Gas Power Cycles - PartI _ March 2012

14/3/2012

1

Chapter 9

GAS POWER CYCLES

Mehmet KanogluUniversity of Gaziantep

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Thermodynamics: An Engineering Approach Seventh Edition in SI Units

Yunus A. Cengel, Michael A. Boles

McGraw-Hill, 2011

2

Objectives• Evaluate the performance of gas power cycles for which the

working fluid remains a gas throughout the entire cycle.

• Develop simplifying assumptions applicable to gas power

cycles.

• Review the operation of reciprocating engines.

• Analyze both closed and open gas power cycles.

• Solve problems based on the Otto, Diesel, Stirling, and

Ericsson cycles.

• Solve problems based on the Brayton cycle; the Brayton cycle

with regeneration; and the Brayton cycle with intercooling,

reheating, and regeneration.

• Analyze jet-propulsion cycles.

• Identify simplifying assumptions for second-law analysis of

gas power cycles.

• Perform second-law analysis of gas power cycles.

14/3/2012

2

AN OVERVIEW OF RECIPROCATING ENGINES

Basic Components of IC Engines

�Cylinder, piston, inlet valve and

exhaust valve.

�Piston moves from the top dead

center (TDC) to the bottom dead

center (BDC).

�Clearance volume, Vc is a spacing

between the top of the piston and

the valve’s heads when the piston is

at the end of the delivery stroke.

�Swept volume or displacement

volume, Vs is the volume between

TDC and BDC.

piston

bore

Stroke (Vswept)

TDC

BDC

Inlet valve (air)

Exhaust valve

(gas)

Vc

Engine Classification

Reciprocating internal combustion (IC) engines are classified into

two general categories, depending on how the combustion process

in the cylinder is initiated, i.e.:

a) Spark-ignition (SI) engines;

b) Compression-ignition (CI) engines.


14/3/2012

3

Description of SI Engines

�Run on liquid fuel such as gasoline or petrol, which is mixed

with air.

�The air-fuel mixture enters the cylinder and is compressed to a

highest pressure and temperature. A spark from a spark-plug

ignites the combustible air-fuel mixture.

� It burns and combustion gases are produced.

�The high pressure of the gases pushes the piston downwards,

producing a power stroke of the piston.

�The crankshaft transforms the reciprocating motion into

rotational motion (rpm), which is carried by gears and drive

shaft systems to the wheels, causing the vehicle to move.


Description of CI Engines

�Run on diesel liquid fuel.

�The fresh atmospheric air enters the cylinder in which it is

compressed to about 1/22 of its original volume, causing its

temperature to raise to about 540ºC or higher.

�Diesel fuel is then injected into the compressed air.

�The heat of compression of the air causes the diesel to burn.

�Thus producing high temperature combustion gases.

�The combustion gases pushes the piston downward during

the power stroke of the piston.

�As in the SI engines, the reciprocating motion is transformed

into rotational motion.


14/3/2012

4

IN BOTH ENGINES, THE COMBUSTION GASES ARE

EVENTUALLY EXHAUSTED OUT OF THE CYLINDER SO

THAT FRESH-AIR MIXTURE CAN BE INDUCED INTO THE

CYLINDER TO CONTINUE THE THERMODYNAMICS

CYCLES – therefore working on an open cycle is the

characteristics of all internal combustion engines since the

working fluid does not undergo a complete thermodynamic

cycle.


Classification by Cycles

Reciprocating internal

combustion engines operate

either on two-stroke or four-

stroke cycle.

Four-stroke Cycle

�Most automotive engines

operate on a 4-stroke cycle.

�Every fourth piston stroke is

the power stroke.

�The crankshaft makes two

revolutions to complete the

cycle.


Four-stroke Cycle

The sequence of events in

this cycle is as follows:

� Intake stroke: The intake

valve opened. The piston

moving downward, allowing

the air fuel mixture to enter

the cylinder.

�Compression stroke: The

intake valve closed. The

piston is moving upward,

compressing the mixture.

14/3/2012

5


Classification by Cycles

Four-stroke Cycle

�Power stroke: The ignition

system delivers a spark to

the spark plug to ignite the

compressed mixture. As the

mixture burns, it creates

high pressure that pushes

the piston down.

