How a Hot Bulb Engine Works

Making it Go

Hot bulb engines share the same basic components as the vast majority of other internal combustion engines. The detonation, or combustion of gases, pushes a piston housed inside a cylinder. The piston is connected to a flywheel via a crankshaft and connecting rod. This allows the engine to convert heat energy (the combustion) to mechanical energy at the flywheel. The flywheel then drives whatever mechanical component is attached to it.

Unlike gasoline and diesel engines, combustion in a hot bulb engine takes place in a separate chamber referred to as the "hot bulb," or "vaporizer." Essentially the hot bulb extends horizontally off the front of the engine, usually nearest the cylinder. Most hot bulbs looked something like a darning mushroom. The bulb contains a plate of metal, almost like a tea cup saucer, that would heat along with the bulb.

A fuel nozzle, usually a small metered orifice valve, dripped fuel into the hot bulb. The fuel would hit the metal plate, vaporize, mix with air and ignite. A narrow passage connected the bulb and the cylinder. The expanding gases would shoot down the small passage and move the piston in the cylinder.

Gas engines use electricity to fire a spark plug and rotate the crankshaft to get the engine going. Hot bulb engines do not have this luxury. On a mild day -- about 60 degrees Fahrenheit (15.6 degrees Celsius) -- the bulb must be heated for anywhere from two to five minutes, and up to half an hour on cold days or on larger engines. This initial heat, developed with a blow torch in the early days and later through coil and spark plugs, vaporizes the first charge of fuel.

An operator spun the engine's flywheel, the biggest and heaviest part of the entire assembly, (often weighing hundreds of pounds on even the small engines), by hand until the combustion process was going and the engine was up and running.

Once the engine was up and running the heat of combustion would keep the bulb hot enough to keep vaporizing fuel, and the engine would be largely self-sustaining. However, if the load on the engine dropped, or it was used in a very cold environment, the bulb would need periodic or even constant heating. While seemingly simple and reliable, hot bulb engines could be temperamental and had their fair share of quirks and challenges. The next page will discuss some of those traits.