chapter 9 gas power cycles - به نام یگانه ... · pdf file9-2 9-14 the four processes...

9-1

Chapter 9 GAS POWER CYCLES

Actual and Ideal Cycles, Carnot cycle, Air-Standard Assumptions 9-1C The Carnot cycle is not suitable as an ideal cycle for all power producing devices because it cannot be approximated using the hardware of actual power producing devices. 9-2C It is less than the thermal efficiency of a Carnot cycle. 9-3C It represents the net work on both diagrams. 9-4C The cold air standard assumptions involves the additional assumption that air can be treated as an ideal gas with constant specific heats at room temperature. 9-5C Under the air standard assumptions, the combustion process is modeled as a heat addition process, and the exhaust process as a heat rejection process. 9-6C The air standard assumptions are: (1) the working fluid is air which behaves as an ideal gas, (2) all the processes are internally reversible, (3) the combustion process is replaced by the heat addition process, and (4) the exhaust process is replaced by the heat rejection process which returns the working fluid to its original state. 9-7C The clearance volume is the minimum volume formed in the cylinder whereas the displacement volume is the volume displaced by the piston as the piston moves between the top dead center and the bottom dead center. 9-8C It is the ratio of the maximum to minimum volumes in the cylinder. 9-9C The MEP is the fictitious pressure which, if acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle. 9-10C Yes. 9-11C Assuming no accumulation of carbon deposits on the piston face, the compression ratio will remain the same (otherwise it will increase). The mean effective pressure, on the other hand, will decrease as a car gets older as a result of wear and tear. 9-12C The SI and CI engines differ from each other in the way combustion is initiated; by a spark in SI engines, and by compressing the air above the self-ignition temperature of the fuel in CI engines. 9-13C Stroke is the distance between the TDC and the BDC, bore is the diameter of the cylinder, TDC is the position of the piston when it forms the smallest volume in the cylinder, and clearance volume is the minimum volume formed in the cylinder.

PROPRIETARY MATERIAL. © 2006 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, you are using it without permission.

9-2

9-14 The four processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the net work output and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A-17. Analysis (b) The properties of air at various states are

P

qout

qin 3

4

2

1

qout

qin 3

4

2

1

( )

( )

( ) kJ/kg 828.149.101310kPa 2668

kPa 100

kPa 2668kPa 800K 539.8K 1800

1310kJ/kg 1487.2

K 1800

K 539.8kJ/kg 389.22

088.111.386kPa 100kPa 800

386.1kJ/kg 300.19

K300

43

4

22

33

2

22

3

33

33

2

2

1

2

11

34

3

12

1

=→===

===→=

==

→=

==

→===

==

→=

hPPP

P

PTT

PT

PT

P

Pu

Tu

PPP

P

Ph

T

rr

r

rr

r

vv

T v

T

From energy balances,

kJ/kg570.1 9.5270.1098

kJ/kg 527.919.3001.828

kJ/kg 1098.02.3892.1487

outinoutnet,

14out

23in

=−=−=

=−=−=

=−=−=

qqw

hhq

uuq

s

(c) Then the thermal efficiency becomes

51.9%===kJ/kg1098.0

kJ/kg570.1

in

outnet,th q

wη


9-3

9-15 EES Problem 9-14 is reconsidered. The effect of the maximum temperature of the cycle on the net work output and thermal efficiency is to be investigated. Also, T-s and P-v diagrams for the cycle are to be plotted. Analysis Using EES, the problem is solved as follows: "Input Data" T[1]=300 [K] P[1]=100 [kPa] P[2] = 800 [kPa] T[3]=1800 [K] P[4] = 100 [kPa] "Process 1-2 is isentropic compression" s[1]=entropy(air,T=T[1],P=P[1]) s[2]=s[1] T[2]=temperature(air, s=s[2], P=P[2]) P[2]*v[2]/T[2]=P[1]*v[1]/T[1] P[1]*v[1]=R*T[1] R=0.287 [kJ/kg-K] "Conservation of energy for process 1 to 2" q_12 -w_12 = DELTAu_12 q_12 =0"isentropic process" DELTAu_12=intenergy(air,T=T[2])-intenergy(air,T=T[1]) "Process 2-3 is constant volume heat addition" s[3]=entropy(air, T=T[3], P=P[3]) {P[3]*v[3]/T[3]=P[2]*v[2]/T[2]} P[3]*v[3]=R*T[3] v[3]=v[2] "Conservation of energy for process 2 to 3" q_23 -w_23 = DELTAu_23 w_23 =0"constant volume process" DELTAu_23=intenergy(air,T=T[3])-intenergy(air,T=T[2]) "Process 3-4 is isentropic expansion" s[4]=entropy(air,T=T[4],P=P[4]) s[4]=s[3] P[4]*v[4]/T[4]=P[3]*v[3]/T[3] {P[4]*v[4]=0.287*T[4]} "Conservation of energy for process 3 to 4" q_34 -w_34 = DELTAu_34 q_34 =0"isentropic process" DELTAu_34=intenergy(air,T=T[4])-intenergy(air,T=T[3]) "Process 4-1 is constant pressure heat rejection" {P[4]*v[4]/T[4]=P[1]*v[1]/T[1]} "Conservation of energy for process 4 to 1" q_41 -w_41 = DELTAu_41 w_41 =P[1]*(v[1]-v[4]) "constant pressure process" DELTAu_41=intenergy(air,T=T[1])-intenergy(air,T=T[4]) q_in_total=q_23 w_net = w_12+w_23+w_34+w_41 Eta_th=w_net/q_in_total*100 "Thermal efficiency, in percent"


9-4

T3 [K] ηth qin,total [kJ/kg] Wnet [kJ/kg] 1500 50.91 815.4 415.1 1700 51.58 1002 516.8 1900 52.17 1192 621.7 2100 52.69 1384 729.2 2300 53.16 1579 839.1 2500 53.58 1775 951.2

5.0 5.3 5.5 5.8 6.0 6.3 6.5 6.8 7.0 7.3 7.5200

400

600

800

1000

1200

1400

1600

1800

2000

s [kJ/kg-K]

T[K]

100 kPa

800 kPa

Air

1

2

3

4

10-2 10-1 100 101 102101

102

103

4x103

v [m3/kg]

P[kPa]

300 K

1800 K

Air

1

2

3

4


9-5

1500 1700 1900 2100 2300 250050.5

51

51.5

52

52.5

53

53.5

54

T[3] [K]

ηth

1500 1700 1900 2100 2300 2500800

1000

1200

1400

1600

1800

T[3] [K]

qin,total

[kJ/kg]

1500 1700 1900 2100 2300 2500400

500

600

700

800

900

1000

T[3] [K]

wnet

[kJ/kg]


9-6

9-16 The four processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the maximum temperature in the cycle and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg.K, cv = 0.718 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (b) From the ideal gas isentropic relations and energy balance, P

qin

q41

q34

3

4

2

1

( )

( ) K 579.2kPa 100kPa 1000K 300

0.4/1.4/1

1

212 =

=

=

− kk

PPTT

( )

( )( ) K 3360==→−⋅=

−=−=

3max3

2323in

579.2KkJ/kg 1.005kJ/kg 2800 TTT

TTchhq p

v (c) ( ) K 336K 3360

kPa 1000kPa 100

33

44

4

44

3

33 ===→= TPPT

TP

TP vv

( ) ( )( ) ( )

( )( ) ( )( )

21.0%=−=−=

=−⋅+−⋅=

−+−=

−+−=+=

kJ/kg 2800kJ/kg 2212

11

kJ/kg 2212K300336KkJ/kg 1.005K3363360KkJ/kg 0.718

in

outth

1443

1443out41,out34,out

qq

TTcTTchhuuqqq

p

η

v

T

q34

q41

qin 3

4

2

1

s

Discussion The assumption of constant specific heats at room temperature is not realistic in this case the temperature changes involved are too large. 9-17E The four processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the total heat input and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A-17E. Analysis (b) The properties of air at various states are

P T Btu/lbm 129.06 Btu/lbm, 92.04R 540 111 ==→= hu

q12

qout1

2 q23

4

3

s

qout

q23

q12

1

2

4

3

v

( )

( ) Btu/lbm 593.22317.01242psia 57.6psia 14.7

1242Btu/lbm 48.849

R 3200

psia 57.6psia 14.7R 540R 2116

Btu/lbm 1.537,R2116Btu/lbm 04.39230004.92

43

4

33

11

22

1

11

2

22

22

in,121212in,12

34

3

=→===

==

→=

===→=

==

=+=+=→−=

hPPP

P

Ph

T

PTT

PT

PT

P

hTquu

uuq

rr

r

vv

T

From energy balance,

Btu/lbm 464.1606.12922.593 38.312300

Btu/lbm 312.381.53748.849

14out

in23,in12,in

23in23,

=−=−=

=+=+=

=−=−=

hhqqqq

hhqBtu/lbm612.38

(c) Then the thermal efficiency becomes

24.2%=−=−=Btu/lbm612.38Btu/lbm464.16

11in

outth q

qη


9-7

9-18E The four processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the total heat input and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 0.240 Btu/lbm.R, cv = 0.171 Btu/lbm.R, and k = 1.4 (Table A-2E).

