Hot-bulb engine

The hot-bulb engine is a type of internal combustion engine in which fuel is ignited by being brought into contact with a red-hot metal surface inside a bulb, followed by the introduction of air (oxygen) compressed into the hot-bulb chamber by the rising piston. There is some ignition when the fuel is introduced, but it quickly uses up the available oxygen in the bulb. Vigorous ignition takes place only when sufficient oxygen is supplied to the hot-bulb chamber on the compression stroke of the engine.

Most hot-bulb engines were produced as one-cylinder, low-speed two-stroke crankcase scavenged units.

The concept of this engine was established by Herbert Akroyd Stuart, an English inventor, at the end of the 19th century. The first prototypes were built in 1886 and production started in 1891 by Richard Hornsby & Sons of Grantham, Lincolnshire, England under the title Hornsby Akroyd Patent Oil Engine under licence. It was later developed in the United States by the German emigrants Mietz and Weiss, by combining it with the two-stroke engine developed by Joseph Day. Similar engines, for agricultural and marine use, were built by J. V. Svensons Automobilfabrik, Bolinders, Lysekils Mekaniska Verkstad, Pythagoras Engine Factory and many other factories in Sweden. Bolinder is now part of the Volvo group.

Akroyd-Stuart's heavy-oil engine (compared to spark-ignition) is distinctly different from Rudolf Diesel's better-known engine, where ignition is initiated through the heat of compression. An oil engine will have a compression ratio between 3:1 and 5:1, where a typical diesel engine will have a compression ratio ranging between 15:1 and 20:1. Furthermore, fuel is injected during the intake stroke and not at the end of the compression stroke as in a Diesel's compression ignition engine.

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