What is a Heat engine?
Below diagram shows working of a heat engine
Th: Temperature of source
Tc: Temperature of sink
Qh: Heat supplied to the engine
Qc: Heat rejected by the engine
W: Work output of the engine
Given: Th > Tc
From the above diagram we can easily figure out that
Work output of heat engine
W = Qh – Qc
Efficiency of heat engine
η = Work output/ Heat input
η = (Qh – Qc)/ Qh
η = 1 – Qc/ Qh
From the second law of thermodynamics we know that Qc can never be equal to zero. Which means that efficiency of a heat engine can never be 100%. Even in the ideal case.
Kelvin-Plank statement of second law
It is impossible for a heat engine to produce net work in a cycle if it exchanges heat only with bodies at a single fixed temperature.
Image source: By P.wormer – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12404233