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1、 UNIT 7 EMISSION CONTROL SYSTEMS TEXT AOverview of Emission Control Systems PROFESSIONAL ENGLISHUNIT 7 EMISSION CONTROL SYSTEMS TEXT AOverview of Emission Control Systems PCV System EGR System Evaporative Control System Air Injection Catalytic ConverterPROFESSIONAL ENGLISH本次课学习目标l掌握排放控制系统的组成l掌握排放控制系
2、统的原理l掌握有关的英文专业术语l了解翻译技巧重复NEW WORDSdilutedailju:tv.冲淡,变淡,变弱,稀释crossoverkrsouvn.转线路,天桥,交叉channeltnln.海峡，水道，信道，频道，通道routeru:tn.路线,路程,通道；v.发送lean（out）li:nv.贫化，变瘦thermal:mla.热的,热量的transducertrnzdju:sn.传感器，变频器，变换器orificerifisn.孔，口，节流孔linearlinia.线的,直线的,线性的potentiometerptenimitn.电位计,电势计fumefju:mn.(浓烈或难闻的)烟
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4、enderrendv.致使，表演，实施NEW WORDSPHRASES AND EXPRESSIONS脉宽调制脉宽调制pulse width modulationEGR阀位置（阀位置（EVP）传感器）传感器EGR valve position(EVP)sensor锥形端锥形端tapered tip通气孔通气孔bleed port背压背压back pressure 大节气门开度（简写为大节气门开度（简写为WOT）wide-open throttle废气再循环阀废气再循环阀EGR valve节气门开度节气门开度throttle opening在最坏的情况下在最坏的情况下at worst空气喷射空气
5、喷射air injection催化转换器催化转换器catalytic converter空气泵空气泵air pump分流阀分流阀diverter valve清污阀清污阀purge valve与与一道（同时），以及，除一道（同时），以及，除之外（还）之外（还）along with活性炭罐活性炭罐charcoal canisterPHRASES AND EXPRESSIONS To minimize pollution of the atmosphere from incompletely burned and evaporating gases and to maintain good driv
6、eability and fuel economy,a number of emission control systems are used on all vehicles.They include positive crankcase ventilation(PCV)system，evaporative emission control(EVAP)system，exhaust gas recirculation(EGR)system，air injection(AIR)system and three-way catalytic converter(TWC)system.PCV Syste
7、m The purpose of the positive crankcase ventilation(PCV)system is to take the vapors produced in the crankcase and piston blow-by,and redirect them into the air/fuel intake system to be burned during combustion.These vapors dilute the air/fuel mixture,they have to be carefully controlled and metered
8、 so as not to affect the performance of the engine.This is the job of the positive crankcase ventilation(PCV)valve.Overview of Emission Control Systems At idle,when the air/fuel mixture is very critical,just a little of the vapors are allowed in to the intake system.At high speed when the mixture is
9、 less critical and the pressures in the engine are greater,more of the vapors are allowed in to the intake system 1.When the valve or the system is clogged,vapors will back up into the air filter housing or at worst,the excess pressure will push past seals and create engine oil leaks 2.If the wrong
10、valve is used or the system has air leaks,the engine will idle rough,or at worst engine oil will be sucked out of the engine.EGR System Exhaust gas recirculation(EGR)systems reduce the amount of oxides of nitrogen produced during the combustion process.The EGR system dilutes the air/fuel mixture wit
11、h controlled amounts of exhaust gas.Since exhaust gas does not burn,this reduces the peak combustion temperatures.Overview of Emission Control SystemsOverview of Emission Control Systems At lower combustion temperatures,very little of the nitrogen in the air will combine with oxygen to form NOX.Most
12、 of the nitrogen is simply carried out with the exhaust gases.For driveability,it is desirable to have the EGR valve opening(and the amount of gas flow)proportional to the throttle opening.Driveability is also improved by shutting off the EGR when the engine is started up cold,at idle,and at full th
13、rottle.Since the NOX control requirements vary on different engines,there are several different systems with various controls to provide these functions.EGR Valves Most of these systems use a vacuum-operated EGR valve to regulate the exhaust gas flow into the intake manifold.Exhaust crossover passag
14、es under the intake manifold channel the exhaust gas to the valve.(Some in-line engines route the exhaust gas to the valve through an external tube.)Typical mounting of the EGR valve is either on a plate under the carburetor or directly on the manifold.Overview of Emission Control Systems The EGR va
15、lve is a vacuum-operated,flow control valve.Opening the EGR valve by control vacuum at the diaphragm,allows exhaust gases to flow through the valve and into the intake manifold.Here,the exhaust gas mixes with the air/fuel mixture.The effect is to dilute or lean out the mixture,so that it still burns
16、 completely but with a reduction in combustion chamber temperatures.There are also various designs of EGR valves.The positive back pressure EGR valve has a bleed port and valve positioned in the center of the diaphragm.A light spring holds this bleed valve open,and an exhaust passage is connected from the lower end of the tapered valve through the stem to the bleed valve.When the engine is running,exhaust pressure is applied to the bleed valve.At low engine speeds,exhaust pressure is not high en