�Exhaust stroke: The

exhaust valve opened. The

piston moves upward as the

burned gases escape from

the cylinder.

�The ignition occurs before

the compression process

end.

� Psys > Patm during the

exhaust stroke.

� Psys < Patm during the intake

stroke.

Classification by Cycles – 4-stroke cycle


14/3/2012

6

Classification by Cycles – 4-stroke cycle

The cylinders are

arranged in a line in

a single bank

The cylinders are

arranged in 2

banks set at an

angle to one

another The cylinders are arranged in

2 banks on opposite sides of

the engine

Performance Criteria of Reciprocating Engines -

Compression Ratio, rv

Compression ratio =

ie.

Note: compression ratio is volume

ratio and it is not a pressure ratio.

piston

bore

stroke

TDC

BDC

Inlet valve (air)

Exhaust valve

(gas)

Vc

(Vs)

c

scv

V

VVr

+=

volumeMinimum

volumeMaximum

14/3/2012

7

�Mean effective pressure (MEP) is a

conceptual/fictitious pressure.

� If it acted on the piston during the

entire power stroke, would produce

the same amount of net work as that

produced during the actual cycle.

� For the same engines size, MEP

can be used as a criteria or

parameter to compare the engines

performance.

� The engine with a larger value of

MEP delivers more net work per

cycle and thus performs better.

Performance Criteria of Reciprocating Engines - Mean

Effective Pressure, MEP

Mean effective

pressure

14

BASIC CONSIDERATIONS IN THE ANALYSIS OF

POWER CYCLES

Modeling is a

powerful

engineering tool

that provides great

insight and

simplicity at the

expense of some

loss in accuracy.

Most power-producing devices operate on cycles.

Ideal cycle: A cycle that resembles the actual cycle

closely but is made up totally of internally reversible

processes.

Reversible cycles such as Carnot cycle have the

highest thermal efficiency of all heat engines operating

between the same temperature levels. Unlike ideal

cycles, they are totally reversible, and unsuitable as a

realistic model.

Thermal efficiency of

heat engines:

The analysis of many complex

processes can be reduced to a

manageable level by utilizing

some idealizations.

14/3/2012

8

15

The ideal cycles are internally reversible, but, unlike the Carnot cycle, they

are not necessarily externally reversible. Therefore, the thermal efficiency

of an ideal cycle, in general, is less than that of a totally reversible cycle

operating between the same temperature limits. However, it is still

considerably higher than the thermal efficiency of an actual cycle because

of the idealizations utilized.

16

The idealizations and simplifications in the

analysis of power cycles:

1. The cycle does not involve any friction.

Therefore, the working fluid does not

experience any pressure drop as it flows in

pipes or devices such as heat exchangers.

2. All expansion and compression processes

take place in a quasi-equilibrium manner.

3. The pipes connecting the various

components of a system are well insulated,

and heat transfer through them is negligible.

Care should be exercised in

the interpretation of the

results from ideal cycles.

On both P-v and T-s diagrams, the area enclosed by the

process curve represents the net work of the cycle.

On a T-s diagram, the ratio of the area

enclosed by the cyclic curve to the

area under the heat-addition process

curve represents the thermal efficiency

of the cycle. Any modification that

increases the ratio of these two areas

will also increase the thermal efficiency

of the cycle.

14/3/2012

9

17

THE CARNOT CYCLE AND ITS

VALUE IN ENGINEERING

P-v and T-s diagrams of a

Carnot cycle.

The Carnot cycle is composed of four totally reversible

processes: isothermal heat addition, isentropic expansion,

isothermal heat rejection, and isentropic compression.

For both ideal and actual cycles: Thermal efficiency

increases with an increase in the average temperature at

which heat is supplied to the system or with a decrease

in the average temperature at which heat is rejected from

the system.

A steady-flow Carnot engine.

18

AIR-STANDARD ASSUMPTIONS

The combustion process is replaced by a

heat-addition process in ideal cycles.

Air-standard assumptions:

1. The working fluid is air, which continuously

circulates in a closed loop and always

behaves as an ideal gas.

2. All the processes that make up the cycle

are internally reversible.

