A balanced diaphragm type of maximum cylinder pressure indicator was designed to give results consistent with engine operating conditions. The apparatus consists of a pressure element, a source of controlled high pressure and a neon lamp circuit. The pressure element, which is very compact, permits location of the diaphragm within 1/8 inch of the combustion chamber walls without water cooling. The neon lamp circuit used for indicating contact between the diaphragm and support facilitates the use of the apparatus with multicylinder engines

Collins, John H , Jr

Spanogle, J A

English

NASA Technical Reports Server

, 
t , 
FILE COpy 
NO. )-W 
TECHNICAL HOTES 
lITATIO _:.AL ADVISORY CO:,::,lI TrEE FO~ AERO:~AUT I CS 
No . 359 
A BALAl :JED DL\PERAGl,l TYPE OF 
lvlAX I~ElT!~ CYLIl~DErt PRESSuRE INDICATOR 
By J . A. Sp2.Jwgle and John :c. ColI i~s, Jr. 
Langley ~;~er.:o ri al Aerontlutj.clll L8.borlltory 
Yas~1iTlgton 
Dccemo~n-, 1930 
FI Opy 
To be return!d to 
the files of the National 
Advisory Comnitt 
for Aeronautics 
Washington, D. C. 
https://ntrs.nasa.gov/search.jsp?R=19930081129 2020-03-17T04:17:47+00:00Z
NATIONAL ADVISORY COl,J.I'': ITTEE FOR AERONAUTICS 
TECHNICAL NOTE NO . 359 
A :9ALAl\fCED DIAPHRAGI,~ TYPE OF }:lAXltJlUM CYLINDER PRESSURE INDI CATOR 
By J. A. Spanogle and John H. Collins, Jr. 
Sum mar y 
A balanced diaphre.gm type of maximum cylindcr pressure 
indicator was designed to give results consistent with engine 
operating conditions. The apparatus consists of a pressure 
elewent, a source of controlled high pressure, and a neon lamp 
circui t. The pressure element, w'hich is very compact, pcrmi ts 
location of the diaphragr.1 wi thin 1/8 inch of the combustion 
chamber walls without water cooling. The neon lamp circuit 
used for indicating contact between the di8.phragm and support 
facili tates the use of the apP2.ratus with mul ticylinder engines. 
In measuring compressi on pressures only one reading is 
taken. Ho lever, in rneasuring maximurJ explosion pressures a 
range of pressure is read which sh:::>ws the vari 2 tion between 
indi vidual cycles. Thi s indic2.tor is in current use at the 
Langley Memorial Aeronautical L2.borE'.tory as the most accurate 
available means for indice. tin6 maxiE1Um cylinder pressures. 
I n t rod u c t ion 
The problem of accurately r:1ee.suri ng the m8.ximuI1 cylinder 
pressures in high-speed interne..l combustion engines is of 
N.A.C.A. Technical Note No. 359 2 
considerable importance in their design, development, and opera-
tion. Efforts towards the solution of this problem have resulted 
in the developmont of a number of instruments, some of which are 
adaptations of those used for measuring pressures on slow-speed 
steam engines, while others arc designed primarily for high-
speGd work (Roferences 1, 2 and 3). 
The indicator under discussion is similar in principle to 
the one described in N.A.C.A. Technic~l Report No. 107 (Reference 
2). It ha s been used only for the measureQont of maximum cyl-
inder presRures, although it ~ould be possible to build up a 
card by the point-to-point method described in Reference 2. 
The indicator as designed and used at the Langley Memorial Aero-
nautical Laboratory fulfills a specific s e t of conditions of 
operation, but can be applied to ~o st engine test work. 
Des i g n 
This maximum cylinder pressure indicator was designed 
particularly for use with experimental, high-speed, compression-
igni tion enginGs. The r eqUirements w"ere that it must wi thstEmd 
the high pressures invol:ved in this work and give accurate read-
ings of thes e pressures. The indicator r:lust also b e easy to 
s ervice and be adapt~ble to seVGral engines, 
