hot bulb

Hot bulb engine


The hotbulb, or hot bulb engine or vaporizing oil engine is a type of internal combustion engine. It is a surface ignition engine in which the superheated fuel is ignited by being brought into contact with oxygen-rich fresh air, rather than by a separate source of ignition, such as a spark plug.

It was perfected 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 the USA 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 Bolinder and Pythagoras engine factory in Sweden. Bolinder is now part of the Volvo group.

Akroyd-Stuart's vaporizing 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 of about 3:1, where a typical Diesel engine will have a compression ratio ranging between 15:1 to 20:1.

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

Operation and working cycle

The hot-bulb engine shares its 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 majority operate on the standard 4-stroke cycle of an Induction Stroke, a Compression Stroke, a Power Stroke and an Exhaust Stroke.

The main feature of the hot-bulb engine is the vaporiser or hot-bulb, a chamber usually cast into the engine block and attached 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 (on later models sometimes electric heating or pyrotechnics was used). The pre-heating time depended on the engine design, the type of heating used and the ambient temperature, but generally ranged from 2-5 minutes for most engines in a temperate climate, to as much as half an hour if operating in extreme cold or the engine was especially large. 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 (The Induction Stroke). During the same stroke, fuel is sprayed into the hot-bulb by a mechanical jerk-pump through a nozzle. Through the action of the sprayer and the heat of the hot-bulb, the fuel instantly vaporises. The air in the cylinder is then forced through the top of the cylinder as the piston rises (The Compression Stroke), through the opening into the hot-bulb, where it is compressed and therefore its temperature rises. The vaporised fuel mixes with the compressed air and ignites due to the heat of the compressed air and the heat applied to the hot-bulb prior to starting. The resulting pressure drives the piston down (The Power Stroke). The piston's action is converted to a rotary motion by the crankshaft-flywheel assembly, to which equipment can be attached for work to be performed. The flywheel stores momentum, some of which is used to turn the engine over during the three strokes when power is not being produced. The piston rises again and the exhaust gases are expelled through the exhaust valve (The Exhaust Stroke). The cycle then starts again.

Once the engine is running, the heat of compression and ignition maintains the hot-bulb at the necessary 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. The fact that the engine could be left unattended for long periods whilst running made hot bulb engines popular choices for powering generators and pumps.


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

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

Another attraction was their safety. A steam engine, with its exposed fire and hot boiler, steam pipes and working cylinder could not be used in flammable conditions such as munitions factories or fuel refineries. Hot-bulb engines also produced cleaner exhaust fumes. A big danger with the steam engine was that if the boiler pressure grew too high and the safety valve failed, a highly dangerous explosion could occur (although this was a relatively rare occurrence by the time the hot-bulb engine was invented). A more common problem was that if the water level in the boiler of a steam engine was allowed to drop too low, the internal structure of the boiler could collapse or melt, also causing dangerous release of high pressure gas. If a hot bulb engine ran out of fuel, it would simply stop. The cooling water was usually a closed circuit, so no water loss would occur unless there was a leak. If the cooling water ran low, the engine would seize through overheating- a major problem, but it carried no danger of explosion.

Compared to both steam and gasoline (petrol) engines, hot-bulb engines are simpler and therefore have less potential 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 combination 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.

Due to the lengthy pre-heating time, hot-bulb engines were nearly always guaranteed to start quickly, even in extremely cold condtions. This made them popular choices in cold regions such as Canada and Scandinavia, where steam engines were not viable and early gasoline and diesel engines could not be relied on to operate.


The reliability of the 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, forestry and marine users, where they were used for pumping and for powering milling, sawing and threshing machinery. Hot-bulb engines were also used on road rollers and tractors.

