performans parametreleri

7/24/2019 Performans Parametreleri

1/18

BC

L

TC

l

VC

s

a

B ( )2/1222

sincos)( alas +=

The cylinder volume at any crank angle is:

))((4

)(2

salB

VV c ++=

Compression ratio:

c

dc

TC

BCc

V

VV

V

Vr

+==

Cylinder volume when piston at TC (s=l+a)

defined as the clearance volume Vc

Maximum displacement, or swept, volume:

LB

Vd4

2

=

Engine Geometry

or most engines ! " # (s$uare engine)


2/18

BC

L

TC

l

VC

s

a

B

( )( )

+=

2/122 sin/

cos1sin

2

alS

S

p

p

Averageand instantaneouspiston speeds are:

dt

dsS

LNS

p

p

=

= 2

%here & is the rotational speed of the crank shaft

in units revolutions per second

( ) 2/1222 sincos alas +=

'verage piston speed for a standard auto engine is

" m*s+ ltimately limited -y material strength+

Therefore engines with large strokes run at lower

speeds those with small strokes can run

at higher speeds+

Mean and Instantaneous Piston Speeds


3/18

Engine Torque and Power

Tor$ue is measured using a dynamometer+

#oad cell

orce .tator

/otor

-

&

The torqueexerted -y the engine is: T = F b with units: J

The power0 delivered -y the engine turning at a speed & and

a-sor-ed -y the dynamometer is:

01 T = (2N) T w/units: (rad/rev)(rev/s)(J) = Watt

&ote:is the shaft angular velocity with units: rad*s


4/18

Indiated !or"

2iven the cylinder pressure data over the operating

cycle of the engine one can calculate the work done

-y the gas on the piston+

The indicated work per cycle is = pdVWi

Compression

%34

0ower

%546ntake

%54

7xhaust

%34

%'5 4

%!3 4


5/18

0i1 %i& * n/ w*units: (k8*cycle) (rev*s) * (rev*cycle)

where N9 crankshaft speed in rev*s

nR9 num-er of crank revolutions per cycle

1 for ;


6/18

Me#anial E$$iieny

.ome of the power generated in the cylinder is used

to overcome engine friction+ The $rition powerisused to descri-e these losses:

0f1 0i< 0-

riction power can -e measured -y motoring the engine+

The mechanical efficiency is defined as:

m 1 0-* 0i1 < (0f* 0i)

Mechanical efficiency depends on throttle position, engine

design, and engine speed+ Typical values for car engines

at %?T are @4A B444 /0M and A B max speed+


7/18

There is a maximum in the -rake power

versus engine speed called the rated

%ra"e power+

't higher speeds -rake power decreases as

friction power -ecomes significant compared

to the indicated power

There is a maximum in the tor$ue versus

speed called ma&imum %ra"e torque(M!T)+

!rake tor$ue drops off:=at lower speeds do to heat losses=at higher speeds it -ecomes more difficult

to ingest a full charge of air+

Ma& %ra"e torque

' "! ( ')*+' #p

,ated %ra"e power

Power and Torque versus Engine Speed


8/18

Indiated Mean E$$etive Pressure -IMEP.

imepis a fictitious cnstantpressure that would produce the samework per cycle if it acted on the piston during the power stroke+

imep = Wi/ !d= ("inR) / (!dN)

s "i= imep !dN / nR= imep #p$p/ (2 nR)imepdoes not depend on engine speed, Dust like tor$ue+

imepis a -etter parameter than tor$ue to compare engines for design and

output -ecause it is independent of engine speed, &, and engine si>e, Vd+

Bra"e mean e$$etive pressure(-mep) is defined as:

R

d

d

R

d

b

n

VbmepT

V

nT

V

Wbmep

=

==

2

2


9/18

Ma&imum BMEP

=The maximum -mep is o-tained at %?T at a particular engine speed

=Closing the throttle decreases the -mep

=or a given displacement, a higher maximum -mep means more tor$ue

=or a given tor$ue, a higher maximum -mep means smaller engine

=Eigher maximum -mep means higher stresses and temperatures in the

engine hence shorter engine life, or -ulkier engine+

=or the same -mep


10/18

Spei$i /uel Consumption

=or transportation vehicles fuel economy is generally given as

mpg, or liters*44 km+

=6n engine testing the fuel consumption is measured in terms of

the fuel mass flow rate+

=The spei$i $uel onsumption0 sfc, is a measure of how efficiently the fuel supplied to the engine is used to produce power,

+ +-sfc 1 mf * 0- isfc 1 mf * 0i (w*units: g*k%


11/18

Bra"e Spei$i /uel Consumption vs Si1e

=!.C decreases with engine si>e due to reduced heat losses

from gas to cylinder wall+

rLr

rL

volumecylinder

areasurfacecylinder 12

2 =

=&ote: cylinder surface to volume ratio increases with -ore diameter+


12/18

Bra"e Spei$i /uel Consumption vs Speed

='t high speeds the -sfc increases due to increased friction

='t lower speeds the -sfc increases due to increased time for heat

losses from the gas to the cylinder and piston wall

=!sfc increases with compression ratio due to higher thermal efficiency

=There is a minimum in the -sfc versus engine speed curve


13/18

Per$ormane Maps

0erformance map is used to display the -sfc over the engines full load

and speed range+ sing a dynamometer to measure the tor$ue and fuelmass flow rate you can calculate:

-mep 1 T n/* Vd 0-1 & T

Constant -sfc contours from a

two


14/18

Com%ustion E$$iieny

=The time for com-ustion in the cylinder is very short so

not all the fuel may -e consumed or local temperatures may not support com-ustion

=' small fraction of the fuel may not react and exits with the

exhaust gas+ The om%ustion e$$iieny is defined as

actual heat input divided -y theoretical heat input:

%here %in1 heat added -y com-ustion per cycle

m&1 mass of fuel added to cylinder per cycle

%'!1 heating value of the fuel (chemical energy per unit mass)

+ +

c

1 Fin

* (mf

FEV

) 1 Fin

* (mf

FEV

)


15/18

T#ermal E$$iieny

=Thermal efficiencies can -e given in terms of -rake or indicated values

=6ndicated thermal efficiencies are typically 4A to G4A and -rake

thermal efficiencies are usually a-out H4A

t1 work per cycle * heat input per cycle

t1 % * Fin1 % * (cmfFEV)

or in terms of ratesI

t1 power out*rate of heat input

+ +

t

1 0*Fin

1 0*(c

mf

FEV

)


16/18

Ar%itrary E$$iieny

-a"a $uel onversion e$$iieny.

&ote: &is very similar to t, the difference is that ttakes into

account only the actual fuel com-usted in the engine+

+/ecall that s&c1 mf* 0-

Thus f1 * (s&cFEV)

+f1 %-* (mfFEV) 1 0-* (mfFEV)


17/18

Volumetri E$$iieny

=Jue to the short cycle time and flow restrictions less than ideal

amount of air enters the cylinder+

=The effectiveness of an engine to induct air into the cylinders is

measured -y the volumetric efficiency which is the ratio of actual

air inducted divided -y the theoretical air inducted:

+ v1 ma* (aVd) 1 n/ma* (aVd&)

where ais the density of air at atmospheric conditions 0o, To for an

ideal gas a="/ RaT andRa= 2* ,J/,-. (at standard conditions a1 +K kg*mH)

=Typical values for %?T are in the range A


18/18

Air4/uel ,atio

=or com-ustion to take place, the proper ratio of air and

fuel

must -e present in the cylinder+

=The air4$uel ratiois defined as

+ + ' 1 ma* mf1 ma* mf=The ideal ' is a-out :, with homogenous com-ustion

possi-le in the range of G to @+

=or a .6 engine the ' is in the range of to K depending

on the operating conditions+

=or a C6 engine, where the mixture is highly non

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