3. The combustion process is replaced by a

heat-addition process from an external

source.

4. The exhaust process is replaced by a

heat-rejection process that restores the

working fluid to its initial state.

Cold-air-standard assumptions: When the working fluid is considered to be

air with constant specific heats at room temperature (25°C).

Air-standard cycle: A cycle for which the air-standard assumptions are

applicable.

14/3/2012

10

Problem 9-21 (page 536)

An ideal gas is contained in a piston-cylinder device and

undergoes a power cycle as follows:

1-2 isentropic compression from an initial temperature

T1 = 20°C with a compression ratio rv

= 5

2-3 constant pressure addition

3-1 constant volume heat rejection

The gas has constant specific heats with cv

= 0.7 kJ/kg.K and

R = 0.3 kJ/kg.K.

a) Sketch the P-v and T-s diagrams for the cycle.

b) Determine the heat and work interactions for each process,

in kJ/kg.

c) Determine the cycle thermal efficiency.

19

20

OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-

IGNITION ENGINES

Actual and ideal cycles in spark-ignition engines and their P-v diagrams.

14/3/2012

11

• The ideal cycle, which closely

resembles the actual operating

conditions of spark-ignition (SI),

or petrol engine, or gas engine,

or high-speed oil engines.

• It consists of four internally

reversible processes:

1-2 Isentropic compression

2-3 Constant-volume heat

addition

3-4 Isentropic expansion

4-1 Constant-volume heat

rejection

= Vmin = Vmax

OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-

IGNITION ENGINES

The Air Standard Cycles

v

p

c

ck =

The Otto Cycle Analysis

= Vmin = Vmax

Compression / expansion

index under the cold aircold air--

standard assumptionsstandard assumptions

14/3/2012

12


V

V ie.,

volumeClearance

meSwept volu volumeClearance

volumeMinimum

volumeMaximum ration Compressio

2

1

2

1 ==

+=

=

v

vrv


= Vmin = Vmax


Ideal Gas Equations

( )1

2

1

1

2

1

1

2

1

2

1

1

2

2

1

)5

)4

)3

−

−

=

=

=

k

k

k

k

v

v

T

T

p

p

T

T

p

p

v

v

2

22

1

11 )2

)1

T

vp

T

vp

RTpv

=

=

Under the coldcold--air standard assumptionsair standard assumptions, the relations

between ‘initial’ and ‘final’ states of isentropic expansion

process or isentropic compression process can be related

by the following equations.

14/3/2012

13


( )( )23

14

23

41

23

4123

supply

1

1

TTcm

TTcm

Q

Q

Q

QQ

Q

W

v

v

netotto

−−

−=

−=

−==η

( )( )23

141TT

TTotto −

−−=∴ η


= Vmin = Vmax

Thermal efficiency of the ideal Otto

cycle under the cold airthe cold air--standard standard

assumptions assumptions


1

3

4

4

3

−

=

k

v

v

T

T

1

2

1

1

2

−

=

k

v

v

T

T


= Vmin = Vmax

Since processes 1-2 and 3-4 are both isentropic (under the cold airthe cold air--

standard assumptionsstandard assumptions), then,

and

14/3/2012

14


1

2

1

2

1

4

3

T

T

v

v

T

Tk

=

=

−

2314 and vvvv ==


= Vmin = Vmax

but


( )( ) 3

4

23

14

4

14

3

23

4

1

3

2

4

1

3

2

1

2

4

3

or

or

11or

rearrange ie.,

T

T

TT

TT

T

TT

T

TT

T

T

T

T

T

T

T

T

T

T

T

T

=−−

−=

−

−=−

==


( )( )

( )

11 ie.,

1111

1otto

1

2

13

4

23

14

−

−

−=

−=−=

−−

−=∴

k

v

kotto

r

v

vT

T

TT

TT

η

η

The above equation is only valid for the ideal Otto

cycle under the cold airthe cold air--standard assumptionsstandard assumptions.

The above equation shows that under the cold airthe cold air--

standard assumptionsstandard assumptions, the thermal efficiency of an

ideal Otto cycle depends on the compression ratio

of the engine and the specific heat ratio of the

working fluid.

