-
7/27/2019 Engine Working Loads
1/4
ENGINE WORKING LOADSL. Introduction. Internalcombustion roduces
highpressurehatactsuponapiston;
heconsequencesrehereexplored.Thereare
hreevariableswhichbroadlydefinehedesign f a singlecylindernternal
ombustionngine theboreB,thestrolrcS= 2r and heconnectingod tength
These reshownn Fig.I.
F'igureWeare nterestedn the orquesupplied y theengine,he
hrusVtensionn theconnectingod (ensure obuckling, mplicationsor
fatigueof big-endbolts)and heside-thrust nthecylinder-wall
implicationsor wearand nternal osses).2. SomehistoryThe
19966-cylinderWl 24)Mercedes enz8300dieselengine asboreB - 87rnm
strokes = 84 mm. Thesweptvolume s
^ _ a7 L I I T
x84x 6 -2.996x106mm'; but I cm=10mm, so I cm3:1000mm3, nd
heasweptvolume is 2996cffi3 or 3-litres. Since he bore is greater
han the stroke, heengine s saidto over-square. Early
automotivepracticewas for engines o be under-s(luare; his was
encouraged y the impositionof Vehicle ExciseDuty (VED)
baseduponheRAChorse-powerormulaRAChp=ry where is heboren
inches,2.5n isthenumber fcylinders,atherhanCO,
emissionscurrentlymployed. incethis formula is independentof stroke
S,t was advantageouso achievea given
-",.I
-
7/27/2019 Engine Working Loads
2/4
capacityby having a small bore but largesfoke. For example, he
JaguarXK 3.8 litreengine 1950's,1960's)hadaboreof 87 ffiffi,but a
strokeof 106mm. A long strokehas he advantage f higher torqueat low
speed, ower side-thrust,but thedisadvantage f higher piston speed
andhencemechanical osses)andhigher pistonacceleration
andhencevarious dynamicstresses)or a given enginespeed. The
trendtowardsover-squareengines riginated n theUSA in the
1950's.
3. Somecalculations. SupposeF is the net force on thepiston
crown,that is gaspressurex piston crown areaminus the force
necessaryo acceleratehe piston,which is
F=p(e),q-ffi*""@'r["o,.ry)
(b)
Figure2 G
From he ree-body iagram f thepiston,Fig.2a,onehasF = ecosl, orQ
= FsecQ, fid G = QsinL= FtanQ whereQisthecompressiveorce n
theconnectingod,andG is theside-thrust rovidedby thecylinderwall.
ForagivenF, theside-thrust ill besmallwhen/ is small; or agiven
and0,themagnitudeof G decreasess increases:hus a longconnectingod
leads o smallerside-thrust,and so ess rictional loss,and esswear.
On heotherhand,ong odengineswith
aparticularstrokebuildsuctionabovehepiston
essquickly,sincehepistonmovesaway rom TDC moreslowly; his resultsn
a lower nlet portairvelocity,which educesow speedorque.Typically,
heratio lf r rangesrom3 to 4.
-
7/27/2019 Engine Working Loads
3/4
sin =lsinl ,
The orque ppliedo thecranks z = exoN:Fse clxoN, butoNsecQ= oM,
leadingo T= l7xoM. Thus o, 1 given F,orquewill be arge fdistanceOM
is large.
ReferringtoFig., Op=/coS +lcos|,ood *=tan Q,soOPoM=oPtan0,or
oM=rcosetu t+rs in l . Now s ing= ls in l , soeos{ = l - (r l t )
'z sinzQ,snd tan|= ThusOM
canbeexpresseds
From heabove,t is clear hatdistance M increasess hecrank hrowr
(which sone-halfof thestroke, = sl}) increases.n turno
nemayconcludehata longstrokeenginewill providea larger orque,all
other actorsbeingequal whichtheyneverare' Thusa long-stroke
inglecylinder500ccengine,as n a 1950/60,sPanther,s ideal or
amotorcycle ombination.Motorbikeandsidecar!)4.
Tuming-momentdiagram. A typicar v-diagramis shownn Fig.3.
Figure3Nextonemayconstnrct furning-moment iagram, s n Fig.4.
l-(rlt)'zsin2Q
-
7/27/2019 Engine Working Loads
4/4
,a *# , _r_i
tNotethatwhen hetorque s negative,heenginemustbedrivenby
anextemalsourceof energy henceheneed or a flywheel. The ncreasen
torque owards heendofeachstoke s due o retardationof
thepistonadding o thegaspressure.Themeanoraverageorque s
4nt @)d0T - J - '- mean 4n5. Multi-cylinder engine.Fora 4
cylinderengine,with firing order l342,onecansuperimpose firther
three orquecurves,eachshiftedby n radians.Theoveralltorquecurvewill
thenbe much ess peaky"anda smaller lywheelwill berequired.
4
