Automobile Emissions Essay Research Paper ABSTRACTPollution from — страница 4

capable of meeting current American standards while maintaining satisfactory driveability, power output, and fuel economy without the use of catalyst units in the exhaust system. 3.2 THE USE OF CATALYSTS FOR EMISSION CONTROL The concept of using a catalyst to convert carbon monoxide, hydrocarbons, and nitrogen oxides to less environmentally threatening compounds such as nitrogen, water and carbon dioxide was a well established practice prior to the need arising from motor vehicle emissions. However, rapid changes in exhaust gas temperature, volume and composition were features not previously encountered in chemical and petroleum industry applications. Other unique requirements were the control of emissions such as ammonia, hydrogen sulfide and nitrous oxide which could result

from secondary catalytic reactions and for the catalyst system to maintain its performance after high temperature excursions up to 1000?C and in the presence of trace catalyst poisons such as lead and phosphorous.7 The principal reactions on automobile exhaust Catalysts are as follows: Oxidation Reactions: 2CO + O2 ? 2CO2 4HC + 5O2 ? 4CO2 + 2H3O Reduction Reactions: 2CO + 2NO ? 2CO2 + N2 4HC + 10NO ? 4CO2 + 2H3O + 5N2 By the nature of the oxidation and reduction reactions which are involved in the removal of carbon monoxide, hydrocarbons and nitrogen oxides and the operating characteristics of the preferred catalyst, several combinations of engine/catalyst systems have been used since catalysts were introduced on American cars in 1975. 3.2.1 The Carbon Monoxide/Hydrocarbon

Oxidation Catalyst Concept When emission control is primarily concerned with carbon monoxide and hydrocarbons and not with nitrogen oxide, such as is the case in the European “Euronorms” standards, oxidation catalysts are used. Key features of this system are the use of a secondary air supply to the exhaust gas stream to ensure oxidizing conditions under all engine operating loads and the use of exhaust gas recirculation (EGR) to limit nitrogen oxide emissions from the engine. A schematic of this system is shown in Figure 3.1. Figure 3-1 The Oxidation Catalyst This System was used initially in America to meet interim emission standards and is likely to be adopted to meet similar standards on medium and smaller engine cars (less than 2 litter engines) in Europe. 3.2.2 Dual Bed

and Threeway Catalyst Concepts In order to overcome the limitations imposed by the use of EGR and to meet more rigid nitrogen oxide standards, catalysts capable of reducing nitrogen oxide emissions are necessary. Initially, as a result of the difficulty of controlling air/fuel ratios to the tolerances required by a single catalyst unit, a dual catalyst bed was used. In order to ensure reducing conditions in the first catalyst bed, where nitrogen oxides were reacted, the engine was tuned slightly rich of the stoichiometric ratio. Secondary air was then injected into the exhaust stream ahead of the second catalyst bed (oxidation bed) to complete the removal of carbon monoxide and hydrocarbons. With developments in engine control and catalyst technology involving widening the

air/fuel operating window for 90 % removal of hydrocarbons, carbon monoxide and nitrogen oxides, the dual bed system has been replaced with a single threeway catalyst unit. A schematic of this system is shown in Figure 3.2. Figure 3-2 The Three-way Catalyst Key features of this system, in addition to the catalyst unit, are an electronically controlled air/fuel management system incorporating in its most advanced form, the use of an oxygen sensor to monitor and control exhaust gas combustion. Systems such as this are now universal on American and Japanese cars and in those countries that have adopted similar emission standards. The performance of the Threeway Catalyst system is summarized in Table 3.2 and Table 3.3. Cold ECE 15 HC + NOX NOX CO cycle, g/test Without Catalyst With

Catalyst Without Catalyst With Catalyst Without Catalyst With Catalyst PEUGEOT 205 18.3 8.5 7.8 5.8 26.3 8.8 FIAT UNO 45 15.2 4.1 6.2 2.7 26.7 9.8 VW GOLF C 16.1 6.4 5.7 2.0 50.5 42.7 ROVER 213 12.3 5.2 3.6 1.4 46.7 27.5 Table 3-2 Emission Levels from small vehicles Polycyclic Aromatic Emissions, mg/mile Hydrocarbon Without Catalyst With Catalyst phenanthrene 1.85 0.16 anthracene 0.61 0.04 fluoranthrene 2.27 0.23 pyrene 2.91 1.50 perylene 1.21 0.40 benzo(a)pyrene 0.94 0.17 benzo(e)pyrene 2.76 0.41 dibenzopyrenes 0.28 0.23 coronene 0.41 0.27 Table 3-3 Polycyclic Aromatic Hydrocarbon Emissions from a Programmed Combustion Engine 3.2.3 Lean Burn Catalyst Systems Engine operations with air/fuel ratios of 20:1 is a good way of reducing nitrogen emissions and improving fuel economy.