Diesel-powered vehicles

Sir,

Your article on diesel-powered vehicles in the January issue strayed somewhat from the path of historical accuracy in giving the credit for this engine to Rudolf Diesel alone. Indeed, Diesel's only contribution to the diesel engine we know today is the name, for the essential features, both of the present engine and of the true diesel engine, were described and patented by British engineers some years before Diesel's patent of 1892.

The basic feature of the diesel engine is, of course, the injection of liquid fuel directly to the cylinder in a finely-divided state so that it is ignited by the heat of compression. William Priestman patented an engine in which direct injection was employed to disperse a heavy fuel oil in 1885, a combination of hydraulic pressure and air blast being employed. Although Priestman never brought this engine to the level of reliability necessary for commercial production, and used an electric spark for ignition, he was certainly aware of the possibility of compression ignition and it was the air-blast injection technique that eventually permitted Diesel— or to be more accurate the engineers at Maschinenfabrik Augsberg-Nurnberg — to produce a successful compression ignition engine twelve years later. The subsequent Priestman engine—the first commercial engine to use a low-volatility fuel—powered the first successful motor boat (1892), the first successful i/c locomotive (1894) and a pioneer motor lorry (1897), achievements that have been largely forgotten.

Herbert Akroyd Stuart's patent of 1890 described an engine in which the fuel was injected and atomised by pressure alone and ignited by compression, which is precisely the principle of today's diesel engine. Unfortunately he could not obtain fuel pumps suitable for such an engine and resorted to a lower pressure coupled with a hot bulb for production engines, which were built by Richard Hornsby & Sons Ltd. The first Hornsby-Akroyd engine was going into regular service while the ink was drying on Diesel's "master patent"!

What Diesel did was to postulate an engine which embodied the ideas of several other engineers with the addition of two original ones of his own, namely the use of uncooled cylinders to obtain maximum cycle temperature and the use of pulverised coal instead of oil. After five years of fruitless experiments, which proved that his own ideas were impractical, he handed the job over to MAN who proceeded to make a success of it. A. F. Evans, in a history of the oil engine published in 1931, stated that Diesel actually purchased the idea of a compression ignition engine from the French engineer Emil Capataine, and as Evans was a professional engineer who knew most of the parties involved his opinions are worth noting. Diesel's real contribution was, like that of Baird to the development of television, not in the role of inventor but of demonstrator and salesman. He undoubtedly did much to publicise the engine and create the demand that stimulated research to produce a more compact unit suitable for automotive purposes.

By 1930 the wheel had turned a full circle, the perfection of fuel pumps making possible an airless-injection engine as postulated by Akroyd Stuart forty years earlier and rendering obsolete the more complicated "Diesel" engine. It is odd that the same thing has now happened with the petrol engine, for the originators of the four-stroke cycle (Beau de Rochas and Otto) believed that stratification was necessary to achieve maximum efficiency. If the Honda CVCC engine lives up to its promises their ideas, long held to be wrong, will have been vindicated.

Kenilworth C. R. Weaver