PAnalysis (b) ( )

( )( )

( )

( ) ( )( ) Btu/lbm 217.4R22943200RBtu/lbm 0.24

psia 62.46psia 14.7R 540R 2294

R 2294R540Btu/lbm.R 0.171Btu/lbm 300

2323in,23

11

22

1

11

2

22

2

2

1212in,12

=−⋅=−=−=

===→=

=−=

−=−=

TTchhq

PTT

PT

PT

PT

TTTcuuq

P

vv

v

qout

q23

q12

1

2

4

3

v

Process 3-4 is isentropic: ( )

( )

( ) ( )( ) Btu/lbm 378.55402117Btu/lbm.R 0.240

4.217300

R 2117psia 62.46

psia 14.7R 3200

1414out

in,23in,12in

0.4/1.4/1

3

434

=−=−=−=

=+=+=

=

=

=

−

TTchhq

qqq

PP

TT

p

kk

Btu/lbm517.4

T

q12

qout1

2 q23

4

3

(c) 26.8%=−=−=Btu/lbm 517.4Btu/lbm 378.5

11in

outth q

qη s

9-19 The three processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the heat rejected and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg.K, cv = 0.718 kJ/kg·K, and k = 1.4 (Table A-2).

Analysis (b) ( )

( ) K 579.2kPa 100kPa 1000

K 3000.4/1.4/1

1

212 =

=

=

− kk

PP

TT P

qout

qin

1

2 3

v

( ) ( )( )( )( ) K 12662.579KkJ/kg 1.005kg 0.004kJ 2.76 33

2323in

=→−⋅=

−=−=

TT

TTmchhmQ p

Process 3-1 is a straight line on the P-v diagram, thus the w31 is simply the area under the process curve,

( )

( ) kJ/kg 7.273KkJ/kg 0.287kPa 1000K 1266

kPa 100K 300

2kPa 1001000

22area

3

3

1

11331

1331

=⋅

−

+=

−

+=−

+==

PRT

PRTPPPP

w vv

T

Energy balance for process 3-1 gives

( )[ ]( ) ( )( )[ ] kJ 1.679=⋅+−=

−+−=−−−=

−=−−→∆=−

K1266-300KkJ/kg 0.718273.7kg 0.004)(

)(

31out31,31out31,out31,

31out31,out31,systemoutin

TTcwmTTmcmwQ

uumWQEEE

vv

qin

qout 1

2 3

s

(c) 39.2%=−=−=kJ 2.76kJ 1.679

11in

outth Q

Qη


9-8

9-20 The three processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the net work per cycle and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A-17. Analysis (b) The properties of air at various states are P

qout

qin

1

2

3

( )

( )

( ) ( )( )

( ) ( )( )

kJ0.422 651.1073.2

kJ 1.651kJ/kg290.16840.38kg 0.003

kJ 2.073kJ/kg206.91897.91kg 0.003

kJ/kg 840.3851.8207.2kPa 380

kPa 95

2.207kJ/kg, 897.91

K 1160K 290kPa 95kPa 380

kJ/kg 290.16kJ/kg 206.91

K 029

outinoutnet,

13out

12in

32

3

2

11

22

1

11

2

22

1

11

23

2

=−=−=

=−=−=

=−=−=

=→===

==→

===→=

==

→=

QQW

hhmQ

uumQ

hPPP

P

Pu

TPP

TT

PT

P

hu

T

rr

r

vv

v

T

qout

qin

2

3 1

s(c) 20.4%===

kJ 2.073kJ 0.422

in

outnet,th Q

Wη

9-21 The three processes of an air-standard cycle are described. The cycle is to be shown on P-v and T-s diagrams, and the net work per cycle and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg.K, cv = 0.718 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (b) From the isentropic relations and energy balance,

( )

( )( )

( ) ( )

( )( )( )

( ) ( )

( )( )( )

kJ0.39 48.187.1

kJ 1.48K290780.6KkJ/kg 1.005kg 0.003

kJ 1.87K2901160KkJ/kg 0.718kg 0.003

K 780.6kPa 380

kPa 95K 1160

K 1160K 290kPa 95kPa 380

outinoutnet,

1313out

1212in

0.4/1.4/1

2

323

11

22

1

11

2

22

=−=−=

=−⋅=

−=−=

=−⋅=

−=−=

=

=

=

===→=

−

QQW

TTmchhmQ

TTmcuumQ

PP

TT

TPP

TT

PT

P

p

kk

v

vv

P

qout

qin

2

3 1

s

qout

qin

1

2

3

v

T

(c) 20.9%===kJ1.87kJ0.39

in

netth Q

Wη


9-9

9-22 A Carnot cycle with the specified temperature limits is considered. The net work output per cycle is to be determined. Assumptions Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg.K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg.K, and k = 1.4 (Table A-2). Analysis The minimum pressure in the cycle is P3 and the maximum pressure is P1. Then,

or,

( )

( )( ) kPa 3.935

K 300K 900

kPa 201.4/0.41/

3

232

/1

3

2

3

2

=

=

=

=

−

−

kk

kk

TT

PP

PP

TT

T

The heat input is determined from

Then,

( )

( ) ( )( )( )

( )( ) kJ 0.393===

=−=−=

=⋅=−=

⋅=⋅−=−=−

kJ 0.58890.667

.7%66K 900K 300

11

kJ 0.5889KkJ/kg 0.2181K 900kg 0.003

KkJ/kg 0.2181kPa 2000kPa 935.3

lnKkJ/kg 0.287lnln

inthoutnet,

th

12in

1

20

1

212

QW

TT

ssmTQ

PP

RTT

css

H

L

H

p

η

η

1

4 qout

qin 2

3

900

300

s

9-23 A Carnot cycle with specified temperature limits is considered. The maximum pressure in the cycle, the heat transfer to the working fluid, and the mass of the working fluid are to be determined. Assumptions Air is an ideal gas with variable specific heats. Analysis (a) In a Carnot cycle, the maximum pressure occurs at the beginning of the expansion process, which is state 1.

