original engine operated on the four-stroke cycle (induction, compression, power and exhaust), and Hornsby continued to build engines to this design, as did several other British manufacturers such as Blackstone and Crossley. Manufacturers in Europe, Scandinavia and in the United States built engines working on the two-stroke cycle with crankcase scavenging. The latter type formed the majority of hot-bulb engine production. The flow of gases through the engine is controlled by valves in four-stroke engines, and by the piston covering and uncovering ports in the cylinder wall in two-strokes. In the hot bulb engine, combustion takes place in a separated combustion chamber, the "vaporizer" (also called the "hot bulb"), usually mounted on the cylinder head, into which fuel is sprayed. It is connected to the cylinder by a narrow passage and is heated by combustion gases while running; an external flame, such as a blow torch or slow-burning wick, is used for starting; on later models, electric heating or pyrotechnics were sometimes used. Another method was the inclusion of a spark plug and vibrator-coil ignition; the engine would be started on petrol (gasoline) and switched over to oil after warming to running temperature. The pre-heating time depends on the engine design, the type of heating used and the ambient temperature, but for most engines in a temperate climate generally ranges from 2 to 5 minutes to as much as half an hour if operating in extreme cold or the engine is especially large. The engine is then turned over, usually by hand, but sometimes by compressed air or an electric motor. Once the engine is running, the heat of compression and ignition maintains the hot bulb at the necessary temperature, and the blow-lamp or other heat source can be removed. Thereafter, the engine requires no external heat and requires only a supply of air, fuel oil and lubricating oil to run. However, under low power the bulb could cool off too much, and a throttle can cut down the cold fresh air supply. Also, as the engine's load is increased, so does the temperature of the bulb, causing the ignition period to advance; to counteract pre-ignition, water is dripped into the air intake. Equally, if the load on the engine is low, combustion temperatures may not be sufficient to maintain the temperature of the hot bulb. Many hot-bulb engines cannot be run off-load without auxiliary heating for this reason. The