12/4/2016 dr.ismail mahmoud metwally el semary shoubra/mechanical... · 4 • air-standard analysis...
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1 Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
2 Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
3
Engine Cycles
Air - Fuel Cycle
Actual Cycle
Theoretical Cycle
Air Standard Cycle
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
4
• Air-standard analysis is used to perform elementary
analyses of IC engine cycles.
• Simplifications to the real cycle include:
1) Fixed amount of air (ideal gas) for working fluid
2) Combustion process not considered
3) Intake and exhaust processes not considered
4) Engine friction and heat losses not considered
5) Specific heats independent of temperature
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Process 1 2 Isentropic compression
Process 2 3 Constant volume heat addition
Process 3 4 Isentropic expansion
Process 4 1 Constant volume heat rejection
v2 TC
TC
v1 BC
BC
Qout
Qin
Air-Standard Otto cycle
3
4
2
1
v
v
v
vr
Compression ratio:
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
6
21121122
21
21
1
2
112
1
1
1
2
112
1
)(
1
0
and
TTCvk
TTR
k
vPvPw
q
rPv
vPPrT
v
vTT
k
c
k
k
c
k
Process 1-2 Isentropic Compression.
All valves closed.
Thermodynamic Analysis of Air-Standard Otto Cycle:
Process 2-3 Constant Volume Heat Input. All valves closed.
0
and
that so
32
2332
.3.3
2
3
2
323
w
TTCvqq
PPTT
P
P
T
TvvV
in
MaxMax
TDC
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
7
43343344
43
43
34
334
1
3
1
4
334
1
)(
1
0
1 and
1
TTCvk
TTR
k
vPvPw
q
rP
v
vPP
rT
v
vTT
k
c
kk
c
k
0
that so
54
414554
4
5
4
5145
w
TTCvTTCvqq
P
P
T
Tvvvv
out
BDC
Process 3-4 Isentropic Expansion All valves closed.
Process 4-5 Constant Volume Heat Rejection Exhaust valve open and intake valve closed.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Thermal efficiency of Otto Cycle:
1
2
1
2
32
1
41
23
14
111
1
1
111
k
cOtto
in
out
in
netOtto
rT
T
T
TT
T
TT
TTCv
TTCv
q
q
q
w
The effect of compression ratio
on thermal efficiency of Otto
Cycle:
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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The effect of variation of specific heat constant γ = Cp/Cv on
thermal efficiency of Otto Cycle:
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Air-Standard Diesel cycle Process 1 2 Isentropic compression
Process 2 3 Constant pressure heat addition
Process 3 4 Isentropic expansion
Process 4 1 Constant volume heat rejection
2
3
2
3
T
T
v
vr offcut
Cut off ratio:
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
11
21121122
21
21
12
112
11
1
2
112
1
)(
1
0
and
TTCvk
TTR
k
vPvPw
q
rPv
vPPrT
v
vTT
kc
kk
c
k
23332
2332
22
32323
and
vvPw
TTCpqq
rTv
vTTPP
in
offcut
Process 1-2 Isentropic Compression Stroke.
Process 2-3 Constant Pressure Heat Input. (Combustion)
Thermodynamic Analysis of Air-Standard Diesel Cycle:
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Process 3-4 Isentropic Power or Expansion Stroke.
43343344
43
43
off 3
4
334
1
off 3
1
4
334
1
)(
1
0
and
TTCvk
TTR
k
vPvPw
q
r
rP
v
vPP
r
rT
v
vTT
k
c
cut
kk
c
cut
k
Process 4-5 Constant Volume Heat Rejection. (Exhaust stroke)
0
that so
54
414554
4
5
4
5145
w
TTCvTTCvqq
P
P
T
Tvvvv
out
BDC
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Thermal efficiency of Diesel Cycle:
1
111
11
off
koff
1
23
14
cut
cutt
k
cDiesel
in
out
in
netDiesel
rk
r
r
TTCp
TTCv
q
q
q
w
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Air-Standard Dual cycle Process 1 2 Isentropic compression
Process 2 3 Constant volume heat addition
Process 3 4 Constant pressure heat addition
Process 4 5 Isentropic expansion
Process 5 1 Constant volume heat rejection
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Thermodynamic Analysis of Air-Standard Dual Cycle:
21121122
21
21
12
112
11
1
2
112
1
)(
1
0
and
TTCvk
TTR
k
vPvPw
q
rPv
vPPrT
v
vTT
kc
kk
c
k
0
that so
32
2332
2
3
2
323
w
TTCvqq
P
P
T
TvvV
in
TDC
Process 1-2 Isentropic Compression Stroke.