(Table A-17) T P

T P

r

r

1

4

1

4

238

2 379

= → =

= → =

1200 K

350 K .T

Wnet = 0.5 kJ

1

4Qout

Qin 2

3

1200

( ) max41 kPa 300379.2

238

4

1 PPP

PP

r

r==== kPa30,013

350(b) The heat input is determined from

s

( ) ( ) kJ0.706 0.7083/kJ 0.5/

%83.70K 1200K 350

11

thoutnet,in

th

===

=−=−=

η

η

WQ

TT

H

L

(c) The mass of air is

( ) ( )

( )( ) ( )( )

kg0.00296 kJ/kg 169.15kJ 0.5

kJ/kg169.15K3501200KkJ/kg0.199

KkJ/kg 0.199

kPa 150kPa 300

lnKkJ/kg 0.287ln

outnet,

outnet,

12outnet,

21

3

403434

===

=−⋅=−−=

−=⋅−=

⋅−=−−=−

wW

m

TTssw

ss

PP

Rssss

LH

oo


9-10

9-24 A Carnot cycle with specified temperature limits is considered. The maximum pressure in the cycle, the heat transfer to the working fluid, and the mass of the working fluid are to be determined. Assumptions Helium is an ideal gas with constant specific heats. Properties The properties of helium at room temperature are R = 2.0769 kJ/kg.K and k = 1.667 (Table A-2). Analysis (a) In a Carnot cycle, the maximum pressure occurs at the beginning of the expansion process, which is state 1.

or,

( )

( )( ) kPa 6524=

=

=

=

−

−

71.667/0.661/

4

141

/1

4

1

4

1

K 350K 1200

kPa 300kk

kk

TT

PP

PP

TT

Wnet = 0.5 kJ

350

1200 1

4

Qin 2

3

T

(b) The heat input is determined from s

( ) ( ) kJ0.706 0.7083/kJ 0.5/

70.83%K 1200K 350

11

thoutnet,in

th

===

=−=−=

η

η

WQ

TT

H

L

(c) The mass of helium is determined from

( )

( )( ) ( )( )

kg 0.000409===

=−⋅=−−=

−=⋅−=

⋅−=−=−

kJ/kg 1223.7kJ 0.5

kJ/kg 1223.7K3501200KkJ/kg 1.4396

KkJ/kg 1.4396

kPa 150kPa 300

lnKkJ/kg 2.0769lnln

outnet,

outnet,

12outnet,

21

3

40

3

434

wW

m

TTssw

ss

PP

RTT

css

LH

p


9-11

9-25 A Carnot cycle executed in a closed system with air as the working fluid is considered. The minimum pressure in the cycle, the heat rejection from the cycle, the thermal efficiency of the cycle, and the second-law efficiency of an actual cycle operating between the same temperature limits are to be determined. Assumptions Air is an ideal gas with constant specific heats. Properties The properties of air at room temperatures are R = 0.287 kJ/kg.K and k = 1.4 (Table A-2). Analysis (a) The minimum temperature is determined from

( )( ) ( )( ) K 350K750KkJ/kg 0.25kJ/kg 10012net =→−⋅=→−−= LLLH TTTTssw

The pressure at state 4 is determined from

or

( )

( )

kPa 1.110K 350K 750kPa 800 4

1.4/0.4

4

1/

4

141

/1

4

1

4

1

=→

=

=

=

−

−

PP

TT

PP

PP

TT

kk

kk

qout

wnet=100 kJ/kg

750 K 1

4

qin 2

3

T

s

The minimum pressure in the cycle is determined from

( ) kPa 46.1=→⋅−=⋅−

−=∆−=∆

33

3

40

3

43412

kPa 110.1lnKkJ/kg 0.287KkJ/kg 25.0

lnln

PP

PP

RTT

css p

(b) The heat rejection from the cycle is kgkJ/ 87.5==∆= kJ/kg.K) K)(0.25 350(12out sTq L

(c) The thermal efficiency is determined from

0.533=−=−=K 750K 350

11thH

L

TT

η

(d) The power output for the Carnot cycle is

kW 9000kJ/kg) kg/s)(100 90(netCarnot === wmW &&

Then, the second-law efficiency of the actual cycle becomes

0.578===kW 9000kW 5200

Carnot

actualII W

W&

&η


9-12

Otto Cycle 9-26C The four processes that make up the Otto cycle are (1) isentropic compression, (2) v = constant heat addition, (3) isentropic expansion, and (4) v = constant heat rejection. 9-27C The ideal Otto cycle involves external irreversibilities, and thus it has a lower thermal efficiency. 9-28C For actual four-stroke engines, the rpm is twice the number of thermodynamic cycles; for two-stroke engines, it is equal to the number of thermodynamic cycles. 9-29C They are analyzed as closed system processes because no mass crosses the system boundaries during any of the processes. 9-30C It increases with both of them. 9-31C Because high compression ratios cause engine knock. 9-32C The thermal efficiency will be the highest for argon because it has the highest specific heat ratio, k = 1.667. 9-33C The fuel is injected into the cylinder in both engines, but it is ignited with a spark plug in gasoline engines.


9-13

9-34 An ideal Otto cycle with air as the working fluid has a compression ratio of 8. The pressure and temperature at the end of the heat addition process, the net work output, the thermal efficiency, and the mean effective pressure for the cycle are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The gas constant of air is R = 0.287 kJ/kg.K. The properties of air are given in Table A-17. Analysis (a) Process 1-2: isentropic compression.

621.2kJ/kg214.07

K3001

11 =

=→=

r

uT

v

P

750 kJ/kg 2

3

1

4

v

( )

( ) ( ) kPa 1705kPa 95K 300K 673.1

8

kJ/kg 491.2K 673.1

65.772.621811

11

2

2

12

1

11

2

22

2

2

1

2112

=

==→=

==

→====

PTT

PT

PT

P

uT

r rrr

v

vvv

vvv

vv

Process 2-3: v = constant heat addition.

588.6

kJ/kg 241.217502.4913

3in23,2323in23, =

=→=+=+=→−=

r

Tquuuuq

v

K1539

( ) kPa3898 kPa 1705K 673.1K 1539

22

33

2

22

3

33 =

==→= P

TT

PT

PT

P vv

(b) Process 3-4: isentropic expansion.

( )( )kJ/kg 571.69

K 774.570.52588.68

4

4

2

1334 =

=→====

uT

r rrr vvv

vv

Process 4-1: v = constant heat rejection. q u uout 571.69 214.07 357.62 kJ / kg= − = − =4 1

kJ/kg 392.4=−=−= 62.357750outinoutnet, qqw

(c) 52.3%===kJ/kg 750kJ/kg 392.4

in

outnet,th q

wη

(d) ( )( )

( )( )kPa495.0

kJmkPa

1/81/kgm 0.906kJ/kg 392.4

)/11(MEP

/kgm0.906kPa95

K300K/kgmkPa0.287

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

=

⋅

−=

−=

−=

==

==⋅⋅

==

rww

r

PRT

vvv

vvv

vv


9-14

9-35 EES Problem 9-34 is reconsidered. The effect of the compression ratio on the net work output and thermal efficiency is to be investigated. Also, T-s and P-v diagrams for the cycle are to be plotted. Analysis Using EES, the problem is solved as follows: "Input Data" T[1]=300 [K] P[1]=95 [kPa] q_23 = 750 [kJ/kg] {r_comp = 8} "Process 1-2 is isentropic compression" s[1]=entropy(air,T=T[1],P=P[1]) s[2]=s[1] T[2]=temperature(air, s=s[2], P=P[2]) P[2]*v[2]/T[2]=P[1]*v[1]/T[1] P[1]*v[1]=R*T[1] R=0.287 [kJ/kg-K] V[2] = V[1]/ r_comp "Conservation of energy for process 1 to 2" q_12 - w_12 = DELTAu_12 q_12 =0"isentropic process" DELTAu_12=intenergy(air,T=T[2])-intenergy(air,T=T[1]) "Process 2-3 is constant volume heat addition" v[3]=v[2] s[3]=entropy(air, T=T[3], P=P[3]) P[3]*v[3]=R*T[3] "Conservation of energy for process 2 to 3" q_23 - w_23 = DELTAu_23 w_23 =0"constant volume process" DELTAu_23=intenergy(air,T=T[3])-intenergy(air,T=T[2]) "Process 3-4 is isentropic expansion" s[4]=s[3] s[4]=entropy(air,T=T[4],P=P[4]) P[4]*v[4]=R*T[4] "Conservation of energy for process 3 to 4" q_34 -w_34 = DELTAu_34 q_34 =0"isentropic process" DELTAu_34=intenergy(air,T=T[4])-intenergy(air,T=T[3]) "Process 4-1 is constant volume heat rejection" V[4] = V[1] "Conservation of energy for process 4 to 1" q_41 - w_41 = DELTAu_41 w_41 =0 "constant volume process" DELTAu_41=intenergy(air,T=T[1])-intenergy(air,T=T[4]) q_in_total=q_23 q_out_total = -q_41 w_net = w_12+w_23+w_34+w_41 Eta_th=w_net/q_in_total*100 "Thermal efficiency, in percent" "The mean effective pressure is:" MEP = w_net/(V[1]-V[2])"[kPa]"


