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MR No. E5D30a

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

ORIGINAllY ISSUED

April 1945 as Memorandum Report E 5D30a

A COOLING-CORRELATION EQUATION FOR A DOUBLE-ROW RADIAL

ENGINE BASED ON THE TEMPERATURE OF THE EXHAUST-VALVE SEAT

By James M. Jagger and Fred O. Black, Jr.

Aircraft Engine Research Laboratory Cleveland, Ohio FIL E C P'

WASHINGTON

To be ratUl II ·d \ 0 the .tiles of lhe ~i1tiollBI

Advisory Com,,; tee for Aeronautics

Washin,ton, D. C

NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre-viously held under a security status but are now unclassified. Some of these reports were not tech-nically edited. All have been reproduced without change in order to expedite general distribution.

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NACA MR No . E5D30a

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

MBMORANDUH REPORT

for the

Army Air F'orces, Air rr'echnical Service Command

_a.. COOIJNr..-CORRELATION EQUATION FOR A DOUBLE-:qCW ~ADIAL EK~INE

BASED ON THE TEMPERATURE OF THE "EXHAUSIJLVALVE SEAT

By James M. Jagger and Fred O. Black, Jr.

S\1$.1ARY

A cooling- correlation e~uation was obtained based on the average temperature of the exhaust-vc~ve seats of a large double-row radial engine" The exhaust-valve seat was chosen as the basis for the correlation because endurance tests have shown it to be the critical region of this engine cylinder. From this correlation equation prediotions may be made of the cooling-air pressure drop required to cool the critical region of the cylinders.

The accuracy of predicting engine cooling-air pressure-drop requirements} based on the cooling correlation ,. was substantially increased when a modified carburetor Nas used.

INTRODUCTION

At the request of the Ar'f'fW Air Forces, Air Technical Service Command, tests have been conducted at the NACA Cleveland Laboratory to improve the cooling of a large double- row radial air'c:.:aft engine. In connection with the program, a stu~v has been made of the coolin~ characteristics of the engine by the NACA cooling correlation method. (See reference 1.) Cooling correlations have previously been based on the temperatures of the rear spark-plug boss and the rear spark-plug gasket. (See reference 2.)

The exhaust- valve seat has been shown to be a critical r pgion of the rear-row cylinders. The ini tial failure in endurance tests made at

2 NACA MR No, E5D30a

excescive cylinder temueratures was that of the exhaust~valve seat. Operation at this condition resul ted in burnjng of the exhaust-valve seat and warping of the exhaust valve and the exhaust-valve guide.

A cooling-'correlation equation baue,:' un -C~~8 c:L'i tical cyhnder temperature was successfully developed This correlation is compared with one based on the temperature of the rear sPer~plug boss . Tests were made wi th a nlodified carburetor and a standard carburetor ; the modified carburetor produced a more consistent relation between the average tempera.ture of all engine cylinders and the temperature of the hottest cylinder .

ANALYSIS

The NACA cooling correlation method (reference 1) is used to present the cooling characteristics of the engine tested.

where

C

6p

The correlation equation is of the form

T T h - a -'11 'T g - h

W e

= C 106p)y ;:: .'W x/y )

C \-(5 e_6

-p_;

r eference cylinder-head temoerature, OF

cooling-air stagnation temperature in front of engine, OF

o mean effec"tive gas temperatures F

constant

weight flqw of engine charge air, pounds per second

ratio of density of cooling air in front of engine to density of standard sea-level air

cooling-air pressure drop> inches of water

(1)

The mean effective gas temperature Tg represents the mean gas

temperature effective in the transfer of heat from the combustion gases to the cylinder head. Wi th constant spark advance and exhaust back pressures Tg is principally dependent upon fuel-air ratio and the t6mperatrrre of the charge air entering the cylinder and is represented by the equation

3

(2)

where Irg80 is the mean effective gas tempera ture for a charge-air

temperature of 800 F wi thout supercharging and 6 Tg represents the

variation of Tg with manifold-air temperature Tm' For a fuel-air

ratio of 0,08) a value of 11500 F is assumed for tho reference mean effective gas temperature of the cylinder head o (See reference 1 0 ) II'he r elation has been empirically de termined for the cylinder heads to be approxima tely

The manifold-air temperature T m is calcul~ted from the carburetor

inlet-air t emperature Tc and the theoretical supercharger temperature rise assuming no fuel vaporizationo For the engine tested, the r elation is

;' N \2 Tm = Tc + 19.8 \.iOOO~)

where N is the engine speed, rpm.