J.V. Svensons Motorfabrik, i Augustendal in Stockholm Sweden used hot bulb engines in their Typ 1 motor plough, produced from 1912 to 1925. Munktells Mekaniska Verkstads AB, in Eskilstuna, Sweden, produced agricultural tractors with hot bulb engines from 1913 onwards. Heinrich Lanz Mannheim AG, in Mannheim, Germany, started to use hot bulb engines in 1921, in the Lanz Bulldog HL. Other well known tractor manufacturers that used bulb engines were Bubba, Gambino, Landini and Orsi in Italy, HSCS in Hungary, SFV in France Ursus in Poland, and Marshall in England.

At the start of the 20th century there were several hundreds of European manufacturers of hot bulb engines for marine use. In Sweden alone there were over 70 manufacturers, of which Bolinders is the best known (in the 1920s they had about 80% of the world market). The Norwegian SABB was a very popular hot bulb engine for small fishing boats and many of them are still in working order. In America Standard, Weber, Reid, Stickney, Oil City, and Fairbanks Morse built hotbulb engines.

A limitation of the design of the engine was that it could only run over quite a narrow (and slow) speed band, typically 50-300 R.P.M.. This made the hot-bulb engine difficult to adapt to automotive uses other than vehicles such as tractors, where speed was not a major requirement. This limitation was of little consequence for stationary applications, where the hot-bulb engine was very popular.

Owing 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) and pumping/drainage duties.

The hot-bulb engine was invented at the same time that dynamos and electric light systems were perfected, and electricity generation was one of the hot-bulb engines main uses. The engine could achieve higher R.P.M. than a standard reciprocating steam engine (although high-speed steam engines were developed during the 1890s), and its low fuel and maintenance requirements (including the ability to be operated and maintained by only one person) made it ideal for small-scale power supply. Generator sets driven by hot-bulb engines were installed in numerous large houses (especially in rural areas) in Europe, as well as in factories, theatres, lighthouses, radio stations and many other locations where a centralised electricity grid was not available. Usually the dynamo or alternator would be driven off the engine's flywheel by a flat belt, to allow the necessary 'gearing up'- making the generator turn at a faster speed than the engine. Companies such as Armstrong Whitworth and Boulton Paul manufactured and supplied complete generating sets (both the engine and generator) from the 1900s to the late 1920s, when the formation of national grid systems throughout the world and the replacement of the hot-bulb engine by the diesel engine caused a drop in demand.

The engines were also used in areas where the fire of a steam engine would be an unacceptable fire risk. Akroyd-Stuart developed the world's first oil-engined locomotive (the 'Lachesis') for the Royal Arsenal, Woolwich, where the use of locomotives had previously been impossible due to the risk. Hot-bulb engines proved very popular for industrial engines in the early 20th century, but lacked the power to be used in anything larger.

Compression ignition

Herbert Akroyd Stuart was always keen to improve the efficiency of his engine. The obvious way to do this was to raise the compression ratio to increase the engine's thermal efficiency. However, above ratios of around 8:1 the fuel oil in the vapouriser would ignite before the piston reached the limit of its travel. This pre-detonation caused rough running, power loss and ultimately engine damage (see engine knocking for more information). Working with engineers at Hornsby's, Akroyd Stuart developed a system whereby the compression ratio was increased to as much as 18:1 and fuel oil was delivered to the cylinder only when the piston reached top dead centre, thus preventing pre-ignition.

This system was patented in October 1890 and development continued. In 1892 (5 years before Rudolf Diesel's first prototype), engineers at Hornsby's built an experimental engine. The vapouriser was replaced with a standard cylinder head and used a high-pressure fuel nozzle system. The engine could be started from cold and ran for 6 hours, making it the world's first internal combustion engine to run on purely compression ignition. However, to build a fully practical fuel injection system required using machining techniques and building to tolerances that were not possible to mass produce at the time. Hornsby's was also working at full capacity building and selling hot-bulb engines, so these developments were not pursued.


