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 hotbulb
tractors and their many imitators had this feature. This reduced oil consumption considerab.ly
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.
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 eficiency.
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.
Two-stroke engines
Advantages
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.
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 occu,r although this was a relatively rare occurrence by the time the ho-bt ulb 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.
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 biofue[l8.]
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.
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 hotbulb
tractors and their many imitators had this feature. This reduced oil consumption considerab.ly
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.
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 eficiency.
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.
Two-stroke engines
Advantages
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.
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 occu,r although this was a relatively rare occurrence by the time the ho-bt ulb 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.
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 biofue[l8.]
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.
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