE HORSEPOWER

24

30NOTE:

(1) THE FREQUENCY RANGE SHALL BEICYCLED LOGARITHMICALLY FROMMINIMUM TO MAXIMUM IN 30 MINUTES

14 FOR TOTAL PERIOD SPECIFIED20g

15

I I I

SI I I

"-_ I I

5 II!

NOTE: (2) DWELL 30 MINUTES AT EAGH RESONANCE0I

5 14 23 104 u0 2(000FREQUENCY IN CPS

0., _

'Cl ACCELERATION LEVELS AREI±G (PEAK)I . 36 ,

I i *

Lal 0.01 -X I I

z

-I __I

Ii II

zNLI IIIII IXI

: U. uil Ed< I t

SI I I\I I

Si I NSIII I

I LEGEND:

I AVERAGE .IUFFI ER ACCELERATION

SMIAXIMU'I MUFFLER ACCELERATION

I 1I I I

-, 14 .3 ,.4 ,,00FiiEQUENCY IN Cl-s

FIG. 20 MIL-STD-810A. VIBRATION TEST SPECIFICATION FOREQUIPMENT MOUNTED DIRECTLY ON AIRCRAFT ENGINES

corresponding to the maximum measure. accelerations. For effectivesinusoidal or random vibratioa testing, it is necessary that theexhaust system be heated to maximum operating temperatures asdefined in this report.

Althouah the vibration ezcitation of the engine with respect tothe exhaust system at the muunting flange locations was defined as astationary process; i.e., its statistical properties do not changewith time, random type vibration testing of engine exhaust systemswas preferred only for the feature of testing under a continuiu oefrequencies. The engine excitation of the exhaust system consistedof periodic complex vibration vaveforms; the vwveforms, however, wereout of phase at the exhaust mounting flanges so they were phased bythe firing order of the engine. Since a rigtd test fixture wasrequired for mounting the exhaust systems on the vibration exciter,testing under broadband frequencies vas believed to be more realisticthan testing only under a single discrete frequency and varying thatfrequency from minimun to maximtm.

26

ODNCLUSIONS

Based upon the results of in-flight tests and ground tests reportedherein on four-cycle, horizontally-opposed, air cooled engines, it isconcluded that:

1. Engine exhaust gas temperatures may be estimated by utilizingt!,e temperature data and the theoretical effects of compression ratioas presented in this report.

2. Baffles and diffusers within the mufflers of engines with highcompression ratios on the order of 8.5:1 are probably operating under bmaximum temperatures (1500oF to 16000F) that are marginal with respectto producing high temperature oxidation of currently used materials.

3. Maximum temperature of the exhaust system components exposed

to cooling air on their outer surfaces was measured at levels of1200oF and below. Rapid high temperature oxidation of the materials A(AISI 321 and 347 stainless steels) from which these components werefabricated is not expected at this temperature level.

4. The observed wide variation in metal temperatures of themuffler outer walls reflect uneven heating. This can effect highthermal stresses that contribute to the initiation of crack typefailures.

5. Increased horsepower tends to increase engine vibrationintensity.

6. The engine vibration vaveform is periodic but consists of acomplex, continuous frequency spectrum with significant amplitudes incertain narrowband frequency limits.

7. TIV NIL-STD-8lOA, Vibration Test Specification, for equipmentmounted directly on aircraft engines is appropriate for sinusoidalvibration testing of ecgine exhaust systems in the vertical axis orcritical motion sensitive direction.

8. The complex engine vibration wave with phase variationsbetween cylinders imposes a severe requirement for realistic vibrationtesting of exhauat systems. These conditions would be more clostilysimulated by random-type vibration testing as opposed to sinusoidaltesting.

27

RECO(MENDATIONS

Based upon the resultj of the in-flight tests and ground testsreported herein, it is recommended that:

I. A material more remistant than AISI 321 and 347 stainlesdsteels to high temperatures and the products of combustion be used forthe muffler baffles and diffusers.

