Tech Briefs

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April 2002
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Aftertreatment strategies for small diesels

In a study by researchers from Fiat and Emitec, a small catalyst was mounted before the turbocharger to take advantage of the thermal energy of the gases.

According to researchers from Fiat and Emitec who presented a paper during the Diesel Exhaust Emission Control Technical Session at Congress, the number of diesel-powered vehicles has increased significantly in recent years, mainly because of their low fuel consumption. In contrast to vehicles using gasoline engines, diesel cars had no need of exhaust emissions controls in Europe. With more restrictive regulations and further reductions of emissions limits expected in the future, aftertreatment strategies have become crucial for small passenger cars.

Catalytic converters are more difficult to apply to diesels. The exhaust temperature is low and the engine runs lean, preventing NOx control with a three-way catalyst. Catalytic converters on diesel engines often fall below light-off temperature during deceleration and idling modes of the European driving cycle. Soot particles in the exhaust must also be contained.

In a turbocharged engine, the turbocharger acts as a heat sink, reducing exhaust gas temperature downstream by 50-150°C (90-270°F). During research, a very small catalyst was mounted before the turbocharger to take advantage of the thermal energy of the gases. Despite a small catalyst volume of 0.025-0.045 L (1.5-2.7 in³), high efficiencies were observed.

Another possibility for catalyst optimization of a diesel exhaust system is a hybrid catalyst that uses different thermal masses within one substrate mantle. The object obtains fast light-off with improved heat storage.

Engine-related reductions in NOx or particulate emissions from diesel engines can only be achieved, based on today's combustion processes, to the detriment of one or both components. As a result, it is likely that particulate reduction will require filters. Flow-through particulate traps are a new development that prevent plugging when regeneration fails to work.

The research done on a 1.9-L common-rail engine showed that installing a small catalyst in front of the turbocharger eliminated the need for the under-floor catalyst. The small volume reduces the cost of the catalyst in noble metals and canning materials. The substitution of a hybrid catalyst for a metal substrate catalyst with the same cell density reduces HC and carbon monoxide emissions. The flow-through particulate trap shows a reduction of 22% compared to the production system. No significant power loss is measured from the production system combined with a pre-turbocharger catalyst.

- John Fobian

VW exhaust aftertreatment by OMG

Engineers from Volkswagen and OMG created an exhaust gas aftertreatment system during development of FSI engines to meet EU IV emissions standards. Click to enlarge

To reduce the fuel consumption of its vehicle fleet, Volkswagen AG developed spark-ignition engines with direct fuel injection. Those who attended the Advanced Catalysts and Substrates Technical Session at the SAE Congress learned how engineers first had to develop a suitable exhaust gas aftertreatment system to launch this new engine concept with stratified lean operation mode while meeting the stringent EU IV emissions standards. This feat was achieved as part of an intensive cooperation between Volkswagen AG and OMG, formerly dmc² Degussa Metals Catalysts Cerdec AG.

Due to intensive cooperation of the development partners involved, it was possible to implement for the first time an exhaust gas aftertreatment system based on NOx-storage-catalyst technology for the emissions standards in the Volkswagen Lupo FSI (fuel stratified injection). The fuel consumption of the Volkswagen Lupo FSI is 4.9 L/100 km. The emissions and fuel consumption targets were successfully achieved by:

• A general exhaust gas aftertreatment concept with a NOx-storage catalyst and on-demand purging functions based on NOx-sensor control.
• Improvement of the aging stability of the catalyst system by developing a stable NOx-storage catalyst and light-off catalyst formulation, engine-out emissions reduction, and exhaust gas cooling devices.
• Improvement in the desulfation characteristics of the NOx storage catalyst and development of an active desulfation process.

European fuel-quality standards required the development of a new active and lambda-controlled desulfation process. Although the Volkswagen FSI exhaust gas aftertreatment concept can withstand high fuel sulfur contents, sulfur-free fuel is required for best fuel economy. With widespread available sulfur-free fuel, advanced NOx storing components with a wider NOx activity window can be used, which leads to more frequent use of fuel-saving lean operation mode.

Since spring 2000, several mineral oil companies have introduced sulfur-free fuel with less than 10 ppm in Germany. Volkswagen supports this trend, hoping that within a short time period the availability of sulfur-free fuel will be extended to other countries.

As soon as fuels with less than 10 ppm are available within the European Union on a widespread scale, new NOx storing components with wider NOx activity windows and higher desulfation temperatures can be employed. When catalyst deactivation occurs very slowly due to low sulfur concentrations, the chance for a "natural" desulfation to be performed before active sulfur regeneration must be initiated increases. Fuel economy will thus increase not only by rare desulfation. The main advantage will be more frequent use of lean-burn and stratified engine operation mode, especially at higher catalyst temperatures.

The researchers believe that worldwide improvements in fuel quality standards are needed for market penetration of FSI engines. As direct gasoline injection is the technology with the highest potential in reduction of CO₂ and fuel consumption, any initiative will be supported that promotes the FSI strategy.

- Linda Trego

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