The diesel is a clear winner, but maintaining its high fuel efficiency while reducing its emissions of particulate matter and nitrogen oxides is a challenge. However, today's diesels have a potentially fatal flaw. They are major contributors to ambient concentrations of both PM and NOx, pollutants that have adverse health and environmental impacts, including the formation of ozone. As a result, today's diesel fleet is substantially dirtier than today's gasoline fleet. Today, about a quarter of the total fuel used in vehicles and off-road equipment is diesel.
Diesels are now in the emissions-control spotlight.
The PM standard takes effect in 2007 and includes finer particles than previously limited. The NOx standard will be phased in from 2007 to 2010, and a sulfur cap of 15 ppm for highway diesel fuel will be phased in starting in 2006. Speakers from Europe and Japan noted that the new US regulations are somewhat stricter than those in other countries, though the gap is narrowing. Many European countries typically place more emphasis on limiting emissions of CO 2 and other greenhouse gases, and tax incentives are used to encourage diesel use and fuel cleanup. Symposium participants disagreed considerably about the timing and appropriateness of the evolving US diesel regulations.