आधुनिक आवर्तसारणीWifistudy | Neeraj Sir 23 Comparison of Otto,...

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Wifistudy | Neeraj Sir 14

Analysis of Gas Power Cycles Used in IC engines

➢ An important application of thermodynamics is the analysis of power cycles through whichthe energy absorbed as heat can be continuously converted into mechanical work.

➢ A thermodynamic analysis of the heat engine cycles provides valuable informationregarding the design of new cycles or for improving the existing cycles.

Assumptions

➢ The working substance consists of a fixed mass of air and behaves as a perfect gas.

➢ There is no change in the chemical equilibrium of the working fluid and also composition.

➢ The specific heats Cp and Cv of air do not vary with temperature.


Otto Cycle

➢ Otto cycle consists following processes.

➢ Isentropic Compression

➢ Constant volume heat addition

➢ Isentropic Expansion

➢ Constant volume heat rejection

V

P

1

2

3

4

Swept Volume

Vs = V1 – V2Vs = V1 – V2

Clearance Volume

Vc = V2

Vc = V2

Win

Qin Wout

Qout


Efficiency of Otto Cycle

➢ As we know efficiency is given by :

➢ By solving this we get :

𝜼 =𝑾𝒏𝒆𝒕

𝑸𝒊𝒏

=𝑾𝒐𝒖𝒕 −𝑾𝒊𝒏

𝑸𝒊𝒏

➢ Compression ratio r : 𝒓 =𝑽𝟏𝑽𝟐

➢ So efficiency :

𝜼 = 𝟏 −𝑽𝟐𝑽𝟏

(𝜸 −𝟏)

𝜼 = 𝟏 −𝟏

𝒓(𝜸 −𝟏)


Effect of compression ratio on Efficiency of Otto Cycle

➢ So efficiency : 𝜼 = 𝟏 −𝟏

𝒓(𝜸 −𝟏)


Diesel Cycle

➢ Diesel Cycle consists following processes.

➢ Isentropic Compression

➢ Constant pressure heat addition

➢ Isentropic Expansion

➢ Constant volume heat rejection

V

P

1

2 3

4

Swept Volume

Vs = V1 – V2

Vs = V1 – V2

Clearance Volume

Vc = V2

Vc = V2

Win

Qin

Wout

Qout


Some Important Terms

V

P

1

2 3

4

Cut off Ratio

Vs = V1 – V2

Expansion Ratio

Vc = V2

Win

Qin

Wout

Qout

𝝆 =𝑽𝟑𝑽𝟐

𝒓𝒆 =𝑽𝟒𝑽𝟑


Efficiency of Diesel Cycle

➢ Efficiency is given by :

𝜼 = 𝟏 −𝟏

𝒓 𝜸 −𝟏

𝝆𝜸 − 𝟏

𝜸(𝝆 − 𝟏)


Dual Cycle

➢ In dual cycle heat addition takes place by 2 process.

➢ Cycle is shown in figure.

Pressure Ratio

𝜶 =𝑷𝟑𝑷𝟐

Cut off Ratio

𝝆 =𝑽𝟒𝑽𝟑


Efficiency of Dual Cycle

➢ Efficiency is given by :

𝜼 = 𝟏 −𝟏

𝒓 𝜸 −𝟏

𝜶𝝆𝜸 − 𝟏

𝜶 − 𝟏 + 𝜶 𝜸(𝝆 − 𝟏)


Comparison of Otto, Diesel and Dual Cycles

➢ Case 1: Same Compression Ratio and Heat Addition:

Otto cycle has the highest work output and efficiency. Diesel cycle has the least efficiency andDual cycle having the efficiency between the two.

➢ Case 2: Same Compression Ratio and Heat Rejection:

Otto cycle has the highest efficiency. Diesel cycle has the least efficiency and Dual cyclehaving the efficiency between the two.



➢ Case 3: Same Peak Pressure, Peak Temperature and Heat Rejection:

The Diesel cycle efficiency is greater than the Otto cycle efficiency and Dual cycle having theefficiency between the two.

➢ Case 4: Same Maximum Pressure and Heat Input:




➢ Case 5: Same Maximum Pressure and Work Output:


For the same compression ratio:


(A) Otto cycle is more efficient than the Diesel cycle

(B) Diesel cycle is more efficient than the Otto cycle

(C) Both Otto and Diesel cycles are equally efficient

(D) None of these

Compression ratio of IC engines is:


(A) The ratio of volumes of air in cylinder before compression stroke and aftercompression stroke

(B) Volume displaced by piston per stroke and clearance volume in cylinder

(C) Ratio of pressure after compression and before compression

(D) Swept volume / cylinder volume

For the same maximum pressure and peak temperature, which cycle will be most efficient-


(A) Otto cycle

(B) Diesel cycle

(C) Both Otto and Diesel cycles are equally efficient

(D) Dual cycle

Air standard efficiency of a diesel cycle is dependent uponA. Ratio of specific heatsB. Cut off ratioC. Adiabatic compression ratio


(A) Only A

(B) Only B

(C) A, B and C

(D) Only C

For a 4-stroke diesel engine, the compression ratio is 21 : 1 and the cut off ratio is 2 : 1.What is its expansion ratio?


(A) 7 : 1

(B) 10.5 : 1

(C) 12 : 1

(D) 19 : 1

For an engine operating on air standard Otto cycle, the clearance volume is 10% of theswept volume. The specific heat ratio of air is 1.4. The air standard cycle efficiency is


(A) 38.3%

(B) 39.8%

(C) 60.2%

(D) 61.7%

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आधुनिक आवर्तसारणीWifistudy | Neeraj Sir 23 Comparison of Otto,...

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