Theory and Construction of a Rational Heat Motor (; English: Theory and construction of a rational heat motor with the purpose of replacing the steam engine and the internal combustion engines known today) is an essay written by German engineer Rudolf Diesel. It was composed in 1892, and first published by Springer in 1893. A translation into English followed in 1894. One thousand copies of the German first edition were printed. In this essay, Rudolf Diesel describes his idea of an internal combustion engine based on the Carnot cycle, transforming heat energy into kinetic energy using high pressure, with a thermal efficiency of up to 73%, outperforming any steam engine of the time.
Diesel sent copies of his essay to famous German engineers and university professors for spreading and promoting his idea. He received plenty of negative feedback; many considered letting Diesel's heat engine become reality unfeasible, because of the high pressures of 200–300 atm (20.3–30.4 MPa) occurring, which they thought machines of the time could not withstand. Only few found the actual mistake in Diesel's theory: Isothermal-adiabatic compression, which the theory is based on, is impossible. Even with almost isothermal-adiabatic compression, an engine could not operate because of the lean air-fuel mixture. In other words, an engine as described in the essay would require so much compression work that it could not perform any useful work.
Yet, some scientists of the time praised Diesel's idea, which would lead into Maschinenfabrik Augsburg and Krupp Essen forming a consortium for building Diesel's engine. Diesel, who was then ordered to build his own engine, realised his mistake and considered using a modified combustion process. Key changes are the way of compression, which is only adiabatic in the modified combustion process, the pressure, which Diesel reduced significantly, and the fuel injection, where Diesel increased the fuel quantity. In 1897, after four years of work, Diesel had successfully finished his rational heat motor using his modified combustion process. This engine became known as the Diesel engine. Publicly, Diesel never admitted that he had to use a different combustion process from that one he described in his essay, because this would have rendered his heat motor patent obsolete.
Content summary
Theory and Construction of a Rational Heat Motor has nine chapters in total. The first chapter describes the theory of combustion, and is separated into five individual combustion processes, out of which the third is the constant pressure process used for the rational heat motor. Therefore, it is described in more detail in this article. In the second chapter, Diesel describes how he intends to design and build an engine with an indicated power of 100 PS. With the third chapter, Diesel tries to address using a process with adiabatic compression only; the fourth chapter describes designing a real motor for this modified process. The fifth chapter addresses yet another modified process, with an incomplete expansion phase, but Diesel does not include a design concept. Furthermore, he considers a completely closed cycle in the sixth chapter and using his invention as a refrigerator in the seventh chapter. His theories on how to use a rational heat motor are described in the eighth chapter. The ninth chapter includes additional comments. His additional work Nachträge zur Bröschüre is not included in the original essay, but in newer editions, it serves as a tenth chapter.
Theorie der Verbrennung (Theory of combustion) Konstruktion des vollkommenen Motors (Construction of the ideal motor) Erste Abweichung vom vollkommenen Process (First deviation from the ideal process) Konstruktion des Motors für den abweichenden Process (Construction of the motor for the deviating process) Andere Abweichungen vom vollkommenenen Process (Additional deviations from the ideal process) Der neue Motor als geschlossene Maschine (The new motor in a closed cycle) Der neue Motor als Kälteerzeugungsmaschine (The new motor as a refrigerator) Anwendungen des neuen Motors (Applications of the new motor) Schlussbemerkungen (Conclusions) Nachträge zur Broschüre (Supplement of the brochure)
Theory of combustion and construction of the ideal motor
The principle
Diesel's idea of a rational heat motor was designing a cycle that would allow maximum heat utilisation, based on the Carnot cycle. To overcome the low efficiency of steam and combustion engines of the time, Diesel wanted to build an entirely new type of internal combustion engine. In the 1890s, regular gas engines were capable of transforming only 6% of the fuel energy into kinetic energy; good triple expansion steam engines were slightly better than that, they could convert 7.2% of the fuel energy into kinetic energy. Diesel said that his rational heat motor has a thermal efficiency of 73%, thus being capable of converting approximately ″6 to 7-times as much″ chemical energy into kinetic energy, meaning that it has an efficiency of approximately 50%. Diesel even claimed that future versions of his motor would have an even higher efficiency.
