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JULY 5TH, 1945 LI H T
Survey of Britain s Most Powerful Radial Engine : An Example of.
r l^ OgiCal Layout to Achieve Com pactness with PowerT has been
eommbn knowledge far some time past thatthe Bristol Aeroplane Co.,
Ltd., have been engaged withthe production of a larger and improved
model in theage of radial, air-cooled, sleeve-valve engines with
whichey have for so long enhanced their reputation. Thismmon
knowledgethe result of unofficial "leaks"ibraced the facts that the
new engine was an 18-cylinderlit of over 2,000 h.p. and was called
the Centaurug..ther than this nothing much was generally known
untilnctioned reference, to the engine was made with thelease of
the Short Shetland flying boat Flight,May 17th,145), when it was
revealed that the Centaurus was of'er 2,500 h.p.Even now we are not
permitted to give any indicationthe power output other than
at it is well in excess of500 h.p., so that, in
assessingit'cific qualities' relating>wer, one canily use
the;ure of 2,500 as'datum andite that theuated result ismewhat
loweran the factualdue.Probably theost impressiveature to strikee
casual ob-rver of themtaurus is thempactness ofe unit in rela-)n to
its poweritput, and, as airollary, the achieve-ent of a clean and
efn-ent fairing by refine-.ent of design. It is,course, obvious
thatmiponent in the engine is thesuit of months, in some cases;ars,
of development, and thatc only means by which such WOLIC-anship can
bear fruit is by way ofie hard road of technical ability fol-wed
strongly by the incessant trial"id error of testing. By such means
a tremendous fundEdata is amassed, such data being the "history"
onlyuough which the future may be discerned. Lookingack in order to
look forward applies equally in the tech-ical as in the ordinary
world. .
Specific PowersFrom the aspect of arrangement analysis the
Centaurusbasically similar to its immediate forerunner, the
Her-jles, with the chief differences that it has four more
flinders and that entirely separate individual gear trainsre
embodied front arid rear for driving the sleeves in the-spective
cylinder* banks. Swept volume is naturallyirger, but so is the unit
swept volume as, although theylinder bores remain the same at
5.75m., the stroke of theentaurus is 7.0m. as compared with the
6.5m. of thelercules. This gives the new engine a capacity of
53.6tres (3,270 cu. in.) in contrast to the Hercules' 38.7
litres2.360 cu. in.), an increment of 38.5 per cent. In terms
ofapacity and power output, at the restricted Centaurus
every
Compactness without complication.Note the backswept exhaust
stackcooling muffs and the collected exhausttail pipes.
power figure of 3 500 the b.h.p. /li tre of both
HerculesCentaurus is 46.5, although we know that the latter
engineis somewhat better than this, as may be rough y indicatedby
the figures for b.h .p ./ sq. in. of piston area, thesebeing
respectively:. Hercules 4.93. and Centaurus betterthan 5.34. f:
Cooling Arrangemen ts
As an indication of refinement in the design of the cowl-ing,
if. we take as a datum the frontal area of the Herculesat 2,122 sq.
in. and give it the value of unity, then theCentaurus frontal area
of 2,402 sq. in. gives a comparativeratio of 1.13:1, which is well
below the relative h.p..ratioof 1.385 :1, itself a conservative
figure. Put another way,this means that for an increase inpower
output of 38.5 per cent, overthe Hercules, the Centaurus has onlya
13 per cent, greater frontal area,its specific' power thus
being150.5 h.p./sq. ft.
It must be appreciated thatthe goal of really close cowling...-
. . ,.., . -.. is hindered in at-. / tainment by the-very critical
fac-tors of cooling,and Bristol'sachievement isrendered the
moredeserving of praiseby virtue of theirhaving obtained,with it,
an en-hanced coolingefficiency. To a- certain extent theemployment
of acooling fan driven from the air-screw spinner can be
deemedpartially responsible for the in-crease in cooling
efficiencybut by no means wholly.
As the Centaurus is designed- as apower plant, the cowling,
fairing andbaffling arrangements naturally comeinto the picture in
addition to theengine per se, so whilst on the subject of cowling
we might as well dealwith it before going on tc the engine'proper.
The subject is well worthy of study.