2. The thermal and -Abration data be utilized forcalculating the requirements of material and material thickness forengine exhaust system applications.

28

REFERENCES

1. McCunn, T. H., Metallurgical Evaluation ot failed AircraftStainless Steel Exhaust System Components, Federal AviationAdministration, Aircraft Development Service, Technicalreport FAA-ADS-28, 1/65

2. Slusher, G. R., Analysis of Engine Exhaust System Failuresin General Aviation Aircraft, Fcderal Aviation Administration,Aircraft Development Service, Technical Report FAA-ADS-29. 9/64

3. Slusher, G. R., Evaluation of Low-Cost Carbon MoncxideIndicators, Federal Aviation Administration, AircraftDevelopment Service, Technical Report FAA-ADS-80. 7/66

4. Keller, A. C., Considerations in the Analysis of ArbitraryWaveforms, Spectral Dynamics Corporation of San Diego,California.

5. MIL-STD-810A, USAF, Military Standard, Environmental TestMethods for Aerospace and Ground Equipment.

6. Obert, Edward, F., Thermodynamics, McGraw-Hill Book Comoany, Inc.

29

APPENDIX 1

DESCRIPTION OF INSTRWMENTATION

Exhaust System Teziperatures

Two types of chromel-aluuel thermocouples were utilized for exhaustsystem temperature measurement. Metal-sheathed, ceramic-insulatedthermocouples were immersed into t'-x exhaust gas stream at the stack ormanifold location and at the tailpipe location for measurement of thegas temperatures. Open tip thermocouples were installed either undermetal clamps or welded to the surface for measurement of the exhaustsystem metal temperatures. Metal temperatures were monitored atlocations on the stack, manifold, tailpipe, and two places on themuffler outer wall. Typical thermocouple locations are shown inFigures 1, 1.1 and 1.2 of this report.

The thermocouple signals or temperatures ware recozded on anoscillograph when in flight; while on the ground, the tew'peratures wererecorded manually from visual observations on a precision direct-readingputentiometer.

Accuracy of the temperature data reduced from the oscillographrecords was +lOOF as calculated from reading accuracy. Temperaturedata were recorded to +5 0 F reading accuracy utilizing the direct-readingpotentiometer.

Engine and Exhaust System Vibration

Piezoelectric quartz accelerometers were utilized for engine exhaustsystem vibration measurement. The crystal transducers generate anelectrical charge output signal proportional to the acceleration input.Basic sensitivity of these devices is "unit charge per unit acceleration"and is expressed as "picocoulombs per g (pCb/g)." A charge amplifer wasrequired to convert the high-impedance charge output of the transducerto a low-impedance voltage current signal necessary for recording anddisplay purposes.

The quartz accelerometers were selected for vibration measurementon the engines and exhaust systems because they feature high linearityup to temperatures of 5000F, and their high natural frequency permittedfrequency response within 5 percent up to 8000 cps. Since accelerometersensitivity was only one pCb/g, however, the accelerometers, amplifiers,cables and recording instruments were calibrated as systems on anelectro-dynamic vibration test system.

Figures 1, 1.1 and I.? of this report shows the installation oftypical vibration instrumentation on an engine and muffler. An accelero-meter was installed on the head of a cap nut fabricated to fit the engineexhaust flange stud to measure the engine input vibration to the ekhaust

P

1-1

LEGEND

k ACCELERATION

0 EXHAUST GAS TOTAl TEMPERATURE

X METAL TEMPERATURE

CABIN HEATERAIR SHROUD

SMUFFLER OUTER WALL

HEAT EXCHANGER SURFACE

STACKS - ENGINEEXHAUST INLET-

CARBURETOR HEAT LEFT CYLINDERAIR SHROUD , BANKREMOVED

STACKS - ENGINE

EXHAUST INLETRIGHT CYLINDERBANK

STACK ASSEMBLY ENGINE EXHAUST

TAIL PIPEENGINE EXHAUST.

