fuel and combustion takes place. The piston is driven down the cylinder. As it descends, the piston first uncovers the exhaust port. The pressurised exhaust gases flow out of the cylinder. A fraction after the exhaust port is uncovered, the descending piston uncovers the transfer port. The piston is now pressurising the air in the crankcase, which is forced through the transfer port and into the space above the piston. Part of the incoming air charge is lost out of the still-open exhaust port to ensure all the exhaust gases are cleared from the cylinder, a process known as "scavenging". The piston then reaches the bottom of its stroke and begins to rise again, drawing a fresh charge of air into the crankcase and completing the cycle. Induction and compression are carried out on the upward stroke, while power and exhaust occur on the downward stroke. A supply of lubricating oil must be fed to the crankcase to supply the crankshaft bearings. Since the crankcase is also used to supply air to the engine, the engine's lubricating oil is carried into the cylinder with the air charge, burnt during combustion and carried out of the exhaust. The oil carried from the crankcase to the cylinder is used to lubricate the piston. This means that a two-stroke hot-bulb engine will gradually burn its supply of lubricating oil, a design known as a "total-loss" lubricating system. There were also designs that employed a scavenge pump or similar to remove oil from the crankcase and return it to the lubricating-oil reservoir. Lanz hot-bulb tractors and their many imitators had this feature. This reduced oil consumption considerably. In addition, if excess crankcase oil is present on start up, there is a danger of the engine starting and accelerating uncontrollably to well past the speed limits of the rotating and reciprocating components. This can result in destruction of the engine. There is normally a bung or stopcock that allows draining of the crankcase before starting. The lack of valves and the doubled-up working cycle also means that a two-stroke hot-bulb engine can run equally well in both directions. A common starting technique for smaller two-stroke engines is to turn the engine over against the normal direction of rotation. The piston will "bounce" off the compression phase with sufficient force to spin the engine the correct way and start it. This bi-directional running was an advantage