# Thermal Engineering Questions and Answers – Diesel Cycle (or) Constant Pressure Cycle

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This set of Thermal Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Diesel Cycle (or) Constant Pressure Cycle”.

1. Which cycle is idealized cycle for the compression ignition engines?
a) Otto cycle
b) Diesel cycle
c) Dual cycle
d) Bryton cycle

Explanation: Compression ignition engine that uses fuel injection to compressed air for combustion. An air standard diesel cycle is the idealized cycle for the compression ignition engines.

2. In diesel cycle heat rejection occurs at ___________
a) Reversible constant volume process
b) Reversible constant pressure process
c) Irreversible constant volume process
d) Irreversible constant pressure process

Explanation: Diesel cycle consist of one isobaric process, one isochoric process and two adiabatic process. Heat rejection process is isochoric process.

3. In diesel cycle heat addition occurs at ___________
a) Reversible constant volume process
b) Reversible constant pressure process
c) Irreversible constant volume process
d) Irreversible constant pressure process

Explanation: Diesel cycle consist of one isobaric process, one isochoric process and two adiabatic process. Heat rejection process is isobaric process.

4. In diesel engine fuel is ignited by ___________
a) Fuel injection
b) Spark
c) Heat resulting from compressing air
d) Heat resulting from compression of air fuel mixture

Explanation: Compression ignition engine that uses fuel injection to compressed air for combustion. An air standard diesel cycle is the idealized cycle for the compression ignition engines.

5. The thermal efficiency of diesel cycle having fixed compression ratio, with increase in cut-off ratio.
a) Increases
b) Decreases
c) Be independent
d) Data insufficient

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

Increase in cut-off ratio will increase in term $$\frac{ρ^γ-1}{(ρ-1)r^{γ-1}}$$ therefore decrease in efficiency of diesel cycle.

6. The thermal efficiency of diesel cycle having fixed cut-off ratio, with increase in compression ratio.
a) Increases
b) Decreases
c) Be independent
d) Data insufficient

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

Increase in compression ratio will decrease in term $$\frac{ρ^γ-1}{(ρ-1)r^{γ-1}}$$ therefore increase in efficiency of diesel cycle.

7. An air standard diesel cycle consist of ___________
a) One constant pressure process and three adiabatic process
b) One constant pressure process, one constant volume process and two adiabatic process
c) Two constant volume process and two adiabatic process
d) Two constant pressure process and two adiabatic process

Explanation: The sequence of processes in diesel cycle is isentropic compression, isobaric heat addition, isentropic expansion and isochoric heat rejection. Therefore an air standard diesel cycle consist of one constant volume, one constant pressure and two adiabatic process.

8. The compression ratio (rk) of diesel cycle is equal to ___________
a) rk = volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression
b) rk = volume of the cylinder at the end of the compression/ volume of the cylinder at the beginning of the compression
c) rk = clearance volume/ volume of the cylinder at the beginning of the compression
d) rk = volume of the cylinder at the end of the compression/clearance volume

Explanation: rk = $$\frac{swept \, volume+clearance \, volume}{clearance \, volume}$$
rk = volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression.

9. For same peak pressure and heat input _________
a) Otto cycle is more efficient
b) Diesel cycle is more efficient
c) Dual cycle is more efficient
d) Both diesel and otto cycle are equally efficient

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

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

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

11. What is the efficiency of diesel cycle if γ and T denotes the specific heat ratio and temperature respectively?
a) 1 – $$\frac{T4-T1}{T3-T2}$$
b) 1 – $$\frac{T4-T1}{γ(T3-T2)}$$
c) 1 – $$\frac{γ(T4-T1)}{T3-T2}$$
d) 1 – $$\frac{T4-T1}{(γ-1)(T3-T2)}$$

Explanation: Qaddition = mcp(T3 – T2)
Qrejection = mcv(T4 – T1)
Efficiency of diesel engine = $$\frac{Q1-Q2}{Q1}$$
Efficiency of diesel engine = 1 – $$\frac{T4-T1}{γ(T3-T2)}$$

12. Scavenging in diesel cycle means___________
a) Air used for combustion under pressure
b) Forced air for cooling the cylinder
c) Burnt air containing products of combustion
d) Air used for forcing burnt gases out of engines cylinder during exhaust stroke

Explanation: Scavenging is defined as forcing burnt gases out of engines cylinder during exhaust stroke. Therefore scavenging in diesel cycle means air used for forcing burnt gases out of engines cylinder during exhaust strokes.

13. A diesel engine is usually more efficient than a spark ignition engine because ___________
a) Diesel being a heavier hydrocarbon, release more heat per kg than gasoline
b) The air standard efficiency of diesel engine is higher than the otto cycle, at a fixed compression ratio
c) The compression ratio of diesel engine is higher than petrol engine
d) Self ignition temperature of diesel engine is higher than that of gasoline

Explanation: Diesel engine actually have a higher peak temperature than petrol engines. A diesel engine is usually more efficient than a spark ignition engine because the compression ratio of diesel engine is higher than petrol engine.

14. The combustion in compression ignition engine is __________
a) Homogenous
b) Heterogeneous
c) Laminar
d) Turbulent

Explanation: In compression ignition engine fuel injection system is used. Therefore fuel is injected to compressed air and hence air fuel mixture is heterogeneous.

15. Which of the following is compressed in diesel engine?
a) Air
b) Air and fuel
c) Air and lubricating oil
d) Fuel 