(4 )

In order to distinguish between the correlations presented, the subscript 1 r efers to the correlation based on the average temperature of the 18 exhaust-valve sea ts and the subscript 2 refers to the correlation based on the aver age temperature of the 18 rear spark-plug bosses.

ENGINE TEST EQUIPMENT

The 3350-cubic- inch-displacement engine .tested -was fi tted wi th a cowling from a four-engine airplane and installed in an engine test stand. (See fi g . 1.) The cooling-air flow was provided by an exhauster" which reduced the pressure at the rear of the engine . The combustion air was supplied by a blower to the ducting forward of the intercooler.

4 NACA MR No. E5D30a

The cylinder-head. temperatures were measured by thermocouples . The positions of the therffiocouples used to measure the temperatures of the exhaust· valve seat and rear spark--plug boss are shown in figure 2. The engine cooling-air pressure drop was obtained by taking the difference be'~weon the average of five total-pressure measurements on each front· · row cylinder and the average of three static-pressure measurements taken at the rear of each rear-·row cylinder. The nasi tion of these pressure tubes on the engine cylinders is shown in figure 3., .

TEST PROCEDURE

Two series of tests were made to establish the correlation at a fuel-air ratio of O,G8 with the modified carburetor~

10 Cooling-air flow was varied whilf' fuel-air ratio, CArburetor-air temperature, engine speed , find charge-air flm·J were held constant to determine the variation of engine temperature with cooling-air flow .

20 Charge-air flow Was varied ;hile fuel-air ratio, carburetor- air temperature, engine speed~ and cooling-air flow were held constant to determine the variation of engin0 temperature with charge- air flow •

. In order to determine the effect of fuel- air r a tio on mean effective gas temperature , tests were made \I!i th the modifi "d and the standard carburetors at- various fuel-air ratios, powers , and engine speeds at a constant carburetor- air temperature.

RESULTS Jll1TI DISCtGSION

Figure 4 establishes the relation between the cooling temperature differential (Tt - Ta)/(Tg - Tt) and the cooling- air pressure drop

aL'1p. The slope of the curve in figure 4(a) based on the temperature of the exhaust- valve seat determines the exponent Yl in the equation

T T W Xl hI - a = C ..

A comparison of figures 4(a) and 4(b) shows that the temperature of the cri tical r8gion of the cylinder is less affected by engine cooling--air flow than is the temperature of the rear spark--plug boss~

5

Figure 5 establishes the relation between the cooling temperature differential (Th '- Ta)/('rg -. Th) and the weight. flow of engine charge air We- ',Ehe slopes of the curves in figure 5(a) and figure 5(b) determine tbe e~~onents Xl and x2 in equations (5) and (6)~respectively. The slopes of the curves i.n figure 5 are essentially the same, ""hieh indicates that the temperatures of the critical region of the c;)'linder and the rear spark-plug boss are equally affected by the v,'eight fl ow of engine charge alr.

When these exponents are substituted in equation (1) and the constants evaluated, the cooling-correlation eq,uations en and (8) are determined based on the average terr:perature of the exhaus tr-valve seat and the average temperatur e of the rear spark-plug boss r respectivelyo

/" )\0 . 20 Thl _ T a / We

2" 50 1 ('7)

l.r----T-- = Go 576 \ / gl -, .b~ \ '-, ab.p

/ ' 0.29 T T / W 1. 79 112- a I e ---~ = 0.500 ,---Tg2 - h2 _O'Dp -'

(8)

A plot of equations (7) and (8) is presented in figure 6.

The variation of the reference mean effective gas temperature Tg80 with fuel- air ratio based on tests with the modified carburetor is shown in figure 7. The reference rllean effective gas temperature was calculated by solving equations (7) and (8) for Tg and then calculating the reference mean effective gas tempera.ture Tg80 using equations (2)9 (3), and (4) , Similar curves shoving the variation of the reference mean effective gas temperature with fuel-air ratio , based on tests using the standard carburetor are shown in figure 8 .

Figures 7 and 8ohow that the scatter of test results based on the temperature of the exhaust-valve seat is less than the scatter when based on the temperature of tbe rear spark-plug boss o Figure 7 ~ based on results obtainod with the modified carburetor, shows the effect of improved fuel-air-ratio distribution in tha.t it closely approximates lli'1published single-cylinder test data. The curves have a sr~rp peak at a fuel-ai r ratio of approximately 0 . 068 and the range of stable lean engine operation has been extended to a fuel-air r a tio of 0~052

r o

APPIJCATION OF RESULTS

N.ACA MR No 0 E5D30a

Inasmuch as the correlation is based on the average temperature of all engine cyli..n(:tE.::s v ' whereas the most impor·~ant temperature is that of the hottest cylinder) it is necessary that aCCUl'ate info~· ation be available relating the temperature of the hottest c.:yJinder to the average temperature of all cylinders.

