New Emissions Targets Have Diesel Engine Industry Embracing Exhaust Gas Post-Combustion Treatment

Diesel Progress North American Edition, Dec, 1999 by W. Addy Majewski

A few years ago, discussions could be heard about the future powerplant of choice in the heavy-duty vehicle market. Would the diesel withstand the pressures from competing engine technologies? It has. Today's discussions focus on whether diesel can expand into the light-duty vehicle market in North America. And the expectations are that it will, especially in such applications as light-duty trucks, SUVs, or minivans. There is only one condition: diesel has to be clean.

In the popular perception, the diesel engine is still dirty. Not everyone, even among those who publicly speak in the clean air debate, realizes the progress that has been achieved over the last years or the environmental benefits of diesels. These benefits, relative to spark-ignited (SI) engines, include low carbon monoxide (CO) and hydrocarbons (HC) emissions, as well as low carbon dioxide (greenhouse gas) emissions. Contribution of diesels to the U.S. nationwide emission inventory of CO and VOC from all mobile sources is only 4 percent and 9 percent, respectively. The toxicity of CO has been established years ago beyond any doubt, while many hydrocarbons are characterized by high smog forming reactivity. Diesel engines provide an easy and efficient way of reducing these emissions. The downsides of diesels are emissions of oxides of nitrogen ([NO.sub.x]) and particulate matter (PM). Contribution of diesels is estimated at 42 percent of [NO.sub.x] and 65 percent of PM-10 from all mobile sources in the U.S.

Emissions of [NO.sub.x], a known smog-forming gas, continue to be of primary importance for the air quality in all large urban areas, including California and the U.S. Northeast. Emissions of PM, even though medical authorities have not yet reached a consensus on their health effects, are in the focus of emission regulatory agencies. Furthermore, health studies and regulatory initiatives scrutinize some of currently unregulated emissions, such as PM number concentrations or organic air toxics associated with diesel particulates.

Diesel emission control technology is about to see a shift of attitude in relation to exhaust gas aftertreatment. [NO.sub.x] and PM emission standards introduced throughout the 1990s were met primarily by advances in combustion technologies, such as electronic engine and fuel injection control, turbocharging and intake air improvements, combustion chamber design and, for the 2004 heavy-duty (HD) diesel standard, exhaust gas recirculation. Coming regulations, notably the new 2007 HD standard to be proposed by the EPA in 2000, have a clear intention of forcing post-combustion treatment of diesel exhaust gas in the form of particulate filters and advanced catalyst systems. The diesel engine is heading towards embracing a system approach of the engine-aftertreatment package, a philosophy seen in the development of the gasoline engine for almost two decades.

The first diesel emission standard forcing advanced exhaust gas aftertreatment was the heavy-duty Euro IV/V regulation (2005/2008), still pending its final approval by the European Parliament. In the U.S., the EPA proposed earlier this year Tier 2 light-duty standards, with [NO.sub.x] and PM limits far below the capabilities of today's diesel engines. During public discussion of that proposal, light-duty engine and vehicle manufacturers expressed willingness to introduce diesel particulate filters and [NO.sub.x] trap-catalyst systems. Since several of these new post-combustion treatment technologies are intolerant to sulfur, engine and emission aftertreatment manufacturers requested that the Tier 2 emissions standard be accompanied by ultra low sulfur fuel legislation.

In the coming year, a number of regulatory actions will be taken by the EPA in regards to HD diesel engines. Although no official emission limits have been proposed yet, the EPA released emission targets for future regulations, as well as a view on the issue of diesel fuel quality. Coming regulations for new heavy-duty engines include several components, as follows:

* Amendments to the 2004 HD highway engine emission regulation, published as a proposed rule in October, introduce new heavy-duty emission standards for Otto-cycle engines. For diesels, the proposal reaffirms the 2.4 g/bhp-hr [NO.sub.x] HC engine standard for vehicles above 8500 lb. GVWR. New test procedures (steady-state European ESC cycle) are added for heavy-duty diesel engines and vehicles. Engines will have to meet the emission standards on both FTP and ESC tests. In addition, the proposal introduces Not-To-Exceed (NTE) limits, to be met under any testing condition (no specific testing procedure).

* New emissions standards for highway diesel engines will target emission levels of about 0.5 g/bhp-hr for [NO.sub.x] and 0.01 g/bhp-hr for particulate matter. Current standards amount to 4 g/bhp-hr [NO.sub.x] (2.4 g/bhp-hr [NO.sub.x] HC in 2004) and 0.1 g/bhp-hr for PM. The new limits are considered "aftertreatment-forcing" by the EPA. The maximum diesel fuel sulfur level will be set somewhere between 5 to 40 ppm.