Diesel Emission Standards for Heavy-Duty
On- and Off-Road Engines

On- and off-road diesel emissions are regulated at the federal level through engine emissions standards that apply to new engines and through regulations on diesel fuel. In addition, the State of Colorado regulates diesel smoke emissions through its Diesel Inspection/Maintenance Program. (See the insert on “Colorado’s Diesel Inspection/Maintenance Program” for more information.)

The U.S. Environmental Protection Agency (EPA) has regulated on-road diesel emissions at the federal level through the Clean Air Act (CAA) since 1970. It was only 13 years ago in 1990 that EPA amended the CAA to include regulations for off-road diesel vehicles. The standards for on- and off-road engines set maximum allowable levels of emissions for new engines and diesel fuel.

EPA has made many revisions to the CAA over the past thirty years, mandating more stringent standards to reduce diesel emissions. In 1977, the CAA mandated 90 percent reductions for levels of carbon monoxide (CO) and hydrocarbons (HC) for heavy-duty, on-road vehicles by 1984, and a 75 percent reduction in nitrogen oxide (NOx) emissions by 1985. Further amendments to the CAA in 1990 and recent regulations to further reduce NOx emissions from heavy-duty diesel engines by over 50 percent from 1998 levels have resulted in significant reductions.

Advances in diesel engine technology have also made diesel engines significantly cleaner in recent decades. (See the insert on “Advances in Diesel Engine Technology” for more information.) However, as EPA continues to learn more about the adverse health effects from exposure of diesel exhaust, it will continue to propose new programs and regulations to protect public health.

The following sections will provide more detailed information on existing federal regulations for both on- and off-road diesel engines and fuel. In addition, it will provide information on upcoming issues and provide resources to find out more about existing and future emissions standards.

On-Road Heavy-Duty Diesel Emissions Standards

Historical Certified Emissions Standards

1970-85 - Emissions standards for heavy-duty diesel engines were introduced in 1970 with a 13-mode, steady state test procedure, which continued through the 1983 model year. In 1974, HC+NOx and CO standards were introduced. Additionally, opacity smoke standards were tightened to levels (Accel=20%, Lug=15%, and Peak=50%), which are current today. The first NOx emissions standard (10.7 g/bhp-hr) and an optional transient test were added in 1984. The steady state test was eliminated in 1985.

1988-98 - A particulate matter (PM) standard (0.6 g/bhp-hr) was added in 1988 and PM and NOx standards continued to be tightened (6.0 g/bhp-hr in 1990 and 5.0 g/bhp-hr in 1991) to their current levels (4.0 g/bhp-hr in 1998). Technological changes began in 1990 with turbocharging and retarded fuel injection timing, among others. Initial use of electronic controls started in 1991 and was used on most engines by 1994 and nearly all engines by 1998. Oxidation catalysts and improvements to combustion chamber design were introduced in 1994. Further improvements in turbocharging continued through 1998.

Future Certified Emissions Standards

2002-04 – A NOx+HC standard of 2.5 g/bhp-hr, with an HC limit of 0.5 g/bhp-hr was introduced in 2002. In a Consent Decree with the federal government, six engine manufacturers agreed to produce engines meeting the 2004 standard by October 2002.

2007 – The standards will be lowered in 2007 to 0.01 g/bhp-hr for PM and 0.20 g/bhp-hr for NOx. In order to achieve the 2007 standards, an ultra-low sulfur fuel (sulfur content 15 ppm) is required and after-market technology is available. This equipment includes catalyzed traps, oxidation catalysts, NOx absorbers, and selective catalytic reduction systems. (See the insert on “Emission Control Technology” for more information.)

Off-Road Heavy-Duty Diesel Emissions Standards

Emissions standards for today’s off-road engines are relatively lenient compared to those standards for on-road engines. To date, the standards focus mostly on NOx reductions, however, future programs will focus on significant reductions (up to 90 percent) from both PM and NOx.

Current Emissions Standards

Tier 1 standards for off-road engines were established in 1994 and were phased in between 1996 and 2000 for various horsepower ratings. Tier 1 standards will remain in effect until the Tier 2 and/or Tier 3 standards take effect. Tier 2 and Tier 3 standards were established in 1998 to be phased in for various horsepower ratings between 2001 and 2008.

Future Emissions Standards

In April 2003, EPA proposed a national program to further reduce emissions from off-road diesel engines used in the industrial, construction, and agricultural industries by using engine and fuel controls to achieve greater emissions reductions for both NOx and PM. The plan will call for engine manufacturers to produce new engines with advanced emission control technologies. In order to allow engine manufacturers sufficient time, the proposed standards could potentially take effect as early as 2008 and be fully phased in by 2014. The engines affected by this new proposal “currently account for about 44 percent of total mobile source diesel PM emissions and about 12 percent of total NOx emissions from mobile sources nationwide,” according to EPA.

Off-Road Fuel Standards

Current off-road fuel standards allow up to 5,000 ppm sulfur content, however, EPA estimates current fuel has sulfur levels of about 3,400 ppm on average nationwide. According to preliminary findings (April 2003) of a Colorado Diesel Emission Study, conducted by the Colorado Department of Public Health and Environment, off-road fuel samples contained a median average of 385 ppm sulfur, well below national levels assumed by EPA and below the level EPA proposed under a new program to reduce emissions from off-road vehicles.

Under EPA’s new proposal, a limit of 500 ppm sulfur content will take effect in 2007 for all off-road equipment, the current standard for on-road equipment. A second step – to limit fuel to 15 ppm in sulfur content – would take effect in 2010, three years after the 15 ppm standard takes effect for on-road equipment.

Contact Information

For more information, contact the U.S. EPA’s Office of Transportation and Air Quality at 734-214-4333 or visit the web site at www.epa.gov/otaq.

Sources

Diesel Technology Forum – www.dieselforum.com
Engine Manufacturers Association – www.engine-manufacturers.org
Report to the Regional Air Quality Council and the Air Quality Control Commission – Reducing Diesel Emissions in the Denver Region, May 2002
U.S. Environmental Protection Agency – www.epa.gov/otaq