It was deCided t o use the prinCiple of balanCing pressures 
on either side of a diaphragn of negligible stiffness, because 
of the small mass of the r.Joving element and tho short distance 
N.A.C.A. Technical lote fo. 359 3 
of travel for mak ing and b r 8aking th8 c : mtact . The most serious 
argument against the use of the diaphrag~ was th8 c ons i de r ation 
of heat dissipation 'fThen located near the c or.lbust ion chamber 
1.lITall. Pr ev ious experience had sho'7n, h01Qever, that a small 
dial;leter diaphragn reT:mined sufficiently cOJl to prGvent rupture 
and it ~as thought that any heat offcct should increase the 
sensi tivi ty of tho diaphre.gm. Therofore, it was de Cided to 
operatG the diaphr~gl!l· wi thout c ooling (Figure 1). 
Since the pressure eleDent s crews into a metric spark 
plug hole, the diameter of the diaphragD is lir:d ted to 1/2 inch 
and is clamped down to 3/8 inch free dianetcr . The diaphragr.1 
is made of Swedish blue ter.1pered spring stoel, which is suffi-
Ciently s atisfactory, so that no other Later ial hBS been tried. 
The diaphragm used is a s thin as the particul2..r pressures to be 
measured wi 11 perr:1 i t, for i!1 research 70r K it is better to have 
an occasional failure than to sacrifice sensitivity by using a 
thick diaphragm. The thickne ss of the diaphreg!:l for us c "J i th 
pressures up to 800 pounds per squa.ra inch has been 0.004 of Em 
inch. A diaphragm 0.008 of an inch thick has been used lqi th 
pressures up to 1500 pounds per square inch. 
The diaphrag~:1 is rJounted in the pressure e 1 0ment (Fi gure 
2) betNeen supports which allo~;J' it approxi::1ately 0.005 of an 
inch movement at its center and restrain it so that it is nover 
distorted beyond the e lastic lioit. The out~r support is dished 
to allo:] this r.1ovement . The inner support is l ess than 0.0005 
of an inch from, but not in c ontact '7 i th tile diaphragrJ in its 
N.A.C.A. Technical Note No. 359 4 
normal position. 
Using Morley 's formula for deflection of a clamped dia-
phragm (Reference 4) a pressure of less than 4 pounds per square 
inch will cause the 0.004-inch thick diaphragm to make contact 
from its extreme position . This introduces an error of less 
than one per cent over its normal working range. The 0.008-
inch diaphragm woul d be about 2 per cent in error at 1500 
pounds per square inch. 
The balancing pressure is provided from an air bottle 
connected to the pressure element through a control block on 
which a Bourdon spring gauge is mounted (Figure 1). A small 
neon lamp is mounted directly above the gauge. The lamp is 
made for operation on 1lO-volt alternating current, but is 
supplied with about l70-volt direct current from a bridge 
acrass the engine dynamometer circuit. For a single-cylinder 
engine only one lamp is used ; however, for multicylinder 
engines a bank of neon l amps are used, each one for an indi-
vidual cylinder, and readings of maximum cylinder pressure are 
taken simultaneously. 
Ope rat ion 
In the assembly of the pressure e l ement it is most impor-
tant to have the proper clearance between the diaphragm and the 
inner support. With this clearance of less than 0.0005 of an 
inch and the correct thickness of diaphragm, contact bet11Jeen 
N.A.C.A. Technical Note No . 359 5 
the diaphragm and the inner support will be effected by a pres-
sur e wi thin the error of com:nercial Bourdon spring gauges sui t-
able for measuring the pre ssure. 
In measuring compression pre ssures during motoring runs, 
the variation in pressu~e of i~dividual cycles is 8 0 slight 
t~at only one pr essur e reading need be taken. This reading is 
taken by rai s ing the controlled a ir pr essure and reading the 
