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How a Hot-Bulb Engine Works?| EngineDIY


What is a hot-bulb engine?

The hot-bulb engine is a type of internal combustion engine in which fuel ignites by coming in 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 or two-cylinder, low-speed two-stroke crankcase scavenged units.

Mietz and Weiss Hot-Surface Ignition Engine, circa 1897

Mietz and Weiss Hot-Surface Ignition Engine, circa 1897

Early diesel engines were various types of heat bulb or heat pipe engines. These engines rely on heated surfaces in the combustion system to aid in fuel vaporization and auto-ignition. Unlike diesel engines, these engines therefore do not rely solely on the heat of compression for ignition. In 2002, the Society of Automotive Engineers (SAE) published 2002-01-15 - Early Swedish Hot Bulb Engines - Efficiency and Performance Comparison with Contemporary Gasoline and Diesel Engines, by Olof Erlandsson, Lund Institute of Technology. Technical summary. A comparison of a hot-ball engine and a diesel engine over the same period. These engines are mainly two-stroke engines, but there are also four-stroke engines. The Hornsby Akroyd engine is a very successful example.

The advantages of a two-stroke engine are relatively high cycle temperatures and short surface cooling times between firings. In addition, the residual heat of a large amount of hot air is not dissipated, promoting the spontaneous combustion process. The overall simplicity of the early two-stroke engine designs also had a positive impact on the two-stroke design. These engines typically use a low pressure fuel injection/metering system to direct fuel to the combustion chamber and rely heavily on the heat from the combustion chamber surface for vaporization. A portion of a combustion system or burner is heated externally, typically by a burner or flare, to produce hot burner walls that promote vaporization and auto-ignition. During operation, the combustion chamber remains hot and the heat is dissipated to the outside. Compared to a true diesel engine, the lower compression ratio reduces structural stress, which reduces engine weight and internal friction. The compression ratio is usually between 6:1 and 12:1. These engines are used in low speed stationary engines or agricultural tractors. Most are single cylinders. These engines were generally more efficient than gasoline engines of the time and competed with earlier diesel engines. The hot bulb engine is often seen as a precursor to the diesel engine with a preheat chamber.

The fuel injection timing of a hot valve engine is usually much faster than that of a diesel engine. This gives the fuel time to evaporate and mix with the air forced into the hot chamber near top dead center (TDC). Therefore, combustion occurs primarily through spontaneous combustion of the mixture near the hot surface of the "hot bulb". The remainder of the combustion is auto-ignition through a gradual flame front and/or temperature increase.



How does a hot bulb engine work?

The reliability of hot bulb engines, their ability to run on a wide range of fuels and the fact that they can run for hours or days at a time make them very popular with agricultural, forestry and marine users, where they are used to pump and power milling, sawing and threshing machinery. Hot bulb engines are also used in road rollers and tractors.

Gas engines use electricity to ignite the spark plugs and turn the crankshaft to start the engine. Hot valve engines do not have that luxury. On warm days (around 60 degrees Fahrenheit (15.6 degrees Celsius)), the bulb needs to be heated for two to five minutes, and up to 30 minutes on cold days or on larger engines. This initial heat is generated in the torch for the first few days and then in the ignition coils and spark plugs to vaporise the first charge of fuel.

The operator turned the engine flywheel, the largest and heaviest part of the entire assembly (even small engines often weigh several hundred pounds), by hand until the combustion process started and the engine was running.

Once the engine was running, the heat of combustion kept the pistons hot enough and the fuel continued to vaporise, allowing the engine to almost completely self-serve. However, if the load on the engine is reduced or the engine is used in very cold environments, the valves must be heated regularly or continuously. Although seemingly simple and reliable, hot-bulb engines were fickle and had quirks and problems.


What is a hot bulb engine used for? 

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

What is a hot-bulb tractor?

From the 1910s to the 1950s, hot-bulb engines were more economical to manufacture with their low-pressure crude-fuel injection and had a lower compression ratio than Diesel's compression-ignition engines.
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 to monitor pressure to prevent overpressure and a resulting explosion. If fitted with automatic lubrication systems and a governor to control engine speed, a hot-bulb engine could be left running unattended for hours at a time.

Lanz Bulldog Schlepper (Typ HL)
by KaHe

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 dropped too low, the lead plug in the crown of the furnace would melt, extinguishing the fire. If a hot-bulb engine ran out of fuel, it would simply stop and could be immediately restarted with more fuel. The water cooling was usually 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.
A 2-cylinder 70BHP hot-bulb engine built by W.H. Allen & Sons, 1923. The engine is on display at the Internal Fire Museum of Power, Tangygroes, Wales, UK.

Compared with steam, petrol (Otto-cycle), and compression-ignition (Diesel-cycle) engines, hot-bulb engines are simpler, and therefore have fewer 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.

Another big attraction with the hot-bulb engine was its ability to run on a wide range of fuels. Even poorly combustible fuels could be used, since a combination of vaporiser and compression ignition meant that such fuels could be made to burn. The usual fuel was fuel oil, similar to modern-day diesel fuel, but natural gas, kerosene, crude oil, vegetable oil or creosote could also be used. This made the hot-bulb engine very cheap to run, since it could be run on readily 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.[2]

Due to the lengthy pre-heating time, hot-bulb engines usually started easily, even in extremely cold conditions. This made them popular choices in cold regions, such as Canada and Scandinavia, where steam engines were not viable and early petrol and diesel engines could not be relied upon to operate. However, it also makes them unsuitable for short time running use, especially in an automobile.


1. "Archived copy". Archived from the original on 2010-10-10. Retrieved 2008-12-04. [1]
2.  "Energy Saving Now!". Archived from the original on 2006-05-27. Retrieved 2006-05-28. [2]


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