Gas exchange proc..doc

7/25/2019 Gas exchange proc..doc

http://slidepdf.com/reader/full/gas-exchange-procdoc 1/28

GAS EXCHANGE PROCESSES

In a spark ignition engine, the intake system typically consist

of an air filter, a car!retor an" throttle or f!el in#ector an"

throttle $ith in"i%i"!al f!el in#ectors in each intake port,

an" intake manifol"& '!ring the in"!ction process, press!re

losses occ!r as the mi(t!re passes thro!gh or each of these

components& )here is an a""itional press!re "rop across the

intake port an" %al%e&

)he e(ha!st system typically consists of an e(ha!st

manifol", e(ha!st pipe, often a catalytic con%erter for

emission control, an" a m!ffler or silencer&

)he "rop in press!re along the intake system "epen"s onengine spee", the flo$ resistance of the elements in the



system, the cross*sectional area thro!gh $hich the fresh

charge mo%es, an" the charge "ensity&

)he !s!al practice is to e(ten" the %al%e open phases eyon"

the intake an" e(ha!st strokes to impro%e emptying an"

charging of the cylin"ers an" make the !se of the inertia of

the gases in the intake an" e(ha!st systems&

)he e(ha!st process !s!ally egins + to - CA efore .'C&

/ntil ao!t .'C the !rne" cylin"er gases are "ischarge"

"!e to the press!re "ifference et$een the cylin"er an" the

e(ha!st system& After, .'C the cylin"er is sca%enge" y the

piston as it mo%es to$ar" )'C& )he terms lo$"o$n an"

"isplacement are !se" to "enote these t$o phases of the

e(ha!st process&

)he e(ha!st %al%e closes 01 to 2 CA after )'C an" the inlet

%al%e opens 0 to 3 CA efore )'C&

.oth %al%es are open "!ring an o%erlap perio" an" $hen

pi4pe 0, ackflo$ of e(ha!ste" gas into the cylin"er an" of

cylin"er gases into the intake $ill !s!ally occ!r&

)he a"%antage of %al%e o%erlap occ!rs at high engine spee"s

$hen the longer %al%e open perio"s impro%e %ol!metric

efficiency& As the piston mo%es past )'C an" the cylin"er

press!re falls elo$ the intake press!re, gas flo$ from the

intake into the cylin"er& )he intake %al%e remains open !ntil

1 to 5 CA after .'C so that fresh charge may contin!e to

flo$ into the cylin"er after .'C&

In a "iesel engine intake system, the car!retor or E6I

7 electronic f!el in#ection8 system the throttle plate are

asent& CI engines are more fre9!ently t!rocharge"&

:hen the e(ha!st %al%e opens the !rne" cylin"er gases are

fe" to a t!rine $hich "ri%es a compressor $hich

compresses the air prior to entry to the cylin"er&

'!e to the time %arying %al%e open area an" cylin"er

%ol!me, gas inertia effects an" $a%e propagation in the

intake an" e(ha!st systems the in the intake, the cylin"er,

an" the e(ha!st "!ring these gas e(change processes %ary in

a complicate" $ay&



;ol!metric Efficiency

;ol!metric efficiency is !se" as an o%erall meas!re of theeffecti%eness of a fo!r stroke cycle engine an" its intake an"

e(ha!st systems as an air p!mping "e%ice&

It is "efine" as<

)he air "ensity ρa, can e e%al!ate" at atmospheric

con"itions= η% is then the o%erall %ol!metric efficiency&

Or it can e e%al!ate" at inlet manifol" con"itions= η% then

meas!res the p!mping performance of the cylin"er, inlet

port, an" %al%e alone&

;ol!metric efficiency is affecte" y the follo$ing<

f!el, engine "esign, an" engine operating %ariales<

NV

m2

d0,a

av

ρ=η



0& 6!el type, f!el4air ratio, fraction of f!el %apori>e" in the

intake system, an" f!el heat of %apori>ation=

3& ?i(t!re temperat!re as infl!ence" y heat transfer=

2& Ratio of e(ha!st to inlet manifol" press!res=

+& Compression ratio=

1& Engine spee"

