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US EPA adopts Tier 4 nonroad engine emission standards

11 May 2004

The US Environmental Protection Agency (EPA) adopted its long expected landmark diesel emission regulation—the final Tier 4 emission standards and diesel fuel rule for new nonroad diesel engines. The new standards, which were proposed in April 2003, mandate emission reductions of particulate matter (PM) and oxides of nitrogen (NOx) on the order of 90%. New diesel engines in most power categories will have to be fitted with advanced emission aftertreatment devices, such as particulate filters and NOx reduction catalysts. Various provisions of the new regulation become effective from 2008 to 2015; for most engine categories, the Tier 4 standards will be phased-in over the period 2011-2014.

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The stringency and the “aftertreatment-forcing” character of the new rule is consistent with the Tier 2 emission standards for light-duty vehicles and the 2007/2010 standards for heavy-duty highway engines. Nonroad engines—with emission standards more relaxed than those for highway engines—have become a significant source of PM and NOx emissions in the USA. According to EPA emission inventory models, nonroad engines are responsible for 47% of the total PM emissions and 25% of NOx emissions from all mobile sources.

The adopted Tier 4 emission standards apply to new mobile nonroad diesel engines, such as those used in construction, agricultural, and industrial equipment. The standards do not apply to locomotive and marine engines, which are covered by separate EPA regulations. Together with the Tier 4 rule, the EPA issued an Advance Notice of Proposed Rulemaking (ANPRM) outlining the intended future emission standards for diesel locomotives and marine diesel engines. Exempted from Tier 4 standards are also mining engines; mining engine emissions and air quality in mines remain under the jurisdiction of Mine Safety and Health Administration (MSHA).

An important part of the new rule is a low sulfur diesel fuel program, which is implemented in order to enable the use of sulfur-sensitive catalytic emission control technologies on nonroad engines. The fuel program introduces the following sulfur limits:

The maximum sulfur content in nonroad diesel fuels is currently not regulated by the EPA. Nonroad fuels meet an industry specification of 0.5% (5000 ppm) sulfur, with an average in-use content of about 3000 ppm (for comparison, sulfur level in highway fuels, currently at 500 ppm, will be capped at 15 ppm from June 2006).

Tier 4 emission standards for PM and NOx and their phase-in periods are listed in the following table.

Tier 4 Emission Standards, g/kWh (g/bhp-hr)
Rated Power, P Year/Phase-in PM NOx
P < 19 kW (25 hp) 2008 0.40 (0.30) 7.5* (5.6)
19 ≤ P < 56 kW (75 hp) 2013 0.03 (0.022) 4.7* (3.5)
56 ≤ P < 130 kW (175 hp) 2012-2014 0.02 (0.015) 0.40 (0.30)
130 ≤ P < 560 kW (750 hp) 2011-2014 0.02 (0.015) 0.40 (0.30)
P ≥ 560 kW (750 hp) generators 2011-2015 0.03 (0.022) 0.67 (0.50)
all other 0.04 (0.03) 3.5 (2.6)
* NOx + NMHC

The EPA estimates that the average cost increase for 15 ppm S fuel will be 7 cents per gallon. This figure would be reduced to 4 cents by anticipated savings in maintenance costs due to low sulfur diesel. The cost of Tier 4 engines is anticipated to increase by about 20%. For most mobile equipment categories, the cost increase of Tier 4 compliant equipment is estimated at 1-3% as a fraction of total equipment price. For example, for a 175 hp bulldozer that costs approximately $230,000 it would cost up to $6,900 to add the advanced emission controls and to design the bulldozer to accommodate the modified engine.

It is not yet clear how well are the Tier 4 standards harmonized with European regulations. In January 2003, the EU proposed its Stage III/IV aftertreatment-forcing standards for nonroad engines, but the final Directive has been delayed, in part due to standard harmonization between EU and US authorities.

Source: US EPA