fact that the engine can be left unattended for long periods while running made hot-bulb engines a popular choice for applications requiring a steady power output, such as farm tractors, generators, pumps and canal boat propulsion. Four-stroke engines Air is drawn into the cylinder through the intake valve as the piston descends (the induction stroke). During the same stroke, fuel is sprayed into the vaporizer by a mechanical (jerk-type) fuel pump through a nozzle. The injected fuel vapourises on contact with the hot interior of the vaporizer but the heat is not sufficient to cause ignition. The air in the cylinder is then forced through the opening into the vaporizer as the piston rises (the compression stroke), where it is lightly compressed (a ratio of around 3:1) - this is not sufficient to cause significant temperature rise of the air charge, which is mostly caused by the air being heated by contact with the internal surfaces of the hot bulb (red hot due to external heating applied before starting or due to the maintained heat of combustion as the engine runs). The compression stroke mostly serves to create a turbulent movement of air from the cylinder into the vaporizer, which mixes with the pre-vaporized fuel oil. This mixing, and the increase in oxygen content as the air is lightly compressed into the vaporizer, causes the fuel oil vapour to spontaneously ignite. The combustion of the fuel charge is completed in the hot bulb, but creates an expanding charge of exhaust gases and superheated air. The resulting pressure drives the piston down (the power stroke). The piston's action is converted to a rotary motion by the crankshaft-flywheel assembly, to which equipment can be attached for work to be performed. The flywheel stores momentum, some of which is used to turn the engine when power is not being produced. The piston rises, expelling exhaust gases through the exhaust valve (the exhaust stroke). The cycle then starts again. Two-stroke engines The basic action of fuel injection and combustion is common to all hot-bulb engines, whether four- or two-stroke. The cycle starts with the piston at the bottom of its stroke. As it rises, it draws air into the crankcase through the inlet port. At the same time fuel is sprayed into the vaporiser. The charge of air on top of the piston is driven into the vaporiser, where it mixes with the atomised