catalytic converter 汽车排气净化系统.pdf

EMISSION SUB SYSTEMS - Catalytic Converter Catalytic Converter Regardless of how perfect the engine is operating, there will always be some harmful by- products of combustion. This is what necessitates the use of a Three-Way Catalytic (TWC) Converter. This device is located in-line with the exhaust system and is used to cause a desirable chemical reaction to take place in the exhaust flow. Essentially, the catalytic converter is used to complete the oxidation process for hydrocarbon (HC) and carbon monoxide (CO), in addition to reducing oxides of nitrogen (NOx) back to simple nitrogen and carbon dioxide. TWC Construction Two different types of Three-Way Catalytic Converters have been used on fuel injected Toyota vehicles. Some early EFI vehicles used a pelletized TWC that was constructed of catalyst coated pellets tightly packed in a sealed shell, while later model vehicles are equipped with a monolith type TWC that uses a honeycomb shaped catalyst element. While both types operate similarly, the monolith design creates less exhaust backpressure, while providing ample surface area to efficiently convert feed gases. Page 1 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved. EMISSION SUB SYSTEMS - Catalytic Converter The Three-Way Catalyst, which is responsible for performing the actual feed gas conversion, is created by coating the internal converter substrate with the following key materials: • Platinum/Palladium; Oxidizing catalysts for HC and CO • Rhodium; Reducing catalyst for NOx • Cerium; Promotes oxygen storage to improve oxidation efficiency The diagram below shows the chemical reaction that takes place inside the converter. TWC Operation As engine exhaust gases flow through the converter passageways, they contact the coated surface which initiate the catalytic process