-& Intake an" e(ha!st manifol" an" port "esign

5& Intake an" e(ha!st %al%e geometry, si>e, lift, an" timings&

)he effects of se%eral of the ao%e gro!ps of %ariales are

essentially 9!asi stea"y in nat!re= their impact is

in"epen"ent of spee" or can e "escrie" a"e9!ately in

terms of mean engine spee"& Ho$e%er, many of these

%ariales ha%e effects that "epen" on the !nstea"y flo$ an"

press!re $a%e phenomena that accompany the time %arying

nat!re of the gas e(change processes&

@!asi*Static Effects

6or the i"eal cycle , an e(pression for %ol!metric efficiency

can e "eri%e" $hich is a f!nction of the follo$ing %ariales<

• Intake mi(t!re press!re pi=

• )emperat!re )i=

• 6!el4air ratio 764A8=

• Compression ratio rc=

• E(ha!st press!re pe=

• ?olec!lar $eight ?=

• Specific heats ratio γ&

)he o%erall %ol!metric efficiency is<

:here m is the mass in the cylin"er at point 0 of the cycle

)he ao%e e(pression forη

% can e $ritten

1c

c

0,a

r

do,a

av

V)1r (

r

)]A/F(1[

)1(m

V

m

−+ρ

χ−=

ρ=η

11i TM

R ~

mV p = 0,a

a

0,a0,a TM

R ~

p ρ=

)]}1() p

p[()1r (

1

1r

r {)]A/F(1[

1)

T

T)(

p

p)(

M

M(

i

e

cc

c

i

0,a

0,a

i

a

v −γ +

−γ −

−+=η



6or 7pe4pi80 the term in B is !nity

Effect of f!el composition, phase, an" f!el4air ratio

In a SI engine , the press!re of gaseo!s f!el in the intake

system re"!ces the air partial press!re elo$ the mi(t!re

press!re& 6or mi(t!res of air, $ater %apor, an" gaseo!s or

e%aporate" f!el it can $rite the intake manifol" press!re as

the s!m of each componentDs partial press!re<

pi pa,i pf,i p$,i

$hich $ith the i"eal gas la$ gi%es

)he $ater %apor correction is !s!ally small 7≤&28& )his

Ratio, pa,i4pi , for se%eral common f!els as a f!nction of

Note that this ratio only e9!als the engine operating f!el4air

ratio if the f!el is f!lly %apori>e"&

6or con%entional li9!i" f!els s!ch as gasoline 7petrol8 the

effect of f!el %apor, an" therefore f!el4air ratio, is small&

6or gaseo!s f!els an" for methanol %apor, the %ol!metric

efficiency is significantly re"!ce" y the f!el %apor in the

intake mi(t!re&

6raction f!el %apori>e", heat of %apori>ation, an" heat

transfer

6or a constant*press!re flo$ $ith li9!i" f!el e%aporation

an" $ith heat transfer, the stea"y*flo$ energy e9!ation is

1

w

a

a

w

f

a

a

f

i

i,a)]

M

M)(

m

m()

M

M)(

m

m(1[

p

p −++=

)m/m(af

!,f f aaAV,f f e!,f f eaa )"m"m(#]"m"m)1("m[ ++=χ+χ−+



:here< χe is the mass fraction e%aporate" an" the s!script

"enote < a, air properties= f , f!el properties= F, li9!i"= ;,

%apor= ., efore e%aporation= A, after e%aporation&

Appro(imating the charge in enthalpy per !nit mass of each

component of the mi(t!re y cp ), an" $ith hf,; hf,F hf,F;

)he pre%io!s e9!ation ecomes

Since cf,F ≈ 3cp,a the last term in the "enominator can e often

e neglecte"&If no heat transfer to inlet mi(t!re occ!rs, the mi(t!re

temperat!re "ecreases as li9!i" f!el is %apori>e"&

6or complete e%aporation of isooctane, $ith Φ 0,

)A*). * 0°C&

6or methanol !n"er the same con"itions )A ). * 03°C

E(perimental "ata sho$ that the "ecrease in air temperat!re

that accompanies li9!i" f!el e%aporation more than offsetsthe re"!ction in air partial press!re "!e to the increase"

amo!nt of f!el %apor< for the same heating rate, %ol!metric

efficiency $ith f!el %apori>ation is higher y a fe$ percent&

)he i"eal cycle e9!ation for %ol!metric efficiency sho$ that

the effect of gas temperat!re %ariations, meas!re" at entry

to the cylin"er, is thro!gh the factor 7)a,4)i8&

Engine test "ata in"icate that a s9!are root "epen"ence of

%ol!metric efficiency on temperat!re ratio is closer to real

eha%ior& )he s9!are root "epen"ence is stan"ar"