The main body of the engine in way of the cylinder-'banks is
closely cowled by four panels, two per side, hingedtogether at top
and bottom on the fore and aft centreplane of the engine and
secured together with three toggle-type fasteners on each side. The
latter are recessed togive a smooth surface line. Forward of the
cowling panelsis the-aerofoil-section annular nose fairing which,
with thevery unusual spinner extension fairing (fitted around
thereduction gear casing), provides a smoothly contoured,
butsurprisingly small, annular intake duct in the entry of
whichrotates the- 18-blade cooling fan fitted to the rear of
thespinner.It may thus be seen that cooling air entrained is
ob-structed not at all until it reaches the front cylinders.Having
done its due amount of fin circulation the air isthen allowed to
escape to the outside atmosphere throughcooling gills in the skirt
ring of the main cowling panels.In consideration of the amount of
development designwhich has gone into producing the Centaurus, the
instance
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4 LI HT
BRISTOL CENTAURUS cylinder and head finning is interesting. By
experimenthas been shown that the optimum pitch for fin locationin
the region of o.iin.- the pitch on the Centfturus is[izin. and the
cooling area of finning to each cylinderid head is no less than
32.47 sq. ft.Not the least novel feature of the cooling
arrangementsthe fact that, unexpectedly enough, the front,row
offlinders, due to the amazing control of the cooling air-low,
actually runs a trifle hotter than the rear row.pressure drop in
the cooling air through, or, rather, acrosshe cylinders is the
rather high figure of 10 inches of water,3d, very shrewdly, this is
put to incidental, yet none theuseful, advantage in cooling the
rear-swept front-ylinder exhaust pipes. These are shrouded' with
tubularjufis,open at each end, which extend nearly to the
trans-.jterse centre plane of the engine, and the pressure
differ-ential between front and rear open ends induces an air-Bow
through the muffs which effectively cools the exhaust
ip s whilst, at the same time, isolating their cooling ajfprom
the ordinary flow to the rear cylinders.Limits of Cooling
The cooling efficiency of the Centaurus is nearly as highit is
possible to get. There are for the future the addi-tional
refinements of a geared cooling fan running at higher[speed so
inducting a greater mass flow,'and also the closerpitching of the
cooling fins. Nevertheless, it would appearthat the ultimate is at-
least within sight, and from thisaspect it is a reasonable
assumption that some limit willsooner or later enforce a barrier
against the use of morepotent fuels and increase of engine rating
by stepping upthe b.m.e.p. This reflection excludes consideration
of the(use of water/methanol injection and kindred devices
whichconceivably might have an increasingly large part to play.The
reduction gear is the well-tried Bristol-Farman typeIbevel
epicyclic which is so well known as to render a[further description
superfluous. One element is, however,[worthy of note; the rear
driving bevel and the front[stationary (sun) bevel ring are both
located on annularIseatings, the bearing faces of which are
spherically curved.I This measure ensures that thedriving and sun
bevels can-[each rock slightly and so permit the load to be
distributedI equally among all three planet pinnions. The three[
spokes on which the planets are carried are integral[with the
airscrew shaft, the tailpiece of which is supportedin a bearing-in
the forward end of the crankshaft. Airscrew' thrust is carried by a
large-diameter ball-bearing housedin the front of the r.g. casing
forward of the gear itself.
