UNIT 7 EMISSION CONTROL SYSTEMS TEXT A Overview of Emission Control Systems,PROFESSIONAL ENGLISH,UNIT 7 EMISSION CONTROL SYSTEMS TEXT A Overview of Emission Control Systems PCV System EGR System Evaporative Control System Air Injection Catalytic Converter,PROFESSIONAL ENGLISH,本次课学习目标,掌握排放控制系统的组成 掌握排放控制系统的原理 掌握有关的英文专业术语 了解翻译技巧重复,NEW WORDS,PHRASES AND EXPRESSIONS,Overview of Emission Control Systems To minimize pollution of the atmosphere from incompletely burned and evaporating gases and to maintain good driveability 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 System 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 so as not to affect the performance of the engine. This is the job of the positive crankcase ventilation (PCV) valve.,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 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 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 with controlled amounts of exhaust gas. Since exhaust gas does not burn, this reduces the peak combustion temperatures.,At lower combustion temperatures, very little of the nitrogen in the air will combine with oxygen to form NOX. Most 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 throttle. 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 passages 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.,The EGR valve 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 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 enough to close the bleed valve. If control vacuum is supplied to the diaphragm chamber, the vacuum is bled off through the bleed port, and the valve remains closed. （略）,In a negative back pressure EGR valve, a normally closed bleed port is positioned in the center of the diaphragm. 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 at lower speeds, there is a high-pressure pulse in the exhaust system. However, between these high-pressure pulses there are low-pressure pulses. As the engine speed increases, more cylinder firings occur in a given time, and the high-pressure pulses become closer together in the exhaust system.（略） A digital EGR valve contains up to three electric solenoids that are operated directly by the PCM. Each solenoid contains a movable plunger with a tapered tip that seats in an orifice. When any solenoid is energized, the plunger is lifted and exhaust gas is allowed to recirculate through the orifice into the intake manifold.