TAILIEUCHUNG - The Science and Technology of Materials in Automotive Engines Part 11

Tham khảo tài liệu 'the science and technology of materials in automotive engines part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 238 Science and technology of materials in automotive engines The development of catalysts to reduce NOx The need to decrease CO2 while at the same time keeping fuel consumption low forces engines to operate under lean combustion conditions. Stable operation is now possible at an air fuel ratio of 50. These conditions meant that the air fuel ratio is beyond the lambda window and the normal threeway catalyst cannot reduce NOx under such high oxygen concentrations. Catalysts that reduce NOx under high oxygen concentrations are called lean NOx catalysts. Two types have been introduced selective NOx reduction catalysts and NOx storage reduction catalysts. Selective NOx reduction catalysts include PT-Ir ZSM-58 and Ir BaSO4 9 and assist the reduction of NOx by HC in high-oxygen environments. Some have already been marketed but further development is required. The NOx storage reduction catalyst10 11 stores NOx temporarily as a form of nitric acid salt NO- Fig. reducing NOx in the exhaust gas. The NO- adsorbents are alkali metals or alkaline-earth metals such as BaCO3. If combustion takes place in the rich state with higher CO and HC the accumulated NO- is separated and reduced. Mechanism showing trap and reduction of NOx. The trapping process is NO O2 NO2 and BaO NO2 BaNO3. The regeneration process is BaNO3 CO BaO N2 CO2 The rich state occurs during acceleration or is generated by an intentional fuel control the latter being known as rich spike. This system can serve to decrease fuel consumption and clean the exhaust gas and was first marketed in a direct injection lean-burn engine by Toyota. One problem with this kind of catalyst is that the adsorbent also traps sulfur and the sulfuric compounds decompose at higher temperatures than NOx. Accumulated S hinders the activity of adsorbents and shortens the life of the catalyst. Therefore the sulfur content of the petrol must be kept as low as possible. The catalyst 239 Controlling pollutants at cold start

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