The crankcase of the Centaurus is unorthodox in thatit is
composed of three sectionsfront, centre, and rearthe mating faces
of which are disposed on the transverse ,centre planes of the
cylinder banks..' Each section has adiaphragm, those of the front
and rear supporting thesleeve drive bearings and isolating the gear
trains fromthe crank throws, and that of the centre provides a
sup-port for the main central crankshaft bearing. The frontand rear
sections of the casing are secured to the centralsection .by hollow
bolts on each side of each cylinder.Mating faces are
metal-to-metal, there being no gaskets,this in itself being a
tribute to the Bristol productionefficiency.Both sets oi sleeve
drives are composed of spur geartrains powered from a common input
gear mounted on
the crankshaft. A description of one set suffices for both,as
front and fear Urives are identical. Meshing with thecentral input
gear are three equispaced intermediate gearseach of which drives
the middle wheel of a separate trainof three, each of the three
wheels driving a sleeve crank ;h three sleeve-driving wheels are
connected in a train bydler wheels^one between adjacent pairs. 'A
built-up, nitrided crankshaftalso in three sections6 employed,-the
front and fear sections, which include thebalance weights, being
attached to the central section by
mi J \ \ . : \ : T : BALANCED * -
two-bolt maneton joints. Each of the balance weights.contain two
2.875m. dia., 3.52 1b. Saloman ball-typevibration absorbers which
have proved to be very effectivein de-tuning both torsional and
flexural vibration. Ex-amination of the double curvature seat track
of the ballenclosures has established that 'the ball movement
citherlongitudinally or transversely does not exceed
0.75111.Big-end bearings carry-on the unique Bristol precedentof
employing white-metalled sleeves shrunk on to thecrankpins, in
contrast to the more usual floating-bushtype bearing. Each crankpin
carries a master connectingrod to which the other eight very slim
con-rods are articu-
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. .JOLY 5TH I945
SL VDRIVINGRANK
SLEEVE VALVEDRIVING GEARSFOR BACK ROWOF CYLINDERS
SUPERCHARGERDRIVING GEARCENTRIFUGES MOUNTINGFOR STARTER
ELF ALIGNINGCRANKSHAFT
ROLLER BEARINGS
IIMII T\ :PORT MAIN 1AIR ENTRYTO
SUPERCHARGER y < i
IGNITIONHARNESS
I-UELINJECTOR
UNIT
EXHAUSTOUTLETS
V
lated ; incidentally, the ordinary con-rods are wider acrossthe
flanges than they are in depth of web. It is of interest .-., , . "
, - . - . , .-.to note that the master-rod big-end bearing is of
12.05 j in. projected area, whilst the wrist-pin bearing area ofthe
normal rods s 1.583 sq. in., as against a httle-end a r r a n g L e
n t and structural details of the Centaurus. Carefularea of 1.575
sq. in. Oil ducts in the crankshaft lubricate profiling of the
cooling air intake entry is a noteworthythe main and big-end
bearings and also feed jets in the - feature of the design. The
sectioned cylinder at the topbalance weights which spray the
pistons, sleeves and little- . of the drawing clearly shows the
relative disposition of piston "ends wrist-pins are lubricated from
the big-end supply and rod assembly with cylinder, sleeve, head and
porting. .
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16 LI H T JU LY 5TH, 1945
B R I S T O LC E N T A U R U S1throu gh spring-loaded oil
retainers .The three high-capacity main bearingsare of large
diameter and of thespherical roller, self-aligning type,crankshaft
end thrust being restrainedby the central bearing.
Maximumangularity of the master rods is13 deg. 38 min., but some of
thenormal rods (which are not evenlyspaced on the ma ster) reach an
angu- _..-;laruty of som ething over .20 deg. ' 'Cylinders are
machined from forged billets, each barrelbeing retained by 16
large-diameter s tuds , the nu ts, forwhich are locked in position
b y spring locking plates. Aspreviously mentioned, the heads are
deeply indented anda r e addition ally, m ade in two sections. The
top half is acasting (including fins) into which the forged and
machinedbottom half is shrunk.
Sleeves are of the latest stiffened type and as masterrod
sleeves have a harder time than the others, they areshell pressed,
in contrast to normal sleeves, which are cast.There are five ports
in each sleeve, two each for inlet andexhaust, an d one which is a
common port for both. Port-ing areas are respective ly: inlet, 6^65
sq. in., ex hau st,4.00 sq. in.Pistons are very short-skirted and
each is fitted with twowedge-section gas rings, a channel-section
scraper ring anda norm al typ e bottom scraper ring. Projected area
of thegudgeon pin in the piston is 3.23sq. in. and , as the rod
little-end bearing area is 1.575 sq. in., theratio is 0.487 : 1
.The Centaurus is aspirated by atwo-speed, single-stage blower
oper-ated in conjunction with a Hobson-R.A .E. fuel injector. Air
is en-trained on both sides of the engineand ducted to the
respective mainentries on each side of the drivecasing, where it is
fed into a beau-tifully formed turbine-type entryto the
double-shrouded impeller.This discharges through a diffuservane
ring to the several inductionpipes serving the cylinders. All
theinduction pipes are the same lengthin order to ensure equal
distribu-tion, and terminate in cast light-alloy branch yokes which
arebolted to the cylinders and dis-
siavtPOPftT
/
VALVEVALVt
// / EXHAUVi/
\ \
\ \\ \\
\ \. \
> 4Q 2 14 4 0Bristol engine development of specificpower and
specific weight against year.
