Page 63 - Entrophy Analysis in Thermal Engineering Systems

CHAPTER FIVE

Most efficient engine

5.1 Introduction
Heat engines have played a significant role in modernization of the
mankind’s life. Early engines had a very low efficiency, so it was one of
the challenges of engineers to find new methods and designs to increase
the power production efficiency. Sadi Carnot, a French military engineer,
was determined to answer a central question of his ear: “whether the motive
power of heat is unbounded,” and “whether the possible improvements in
steam engines have an assignable limit, a limit which the nature of the things
will not allow to be passed by any means whatever” [1].
In his investigation to determine an upper limit for the efficiency of heat
engines, Carnot had considered certain design constraints: (i) the quantity of
heat is given, and (ii) the highest and lowest temperatures experienced by
engine are fixed. In his era, neither the first law nor the second had fully been
realized and formulated. His analysis was based on (i) the caloric theory
where heat was sought to be an indestructible substance, which would trans-
fer between two bodies with different temperatures, and (ii) the empirical
laws of Boyle-Mariotte and Dalton-Gay Lussac.
Carnot’s investigation led him to propose a design of ideal engine. Since
then, the Carnot cycle has been used as a reference to measure the effective-
ness of other engines. In the comparison of the performance of an engine with
that of a Carnot cycle, it is traditionally assumed that both engines operate
between the same high- and low-temperature thermal reservoirs—the second
design constraint of Carnot. Under this condition, a class of heat engines with
two isothermal processes (i.e., Stirling, Carnot, and Ericsson engines) are the
most efficient engines. The reason for this is adequately given by Rankine [2]:
As the conversion of heat into expansive power arises from changes of volume
only, and not from changes of temperature, it is obvious, that the proportion of
the heat received which is converted into expansive power will lie the greatest pos-
sible, when the reception of heat, and its emission, each take place at a constant
temperature.

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Entropy Analysis in Thermal Engineering Systems 55
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