USA: Nonroad Diesel Engines

Background

Emission standards for nonroad (or off-road) engines and vehicles are set by the US EPA. In most cases, federal nonroad engine regulations also apply in California, whose authority to set emission standards for new nonroad engines is limited. The federal Clean Air Act Amendments of 1990 (CAA) preempt California’s authority to control emissions from new farm and construction equipment under 175 hp [CAA Section 209(e)(1)(A)] and require California to receive authorization from the federal EPA for controls over other off-road sources [CAA Section 209 (e)(2)(A)].

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The important steps in nonroad engine emission regulations include:

• Tier 1-3 Standards. The first federal standards (Tier 1) for new nonroad diesel engines were adopted in 1994 for engines over 37 kW (50 hp), to be phased-in from 1996 to 2000. In 1996, a Statement of Principles (SOP) pertaining to nonroad diesel engines was signed between EPA, California ARB and engine makers (including Caterpillar, Cummins, Deere, Detroit Diesel, Deutz, Isuzu, Komatsu, Kubota, Mitsubishi, Navistar, New Holland, Wis-Con, and Yanmar). On August 27, 1998, the EPA signed the final rule reflecting the provisions of the SOP [2787]. The 1998 regulation introduced Tier 1 standards for equipment under 37 kW (50 hp) and increasingly more stringent Tier 2 and Tier 3 standards for all equipment with phase-in schedules from 2000 to 2008. The Tier 1-3 standards are met through advanced engine design, with no or only limited use of exhaust gas aftertreatment (oxidation catalysts). Tier 3 standards for NOx+HC are similar in stringency to the 2004 standards for heavy-duty onroad engines, however Tier 3 standards for PM were never adopted.
• Tier 4 Standards. On May 11, 2004, the EPA signed a final rule introducing Tier 4 emission standards, which were phased-in over the period of 2008-2015 [2786]. The Tier 4 standards require that emissions of PM and NOx be further reduced by about 90%. These emission reductions have been achieved through the use of advanced exhaust gas aftertreatment technologies, with most Tier 4 engine families using urea-SCR catalysts for NOx control. Some Tier 4 engines also utilize a DPF, while others meet the standards without particulate filters.
• Tier 5 Standards. In November 2021, the California Air Resources Board held the first public workshop on the development of Tier 5 emission standards that will seek to further reduce NOx and PM emissions by 50-90%, depending on the engine power category, in the 2028-2030 timeframe. The considered changes also include a new low load certification test cycle (LLAC), extended full useful life (FUL) and emission warranty periods, OBD requirements, and more. However, due to the California preemption, California Tier 5 standards would have a limited scope and could produce only very limited emission reductions, unless a corresponding nonroad engine regulation be adopted by the EPA.

Nonroad Diesel Fuel. At the Tier 1-3 stage, the sulfur content in nonroad diesel fuels was not limited by environmental regulations. The oil industry specification was 0.5% (wt., max), with the average in-use sulfur level of about 0.3% = 3,000 ppm. To enable sulfur-sensitive control technologies in Tier 4 engines—such as catalytic particulate filters and NOx adsorbers—the EPA mandated reductions in sulfur content in nonroad diesel fuels, as follows:

• 500 ppm effective June 2007 for nonroad, locomotive and marine (NRLM) diesel fuels
• 15 ppm (ultra-low sulfur diesel) effective June 2010 for nonroad fuel, and June 2012 for locomotive and marine fuels

The US nonroad emission standards are harmonized to a certain degree with European nonroad emission standards.

EPA emission standards for nonroad diesel engines are published in the US Code of Federal Regulations, Title 40, Part 89. Regulatory text, fact sheets and related documents are available from the EPA web site [2788].

Applicability

The nonroad standards cover mobile nonroad diesel engines of all sizes used in a wide range of construction, agricultural and industrial equipment. The EPA definition of the nonroad engine is based on the principle of mobility/portability, and includes engines installed on (1) self-propelled equipment, (2) on equipment that is propelled while performing its function, or (3) on equipment that is portable or transportable, as indicated by the presence of wheels, skids, carrying handles, dolly, trailer, or platform [40 CFR 1068.30]. In other words, nonroad engines are all internal combustion engines except motor vehicle (highway) engines, stationary engines (or engines that remain at one location for more than 12 months), engines used solely for competition, or engines used in aircraft.

Effective May 14, 2003, the definition of nonroad engines was changed to also include all diesel powered engines—including stationary ones—used in agricultural operations in California. This change applies only to engines sold in the state of California; stationary engines sold in other states are not classified as nonroad engines.

