The hotbulb, or hot bulb engine or vaporizing oil engine is a type of internal combustion engine. It is a surface ignition engine in which the superheated fuel is ignited by being brought into contact with oxygen-rich fresh air, rather than by a separate source of ignition, such as a spark plug. The engines were usually one cylinder, four-stroke units, although following Mietz & Weiss' developments in the USA, 2-stroke versions were constructed. From wikipedia article full version here