chapter 9 gas cycles - part i

8/10/2019 Chapter 9 Gas Cycles - Part I

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Chapter 9

GAS POWER CYCLES(Part 1a)

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

Thermodynamics: An Engineering Approach, 6thEdition

Yunus A. Cengel, Michael A. Boles

McGraw-Hill, 2008


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Objectives

1. Evaluate the performance of gas power cycles.2. Develop simplifying assumptions applicable to gas power cycles.

3. Review the operation of reciprocating engines.

4. Analyze both closed and open gas power cycles.

5. Solve problems based on the Otto and Diesel cycles.

6. Solve problems based on the Brayton cycle; Brayton cycle with regeneration;

and Brayton cycle with intercooling, reheating, and regeneration.

7. Identify simplifying assumptions and perform second-law analysis on gas

power cycles.


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Basic Considerations In Power Cycles Analysis

The analysis of many complex

processes can be reduced to a

manageable level by utilizing

some idealizations.

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 is called an ideal cycle.

Recall: Thermal efficiency of heat engines

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.


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1. The cycle does not involve any friction. Therefore,

the working fluid does not experience any pressuredrop as it flows in pipes or heat exchangers.

2. All expansion and compression processes take

place in a quasi-equilibriummanner.

3. The pipes connecting the various components of a

system are well insulated, so 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 curverepresents the thermal efficiency of the

cycle.

Idealizations (simplifications) in the analysis of power cycles


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Carnot Cycle - Its Value In Engineering

P-vand T-sdiagrams of a Carnot cycle.

Example: A steady-flow Carnot engine.

The Carnot cycle is composed of 4 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 theaverage temperature at which heat is rejected

from the system.


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Air-standard Assumptions

The combustion process is replaced by a

heat-addition process in ideal cycles.

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 combustionprocess is replaced by a

heat-additionprocess from an external

source.

4. The exhaustprocess is replaced by a

heat-rejectionprocess that restores the

working fluid to its initial state.

Cold-air-standard assumptions: When the working fluid is considered to be airwith constant specific heats at room temperature(25C).

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

applicable.


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Chapter 9

GAS POWER CYCLES(Part 1b)




McGraw-Hill, 2008


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ProblemOtto Cycle

934

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

compression process, air is at 95 kPa and 27C, and 750 kJ/kg of heat is

transferred to air during the constant-volume heat-addition process. Assuming

that the specific heats are constant with temperature, determine:a) the pressure & temperature at the end of heat addition process,

b) the net work output,

c) the thermal efficiency, and

d) the mean effective pressure for the cycle.

Answers: (a) 3898 kPa, 1539 K, (b) 392.4 kJ/kg, (c) 52.3 percent, (d ) 495 kPa


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Overview of Reciprocating Engines

The reciprocating engine (basically a pistoncylinder device) is an invention that

has proved to be very versatile and has a wide range of applications.

Reciprocating engine is the

powerhouse of the vast majority of

automobiles, trucks, light aircraft,

ships, electric power generators,

and many other devices.


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Basic Components

Compression ratio:

The piston reciprocates in the cylinder between two fixed positions called the top dead

centre (TDC) - the position that forms the smallest volume in the cylinder - and the bottom

dead centre (BDC) - position that forms the largest volume in the cylinder.

The distance between TDC and BDC is called the stroke of

the engine. The diameter of the piston is called the bore.


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Performance Characteristics

Classifications of IC Engines:

1. Spark-ignition (SI) or Petrol engines

2. Compression-ignition (CI) or Diesel

engines

Mean effective pressure (MEP):

A fictitious pressure that, if it is acted on the piston

during the entire power stroke, would produce the

same amount of net work as that produced during theactual cycle.

Net work output per cycle:


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Otto Cycle: Ideal Spark-Ignition Engines Cycle

Actual and ideal cycles in spark-ignition engines on a P-vdiagram.