OUTLET

FIG. 1. 1 INSTRUMENTATION LOCATION, CROSS-OVERTYPE EXHAUST SYSTEM

1-2

LEGEND

h ACCELERATION

0 EXHAUST GAS TOTAL TEMPERATURE

X METAL TEMPERATURE STACKS - ENGINE EXHAUST INLET -

RIGHT CYLINDER BANK

STACKS -ENGINE

EXHAUST INLET -LEFT CYLINDER BANK

MUFFLER OUTER W LHEAT EXCHANGER SURFACE

TAIL PIPE-ENGINEEXHAUST OUTLET

CARBURETOR HEATERAIR SHROUD

CABIN HEATERAIR SHROUD

CABIN -E,,0"AIR ATTACHMENT

RAM AIR INLETATTACHMENT

FIG. 1.2 INSTRUMENTATION LOCATION, SEPARATETYPE FXHAUSr SYSTEM

1-3

system in the vertical axis. Measurement of engine input vibration alongthe lateral axis was accomplished with an accelerometer installed on thehead of a valve cover cap screw. Engine longitudinal vibration wasmonitored with an accelerometer installed on the flat of a special intakeflange nut and stud. The accelerometer was positioned with the sensitiveaxis located 900 from the axis of the stud. Vibration was measured onboth ends of the mufflers or heat exchangers with an accelerometer in-stalled in the vertical axis.

The vibration signals vere originally recorded on an oscillographfor analysis in the form of histories both in flight and on the ground.Because of high harmonic content, however, the ground data were alsorecorded from visual observations on a sound and vibration waveanalyzer. The wave analyzer incorporated a tuneable one-third octavefilter; for detail analysis, a tuneable one-tenth octave filter; andall-pass range for measurement of the total broadband signal. Inaddition, the vibration signals were recorded on magnetic tape forconventional spectral analysis.

1-4

APPENDIX 2

BASIC OTTO ENGINE CYCLE THEORY

AS RELATED TO EXHUST GAS TEMqPERATUPE

In the study of the factors influencing engine exhaust gastemperatures, a series of simplifying assumptions are made, theanswers are calculated, and then compared with the observations.In this manner the important requirement for theory has beenconsidered even though the calculations often produce quantitativelyinaccurate results.

Four-cycle gasoline engine operation effects a cycle of pressureand temperature change on the gas which may be classified roughly asthe Otto Cycle or constant volume cycle. A pressure-volume diagramfor the Otto Engine Cycle is presented in Figure 2.1.

Assumptions.

1. Constant pressure intake.

2. Adiabatic compression.

3. Constant volume combustion.

4. Adiabatic expansion.

5. Constant volume pressure drop andrejection of gases and heat.

6. Constant pressure rejection of gases.

2-1

___NOTE:

SEE REFERENCE NO. 6 (p. 29)THE ACTUAL CYCLEREPRESENTED BY DASHED

-- LINES IS REPLACED WITHAN IDEALIZED CYCLE REP-RESENTED BY THE SOLIDLINES

it

1:4 -4 - - ..... I I

124k~5

I 777,

VOLUME

1-2 CONSTANT PRESSURE INTAKE2-3 REVERSIBLE ADIABATIC COMPRESSION3-4 RE'ZR.SIBLE CONSTANT VOLUME COMBUSTION4-5 REVERSIBLE ADIABATIC EXPANSION5-2 CONSTANT VOLUME REJECTION OF HEAT2-1 CONSTANT PRESSURE EXHAUST

FIG. Z. I PRESSURE-VOLUME DIAGRAM, OTTO ENGINE CYCLE

2-2

Definition of Equations and Symbols

Pvl< - C - equation for an adiabatic reactionP - absolute pressurev - specific volumeK - cp/cv - ratio of specific heat#C - constantv2/v3 - r - compression ratiov2 - volume before compression%13 volume after compressionr - compression ratiov5/v6 - e - expansion ratiov4 - volume before expansionv5 - volume after expansione - expansion ritioPV -RT - perfect gas lawH - mass of gasR - gas constantT a absolute temperatureD a displacementH - quantity of heath - heating value per lb. of charge

2&-3

Then-

•3 - r - compression ratio

and

v5vZ - a - expansion ratio.

