lesso 7轮机英语.ppt

Lesson 7 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 of exhaust gases. This is the gas exchange. Scavenging is the removal of exhaust gases by blowing in fresh air. Charging is the filling 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 to the cylinder by blowing it in under pressure. Older engines were ‘naturally aspirated(自然吸气)’—taking fresh air only at atmospheric pressure. Increasing the density of the charge air by the use of a suitable compressor between the air intake and the cylinder, the weight of air induced per working stroke is increased. Thereby 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 on most modern diesel engine types by use of exhaust gas turbocharging, in which a turbine wheel driven by exhaust from the engine is rigidly coupled to a centrifugal type air compressor. Both four-stroke and two-stroke cycle engines may be pressure charged. 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 the increased pressures and thermal loads which results. It is essential to the scavenging process that the air entering the cylinder is at a higher pressure than the gas in the exhaust manifold. On two-stroke diesels an electrically driven auxiliary blower 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 ends of a single shaft an exhaust gas driven turbine and an air compressor. The compressor and the turbine are