Process 2-3 Constant Volume Heat Input. (Combustion)
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Process 3-4 Constant Pressure Heat Input. (Combustion)
34443
3443
3
3
43434
and
vvPw
TTCpqq
rTv
vTTPP
in
offcut
Process 4-5 Isentropic Power or Expansion Stroke.
54454455
54
54
off 3
5
445
1
off 3
1
5
445
1
)(
1
0
and
TTCvk
TTR
k
vPvPw
q
r
rP
v
vPP
r
rT
v
vTT
k
c
cut
kk
c
cut
k
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Process 5-6 Constant Volume Heat Rejection. (Exhaust stroke)
0
that so
65
515665
5
6
5
6156
w
TTCvTTCvqq
P
P
T
Tvvvv
out
BDC
Thermal efficiency of Dual cycle:
11
111
11
off
koff
1
3423
15
pcutp
cuttpk
cDual
in
out
in
netDual
rrrk
rr
r
TTCpTTCv
TTCv
q
q
q
w
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
18 Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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The comparison of Otto, Diesel and Dual cycles can be made on the basis
of compression ratio, maximum pressure, maximum temperature, heat
input, work output etc.
(A) For same compression ratio and same heat input:
When compression ratio is constant process (1-2) remains the same for all the
three cycles. And same heat is transferred in all three cycles.
QL (heat rejected) from Otto cycle < QL from Dual cycle < QL from Diesel cycle
So that:
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
12/4/2016 Dr.Ismail Mahmoud Metwally El_semary 20
B: For the same compression ratio and heat rejected.
When compression ratio is constant process (1-2) remains the same for all the three cycles. And
same heat reject in all three cycles.
QH (heat added) from Otto cycle > QH from Dual cycle > QH from Diesel cycle
So that:
21
(C) For same maximum pressure and same heat input:
For the same maximum pressure 3.3’. and 3” must be on same pressure line and
for the same heat input the area should be equal.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
QL (heat rejected) from Diesel cycle < QL from Dual cycle < QL from Otto cycle
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(D) For same pressure and temperature:
It is clear from the figure that the heat rejected by all three cycles, Otto, Diesel and Dual
cycle remains the same .
QH (heat added) from Diesel cycle > QH from Dual cycle > QH from Otto cycle
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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• Air-fuel cycle
Is the theoretical cycle based on the actual properties of the cylinder
contents.
• Simplifications to the Theoretical Air-Fuel Cycle approximation.
1- The actual composition of the cylinder contents.
2- The variation in the specific heat of the gases in the cylinder.
3- The dissociation effect.
4- The variation in the number of moles present in the cylinder as the
pressure and temperature change.
5- No chemical changes in either fuel or air prior to combustion.
6- Combustion takes place instantaneously at top dead center.
7- All processes are adiabatic.
8- The fuel is mixed well with air.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Variation of specific heats:
All gases, except mono-atomic gases,
show an increase in specific heat as
temperature increase. The specific heat
may be written in the form:
C = a+bT+cT2
where T is the absolute temperature and
a, b and c are constants for any specific
gas.
Cp and Cv increase with temperature but
decrease as the temperature increase.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Dissociation effect:
The effect of dissociation is a suppression of a part of the heat release
during combustion and the liberation of it as expansion proceeds
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The actual cycle (Otto Cycle)
1 atm
The actual cycle experienced by internal combustion engines is an open
cycle with changing composition, actual cycle efficiency is much lower than
the air standard efficiency due to various losses occurring in the actual
engine.
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P- Diagram
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Valve overlap
Exhaust gas
residual
29 Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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1. Leakage :- A small amount of combustion pressure can leak past piston rings, Valves or Gasket.
To find this problem make a Compression Test by measure the pressure inside the
combustion chamber in the compression stroke and compared this with the
standard value if P test less than P standard there is leakage.
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The loss of work due to piston movement during combustion process.
Where, combustion does not occur "instantaneously", as a result, there is some
piston motion during combustion, so combustion does not occur at constant
volume.
2. Time Losses:-
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Ignition point: The point at which ignition starts
Spark Advance: “Number of CAD BTD at which Ignition Starts”
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3. Heat Loss:-
The loss of work due to heat transfer during compression, combustion and
expansion strokes
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4. Exhaust loss:-
The loss of work due to opening the exhaust valve before B.D.C. by the hot
exhaust gasses
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5. Pumping work:-
The work required to pump the charge inside the cylinder and exhaust
gases outside the cylinder.
