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Emissions for Standby and Emergency Generators

This article provides information on the definition of emissions and how to select the emission tier an engine falls under. When selecting a generator set, an important consideration is application for the following reasons:

Emergency Generator Applications – Do not have to maintain any emissions requirements for the below instances:


Major Power Source – The generator complies with emissions regulations. All major engine manufacturers publish individual emission charts. These charts reflect the EPA tier the engine is certified for.


What are Emissions?

The exhaust of a diesel engine contains by products from combustion. For environmental safety, the below emissions are monitored:

  • Nitrogen Oxide (NOx) – Occurs naturally as a result of bacterial process, biological growth, decay, lightning, forest fires and burning of fossil fuels. Colorless non-toxic gas (referred to as laughing gas).
  • Hydrocarbons (HC) – Organic chemical compounds that consist entirely of carbon and hydrogen. They range from simple molecules such as methane to polymers such as polystyrene.
  • Carbon Monoxide (CO) – Colorless, odorless, toxic gas produced from incomplete burning of fossil fuels.
  • Particulate Matter (PM) – The sum of all solid and liquid particles suspended in the air.

Emissions History

Prior to July of 2006 regulations for stationary diesel engines (primarily used for power generation) were in contrast to regulations for non-road diesel engines. There were no federal emissions regulations for stationary engines. Emission regulations were governed by state and local authorities. These requirements were governed by hours of operation.

In July of 2006 the Environmental Protection Agency (EPA) finalized plans to regulate emissions from stationary diesel engines. Beginning January 1, 2007 the New Source Performance Standards (NSPS) harmonized emission requirements for stationary diesel engines with existing EPA non-road regulations.

The EPA established two separate categories of “nonroad” applications, listed below:

Mobile – Farm equipment, construction equipment, trailerized generator sets and other portable industrial engines used on temporary off-road applications.
Stationary – Any engine that is permanently installed and used as a power source. Including standby generator sets, onsite prime and distributed energy power systems. Trailerized engines and generator sets are considered to be stationary when they are installed at a single location for more than 12 months.

EPA regulations are divided into emissions Tiers. Each engine manufacturer (such as Cummins, Caterpillar or Detroit) certifies the engine with the EPA. The certification (Tier) of the engine may not be the same between two manufacturers. To calculate Emission Tier (Table 1) for a Cummins engine:

  • Check generator size in Kw or Hp
  • Match date of production of engine
  • Check Tier (color coded) generator set is associated with


Regulations for Standby and Emergency Generators

Reciprocating Internal Combustion Engines (RICE)

The EPA defines RICE as engines that use pistons that alternatively move back and forth to convert pressure into rotating motion. RICE engines are used in manufacturing plants to generate electricity. In addition, engines are used in emergencies to produce electricity for various applications.

The EPA regulates RICE because common combustion sources, collectively, can have a significant impact on air quality and public health. The EPA regulates all RICE stationary engines.  Requirements are based on the following:

  • New or existing engine
  • Engine area or major power source
  • Engine emergency standby power source

The EPA expanded the number and type of stationary RICE that must comply with federal requirements, to include:

  • National Emission Standard for Hazardous Air Pollutants (NESHAP) – For Reciprocating Internal Combustion Engines (RICE). 40 Code of Federal Regulations Part 63, Subpart ZZZZ. The RICE rule.
  • New Source Performance Standards (NSPS) – Standards of performance for stationary spark ignition engines. 40 CFR, Part 60, Subpart JJJJ. The spark ignition NSPS rule.
  • Standards of Performance for Stationary Compression Ignition Internal Combustion Engines. 40 CFR, Part 60, Subpart IIII. The compression Ignition NSPS rule.

