10 Engine Heat Transfer

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Peak gas temperature is about 2500 K

Heat flux from gases to combustion chamber walls can reach10 MWm2

!ombustion chamber surface temperatures must be less than

"00#! for cast iron an$ %00#! for aluminum allo&s to pre'entfatigue cracking

(as)si$e surface of the c&lin$er wall must be kept below1*0#! to pre'ent $eterioration of the lubricating oil film

+park plugs an$ 'al'es must be kept cool to a'oi$ knock an$preignition

,n general- heat transfer affects engine performance- efficienc&an$ emission

Importance of engine heat transferImportance of engine heat transfer


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Heat transfer and engine energy balanceHeat transfer and engine energy balance

( )f f a a b cool misc f a em h m h P Q Q m m h+ = + + + +& && & & &

, ,b cool misc e ic e s f LHV P Q Q H mh m Q+ + + + =& & & & &


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Modes of heat transferModes of heat transfer

Conduction

Convection

Radiation

. k T= &

( )c wq h T T = &

( )" "1 2q T T= &


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(as si$e

Wall

!oolant si$e

( ) ( )" ", , ,CV R c g g w g g w g q q q h T T T T = + = + & & &

( ), ,w g w cCN

w

k T Tq q

t

= =& &

( ), ,CV c c w c cq q h T T = = & &

Overall heat transfer processOverall heat transfer process

Heat flow across cylinder wall


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Convective heat transferConvective heat transfer

Diensional analysisDiensional analysis

!unctional for of relationships which govern the gas"side heat transfer

coefficient is of the for

!orulas for calculation heat transfer coefficient are based on relationship

1 1 0, , , , , , , , ..., , , , ...,p pc ch

c m n

p

S B ch B qBF r R ! !k k L c NT

=

&

Nu Re Prm na=


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Heat transfer correlationsHeat transfer correlations

Heat transfer correlations fall into three t&pes inten$e$ topre$ict

/ time)a'erage$ heat flux to combustion chamber walls

/ instantaneous spatiall& a'erage$ heat flux to walls

/ instantaneous local heat fluxes

Main parameters use$ are

/ 'elocit& to calculate e&nol$s number

/ gas temperature at which gas properties are calculate$

/ gas temperature use$ in con'ecti'e heat transfer e.uation


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Correlations for tie"averaged heat flu#Correlations for tie"averaged heat flu#

Taylor and Toong$ Data fro %& different engines

'ssuptions( coolant and wall teperatures vary little between designs and

effects of geoetry is sall

Thus) at a given fuel*air ratio) convective part of heat flu# correlate with Re

'verage effective gas teperature +governing teperature,

-usselt

Reynolds

( ) 0,c g a"h T T # =

( )( ) ( )2"

" ,,

Nu

g g a cg a c g

QB Q

Bk T TB T T k = =

& &

( )2

"

"

Re

g

mB m

BB

= =

& &


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Correlations for instantaneous spatialCorrelations for instantaneous spatial

averaged coefficientsaveraged coefficients


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Correlations for instantaneous localCorrelations for instantaneous local

coefficientscoefficients

.e!euvre) Dent and /ulaian$ 'pplied to DI diesel with swirl$

correlation forula

heat flu# at radius r

'lternative approach is to use 0onal odeling$ 1onal odels are ore accurate

than global ones

0 %%%0 *

0 0%

..

. pc

ch l $l

k k

=

( ) ( ) ( )

0 *2

0 02%

.

. g wk r

q r T r T r r

=

&


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Radiative heat transferRadiative heat transfer

wo sources of ra$iati'e heat transfer are/ high)temperature burne$ gases

/ soot particles in the $iesel engine flame

a$iation from soot particles in the $iesel engine flame is

about fi'e times the ra$iation from gaseous combustionpro$ucts

a$iati'e heat transfer in con'entional +, engines can beneglecte$

,n $iesel engines ra$iati'e heat transfer is 20 to %5 percent ofthe total heat transfer

a$iati'e heat transfer term is usuall& incorporate$ intocorrelations for con'ecti'e heat transfer


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Theral loading and coponent teperaturesTheral loading and coponent teperatures

Heat flux to combustion chamber walls 'aries with engine$esign an$ operating con$itions

Heat flux to 'arious parts of combustion chamber is not thesame

3onuniform heat flux an$ the $ifferent thermal resistancesbetween locations on the combustion chamber surface an$cooling flui$ result in nonuniform temperature $istribution

within engine components


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Coponent teperature distributionCoponent teperature distribution