The nonroad diesel emission regulations are not applicable to all nonroad diesel engines. Exempted are the following nonroad engine categories:

• Engines used in railway locomotives; those are subject to separate EPA regulations.
• Engines used in marine vessels, also covered by separate EPA regulations. Marine engines below 37 kW (50 hp) are subject to Tier 1-2—but not Tier 4—nonroad standards. Certain marine engines that are exempted from marine standards may be subject to nonroad regulations.
• Engines used in underground mining equipment. Diesel emissions and air quality in mines are regulated by the Mine Safety and Health Administration (MSHA).
• Hobby engines (below 50 cm³ per cylinder)

Examples of regulated applications include farm tractors, excavators, bulldozers, wheel loaders, backhoe loaders, road graders, diesel lawn tractors, logging equipment, portable generators, skid steer loaders, or forklifts.

A new definition of a compression-ignition (diesel) engine was introduced in the 1998 rule, consistent with definitions established for highway engines. The definition focuses on the engine cycle, rather than the ignition mechanism, with the presence of a throttle as an indicator to distinguish between diesel-cycle and otto-cycle operation. Regulating power by controlling the fuel supply in lieu of a throttle corresponds with lean combustion and diesel-cycle operation. This language allows the possibility that a natural gas-fueled engine equipped with a spark plug is considered a compression-ignition engine.

Tier 1-3 Emission Standards

The 1998 nonroad engine regulations were structured as a 3-tiered progression. Each tier involved a phase-in (by horsepower rating) over several years. Tier 1 standards were phased-in from 1996 to 2000. The more stringent Tier 2 standards took effect from 2001 to 2006, and yet more stringent Tier 3 standards phased-in from 2006 to 2008 (Tier 3 standards applied only for engines from 37-560 kW).

Tier 1-3 emissions standards are listed in Table 1. Nonroad regulations use the metric system of units, with regulatory limits expressed in grams of pollutant per kWh.

Manufacturers who signed the 1998 Consent Decrees with the EPA may have been required to meet the Tier 3 standards one year ahead of schedule (i.e. beginning in 2005).

Voluntary, more stringent emission standards that manufacturers could use to earn a designation of “Blue Sky Series” engines (applicable to Tier 1-3 certifications) are listed in Table 2.

Engines of all sizes had to meet smoke standards of 20/15/50% opacity at acceleration/lug/peak modes, respectively.

The regulations included several other provisions, such as averaging, banking and trading of emission credits and maximum “family emission limits” (FEL) for emission averaging.

Tier 4 Emission Standards

The Tier 4 emission standards—phased-in from 2008 through 2015—introduce substantial reductions of NOx (for engines above 56 kW) and PM (above 19 kW), as well as more stringent HC limits. CO emission limits remain unchanged from the Tier 2-3 stage.

Engines up to 560 kW. Tier 4 emission standards for engines up to 560 kW are listed in Table 3.

In engines of 56-560 kW rated power, the NOx and HC standards were phased-in over a few year period, as indicated in the notes to Table 3. The initial standards (PM compliance) are sometimes referred to as the ‘interim Tier 4’ (or ‘Tier 4i’), ‘transitional Tier 4’ or ‘Tier 4 A’, while the final standards (NOx/HC compliance) are sometimes referred to as ‘Tier 4 B’.

As an alternative to introducing the required percentage of Tier 4 compliant engines, manufacturers may certify all their engines to an alternative NOx limit in each model year during the phase-in period. These alternative NOx standards are:

• Engines 56-130 kW:
  • Option 1: NOx = 2.3 g/kWh = 1.7 g/bhp-hr (Tier 2 credits used to comply, MY 2012-2013)
  • Option 2: NOx = 3.4 g/kWh = 2.5 g/bhp-hr (no Tier 2 credits claimed, MY 2012-2014)
• Engines 130-560 kW: NOx = 2.0 g/kWh = 1.5 g/bhp-hr (MY 2011-2013)

Engines Above 560 kW. Tier 4 emission standards for engines above 560 kW are listed in Table 4. The 2011 standards are sometimes referred to as ‘transitional Tier 4’, while the 2015 limits represent final Tier 4 standards.