ass!mption in engine test "ata re"!ction&

!,f a, p

!V,f ea

Ac)A/F(c

")A/F()m/#(TT

+

χ−=−



Effect of inlet an" e(ha!st press!re ratio an" compression

ratio

As the press!re ratio 7pe4pi8 an" compression ratio are

%arie", the fraction of the cylin"er occ!pie" y the resi"!al

gas at the intake press!re %aries&

As this %ol!me increases so %ol!metric efficiency "ecreases&

)hese effects on i"eal cycle %ol!metric efficiency are gi%eny term B , an" for γ 0&2 these efects are sho$n in ne(t

fig!re&



Comine" @!asi *Static an" 'ynamic effects

:hen gas flo$s !nstea"ily thro!gh a system of pipe,

chamers, ports, an" %al%es, oth friction, press!re, an"

inertial forces are present&

)he relati%e importance of these forces "epen"s on gas

%elocity an" the si>e an" shape of these passage an" their

#!nctions&

.oth 9!asi*stea"y an" "ynamic effects are !s!ally

significant&

:hile the effects of changes in engine spee", an" intake an"

e(ha!st manifol", port an" %al%e "esign are interrelate",

se%eral separate phenomena $hich affect %ol!metric

efficiency can e i"entifie"&

6rictional losses

'!ring the intake stroke, "!e to friction in each part of

intake system, the press!re in the cylin"er pc is less than the



atmospheric press!re patm y an amo!nt "epen"ent on

s9!are of the spee"&

)his total press!re "rop is the s!m of the press!re loss in

each component of the intake system< air filter, car!retor

an" throttle, manifol", inlet port, an" inlet %al%e&

Each loss is a fe$ percent, $ith the port an" %al%e

contri!ting the largest "rop&

As a res!lt, the press!re in the cylin"er "!ring the perio" in

the intake process $hen the piston is mo%ing at close to its

ma(im!m spee" can e 0 to 3 percent lo$er than

atmospheric&

6or each component in the intake an" the e(ha!st system,

.erno!lliDs e9!ations gi%es

:here< ξ # is the resistance coefficient for that component

$hich "epen"s on its geometric "etails, an"

% # is the local %elocity&

Ass!ming the flo$ is 9!asi*stea"y, % # is relate" to the mean

piston spee" y

:here< A # an" Ap are the component minim!m flo$ area the

piston area&

)he total 9!asi*stea"y press!re loss "!e to friction is<

)his e9!ation in"icates the importance of large component

flo$ areas for re"!cing frictional losses, an" the "epen"ence

of these losses on engine spee"&

)he fig!re is an e(ample of press!re losses "!e to friction

across the air filter, car!retor, throttle, an" manifol"