9-15

rcomp ηth MEP [kPa] wnet [kJ/kg]

5 43.78 452.9 328.4 6 47.29 469.6 354.7 7 50.08 483.5 375.6 8 52.36 495.2 392.7 9 54.28 505.3 407.1

10 55.93 514.2 419.5

4.5 5.0 5.5 6.0 6.5 7.0 7.5200

400

600

800

1000

1200

1400

1600

s [kJ/kg-K]

T [K

]

95

kPa

3900 kPa

0.9

0.11 m3/kg

Air

1

2

3

4

10-2 10-1 100 101 102101

102

103

104

v [m3/kg]

P [k

Pa]

300 K

1500 K

Air

1

2

3

4

s2 = s1 = 5.716 kJ/kg-K

s4 = 33 = 6.424 kJ/kg-K


9-16

5 6 7 8 9 10320

340

360

380

400

420

rcomp

wnet

[kJ/kg]

5 6 7 8 9 10450

460

470

480

490

500

510

520

rcomp

MEP[kPa]

5 6 7 8 9 1042

44

46

48

50

52

54

56

rcomp

ηth


9-17

9-36 An ideal Otto cycle with air as the working fluid has a compression ratio of 8. The pressure and temperature at the end of the heat addition process, the net work output, the thermal efficiency, and the mean effective pressure for the cycle are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression.

( )( )

( ) ( ) kPa 1745kPa 95K 300K 689

8

K 6898K300

11

2

2

12

1

11

2

22

0.41

2

112

=

==→=

==

=

−

PTT

PT

PT

P

TTk

v

vvv

v

v

P

750 kJ/kg 2

3

1

4

vProcess 2-3: v = constant heat addition.

( )

( )(K1734

K689KkJ/kg 0.718kJ/kg 750

3

3

2323in23,

=−⋅=

−=−=

TTTTcuuq v

)

( ) kPa4392 kPa 1745K 689K 1734

22

33

2

22

3

33 =

==→= P

TT

PT

PT

P vv


( ) K 75581K 1734

0.41

4

334 =

=

=

−k

TTv

v

Process 4-1: v = constant heat rejection. ( ) ( )( ) kJ/kg 327K300755KkJ/kg 0.7181414out =−⋅=−=−= TTcuuq v

kJ/kg423 327750outinoutnet, =−=−= qqw

(c) 56.4%===kJ/kg750kJ/kg423

in

outnet,th q

wη

(d) ( )( )

( )( )kPa534

kJmkPa

1/81/kgm 0.906kJ/kg 423

)/11(MEP

/kgm 0.906kPa 95

K 300K/kgmkPa 0.287

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

=

⋅

−=

−=

−=

==

==⋅⋅

==

rww

r

PRT

vvv

vvv

vv


9-18

9-37 An ideal Otto cycle with air as the working fluid has a compression ratio of 9.5. The highest pressure and temperature in the cycle, the amount of heat transferred, the thermal efficiency, and the mean effective pressure are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression.

P

( )( )

( ) ( ) kPa 2338kPa 100K 308K 757.9

9.5

K 757.99.5K 308

11

2

2

12

1

11

2

22

0.41

2

112

=

==→=

==

=

−

PTT

PT

PT

P

TTk

v

vvv

v

v

Qout Qin

2

3

1

4

vProcess 3-4: isentropic expansion.

( )( ) K 1969==

=

−0.4

1

3

443 9.5K 800

k

TTv

v


( ) kPa 6072=

==→= kPa 2338

K 757.9K 1969

22

33

2

22

3

33 PTT

PT

PT

P vv

(b) ( )( )

( )( )kg10788.6

K 308K/kgmkPa 0.287m 0.0006kPa 100 4

3

3

1

11 −×=⋅⋅

==RTP

mV

( ) ( ) ( )( )( ) kJ 0.590=−⋅×=−=−= − K757.91969KkJ/kg 0.718kg106.788 42323in TTmcuumQ v

(c) Process 4-1: v = constant heat rejection.

( ) ( )( )( ) kJ0.2 40K308800KkJ/kg 0.718kg106.788)( 41414out =−⋅×−=−=−= −TTmcuumQ v

kJ 0.350240.0590.0outinnet =−=−= QQW

59.4%===kJ 0.590kJ 0.350

in

outnet,th Q

Wη

(d)

( )( )kPa 652=

⋅

−=

−=

−=

==

kJmkPa

1/9.51m 0.0006kJ 0.350

)/11(MEP

3

31

outnet,

21

outnet,

max2min

rWW

r

VVV

VVV


9-19

9-38 An Otto cycle with air as the working fluid has a compression ratio of 9.5. The highest pressure and temperature in the cycle, the amount of heat transferred, the thermal efficiency, and the mean effective pressure are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression.

P

( )( )

( ) ( ) kPa 2338kPa 100K 308K 757.9

9.5

K 757.99.5K 308

11

2

2

12

1

11

2

22

0.41

2

112

=

==→=

==

=

−

PTT

PT

PT

P

TTk

v

vvv

v

v

800 K

Polytropic

Qout

Qin

2

3

1308 K

4

vProcess 3-4: polytropic expansion.

( )( )( )( )

kg10788.6K 308K/kgmkPa 0.287

m 0.0006kPa 100 43

3

1

11 −×=⋅⋅

==RTP

mV

( )( )

( ) ( )( )( )kJ 0.5338

1.351K1759800KkJ/kg 0.287106.788

1

9.5K 800

434

34

35.01

3

443

=−

−⋅×=

−−

=

==

=

−

−

nTTmR

W

TTn

K 1759v

v

Then energy balance for process 3-4 gives

( )( ) ( )

( )( )( ) kJ 0.0664kJ 0.5338K1759800KkJ/kg 0.718kg106.788 4in34,

out34,34out34,34in34,

34out34,in34,

outin

=+−⋅×=

+−=+−=

−=−

∆=−

−QWTTmcWuumQ

uumWQ

EEE system

v

That is, 0.066 kJ of heat is added to the air during the expansion process (This is not realistic, and probably is due to assuming constant specific heats at room temperature). (b) Process 2-3: v = constant heat addition.

( ) kPa 5426=

==→= kPa 2338

K 757.9K 1759

22

33

2

22

3

33 PTT

PT

PT

P vv

( ) ( )( )( )( ) kJ 0.4879K757.91759KkJ/kg 0.718kg106.788 4

in23,

2323in23,

=−⋅×=

−=−=−Q

TTmcuumQ v

Therefore, kJ 0.5543=+=+= 0664.04879.0in34,in23,in QQQ (c) Process 4-1: v = constant heat rejection. ( ) ( ) ( )( )( ) kJ 0.2398K308800KkJ/kg 0.718kg 106.788 4

1414out =−⋅×=−=−= −TTmcuumQ v kJ 0.31452398.05543.0outinoutnet, =−=−= QQW

56.7%===kJ 0.5543kJ 0.3145

in

outnet,th Q

Wη

(d)

( )( )kPa 586=

⋅

−=

−=

−=

==

kJmkPa

1/9.51m 0.0006kJ 0.3145

)/11(MEP

3

31

outnet,

21

outnet,

max2min

rWW

rV

VVV

VV


9-20

9-39E An ideal Otto cycle with air as the working fluid has a compression ratio of 8. The amount of heat transferred to the air during the heat addition process, the thermal efficiency, and the thermal efficiency of a Carnot cycle operating between the same temperature limits are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A-17E. Analysis (a) Process 1-2: isentropic compression. P

540

2400 R

qout qin

2

3

1

4

v

T 32.144Btu/lbm92.04

R5401

11 =

=→=

r

uv

( ) Btu/lbm 11.28204.1832.144811

21

2222

=→==== ur rrr vv

v

vv


Btu/lbm 241.42=−=−=

==

→=

28.21170.452

419.2Btu/lbm 452.70

R2400

23

33

3

uuq

uT

in

rv


( )( ) Btu/lbm 205.5435.19419.28 43

4334

=→==== ur rrr vvv

vv

Process 4-1: v = constant heat rejection. Btu/lbm 50.11304.9254.20514out =−=−= uuq

53.0%=−=−=Btu/lbm 241.42Btu/lbm 113.50

11in

outth q

qη

(c) 77.5%=−=−=R 2400

R 54011Cth,

H

L

TT

η

9-40E An ideal Otto cycle with argon as the working fluid has a compression ratio of 8. The amount of heat transferred to the argon during the heat addition process, the thermal efficiency, and the thermal efficiency of a Carnot cycle operating between the same temperature limits are to be determined. Assumptions 1 The air-standard assumptions are applicable with argon as the working fluid. 2 Kinetic and potential energy changes are negligible. 3 Argon is an ideal gas with constant specific heats. Properties The properties of argon are cp = 0.1253 Btu/lbm.R, cv = 0.0756 Btu/lbm.R, and k = 1.667 (Table A-2E). Analysis (a) Process 1-2: isentropic compression.

