Abstract
Modern diesel automotive engines can improve the fuel efficiency of passenger cars and light-duty trucks compared to the current port-injected, spark-ignited gasoline engine. Unfortunately, the stringent North American NOx emissions standards pose a major challenge for the rapid expansion of diesel engines in the North American market.
Although urea selective catalytic reduction (SCR) systems have demonstrated good NOx control performance and fuel sulfur tolerance, the lack of urea dispensing infrastructure and regulatory uncertainties concerning urea re-fill strategies have limited the adoption of this technology for the North American car and light-truck market.
The lean NOx trap (LNT) offers an alternative to urea SCR, but it faces a different set of challenges such as: limited temperature NOx performance, low tolerance to sulfur and the need for high precious metals loadings.
The LNT chemically stores NOx during lean operation, and then requires frequent "NOx purging" with rich exhaust reductants. In the fuel-rich environment, the NOx is released and can then be converted to N2 using an oxidizing-reducing catalyst. To understand the performance capabilities and limitations of lean NOx traps with diesel exhaust, several approaches to generating the rich exhaust feedstream were evaluated on experimental light-duty diesel trucks. Tests were conducted using rich diesel exhaust feedstreams generated by modifying the combustion modes, or by processing diesel fuel on-board with a partial oxidation converter.
The results were analyzed to better understand the chemical and physical factors that limit diesel LNT performance. The results from this activity lead to greater understanding on the role of reductant type on resulting catalyst and aftertreatment system performance. By incorporating the positive features of the development activity into future system designs, lower emissions and improved performance can be achieved.
Keywords: Diesel, Emission, Aftertreatment, Reforming, NOx