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Catalyzed soot filters are being fitted to an increasing range of diesel-powered passenger cars in Europe. While the initial applications used silicon carbide wall-flow filters, oxide-based filters are now being successfully applied. Oxide-based filters can offer performance and system cost advantages for applications involving both a catalyzed filter with a separate oxidation catalyst, and a catalyzed filter-only that incorporates all necessary catalytic oxidation functions. Advanced diesel catalyst technologies have been developed for alternative advanced oxide filter materials, including aluminum titanate and advanced cordierite. In the development of the advanced catalyzed filters, improvements were made to the filter material microstructures that were coupled with new catalyst formulations and novel coating processes that had synergistic effects to give enhanced overall performance.

The tightening of Heavy Duty Diesel (HDD) emissions legislation throughout the world is leading to the development of emission control devices to enable HDD engines to meet the new standards. One system which has shown great promise in controlling PM emissions is the Continuously Regenerating Trap (CRT®) system. This system will be referred to as the CR-DPF for the remainder of this paper. Stringent durability requirements will be introduced alongside the new legislative emission limits, so it is essential that DPF systems are made to be as robust as possible. In Europe the systems are expected to need to meet a durability target of 500,000 km, while in the US this will be approximately 700,000 km (435,000 miles). This paper reports on the development of a greatly improved oxidation catalyst for these CR-DPF applications. Field and engine bench studies revealed that the previous catalyst could be poisoned by sulfur build-up during prolonged operation at low temperatures.

The low exhaust gas temperatures experienced on light duty Diesel vehicles present a very challenging environment for the successful operation of catalytic aftertreatment. To meet the future more severe legislation, Diesel engines are being developed with greater combustion efficiencies and advanced fueling control. These engine developments may produce lower particulate matter (PM) and nitrogen oxides (NOx) emissions, but increased hydrocarbon (HC) and carbon monoxide (CO) emissions may occur. As a result of these engine changes exhaust gas temperatures may reduce still further. These factors demand catalysts with high oxidation activity at low temperatures. This paper reviews oxidation catalyst technology developed for light duty Diesel vehicles and the factors affecting their performance. Results obtained on synthetic gas rigs, bench engines and vehicles are presented. A discussion oh the effect of the level of sulfur (S) present in Diesel fuel on aftertreatment is given.

The lower temperatures encountered with European Stage III/IV turbo charged direct injection (DI) Diesel engines with additional features such as cooled exhaust gas recirculation (EGR), compared to Stage II engines, means that modern light duty Diesel engine exhaust gas will rarely exceed temperatures of around 550° - 650°C under full load conditions, and under normal driving conditions, temperatures as low as 120°C will be common. The development of high activity Diesel oxidation catalysts (DOCs) having good low temperature performance is therefore key for achieving hydrocarbon (HC) and carbon monoxide (CO) conversions to meet Stage III and IV legislation. It is shown that extended operation of conventional DOC technology, at the lower temperatures encountered on modern Diesel engines, introduces an important mechanism of catalyst deactivation by accumulation of soot/coke and associated sulfur.

The drive towards improved fuel economy and lower emissions for Diesel vehicles requires the development of catalysts capable of converting not only carbon monoxide (CO) and hydrocarbon (HC), but also particulate matter (PM) and nitrogen oxides (NOx) in a lean exhaust environment. This paper reviews the approaches that are being considered for this purpose, for light duty Diesels, together with factors that may influence catalyst performance such as components in the fuel and quality of the lubricant.