]lgure 9 shows the difference between the temperature of the hottest exhaust-valve seat and the average temperature of the exhaust-valve seats for all cylinders plotted against fuel- air ratio for each power condition investigatedo A similar plot of the difference between the telfIperature of the hottest rear spark-plug boss and the average te~perature of the rear spark-plug bosses for all cylinders is given in figure 100 In addi tion to the data obtained with the standard &ld modified carburetors in the present tests (engine A): unpublishei data are inclu.ded from various engines of the same model (B f Of and D) equipped with the standard carburetor. These data indicate no consistent relation bevtJeen maximum and average cylinder temperatuTes for engines e~uipped with the standard carburetor" The engine equipped \"i th the modified carburetor, however, produced a consistent pattern in repeated tests and an accurate determination cf the average temperature for all cylinders may be obtained from a given temperature of the hottest cylinder,

Another important parameter required for cooling-correlation predictions is the weight flow of engine crillTge air . The variation of the brake \3pecific air consumption wi th fuel···air ratio for various engine operating conditions is shown in figure 11.

An accurate determination of these parameters substantially increases the a~curacy of predicting engine cooling-air pressure-drop requirements based on the cooling correlation.

SUMMARY OF RESULTS

From test-stand results to determine the cooling cOl~elation of large double-row radial enginepwi th a 335(}·cubic·-inch displace-mente t~e following results may be summarized:

1. A satisfactory cooling correlation equation based on the temperature of the exhaust-valve seat "ms obtained.

NAC.A MR No. E5D30a. 7

2. Improved fuel-air-ratio distribution to the engine cyl-inders produced by the modified carburetor substan~ia11y increased the accuracy of predicting engine cooJ.ing~air pressure-drop require-ments based on the cooling correlation.

Aircraft Engine Research Laboratory, National Advisory Committee for Aeronautics,

Cleveland, Obi 0 1 April 30. 1945.

REFERENCES

1. Pinkel, Benjamin~ Heat-·'ITansfer Processes in AiI'-Cooled Engine Cylinders . NACA Rep. No, 612, 1938.

2. Koenig l Robert J . rand Engelmanv Helmuth W.: A Cooling Cor-relation of the Wright Th-2600-8 Engine Showing the Effect of Water as an Internal Coolant~ NACA ARB No. E5A29\ 1945.

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HACA MR Ho. E5D30a

Figure I. - Test-stand installation of 3350-cubic-inch-displacement engine fitted with cowling from four-engine air p I an e.

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MACA MR No. E5D30a

Exhaust-valve-seat thermocouple

Rear spark-plug-boss thermocoup Ie

NATIONAL ADVISORY COMMITTEE fOR AERONAUTICS

(a) Cutaway of cylinder.

Figure 2. Location of thermocouples installed in cylinder.

NACA MR No. E5D30a

Thermocouple junction

(b) Section throu g h val ve seat.

Rear spark-plug-boss thermocouple

In t ak e valve

Rear

F r on t

Exhaust-valve-seat therlllocouple

Exhaust valve

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

I C) Looking into b arrel.

Figure 2. Concluded~ Lac at ion oft h e rm 0 co u pie sin 5 t a I led i n cy lin d e r.

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NACA MR No. £5030a

---tt--- P3

Right side view Rear view

(a) Static-pressure tubes on rear-row cyl inder.

Front view

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

,-----I I ===-tt-____ HS I I L ____ .J

Right side view

(b) Total-pressure tubes on front-row cyl inder.

Figure 3. - Location of cooling-air pressure tubes installed one y lin d e r s .

MACA MR No. ESD30a

-r-4 .9 E-4.s::

.8 , NATIONAL ADVISORY

COMMITTEE FOR AERONAUTICS Slope = -0 20 r-4 bO .7 E-4 -'- .6 -cu

E-4 • .5 r-4

t- __

-0-t---t--I.r""I ~ r----o.- I-

~ -5 6 7 8 9 10 20 30

cAp, in. water

Ca) Based on temperature of exhaust-valve seat Thl •

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.5 t-_ Slope - -0 ~29 -

"-(). r--r--re>-I---.. ---1---0... .4 --- ....

