Thermodynamics Questions and Answers – Otto Cycle

This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Otto Cycle”.

1. The Otto cycle is the
a) air standard cycle of CI engine
b) air standard cycle of SI engine
c) vapour power cycle of CI engine
d) vapour power cycle of SI engine

Explanation: The Otto cycle is air standard cycle and is used in SI engine.

2. In a four-stroke internal combustion engine,
a) the piston does four complete strokes within cylinder
b) for each cycle, the crankshaft completes two revolutions
c) both of the mentioned
d) none of the mentioned

Explanation: This is the functioning of a four-stroke internal combustion engine.

3. The correct sequence of strokes in a four-stroke SI engine is
a) intake->compression->exhaust->expansion
b) intake->expansion->compression->exhaust
c) intake->exhaust->compression->expansion
d) intake->compression->expansion->exhaust

Explanation: The correct sequence is intake->compression->expansion->exhaust and expansion stroke is also called power stroke.

4. The spark plug fires shortly before the ____ stroke.
a) compression
b) expansion
c) intake
d) exhaust

Explanation: The spark plug fires shortly before the piston reaches TDC and after this ignition the expansion stroke takes place.

5. The pressure in cylinder is ____ the atmospheric value during exhaust stroke and ____ it during intake stroke.
a) above, below
b) below, above
c) equal to, equal to
d) equal to, above

Explanation: This is done to ensure that all the exhaust gases are thrown out of the cylinder and enough amount of intake mixture enters the cylinder.
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6. In a two-stroke engine, the four functions performed in SI engine are done in which two strokes?
a) expansion stroke and compression stroke
b) intake stroke and exhaust stroke
c) compression stroke and power stroke
d) compression stroke and expansion stroke

Explanation: In a two-stroke engine, these two strokes perform all the functions.

7. A two-stroke engine is used in motorcycles and scooters.
a) true
b) false

Explanation: The reason is that these vehicles need engines of small size and weight.

8. In a two-stroke engine,
a) the crankcase is sealed
b) the outward motion of piston is used to pressurize the air-fuel mixture
c) the intake and exhaust valves are replaced by opening in lower part of cylinder wall
d) all of the mentioned

Explanation: These are the modifications done in a two-stroke engine.

9. The two-stroke engine is ____ the four-stroke engine.
a) more efficient than
b) less efficient than
c) equally efficient to
d) none of the mentioned

Explanation: The reason being incomplete removal of exhaust gases in a two-stroke engine.

10. The two-stroke engine is
a) simple and expensive
b) high power-to-weight ratio
c) low power-to-volume ratio
d) all of the mentioned

Explanation: The two-stroke engine is inexpensive and has high power-to-volume ratio.

11. The intake and exhaust processes are not considered in the p-V diagram of Otto cycle.
a) true
b) false

Explanation: The reason is these two processes cancel each other.

12. The efficiency of Otto cycle is given by (rk is the compression ratio)
a) 1/(rk)^(ɣ-1)
b) 1 – 1/(rk)^(ɣ)
c) 1 – 1/(rk)^(ɣ-1)
d) 1/(rk)^(ɣ)

Explanation: This is the expression for efficiency of Otto cycle and rk=compression ratio=Vmax/Vmin.

13. Which of the following statement is true?
a) efficiency of air standard cycle is a function of compression ratio and temperature levels
b) higher the compression ratio, higher will be the efficiency
c) efficiency is dependent on the temperature levels at which the cycle operates
d) all of the mentioned

Explanation: The efficiency of air standard cycle = 1 – 1/[(rk)^(ɣ-1)] and it does not depend on the temperature levels.

14. ɣ for air is equal to
a) 1.0
b) 1.2
c) 1.3
d) 1.4

Explanation: In sir standard cycle, air is the working fluid and ɣ for air is 1.4.

15. The Otto cycle consists of
a) two reversible isotherms and two reversible isobars
b) two reversible isochores and two reversible adiabatics
c) two reversible isotherms and two reversible isochores
d) two reversible isobars and two reversible adiabatics