RICE Rule Does Not Apply

The EPA has determined the RICE rule does not apply to the following applications:

1. Motor vehicles, or to non-road engines which are:

  • Self-propelled (tractors or bull dozers)
  • Propelled while performing designed function (lawn mowers)
  • Portable or transportable, including:
    • Has wheels, skids, carrying handles, dolly, trailer or platform
    • A portable non-road engine becomes stationary if in same location for more than 12 months, or for full annual operating period of a season)

2. Existing Emergency Engines – Located at residential, institutional, or commercial area sources, and not used for local reliability. Engine must meet Subpart ZZZZ emergency engine operational requirements:


  • Emergency engines can operate  for 100 hours per year for any combination of the following:
    • Maintenance and testing
    • Emergency demand response (in situations when blackout is imminent or the reliability coordinator has declared and Energy Emergency Alert Level 2 as defined in the North American Reliability Corporation (NERC), Reliability Standard
    • A deviation of voltage or frequency of five percent or greater below the standard voltage or frequency
  • 50 Hours per Year of the 100 hours per year allocation can be used for:
    • Non-emergency situations, if financial arrangement has not been made
    • Local reliability as a part of a financial arrangement with another entity if specific criteria meeting existing RICE at area sources of Hazardous Air Pollutants (HAP) only
    • Peak shaving until May 3, 2014. Peak shaving is a load management program

RICE Rule Applies

The EPA has determined the RICE rule applies to the following applications:

Engines greater than 500 hp and used as a major source of power including:

  • Existing engines constructed before 12/19/2002
  • New engines constructed on or after 12/19/2002
  • Reconstructed engines, construction began on or after 12/19/2002

Engines less than or equal to 500 hp and used as a major source of power including:

  • Existing engines constructed before 6/12/2006
  • New engines constructed on or after 6/12/2006
  • Reconstructed engines, construction began on or after 6/12/2006


EPA Finalizes Rule

In January 2013 NESHAP rule for backup generators allowed them to operate for up to 100 hours per year without requiring the use of Ultra-Low Sulfur Diesel (ULSD) fuel. Three petitions were filed. In January 2015 EPA responses to the petitions were:

  • The timing requirement to utilize ULSD – EPA rejected arguments that ULSD is already sufficiently available to require its use immediately
  • The timing and required information for reporting with respect to emergency engines – EPA rejected request to move the up the reporting, finding it would be unduly burdensome
  • Criteria for operation in non-emergency situations – EPA rejected arguments that its criteria for allowing such operations were “too indistinct and expansive”

Summary/Regulation Application


Beginning 2007 EPA finalized plans to harmonize emission requirements for stationary diesel engines with existing EPA non-road regulations. All engines were divided into Tiers. Classification for tiers are categorized by production date and engine/generator size. January 2015 EPA finalized 40 CFR 63, Subpart ZZZZ.

Regulation Application

Applying EPA RICE regulations to the emergency generator application for each facility can be an extensive or relatively simple process depending on generator engine size, date of engine production, and utilization of generator.

In the example below, the facility is a medium size but needs a backup power source that has the ability to provide 100% operational capability during power outages. Engineering calculations indicate a 1500 Kw generator will meet operational demands. A Cummins 1500 Kw generator, produced in 2008 (Figure 1) is selected from the inventory.

Figure 1, Cummins 1500 Kw Generator

Follow the steps below to determine the applicable regulations:

Follow the steps below to determine the applicable regulations:

1. Determine if RICE rule applies.

  • RICE rule does not apply because:
    • Generator set is used for power outage emergency (unlimited use for these emergencies).

2. Determine if any state regulations apply. Virginia Link is provided as an example.

The engine for this application can be operated for an unlimited amount of hours to supply emergency power and can operate for up to 100 hours of non-emergency conditions.

Generator Used as Major Power Source

The RICE rule would apply if the application of the generator set listed above was considered a “major source of power” . Cummins manufactured this engine to meet Tier 2 EPA certification requirements.

If a generator set is required to follow RICE applications, the following steps will aid in maintaining generator emissions:

  • Make sure to maintain generator set to manufacturer standards. Items such as:
    • Engine service
    • Cooling system service
    • Air system service (to include engine air supply and compartment supply)
    • Fuel system (including fuel filter change and fault code monitoring)


  • Utilization of ultra-low or low sulfur fuels
  • Utilization of exhaust after treatment systems

For an example of diesel emission control strategies proceed to Cummins EPA.

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