5 6 7 8 9 10 20 30 aAp, in. water

(b) Based on temperature of rear spark-plug boss Th2.

Figure 4. - Variation of cooling temperature differential (Th-Ta)/(Tg-Th) with sea-level cooling-air pre s s ure drop d6P. Charge-air flow, 9800 pounds per hourj fuel-air ratio, 0.08.

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fUCA MR No. £5D30a

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r-4 .7 .s:: f-i

I .6 r-4 bO

f-i ....- .5 "-tIS

8 I .4 r-4

~

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

..-

~;

v V V ISlone =

~ ..-..-1..-/

4

~ V

/

5 6 7 8 9 10 We x 3600, lb/hr

1>.50

(a) Based on temperature of exhaust-valve seat Th1 •

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..-- .5 ~ ~ • .4 ~ bD

E-t -"-- .3 as E-t • ~ ef

..-

/'" V

:/ VD

~ Slope = 0.52 l~

,../

v ....

4 5 6 7 8 9 10 We x 3600, lb/hr

(b) Based on temperature of rear spark-plug boss Th2 •

Fi~ure 5. - Variation of cooling temperature differential (Th-Ta)/{Tg-Th) with charge-air flow We. Cooling-air pressure drop, 18 inches of water; fuel-air ratio, 0.08.

.... ~ I .... bO

8 -........... ,.... "' 8 I .... ~

~

t= I ~ bO

8 -.......... "' 8 I ~

t=

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.7 COMMITTEE FOR AERONAUTICS

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.3 s---

.2. 1

---~ - '----

.2

H:>-I--I--y--

- f---o-~

f..----0

.3 .4 .5 .6.7.8.91.0

we2 . 50/06P, 1b/Csec)(ln. water)

r.---~

Slope = O.

2.0

Ca) Based on temperature of exhaust-valve seat Th1 •

v ;0 ,£V .0- SloDe ;; O.

~ r-o-

...0---~ ....--.----- -_._-_._ - -

.2 .3 .4 .5 .6 .7 .8 .9 1.0 2.0

We I. 79/ o"lIp , lb/(sec)(in . water)

(b) Based on temperature o f rear spark-plug boss ThZ '

~O

3.0 4.0

a9

3.0 4.0

Fi gure 6 . - Variation of coolin g temperature differential ( Th-Ta) /(Tg-Th) with ratio of charge-air flow to coolin g-air pressure drop . Fuel-air ratio, ~ . 03 .

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NACA MR No. E5D30a

1300

Bake Eng! ~e NATIONAL ADVISORY hi)rse- spe ~d COMMITTEE FOR AERONAUTICS

P pwer (rp ~)

° 000 1801) + 340 210;> rr x 500 220~ ~ 0 '700 230i) + L>

NACA MR No. E5D30a

NATIONAL ADVISORY B t-ake Eng1 e COMMITTEE FOR AERONAUT ICS h ~rse- spe d

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0 000 180 ~

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1200 000 240

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J ~ ~ 1000

b) B sed pn te Ilpera "ure ~t re r sp rk-p ugb ~ss rn.2-

~05 .06 .07 .08 .09 .10 .11 Fuel-air ratio

Figure 8. - Variation of reference mean effective gas temperature Tg80 with fuel-air ratio for cylinder heads obtained with standard carburetor.

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N a

•

0 ~

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NACA MR Ho. E5D30a

120 N~TION~L ADVISORY

COMMITTEE FOR AERONAUTICS

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0 0 ~ tr!I o 0

~ f++ 0 ,,

NACA MR No. E5D30a

80 rake ngtn lorse (pee

~ ~ower rpm .... -+= 40

- i- 1000 1800

o NATIONAL ADVISORY

COMMITTEE fOR AERONAUTICS

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80 x B Sta ~dard

2200 2600 -- Aver ge f r 40 modi le-d arbu etor

.&. ()

~05 .06 .07 .OS .10 .11 Fue1-air ratio

Figure 10. - Rear spark-plug-boss temperature spreads obtained with standard and modified carburetors.

- - - - -- ~ -- ~---

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MACA MR No. ES030a

9

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u ,.. .... ce 7 u .... '-4 ....

1340 ~ 2100 2000 ~400 () "-

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+ NATIONAL ADVISORY . COMMITTEE FOR AERONAUTICS 1500 2200 .06 .07 .08 .09 .10 .11

Fuel-air ratio Figure 11.- Comparison ot brake spec1tic air consumption

obtained with standard and modIfied carburetors .

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