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The hot bulb engine is a type of internal combustion engine; more specifically, it is a compression ignition engine, in which the fuel is ignited by being suddenly exposed to high temperature and the pressure of a compressed gas, rather than by a separate source of ignition, such as a spark plug, as is the case in the gasoline engine.

It was invented by Herbert Akroyd Stuart in 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 USA by the German emigrants Miez and Weiss by combining it with the two-stroke engine developed by Robert Day.

Akroyd-Stuart's compression ignition engine (compared to spark-ignition) was invented two years earlier than Rudolf Diesel's better-known engine working on similar principles.

The engines were usually one cylinder, four-stroke units, although following Miez & Weiss' developments in the USA, 2-stroke versions were constructed.

Operation and Working Cycle

The hot-bulb engine shares it's basic layout with nearly all other internal combustion engines, in that it has a piston inside a cylinder connected to a flywheel via a connecting rod and crankshaft. The flow of gases through the engine is controlled by valves.

The main feature of the hot-bulb engine is the vaporiser or 'hot-bulb]], a chamber usually cast into the engine block and attatched to the main cylinder by a narrow opening. Prior to starting the engine from cold, this vaporiser is heated externally by a blow-lamp or slow-burning wick for as much as half an hour. The engine is then turned over, usually by hand but sometimes by compressed air or an electric motor.

Air is drawn into the cylinder through the intake valve as the piston descends and then forced through the top of the cylinder as the piston rises, through the opening into the hot-bulb, where it is compressed and therefore its temperature rises. Fuel is forced into the hot-bulb through an injector by a mechanical pump, where it ignites due to the heat of the compressed air and the heat applied to the hot-bulb prior to starting. The fuel ignites, driving the piston down. This is the power stroke. The piston's action is transferred to the crankshaft and flywheel, to which equipment can be attatched for work to be performed. The piston rises again and the exhaust gases are expelled through the exhaust valve. The cycle then starts again.

Once the engine is running, the heat of compression and ignition maintains the hot-bulb at the neccessary temperature and the blow-lamp or other heat source can be removed. From this point the engine requires no external heat and requires only a supply of air, fuel oil and lubricating oil to run.

Advantages

At the time the hot-bulb engine was invented, it's great attractions were it's economy, simplicity and ease of operation in comparison to the steam engine, then the dominant source of power in industry. Steam engines acheived an average thermal efficency (the amount of heat generated that is actually turned into useful work) of around 6%. Hot-bulb engines could easily acheive 12% thermal efficency.

The hot-bulb engine is much simpler than the steam engine to construct and operate. Steam engines require at least one person to monitor the boiler and add water and fuel as needed. If fitted with automatic lubrication systems and a governor to control the fuel supply, a hot-bulb engine could be left unattended for hours at a time once running.

Compared to both steam and gasoline (petrol) engines, hot-bulb engines are simpler and therefore have less potentional problems. There is no electrical system as found on a petrol engine, and no external boiler and steam system as on a steam engine.

A big attraction with the hot-bulb engine was its ability to run on a wide range of fuels. Even poor-burning fuels could be used since a combiantion of vaporiser- and compression-ignition meant that such fuels could be made to combust. The usual fuel used was Fuel Oil, similar to modern-day diesel, but natural gas, kerosene, paraffin, crude oil, vegetable oil, creosote and even in some cases coal dust were used in hot-bulb engines. This made the hot-bulb engine very cheap to run, since it could be run on cheaply available fuels. Some operators even ran engines on used engine oil, thus providing almost free power.Recently, this multi-fuel ability has led to an interest in using hot bulb engines in developing nations where they can be run on locally produced biofuel.

Uses

The reliability of hot-bulb engine, their ability to run on many fuels and the fact that they can be left running for hours or days at a time made them extremely popular with agricultural and forestry users, where they were used for pumping and powering milling, sawing and threshing machinery. Hot-bulb engines were used on road-rollers and tractors such as the Lanz Bulldog and the Landini.

Due to the lengthy pre-heating time, hot-bulb engines only found favour with users who needed to run engines for long periods of time, where the pre-heating process only represented a small percentage of the overall running period. This included marine use (especially in fishing boats), electricity generation (especially in remote areas where coal was not easily available for steam engines) and pumping duties.

Replacement

From around 1910, the diesel engine was improved dramatically, with more power being available at greater efficencies than the hot-bulb engine could manage (Diesel engines can acheive nearly 50% efficency is designed with maximum economy in mind). Diesel engines offered greater power for a given engine size due to the more efficent combustion method (they had no hot-bulb, relying purely on compression-ignition) and greater ease of use as they required no pre-heating.

The hot-bulb engine was limited in its scope in terms of speed and overall power-to-size ratio. To make a hot-bulb engine capable of powering a ship or locomotive, it would have been prohibitivley large and heavy. The hot-bulb engines used in Landini tractors were as much as 20-litres in capacity for relativly low power outputs. Hot-bulb engines are difficuly to make in multi-cylinder versions as well as creating even combustion throughout the multiple hot-bulbs is a complex business.

With the development of small-capacity, high-speed diesel engines in the 1930s and 1940s, hot-bulb engines fell dramatically out of favour. The last large-scale manufacturer of hot-bulb engines stopped producing them in the 1950s and they are now virtually exctinct in commercial use, except in very remote areas of the developing world.

Difference from the Diesel Engine

The hot-bulb engine is often confused with the diesel engine, and it is true that the two engines are very similar.

The main difference is that:

-The hot-bulb engine uses both compression-ignition and the heat retained in the vaporiser to ignite the fuel.

-The diesel engine uses just compression-ignition to ignite the fuel, and it operates at pressures many times higher than the hot-bulb engine.

Due to the much greater and longer-term success of the diesel engine, today hot-bulb engines are sometimes called 'semi-diesels' because they partly use compression-ignition in their cycle.

However, Diesel's original engine design used compressed air to blast the fuel into the cylinder. This complex and heavy system limited the speed the engine could run at and the minimum size a diesel engine could be built to. Only when Akroyd-Stuart's mechanical pump-and-injector system that he developed for his hot-bulb engine was adapted by Robert Bosch for use in diesel engines were high-speed diesel engines practical.

External links

• The Stationary Engine Club of Sweden
• A video on the history of the Diesel Engine, which also contains a demonstration and illustration of the hot-bulb engine's working cycle
• An article from Gas Engine Magazine on Mietz & Wiess hot-bulb engines

• Engine technology