linguistic:voice_change, structural:summarization
Background

Thomas Newcomen invented the first practical piston-operated steam engine in 1712. Some 50 years after that, James Watt made his celebrated improvements, which were responsible for greatly increasing the usefulness of steam engines. When Carnot became interested in the subject in the 1820s, steam engines were in increasingly wide application in industry and their economic importance was widely recognized. Compound engines (engines with more than one stage of expansion) had already been invented, and there was even a crude internal combustion engine, known as the pyréolophore and built by the brothers Claude and Nicéphore Niépce, with which Carnot was familiar and which he described in some detail in his book.

That practical work on steam engines and the intuitive understanding among engineers of some of the principles underlying their operation co-existed, however, with an almost complete lack of a scientific understanding of the physical phenomena associated with heat. The principle of conservation of energy had not yet been clearly articulated and the ideas surrounding it were fragmentary and controversial. Carnot himself accepted the view, prevalent in France and associated with the work of Antoine Lavoisier, that heat is a weightless and invisible fluid, called " caloric ", which may be liberated by chemical reactions and which flows from bodies at higher temperature to bodies at lower temperature.

In his book, Carnot sought to basic questions: Is there a limit to the work that can be generated from a given heat source? and Can the performance of an engine be improved by replacing steam with a different working fluid ?. Engineers in Carnot's time had tried, using highly pressurized steam and other fluids, to improve the efficiency of engines. In these early stages of engine development, the efficiency of a typical engine —the useful work it was able to do when a given quantity of fuel was burned— was only about 5–7%.

Carnot's book was only 118 pages long and covered a wide range of topics about heat engines in what Carnot must have intended to be a form accessible to a wide public. He made minimal use of mathematics, which he confined to elementary algebra and arithmetic, except in some footnotes. Carnot discussed the relative merits of air and steam as working fluids, the merits of various aspects of steam-engine design, and even included some ideas of his own regarding possible practical improvements. However, the central part of the book was an abstract treatment of an idealized engine (the Carnot cycle) with which the author sought to clarify the fundamental principles that govern all heat engines, independently of the details of their design or operation. This resulted in an idealized thermodynamic system upon which exact calculations could be made, and avoided the complications introduced by many of the crude features of the contemporary steam engines.