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The post-1998 diesel engine emissions challenge was put forth in July 1995 by the Statement of Principles (SOP) signed by the manufacturers of heavy duty engines, the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). Through this SOP, the signatories agreed to reduce the on-highway diesel engine NOx emissions by 50% from the legislated 1998 4.0 g/bhp.hr to 2.0 g/bhp.hr by the year 2004 with no increase over the 1998 particulate matter legislated level set at 0.1 g/bhp.hr. There are provisions in the SOP for the optional grouping of the gaseous hydrocarbons and NOx, limiting them at a combined value of 2.5 g/bhp.hr with a 0.5 g/bhp.hr hydrocarbon limit. In North America, particulate matter emissions standards were first imposed on heavy duty diesel engines in 1988. Since then, the NOx and particulate matter were balanced by taking advantage of the trade-off between the two pollutants inherent in diesel engines.

The adoption of significantly more stringent tailpipe emission standards is leading the domestic car manufacturers and oil companies to reexamine the composition of commercial gasolines. One offshoot of this examination is a cooperative inter-industry Auto/Oil Air Quality Improvement Research Program. This study will attempt to determine the potential reductions in total vehicle emissions and resultant improvements in air quality from the use of reformulated gasolines. One important aspect of the program is to determine the effect of variable aromatic and olefinic hydrocarbon content as well as variable levels of oxygenates on emission levels from a statistically-designed vehicle program. Much laboratory, engine and vehicle emission data has been generated over the past two decades that is potentially useful in interpreting the tailpipe emission patterns associated with proposed fuel composition changes.