In a memoir published, by the Institution of Mechanical Engineers it is stated that "the achievements of Herbert Akroyd Stuart form a landmark in British engineering." The most notable of these was his invention of the compression-ignition heavy oil engine so widely used to-day and more often than not confused with the diesel engine, which came into existence at least seven years after it.
Herbert Akroyd Stuart was the son of an engineer, Charles Stuart, who had a small works at Bletchley (Bucks). Akroyd Stuart was born in Yorkshire on January 28,1864. He received a good general education at the Grammar School, Newbury (Berks), and a sound technical education at the City and Guilds of London College, Finsbury, a well-known centre of advanced engineering study in his day. Later he was employed in his father's works, to the management of which he succeeded on the death of his father.
Stuart was at first employed on general work, and it is recorded that his attention was directed to the possibilities of the heavy oil engine by an accident in which he narrowly escaped severe burns. Examining some machinery for tinning steel plates he dropped some oil on to molten metal, and the resultant vapour immediately burst into flame on contact with a light he carried. The possibility of using oil vapour in an engine occurred to him and he started to experiment. This was in 1886.
The gas engine had been successfully introduced some ten years before, and Priestman Bros., of Hull, were working on an engine running on ordinary paraffin oil such as is used in domestic lamps. This engine was put on the market in 1888. But in the gas engine and in the paraffin engine the charge of gas, or oil vapour, and air drawn together into the cylinder had to be ignited from some external source, as has the petrol engine used in the ordinary motor car.
Stuart started on the principle of externally-derived ignition and tried a number of methods of effecting it, for he shared the belief (then current) that the oil vapour and the air forming the charge had to be thoroughly mixed before it was compressed by the return stroke of the piston and ignited. He had, however, a good deal of trouble from premature ignition, and finally thought of the novel plan of filling the cylinder with air alone on the first part of the suction stroke, and compressing it during the latter part, this compression causing, by a natural law, the evolution of heat.
The compressed air in the heavy oil engine is at a sufficiently high temperature towards the end of the suction stroke to ignite the charge when this is introduced into it in a fine spray. The discovery of this fact is entirely due to Herbert Akroyd Stuart. The charge is sprayed by means of a pump and ignition takes place when the piston may be said to be momentarily pausing before returning on the working stroke. The charge is thus burned when the volume of the compression space is virtually constant.
The principle was fully developed by 1890, in which year Stuart took out a patent for the engine. By the end of that year the first dozen automatically firing heavy oil engines were built. Another distinct novelty in the Akroyd Stuart engine was the adoption of a separate compression chamber united to the cylinder end by a bottle neck ; and yet another, the cooling of the compression chamber by a water jacket. In 1891 Stuart granted a licence to Richard Hornsby and Sons, of Grantham (Lines), for the manufacture of the engine which, with some modifications in design, was then known as the Hornsby-Akroyd engine.
The engine which was patented by Diesel in 1892 did not assume a really practicable form until 1897. In this engine the oil was injected not by a pump but by a jet of compressed air at a high pressureâ€”from 1,000 lb. to 1,200 lb. per sq. in. This involved the addition of a multi-stage air compressor which, not infrequently, gave rise to numerous troubles.
When the last of Stuart's master patents had run out in 1906 a number of engine builders took up his "airless injection" principle and, with some differences in detail, the type has now become an important prime mover on land, at sea and in the air.
Why the true inventor has not been more widely recognized is puzzling. The late Professor William Robinson said : "It is interesting to note that Germans are now building the British airless-injection engine, and all the credit is given to Diesel. Not only so, but the Akroyd precombustion chamber with the narrow passage to give forced turbulence ... is used to-day as a new device and patented with slight modification." This was true in 1930 and the position has not altered since that date. Moreover, Stuart's invention of the water-cooled combustion chamber in 1892 has been generally used since the patent lapsed without acknowledgment of its origin.
Herbert Akroyd Stuart would, therefore, seem to have initiated those changes in oil engine construction which are most likely to endure, though recognition of his genius is both scant and tardy. Having emigrated to Australia in the year 1900, he died at Claremont, Western Australia, in 1927.
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