For the Otto Cycle, a - r

A. For an adiabatic compression and expansion:

Compression Expansion

P3 V3 k , 2v2k P4 v4 k" p5V5k

P3 k P4 kP2 P5

Perfect Gas Law P4 v4 - H R T4

P3 v3 M R T3 Psv5 - H R T5

P2 v2 M R T2 r 4 .P4 I k-I

and T 3 P3 1 k.-1T2 P2 r

Displacement - D - v2 -vl

and since v2 - rVl

it follows that vI- DT-1

B. Combustion remperature increase:

If H BTU's heat gze added to '"' lbs. of gas:

H- WCv 6~T

The amount of heat added is a function of the charge weight and:

V " h - Heating value (per lb. of charge)

2-4

Then:

AT h

The combustion temperature rise is ca?•ulated by dividing "h"heating valve of the charge by "Cv" specif'c heat at constant volume.The combustion temperature rise is independent of the volume of thecombustion chambers, of the amount of gas contained therein, and ofthe gas temperature and pressure of that gas.

At the start of the intake stroke, the clearance volume "vl"will be filled with exhaust gas which has no heating value. Themass of 0as contained in "v 2" cu. in. must be heated by (v2-vl) hohere "h is the heating vasue, if the temperature and pressureof tlee residual exhaust gas is alto T2 and r2:

(v 2 -v 1 ) h v 2 cvAT

v2 vI h v2 1V Io r-lAT- __ 1v 2 TV v 2 cvo C

AT r-1C

The combustion temperature rise equals r-1 time a medium conatant"C" where C " Heating value

specific Heatand:

T4

5 r

It was concluded that:

a. The only engine factor having an influence onthe combustion temperature and the exhaust gastemperature is the compression ratio.

b. As far as the gas is concerned, onl\ the heatingvalue of the fuel and the specific heat influencethe temp-rature rise; however, since they aregencra]! ctonstant i-nder given condition, the fuel-to-air mixturi in.fl zces the temperature riseduring combustion and ultimately the exhaujst gastemperature.

2-5

c. Calculation of compression temperatre rise froma compression ratto of 7.0:1 to 8.5:1.

T3/T2 r rk-l

k-I k-1iAT 3 - (T 8. 5 - T7 . 0) - (r- 8 . 5 - r-.o) T2

K - 1.392 at 800°R average temperature

T2 = 520OR standard day1.392-1 1.392-1

AT3 - (8.5 -7.0 ) 520OR

WT3 - 920F

Teat results agree with che theoretical change in compresciontcmper.tur'e from A ratio of 7.0"1. to 8.5:1. The theoretical changein compression temperature was utilized for establishment of thecurve slopes in Figures 3 and 4 of this report.

2-6

APPENDIX 3

ENGINE EXHAUST GAS AND METAL TEMPERATURES

Detail measurements of exhaust gas and metal temperatures werecorrected to Standard Day condit-lons and plotted as a function ofcorrected engine crankshaft speed for fiv? aircraft in .light andfor seven engine installations on the ground. Five illustrations(Figures 3.1, 3.2, 3.3, 3.5, and 3.7 ) compare exhaust gas and metaltemperatures measured in flight with the identical temperatures asmeasured on the ground. The ground data include both rich and leancarburetor fuel-to-air mixt'ires. The exhaust gas and metal temperaturesmeasured on the ground on two engine installations are presented inFigures 3.4 and 3.6 for both rich and lean carburetor fuel-to-airmixtures.