Other Provisions. The Tier 4 regulation and later amendments include a number of additional provisions:

• Smoke Opacity—Existing Tier 2-3 smoke opacity standards and procedures continue to apply in some engines. Exempted from smoke emission standards are engines certified to PM emission standards at or below 0.07 g/kWh (because an engine of such low PM level has inherently low smoke emission).
• Crankcase Ventilation—The Tier 4 regulation does not require closed crankcase ventilation in nonroad engines. However, in engines with open crankcases, crankcase emissions must be measured and added to exhaust emissions in assessing compliance.
• DEF Refill Interval—For SCR-equipped nonroad diesel engines, a minimum DEF (urea solution) refill interval is defined as at least as long (in engine-hours) as the vehicle’s fuel capacity [3408].
• Ammonia Emissions—While ammonia emissions are unregulated, the EPA recommends that ammonia slip should be below 10 ppm average over the applicable test cycles [3693].
• Emergency Operation—To facilitate the use of certain nonroad engines in temporary emergency situations, the engines can be equipped with an AECD to override performance inducements related to the emission control system—for example, to allow engine operation without urea in the SCR system during an emergency [3408]. This flexibility is intended primarily for engines used in construction equipment and portable equipment used for temporary power generation and flood control.
• ABT Program—Similarly to earlier standards, the Tier 4 regulation includes such provisions as averaging, banking and trading of emission credits and FEL limits for emission averaging.

Test Cycles and Fuels

Nonroad engine emissions are measured on a steady-state test cycle that is equivalent to the ISO 8178 C1, 8-mode steady-state test cycle. Other ISO 8178 test cycles are allowed for selected applications, such as constant-speed engines (D2 5-mode cycle), variable-speed engines rated under 19 kW (G2 cycle), and marine engines (E3 cycle).

Transient Testing. Tier 4 standards have to be met over both the steady-state test and the nonroad transient cycle, NRTC. The transient testing requirements started with MY 2013 for engines below 56 kW, MY 2012 for 56-130 kW, and MY 2011 for 130-560 kW engines. Engines above 560 kW are not tested on the transient test. Also constant-speed, variable-load engines of any power category are not subject to transient testing. The NRTC protocol includes a cold start test. The cold start emissions are weighted at 5% and hot start emissions are weighted at 95% in calculating the final result.

Tier 4 nonroad engines must also meet not-to-exceed standards (NTE), which are measured without reference to any specific test schedule. The NTE standards became effective in 2011 for engines above 130 kW; in 2012 for 56-130 kW; and in 2013 for engines below 56 kW. In most engines, the NTE limits are set at 1.25 times the regular standard for each pollutant. In engines certified to NOx standards below 2.5 g/kWh or PM standards below 0.07 g/kWh, the NTE multiplier is 1.5. The NTE standards apply to engines at the time of certification, as well as in use throughout the useful life of the engine. The purpose of the added testing requirements is to prevent the possibility of “defeating” the test cycle by electronic engine controls.

Certification Fuels. Fuels with sulfur levels no greater than 0.2 wt% (2,000 ppm) were used for certification testing of Tier 1-3 engines. From 2011, all Tier 4 engines are tested using fuels of 7-15 ppm sulfur content. The transition from the 2000 ppm S specification to the 7-15 ppm specification took place in the 2006-2010 period (see Certification Diesel Fuel).

A change from measuring total hydrocarbons to nonmethane hydrocarbons (NMHC) has been introduced in the 1998 rule. Since there is no standardized EPA method for measuring methane in diesel engine exhaust, manufacturers can either use their own procedures to analyze nonmethane hydrocarbons or measure total hydrocarbons and subtract 2% from the measured hydrocarbon mass to correct for methane.

Environmental Benefit and Cost

1998 Regulation

At the time of signing the 1998 rule, the EPA estimated that by 2010 NO_x emissions would be reduced by about a million tons per year, the equivalent of taking 35 million passenger cars off the road.

The costs of meeting the emission standards were expected to add under 1% to the purchase price of typical new nonroad diesel equipment, although for some equipment the standards may cause price increases on the order of 2-3%. The program was expected to cost about $600 per ton of NO_x reduced.

Tier 4 Regulation

When the full inventory of older nonroad engines are replaced by Tier 4 engines, annual emission reductions are estimated at 738,000 tons of NOx and 129,000 tons of PM. By 2030, 12,000 premature deaths would be prevented annually due to the implementation of the proposed standards.

The estimated costs for added emission controls for the vast majority of equipment was 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.

EPA estimated that the average cost increase for 15 ppm S fuel would be 7 cents per gallon. This figure would be reduced to 4 cents by anticipated savings in maintenance costs due to low sulfur diesel.