4. Practicle Cycles

8/10/2019 4. Practicle Cycles

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Mean Efective Pressure - MEPthe theoretical constant pressure which iexerted on the piston or the entire powerstroke would produce work equal to thework o the cycle.

MEP= work/V-V! "ndicated MEP was ound usin# indicator

cards and a plani$eter which #ave areainside the curve %work&. V-V! is known

%displace$ent& so MEP could 'ecalculated. (oday electronics andtransducers are used to #et MEP andindicator cards.

)//!**

MEP


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)//!**!


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)//!** +

Indicated Efective Pressure


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(he area under the curve %as deter$ined 'y theplani$eter & is the work which is equal to the area in

the rectan#le co$prised o the Mean Efective Pressureti$es the chan#e in volu$e %'y de,nition&. n the dia#ra$ the chan#e in volu$e or trace len#th is

$easured.(he hei#ht o the trace depends on the type o sprin#

used in the indicator. (he sprin# scale is #iven in $$per k#/c$!.

Exa$ple n indicator card is taken with a sprin# scaleo $$ = .0 k#/c$!. (he plani$eter #ives an area o

!! c$!. " the trace len#th is !.0 c$ what is the MEP1rea/len#th = hei#ht !!/!.0 = .2) c$.2) c$ = 2.) $$ 2.) $$ x .0 k#/c$! = !).3k#/c$!

)//!** 3

Indicated Efective Pressure


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)//!**4 0


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)//!** )

Draw Cards


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5yl hp = P67/++8*** per cylinder

P= $ep8 psi 6= stroke8 eet= area o piston8 in!

7 = nu$'er o power strokes per $inute

= rp$ or ! stroke cycle % powerstroke per revolution& = rp$ x 9 or 3 stroke cycle % power

stroke every two revolution&

++8*** tl'/$in/hp

En#ine hp = %P67/++8***& x : o cylinders

)//!** 2

Power


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Power per cylinder in ;


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! stroke has twice as $any power strokes at a #ivenrp$ so we theoretically #et twice as $uch hp

>?( ! stroke has 'reathin# and coolin# pro'le$s. 3stroke has exhaust and intake strokes to clear andrechar#e cylinder. ! stroke $ust acco$plish thisaround >@5. ! stroke has less ti$e to re$ove heat

ro$ liner and co$'ustion cha$'er to keep the$within accepta'le li$its. >MEP is lower in a ! strokeas a$ount o uel added is li$ited 'y coolin#pro'le$s

! stroke has a 'etter wei#ht to AP ratio andproduces $ore unior$ torque %twice as $any powerstrokes at a #iven BPM

Modern tur'ochar#ed ! and 3 stoke en#ines are o

a'out equal eCciency. )/** 4

stroke


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Ftoichio$etric %ideal& air/uel ratio %G& opetroleu$ is 3-0. ctual en#ine $assratio o air to uel is !0-+* as not idealco$'ustion conditions.

$fuel = $air/ G

H/dd = $fuel x Aeatin# Valve o uel H/dd is li$ited 'y $fuelas there is a li$it how

$uch uel can 'e added at a #iven BPMwithout s$okin# % s$oke li$it& I $eans

$ore uel 'ein# inJected than can 'eco$pletely 'urned.

)//!**

ir!"ue# $atio


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torque = a'ility to do work = orce%l's& x crank radius B % eet&

"ncrease torque 'y increasin# the orce %pressure&or 'y len#thenin# the crank radius

usin# Prony 'rake we can ,nd >AP which is

>AP= !KGB7/++8***G= orce8 l's B=radius o 'rake8 eet 7=BPM

%++8***/!K = 0!0!& >AP = !KGB7/++8*** = GB7/0!0!

(orque = GB >AP = GB7/0!0! = (7/0!0! or (orque= 0!0!

>AP/7 = 0!0! AP/BPM

)//!** !

%or&ue


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(orque = GB G = Pressure x reaB and are ,xed or a #iven en#ine so

torque delivered is directly proportional tothe pressure

@iesel en#ine is torque-li$ited as torque isli$ited 'y >MEP and >MEP is li$ited 'y Hadd

)//!** +

%or&ue


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Volu$e at >@5 %V&

Volu$e at (@5 or clearance volu$e %V! & @isplace$ent or swept volu$e %Vpd& = area o

piston x stroke

V= VpdD V! rc= V/V! = %VpdD V! &/ V! = %Vpd/ V! & D

percent clearance %c& = V!/ Vpd

rc= %Vpd/ V! & D = %/c& D or / c= rc

I c= /% rcI &

)//!** 3

Percent C#earance 'c(


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)//!** 0


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Liven V=** and Vpd= 4*

V= VpdD V! then V!=*rc= V/V! = **/* = *

c= /% rcI & = /%*-& = /4

Liven rc=* and V=**

V!= V/rc= **/* = *

/ c= rcI = * - = 4 c= /4

Vpd= V I V! = ** -* = 4*

)//!**)

Percent C#earance


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volu$etric eCciency is the ratio o the

actual wei#ht o air trapped in thecylinder to the theoretical wei#ht 'ased onthe piston displace$ent and thepressure/te$perature o the air.

Volu$etric eCciency is efected 'y portdesi#n8 nu$'er8 sie and lit o valves8and 'y valve ti$in#.

(ur'o char#ed or 'lown en#ines havevolu$etric eCciencies #reater than ** N

(he de#ree o eli$ination o exhaust#ases also efects the volu$etric

eCciency. )//!** 2

)o#umetric E*cienc+


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Volu$etric eCciency is reduced 'y

5ylinder 'ein# hot increaseste$perature o air thus reducin#density

Exhaust #ases not co$pletelypur#ed dilute and heat inco$in# airchar#e

Valve and $aniold restrictionsi$pede Oow o air in and exhaust#ases out

)//!**

)o#umetric E*cienc+


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Piston $otion is cyclic non-sinusoidal .

@irection o piston $otion reverses at (@5 and>@5 so piston speed at (@5 and >@5 is ero.

Piston speed varies over the len#th o the stroke Ftartin# at (@5 piston travel is hal the

downward stroke at 4* de#rees and ro$ 4*-*de#rees the piston travel is hal the stroke.

Gro$ >@5 to !2* de#rees the piston travel is hal the stroke and ro$ !2* de#rees to (@5 it is

hal the stroke. Gor a #iven an#le o crank rotation piston travel

is #reater around (@5 than >@5.

)//!** 4

Piston Motion


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)//!** !*

Piston %rave#


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)//!** !

%heoretica# C+c#e

t # 4 St k % , h d


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)//!** !!

ctua# 4 Stroke %ur,ocharged


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)//!** !+

-atura##+ spirated 4 stroke


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)//!** !0

%wo stroke with e/haust va#ve

E t i


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)//!** !)

Enterprise

EMD


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)//!** !2

EMD


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)//!** !

"u##+ ported 2 stroke P)

- t ## i t d 4 t k P)


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)//!** !4

-atura##+ aspirated 4 stroke P)


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4 Stroke 0 2 Stroke 1ring ! crankarrangement

4. Practicle Cycles

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