Equipment Type

Diesels Lunge Toward the Emissions Goal Line

With 90 percent of pollutants removed from diesel exhaust, government and industry unleash an all-out effort to do away with the final foul fraction

November 01, 2005

September 2006

Ultra-low-sulfur diesel fuel (15 parts per million sulfur) required for highway diesels. EPA estimates this highly refined fuel will cost 4.5 to 5 cents per gallon more than 500-ppm-sulfur diesel.


Lower sulfur diesel fuel (500 ppm sulfur) required for non-road diesels.


Compared to 1988 engines, both NOx and particulate emissions from new on-highway diesels are reduced by about 98 percent Ultra-low sulfur diesel fuel (15 ppm sulfur) required for non-road diesels


The full inventory of non-road diesels are replaced by Tier 4 engines, reducing NOx emissions by more than 1.7 million tons per year. Every year, 12,000 premature deaths are prevented. Source: EPA

The Clean Air Act that created the Environmental Protection Agency in the early 1970s set a distant, ambitious goal to clean America's air enough to protect public health, especially for babies, the elderly, and others with respiratory weakness.

Manufacturers of heavy-duty diesel engines have cut nearly 90 percent of pollutants leaving their products' exhaust stacks, and they are striving to clean up what may turn out to be the hardest 8 percent of all.

The next major deadline for on-highway trucks, EPA emission standards for 2007, brings draconian change. Allowable levels of the most critical diesel pollutants — nitrogen oxides (NOx) and particulate matter (PM) — are both to be cut by a factor of 10. The PM limit applies across the board in 2007, while the NOx limit will be phased in through 2010. Manufacturers will even be expected to control crankcase emissions.

To support the technologies necessary to produce this extraordinary change, EPA will impose a limit on sulfur allowable in diesel fuel. Today's on-highway fuel at 500 parts per million (ppm) sulfur will foul the catalytic filters and traps that are necessary to meet the NOx and PM limits. Highway diesel fuel sold at the terminal can include no more than 15 ppm sulfur as of July 15, 2006. Retail fuel stations and wholesale purchasers will get this ultra-low-sulfur diesel (ULSD) in September.

Effective June 2007, non-road diesels, which include construction and agricultural equipment, will be required to burn the equivalent of today's low-sulfur fuel at 500 ppm sulfur. By June 2010, non-road equipment will be restricted to using ULSD.

It's no coincidence that off-road equipment will have to use the same fuel as on-highway trucks. They will likely be using much of the same emissions-reduction technology by then. Most of the disparities in exhaust limits between construction equipment and highway trucks will gradually disappear by 2011. In that year, on-highway NOx is expected to be 0.2 grams per brake horsepower hour and PM will be 0.01 grams. Non-road engines from 175 to 750 horsepower, under Tier 4 regulations, will be allowed only 0.4 grams of NOx and 0.02 grams of PM. By 2014, 75 to 750 horsepower non-road engines will be allowed just 0.3 grams of NOx and 0.015 grams of PM.

Implementation of non-road engine-emissions limits had been a progression. Each tier was phased in by horsepower rating over several years. Tier 1 standards were completely implemented by 2000. More-stringent Tier 2 standards will be in full effect in 2006. Tier 3 limits began being phased in this year for some makes of over-175-hp diesels and will be complete in 2008.

Tier 3 NOx standards are similar to the 2004 on-highway limits. But new standards for PM were never adopted, so the limits remain the same as in Tier 2. It allowed a variety of solutions for off-road equipment.

Caterpillar is one of the only engine makers to choose exhaust aftertreatment devices, just as they have been with truck engines. Charge-air coolers have become common in construction equipment, though, as the cooling of intake air lowers combustion temperature and moderates NOx production. Some designers have chosen to use exhaust-gas recirculation, claiming the more complex technology delivers improved performance in certain engine sizes.

In 2004, EPA signed the final rule introducing Tier 4 emission standards for non-road engines, which are to be phased in from 2008 to 2015. Tier 4 slashes PM and NOx emissions by an additional 90 percent and brings them to near parity with on-highway engines.

At the time of signing the 1998 rule that introduced Tiers 1, 2 and 3, the EPA estimated that NOx emissions in 2010 would be reduced by about a million tons per year — equivalent to taking 35 million passenger cars off the road. Now EPA anticipates that when the full inventory of older non-road engines are replaced by Tier 4 engines, annual emissions will be cut by an additional 738,000 tons of NOx and 129,000 tons of PM. By 2030, 12,000 premature deaths would be prevented annually.

Costs for added emission controls for most off-road equipment was estimated at 1 to 3 percent of the total equipment price. The EPA also estimates that the 2007 on-highway emission standards will raise vehicle costs $1,200 to $1,500, compared to the typical cost of a new heavy-duty truck at up to $150,000.

EPA estimates that the average cost of ULSD will be 7 cents per gallon more than today's on-highway fuel. The agency expects that cost to be offset somewhat by 4 cents per gallon in anticipated reduction of maintenance costs due to cleaner-burning low-sulfur diesel.

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