Despite relying on compression ignition, Diesel says that he never purposely designed his motor with this specific characteristic. In his patent DRP 67 207, Diesel describes that a motor with the best thermal efficiency would automatically have compression ignition. In his 1913 book Die Entstehung des Dieselmotors, he denies that compression ignition is a key feature of his motor:
On page 16 of Theory and Construction of a Rational Heat Motor, Diesel writes that ignition in his rational heat motor takes place either by means of artificial ignition or compression ignition:
The third process
The Diesel process is a hypothetical constant-pressure model, with four distinct processes, a so-called cycle, meaning that these four distinct processes can be repeated over and over again. These distinct processes are the same processes that can be found in a four-stroke engine: intake, compression, combustion, exhaust. All four strokes combined form the cycle, which is supposed to result in useful work. The Diesel process uses a special compression stroke based on the idea that a gas can be compressed in a combined isothermal-adiabatic way. Isothermal means that the temperature during compression does not change, thus requiring heat dissipation; adiabatic means that the gas changes its volume, but without heat dissipation. This means that there is no waste heat.
For the means of compression, Diesel intended using a notional compression cylinder. This process requires work and consists of four phases: Intake, sucking in uncompressed air, requiring work. Compression with an isothermal curve (or a different curve), dissipating heat and requiring work. Compression with an adiabatic curve, requiring work. Emitting the combustion air, requiring work.
For the distinct combustion process, Diesel intended using a notional expansion cylinder. Again it consists of four phases: Gas entering with constant pressure, requiring work. Combustion with any curve, producing work, dissipating heat. Expansion of the gas with an adiabatic curve until atmospheric pressure is reached, producing work. Exhausting the gas, requiring work.
Adding these phases will result in a diagram similar to a Carnot diagram as shown on the right. Because fuel will be added to the gas, the start position 1 will not be identical with the end position 1, meaning that there is always slightly more work required. However, Diesel considered using a very lean air-fuel mixture, thus resulting in the amount of extra work required being insignificant. In theory, the combustion process ends at position 4 of the diagram. But this is not the end of the work formation taking place in the two phases combustion (3–4) and expansion (4–1), as explained. Diesel considered an isothermal expansion phase unfeasible, because it would cause a gigantic expansion cylinder, resulting in a very large and unpractical engine. This is why gas expansion is adiabatic and only taking place until atmospheric pressure is reached. He considered the additional work required, resulting in overall work loss, ″unimportant″, because he considered making a real motor more reasonable than focussing on the best efficiency. This resulted in the original Diesel diagram.
Wrong conclusion
Diesel's theory had three major problems:
Isothermal-adiabatic compression, which is supposed to result in high efficiency, is not possible; hence, Diesel later revised his process. To reach a high thermal efficiency, Diesel intended using a high compression of 200–300 atm (20.3–30.4 MPa), which would result in too much friction loss. As thermal efficiency does not depend on the temperatures, Diesel intended reducing the combustion temperature by using a lean air-fuel mixture of 99.324:1, which would not allow combustion.
In fact, a high compression ratio increases efficiency, however, only to a certain point, because, like Diesel figured, too much heat energy would have to be dissipated and too much friction would occur, which could not be compensated by the engine's work. On the other hand, a compression ratio chosen too low results in insufficient heat utilisation. When designing his theory, Diesel already considered reducing compression to 90 atm (9.1 MPa), which he thought would result in only 5% thermal efficiency loss, but in a significant increase in actual efficiency, yet he recommends increasing pressure as much as possible. His solution for the heat dissipation problem was still wrong: He decided to use more air, resulting in an air-fuel mixture which is too lean. Such an air-fuel mixture cannot provide any work, because it cannot combust, not even with artificial ignition.
Criticism
As mentioned, Diesel was mostly criticised for his idea of a heat motor, but also received positive feedback. However, most critics did not criticise the theory's flaw, but that Diesel's heat engine used very high amounts of pressure to operate. Diesel himself acknowledged the feedback:
Wilhelm Züblin, engineer of Sulzer, and Professor Alois Riedler came to the conclusion that Diesel's motor could require so much compression work that it could possibly not perform any useful work. In his 1887 work Theorie der Gasmotoren, Otto Köhler had already addressed that an ideal cycle is not suitable for a real motor, coming to the same conclusion as the former. He had foreseen the problem of friction loss rendering the motor work useless and, in a letter addressed to Diesel's friend Venator, he considered the pressure required too high:
Other critics rather feared that the material would not withstand the enormous strain, but otherwise did not criticise the mistake of Diesel's theory:
Others praised Diesel and the theory:
Diesel realising his theory′s flaw