Mechanical Engineering Thermodynamics II- Lecture 03_27 Sep

7/27/2019 Mechanical Engineering Thermodynamics II- Lecture 03_27 Sep

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1

OTTO CYCLE AND DIESEL CYCLE

ME Thermodynamics II

MCB 2063

LECTURE 3


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2

Air Standard Assumptions

2

The actual gas power cycles are complex. Therefore air-

standard assumptions are used for the purpose of analysis :

- The working fluid is air, continuously circulates in a closed

loop and always behaves as an ideal gas.

- All the processes that make up the cycle are internally

reversible.

- The combustion process is replaced by a heat-additionprocess

- The exhaust process is replaced by a heat-rejection process


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3

3

- Another assumption to simplify the analysis is using

air-standard assumptions in which air has constant specific

heats at room temperature of 25C.

- The assumption is known as cold-air-standardassumptions.

- A cycle for which the air-standard assumptions are

applicable is frequently referred to as an air-standardcycle.

Air Standard Assumptions


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Otto Cycle: The Ideal Cycle for SI Engines


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5

The Otto Cycle

5

The mean effective pressurecanbe used as a parameter tocompare the performancesofreciprocating engines of equalsize.

The engine with a larger valueof MEP will del iver more networkper cycle and thus will

perform better.

Mean Effective Pressure (MEP)

d

netnet

V

W

VV

WMEP

minmax


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The Otto Cycle

6

The Ideal P-Vand T-s Diagram for Otto Cycle


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The Otto Cycle

7

1-2

Isentropic Compression

2-3

Constant Volume Heat Addition

3-4

Isentropic Expansion

4-1

Constant Volume Heat Rejection

The Ideal Process ofP-Vfor Otto Cycle


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8

The Otto Cycle

8

Thermal Efficiency - the ratio of the work output of a heatengine to the heat input expressed in the same units of

energy.

Basic indicator of the engine performance

Thermal efficiency of Otto Cycle :

in

outin

in

netthq

qq

q

W

Heat

Work


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9

The Otto Cycle

9

Thermal efficiency of Otto cycle

under air standard assumptions :

k= specific heat ratio, Cp/Cv

1

11

kth r


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10

The Otto Cycle

10

In Otto Cycle, no work is involved during the two heat

transfer processes since both take place at constant volume.

Therefore :

(kJ/kg))(

(kJ/kg))(

1414

2323

TTcuuq

TTcuuq

vout

vin

(kJ/kg)outinnet qqW


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Engine Capacity

Compression Ratio

Thermal Efficiency

C

CD

C

T

CCDT

V

VV

V

V

CRr

VSB

VVV

4

2

in

ou tin

in

netth

q

qq

q

W

Heat

Work

The Otto Cycle


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Pressure Volume - Temperature

Pressure Volume

Temperature Volume

Pressure - Volume

kk

kk

V

V

P

P

V

V

P

P

V

V

T

T

V

V

T

T

T

VP

T

VP

T

VP

T

VP

mRTVP

3

4

4

3

2

1

1

2

1

3

4

4

3

1

2

1

1

2

4

44

3

33

2

22

1

11

111

and

and

The Otto Cycle


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13

The Otto Cycle

13


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The Otto Cycle

14

EXAMPLE 92 The Ideal Otto Cycle Page 494 :

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

beginning of the compression process, air is at 100 kPa and

17C, and 800 kJ/kg of heat istransferred to air during the

constant-volume heat-addition process. Accountingfor the

variation of specific heats of air with temperature, determine

(a)the maximum temperature and pressure that occur during the

cycle,

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

(d ) the mean effectivepressure for the cycle.


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15

The Otto Cycle

15

The Ideal P-Vand T-s Diagram for Otto Cycle


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Q1

The air at the beginning of the compression stroke of an air-standard Otto cycle is at 95 kPa and 22C and the cylinder

volume is 5600 cm3. The compression ratio is 9 and 8.6 kJ are

added during the heat addition process. Determine :

Q2

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 a

polytropic constant of 1.3. Using constant specific heats at 800K

determine :

.

The Otto Cycle


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The Otto Cycle

17


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18

18

In Diesel Engine, the fuel injection process in diesel enginesstarts when the piston approaches TDC and continues

during the first part of the power stroke.

Therefore, the combustion process in these engines takes

place over a longer interval. Because of this longer duration,

the combustion process in the ideal Diesel cycle is

approximated as a constant-pressure heat-addition

process.

In fact, this is the only process where the Otto and the

Diesel cycles differ.

The Diesel Cycle


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19

19

1-2

Isentropic Compression

2-3

Constant Pressure Heat Addition

3-4

Isentropic Expansion

4-1

Constant Volume Heat Rejection

The Diesel Cycle

The Ideal Process ofP-Vfor Diesel Cycle


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The Diesel Cycle

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The Ideal P-Vand T-s Diagram for Diesel Cycle


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21

In Diesel cycle, similar to Otto cycle, no work is involved

during the two heat transfer processes but heat added at

constant pressure while heat rejected at constant volume.

Therefore :

(kJ/kg))(

(kJ/kg))(

1414

2323

TTcuuq

TTchhq

vout

pin

The Diesel Cycle


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2

3

V

Vrc

Cut off ratio, the ratio of cylinder volume

before and after the combustion process

Thermal efficiency

of Diesel cycle

)1(

111

1

c

k

c

kth rk

r

r

k= specific heat ratio, 1.4

The Diesel Cycle

2

1

V

Vr The compression ratio for Diesel Cycle


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Compression ratio and cutoff ratio of diesel engine

The Diesel Cycle


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Pressure Volume - Temperature

Pressure Volume

Temperature Volume

Pressure - Volume

kk

kk

V

V

P

P

V

V

P

P

V

V

T

T

V

V

T

T

T

VP

T

VP

T

VP

T

VP

mRTVP

3

4

4

3

2

1

1

2

1

3

4

4

3

1

2

1

1

2

4

44

3

33

2

22

1

11

111

and

and

The Diesel Cycle


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EXAMPLE 93 The Ideal Diesel Cycle Page 498 :

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

compression ratio of 18 and a cutoff ratio of 2. At the beginning

of the compression process, the working fluid is at 100 kPa,

27C, and 1917 cm2. Utilizing the cold-air standard

assumptions, determine

(a) the temperature and pressure of air at the end of each process

(b) the net work output,

(c) the thermal efficiency, and

(d ) the mean effectivepressure for the cycle.

The Diesel Cycle

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Mechanical Engineering Thermodynamics II- Lecture 03_27 Sep