汽车专业 翻译 中英文（全）automobile emissions 学位论文.doc

Lesson 6- Automobile Emissions Traffic Jam Background Information Emissions from an individual car are generally low, relative to the smokestack image many people associate with air pollution but in numerous cities across the country, the personal automobile is the single greatest polluter with emissions from millions of vehicles on the road. Driving a private car is probably a typical citizen’s most “polluting” daily activity. The Clean Air Act of 1970 gave EPA broad authority to regulate motor vehicle pollution, and the agency’s emission control policies have become progressively more stringent since the early 1970’s. EPA standards dictate how much pollution autos may emit but automakers decide how to achieve the pollution limits. The emission reductions of the 1970’s came about because fundamental improvements in engine design plus the addition of charcoal canisters to collect hydrocarbon vapors and exhaust gas recirculation valves to reduce nitrogen oxides. The advent of “first generation” catalytic converters in 1975 significantly reduced hydrocarbon and carbon monoxide emissions. The use of converters provided a huge indirect benefit as well. The converters lead to the widespread introduction of unleaded gasoline because lead inactivates the catalyst. This resulted in dramatic reductions in ambient lead levels and alleviated many serious environmental and human health concerns associated with lead pollution. The next major milestone in vehicle emission control technology came in 1980-81. In response to tighter standards, manufacturers equipped new cars with even more sophisticated emission control systems. These systems generally include a “three-way” catalyst, an on-board computer, and an oxygen sensor. This equipment helps optimize the efficiency of the catalytic converter. Provisions of the 1990 Clean Air Act are further reducing vehicle emissions. Mobile source provisions include even tighter tailpipe standard