P

( )( ) R 21618R 540 0.6671

2

112 ==

=

−k

TTv

v

Process 2-3: v = constant heat addition. ( ) ( )( ) Btu/lbm 18.07=−=−=−= R 21612400Btu/lbm.R 0.07562323in TTcuuq v

qout

qin 2

3

1

4

v(b) Process 3-4: isentropic expansion.

( ) R 60081R 2400

0.6671

4

334 =

=

=

−k

TTv

v

Process 4-1: v = constant heat rejection. ( ) ( )( ) Btu/lbm 4.536R540600Btu/lbm.R 0.07561414out =−=−=−= TTcuuq v

74.9%=−=−=Btu/lbm 18.07Btu/lbm 4.536

11in

outth q

qη

(c) 77.5%=−=−=R 2400R 540

11Cth,H

L

TT

η


9-21

9-41 A gasoline engine operates on an Otto cycle. The compression and expansion processes are modeled as polytropic. The temperature at the end of expansion process, the net work output, the thermal efficiency, the mean effective pressure, the engine speed for a given net power, and the specific fuel consumption are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at 850 K are cp = 1.110 kJ/kg·K, cv = 0.823 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.349 (Table A-2b). Analysis (a) Process 1-2: polytropic compression

Qout

4

3

2

Qin

1

( )( )

( )( ) kPa 199510kPa 100

K 4.66410K 333

1.3

2

112

1-1.31

2

112

==

=

==

=

−

n

n

PP

TT

v

v

v

v

Process 2-3: constant volume heat addition

( ) K 2664kPa 1995kPa 8000K 664.4

2

323 =

=

=

PP

TT

( )

( )( ) kJ/kg 1646K664.42664KkJ/kg 0.823 2323in

=−⋅=−=−= TTcuuq v

Process 3-4: polytropic expansion.

( ) K 1335=

=

=

− 1-1.31

4

334 10

1K 2664n

TTv

v

( ) kPa 9.400101kPa 8000

1.3

1

234 =

=

=

n

PPv

v

Process 4-1: constant voume heat rejection. ( ) ( )( ) kJ/kg 8.824K3331335KkJ/kg 0.8231414out =−⋅=−=−= TTcuuq v

(b) The net work output and the thermal efficiency are kJ/kg 820.9=−=−= 8.8241646outinoutnet, qqw

0.499===kJ/kg 1646kJ/kg 820.9

in

outnet,th q

wη

(c) The mean effective pressure is determined as follows

( )( )

( )( )kPa 954.3=

⋅

−=

−=

−=

==

==⋅⋅

==

kJmkPa

1/101/kgm 0.9557kJ/kg 820.9

)/11(MEP

/kgm 0.9557kPa 100

K 333K/kgmkPa 0.287

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

rww

r

PRT

vvv

vvv

vv

(d) The clearance volume and the total volume of the engine at the beginning of compression process (state 1) are


9-22

33

m 0002444.0m 0022.0

10 =→+

=→+

= cc

c

c

dc VV

VV

VVr

31 m 002444.00022.00002444.0 =+=+= dc VVV

The total mass contained in the cylinder is

( )( )kg 0.002558

K 333K/kgmkPa 0.287)m 444kPa)/0.002 100(

3

3

1

11 =⋅⋅

==RT

VPmt

The engine speed for a net power output of 70 kW is

rev/min 4001=

⋅==

min 1s 60

cycle)kJ/kg kg)(820.9 002558.0(kJ/s 70rev/cycle) 2(2

net

net

wmW

nt

&&

Note that there are two revolutions in one cycle in four-stroke engines.

(e) The mass of fuel burned during one cycle is

kg 0001505.0kg) 002558.0(

16AF =→−

=→−

== ff

f

f

ft

f

a mm

mm

mmmm

Finally, the specific fuel consumption is

g/kWh 258.0=

==

kWh 1kJ 3600

kg 1g 1000

kJ/kg) kg)(820.9 002558.0(kg 0001505.0

sfcnetwm

m

t

f

Diesel Cycle 9-42C A diesel engine differs from the gasoline engine in the way combustion is initiated. In diesel engines combustion is initiated by compressing the air above the self-ignition temperature of the fuel whereas it is initiated by a spark plug in a gasoline engine. 9-43C The Diesel cycle differs from the Otto cycle in the heat addition process only; it takes place at constant volume in the Otto cycle, but at constant pressure in the Diesel cycle. 9-44C The gasoline engine. 9-45C Diesel engines operate at high compression ratios because the diesel engines do not have the engine knock problem. 9-46C Cutoff ratio is the ratio of the cylinder volumes after and before the combustion process. As the cutoff ratio decreases, the efficiency of the diesel cycle increases.


9-23

9-47 An air-standard Diesel cycle with a compression ratio of 16 and a cutoff ratio of 2 is considered. The temperature after the heat addition process, the thermal efficiency, and the mean effective pressure are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The gas constant of air is R = 0.287 kJ/kg.K. The properties of air are given in Table A-17.

P

qout

qin 3 2

1

4

v

Analysis (a) Process 1-2: isentropic compression.

2.621kJ/kg214.07

K3001

11 =

=→=

r

uT

v

( )kJ/kg 90.98K 862.4

825.382.6211611

2

2

1

2112 =

=→====

hT

r rrr vvv

vv

Process 2-3: P = constant heat addition.

( )( ) 546.4kJ/kg 1910.6

K 862.4223

322

2

33

2

22

3

33==

→====→=r

hTTT

TP

TP

vv

vvv K 1724.8

(b) kJ/kg019.719.8906.191023in =−=−= hhq

Process 3-4: isentropic expansion.

( ) kJ/kg659.737.36546.42

1622 4

2

4

3

43334

=→===== urrrrr vv

v

vv

v

vv

Process 4-1: v = constant heat rejection.

56.3%=−=−=

=−=−=

kJ/kg 1019.7kJ/kg 445.63

11

kJ/kg 445.6307.2147.659

in

outth

14out

qq

uuq

η

(c)

( )( )

( ) ( )( )kPa675.9

kJmkPa

1/161/kgm 0.906kJ/kg 574.07

/11MEP

/kgm 0.906kPa 95

K 300K/kgmkPa 0.287

kJ/kg 574.0763.4457.1019

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

outinoutnet,

=

⋅

−=

−=

−=

==

==⋅⋅

==

=−=−=

rww

r

PRT

qqw

vvv

vvv

vv


9-24

9-48 An air-standard Diesel cycle with a compression ratio of 16 and a cutoff ratio of 2 is considered. The temperature after the heat addition process, the thermal efficiency, and the mean effective pressure are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression. P

qout

qin 3 2

1

4

v

( )( ) K909.416K300 0.41

2

112 ==

=

−k

TTv

v


( )( ) K1818.8====→= K909.422 222

33

2

22

3

33 TTTT

PT

Pv

vvv

(b) ( ) ( )( ) kJ/kg 913.9K909.41818.8KkJ/kg 1.0052323in =−⋅=−=−= TTchhq p


( ) K791.7162K1818.8

2 0.41

4

23

1

4

334 =

=

=

=

−− kk

TTTv

v

v

v


( ) ( )( )

61.4%=−=−=

=−⋅=−=−=

kJ/kg 913.9kJ/kg 353

11

kJ/kg353K300791.7KkJ/kg0.718

in

outth

1414out

qq

TTcuuq

η

v

(c)

( )( )

( ) ( )( )kPa660.4

kJmkPa

1/161/kgm 0.906kJ/kg 560.9

/11MEP

/kgm 0.906kPa 95

K 300K/kgmkPa 0.287

kJ/kg560.93539.913

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

outinnet.out

=

⋅

−=

−=

−=

==

==⋅⋅

==

=−=−=

rww

r

PRT

qqw

vvv

vvv

vv


9-25

9-49E An air-standard Diesel cycle with a compression ratio of 18.2 is considered. The cutoff ratio, the heat rejection per unit mass, and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A-17E.