Cylinder baffles, by mean s of which the airis directed to cover
the optimum fin area.
ENGINE
. These two details show the disposal of, head finning and the
cooling air flowr^ -. ' pa ths.C Y L I N D E RS U E V E
mounted above the clutches and are readily accessible
forcleaning. The clutches themselves are driven by a spring-drive
gear on the tail of the crankshaft to protect the drivefrom cyclic
torqu e variatio ns. Gearratios are: low, 6.761:1, and high,9.03:1,
which, at 2,700 r.p.m.(crankshaft) result in an impellertip speed
of respectively 1,012 and1,325 feet/s ec. Maximum blowercompression
ratio is achieved at2,700 engine r.p.m. in S gearand equals 3-5:1
.Selection of blower gear isgoverned by a valve which variesthe
supply of oil to the clutches asrequired, and the valve
determinesthe gear according to the inductioncharge requirements of
the engine,this being linked up with theHob son-R.A .E. injector.
This in-jector was developed by Farn-borough and is manufactured
byClaudel Hobson. It meters the cor-rect amount of fuel into the
blowerentry according to* an integrationof blower and back pressure
differ-,ential with charge temperatureexisting for a given engine
speed.The injector nozzles are fitted inthe twin air entries to the
blowerimmediately down-stream ofthe butterfly throttles.
Mixtureratio for f ullpower is roughly 10 :1by weight, and for
economic cruis-ing conditions approximately 16:1by weight.At the
rear of the superchargerdrive shaft is a gear box contain-ing the
drives for the superchargeritself and the various engine
acces-sories, such as fuel and oilpumps, magnetos, tachometer,
en-gine starte r, etc. The twin mag-netos which, externally,
lookstrangely like little centrifugalblowers, are mounted on
rakingfaces to port and starboard of
PLATE
OTTOM (-Y U N D t R IS KRKNG PLUGS
' ^ ' . '
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5TH, 1945' * : . ;- - . :r:
LI H T
BRISTOL CENTAURUSj , : (mentioned gear bo x, whilst theengine s
ta r te risi ted vert ically aboveitand engages thro ugha
doggeding-e following major i tems on the Cen tau rus are supp
liedie underm ention ed f irms : ignetos British Thomson
-Houston.uition Ha rnes s Plessey orM arconi. ..arking Plugs Lodge
. , ' -jxible Pipe s Supe rflexit or Avery. -J Mater ial: H igh
Duty Alloys.
n s : \ Ma chining: Well worthy orBristol.ton Rings Brico, We
lhvorth y,orHepworth andGrandage.~ .ector Hobson. _ ' .
BRISTOLType ...Bore and strokeCompression ratio
Swept volumeDiameter over cylindersAirscrew reduction gear
ratio..Airscrew rotationAirscrew shaft size ... ..Fuel
specificationOil specificationOil consumption(average)Engine weight
(bare, dry)Firing order
C E N T A U R U S18-rylinder, two-row radial,air-cooled,
sleeve-valve, withtwo-speed supercharger andHobson-K.A.E.
injector.5.75 in.x7,0 in.7. 8 :1 . -S.-270 cu. in. (33.0 l i
tres).55.3in.0.444 engine speed. ' -L.H. tractor.S.B.A.C. standard
No.fi.D.E.D. 247;. (100/130 grade).D.E:D. 2472.2.5 g.p.h.at2,400
r.p .m .; 1.12f:.p.h.at 1,000 r.p.ni.. 2,780 1b.1, 12,5,16,ft,2,13,
0, 17, 10, 3, 14,7, 18, 11, 4, 15,8.
Something ofPast Effort andFuture Intentions Together
withAnnouncements of NewEngines and AircraftOMPAKABLY with most
other major concerns ,allof whichgo tom a k etheBrit ish aircraf t
ind ustry ,onlya small proportionofthe work th e Bristol A ero-Co.