plen!m of stan"ar" fo!r cylin"er engine intake system&

2

$ $ $ v p ρξ=∆

p p $ $ A%Av =

2

$

p

$

2

p

2

$ $ $ca&m )A

A(%v p p p ∑∑ ξρ=ρξ=∆=−



E9!i%alent flo$* "epen"ent press!re losses in the e(ha!st

system res!lt in the e(ha!st port an" manifol" ha%ing

a%erage press!re le%els that are higher than atmospheric&

)he fig!re sho$ the time a%erage" e(ha!st manifol" ga!ge

press!re as a f!nction of inlet manifol" %ac!!m 7$hich

%aries in%ersely to loa"8 an" spee" for a fo!r cylin"er

a!tomoile SI engine&

At high spee"s an" loa"s the e(ha!st manifol" operates at

press!re s!stantially ao%e atmospheric&

Ram effect

)he press!re in the inlet manifol" %aries "!ring each

cylin"ersD intake process "!e to the piston %elocity %ariation,

%al%e open area %ariation an" the !nstea"y gas flo$ effects

that res!lt from these geometric %ariation& )he mass of air



in"!cte" into cylin"er, an" hence the %ol!metric efficiency, is

almost entirely "etermine" y press!re le%el in the inlet port

"!ring the short perio" efore the inlet %al%e is close"&

At higher engine spee", the inertia of the gas in the intake

system as the intake %al%e is closing increases the press!re in

the port an" contin!es the charging process as the piston

slo$s "o$n aro!n" .'C an" starts the compression stroke&

)his effect ecomes progressi%ely greater as engine spee" is

increase"&

)he inlet %al%e is close" some + to - CA after .'C, in part

to take a"%antage of this ram phenomena&

Re%erse 6lo$ into the Intake

.eca!se the inlet %al%e closes after the start of the

compression stroke, a re%erse flo$ of fresh charge from the

cylin"er ack into the intake can occ!r as the cylin"er

press!re rise "!e to piston motion to$ar" )'C&

)his re%erse flo$ is largest at the lo$est engine spee"&

It is an ine%itale conse9!ence of the inlet %al%e closing time

chosen to take a"%antage of the ram effect at high spee"s&

)!ning



)he p!lsating flo$ from each cylin"ersD e(ha!st process sets

!p press!re $a%e in the e(ha!st system&

)hese press!re $a%es propagate at the local so!n" spee"

relati%e to the mo%ing e(ha!st gas&

)he press!re $a%es interact $ith the pipe #!nctions an"

r!ns in the e(ha!st manifol" an" pipe&

)hese interactions ca!se press!re $a%es to e reflecte" ack

to$ar" the engine cylin"er&

In m!lti cylin"er engines, the press!re $a%es set !p y each

cylin"er, transmitte" thro!gh the e(ha!st an" reflecte" from

the en", can interact $ith each other&

)hese press!re $a%es may ai" or inhiit the gas e(changes

processes&

:hen they ai" the process y re"!cing the press!re in the

e(ha!st port to$ar" the en" of the e(ha!st process, the

e(ha!st system is sai" to e t!ne"&

)he time *%arying inlet flo$ to the cylin"er ca!ses e(pansion

$a%es to e propagate" ack into the inlet manifol"&

)hese e(pansion $a%es can e reflecte" at the open en" of

the manifol" ca!sing positi%e press!re $a%es to e

propagate" to$ar" the cylin"er&



If the timing of these $a%es is appropriately arrange", the

positi%e press!re $a%e $ill ca!se the press!re at the inlet

%al%e at the en" of intake process to e raise" ao%e the

nominal inlet press!re&

)his $ill increase the in"!cte" air mass&

S!ch an intake system is "escrie" as t!ne"&

?etho"s $hich pre"ict the !nstea"y flo$s in the intake an"

e(ha!st systems of ICE $ith goo" acc!racy ha%e een

"e%elope"&

)he amplit!"e of the press!re $a%es increases s!stantially

$ith increasing engine spee"&

)he primary fre9!ency in oth the intake an" e(ha!st

correspon"s to the fre9!ency of the in"i%i"!al cylin"er

intake an" e(ha!st processes&

Higher harmonics that res!lt from press!re $a%es in oth

the intake an" e(ha!st are important also&



;ariation $ith Spee", an" ;al%e Area, Fift, an" )iming

6lo$ effects on %ol!metric efficiency "epen" on %elocity of

fresh mi(t!re in the intake manifol", port, an" %al%e&

Focal %elocities for 9!asi*stea"y flo$ are e9!al to the %ol!me

flo$ rate "i%i"e" y the local cross*sectional area&

Since the intake system an" %al%e "imensions scale

appro(imately $ith the cylin"er ore, mi(t!re %elocities in

intake system $ill scale $ith piston spee"&

Hence, %ol!metric efficiencies as a f!nction of spee", for

"ifferent engines, sho!l" e compare" at the same mean

piston spee"&

Ne(t fig!re sho$s typical c!r%es of %ol!metric efficiency

%ers!s mean piston spee" for a fo!r cylin"er in"irect*

in#ection "iesel engine an" si( cylin"er SI engine&

)he %ol!metric efficiencies of SI engines are !s!ally lo$er

than "iesel %al!es "!e to flo$ losses in the car!retor an"

throttle, intake manifol" heating, the presence of f!el %apor,

an" a higher resi"!al gas fraction&

)he "iesel c!r%e $ith its "o!le peak sho$s the effect of

intake t!ning&

)he shape of these %ol!metric efficiency %ers!s mean piston

spee" c!r%es can e e(plaine" $ith the ai" of ne(t fig!re&

)he fig!re sho$s, in schematic form, ho$ the effect on

%ol!metric efficiency of each of the "ifferent "escrie"

