A hotbulb engine is an early diesel engine.
You heat the bulb and the diesel is injected in the head towards the pre heated bulb (plate) and so will ignite through the heat.
In the early days they couldn't really reach the high compression of the internal combustion chamber and the high presure of the injector in order to start an engine and keep it working. They pre heated a bulb or plate and some engines needed constant fire beneath the bulb when they didn't work hard.
Others would keep themselves warm enough while working so they didn't need the constant flame.
Often this are two stroke engines and the compression lies in the middel between the Gas or petrol engine and the full diesel engine.