PAnalysis (a) Process 1-2: isentropic compression.

3000 R

qout

qin 32

1

4

v

T 32.144Btu/lbm 40.92

R 5401

11 =

=→=

r

uv

( )Btu/lbm 402.05R 1623.6

93.732.1442.18

11

2

2

1

2112 =

=→====

hT

r rrr vvv

vv


1.848===→=R 1623.6

R 3000

2

3

2

3

2

22

3

33

TT

TP

TP

v

vvv

(b)

Btu/lbm 388.6305.40268.790

180.1Btu/lbm 790.68

R 3000

23in

33

3

=−=−=

==

→=

hhq

hT

rv


( ) Btu/lbm 91.250621.11180.1848.1

2.18848.1848.1 4

2

4

3

43334

=→===== urrrrr vv

v

vv

v

vv


(c) 59.1%

Btu/lbm 158.87

=−=−=

=−=−=

Btu/lbm 388.63Btu/lbm 158.87

11

04.9291.250

in

outth

14out

qq

uuq

η


9-26

9-50E An air-standard Diesel cycle with a compression ratio of 18.2 is considered. The cutoff ratio, the heat rejection per unit mass, and the thermal efficiency are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 0.240 Btu/lbm.R, cv = 0.171 Btu/lbm.R, and k = 1.4 (Table A-2E). Analysis (a) Process 1-2: isentropic compression. P

qin 32

1

4

v

( )( ) R172418.2R540 0.41

2

112 ==

=

−k

TTv

v


1.741===→=R 1724R 3000

2

3

2

3

2

22

3

33

TT

TP

TP

v

vvv

(b) ( ) ( )( ) Btu/lbm 306R17243000Btu/lbm.R 0.2402323in =−=−=−= TTchhq p


( ) R 117318.21.741R 3000

741.1 0.41

4

23

1

4

334 =

=

=

=

−− kk

vTTT

v

v

v


(c)

( )( )( )

64.6%

Btu/lbm 108

=−=−=

=−=−=−=

Btu/lbm 306Btu/lbm 108

11

R0541173Btu/lbm.R 0.171

in

outth

1414out

qq

TTcuuq

η

v


9-27

9-51 An ideal diesel engine with air as the working fluid has a compression ratio of 20. The thermal efficiency and the mean effective pressure are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression.

P

qout

qin 32

1

4

v

( )( ) K 971.120K 293 0.41

2

112 ==

=

−k

TTV

V


2.265K971.1K2200

2

3

2

3

2

22

3

33 ===→=TT

TP

TP

V

VVV


( )

( ) ( )( )( ) ( )( )

63.5%===

=−=−=

=−⋅=−=−=

=−⋅=−=−=

=

=

=

=

=

−−−

kJ/kg 1235kJ/kg 784.4

kJ/kg 784.46.4501235

kJ/kg 450.6K293920.6KkJ/kg 0.718

kJ/kg 1235K971.12200KkJ/kg 1.005

K 920.620

2.265K 2200265.2265.2

in

outnet,th

outinoutnet,

1414out

2323in

0.41

3

1

4

23

1

4

334

qw

qqw

TTcuuq

TTchhq

rTTTT

p

kkk

η

v

V

V

V

V

(b) ( )( )

( ) ( )( )kPa933

kJmkPa

1/201/kgm 0.885kJ/kg 784.4

/11MEP

/kgm 0.885kPa 95

K 293K/kgmkPa 0.287

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

=

⋅

−=

−=

−=

==

==⋅⋅

==

rww

r

PRT

vvv

vvv

vv


9-28

9-52 A diesel engine with air as the working fluid has a compression ratio of 20. The thermal efficiency and the mean effective pressure are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression. P

Polytropic

qout

qin 32

1

4

v

( )( ) K 971.120K 293 0.41

2

112 ==

=

−k

TTV

V


2.265K 971.1K 2200

2

3

2

3

2

22

3

33 ===→=TT

TP

TP

V

VVV

Process 3-4: polytropic expansion.

( )

( ) ( )( )( ) ( )( ) kJ/kg 526.3K 2931026KkJ/kg 0.718

kJ/kg 1235K 971.12200KkJ/kg 1.005

K 102620

2.265K 2200r

2.2652.265

1414out

2323in

0.351n

3

1n

4

23

1

4

334

=−⋅=−=−=

=−⋅=−=−=

=

=

=

=

=

−−−

TTcuuq

TTchhq

TTTT

p

n

v

V

V

V

V

Note that qout in this case does not represent the entire heat rejected since some heat is also rejected during the polytropic process, which is determined from an energy balance on process 3-4:

( ) ( )( )

( )( )(

kJ/kg 120.1K 22001026KkJ/kg 0.718kJ/kg 963

kJ/kg 9631.351

K 22001026KkJ/kg 0.2871

34out34,in34,34out34,in34,

systemoutin

34out34,

=−⋅+=

−+=→−=−

∆=−

=−

−⋅=

−−

=

TTcwquuwq

EEEnTTR

w

v

)

which means that 120.1 kJ/kg of heat is transferred to the combustion gases during the expansion process. This is unrealistic since the gas is at a much higher temperature than the surroundings, and a hot gas loses heat during polytropic expansion. The cause of this unrealistic result is the constant specific heat assumption. If we were to use u data from the air table, we would obtain ( ) kJ/kg 1.128)4.18723.781(96334out34,in34, −=−+=−+= uuwq

which is a heat loss as expected. Then qout becomes kJ/kg 654.43.5261.128out41,out34,out =+=+= qqq

and

47.0%===

=−=−=

kJ/kg 1235kJ/kg 580.6

kJ/kg 580.64.6541235

in

outnet,th

outinoutnet,

qw

qqw

η

(b) ( )( )

