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.
Four-stroke Hornsby-Akroyd oil engine
The concept of this engine was established by Herbert Akroyd Stuart, an English inventor. 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.
Two-stroke hot-bulb engines
Some years later, Akroyd-Stuart's design was further developed in the United States by the German emigrants Mietz and Weiss, who combined the hot-bulb engine with the two-stroke scavenging principle, developed by Joseph Day to provide nearly twice the power, as compared to a four-stroke engine of the same size. Similar engines, for agricultural and marine use, were built by J. V. Svensons Motorfabrik, Bolinders, Lysekils Mekaniska Verkstad, AB Pythagoras and many other factories in Sweden.
Comparison to the Diesel engine
Akroyd-Stuart's engine was the first internal combustion engine to use a pressurised fuel injection system and also the first using a separate vapourising combustion chamber. It is the forerunner of all hot-bulb engines, which could be considered predecessors to the similar Diesel engine, developed a few years later.
However, the Hornsby-Akroyd oil engine and other hot-bulb engines are distinctly different from Rudolf Diesel's design, where ignition occurs through the heat of compression alone: An oil engine will have a compression ratio between 3:1 and 5:1, where a typical diesel engine will have a much higher compression ratio usually between 15:1 and 20:1, making it more efficient. Also the fuel is injected easily during the early intake stroke and not at the peak of compression with a high-pressure Diesel injection pump.
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 by a connecting rod and crankshaft. Akroyd-Stuart's original engine operated on the four-stroke cycle (induction, compression, power and exhaust), and Hornsby continued to build engines to this design, as did several other British manufacturers such as Blackstone and Crossley. Manufacturers in Europe, Scandinavia and in the United States built engines working on the two-stroke cycle with crankcase scavenging. The latter type formed the majority of hot-bulb engine production. The flow of gases through the engine is controlled by valves in four-stroke engines, and by the piston covering and uncovering ports in the cylinder wall in two-strokes.
In the hot bulb engine, combustion takes place in a separated combustion chamber, the "vaporizer" (also called the "hot bulb"), usually mounted on the cylinder head, into which fuel is sprayed. It is connected to the cylinder by a narrow passage and is heated by combustion gases while running; an external flame, such as a blow torch or slow-burning wick, is used for starting; on later models, electric heating or pyrotechnics were sometimes used. Another method was the inclusion of a spark plug and vibrator-coil ignition; the engine would be started on petrol (gasoline) and switched over to oil after warming to running temperature.
The pre-heating time depends on the engine design, the type of heating used and the ambient temperature, but for most engines in a temperate climate generally ranges from 2–5 minutes to as much as half an hour if operating in extreme cold or the engine is especially large. The engine is then turned over, usually by hand, but sometimes by compressed air or an electric motor.
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. Thereafter, the engine requires no external heat and requires only a supply of air, fuel oil and lubricating oil to run. However, under low power the bulb could cool off too much, and a throttle can cut down the cold fresh air supply. Also, as the engine's load is increased, so does the temperature of the bulb, causing the ignition period to advance; to counteract pre-ignition, water is dripped into the air intake. Equally, if the load on the engine is low, combustion temperatures may not be sufficient to maintain the temperature of the hot bulb. Many hot-bulb engines cannot be run off-load without auxiliary heating for this reason.
The fact that the engine can be left unattended for long periods while running made hot-bulb engines a popular choice for applications requiring a steady power output, such as farm tractors, generators, pumps and canal boat propulsion.
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 vaporizer by a mechanical (jerk-type) fuel pump through a nozzle. 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 vaporizer, where it is compressed and its temperature rises. The vaporized fuel mixes with the compressed air and ignites primarily due to the heat of the hot bulb generated while running, or heat applied to the hot bulb prior to starting. By contracting the bulb to a very narrow neck where it attaches to the cylinder, a high degree of turbulence is set up as the ignited gases flash through the neck into the cylinder, where combustion is completed. 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 when power is not being produced. The piston rises, expelling exhaust gases through the exhaust valve (the exhaust stroke). The cycle then starts again.
The cycle starts with the piston at the bottom of its stroke. As it rises, it draws air into the crankcase through the inlet port. At the same time fuel is sprayed into the vaporiser. The charge of air on top of the piston is compressed into the vaporiser, where it is mixed with the atomised fuel and ignites. The piston is driven down the cylinder. As it descends, the piston first uncovers the exhaust port. The pressurised exhaust gases flow out of the cylinder. A fraction after the exhaust port is uncovered, the descending piston uncovers the transfer port. The piston is now pressurising the air in the crankcase, which is forced through the transfer port and into the space above the piston. Part of the incoming air charge is lost out of the still-open exhaust port to ensure all the exhaust gases are cleared from the cylinder, a process known as "scavenging". The piston then reaches the bottom of its stroke and begins to rise again, drawing a fresh charge of air into the crankcase and completing the cycle. Induction and compression are carried out on the upward stroke, while power and exhaust occur on the downward stroke.
A supply of lubricating oil must be fed to the crankcase to supply the crankshaft bearings. Since the crankcase is also used to supply air to the engine, the engine's lubricating oil is carried into the cylinder with the air charge, burnt during combustion and carried out of the exhaust. The oil carried from the crankcase to the cylinder is used to lubricate the piston. This means that a two-stroke hot-bulb engine will gradually burn its supply of lubricating oil, a design known as a "total-loss" lubricating system. There were also designs that employed a scavenge pump or similar to remove oil from the crankcase and return it to the lubricating-oil reservoir. Lanz hot-bulb tractors and their many imitators had this feature. This reduced oil consumption considerably.
In addition, if excess crankcase oil is present on start up, there is a danger of the engine starting and accelerating uncontrollably to well past the speed limits of the rotating and reciprocating components. This can result in destruction of the engine. There is normally a bung or stopcock that allows draining of the crankcase before starting.
The lack of valves and the doubled-up working cycle also means that a two-stroke hot-bulb engine can run equally well in both directions. A common starting technique for smaller two-stroke engines is to turn the engine over against the normal direction of rotation. The piston will "bounce" off the compression phase with sufficient force to spin the engine the correct way and start it. This bi-directional running was an advantage in marine applications, as the engine could, like the steam engine, drive a vessel forward or in reverse without the need for a gearbox. The direction could be reversed either by stopping the engine and starting it again in the other direction, or, with sufficient skill and timing on the part of the operator, slowing the engine until it carried just enough momentum to bounce against its own compression and run the other way. This was an undesirable quality in hot-bulb-powered tractors equipped with gearboxes. At very low engine speeds the engine could reverse itself almost without any change in sound or running quality and without the driver noticing until the tractor drove in the opposite direction to that intended. Lanz Bulldog tractors featured a dial, mechanically driven by the engine, that showed a spinning arrow. The arrow pointed in the direction of normal engine rotation; if the dial spun the other way, the engine had reversed itself.