The piston executes four complete strokes within the cylinder. The crankshaft

completes two revolutions for each thermodynamic cycle.

These engines are called four-stroke IC engines.


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T-s Diagram of Ideal Otto Cycle

IC Engines Classifications:

Four-stroke cycle

1 cycle = 4 stroke = 2 revolutions of crankshaft

Two-stroke cycle

1 cycle = 2 stroke = 1 revolution of crankshaft

Sequence of processes:


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In two-stroke engines, all four functions described earlier are executed in two

strokes: the power and compression stroke.

Generally less efficient, but are relatively simple and inexpensive. They have high

power-to-weight and power-to-volume ratios.

Two-Stroke IC Engines


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Thermal Efficiency of Otto Cycle

The heat supplied to the working fluid during

constant-volume heating (combustion),

The heat rejected from the working fluid during

constant-volume cooling (exhaust),

Thermal efficiency,

Temperature-volume relation,

Compression ratio,

Cold-air standard assumption.


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ProblemOtto Cycle

934B

Reconsider the ideal Otto cycle in Problem 9-34. Assuming that the specific

heats vary with temperature, determine:

a) the pressure & temperature at the end of heat addition process,

b) the net work output,c) the thermal efficiency, and

d) the mean effective pressure for the cycle.

Answers: (a) 3898 kPa, 1539 K, (b) 392.4 kJ/kg, (c) 52.3 percent, (d ) 495 kPa


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ProblemOtto Cycle

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37The compression ratio of an air-standard Otto cycle is 9.5. Prior to the isentropic

compression process, the air is at 100 kPa, 35C, 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 and pressure in the cycle;b) the amount of heat transferred in, in kJ;

c) the thermal efficiency; and

d) the mean effective pressure.

Answers: (a) 1969 K, 6072 kPa, (b) 0.59 kJ, (c) 59.4 percent, (d) 652 kPa


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Problem

9

39E

An ideal Otto cycle with air as the working fluid has a compression ratio of 8. The

minimum and maximum temperatures in the cycle are 300 K and 1340 K.

Accounting for the variation of specific heats with temperature, determine:

a) the amount of heat transferred to the air during heat-addition process,

b) the thermal efficiency, andc) the thermal efficiency of a Carnot cycle operating between the same

temperature limits.

Otto CycleClass Exercise


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Premature ignition of the fuel produces audible noise called engine knock. It hurts

performance and causes engine damage.

Autoignition places upper limit on compression ratios that can be used in SI engines.Specific heat ratio, kaffects the thermal efficiency of the Otto cycle.

Engine Knock (Autoignition)


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ProblemOtto Cycle

9

41A four-cylinder, four-stroke, 2.2-L gasoline engine operates on the Otto cycle with

a compression ratio of 10. The air is at 100 kPa and 60C at the beginning of the

compression process, and the maximum pressure in the cycle is 8 MPa. The

compression and expansion processes may be modeled as polytropic with an

index of 1.3. Using constant specific heats at 850 K, determine:

a) the temperatureat the end of the expansion process,

b) the net work output and the thermal efficiency,

c) the mean effective pressure,

d) the engine speed for a net power output of 70 kW, and

e) the specific fuel consumption, in g/kWh, defined as the ratio of the mass

of the fuel consumed to the net work produced.

Note: The airfuel ratio, defined as the amount of air divided by the amount of

fuel intake, is 16.


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Chapter 9

GAS POWER CYCLES(Part 1c)




McGraw-Hill, 2008


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ProblemDiesel Cycle

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47An air-standard Diesel cycle has a compression ratio of 16 and a cutoff ratio of

2. At the beginning of the compression process, air is at 95 kPa and 27C.

Accounting for the variation of specific heats with temperature, determine:

a) the temperature after the heat-addition process,

b) the thermal efficiency, andc) the mean effective pressure.