( ) ( )( )kPa 691=

⋅

−=

−=

−=

==

==⋅⋅

==

kJmkPa 1

1/201/kgm 0.885kJ/kg 580.6

/11MEP

/kgm 0.885kPa 95

K 293K/kgmkPa 0.287

3

31

outnet,

21

outnet,

max2min

max3

3

1

11

rww

r

PRT

vvv

vvv

vv


9-29

9-53 EES Problem 9-52 is reconsidered. The effect of the compression ratio on the net work output, mean effective pressure, and thermal efficiency is to be investigated. Also, T-s and P-v diagrams for the cycle are to be plotted. Analysis Using EES, the problem is solved as follows: Procedure QTotal(q_12,q_23,q_34,q_41: q_in_total,q_out_total) q_in_total = 0 q_out_total = 0 IF (q_12 > 0) THEN q_in_total = q_12 ELSE q_out_total = - q_12 If q_23 > 0 then q_in_total = q_in_total + q_23 else q_out_total = q_out_total - q_23 If q_34 > 0 then q_in_total = q_in_total + q_34 else q_out_total = q_out_total - q_34 If q_41 > 0 then q_in_total = q_in_total + q_41 else q_out_total = q_out_total - q_41 END "Input Data" T[1]=293 [K] P[1]=95 [kPa] T[3] = 2200 [K] n=1.35 {r_comp = 20} "Process 1-2 is isentropic compression" s[1]=entropy(air,T=T[1],P=P[1]) s[2]=s[1] T[2]=temperature(air, s=s[2], P=P[2]) P[2]*v[2]/T[2]=P[1]*v[1]/T[1] P[1]*v[1]=R*T[1] R=0.287 [kJ/kg-K] V[2] = V[1]/ r_comp "Conservation of energy for process 1 to 2" q_12 - w_12 = DELTAu_12 q_12 =0"isentropic process" DELTAu_12=intenergy(air,T=T[2])-intenergy(air,T=T[1]) "Process 2-3 is constant pressure heat addition" P[3]=P[2] s[3]=entropy(air, T=T[3], P=P[3]) P[3]*v[3]=R*T[3] "Conservation of energy for process 2 to 3" q_23 - w_23 = DELTAu_23 w_23 =P[2]*(V[3] - V[2])"constant pressure process" DELTAu_23=intenergy(air,T=T[3])-intenergy(air,T=T[2]) "Process 3-4 is polytropic expansion" P[3]/P[4] =(V[4]/V[3])^n s[4]=entropy(air,T=T[4],P=P[4]) P[4]*v[4]=R*T[4] "Conservation of energy for process 3 to 4" q_34 - w_34 = DELTAu_34 "q_34 is not 0 for the ploytropic process" DELTAu_34=intenergy(air,T=T[4])-intenergy(air,T=T[3]) P[3]*V[3]^n = Const w_34=(P[4]*V[4]-P[3]*V[3])/(1-n) "Process 4-1 is constant volume heat rejection" V[4] = V[1] "Conservation of energy for process 4 to 1" q_41 - w_41 = DELTAu_41 w_41 =0 "constant volume process" DELTAu_41=intenergy(air,T=T[1])-intenergy(air,T=T[4])


9-30

Call QTotal(q_12,q_23,q_34,q_41: q_in_total,q_out_total) w_net = w_12+w_23+w_34+w_41 Eta_th=w_net/q_in_total*100 "Thermal efficiency, in percent" "The mean effective pressure is:" MEP = w_net/(V[1]-V[2])

rcomp ηth MEP [kPa] wnet [kJ/kg] 14 47.69 970.8 797.9 16 50.14 985 817.4 18 52.16 992.6 829.8 20 53.85 995.4 837.0 22 55.29 994.9 840.6 24 56.54 992 841.5

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5200400600800

10001200140016001800200022002400

s [kJ/kg-K]

T[K]

95 kP

a

340

.1 kP

a

592

0 kPa

0.044

0.1 0

.88 m3/kg

Air

2

1

3

4

10-2 10-1 100 101 102101

102

103

104

101

102

103

104

v [m3/kg]

P[kPa] 293 K

1049 K

2200 K

5.69

6.74 kJ/kg-K

Air


9-31

14 16 18 20 22 24790

800

810

820

830

840

850

rcomp

wnet

[kJ/kg]

14 16 18 20 22 2447

49

51

53

55

57

rcomp

ηth

14 16 18 20 22 24970

975

980

985

990

995

1000

rcomp

MEP[kPa]


9-32

9-54 A four-cylinder ideal diesel engine with air as the working fluid has a compression ratio of 17 and a cutoff ratio of 2.2. The power the engine will deliver at 1500 rpm is to be determined. Assumptions 1 The cold air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.4 (Table A-2). Analysis Process 1-2: isentropic compression.

P

Qout

Qin 32

1

4

v

( )( ) K 101917K 328 0.41

2

112 ==

=

−k

TTV

V


( )( ) K 2241K 10192.22.2 222

33

2

22

3

33 ====→= TTTT

PT

Pv

vvv


( )

( ) ( )

( ) ( )( )( )( )

( )( ) kW 46.6===

=−=−=

=−⋅×=−=−=

=−⋅×=

−=−=

×=⋅⋅

==

=

=

=

=

=

−

−

−

−−−

kJ/rev 1.864rev/s 1500/60

kJ/rev 864.1174.1038.3

kJ 174.1K328989.2KkJ/kg 0.718kg10473.2

kJ 3.038K)10192241)(KkJ/kg 1.005)(kg10.4732(

kg10473.2)K 328)(K/kgmkPa 0.287(

)m 0.0024)(kPa 97(

K 989.217

2.2K 22412.22.2

outnet,outnet,

outinoutnet,

31414out

32323in

33

3

1

11

4.01

3

1

4

23

1

4

334

WnW

QQW

TTmcuumQ

TTmchhmQ

RTP

m

rTTTT

v

p

kkk

&&

V

V

V

V

V

Discussion Note that for 2-stroke engines, 1 thermodynamic cycle is equivalent to 1 mechanical cycle (and thus revolutions).


9-33

9-55 A four-cylinder ideal diesel engine with nitrogen as the working fluid has a compression ratio of 17 and a cutoff ratio of 2.2. The power the engine will deliver at 1500 rpm is to be determined. Assumptions 1 The air-standard assumptions are applicable with nitrogen as the working fluid. 2 Kinetic and potential energy changes are negligible. 3 Nitrogen is an ideal gas with constant specific heats. Properties The properties of nitrogen at room temperature are cp = 1.039 kJ/kg·K, cv = 0.743 kJ/kg·K, R = 0.2968 kJ/kg·K, and k = 1.4 (Table A-2).

PAnalysis Process 1-2: isentropic compression.

Qout

Qin 32

1

4

v

( )( ) K 101917K 328 0.41

2

112 ==

=

−k

TTV

V


( )( ) K 2241K 10192.22.2 222

33

2

22

3

33 ====→= TTTT

PT

Pv

vvv


( )

( )( )( )( )

( ) ( )( )( )( )( ) ( )

( )( )( )

( )( ) kW 46.6===

=−=−=

=−⋅×=−=−=

=−⋅×=

−=−=

×=⋅⋅

==

=

=

=

=

=

−

−

−

−−−

kJ/rev 1.863rev/s 1500/60

kJ/rev .8631175.1037.3

kJ 1.175K328989.2KkJ/kg 0.743kg102.391

kJ 3.037K10192241KkJ/kg 1.039kg102.391

kg10391.2K 328K/kgmkPa 0.2968

m 0.0024kPa 97

K 989.2172.2K 22412.22.2

outnet,outnet,

outinoutnet,

31414out

32323in

33

3

1

11

0.41

3

1

4

23

1

4

334

WnW

QQW

TTmcuumQ

TTmchhmQ

RTP

m

rTTTT

p

kkk

&&

v

V

V

V

V

V

Discussion Note that for 2-stroke engines, 1 thermodynamic cycle is equivalent to 1 mechanical cycle (and thus revolutions).


9-34

9-56 [Also solved by EES on enclosed CD] An ideal dual cycle with air as the working fluid has a compression ratio of 14. The fraction of heat transferred at constant volume and the thermal efficiency of the cycle are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A-17. P

x

Qout

1520.4 kJ/kg

3

2

1

4

v

Analysis (a) Process 1-2: isentropic compression.

621.2kJ/kg 214.07

K 3001

11 =

=→=

r

uT

v

( )kJ/kg 611.2K 823.1

37.442.6211411

2

2

1

2112 =

=→====

uT

r rrr vvv

vv

Process 2-x, x-3: heat addition,

( ) (

( ) ( )xx

xx

r

huhhuuqqq

hT

−+−=

−+−=+=

==

→=

−−

2.25032.6114.1520

012.2kJ/kg 2503.2

K 2200

32inx,3in2,xin

33

3v

)

