Nicolas Léonard Sadi Carnot (1 June 1796 – 24 August 1832) was a French physicist and military engineer who gave the first successful theoretical account of heat engines, later known as the Carnot cycle. He thereby layed the foundations of the second law of thermodynamics which caused him to be the "Father of thermodynamics", being responsible for such concepts as Carnot efficiency, Carnot theorem, Carnot heat engine, and others. Born in Paris, the first son of the eminent military leader and geometer, Lazare Nicholas Marguerite Carnot, elder brother of Hippolyte Carnot, and uncle of Marie François Sadi Carnot (President of the French Republic (1887-1894), son of Hippolyte Carnot). He was named after the Persian poet Sadi of Shiraz (Carnot 1960, p. xi), his third name being the name he went by. From age 16 (1812), he lived in Paris and 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. He later became an officer in the French army before committing himself to scientific research and becoming the most celebrated of Fourier's contemporaries being interested in the theory of heat. From 1814 on he served in the military. Following the final defeat of Napoleon in 1815, his father went into exile. Back in Carnots times the scientific study of the steam engine had 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 in 1712. Approximately 50 years later Watt established his well known advance to greatly increase the efficiency 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 which he described in his book. (Carnot 1960, p. 56) 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 not yet done. 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 the substitution with fluids other than steam, to improve the efficiency of engines. The Carnot cycle: Carnot proposed to answer two questions about the operation of heat engines: "Is the potential work available from a heat source potentially unbounded?" and "Can heat engines be improved when replacing the steam with some other working fluid or gas?" he tried answering these questions in a memoir, that was published in 1824 when he had reached the age of 28. the memoir was entitled “Réflexions sur la puissance motrice du feu” ("Reflections on the Motive Power of Fire"). In his book he intended to cover a wide range of topics on heat engines in a rather popular way. The equations were kept to a minimum and hardly called for anything beyond simple algebra and arithmetic, except for the footnotes randomly. He discussed the relative merits of air and steam as the working fluid, the merits of various points of steam engine design, and came up with a couple of ideas of his very own practical improvements. Yet the most important part of the book he devoted to a rather abstract presentation of the ideal engine that could be used to understand the fundamental principles applicability to all heat engines. Perhaps the most important contribution Carnot has given to thermodynamics was the process of abstraction of the essential features of the steam engine at the time to a general ideal heat engine. This resulted in a model thermodynamic system upon which exact calculations could be made. By doing so, he managed to lay out clear answers to his original two questions. He proofed the efficiency of this idealized engine to be a function 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. No thermal engine operating any other cycle can be more efficient, given the same operating temperatures. He realized by intuition that he could give very definite answers to his two main questions. The Carnot cycle is the most efficient possible engine, not only because of the absence of friction and other incidental wasteful processes but also because of the absence of conduction of heat between parts of the engine at different temperatures. He knew that the mere conduction of heat between bodies at different temperatures was a senseless, irreversible process which had to be eliminated in order for the heat engine to reach its maximum efficiency. He was furthermore quite certain that the maximum efficiency attainable did not depend upon the exact nature of the working fluid. This he stated 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 use the term "efficiency of a reversible heat engine," and by "transfer of caloric," we mean the reversible transfer of heat." He knew his engine would reach the maximum efficiency, but was unable to state what that efficiency would be. Therefore he concluded: The production of motive power in steam engines is due not to actual consumption of the caloric but to its transportation from a warm body to a cold body.In the fall of caloric the motive power evidently increases with the difference of temperature between the warm and cold bodies, but we do not know whether it is proportional to this difference.

Source: Wikipedia