Operation of an engine-flight propeller combination on theground, with the exception of constant speed-variable propeller pitchinstallations, is limited to an engine rotational speed loss thanmaximun rated. In flignt, the engine is unloaded somewhat by theram airflow passing through the propeller and maximum rated speed isavailable. In addition, data recorded under conditions of ambienttemperatures greater than 60OF cvrrects to engine speed levels lessthan those observed.

Because of these factors, it was necessary to extrapolate tomaximu, rated speed some of the cuirves plotted from data recorded ontCe ground. Since the plotted curves of the inflight data, in allcases, extended through rated speed conditions and since the grounddata from constant speed engines (Figure 3.2) also plotted throughrated conditions, the shape of the curves has been indicated. Theextrapolations were accomplished by duplicating (familying) theshape of the curves in those case6 where corrected data existsthrough conditions of maximnum rated speed. In all occurrences,t!ýe continuous curve used to connect data points was broken andthe extrapolation continued with dashed lines or curves.

3-3-1

--!1ili r-r77i

I Sep

1.1A Tr&4 dtA*TLSE

jN TE

As E9A V I .e E I. . . .. W t END . . . . .i.

~~~~T ... . . . .1. T L- EI • / ,i - -N .. . 7 I . . . . • . • . .• .- , - - . . .

* O~tN I ArTKI I II

9 IA .. .- i . : . t . , l~ . . . . .

41 1Ž 17T

I 711

S.. . . . . .. . . . I .D.. . . . . ..24

w * i~ I

1tw

.OD 000 .I.O . . O .4L, G 4R U LE AN

INI I OSOIDAý IT 111.1 1 N ý.I RP.

FIG. 3. 1 EXHAUST GAS AND METAL TEMPERATURES, AIRCRAFT

CODE MODEL "A"

3-2

I. SAN I • IM I LRATL! E L F AN, M ,I • ( 1)CC"Wý NLTA, TE.'I'L KTtA1 El,.-, CA.,C

SMETAC T E1,CR1 MI 1 A. I,Ib' lEll A 1 4IA TI tIII ,A ILC

* MII* ItM-M I il L il L IAA I IC Rl

S E, 1. PI ' E , C* ACIEML A*C.Al 4C A-EM

._l • kA .L •A-I i .11

- - - - I t + C, . - , ,

,+ " - I " , _ _- -_ _

.LII11I •.. .- C ;C A C !A

I ....... . ...

. . . . . - . . . .. . ,- 7 +-' +

- -.. C, - .

HE f- kiX tCR

II"COD MODCEL ''B'

I I -, -. .-. .C *CCCI.. l AC.-CI - ,I C. ,1 , ACM,

FIG. 3. 2 EXHAUST GAS AND META~L TEMPERATURES, AIRCRAFT

CODE MODEL "B"

3-3

I, .. INr

Go S TILM IL. ALA .M PSTAL

*6o -4 METAL TI LMIERAI "AL . TAIL i

I T^! 0! I 'UPE AIIJ L - *NI Al I&MXTAL. TLM"L'LAAL'0E1 . 11'P"4 ES END ,.

4, '~ ETA , TEI RA UIA0 MU LE OIf 4T Vill

EM.LNE -B LEPO LLRSLLiW 16A- CL .. IN!. EILSPI.P.EMENT