By trial and error, we get Tx = 1300 K and hx = 1395.97, ux = 1022.82 kJ /kg. Thus,

and

27.1%===

=−=−=

−

−

kJ/kg 1520.4kJ/kg 411.62

ratio

kJ/kg 411.622.61182.1022

in

in,2

2in,2

qq

uuq

x

xx

(b) cxxx

xx rTT

TP

TP

====→= 1.692K 1300K 220033

3

33

v

vvv

( ) kJ/kg 886.3648.16012.2692.114

692.1692.1 42

4

3

43334

=→===== urrrrr vv

v

vv

v

vv


55.8%=−=−=

=−=−=

kJ/kg 1520.4kJ/kg 672.23

11

kJ/kg 72.23607.2143.886

in

outth

14out

qq

uuq

η


9-35

9-57 EES Problem 9-56 is reconsidered. The effect of the compression ratio on the net work output and thermal efficiency is to be investigated. Also, T-s and P-v diagrams for the cycle are to be plotted. Analysis Using EES, the problem is solved as follows: "Input Data" T[1]=300 [K] P[1]=100 [kPa] T[4]=2200 [K] q_in_total=1520 [kJ/kg] r_v = 14 v[1]/v[2]=r_v "Compression ratio" "Process 1-2 is isentropic compression" s[1]=entropy(air,T=T[1],P=P[1]) s[2]=s[1] s[2]=entropy(air, T=T[2], v=v[2]) P[2]*v[2]/T[2]=P[1]*v[1]/T[1] P[1]*v[1]=R*T[1] R=0.287 [kJ/kg-K] "Conservation of energy for process 1 to 2" q_12 -w_12 = DELTAu_12 q_12 =0 [kJ/kg]"isentropic process" DELTAu_12=intenergy(air,T=T[2])-intenergy(air,T=T[1]) "Process 2-3 is constant volume heat addition" s[3]=entropy(air, T=T[3], P=P[3]) {P[3]*v[3]/T[3]=P[2]*v[2]/T[2]} P[3]*v[3]=R*T[3] v[3]=v[2] "Conservation of energy for process 2 to 3" q_23 -w_23 = DELTAu_23 w_23 =0"constant volume process" DELTAu_23=intenergy(air,T=T[3])-intenergy(air,T=T[2]) "Process 3-4 is constant pressure heat addition" s[4]=entropy(air, T=T[4], P=P[4]) {P[4]*v[4]/T[4]=P[3]*v[3]/T[3]} P[4]*v[4]=R*T[4] P[4]=P[3] "Conservation of energy for process 3 to4" q_34 -w_34 = DELTAu_34 w_34 =P[3]*(v[4]-v[3]) "constant pressure process" DELTAu_34=intenergy(air,T=T[4])-intenergy(air,T=T[3]) q_in_total=q_23+q_34 "Process 4-5 is isentropic expansion" s[5]=entropy(air,T=T[5],P=P[5]) s[5]=s[4] P[5]*v[5]/T[5]=P[4]*v[4]/T[4] {P[5]*v[5]=0.287*T[5]} "Conservation of energy for process 4 to 5" q_45 -w_45 = DELTAu_45 q_45 =0 [kJ/kg] "isentropic process" DELTAu_45=intenergy(air,T=T[5])-intenergy(air,T=T[4]) "Process 5-1 is constant volume heat rejection" v[5]=v[1] "Conservation of energy for process 2 to 3" q_51 -w_51 = DELTAu_51


9-36

w_51 =0 [kJ/kg] "constant volume process" DELTAu_51=intenergy(air,T=T[1])-intenergy(air,T=T[5]) w_net = w_12+w_23+w_34+w_45+w_51 Eta_th=w_net/q_in_total*Convert(, %) "Thermal efficiency, in percent"

rv ηth [%] wnet [kJ/kg] 10 52.33 795.4 11 53.43 812.1 12 54.34 826 13 55.09 837.4 14 55.72 846.9 15 56.22 854.6 16 56.63 860.7 17 56.94 865.5 18 57.17 869

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.50

500

1000

1500

2000

2500

3000

3500

s [kJ/kg-K]

T[K]

100 kPa

382.7 kPa

3842 kPa

6025 kPa

T-s Diagram for Air Dual Cycle

1

2

3

4

5

v=const

p=const

1

2

3 4

5

s=const

10-2 10-1 100 101 102101

102

103

8x103

v [m3/kg]

P[kPa]

300 K

2200 K

P-v Diagram for Air Dual Cycle


9-37

10 11 12 13 14 15 16 17 1852

53

54

55

56

57

58

rv

ηth[%]

10 11 12 13 14 15 16 17 18790

800

810

820

830

840

850

860

870

rv

wnet

[kJ/kg]

9-58 An ideal dual cycle with air as the working fluid has a compression ratio of 14. The fraction of heat transferred at constant volume and the thermal efficiency of the cycle are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K, and k = 1.4 (Table A-2). Analysis (a) Process 1-2: isentropic compression.

( )( ) K 86214K 300 0.41

2

112 ==

=

−k

TTv

v

Process 2-x, x-3: heat addition,

( ) ( )

( ) ( )( )( ) ( )( xx

xpx

xxxx

TT

TTcTTchhuuqqq

−⋅+−⋅=

−+−=

)

−+−=+= −−

2200KkJ/kg 1.005862KkJ/kg 0.718kJ/kg 1520.432

32in,3in,2in

v

P

4

1

2

3

1520.4 kJ/kg

x

Qout

v

Solving for Tx we get Tx = 250 K which is impossible. Therefore, constant specific heats at room temperature turned out to be an unreasonable assumption in this case because of the very high temperatures involved.


9-38

9-59 A six-cylinder compression ignition engine operates on the ideal Diesel cycle. The maximum temperature in the cycle, the cutoff ratio, the net work output per cycle, the thermal efficiency, the mean effective pressure, the net power output, and the specific fuel consumption are to be determined. Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The properties of air at 850 K are cp = 1.110 kJ/kg·K, cv = 0.823 kJ/kg·K, R = 0.287 kJ/kg·K, and k = 1.349 (Table A-2b). Analysis (a) Process 1-2: Isentropic compression

Qout

4

32

Qin

1

( )( )

( )( ) kPa 434117kPa 95

K 7.88117K 328

1.349

2

112

1-1.3491

2

112

==

=

==

=

−

k

k

PP

TT

v

v

v

v

The clearance volume and the total volume of the engine at the beginning of compression process (state 1) are

3

3

m 0002813.0

m 0045.017

=

+=→

+=

c

c

c

c

dcr

V

V

V

V

VV

31 m 004781.00045.00002813.0 =+=+= dc VVV

The total mass contained in the cylinder is

( )( )kg 0.004825

K 328K/kgmkPa 0.287)m 781kPa)(0.004 95(

3

3

1

11 =⋅⋅

==RTP

mV

The mass of fuel burned during one cycle is

kg 000193.0kg) 004825.0(

24 =→−

=→−

== ff

f

f

f

f

a mm

mm

mmmm

AF

Process 2-3: constant pressure heat addition kJ 039.88)kJ/kg)(0.9 kg)(42,500 000193.0(HVin === cf qmQ η

K 2383=→−=→−= 3323in K)7.881(kJ/kg.K) kg)(0.823 004825.0(kJ 039.8)( TTTTmcQ v

The cutoff ratio is

2.7===K 881.7K 2383

2

3

TT

β

(b) 33

12 m 0002813.0

17m 0.004781

===r

VV

23

14

3323 m 00076.0)m 0002813.0)(70.2(

PP ==

===VV

VV β


9-39


( )

( ) kPa 2.363m 0.004781m 0.00076kPa 4341

K 1254m 0.004781m 0.00076K 2383

1.349

3

3

4

334

1-1.349

3

31

4

334

=

=

=

=

=

=

−

k

k

PP

TT

V

V

V

V

Process 4-1: constant voume heat rejection. ( ) ( )( ) kJ 677.3K3281254KkJ/kg 0.823kg) 004825.0(14out =−⋅=−= TTmcQ v

The net work output and the thermal efficiency are kJ 4.361=−=−= 677.3039.8outinoutnet, QQW

0.543===kJ 8.039kJ 4.361

in

outnet,th Q

Wη

(c) The mean effective pressure is determined to be

kPa 969.2=

⋅

−=

−=

kJmkPa

m)0002813.0004781.0(kJ 4.361

MEP3

321

outnet,

VV

W

(d) The power for engine speed of 2000 rpm is

kW 72.7=

==

s 60min 1

rev/cycle) 2((rev/min) 2000kJ/cycle) (4.361

2netnetnWW&&

Note that there are two revolutions in one cycle in four-stroke engines. (e) Finally, the specific fuel consumption is

g/kWh 159.3=

==

kWh 1kJ 3600

kg 1g 1000

kJ/kg 4.361kg 000193.0sfc

netWmf


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