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. Advantages 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. 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. 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.