# Thermal Engineering Questions and Answers – Dual Combustion Cycles

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This set of Thermal Engineering written test Questions & Answers focuses on “Dual Combustion Cycles”.

1. What is the other name of dual cycle?
a) Mixed cycle
b) Otto cycle
c) Diesel cycle
d) Joule Cycle

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

2. 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

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

3. What is the efficiency of dual cycle?
a) 1-$$\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p (ρ-1)}$$
b) 1-$$\frac{1}{r^{γ-1}}\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p}$$
c) 1-$$\frac{1}{r^γ}\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p (ρ-1)}$$
d) 1-$$\frac{1}{r^{γ-1}}\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p (ρ-1)}$$

Explanation: Efficiency of dual cycle = 1-$$\frac{1}{r^{γ-1}}\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p (ρ-1)}$$
Efficiency of otto cycle = 1 – $$\frac{1}{r^{γ-1}}$$
Efficiency of diesel cycle = 1 – $$\frac{ρ^γ-1}{(ρ-1)r^{γ-1}}$$

4. 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

Explanation: 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. 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

Explanation: 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. 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

Explanation: 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. Most high speed compression ignition operates on ________
a) Diesel cycle
b) Otto cycle
c) Brayton cycle
d) Dual cycle

Explanation: 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. 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%

Explanation: Efficiency of dual cycle=1-$$\frac{1}{r^{γ-1}}\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p (ρ-1)}$$
Efficiency of dual cycle= 1-$$\frac{1}{15^{1.4-1}}\frac{(1.4^{1.4}×10.71-1)}{(10.71-1)+1.4×10.71(1.4-1)}$$
Efficiency of dual cycle = 90.6%

9. What is the value of working fluid in dual cycle?
a) 1.0
b) 1.2
c) 1.4
d) 1.8

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

10. 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

Explanation: compression ratio of dual cycle (r)=$$\frac{swept \, volume+clearance \, volume}{clearance \, volume}$$
r=volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression

11. 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

Explanation: ρ = $$\frac{Volume \, at \, which \, fuel \, supply \, cutoff}{Volume \, at \, which \,fuel \,supply \,start}$$
Cutoff ratio of dual cycle is equal to Volume at which fuel supply cutoff to volume at which fuel supply start.

12. 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%

Explanation: Efficiency of dual cycle=1-$$\frac{1}{r^{γ-1}}\frac{(ρ^γ r_p-1)}{(r_p-1)+γr_p (ρ-1)}$$
Efficiency of dual cycle= 1-$$\frac{1}{13^{1.4-1}}\frac{(1.4^1.4×9.28-1)}{(9.28-1)+1.4×9.28(1.4-1)}$$
Efficiency of dual cycle = 63%

13. 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

Explanation: re = $$\frac{V5}{V4} = \frac{V1}{V4}$$
re = $$\frac{V1}{V3}×\frac{V3}{V4}$$
re = $$\frac{V1}{V2}×\frac{V3}{V4}$$
re = r×(1/ρ)
re = r/ρ

14. 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

Explanation: re = r/ρ
re = 15/1.4
re = 10.7

15. 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