14/3/2012

15

}


kr

TT

TT

Q

W

k

v

otto

otto

in

netotto

index and ration compressio offunction 1

1

res temperatuoffunction 1

basic

1

23

14

L

L

L

−−=

−−

−=

=

η

η

η


Summary

These two

equations are

only valid for the

ideal Otto cycle

under the cold the cold

airair--standard standard

assumptionsassumptions.

30

The thermal efficiency of the

Otto cycle increases with the

specific heat ratio k of the

working fluid.

Thermal efficiency of the ideal

Otto cycle as a function of

compression ratio (k = 1.4).

In SI engines, the

compression ratio is

limited by auto-

ignition or engine

knock.

14/3/2012

16

Example 9-1

An ideal Otto cycle has a compression ratio of 8. At the beginning

of the compression process, air is at 95 kPa and 27°C, and 750

kJ/kg of heat is transferred to air during the constant-volume

heat-addition process. Under the cold air-standard assumptions,

determine,

a) the pressure and temperature at the end of the heat

addition;

b) the net work output;

c) the thermal efficiency;

d) the mean effective pressure for the cycle.

31

Prob. 9-37 (page 537)

An ideal Otto cycle with air as the working fluid has a

compression ratio of 8. The minimum and maximum

temperature in the cycle are 300 and 1340 K. Accounting for

the variation of specific heats with temperature, determine,

a) the amount of heat transferred to the air during the

heat-addition process,

b) the thermal efficiency, and

c) the thermal efficiency of a Carnot cycle operating

between the same temperature limits.

14/3/2012

17

Prob. 9-35 (page 537)

The compression ratio of an air-standard Otto cycle is 9.5. Prior

to the isentropic compression process, the air is at 100 kPa,

35°C, and 600 cm3. The temperature at the end of the isentropic

expansion process is 800 K. Using specific heat values at room

temperature, determine,

a) the highest temperature (K) and pressure (kPa) in the

cycle,

b) the heat transferred in, (kJ),

c) the thermal efficiency,

d) the mean effective pressure (kPa).

Prob. 9-36 (page 537)

Repeat Prob. 9-35, but replace the isentropic compression

process by a polytropic expansion process with the polytropic

exponent n = 1.35.

14/3/2012

18

35

DIESEL CYCLE: THE IDEAL CYCLE FOR

COMPRESSION-IGNITION ENGINES

In diesel engines, the spark plug is replaced by a

fuel injector, and only air is compressed during

the compression process.

In diesel engines, only air is compressed during the

compression stroke, eliminating the possibility of

autoignition (engine knock). Therefore, diesel engines can

be designed to operate at much higher compression ratios

than SI engines, typically between 12 and 24.

1-2 isentropic

compression

2-3 constant-

volume heat

addition

3-4 isentropic

expansion

4-1 constant-

volume heat

rejection.

v

p

c

ck =

Compression / expansion

index under the cold aircold air--

standard assumptionsstandard assumptions

p

vv1v2

p3 = p2

1

4

32

pvγ = const



14/3/2012

19

( )1441 . TTcmQ v −=

( )2323 . TTcmQ p −=

Under Under the cold airthe cold air--standard standard

assumptionsassumptions

Heat added to the engine

p

vv1v2

p3 = p2

1

4

32

pvγ = const

Heat rejected from the engine



( )( )

( )( )

( )( )

res temperatuoffunction 1

ie.,

1.

.11

or

basic or

23

14

23

14

23

14

23

41

23

4123

TTk

TT

TTk

TT

TTcm

TTcm

Q

Q

Q

QQ

Q

W

Diesel

p

vDiesel

Diesel

in

netDiesel

−−

−=

−−

−=−/

−/−=−=

−==

η

η

ηη L

Thermal efficiency under the cold airthe cold air--standard assumptionsstandard assumptions



