Automotive engineers determined that the restrictive California air emission standards would be impossible to attain using a traditional gasoline-fueled internal combustion engine. Instead, they focused their efforts on alternative fuels, such as hydrogen and compressed natural gas (CNG), or emerging technologies, like fuel cells or gasoline-electric hybrids.

To achieve the SULEV rating, the BMW 3 Series SULEV must be fueled with specially formulated low-sulfur gasoline; however, there is no loss of performance if the vehicle is fueled with higher sulfur content fuel, although SULEV emissions might not be attained.

The SULEV engine works to accomplish four main goals:

• Reduce engine-out hydrocarbons to restrict the number of hydrocarbons released into the atmosphere upon ignition and throughout the powerband.
• Shorten catalyst light-off time to abbreviate the catalyst (exhaust) period between detonations.
• Improve hydrocarbon trap efficiency through the use of multiple advanced filters, to virtually eliminate hydrocarbon emissions.
• Stabilize catalyst efficiency to improve the efficacy of catalytic exhaust processing, thereby further reducing pollutants.

The federal government and California (along with New York, Massachusetts, Vermont and Maine currently) have their own emissions rules, and having two sets of standards invites confusion. Decades ago -- before there were any national automotive emissions standards -- California mandated its own, more restrictive emissions rules to ameliorate the severe smog problems in California. Over time, these rules have gradually become not only the standard for the state, but also the standard for the nation and the world.

Though there are many different aspects of vehicle emissions that need to be monitored and limited, the three most important factors are considered to be carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbons (HC), and the accompanying particulate matter. Among these, NOx and particulate matter are most directly associated with smog. Besides being linked to smog problems and changes in the upper atmosphere, each of these pollutants is associated with health risks, including respiratory problems and other long-term effects.

Cars and light trucks that only comply with the US Federal Government's 'Tier 1' emissions are the least-green new cars in terms of tailpipe emissions.

California’s TLEV (transitional low-emission) standard is similar to the Tier 1 federal standard, but with tighter limits on hydrocarbons to aid the state’s smog-prone valleys.

LEV (low-emission) vehicles adhere to a tighter standard than most passenger cars now meet. The LEV standard for oxides of nitrogen is half that of TLEV and Tier 1, and the allowance of hydrocarbon emissions is further tightened. Cars complying with LEV standards are now widely available nationwide.

ULEV (ultra-low-emission) vehicles further reduce the permissible levels of hydrocarbons and carbon dioxide emissions by 50 percent, compared to LEV. Many models now meet ULEV standards in California, but not necessarily in other states. In simple terms, cars rated ULEV are up to 50 percent cleaner than LEV cars.

SULEV (super-ultra-low-emission) is the new emissions standard to meet. The SULEV standard—about 90 percent cleaner than LEV—is a big leap forward. It requires emissions equipment to perform within the standards for 150,000 miles, and dramatically reduces the allowed amounts of hydrocarbon and carbon dioxide emissions. Only two non-hybrid gasoline-engine vehicles met SULEV standards for 2002:

• Honda Accord (four-cylinder, automatic only)
• Nissan Sentra CA (a special California model)

Federal 'Tier 2' standards will be required nationwide beginning in 2004. Along with California’s tighter new minimum requirement called LEV II, they require all vehicles to comply with much tighter limits on hydrocarbons and particulate matter—still not as tight as SULEV standards.