新版轮机英语unitGasexchangeprocess.ppt

Lesson 8 Text The Gas Exchange process(换气过程） A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal（排出） of exhaust gases. This is the gas exchange. Scavenging（扫气） is the removal of exhaust gases by blowing in fresh air. Charging（充气） is the filling（充满，填充） of the engine cylinder with a supply or charge of fresh air ready for compression. With supercharging(增压) a larger mass of air is supplied（提供） to the cylinder by blowing it in under pressure. Older engines were ‘naturally aspirated(自然吸气)’—taking fresh air only at atmospheric(大气的） pressure. Increasing the density（密度） of the charge air by the use of a suitable compressor between the air intake（气管入口） and the cylinder, the weight of air induced（促使，引起） per working stroke is increased. Thereby（因此） a greater weight of fuel can be burnt with a consequent increase in the cylinder power output. This increase in charge air density is accomplished（完成，实现） on most modern diesel engine types by use of exhaust gas turbocharging（废气涡轮）, in which a turbine（涡轮） wheel driven by exhaust from the engine is rigidly coupled（刚性连接） to a centrifugal(离心的） type air compressor（压气机）. Both four-stroke and two-stroke cycle engines may be pressure charged（增压）. It should be noted that supercharging is not simply a matter of adding equipment to non-supercharged engines. The supercharged engine must be designed to withstand（经受住） the increased pressures and thermal loads which results. It is essential（必要的） to the scavenging process that the air entering the cylinder is at a higher pressure than the gas in the exhaust manifold（排气总管）. On two-stroke diesels an electrically driven auxiliary blower（辅助鼓风机） is usually provided because the exhaust gas driven turbo-blower cannot provide enough air at low engine speeds. High pressure supercharged engines use turbochargers in series(串联的). The pressurized air is usually cooled to increase the charge air density. The turbo-blower or turbo-charger has on opposite e