How a Hot Bulb Engine Works

Care and Feeding of Hot Bulb Engines

One of the biggest advantages of hot bulb engines was their ability to use any type of crude fuel. Basically, if the fuel could flow through a pipe and if it would burn then a hot bulb engine could likely run on it.

This aspect of their nature made the engines popular along isolated stretches of oil pipelines, which offered a ready supply of unrefined fuel. The machines were primarily stationary, though there were a few antique tractors that used hot bulb engines for propulsion. As a stationary source of power the machines were ideal for industrial use, whether running a small shop or a small sawmill, they provided steady power for a cheap price. However, because of their low power output to size -- a farm tractor needed a hot bulb engine of about 20 liters to function -- the engines were not used in larger industrial applications like powering a mill.

Preston Foster, curator of collections at the Coolspring Power Museum and a professional antique engine restoration specialist, said hot bulb engines were ideal for their time and place, but did have some drawbacks.

For instance, hot bulb engines did not run well on more refined fuels, such as gas or diesel. "It was mostly kerosene and other less refined fuels," Foster said.

The engines, especially the two-stroke variety, were also prone running backwards, to becoming overpowered with fuel and running almost out of control before the governor could catch up. Foster said the engine components were made at a time when engine metallurgy and machining were relatively crude, parts could break easily, and finding replacements was difficult.

On the American-made two-stroke models the engine would occasionally scavenge oil from the crankcase to use as fuel, robbing itself of lubrication.

It was these drawbacks, compounded by improvements in metallurgy and machining, that led to the downfall of the hot bulb engine.