Wechselwirkung Rohemissionen und Verbrauch: Folgen für die Abgasnachbehandlung

Zusammenfassung

Bisher wurden für Personenkraftwagen eher niedriglastige Zyklen zur Ermittlung von Kraftstoffverbrauch und Emissionen herangezogen. Dies hatte zu Folge, dass die Entwicklungsschwerpunkte in der Vergangenheit massiv darauf konzentriert wurden, in diesen eher schwachlastigen Zyklen möglichst verbrauchsgünstig und unauffällig im Emissionsverhalten zu sein.

Die gesetzlichen Regularien diesbezüglich ändern sich derzeit maßgeblich, sowohl in Europa und als auch in weiten Teilen der restlichen Welt. Von den Zulassungsbehörden werden erheblich dynamischere Testzyklen, wie z.B. der WLTC-Zyklus, geplant und eingeführt.

Rein in Bezug auf die Emissionsgesetzgebung geht der Gesetzgeber auch noch einen Schritt weiter und fordert in Zukunft die Einhaltung der Schadstoffgrenzwerte nicht nur innerhalb der vorgeschriebenen Fahrzyklen, sondern auch im gesamten Betriebsspektrum des Fahrzeugantriebs. Diese Real-Driving-Emissions (RDE) sollen nach aktuellen Planungen mit im Fahrzeug mitzuführenden Messstationen (sog. PEMS) ermittelt werden.

Abstract

Emission standards are going to be based on more dynamic and high-load cycles than they are today. At the moment, the NEDC is the standard cycle in Europe. The introduction of the WLTC and the completion of real drive emission standards (RDE) are getting important for the future.

Thus, there is need of new products or a new combination of existing exhaust gas treatment systems to fulfill these emission standards. The emissions shall be reduced in respect of quantity and quality. The balancing act will be to reduce emissions and fuel consumption at the same time.

The conventional strategy for the NEDC consists of optimized emission control in the relevant area of the engine operating map. This is achieved via a combination of engine- based modifications and necessary exhaust gas treatment. The measures will remain the same, although increased effort seems to be likely. This task applies for compression-ignited (CI) as well as spark-ignited (SI) engines.

Development departments of spark-ignited engines have to meet a further challenge. Fine-particle emissions are going to be limited from 2015 on. This applies SI engines with direct injection and lean operation. Modifications of mixture formation, combustion process and probably the exhaust gas treatment will be the result. However, these measures will take effect on fuel consumption. The particle filter for SI engines, for example, raises fuel consumption due to a higher exhaust back pressure and regeneration mode.

In addition, critical conditions for emissions and fuel consumption are during engine start, warm-up and catalyst heating mode. SI engines usually run with an enriched airfuel- mixture at full-load and high engine speed, due to performance and component protection. This causes high fuel consumption and emissions, since the three-way catalyst cannot operate at non-stoichiometric conditions. The only solution will be to avoid an enriched mixture and develop other measures (e.g. intensified charge-air cooling).

Stoichiometric and monovalent CNG-SI engines are a promising solution for decreased limited pollutants and CO2-emissions. However, the exhaust gas treatment of CNG-engines is still not solved satisfactorily.