%aries $ith spee"&



Non spee" "epen"ent effects 7as f!el %apor press!re8 "ropη

%

elo$ 0 percent 7c!r%e A8&

Charge heating in the manifol" an" cylin"er "rops c!r%e A

to c!r%e .& It has a greater effect at lo$er engine spee"s "!e

to longer gas resi"ence times&

6rictional flo$ losses increase as the s9!are of engine spee",

"rop c!r%e . to c!r%e C&

At higher engine spee"s, the flo$ into the engine "!ring at

least part of intake process ecomes choke"&Once this occ!rs, f!rther increases in spee" "o not increase

the flo$ rate significantly so %ol!metric efficiency "ecreases

sharply from c!r%e C to c!r%e '&

)he in"!ction ram effect, at higher engine spee"s, raises

c!r%e ' to c!r%e E&

Fate inlet %al%e closing, $hich allo$s a"%antage to e taken

of increase" charging at higher spee"s, res!lts in a "ecrease

inη

% at lo$ engine spee"s "!e to ackflo$7c!r%es C an" '

to 68

6inally, intake an"4or e(ha!st t!ning can increase the

%ol!metric efficiency o%er part of the engine spee" range,

c!r%e 6 to G&

An e(ample of the effect on %ol!metric efficiency of t!ning

the intake manifol" r!nner is sho$n in the ne(t fig!re&



In an !nstea"y flo$ calc!lation of the gas e(hange processes

of a fo!r cylin"er SI engine, the length of the intake

manifol" r!nners $as increase" s!ccessi%ely y factors of 3&

)he 2+ mm length pro"!ces a "esirale t!ne" %ol!metric

efficiency c!r%e $ith increase" lo$*spee" air flo$ an" flat

mi"*spee" characteristics&

:hile the longest r!nner f!rther increases lo$ spee" air

flo$, the loss in η% at high spee" $o!l" e !nacceptale&

Ne(t fig!re sho$ "ata from a fo!r cylin"er SI engine $hich

ill!strate the effect of %arying %al%e timing an" %al%e lift on

%ol!metric efficiency %ers!s spee" c!r%e&

Earlier than normal inlet %al%e closing re"!ces ack flo$

losses at lo$ spee" an" increases η%&

)he penalty is re"!ce" air flo$ at high spee"s&

Fo$ %al%e lifts significantly restrict engine reathing o%er

the mi"*spee" an" high spee" operation ranges&

Ao%e a critical %al%e lift, lift is no longer a ma#or constraint

on effecti%e %al%e open area&

6FO: )HRO/GH ;AF;ES

Poppet ;al%e Geometry an" )iming

)he main geometric parameters of a poppet %al%e hea" an"

seat are sho$n in the fig!re&



Ne(t fig!re sho$s the proportions of typical inlet an"

e(ha!st %al%es an" ports, relati%e to the %al%e inner seat

"iameter '&

)he inlet port is generally circ!lar, or nearly so, an" thecross*sectional area is no larger than is re9!ire" to achie%e"

the "esire" po$er o!tp!t&

6or the e(ha!st port, the importance of goo" %al%e seat an"

g!i"e cooling, $ith the shortest length of e(pose" %al%e stem,

lea"s to a "ifferent "esign& Altho!gh a circ!lar cross section

is still "esirale, a rectang!lar or o%al shape is often

essential aro!n" the g!i"e oss area&



)ypical %al%e hea" si>es for "ifferent shape" com!stion

chamers in terms of cylin"er ore . are gi%en in )ale

;al%e hea" "iameter in terms of cylin"er ore .

Com!stion Appro(imate

Chamer shape Inlet E(ha!st mean piston spee"

?a(& po$er Jm4sK

******************************************************************************

:e"ge or atht! &+2*&+-. &21*,25. 01

.o$l in piston &+3*&++. &2+*&25. 0+

Hemispherical &+*&1. &+0*&+2. 0

6o!r %al%e

pent roof &21*&25. &3*&23. 3

Each of these chamer shapes imposes "ifferent constraints

on %al%e si>e& Farger %al%e si>es allo$ higher ma(im!m air

flo$s for a gi%en cylin"er "isplacement&

)ypical %al%e timing %al%e lift profile, an" %al%e open areas

for a fo!r stroke SI engine are sho$n in the ne(t fig!re&

SAE "efines %al%e timing e%ents ase" on reference %al%e lift

points<

• Hy"ra!lic lifters& Opening an" closing positions are

the &01 mm %al%e points&

• ?echanical lifters& ;al%e opening an" closing

positions are the points of &01mm lift pl!s the

specifie" lash&



Alternati%ely, %al%e e%ents can e "efine" ase" on ang!lar

criteria along the lift c!r%e&

)he instantaneo!s %al%e flo$ area "epen"s on %al%e lift an"

