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The diesel engine has long been the most energy efficient powerplant for transportation. Moreover, diesels emit extremely low levels of hydrocarbon and carbon monoxide that do not require post-combustion treatment to comply with current and projected standards. It is admittedly, however, difficult for diesel engines to simultaneously meet projected nitrogen oxides and particulate matter standards. Traditionally, measures aimed at reducing one of these two exhaust species have led to increasing the other. This physical characteristic, which is known as NOx/PM tradeoff, remains the subject of an intense research effort. Despite this challenge, there is significant evidence that heavy-duty highway engine manufacturers can achieve substantial emission reductions. Many development programs carried out over the last five years have yielded remarkable results in laboratory demonstrations.

Exhaust gas recirculation (EGR) has long been used in gasoline and light-duty diesel engines as a NOx reduction tool. Recently imposed emission regulations led several heavy-duty diesel engine manufacturers to adopt EGR as part of their strategy to reduce NOx. The effectiveness of this technology has been widely documented, with NOx reduction in the range of 40 to 50 percent having been recorded. An inevitable consequence of this strategy is an increase in particulate emission, especially if EGR was used in high engine load modes. Selective catalytic reduction (SCR), a method for NOx reduction, is widely used in stationary applications. There is growing interest and activity to apply it to mobile fleets equipped with heavy-duty diesel engines. Results of this work indicate that SCR has the potential to dramatically reduce NOx in diesel exhaust. Reductions greater than 70 percent were reported by several including the Institute's previous work (SAE Paper No. 1999-01-3564).