The US Environmental Protection Agency (EPA) issued a number of rules to control emissions of toxic air pollutants from existing stationary reciprocating internal combustion engines (RICE):

  • On June 15, 2004, the EPA issued a rule applicable to several new and existing RICE categories, which included emission standards for certain existing spark ignition (SI) stationary engines.
  • On February 17, 2010, the EPA issued a rule to reduce emissions from existing diesel powered stationary engines (compression ignition, CI, engines).
  • On August 20, 2010, the EPA issued a rule to reduce emissions from existing gas-fired stationary engines (spark ignition, SI, engines).
  • On March 9, 2011, the EPA issued a rule introducing several minor amendments and clarifications to the regulation published on August 20, 2010.

The rules, titled National Emission Standards for Hazardous Air Pollutants (NESHAP) for Reciprocating Internal Combustion Engines, are intended to reduce emissions of toxic air pollutants—such as formaldehyde (HCHO), acetaldehyde, acrolein, methanol and other air toxics—from several categories of previously unregulated stationary engines. The EPA has determined that carbon monoxide (CO) can be often used as an appropriate surrogate for formaldehyde. Since testing for CO emissions has many advantages over testing for emissions of hazardous air pollutants (HAP), most of the emission standards have been finalized in terms of CO as the only regulated pollutant.

The NESHAP standards discussed below are applicable to existing engines. Separate regulations have been adopted to control emissions from new stationary engines.

The NESHAP regulations for stationary engines are published in Title 40, Part 63, Subpart ZZZZ (63.6580) of the Code of Federal Regulations (CFR).


The applicability of the emission standards depends on the classification of the source of air toxics emissions. “Major sources” of air toxics are defined as those that emit 10 short tons per year of a single air toxic or 25 short tons per year of a mixture of air toxics. “Area sources” are those sources that are not “major sources”.

The NESHAP rules are applicable to “existing” diesel and SI engines, as determined by their date of construction or reconstruction:

  • “Area sources” of air toxics emissions: Engines constructed or reconstructed before June 12, 2006.
  • “Major sources” of air toxics emissions:
    • Engines ≤ 500 hp constructed or reconstructed before June 12, 2006.
    • Engines > 500 hp constructed or reconstructed before December 19, 2002.

The emission standards apply to engines used for non-emergency purposes.

Emission Standards

The NESHAP standards are expressed as volumetric, dry CO concentrations (ppmvd) at 15% O2 (with the exception of standards for rich-burn SI engines, expressed as volumetric concentrations of HCHO at 15% O2). The standards must be met during any operating conditions, except during periods of start-up (of maximum 30 minutes). Emissions are tested at 100% load.

Alternative compliance options are available in certain engine categories, expressed as percentage CO or HCHO emission reductions. These reductions can be achieved by retrofitting engines with such controls as oxidation catalysts.

The standards for stationary diesel engines are listed in the following table.

Table 1
NESHAP Emission Requirements for Stationary Diesel (CI) Engines
Engine CategoryEmission StandardAlternative CO Reduction
Area Sources
Non-Emergency 300 < hp ≤ 50049 ppmvd CO70%
Non-Emergency > 500 hp23 ppmvd CO70%
Major Sources
Non-Emergency 100 ≤ hp ≤ 300230 ppmvd CO-
Non-Emergency 300 < hp ≤ 50049 ppmvd CO70%
Non-Emergency > 500 hp23 ppmvd CO70%

Standards for spark ignition, gas-fired stationary engines are summarized in Table 2. The engine designations indicate two- or four-stroke (2S/4S) lean- or rich-burn (LB/RB) gas engines.

Table 2
NESHAP Emission Requirements for Stationary Gas (SI) Engines
Engine CategoryEmission StandardAlternative CO/HCHO Reduction
Area Sources
4SLB, Non-Emergency > 500 hp47 ppmvd CO93% CO
4SRB, Non-Emergency > 500 hp2.7 ppmvd HCHO76% HCHO
Major Sources
2SLB, Non-Emergency 100 ≤ hp ≤ 500225 ppmvd CO-
4SLB, Non-Emergency 100 ≤ hp ≤ 50047 ppmvd CO-
4SRB, Non-Emergency 100 ≤ hp ≤ 50010.3 ppmvd HCHO-
Landfill/Digester Gas, Non-Emergency 100 ≤ hp ≤ 500177 ppmvd CO-
4SRB, Non-Emergency > 500 hp350 ppmvd HCHO76% HCHO

Other Provisions

Diesel Fuel. The diesel rule requires the use of ultra-low sulfur diesel fuel for stationary non-emergency engines greater than 300 hp with a displacement of less than 30 liters per cylinder. The regulation will be fully implemented by 2013.

Crankcase Filtration. Stationary engines above 300 hp must be equipped with closed or open crankcase filtration system in order to reduce metallic HAP emissions.

The regulations specify a number of other requirements and provisions, including work practices for operators of diesel and SI engines.