the geometric "etails of the %al%e hea", seat, an" stem&

)here are three separate stages to the flo$ area "e%elopment

as %al%e lift increases&

6or lo$ %al%e lifts, the minim!m flo$ area correspon"s to a

fr!st!m of a right circ!lar cone $here the conical face

et$een the %al%e an" the seat, $hich is perpen"ic!lar to the

seat, "efines the flo$ area&

6or this stage<

An" the minim!m area is<

:here β is the %al%e seat angle, F% is the %al%e lift, '% is the

hea" "iameter 7 the o!ter "iameter of the seat8, an" $ is the

seat $i"th 7"ifference et$een the inner an" o!ter seat

ra"ii8&6or the secon" stage , the minim!m area is still the slant

s!rface of a fr!st!m of a right circ!lar cone, !t this s!rface

is no longer perpen"ic!lar to the %al%e seat&

)he ase angle of the cone increases from 7 * β8 to$ar"

that of a cylin"er, °&

6or this stage<

an"

$here 'p is the port "iameter, 's is the %al%e stem "iameter,

an" 'm is the mean seat "iameter 7'% $8&

6inally, $hen the %al%e lift is s!fficiently large, the minim!m

flo$ is no longer et$een the %al%e hea" an" seat=

0!

co''i

wv >>

ββ

β+−βπ= 2'i2

!w2(co'!A v

vvm

ββ

>≥β+−−

co''i

w!&aw]w)

*

[(

v

2/12

m

2

'

2

p

2/122

vmm ]w)&aw![(A +β−π=



It is the port flo$ area min!s the sectional area of the %al%e

stem& )h!s, for<

then

)he ma(im!m %al%e lift is normally ao!t 03 percent of the

cylin"er ore&

Inlet %al%e opening 7I;O8 typically occ!rs 0 to 31° efore

)'C

Engine performance is relati%ely insensiti%e to this timing

point& It sho!l" occ!r s!fficiently efore )'C so that

cylin"er press!re "oes not "ip early in the intake stroke&

Inlet %al%e closing 7I;C8 !s!ally falls in the range + to -°

after .'C, to pro%i"e more time for cylin"er filling !n"er

con"itions $here cylin"er press!re is elo$ the intake

manifol" press!re at .'C& I;C is one of the principal

factors that "etermines high spee" %ol!metric efficiency= it

also affects lo$ spee" %ol!metric efficiency "!e to ackflo$

into intake&

E(ha!st %al%e opening 7E;O8 occ!rs 1 to -° efore .'C,

$ell efore the en" of e(pansion stroke, so that lo$ "o$n

β+−−

> &aw]w)*

[(!

2/122

m

2

'

2

p

v

)(*

A 2

'

2

pm −

π=



can assist in e(pelling the e(ha!st gases& )he goal here is to

re"!ce cylin"er press!re to close to the e(ha!st manifol"

press!re as soon as possile after .'C o%er the f!ll engine

spee" range& Note that the timing of E;O affects the cycle

efficiency since it "etermines the effecti%e e(pansion ratio&

E(ha!st %al%e closing 7E;C8 en"s the e(ha!st process an"

"etermines the "!ration of the %al%e o%erlap perio"& E;C

typically falls in the range to 3° after )'C& At i"le an"

light loa", in SI engines 7 $hich are throttle"8 it therefore

reg!lates the 9!antity of e(ha!st gases that flo$ ack into

the com!stion chamer thro!gh the e(ha!st %al%e !n"er

the infl!ence of intake manifol" %ac!!m& At high engine

spee"s an" loa"s, it reg!lates ho$ m!ch of the cylin"er

!rne" gases are e(ha!ste"& E;C timing sho!l" occ!r

s!fficiently far after )'C so that the cylin"er press!re "oes

not rise near the en" of the e(ha!st stroke&

Fate E;C fa%ors high po$er at the e(pense of lo$ spee"

