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Overview

Refrigerators, car engines, and power plants all run up against the same limit: no heat engine can convert 100% of the heat it absorbs into useful work, and this chapter is where students learn exactly why. In NEB and HISSAN board exams, this chapter is dominated by one recurring calculation — Carnot engine efficiency, η = 1 − (T₂/T₁), tested from nearly every possible angle: finding efficiency directly, working backward to find how much the source temperature must rise to hit a target efficiency, or comparing two engines running between different temperature pairs.

A useful pattern to notice: nearly every numerical problem in this chapter reduces to the same handful of formulas — Carnot efficiency, coefficient of performance for refrigerators, and basic heat-work-efficiency relations for petrol/diesel engines. Once these three relations are memorized cold, most numerical problems here become a matter of correctly identifying which temperatures are source and sink (always in Kelvin) rather than learning new physics.

The complete set of past NEB and HISSAN questions on this chapter is below, organized by MCQs, theory questions, and numerical problems. Step-by-step numerical solutions are available on the Solutions page for this chapter.

MCQs

Q12083

If efficiency of petrol is [latex]\eta_1[/latex] and that of diesel engine is [latex]\eta_2[/latex] then, which of the following expression is correct?
a.
[latex]\eta_1 = \eta_2[/latex]
b.
[latex]\eta_1 > \eta_2[/latex]
c.
[latex]\eta_1 < \eta_2[/latex]
d.
[latex]\eta_1 \geq \eta_2[/latex]
Show answer
Correct answer: (C)
Q22082 GIE ‘A’

The maximum efficiency of Carnot engine operating between [latex]27^o\ C[/latex] and [latex]327^o\ C[/latex] is
a.
125%
b.
75%
c.
50%
d.
25%
Show answer
Correct answer: (C)
Q32082 GIE ‘B’

A refrigerator transfers 4000 J of heat from cold reservoir at [latex]-10^oC[/latex] and consumes 1000J heat from cold reservoir at [latex]-10^oC[/latex] and consumes 1000 J of work. The coefficient of performance (COP) of the refrigerator is
a.
2
b.
3
c.
4
d.
5
Show answer
Correct answer: (C)
Q42082

The efficiency of Carnot engine operating between steam point and ice point is
a.
26.8 %
b.
50%
c.
73.2%
d.
100%
Show answer
Correct answer: (A)
Q52081 GIE ‘A’

An ideal refrigerator is working between 0oC and 127oC. The coefficient of performance is
a.
3.25
b.
2.85
c.
2.15
d.
1
Show answer
Correct answer: (C)
Q62081 GIE ‘B’

What is the efficiency of a carnot engine working between steam point and ice point?
a.
27%
b.
37%
c.
50%
d.
75%
Show answer
Correct answer: (A)
Q72081 “B/C’

Given figure represents the block diagram of a heat engine. What is the efficiency of this engine?
a.
25%
b.
50%
c.
60%
d.
75%
Show answer
Correct answer: (B)
Q82081 ‘D’

An ideal heat engine working between temperatures T1 and T2 has efficiency [latex]\eta[/latex]. If both the temperature are raised by 50 K each, the new efficiency of engine will be
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