3ormall& heat flux is the highest

/ in the center of the c&lin$er hea$

/ in the exhaust 'al'e seat region

/ to the center of the piston

Heat flux is lowest to the c&lin$er walls


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Piston teperature distributionPiston teperature distribution


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Cylinder head teperature distributionCylinder head teperature distribution


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Cylinder liner teperature and heat flu#Cylinder liner teperature and heat flu#

distributiondistribution


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2#haust valve teperature distribution2#haust valve teperature distribution


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+pee$- loa$- e.ui'alence ratio- compression ratio- spark orin4ection timing- charge motion- mixture inlet temperature-coolant temperature an$ composition- wall material- wall$eposits effect the magnitu$e of heat flux an$ temperature

$istribution in the engine components Woshni correlation an$ relation for heat transfer flux ma& be

use$ for pre$icting tren$s

2ffect of engine variables2ffect of engine variables

( ) ( ) ( ) ( ) ( )0 2 0 * 0 55 0 *2

W m K % 2 m kPa K m s. . . .

.ch B p T w

=

( )c wq h T T = &


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2ffect of engine variables +/I engines,2ffect of engine variables +/I engines,

/peed and load/peed and load

Relative iportance of heat lossesper cycle decreases as speed 3load increase) but average heattransfer per unit tie increases


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24uivalence ratio24uivalence ratio


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Copression ratioCopression ratio

,ncreasing compression ratio $ecreases total heat flux to the

coolant until rc%10- thereafter heat flux increases slightl& as rc

increases

ffect of changes in compression ratio on component

temperature $epen$s on location6 (enerall&- with increasing

compression ratio7

/ hea$ an$ exhaust 'al'e temperature $ecrease $ue to lower expansion

an$ exhaust stroke temperatures

/ piston an$ spark plug electro$e temperatures increase 8at constantthrottle settings9 $ue to higher peak combustion temperatures


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/par5 tiing/par5 tiing


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/wirl and s4uish/wirl and s4uish

,ncrease$ gas 'elocities- $ue to swirl of s.uish motion- result in

higher heat fluxes

( ) ( ) ( ) ( ) ( )0 2 0 * 0 55 0 *2

W m K % 2 m kPa K m s. . . .

.ch B p T w

=

( )c wq h T T = &


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Inlet teperatureInlet teperature

Heat flux increases linearl& with increasing inlet temperature

,ncrease of 100 K gi'es 1% percent increase in heat flux


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2ffect of engine variables( coolant teperature2ffect of engine variables( coolant teperature


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Heat transfer calculationHeat transfer calculation

:or con$uction an$ con'ection heat flowrate through material la&er

( )h cQ h" T T = &

Heat transfer rate is e.ual for all la&ers

where

otal thermal resistance

( ) ( ) ( ) ( )tot hg c g hg wh w wh wc c wcQ h a T T h " T T h " T T h " T T = = = = &

( ) ( ) ( )hg c tot hg wh g wh wc w wc c cQ T T R T T R T T R T T R= = = = &

1 for con'ection for con$uctionR h" R L k"= =

tot g w c

R R R R= + +


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Heat transfer calculationHeat transfer calculation

:or c&lin$er gas)wall boun$ar& la&er fro

an& instant

!&cle a'erage$ 'alues

+implest le'el is to assume a'erage thickness an$ con$ucti'it& for the c&lin$er

;wall< an$ coolant boun$ar& la&er6 hen

( )g g wQ h " T T = &

g g g w gQ Q C&C'(R h h C&C'(R T T Q h "= = = + & & &

( ) ( ) ( ) ( ) ( )1g g w w w c w w w cQ h " T T R T T k " L T T = = = &


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Heat transfer networ5Heat transfer networ5

:or isolate$ c&lin$er hea$

( )wH cH g g wHT TQ h " T T H

= = &


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:or connecte$ liner an$ piston heat balances- at point TwL

at point TN)'*

at point TwP

gL wLwL c N)'* wL

o!t f

T TT T T T Q Q

F " '

= = + +& &

gP N)'*N)'* wL N)'* oil T TT T T T

' + * B

+ =

+

gP wPN)'* wP T TT T

* B

=


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hermal resistances7

1 2

1 1

L L

Fh " h "

= + ( )1 g PB h "= ( )1 g L" h "=


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Ratio of coolant heat flow rate to

bra5e power as a function of

engine speed$

=ifferent si>e an$ t&pes of engines7

8a9 small automoti'e $iesels?

8b9 larger automoti'e $iesels?

8c9 'arious $iesels

8$9 k i iti i


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10 Engine Heat Transfer

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