14/3/2012

20

c

c

k

v

k

k

rTT

rv

v

T

T

r

TT

v

v

TT

v

v

T

T

23

2

3

2

3

1

211

2

1

21

1

2

1

1

2

ie

3 2 process isobaricfor ratio off-cut

=

→==

=→

=→

= −−

−

L

Thermal efficiency in terms of compression ratio rv and cut-off ratio, rc– under the cold aircold air--standard assumptionsstandard assumptions

p

vv1v2

p3 = p2

1

4

32

pvγ = const



[ ]( )[ ]11

1

ie

Also,

1

12

1

2

1

34

11

4

2

2

3

1

4

3

3

4

−

−−=∴

=

=

=

=

=

=

−

−

−−

−−−

c

k

v

k

cDiesel

k

v

k

c

k

v

cc

k

v

c

k

v

c

kk

rkr

r

r

rT

r

rrT

r

rTT

r

r

v

vx

v

v

v

v

T

T

η

Thermal efficiency in terms of compression ratio rv and cut-off ratio, rc- under the cold aircold air--standard assumptionsstandard assumptions

p

vv1v2

p3 = p2

1

4

32

pvγ = const



14/3/2012

21

41

Thermal efficiency

of the ideal Diesel

cycle as a function

of compression

and cutoff ratios

(k=1.4).

Cutoff

ratio

for the same compression ratio

Prob. 9-52 (page 538)

An ideal diesel engine has a compression ratio of 20 and uses

air as the working fluid. The state of air at the beginning of

the compression process is 95 kPa and 20°C. If the maximum

temperature in the cycle is not to exceed 2200 K, determine:

a) the thermal efficiency, and

b) the mean effective pressure.

Assume constant specific heats for air at room temperature.

14/3/2012

22

Prob. 9-50 (page 538)

An air-standard Diesel cycle has a compression ratio of 18.2.

Air is at 27oC and 100 kPa at the beginning of the compression

process and at 1700 K at the end of the heat addition process.

Accounting for the variation of specific heats with

temperature, determine,

a) the cutoff ratio,

b) the heat rejection per unit mass,

c) the thermal efficiency.

Prob. 9-55 (page 538)

A four-cylinder two-stroke 2.0 L diesel engine that operates

on an ideal Diesel cycle has a compression ratio of 22 and a

cutoff ratio of 1.8. Air is at 70°C and 97 kPa at the beginning of

the compression process. Using the cold-air-standard

assumptions, determine how much power the engine will

deliver at 2300 rpm.

14/3/2012

23

45

QUESTIONS ???

Diesel engines operate at

higher air-fuel ratios than

gasoline engines. Why?

Despite higher power to

weight ratios, two-stroke

engines are not used in

automobiles. Why?

The stationary diesel engines

are among the most efficient

power producing devices

(about 50%). Why?

What is a turbocharger? Why

are they mostly used in

diesel engines compared to

gasoline engines.

P-v diagram of an ideal dual cycle.

Dual cycle: A more realistic

ideal cycle model for modern,

high-speed compression ignition

engine.

46

STIRLING AND ERICSSON CYCLES

A regenerator is a device that

borrows energy from the working

fluid during one part of the cycle

and pays it back (without interest)

during another part.

Stirling cycle

1-2 T = constant expansion (heat addition from the external source)

2-3 v = constant regeneration (internal heat transfer from the working fluid to the regenerator)

3-4 T = constant compression (heat rejection to the external sink)

4-1 v = constant regeneration (internal heat transfer from the regenerator back to the working fluid)

T-s and P- vdiagrams of Carnot, Stirling, and Ericsson cycles.

14/3/2012

24

47

The execution of the Stirling cycle. A steady-flow Ericsson engine.

The Ericsson cycle is very much like the

Stirling cycle, except that the two constant-

volume processes are replaced by two

constant-pressure processes.

Both the Stirling and Ericsson cycles are

totally reversible, as is the Carnot cycle,

and thus:

The Stirling and Ericsson cycles give

a message: Regeneration can

increase efficiency.

Summary

• Basic considerations in the analysis of power cycles

• The Carnot cycle and its value in engineering

• Air-standard sssumptions

• An overview of reciprocating engines

• Otto cycle: The ideal cycle for spark-ignition engines

• Diesel cycle: The ideal cycle for compression-ignition

engines

• Stirling and Ericsson cycles

48

Date post:	09-Feb-2016
Category:	Documents
Upload:	rajwinder-singh
View:	30 times
Download:	10 times

Gas Power Cycles - PartI _ March 2012

Documents