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Combustion usually starts at a single point, and proceeds with a
moving "flame front". Combustion time varies with fuel composition
(which affects flame speed), combustion chamber size and shape,
location and number of ignition sources, engine speed, and other
engine operating conditions.
6. Progressive Burning :-
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Actual cycle efficiency < Air standard cycle efficiency
stair
actr
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Ideal Intake and Exhaust Strokes (Neglecting the effect of the throttle)
The intake & exhaust processes are
often shown as a constant pressure
processes, e.g., 56 and 61
For WOT states 1 and 5 are the same.
Pre
ss
ure
, P
Specific volume, v
EV closes
IV opens
IV closes
EV opens
WOT
Accordingly for Ideal process:
Valves operate instantaneously, intake
and exhaust process are adiabatic and
constant pressure.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Ideal Intake and Exhaust Strokes (Considering the effect of the throttle)
Throttled: Pi < Pe
Supercharged: Pi > Pe
EV closes
IV opens
EV closes
IV opens
6’
EV opens
IV closes (state1)
EV opens
IV closes
1
EV closes
IV opens 6’
Unthrottled: Pi = Pe = 1 atm
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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The pressure at the intake port is significantly lower than atmospheric pressure
Pintake
Throttled
Throttled stroke
Actual Intake and Exhaust Strokes
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Actual Intake and Exhaust Strokes
Engines with superchargers or turbochargers have intake pressures
greater than the exhaust pressure, yielding a positive pump work
Pintake
Compressor
Supercharging
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Actual Exhaust Stroke
Blowdown – At the end of the power
stroke when the exhaust valve opens
the cylinder pressure is much higher
than the exhaust manifold pressure
which is typically at 1 atm (P4 > Pe), so
the cylinder gas flows out through the
exhaust valve and the pressure drops
to Pe.
State 4 (BC)
State 5 (BC)
State 6 (TC)
Pi Ti Pe
The actual exhaust process consists of two phases:
i) Blowdown ii) Displacement
Blowdown Displacement
Products
Displacement – Remaining gas is pushed
out of the cylinder by the piston
moving to TDC.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Valve Opening and Closing In thermo cycles it is assumed the valves open and close instantaneously
In reality a cam is used to progressively open and close the valves, the
lobes are contoured so that the valve land gently on the seat.
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Valve Opening and Closing
In real engines in order to ensure that the valve is fully open during a stroke,
for high volumetric efficiency, the valves are open for longer than 180o.
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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The exhaust valve opens before BDC and closes after TDC and the intake
valve opens before TDC and closes after BDC.
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At TDC there is a period of time called valve overlap where
both the intake and exhaust valves are open.
TDC BDC BDC CA
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Valve timing:-
No. of crank angles during which the valve is open.
Valve overlap:-
No. of crank angles during which the two valves are open at the same time
Definitions:-
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When the intake valve opens BTDC the cylinder pressure is at roughly Pe
Part throttle (Pi < Pe): residual gas flows into the intake port. During intake
stroke the residual gas is first returned to the cylinder then fresh gas is
introduced. Residual gas reduces part load performance.
Supercharged (Pi > Pe): fresh gas can flow out the exhaust valve
Pi
Throttled
Pi < Pe
Pi
Supercharged
Pi > Pe
Pe
Pe
Valve overlap
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@1000 rpm intake duration: 230o = 38.4 ms
@2500 rpm 230o = 15.4 ms
@5000 rpm 230o = 7.7 ms, 285o = 9.5 ms
i
e
BC TC
EVO
IVO
IVC
EVC
180o
i
e
BC TC
EVO
IVO
IVC
EVC
180o
Conventional Performance
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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At high engine speeds less time available for fresh gas intake so need more
crank angles to get high volumetric efficiency large valve overlap
At low engine speed and part throttle valve overlap is minimized by reducing
the number of CA the intake valve stays open.
Overlap
15o
65o
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016
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Honda Variable valve Timing
and lift Electronic Control
(VTEC)
Solenoid Activated Valves
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For ideal valve time:
The intake valve open at TDC and closed at BDC.
The exhaust valve open at BDC and closed at TDC.
For actual valve time:
The intake valve open (10o) before TDC and closed (50o:70o)after BDC.
The exhaust valve open (40o:50o) before BDC and closed (15o:30o) after TDC.
This difference to overcome the mechanical motion of the valve.
The difference between ideal and actual valve timing diagrams of
petrol engine
Dr.Ismail Mahmoud Metwally El_semary 12/4/2016