performans parametreleri
TRANSCRIPT
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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$uare engine)
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BC
L
TC
l
VC
s
a
B
( )( )
+=
2/122 sin/
cos1sin
2
alS
S
p
p
Averageand instantaneouspiston speeds are:
dt
dsS
LNS
p
p
=
= 2
%here & is the rotational speed of the crank shaft
in units revolutions per second
( ) 2/1222 sincos alas +=
'verage piston speed for a standard auto engine is
" m*s+ ltimately limited -y material strength+
Therefore engines with large strokes run at lower
speeds those with small strokes can run
at higher speeds+
Mean and Instantaneous Piston Speeds
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Engine Torque and Power
Tor$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
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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
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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 ;
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Me#anial E$$iieny
.ome of the power generated in the cylinder is used
to overcome engine friction+ The $rition powerisused to descri-e these losses:
0f1 0i< 0-
riction power can -e measured -y motoring the engine+
The mechanical efficiency is defined as:
m 1 0-* 0i1 < (0f* 0i)
Mechanical efficiency depends on throttle position, engine
design, and engine speed+ Typical values for car engines
at %?T are @4A B444 /0M and A B max speed+
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There is a maximum in the -rake power
versus engine speed called the rated
%ra"e power+
't higher speeds -rake power decreases as
friction power -ecomes significant compared
to the indicated power
There is a maximum in the tor$ue versus
speed called ma&imum %ra"e torque(M!T)+
!rake tor$ue drops off:=at lower speeds do to heat losses=at higher speeds it -ecomes more difficult
to ingest a full charge of air+
Ma& %ra"e torque
' "! ( ')*+' #p
,ated %ra"e power
Power and Torque versus Engine Speed
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Indiated Mean E$$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
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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
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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%
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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+
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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
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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
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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$$iieny is defined as
actual heat input divided -y theoretical heat input:
%here %in1 heat added -y com-ustion per cycle
m&1 mass of fuel added to cylinder per cycle
%'!1 heating value of the fuel (chemical energy per unit mass)
+ +
c
1 Fin
* (mf
FEV
) 1 Fin
* (mf
FEV
)
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T#ermal E$$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
)
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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)
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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
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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