Symposium Summary: The Future of Diesel: Scientific Issues 2000 Air Pollution Symposium

December 2000

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The Summer Air Symposium is a continuation of conferences establis hed in 1993 for the purpose of informing decision makers on scientific aspects of important air pollution issues. Participation is limited to invited members of the scientific, regulatory, industrial, and public interest communities. The small size of the group promotes discussions that have led to important progress on ideas for improvements in air pollution control. The conference is co-sponsored by a variety of organizations (see Appendix I).

The objective of the 2000 Symposium was to discuss scientific aspects of diesel engines and fuel, including:

Widespread Diesel: Diesel engines are in widespread use for a number of reasons. They are the most economical engines to own and operate due to their high power density. They are highly reliable and very durable, with an effective infrastructure for fuel distribution. Models are available in a wide power range, and the engine can be modified to operate with alternate fuels.

Diesel Pollution: Diesels have become a large part of the NOx pollution problem. NOx from gasoline-powered vehicles has declined due to emission controls, and stationary-source NOx has also decreased substantially in most parts of the country. However diesel emissions have been essentially uncontrolled for the last thirty years while use of diesel vehicles has been increasing. Diesels also contribute substantially to ambient concentrations of particulate matter (PM), although the variation in emission rates for the in-use fleet is large. It is known that in-use diesel NOx emissions are high from a variety of consistent data and corollary information, however uncertainties remain, such as fleet activity and how representative test cycles are. There are tradeoffs between fuel economy, PM, and NOx in injection timed engines.

Heavy duty diesel trucks dominate heavy duty diesel vehicle emissions, however transit buses and school buses have localized exposures that are of concern. Off-road emissions are more distributed. Construction is the major source, followed by locomotives, agriculture, marine, and commercial.

Diesel Regulation: On June 2, 2000, EPA published proposed Phase 2 rules. These rules attempt to regulate vehicle and fuel as a system, aiming for 90% reduction in PM, NOx, and HC emissions in new vehicles. EPA anticipates very low emissions from these requirements if sulfur levels in fuel are reduced. New federal diesel truck emission standards have been proposed by U.S.EPA, and these will reduce both NOx and PM by 98% from uncontrolled levels. Off-road diesels are as important as buses and trucks, because they emit as much NOx and twice as much PM, and new off-road emission standards are being developed. Greater than 97% reduction in off-road emissions, compared to uncontrolled levels, is achievable. Defining the certification process is as important as the emission standard set, because that is how it is determined whether the standard is met or not.

In Europe, where diesel is a popular choice for automobiles and light duty vehicles, regulations for 2005-2008 are similar to the US for PM but not for NOx. Some European governments are requiring diesel particulate filters for all trucks starting in 2005. NOx limits for 2005 are not stringent, but 2008 limits will require a NOx emission control device. The EU 2005 standards are probably the last time diesel engines will face less stringent regulations than gasoline-powered light duty cars and trucks. Japans current standards are less stringent than Europe for both NOx and PM, and less stringent for US standards for PM but similar for NOx. New long-term standards are currently under discussion. They are not anticipated to be as stringent for PM but similar for NOx compared to future US and European requirements. In general, there is more emphasis on CO2 and greenhouse gases in Europe (and to a lesser extent Japan) whereas in the US, the emphasis is on conventional pollutants.

Issues: Active after-treatment for both NOx and PM may be needed in the future. There are interesting and promising developments with respect to after-treatment, but no proven devices. The control of NOx is the more difficult problem. The fundamental constraint is that there is no catalyst available to facilitate the dissociation of NOx. Selective catalytic reduction is feasible, as are NOx traps that store NOx as nitrate, and plasma systems.

Fuel changes are a key enabling technology. Current California requirements call for 15 ppm sulfur in diesel fuel used in transit buses as of 2002. EPAs proposed Phase II rules call for a sulfur cap of 15 ppm for highway diesel fuel by 2006. This is in contrast to 350 ppm sulfur in current US on-road fuel. Engine manufacturers are calling for ultra-low sulfur fuel, arguing that 15 ppm isnt low enough. However oil companies question whether this can be accomplished. Concerns about ultra-low sulfur fuel include whether enough refineries can upgrade fast enough to meet supply needs, and how contamination and measurement affect the ability to meet standards.

Health concerns will continue to drive further regulation of emissions from diesel engines. EPA has been assessing diesel health risk for 12 years. Diesel exhaust contains carbonaceous particles with some organics adhered, gaseous phase organics, and some metals. All of these have associated toxicological issues. In California, the Multiple Air Toxics Exposure Study (MATES II) found that ~70% of all cancer risk in the South Coast Air Basin due to outdoor air pollution is attributable to diesel PM. MATES II estimates that outdoor toxic air pollution accounts for less than 1% of cancer due to all risk factors. There is a growing interest in the size distribution of particulates. Particulates smaller than 100 nm are low in mass but high in number, and toxicologists need to determine how important they are.

Control of emissions from in-use vehicles is expected to get increasing regulatory attention. Retrofit programs can significantly reduce PM, HC, CO, and toxic emissions. There is also the potential for NOx reductions. California is planning to retrofit 1.25 million engines, mostly mobile and some stationary.


MITEI Author

John Heywood Professor Emeritus

Department of Mechanical Engineering

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