From around 1910, the diesel engine was improved dramatically, with more power being available at greater efficiencies than the hot-bulb engine could manage (Diesel engines can achieve nearly 50% efficiency if designed with maximum economy in mind). Diesel engines offered greater power for a given engine size due to the more efficient 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 prohibitively large and heavy. The hot-bulb engines used in Landini tractors were as much as 20-litres in capacity for relatively low power outputs. Hot-bulb engines are difficult to make in multi-cylinder versions, and creating even combustion throughout the multiple hot-bulbs is a complex business. The hot-bulb engine's low compression ratio in comparison to diesel engines limited its efficiency, power output and speed. Most hot-bulb engines could run at a maximum speed of around 100 rpm, whilst by the 1930s diesel engines capable of 2,000 rpm were being built. Also, due to the design of hot bulb and the limitatations of current technology in regards to the injector system, most hot-bulb engines were single-speed engines, running at a fixed speed, or in a very narrow speed range. Diesel engines can be designed to operate over a much wider speed range, making them more versatile. This made these medium-sized diesels a very popular choice for use in generator sets, replacing the hot-bulb engine as the engine of choice for small-scale power generation. The Hot tube engine addresses the speed limitation and gave great flexibility in operation, although the solution induced a source of weakness in the design.

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 extinct in commercial use, except in very remote areas of the developing world. An exception to this is marine use – hot-bulb engines were widely fitted to inland barges and narrowboats in continental Europe and the UK. The UK's first motor narrowboats Cadbury 1 and Cadbury 2 (1911) were powered by Bolinder single-cylinder hot-bulb engines, and this type became common between the 1920s and the 1950s. With hot-bulb engines being generally long-lived and ideally suited to such a use, it is not uncommon to find vessels still fitted with their original hot-bulb engines today.

Ignoring the obvious differences (electrical heating, differing fuels, high RPMs – at least in the small model aircraft types) the modern Glow Plug engine could be considered the latest incarnation of these "hot spot" ignition based engines.

Differences 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. Aside from the obvious lack of a hot-bulb vaporiser in the diesel engine, the main differences are 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' or 'semi diesel' because they partly use compression-ignition in their cycle.

When both types of engines were made and sold in large numbers, both were classed as 'oil engines', since they ran on fuel oil. Hot-bulb engines were often known as 'Hot Start Oil Engines', because they had to be pre-heated. Similarly, diesel engines were known as 'Cold Start Oil Engines', because they could be started with the engine cold.

There is also a crucial difference in the timing of the fuel injection process:

  • In the hot-bulb engine, fuel is sprayed into the vapouriser during the Induction Stroke as air is drawn into the cylinder.
  • In the diesel engine, fuel is injected into the cylinder in the final stages of the Compression Stroke.

There is a detail difference in the method of fuel injection:

  • The hot-bulb engine uses a medium-pressure mechanical jerk pump to deliver fuel to the cylinder through a sprayer- a simple multi-holed nozzle.
  • In the original diesel engine, fuel was delivered to the cylinder by highly compressed air through an injector, which used a spring-loaded pin to control fuel delivery though the nozzle.

The complex and heavy air-blast system used in early Diesels limited the speed the engine could run at and the minimum size a diesel engine could be built to. This was needed to inject fuel under sufficient pressure for it to enter the highly compressed air in the cylinder. In hot-bulb engines fuel is injected before compression takes place, allowing the lighter, more accurate system to be used. Only when Akroyd-Stuart's mechanical pump-and-sprayer system that he developed for his hot-bulb engine was adapted by Robert Bosch for use in diesel engines (by making the system run at a much higher pressure and combining it with a modified version of Diesel's injector) were high-speed diesel engines practical.


Hot bulb engines were built by a large number of manufacturers, usually in modest series. The Pythagoras Engine Factory in Norrtälje in Sweden is kept as a museum (the Pythagoras Mechanical Workshop Museum), and has a functioning production line and extensive factory archives.

See Also

Fairbanks Morse

External links

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