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lDRTO
ENGINE DESIGN
Piston Stroke
ADVANTAGES EXPLAINED
UIn outwardr appearance many
Imodern cars ore very. similar.
\VWhen it comes to the engine,
however, ,there is often consider
able difference boish in funda
'meatal design and, in the detail
cons'truction, and to the ordlnary
!motorist, tile advantages and dis
'advanitages of this or that design
mlay not ibe apparent.
One of the 'mlost obvious polnts of
differel?ce between one engine ond
another is, of course, in thile number
of cyliliders, 'but wthether .tile engine
Ilasi a long stilroke or shont strolke is
not quite so clear at a glance. In
,ngland, the anetlhod of ,taxation ei-.
courages ithe development of engies
wilth a long stroke, says :thle Dunlop
Perd.riau 'Bulletin, for an increaese inll
til :roe lok ,wll velanliivrsoilease in. en
gine capascity, but ;',uil:lnot .increase
the taxation, wvhich is based on. ,tihe
measurement of Itlie' .boreof 'lhe en.
gine only.
.This brings up t?he qilestion a:s to
whlether a long s? roke or sIort st.roke
is 'better. With a long strolke, gleatel
eve-see is obtained, silnce the cihnklc
is long. IThis ineaiis. tihat. ahighel
gear canl be used.' Thler' alre advan,
:ages *n usinng a hIligh ?gea? in. that the
,lanksllaft and 'transmission. does not
rotate so quickly: fol a given ?roan
speed; thelrefore, the 'bearings will .be
snubject Ito less frinction; also 'onik
nhaft hliip, swhioh usually msanifents
itself only at very hilgh revolutionls,
is less likely 'to occur.
lAlthough 1the crank turns . more
slowly in the long stroke engine, the
piston speid is the same, or, to put
it another way, for a given piston
/speed the crankshlaft 'wvill itul?si more
islowly in 'a long stroke engine ,than
in a short stroke engine; ..therefore,
the former offers no advantage over
the latter in .thile ralbter of friction.
between the'. pisoon and the cylinder
walls.
FRICTIONAL LOSS
On the other Ihand, one of tlhe chief
advantages of a jsho:t stroke is t1hat
there is less anpularity .of the con
neoting sod. that is to say, since the
rllnk is short, the connecting rod is
thrown less out of its. nerpendjrule,
position, and, .therefore; :thle piston
is less violent'y pressed agailnst the
walls of Itlle cvl'ndeilr. Thus. ther1' is
a .reduritons in frictional loss.between
Ilston 'aid cylinder. Incidentally. this
loss,.is one :of. thl grieatest troubles-,"to
contend ssith" iin obtaining higher -er
ficiency. .' ' .-:.:- ? ,
Another point is .hat Wvit tlls ? ,ho
fIrolce ;it is 'possible 'to h.save:. the
gidgeon: pillIsigher up. i: the?lpiston.
thusi preventing the tendecy:: of"'the
nistonl to dhIater; i~?iichl It .hasi if die
pill is .toonear'- the lover elid:. The
disadvaiitages' dste 10o 'connecting rod
angular1lIty illn 'a lonrg-stiroke engine Is
sometimes overcome'by using a lone
-r connliedting rod and 'o i0d.ucing the
ang's' it :makes 'vltl It;he ,perpen
dicular. 'Taking it all round, the ad
Vnl"nag.es of. the shor.ter stroke seem
to outweighl its disadvantages.
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