The Hornsby-Akroyd oil engine was the first successful design of internal combustion engine using "heavy oil" as a fuel. It was the first to use a separate vapourising combustion chamber and is the forerunner of all hot-bulb engines. Oil used for engine fuel must be turned to a vapour state and remain in that state during compression. Furthermore, the combustion of the fuel must be powerful, regular, and complete, to avoid deposits that will clog the valves and working parts of the engine.