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# Thermal Engineering MCQ’s – Dual Combustion Cycles

1 - Question

What is the other name of dual cycle?

a) Mixed cycle

b) Otto cycle

c) Diesel cycle

d) Joule Cycle

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Answer: aExplanation: Dual cycle is combination of otto cycle and diesel cycle. Dual cycle is also known as mixed cycle.

2 - Question

Dual cycle is combination of__________

a) Stirling and diesel cycle

b) Otto and diesel cycle

c) Stirling and otto cycle

d) Otto and Brayton cycle

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Answer: bExplanation: Dual cycle is combination of otto cycle and diesel cycle. Dual cycle is also known as mixed cycle.

3 - Question

What is the efficiency of dual cycle?

a) 1-(ργrp−1)(rp−1)+γrp(ρ−1)

b) 1-1rγ−1(ργrp−1)(rp−1)+γrp

c) 1-1rγ(ργrp−1)(rp−1)+γrp(ρ−1)

d) 1-1rγ−1(ργrp−1)(rp−1)+γrp(ρ−1)

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Answer: d

Explanation: Efficiency of dual cycle = 1-1rγ−1(ργrp−1)(rp−1)+γrp(ρ−1)

Efficiency of otto cycle = 1 – 1rγ−1

Efficiency of diesel cycle = 1 – ργ−1(ρ−1)rγ−1

4 - Question

Dual cycle consist of __________

a) Two constant pressure and two adiabatic process

b) Two constant volume and two adiabatic process

c) One constant pressure process, two constant volume process and two adiabatic process

d) One constant pressure process, one constant volume process and two adiabatic process

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Answer: cExplanation: The sequence of processes in dual cycle is isentropic compression, isochoric heat addition, isobaric heat addition, isentropic expansion and isochoric heat rejection. Therefore a dual cycle consist of two constant volume. One constant pressure and two adiabatic process.

5 - Question

In a dual cycle heat rejection takes place ______

a) At constant volume process

b) At constant pressure process

c) First at constant volume then at constant pressure process

d) First at constant pressure then at constant volume process

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Answer: aExplanation: The sequence of processes in dual cycle is isentropic compression, isochoric heat addition, isobaric heat addition, isentropic expansion and isochoric heat rejection. Therefore, heat rejection takes place at constant volume process.

6 - Question

In a dual cycle heat addition takes place ________

a) At constant volume process

b) At constant pressure process

c) First at constant volume then at constant pressure process

d) First at constant pressure then at constant volume process

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Answer: cExplanation: The sequence of processes in dual cycle is isentropic compression, isochoric heat addition, isobaric heat addition, isentropic expansion and isochoric heat rejection. Therefore, heat addition takes place first at constant volume then at constant pressure process.

7 - Question

Most high speed compression ignition operates on ________

a) Diesel cycle

b) Otto cycle

c) Brayton cycle

d) Dual cycle

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Answer: dExplanation: Compression ratio of dual cycle is 12-16, also dual cycle is combination of diesel cycle and otto cycle. Therefore, most high speed compression ignition operates on dual cycle.

8 - Question

What is the efficiency of dual cycle if compression ratio and cut-off ratio is equal to 15 and 1.4 respectively?

a) 91%

b) 90.8%

c) 90.6%

d) 90%

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Answer: d

Explanation: Efficiency of dual cycle=1-1rγ−1(ργrp−1)(rp−1)+γrp(ρ−1)

Efficiency of dual cycle= 1-1151.4−1(1.41.4×10.71−1)(10.71−1)+1.4×10.71(1.4−1)

Efficiency of dual cycle = 90.6%

9 - Question

What is the value of working fluid in dual cycle?

a) 1.0

b) 1.2

c) 1.4

d) 1.8

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Answer: cExplanation: In air standard dual cycle working fluid is air. The specific heat ratio of air is equal to 1.4.

10 - Question

The compression ratio (r) of dual cycle is equal to ________

a) r = volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression

b) r = volume of the cylinder at the end of the compression/ volume of the cylinder at the beginning of the compression

c) r = clearance volume/ volume of the cylinder at the beginning of the compression

d) r = volume of the cylinder at the end of the compression/clearance volume

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Answer: aExplanation: compression ratio of dual cycle (r)=sweptvolume+clearancevolumeclearancevolume r=volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression

11 - Question

The cut-off ratio (ρ) of dual cycle is equal to ________

a) Volume at which fuel supply cutoff to volume at which fuel supply start

b) Volume at which fuel supply throughout the process to volume at which fuel supply cutoff

c) Volume at which fuel supply cutoff to volume of fuel supply throughout the process

d) Volume at which fuel supply start to volume at which fuel supply cutoff

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Answer: aExplanation: ρ = VolumeatwhichfuelsupplycutoffVolumeatwhichfuelsupplystart Cutoff ratio of dual cycle is equal to Volume at which fuel supply cutoff to volume at which fuel supply start.

12 - Question

What is the efficiency of dual cycle if compression ratio and cut-off ratio is equal to 13 and 1.4 respectively?

a) 68%

b) 63%

c) 60%

d) 65%

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Answer: bExplanation: Efficiency of dual cycle=1-1rγ−1(ργrp−1)(rp−1)+γrp(ρ−1) Efficiency of dual cycle= 1-1131.4−1(1.41.4×9.28−1)(9.28−1)+1.4×9.28(1.4−1) Efficiency of dual cycle = 63%

13 - Question

What is the relation between compression ratio, cutoff ratio and expansion ratio?

a) re = ρ×r

b) re = ρ/r

c) re = 1/ρr

d) re = r/ρ

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Answer: d

Explanation: re = V5V4=V1V4

re = V1V3×V3V4

re = V1V2×V3V4

re = r×(1/ρ)

re = r/ρ

14 - Question

Determine the value of expansion ratio if compression ratio and cutoff ratio is equal to 15 and 1.4 respectively.

a) 10.5

b) 21

c) 11

d) 10.7

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Answer: d

Explanation: re = r/ρ

re = 15/1.4

re = 10.7

15 - Question

Determine the value of cutoff ratio if compression ratio and expansion ratio is equal to 15 and 11 respectively.

a) 1.4

b) 1.36

c) 1.5

d) 1.3

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Answer: b

Explanation: re = r/ρ

ρ = 15/11

ρ =1.36