conventional vehicles, use current technology, and are no more limited in range than current vehicles.

Recently, a much greater use of diesel engines in passenger cars has also been promoted in part as a way to get better fuel economy. Diesel engines have been greatly improved in recent years in noise and smoke production and in cold-starting ability. However, they continue to produce higher NO_x and PM emissions than gasoline engines. Great efforts are now under way to devise emission controls to treat these emissions and meet Tier II emission standards.

Beyond the lead phase-out, several states preceded the federal government in regulating motor-vehicle fuels. In the early 1970s, California imposed limits on the Reid vapor pressure (RVP,¹⁵ a measure of volatility) of gasoline sold in some parts of the state. RVP affects evaporative emissions of gasoline from vehicles as well as from storage tanks and distribution facilities. California was also the first state to impose limits on the sulfur content of gasoline.

In the years preceding adoption of the 1990 CAA Amendments, increased action by states was focused on the need to regulate fuels in addition to motor vehicles. In 1988, Colorado pioneered oxygenate requirements for gasoline—specifying a minimum oxygen content of 1.5% by weight during the winter—to reduce CO emissions. Several other western states soon followed suit.

In 1989, first the northern states and then the federal government took another step in fuel property regulation, imposing RVP limits for gasoline. At the time, RVP limits, as compared with other various mobile- and stationary-source controls, were judged to be by far the most cost-effective means available for reducing VOCs and, in turn, O₃ which had become an important issue because of the pervasive violations of the O₃ NAAQS that occurred during the summer of 1988 (OTA 1989). Average RVP levels had steadily risen during the 1980s, in part because of the use of light hydrocarbons to replace tetraethyl lead as octane enhancers (see Box 4-4).

The 1988 California Clean Air Act charged the Air Resources Board with attending to both vehicles and their fuels in pursuit of the maximum possible emission reductions of VOCs and NO_x. The Atlantic Richfield Company (ARCO) introduced the first reformulated gasoline (RFG), known as EC-1, in Southern California. EC-1 contained reduced amounts of olefins, aromatics, lead, and sulfur as compared to regular unleaded gasoline; had a lower RVP; and had methyl tert-butyl ether (MTBE) added to raise the oxygen content.