tor9!e an" i"le com!stion 9!ality&

)he effect of %al%e geometry an" timing on air flo$ can e

ill!strate" concept!ally y "i%i"ing the rate of change of

cylin"er %ol!me y the instantaneo!s minim!m %al%e flo$

area to otain a pse!"o flo$ %elocity for each %al%e<

:here ; is the cylin"er %ol!me, . is the cylin"er ore, s is

the "istance et$een the pin an" crank a(is, an" Am is %al%e

area&Instantaneo!s pse!"o flo$ %elocity profiles for the e(ha!st

an" intake strokes of a fo!r stroke fo!r cylin"er engine are

sho$n in ne(t fig!re&

Note the appearance of t$o peaks in the pse!"o flo$ %elocity

for oth the e(ha!st an" intake strokes& )he roa"

occ!rring at ma(im!m piston %elocity reflect the fact %al%e

flo$ area is constant at this point&

)he peaks close to )'C res!lt from the e(ha!st %al%e closingan" intake %al%e opening profiles&

θ

π=

θ=

d

d'

A*

d

dV

A

1v

m

2

m

p'



)he peak at the en" of the e(ha!st stroke is important since

it in"icates a high press!re "rop across the %al%e at this

point, $hich $ill res!lt in higher trappe" resi"!al mass&

)he magnit!"e of this e(ha!st stroke pse!"o %elocity peak

"epen"s strongly on the timing of e(ha!st %al%e closing&

)he pse!"o %elocity peak at the start of the intake stroke is

m!ch less important& )hat the pse!"o %elocities early in the

e(ha!st stroke an" late in the intake stroke are lo$ in"icates

that phenomena other than 9!asi stea"y flo$ go%ern the

flo$ rate& )hese are the perio"s $hen e(ha!st lo$ "o$n

an" ram an" t!ning effects in the intake are most

important&

6lo$ Rate an" 'ischarge Coefficients

)he mass flo$ rate thro!gh a poppet %al%e is !s!ally y the

e9!ation for compressile flo$ thro!gh a flo$ restriction&

)his e9!ation is "eri%e" from a one "imensional isentropic

flo$ analysis an" real gas flo$ effects are incl!"e" y meansof an e(perimentally "etermine" "ischarge coefficient C'&

)he air flo$ rate is relate" to the !pstream stagnation

press!re p an" stagnation temperat!re ), static press!re

#!st "o$nstream of the flo$ restriction, an" a reference area

AR characteristic of the %al%e "esign<

:hen the flo$ is choke" i&e&, p) 4 p ≤ J34 7γ 08Kγ47γ * 08 , the

appropriate e9!ation is<

6or flo$ into the cylin"er thro!gh an intake %al%e, p is theintake press!re pi an" p) is the cylin"er press!re&

2/1/)1(

0

T/1

0

T2/1

0

0R ]})

p

p(1[

1

2{)

p

p(

)RT(

pA+m

γ −γ γ −

−γ

γ =

)1(2/)1(2/1

2/1

0

0R )1

2(

)RT(

pA+m

−γ −γ

+γ γ =



6or flo$ o!t of the cylin"er thro!gh an e(ha!st %al%e ,p is

the cylin"er press!re an" p) is the e(ha!st system press!re&

)he %al!e of C' an" the choice of reference area are linke"

together< their pro"!ct, C'AR , is the effecti%e flo$ area of

the %al%e assemly AE& Se%eral "ifferent reference areas ha%e

een !se"& )hese incl!"e the %al%e hea" area π'%34+, the port

area at the %al%e seat π'p34+, the geometric minim!m flo$

area&

)he most con%enient reference area in practice is the so*

calle" %al%e c!rtain area< Ac π'%F% since it %aries linearly

$ith %al%e lift an" simple to "etermine&

Inlet ;al%e

)he fig!re sho$s the res!lts of stea"y flo$ tests on typical

inlet %al%e config!ration n $ith a sharp*cornere" %al%e

seat&

)he "ischarge coefficient ase" on %al%e c!rtain area is a

"iscontin!o!s f!nction of the %al%e lift4 "iameter ratio&)he three segments sho$n correspon" to "ifferent flo$

regimes as in"icate"& At %ery lo$ lifts, the flo$ remains

attache" to the %al%e hea" an" seat, gi%ing high %al!es for

the "ischarge coefficient&

At interme"iate lifts, the flo$ separates from the %al%e hea"

