Nicolas Léonard Sadi Carnot (1 June 1796 – 24 August 1832) was a French physicist and military engineer who, in his 1824 Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines, now known as the Carnot cycle, thereby laying the foundations of the second law of thermodynamics. Technically, he is the world's first thermodynamicist and often called the "Father of thermodynamics", being responsible for such concepts as Carnot efficiency, Carnot theorem, Carnot heat engine, and others.
From age 16 (1812), he attended the École polytechnique where he and his contemporaries, Claude-Louis Navier and Gaspard-Gustave Coriolis, were taught by professors such as Joseph Louis Gay-Lussac, Siméon Denis Poisson and André-Marie Ampère. After graduation, he became an officer in the French army before committing himself to scientific research, becoming the most celebrated of Fourier's contemporaries who were interested in the theory of heat. Since 1814, he served in the military. Following the final defeat of Napoleon in 1815, his father went into exile. He later obtained permanent leave of absence from the French army. Subsequently, he spent time to write his book.
The historical context in which Carnot worked was that the scientific study of the steam engine hardly existed, but the engine was actually pretty far along in its development. It had attained a widely recognized economic and industrial importance. Newcomen had invented the first piston operated steam engine over a century before, in 1712. About 50 years after that, Watt made his celebrated improvements to greatly increase the efficiency and practicality of the engine. Compound engines, with more than one stage of expansion, had already been invented. There was even a crude form of an internal combustion engine, which Carnot was familiar with, and described in some detail in his book. Amazing progress on the practical side had been made, so at least some intuitive understanding of the engine's workings existed. The scientific basis of its operation, however, was almost nonexistent even after all this time. In 1824, the principle of conservation of energy was still immature and controversial, and an exact formulation of the first law of thermodynamics was yet over a decade away. The mechanical equivalent of heat was still two decades away. The prevalent theory of heat was the caloric theory which supposed that heat was a sort of weightless, invisible fluid that flowed when out of equilibrium.
Engineers of Carnot's time had tried various mechanical means, such as high pressure steam, or use of some fluid other than steam, to improve the efficiency of engines. The efficiency, the work generated from a given quantity of fuel, such as from burning a lump of coal, in these early stages of engine development was mere 3%.
Perhaps the most important contribution Carnot made to thermodynamics was the process of abstraction of the essential features of the steam engine as it was known in his day into a more general, idealized heat engine. This resulted in a model thermodynamic system upon which exact calculations could be made, and avoided the complications introduced by many of the crude features in the contemporary versions of the steam engine. By idealizing the engine, he could give clear answers to his original two questions that were impossible to dispute.
He showed that the efficiency of this idealized engine is a function only of the two temperatures of the reservoirs between which it functions. He did not, however, give the exact form of the function, which was later derived to be (T1−T2)⁄T1, where T1 is the absolute temperature of the hotter reservoir. (Note: This equation probably came from Kelvin.) No thermal engine operating any other cycle can be more efficient, given the same operating temperatures.
He saw very clearly, intuitively, that he could give very definite answers to the two questions set before the reader. The Carnot cycle is the most efficient possible engine, not only because of the (trivial) absence of friction and other incidental wasteful processes; the main reason is that there is supposed to be no conduction of heat between parts of the engine at different temperatures. He knew that the mere conduction of heat between bodies at different temperatures is a wasteful, irreversible process and must be eliminated if the heat engine is to have the maximum efficiency.
Regarding the second point, he also was quite certain that the maximum efficiency attainable did not depend upon the exact nature of the working fluid. He stated this for emphasis as a general proposition: "The motive power of heat is independent of the agents employed to realize it; its quantity is fixed solely by the temperatures of the bodies between which is effected, finally, the transfer of caloric." By "motive power of heat," we would today use the term "efficiency of a reversible heat engine," and by "transfer of caloric," we would mean the reversible transfer of heat." He knew intuitively that his engine would have the maximum efficiency, but was unable to state what that efficiency would be.
After the publication of his book in 1824, his book quickly went out of print and for some time his book was very difficult to obtain. For example, Kelvin had great difficulty in getting a copy of Carnot's book. Nowadays, his book in French can be downloaded electronically. An English translation of his book was published by Dover in 1960 and then reprinted by Peter Smith in 1977. Some of his posthumous manuscripts were also translated into English and included. (Please see reference.)
Carnot published his book in the days of steam engines. His theory explained why steam engines using superheated steam were better because of the higher temperature of the hot reservoir involved. Carnot's theory did not help to improve the efficiency of steam engines in the beginning; his theory only helped to explain why an existing practice was better. Only towards the end of the nineteenth century was Carnot's idea that the heat engine efficiency can be increased by increasing the temperature of the hot reservoir put into practice by, for example, Rudolf Diesel (1858-1913) who was quite fascinated by Carnot's theory and designed an engine (diesel engine) in which the temperature of the hot reservoir is much higher than that of a steam engine, resulting in an engine which is more efficient than a steam engine. (Reference: "The Diesel motor", Journal of the Franklin Institute, November 1901.) Thus eventually, Carnot's book had a real impact on the design of practical engines.