Herbert Akroyd Stuart

Herbert Akroyd-Stuart (28 January 1864 – 19 February 1927) was an English inventor who is noted for his invention of the hot bulb engine, or heavy oil engine.

Akroyd-Stuart was born in Halifax, Yorkshire, but lived in Australia for a period in his early years. He was educated at Newbury Grammar School (now St. Bartholomew's School) and Finsbury Technical College in London. He was the son of Charles Stuart, founder of the Bletchley Iron and Tinplate Works, joining his father in the business in 1887.

In 1885, Akroyd Stuart accidentally spilt paraffin oil (kerosene) into a pot of molten tin. The paraffin oil vaporised and caught fire when in contact with a paraffin lamp. This gave him an idea to pursue the possibility of using paraffin oil (very similar to modern-day diesel) for an engine, which unlike petrol proved difficult to vaporise in a carburettor because its volatility is insufficient.

His first prototype engines were built in 1886. In 1890, in collaboration with Charles Richard Binney, he filed Patent 7146 for Richard Hornsby and Sons of Grantham, Lincolnshire, England. The patent was entitled: Improvements in Engines Operated by the Explosion of Mixtures of Combustible Vapour or Gas and Air. One such engine was sold to Newport Sanitary Authority, but the compression ratio was too low to get it started from cold, and it needed a heat poultice to get it going.

Akroyd-Stuart's engines were built from 26 June 1891 by Richard Hornsby and Sons as the Hornsby Akroyd Patent Oil Engine under licence and were first sold commercially on 8 July 1892. It was the first internal combustion engine to use a pressurised fuel injection system.

The Hornsby–Akroyd engine used a comparatively low compression ratio, so that the temperature of the air compressed in the combustion chamber at the end of the compression stroke was not high enough to initiate combustion. Combustion instead took place in a separated combustion chamber, the vaporizer (also called the hot bulb) mounted on the cylinder head, into which fuel was sprayed. It was connected to the cylinder by a narrow passage and was heated either by the cylinder's coolant or by exhaust gases while running; an external flame such as a blowtorch was used for starting. Self-ignition occurred from contact between the fuel-air mixture and the hot walls of the vaporizer. By contracting the bulb to a very narrow neck where it attached to the cylinder, a high degree of turbulence was set up as the ignited gases flashed through the neck into the cylinder, where combustion was completed. As the engine's load increased, so did the temperature of the bulb, causing the ignition period to advance; to counteract pre-ignition, water was dripped into the air intake.

Hot bulb engines were produced until the late 1920s, often being called semi-diesels, even though they were not as efficient as compression ignition engines. They had the advantage of comparative simplicity, since they did not require the air compressor used by early Diesel engines; fuel was injected mechanically (solid injection) near the start of the compression stroke, at a much lower pressure than that of Diesel engines.

Richard Hornsby and Sons built the world's first oil-engined railway locomotive LACHESIS for the Royal Arsenal, Woolwich, England, in 1896. They also built the first compression-ignition powered automobile.