Editing Carnot heat engine (section)

==The Carnot engine and Rudolf Diesel==
In 1892 [[Rudolf Diesel]] patented an [[internal combustion engine]] inspired by the Carnot engine. Diesel knew a Carnot engine is an ideal that cannot be built, but he thought he had invented a working approximation. His principle was unsound, but in his struggle to implement it he developed a practical [[Diesel engine]].

The conceptual problem was how to achieve isothermal expansion in an internal combustion engine, since burning fuel at the highest temperature of the cycle would only raise the temperature further. Diesel's patented solution was: having achieved the highest temperature just by compressing the air, to add a small amount of fuel at a controlled rate, such that heating caused by burning the fuel would be counteracted by cooling caused by air expansion as the piston moved. Hence all the heat from the fuel would be transformed into work during the isothermal expansion, as required by Carnot's theorem.

For the idea to work a small mass of fuel would have to be burnt in a huge mass of air. Diesel first proposed a working engine that would compress air to 250 atmospheres at {{cvt|800|°C|°F|round=50}}, then cycle to one atmosphere at {{cvt|20|°C|°F|round=50}}. However, this was well beyond the technological capabilities of the day, since it implied a compression ratio of 60:1. Such an engine, if it could have been built, would have had an efficiency of 73%. (In contrast, the best steam engines of his day achieved 7%.)

Accordingly, Diesel sought to compromise. He calculated that, were he to reduce the peak pressure to a less ambitious 90 atmospheres, he would sacrifice only 5% of the [[thermal efficiency]]. Seeking financial support, he published the "Theory and Construction of a Rational Heat Engine to Take the Place of the Steam Engine and All Presently Known Combustion Engines" (1893). Endorsed by scientific opinion, including [[Lord Kelvin]], he won the backing of [[Krupp]] and {{lang|de|italic=no|[[Maschinenfabrik Augsburg-Nürnberg|Maschinenfabrik Augsburg]]}}. He clung to the Carnot cycle as a symbol. But years of practical work failed to achieve an isothermal combustion engine, nor could have done, since it requires such an enormous quantity of air that it cannot develop enough power to compress it. Furthermore, controlled fuel injection turned out to be no easy matter.

Even so, the Diesel engine slowly evolved over 25 years to become a practical high-compression air engine, its fuel injected near the end of the compression stroke and ignited by the heat of compression, capable by 1969 of 40% efficiency.<ref>{{cite journal|last=Bryant|first=Lynwood|title=Rudolf Diesel and His Rational Engine|journal=Scientific American|volume=221|issue=2|date=August 1969|pages=108–117|doi=10.1038/scientificamerican0869-108 |jstor=24926442|bibcode=1969SciAm.221b.108B }}</ref>