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. Yet, even with the progress achieved thus far it is evident that engine design measures alone will fall short of meeting future NOx/PM emissions limits.
In the meantime, many advances were achieved in diesel exhaust post-combustion emission control devices (after-treatment) technology. Selective catalytic reduction (SCR), used in the past to reduce NOx from large industrial installations, is now being developed for heavy-duty on-highway diesel engines. Diesel particulate filters (DPF), which are known for their ability in reducing PM, have previously suffered from regeneration, reliability, and durability problems. Now they are being included in plans for serial production. In their advanced development and planned production configurations, both technologies depend heavily on engine speed, load, exhaust temperature, and other characteristics. In fact, the move to integrate engine and post-combustion emission control devices (PC-ECD) into one overall emission control system is gaining popularity.
This paper describes the application of SCR and DPF technology to a heavy-duty on-highway diesel engine. Regeneration of the DPF was further enhanced with a cerium-based fuel-borne catalyst, which reduced the ignition temperature of the collected particulate. Results of this development effort not only met the 2002 EPA heavy-duty diesel NOx and PM standards, but went beyond these requirements to achieve less than 1.5 and 0.05 g/bhp-hr respectively, for these pollutants. Detailed accounting of engine- and DPF-out cerium is also provided.