contd....

Close-Coupled Catalysts

A rapid catalyst light off is achieved by installing the converter very close to the engine to minimize heat losses from the exhaust gases as shown on Fig. 5.20. The catalysts installed so close are called ‘close-coupled' catalysts. These catalysts have the following features;

• The close-coupled catalysts are designed mainly for oxidation of HC.
• In the close-coupled catalysts, CeO₂ is removed from the substrate washcoat as. CeO₂has excellent properties for oxidation of CO.
• The closed coupled catalysts have a small volume to improve light-off characteristics.
• These are designed to oxidize only part of HC and little CO so that generation of very high temperatures in the catalyst bed is prevented.
• HC oxidation in thee close-coupled catalyst raises the exhaust gas temperature so that the main under floor catalyst becomes operational rapidly within a short time. The under-floor catalyst mounted downstream converts the remaining HC and, also the CO and NO_x .

The main 3-way catalyst is installed sufficiently down stream of the exhaust manifold under the floor of the vehicles. These are also termed as ‘underfloor catalytic converters'. The converters should not be exposed to excessively high temperatures during full engine load conditions and steep accelerations when the initial exhaust gas temperature at the inlet of the converter itself are quite high. During accelerations the unburned HC and CO concentrations are higher than the cruising conditions and their oxidation in the converter to achieve typically more than 90% conversion releases high amounts of energy that raises the gas temperatures substantially. However during engine cold start and warm-up conditions, the underfloor catalyst does not function as the gas temperatures are low and it also cools down as it flows from the exhaust port to the converter. With implementation of ULEV and US Tier 2 standards, HC emission control under engine cold start and warm-up operation are also required.
To reduce HC and CO emissions under cold engine conditions mew types of converters have been developed. For the direct injection stratified charge (DISC) engines, NOxemissions under over all lean engine operation are to be controlled. As the 3-way catalysts do not control NOxunder lean engine operation, new types of catalysts to reduce NOX for lean mixture operation have been developed.