This dancing Lanz Bulldog tractor is moving forward and backward, but since its engine is never making a full revolution in any one direction, it’s technically at zero rpm. And it can run forward or backwards equally, shown at 3:45 in the video, when the operator gives it a bit more “gas” and it happens to start moving backwards.
The engine in this Lanz is a two-stroke hot-bulb engine, in a class of its own among the family of internal combustion engines. It’s an archaic old system, and has some unusual features both like and unlike gasoline or diesel engines.
Hot-bulb engine (two-stroke). 1. Hot bulb. 2. Cylinder. 3. Piston. 4. Crankcase
The hot bulb engine is something of a precursor to the diesel engine in that it can burn fuel oils of various kinds, but it has a very low compression ratio and thus does not self-ignite like a diesel. And since it’s crankcase scavenged, it can run in either direction. The one in the video is essentially doing that, but at such a low fuel setting that it never overcomes its compression full, with the cylinder bouncing back down before reaching Top Dead Center, hence the back and forth motion.
Note: at about 2:20 in the video above, there’s an animation. But it’s not of a hot bulb engine, just a regular spark-ignition two stroke gasoline engine. But it does sort of show what’s happening when the engine is running at “zero rpm”.
Rather than try to rewrite Wikipedia’s comprehensive article on these, I’ll just copy some of the salient paragraphs:
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 to 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.
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 driven into the vaporiser, where it mixes with the atomised fuel and combustion takes place. 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.
Got it? Hot bulb engines had low compression ratios, as little as 3:1. But they could burn all kinds of cheap fuel oil including even used motor oil, as the heat of the hot bulb started the vaporization and ignition process. And since oil (and diesel) burn much slower than gasoline vapor, this could be made to work quite effectively. Efficiency of up to 12% was much better than steam engines, but once the diesel engine started to be developed, its significantly greater efficiency eventually doomed the hot bulb engine, although some were made as late as the 1950s for certain applications, particularly inland barges and narrowboats, thus their distinctive chugging sound.
Its simplicity was its greatest asset, as it had no need for an ignition system as in gas engine, or a technically demanding high pressure injection system as in diesels. Also, once the bulb was heated by an external source, they started very easily, a boon in very cold climates. And they could reliably be left to run unattended for long periods of time, such as in pumps and generators. They were widely used in gensets in large private homes, theaters, factories, lighthouses and such, before national grids were built out.
By its nature (crude timing of fuel induction and the slowness of its burning), hot bulb engines are limited to slow engine operation, 300-400 rpm being the typical maximum engine speed. Thus the engines had to have large displacement.
The Lanz Bulldog has become the most prominent example of the breed, being built continuously in Germany from 1921 to 1960, starting with a 6.3 liter single cylinder making 12 hp and ending with a 10.3 liter making 55 hp. Licensed versions and copies were made in a number of European countries, in Poland until 1965. This video shows the process of starting a Lanz from cold, which takes almost ten minutes, starting with the heating of the hot bulb with a torch. Note that when it does start, the first two times it starts running backwards, as it’s not easy to control which direction it’s going to start running. The second time, the operator gets it to start running forwards by slowing it down enough to where the engine “bumps” from backwards to forwards.