Answers: (a) 1724.8 K, (b) 56.3 percent, (c) 675.9 kPa


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Diesel Cycle: Ideal Cycle for CI Engines

The combustion process takes place over a

longer interval - fuel injection starts whenthe piston approaches TDC and continues

during the first part of power stroke.

Hence, combustion process in the ideal

Diesel cycle is approximated as a constant-

pressure heat-addition process.

In diesel engines, only air is compressed during the compression stroke, eliminating

the possibility of autoignition. These engines can be designed to operate at higher

compression ratios, typically between 12and 24.

Fuels that are less refined (thus less expensive) can be used in diesel engines.


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1-2 Isentropic compression

2-3 Constant-pressure heat addition

3-4 Isentropic expansion

4-1 Constant-volume heat rejection.

Sequence of processes:

Note:

Petrol and diesel engines differ only in the

manner the heat addition (or combustion)

process takes place.

It is approximated as a constant volume

process in the petrol engine cycle and as a

constant pressure process in the Dieselengine cycle.


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25Cutoff ratio,

Thermal Efficiency of Diesel Cycle

Heat supplied to the working fluid during the

constant-pressure heating (combustion),

Heat rejected from the working fluid during the

constant-volume cooling (exhaust),

Thermal efficiency of Diesel cycle (general),

- constant specific heats


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ProblemDiesel Cycle

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51An 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

20C. If the maximum temperature in the cycle is not to exceed 2200 K,

determine:

a) the thermal efficiency, andb) the mean effective pressure.

Assume constant specific heats for air at room temperature.

Answers: (a) 63.5 percent, (b) 933 kPa

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ProblemDiesel Cycle

954

A four-cylinder two-stroke 2.4-L diesel engine that operates on an ideal Diesel

cyclehas a compression ratio of 17 and a cutoff ratio of 2.2. Air is at 55C and 97

kPa at the beginning of the compression process.

Using the cold-air standard assumptions, determine how much power theengine will deliver at 1500 rpm.

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For the same compression ratio, thermal efficiency of Otto cycle is greaterthan that

of the Diesel cycle.

As the cutoff ratio decreases, the thermalefficiency of the Diesel cycle increases.

When rc=1, the efficiencies of the Otto

and Diesel cycles are identical.

Thermal efficiencies of large diesel engines

range from about 35 to 40 percent.

Higher efficiency and lower fuel costs

make diesel engines attractive in

applications such as in locomotive engines,

emergency power generation units, largeships, and heavy trucks.


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ProblemDiesel Cycle

9-59A six-cylinder, four-stroke, 4.5-L compression-ignition engine operates on the

ideal Diesel cycle with a compression ratio of 17. The air is at 95 kPa and 55C

at the beginning of the compression process and the engine speed is 2000 rpm.

The engine uses light diesel fuel with a heating value of 42,500 kJ/kg, an airfuel

ratio of 24, and a combustion efficiency of 98 percent. Using constant specific

heats at 850 K, determine:

a) the maximum temperature in the cycle and the cutoff ratio,

b) the net work output per cycle and the thermal efficiency,

c) the mean effective pressure,

d ) the net power output, and

e) the specific fuel consumption, in g/kWh, defined as the ratio of themass of the fuel consumed to the net work produced.

Answers: (a) 2383 K, 2.7 (b) 4.36 kJ, 0.543, (c) 969 kPa, (d ) 72.7 kW, (e) 159 g/kWh


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Approximating the combustion process as

a constant-volume or a constant-pressureheat-addition process is overly simplistic

and not quite realistic.

A better approach would be to model the

combustion process in both SI and CI

engines as a combination of two heat-transfer processes, one at constant volume

and the other at constant pressure.

The ideal cycle based on this concept is

called the dual cycle.

Dual Cycle: Realistic Ideal Cycle for CI Engines

Note: Both the Otto and the Diesel cycles can be obtained

as special cases of the dual cycle.

chapter 9 gas cycles - part i

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