page_title: Herbert Akroyd Stuart
section_title: Akroyd engine and Diesel engine
hierarchical_section_title: Herbert Akroyd Stuart / Oil engines / Akroyd engine and Diesel engine
caption_attribution_description: Plaque in Denmark Street, Bletchley, UK, commemorating the work of Herbert Akroyd Stuart.
context_page_description: Herbert Akroyd-Stuart was an English inventor who is noted for his invention of the hot bulb engine, or heavy oil engine.
context_section_description: Both the Diesel engine, and the Akroyd engine run the same kind of fuel, petroleum oil, which has led to a dispute about whether or not the Diesel engine is based upon the Akroyd engine. The fact that the Diesel engine's operating principle differs from the operating principle Rudolf Diesel describes in his essay Theory and Construction of a Rational Heat Motor further contributed to this. The Akroyd engine was the first functional internal combustion engine that could use petroleum oil as fuel. It was operational in 1891, six sears before the Diesel engine ran for the first time. However, after the Diesel engine had proven successful, "Diesel engine" became the synonym for an engine that ran on any sort of petroleum oil. "Oil engines" that used the Akroyd operating principle were called "Semi-Diesel", and the name "Akroyd", which had been associated with oil engines, fell out of use. Therefore, Herbert Akroyd Stuart sought to replace the term Diesel engine with Akroyd engine in the early 20th century.
  Herbert Akroyd Stuart had two patents, No. 7146 Improvements in Engines Operated by the Explosion of Mixtures of Combustible Vapour or Gas and Air, and No. 15994. In the former, the Akroyd engine's operating principle is described as follows: "... at the desired part of this compression stroke, the supply of liquid hydrocarbon is forced, in a spray form, on to the heated vaporiser, which almost instantly changes it into a gas...". Early Akroyd engines indeed operated on this principle. Rudolf Diesel had a patent on the combustion process described in his essay (DRP 67207). The Diesel engine does neither operate on the process described in the Akroyd patent, nor on the process described in the DRP 67207 patent. It operates on a different operating principle, also invented by Rudolf Diesel (patented in 1893, DRP 82168), which is why Diesel is in fact the Diesel engine's inventor. However, Diesel never admitted that his engine operated on a "secret" operating principle, and claimed that the Diesel engine operates on the (impossible) operating process described in the DRP 67207 patent.
  The key difference between Akroyd and Diesel engine is the ignition: In an Akroyd engine, an ignition device – the so-called "hot bulb" – ignites the fuel, because the compression is too low for compression ignition (<300 kPa). A Diesel engine on the other hand has no ignition devices, the fuel ignites only because of the engine's high compression (>3000 kPa). Since high compression is a key to good efficiency, the Akroyd engine has a much higher fuel consumption (approximately 80% higher) than a Diesel engine.
caption: Plaque in Denmark Street, Fenny Stratford, UK, commemorating the work of Herbert Akroyd Stuart
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