at the inner e"ge of the %al%e seat as sho$n&

An ar!pt "ecrease in "ischarge coefficient occ!rs at this

point&)he "ischarge coefficient then increases $ith increasing lift

since the si>e of the separate" region remains appro(imately

constant $hile the minim!m flo$ area is increasing&

At high lifts, the flo$ separates from the inner e"ge of the

%al%e seat as $ell&



)ypical ma(im!m %al!es of F%4'% &31&

Stea"y flo$ "ischarge coefficient can e !se" to pre"ict

"ynamic performance $ith reasonale precision&

In a""ition to %al%e lift, the performance of the inlet %al%e

assemly is infl!ence" y the follo$ing factors< %al%e seat

$i"th, %al%e seat angle, ro!n"ing of the seat corners, port

"esign, cylin"er hea" shape&





In many engine "esigns the port an" %al%e assemly are !se"

to generate a rotational motion 7s$irl8 insi"e the enginecylin"er "!ring the in"!ction process, or the cylin"er hea"

can e shape" to restrict the flo$ thro!gh one si"e of the

%al%e open area to generate s$irl&

S$irl generation significantly re"!ces the %al%e7 an" port8

flo$ coefficient&

Changes in seat $i"th affect the F%4'% at $hich the shifts in

flo$ regimes& C' increases as seat $i"th "ecreases&

)he seat angleβ

affects the "ischarge coefficient in the lo$lift regime& Ro!n"ing the !pstream corner of the %al%e seat



re"!ces the ten"ency of the flo$ to reak a$ay, th!s

increasing C' at higher lifts&

At lo$ %al%e lifts, $hen the flo$ remains attache",

increasing the Reynol"s n!mer "ecreases the "ischarge

coefficient& Once the flo$ reaks a$ay from the $all, there is

no Reynol"s n!mer "epen"ence of C'&

6or $ell "esigne" ports, the "ischarge coefficient of the port

an" %al%e assemly nee" e no lo$er than of the isolate"

%al%e & If the cross sectional area of the port is not s!fficient

or ra"i!s of the s!rface at the insi"e of the en" is too small,

a significant re"!ction in C' for the assemly can res!lt&

At high engine spee"s, !nless the inlet %al%e is of s!fficient

si>e, the inlet flo$ "!ring part of the in"!ction process can

ecome choke" 7i&e&, reach sonic %elocity at the ma(im!m

%al%e flo$ area8&

Choking s!stantially re"!ces %ol!metric efficiency&

;ario!s "efinitions of inlet ?ach n!mer ha%e een !se" to

i"entify the onset of choking&

)aylor an" team correlate" %ol!metric efficiencies meas!re"

on a range of engine an" inlet %al%e "esigns $ith an inlet

?ach in"e( L forme" from an a%erage gas %elocity thro!gh

the inlet %al%e<

:here Ai is the nominal inlet %al%e area 7π'%34+8, Ci is a

mean %al%e "ischarge coefficient ase" on the area Ai, an" a

is the so!n" spee"&

6rom the metho" !se" to "etermine Ci, it is apparent thatCiAi is the a%erage effecti%e open area of the %al%e 7it is the

a%erage %al!e of C'π'%F%8& L correspon"s closely, therefore,

to the mean ?ach n!mer in the inlet %al%e throat&

)aylorDs correlations sho$ that η% "ecreases rapi"ly for

L ≥&1

An alternati%e e9!i%alent approach to this prolem has een

"e%elope", ase" on the a%erage flo$ %elocity thro!gh the

%al%e "!ring the perio" the %al%e is open&

aA+

%A

ii

p p=



A mean inlet ?ach n!mer $as "efine"<

:here %I is the mean inlet flo$ %elocity the %al%e openperio" an"

)his mean inlet ?ach n!mer correlates %ol!metric

efficiency characteristics etter than the ?ach in"e(&

6or a series of mo"ern small fo!r cylin"er engines, $hen ?i

approaches &1 the %ol!metric efficiency "ecreases rapi"ly&)his is "!e to the flo$ ecoming choke" "!ring part of

intake process&

)his relationship can e !se" to si>e the inlet %al%e for

"esire" %ol!metric efficiency at ma(im!m engine spee"&

Also, if the inlet %al%e is close" too early, %ol!metric

efficiency $ill "ecrease gra"!ally $ith increasing ?i, for

?iM&1, e%en if the %al%e open area is s!fficiently large&

a

vM i

i=

-V.-V+

vi

10)100/(M

θ−θ

η=

Documents

Gas exchange proc..doc