Chuck, I think that is a hot bulb, not hot tube, engine they are not quite the same. This is how I understand the differences...
Hot tube engines used a carburetter but hot bulb engines used and injection pump.
In the case of the hot tube engine as used in some very, very early motor cars the mixture was forced into a heated tube and ignited when criutical pressure and temperature were reached. In the case of the hot bulb engine, as was common on tractors including the Bulldog, fuel oil was pumped into the hot bulb but did not immediately ignite due to lack of oxygen (i.e. too rich) then as the piston came up (forward in most tractors) it forced air into the bulb and when the mixture was right the engine fired.