, L td. , have been engaged upon dur ing the pas t3ars has cometol
ight . How ever , as a resul tof anition staged at Br is tol onJun
e 28 th , we are nowlosiriontogiveafair ly wide surve yoft he c om
pa n y ' s .ties.the preceding pages will be found a descriptione
latest and mo st powerful Bristol engine,theurus;bu t ,inaddi t
iontoth e new:, three new modelsof thewell-Hercules are a lso annou
nced,being designated. respectively00, 120 an d 130- All,ofcourse
,asically similar, the chiefdif-es lying in the superchargers .lark
100 is thela tes t mi l i ta ryles,and has thehighest max i-power
output of thethreeatb .h .p . It e m ploysa
two-speed:hargerofimproved ent ry shape npe.ller design together
withan-K.A.E. fuel injector . Markercules also havethenew
two-blowerandH o b s o n - R . A I E .in-
but opera teatsl ightly reduce d1, these engines hav ing been:ed
primarily for high-a l t i tudeircraft, forw hic h e m ploym e
ntare fatted with a special re arhaving an increased capa c i
ty>ry drive capable of t r a nsm i t -pto 150 h.p . tocope
withtheblowers necessaryforpressuris-Qie Hercules 130isac tua l
lytheodd describedinthe May 24thfFlight, a ndisf i t ted only
withle-speed supercharger , being designed foreconomicionatm ode r
a t e a l t i t ude s .listonthe subjec tofaircraf t power un itsit
isa ppr o-1to reveal th at Bristols hav e been in
terestedforjtimein thevariou s forms of gas t u r b ine s , a nd
,in' carried outt r ia lson theexhaust -dr iven turbo -larger as
early as 1923 and 1924. Theore t ica lwere cont inued, and were
given addi t iona l impetusp. success of Air Comm odore W hi t t le
' s br i l l iant work on je t -propuls ion. It wasfe lt th a
ttheidicated by the i r previous exper ience lay in the
C 6 S242220
fie1 6
U_
2129,o
642
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1920 22 4 6 8193 32 34 J63B 1940 42 44 46Increase in altitude at
whichsuc-cessive engines have maintained
boostequivalenttoJupiteratfull thro ttle,s_ea level.
larger andlonger-range aircraf t , andtheir design wo rk,which
was directed towards the provisionof a gas- turbinefor these types,
was concentrated along specif ic l ines earlyin 19\i. This has
culmina ted in the product ionof a gas-tu r b ineinwhichthe bulkof
the powerisutilised todr ivea variable-pitch airscrewofnormal typ
e;means have a lsobeen provided in thedesign for therecupera t ion
of he a tenergy, which would otherwisebelostinth ejetdischarge .T h
a t thepowerform a ny type sof both civilandmili-ta ry a i rc ra f
tofth e future willbeprovidedby jetp r opu l -sion or the gast u r
b ine is cer ta in .There i s , however , much deve lopmenttobedon
e before the gas turbin e can,for instance,be aneconomical
propo-sition forair l ine ope ration , and it is,therefore ,
prudent toa s sum e t ha t t hereciprocating engine st i l ihas
yearsofuseful life beforeit.The preference of a i r line opera tors
,for radial enginesinthe larger classesof civil transports was fair
ly markedbefore thewar , and it would appe arthat this preference
st i l l stands.The-listofnew aircraf t poweredbyBr is toli s im
press iv e :
Cciitaurus, i&-cylinders. over 2,500 li.h.p.Short Shetland
flying boat.Airspeed A.S.57 airliner.Bristol Type 167 (Brabazon I)
airliner
Hercules, 14-cylinders. 1 675/1 800 b.h.p.Short Sunderland
(lying boat.Handley Page Hermes airliner.Hand ley Page-Hermes t
ransport .Handley Page Halifax transport.Vickers-Armstrong Viking
airliner.Bristol Freighter.Bri-tol Wayfarer aircoach.O ne of the
mer i t s of the Bristolengineist h a t theoriginal design
wasgenerousinconcept ion wi th prec ise ly 'the need forincreas ing
powers in m ind . T he H e r c u le s ,for instance, has been
increased from 1,375 b.h.p.in1937to 1,800 b.h .p .
to-day,againinpowerofover 30 per cen t. ,which has been obta ined
at theexpenseofon ly a bou t10per cent, increase in engine weight .
Bo th HercuJcsandCe nta ur us arecer ta inly capable of de ve lopm
e nt to stillh igher outputs , and may conf ident lybeexpec
tedtocoverthe 2 ,000-3,000 b.h.p . rangeinthe near future. Below
thisrangeit isprob able th at Bristol will also offera deve lop-m e
n tofth e Perseu s nine-cylinder sleeve-valve engine witha powerofa
roun d 1,200 b.p .h . com pared w i th the pre-war