gauge at the po int whe r e the neon l amp stops flickering and 
goes out. The more slowly tile pressure is raised the more 
accurate the r eading wi ll be. However, when running under 
power there is u sually a variati on in the maximum pr essur es 
of different cycles , so that the maximum cylinder pressures 
a re r ead as a pr essur e range rather than as a single pressure. 
This i s done by raising the pressur e until the flickering of 
the lar;1p los es it s r egul ari ty and tal:ing one r eadi ng. The 
second r eadi ng is taken by increasing the pressure until the 
lamp ceases flickering entir ely. 
The troub l es experi enc ed wi th t~li s ins trument are dia-
phragr'l failur es and short c':rcui ts . The diaphr agrl~ f ai lur es 
are u sually due to fa tig-..1e or the er.rployr:;ent of a diaphragr..l 
too thin to withstand the pr essures ~ith which it i s u sed. 
The small amount of current necessary for li ghti ng t he neon 
lamp r.1akes it poss i ole for a st1all p8_rticle of foreign matter 
to c ause tho l aJ:lp t o burn c ~ntiY~uous::'y if the pe_rts a r e not 
olean at asse:noly . Whe n noto r i ng for long peri ods the accunu-
lation of oil nay have a damping effect on the d iaphragm arrd 
N.A.C.A. Technical Note No. 359 
cause the readings to be low. 
No attempt has been made to definitely establish the 
accuracy of this instrument. However, it has been used 
simultaneously with the trappGd-pressure type of valve 
(Reference 3) and, for co~pression pressures and c ombustion 
pressures with low ratGs of pressure rise , the t wo types give 
the same reading, but for high rates of pressure rise, the 
balanced diaphragm type indicates higher pressures. It 
has also been checked against the maxi mum pressures indicated 
by the altered "Farnboro" indicator (Reference 5) and found to 
be in close agreement. 
6 
This indicator shows almost instantly the effect on the 
maximum cylinder pressurcs of any variation in engine operating 
conditions . In the work at this laboratory the instrument has 
been found to give more consistent results than any other type 
available for the measurement of maximum cylinder pressures. 
Langley Memor ial Aeronautical Laboratory , 
National Advisory Committee for Aeronautics, 
Langley Field, Va., November 17, 1930 . 
N.A . C.A. Technica l Note No. 359 
1. Judge, A. W. 
2. Dick inson, H. C. , 
and 
Ne"Vell, F. B. 
3. Hicks, C. ·W. 
4. }.~or l ey, Arthur 
5 . Collins, J. H. Jr. 
Ref ere n c e s 
The Testing of High-Speed 
Cor.1bustion Engines - 1925 . 
A High-Speed Engine Pre s sure 
I ndi cator of the Balanc ed 
Dic"phragr:l Type. N. A. C. A. 
Technical Report No . 107, 
1921. 
TrJ.e Mee.sur enent of Max i r.1Um 
Cyl inder Pressures. N.A. C.A. 
Technical Report No. 294 , 
1928 . . 
Str ength of Materials - 1926. 
Alt erations and Tests of the 
"Fe.rnboro " Engine Ir:dicator. 
N.A . C.A. Technical Note No. 
348 , 1 930 . 
7 
~115 volt A.C. neon lamp 
rrll \ M 
. /i I P 
k ~Witch ~ 
779 ohm, . 5 amp. resi st or Pres sure i ndicator i R 
val ve / v .... >- "" / , 
/' 
Pres sure gauge 7 i 
I 
Cylinder l1eQ,dl ~_\.><-I b{:w.il~;'Ii~~--~ 
Pressure control block 
__ ~compressed air bottle 
Fig.l Diagrammatic sketch of pressure element and auxiliary apparatus -
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N.A.C.A. Technical Note No . 359 
/Electri c connect ion 
Air connection 
.-P'" Insula t i on 
;/ 
Outer support 
Fig.2 Pressure element N.A. C.A. diaphragm t ype 
maximum pressur e i ndicQtor. 
Fi g . 2 
N.A .C.A. Technica1 Note No.359 Fig.3 
Fig . 3 
Pressure element . Pa.rts extended in order of assembly 


A Balanced Diaphragm Type of Maximum Cylinder Pressure Indicator

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