C OMPRLSSLI.L RATKI, . 1:1.PRELPEI.L.ER- 11.LL! P11 GM

T IN 1`I CMI - ALG MIXTUKE .

47 u

14o1

Ico

ON THtGP01G IXUE

IlbOo

:440'

s00 o00 1200 .4..,1 I,0 I0 d, U, 41. 11-4t, l

ENGINLE CRANSHAFIT S!!LL!, N I RkPM

FIG. 3. 3 EXHAUST GAS AND METAL TEMPERATURES, AIRCRAFTCODE MODEL "'t'

3-4

H I I 1 I I I I 1 1

1 EGENI)D1 ~@ GAS TEMPERATURE - MANIFOLDS GAS TEMPERATURE -TAIL PIPE - ------

16- -7 METAL TEMPERATURE -STACK

ig METAL TEMPERATURE -MANIFOLD - I@ METAL. TEMPERATURE -TAILPIIPE

4-.A METAL TEMPERATURE MUFFLER LEFT-------7iIq>~ METAL. TEMPERATURE; MUFFLER LO'GHIT 4L ~J2

ENGINE 145 HORSEPOWER- 300 CUL.LN. DISPLACEMENT j -

t COMPRESSION RATIO 7.0 ,1 I ---

- PROPELLER -FLXED PITCH2 -- 2- - -

I -

- -.- ON THE GROUND -RICH MIXTUR

4 )u U _ _ L ll-

- ------ L L

H

* .. .. ON WlE GROUND) LEAN MIXTURE -

F.NO',NE C.LANK.HAi T SPEED N/I * RP'M

FIG. 3.4 EXHAUST GAS A\ND METAL TEMPERATURFI;, AIRCRAFTCODE MODEL "D"

3-5

GA LPKAI )ILL TAILPIPXAIIAAL TZM4PkIAStlft STACK

160 ME TAI I EW PE kATURE rAi II.Pt - -

IwILTAI. TEMPERftATJ X RIGHT MU4LTTL rK M AeT ENDO . , , , .4oMETAL TtMPILRATURIL RIGHT I UrVI.LK REAR END0

NOTE:I I1 I I I I.

1400 ENGINE10 00 HOKSLY.. ý11 1:0.M0. DISPI.ACEMEZNr

COMPRE.SiJON RATS(, I

P-Pi ~ ~ ~ ___ LkF.D i

* .---. . ON TH FLGHTVO RICH AXTA

400 7{ F 7 ___f 7 7TTTT

ii' -7-

w~

FIG 3. ON EXAS GAS AND MEA TMEAUES, AICRAFT

Cr)DE MODEL ''E"

3-6

I . , F. t -A, I U t N li4- I I Ah • I

MEl AI I EN)) ERAI URE - sIAfEl~o~ I LI Ai 1 I-M ERRAI URE. IItN\;l IF) .-- --- -I I, Of,1)

6 * vI M .l Al 'EI)hl E•RA GIdt A . -U Al .1 ' II'E PEM NL I A ILM -I ERAI UREL -1NI- U . I ER -I I -.FI I-INDM NI A I'IlFMI P RI I IUit E MUkI' It E R II( lI EN) N

N YI E:* EN(GINE- - I I6 u!£iSL-i'G El-)- 5' CU. IN. 1)1.,f 1...\cLi"EN I I .

(C;UKNI R f1,1 UN 8 1 1( , 5:1flU't LI'1; I LR{-l- IXI-•ID l) |' I I -,t ,...,• ,•

S* , . .,,- ,*'* - , -.. .

..N ME GROUND - M

t. N I HE... . . . . E I iXTURE ]

1 -' 1 2800

,d i

•N W\EI- Gi \ .-. i E\- 1 Il•;"'l- "q R' - N'k

YIG. 3. 6 EXHAUST G \S AND METAL£ TEMPERATURES, AIRCRAFT

CODE MODEL "F"

3-7

1. EG ENf

(* C A-' TEMPERATURE -STACK * ***('AS TEldi ERATURE TAlI PIPE ***N ' Al TE'NI PERATt'RE MANIVOIDII

0b~h MFIAI TEMIPERATURE -TAILPIPE

* A PTAl TRMPERAIU.,Z - MUFFLERMEAKTEMIPEHAI tRE MFFI ER

4ý,u -ENI.ANE - u IiURSEOW ER- I'U CCU. IN. 1)114AGMET

- .CJMIR~vIONRATIU h 5:1

YRUPEI I ER-FIXEI) PITCH

4 utt

14l _____ _ _ _ _____________ _ _

w

4U:4

FIG. 3. 7 EXHAUST ET A L TEMPERA I'l'RES, AIRCRAFTCODE N-!,

3-8