Understanding diesel emissions regulation changes

Engineers and designers should understand how continually evolving diesel emissions regulations affect them and their clients.


For nearly two decades the U.S. Environmental Protection Agency has been creating more stringent regulations for the reduction of diesel engine pollutants. The affected industries are entering the fourth and final Tier of the new regulations. Tier 4 final emission regulations are scheduled to go in effect as early as 2013 for engines greater than 25 hp, leaving design engineers, manufacturers, and end users to face tough challenges. 

For more than 50 years, awareness about air pollution and its associated health and environmental risks has been increasing in the U.S. As a result, the U.S. government developed several bills in legislation including the Clean Air Act (CAA) of 1970. The EPA was created in 1971 to implement CAA requirements. 

Stationary diesel generators were not targeted in the original policies. However, they could not escape regulation forever. In July 2006, the EPA issued the New Source Performance Standards (NSPS) for compression ignition engines, which required stationary engines to limit emissions. Various regulation Tiers were published to encourage technological advancements and allow manufacturers time to absorb associated developmental costs. Since Jan. 1, 2007, when these standards went into effect, manufacturers, designers, and owners have been trying to understand and adhere to the constantly evolving emissions requirements.

When NSPS went into effect, initially, the emission regulations were similar for both stationary diesel engines and mobile nonroad engines. It was not until Jan. 1, 2011, that these regulations were divided based on the application type: emergency stationary, stationary (nonemergency), and nonroad mobile. Designers must understand the appropriate applications to properly specify the correct emission requirements for a generator. Only stationary internal combustion engines (ICE) are discussed in this article. 

Application types for generator specification

Figure 1: This graph compares emergency and nonemergency Tier level requirements for varying horsepower engines for the year 2013. Courtesy: ccrd partnersEmergency stationary installations are required to comply only with pre-2011 emissions, while stationary (nonemergency) installations are subject to higher emission standards (see Figure 1). Accordingly, it is important to understand the nuances between the two. The EPA defines an emergency stationary engine as “any stationary internal combustion engine whose operation is limited to emergency situations and required testing and maintenance.” This definition aligns with the National Electrical Code definition of “emergency systems” in Articles 700 and 701, in which they are described as “legally required.” 

Examples of this include installations used to produce power for critical networks or equipment when electric utility power is interrupted. The limitation for testing and maintenance is 100 hours per year when normal power is available. The EPA is very specific that “stationary engines used to supply power to an electric grid or that supply power as part of a financial arrangement with another entity are not considered to be emergency engines.” This applies to any emergency demand load response programs including peak shaving, rate curtailment, interruptible rate programs, continuous base load, cogeneration, and prime-power-rated gensets. Emergency demand load response is becoming increasingly popular with local utility companies as the demand for electricity increases. It is prudent for the designer to discuss with the owner any arrangements the facility has in place or is planning to adopt when evaluating the appropriate requirements for the proper application. Another application not considered to be emergency is storm avoidance. 

To understand the Tier levels, it should be noted that the EPA mandates exhaust emission standards specifically for engines. A generator does not produce emissions; the engine driving the generator does. The emission values are dictated in kW, but this is not necessarily the nominal kW value of a specified genset. A straight conversion of the EPA listed kW to horsepower gives the engine sizes required to meet the emission limits. Manufacturers’ gensets must meet the appropriate Tier levels based on the engine horsepower size provided with the generator. Thus, manufacturers are often required to meet differing Tier levels for the same nominal size generator. 

Figure 2: This graph indicates Tier level requirements for emergency engines to the year 2017. Courtesy: ccrd partners

Designers should be aware that using published charts based on typical EPA kW is not an accurate representation of the Tier level for which a generator set should be specified. For example, a 350 kW generator from one manufacturer is provided with a 755-hp engine (Tier 2), but the same nominal kW from another manufacturer is provided with a 547-hp engine (Tier 3). Figure 2 and Figure 3 indicate current and future EPA Tier requirements based on engine horsepower for emergency and nonemergency engines, respectively. Industry experts speculate that although the tables listing Tier requirements end in 2017, the Tier ratings are anticipated to remain at those levels indefinitely. 

Figure 3: This graph indicates Tier level requirements for nonemergency engines to the year 2017. Courtesy: ccrd partners

Several states and localities have developed control boards that are more stringent than the EPA’s. Currently, California has the most aggressive emission reduction standards. California established the California Air Resources Board (CARB) in 1967 and later identified diesel particulate matter as a toxic air contaminant. As a result, the board approved a risk reduction plan, which instituted the Airborne Toxic Control Measures for Stationary Compression Ignition Engines (ATCM). Since 2006, the ATCM has required all diesel fuel for new and in-use, emergency and nonemergency stationary ICEs having a rated brake horsepower (bhp) of greater than 50 to be either ARB diesel fuel or an alternative diesel fuel such as biodiesel. In addition, new stationary engines greater than 50 bhp are limited to 50 hours per year for maintenance and testing. Although the ARB requirement for hours of maintenance and testing is more stringent than that of the EPA, CARB does allow exceptions on a site-specific basis when enrolled in a specific demand response program, rolling blackout reduction program, or interruptible service contract with prior approval. 

In 2010, CARB amended the stationary diesel engine ATCM to more closely align with the EPA NSPS emergency stationary standards for pollutants, except for diesel particulate matter (PM). The PM limits are more stringent for engines ranging from 50 bhp to 173 bhp. 

To add to the complexity, local air districts may establish even more stringent emission standards and/or more stringent maintenance and testing standards. For example, the South Coast Air Quality Management District (SCAQMD) is the air pollution control agency for all of Orange County and the urban portions of Los Angeles, Riverside, and San Bernardino counties.

Currently, there is a proposed amendment to Rule 1470: Requirements for Stationary Diesel-Fueled Internal Combustion and Other Compression Ignition Engines. The proposed amendment would eliminate the requirements to meet aftertreatment based Tier 4 emission standards for nitrogen oxide (NOX) and align more closely with the ATCM. The proposed rule would retain Tier 4 PM emission standards for new stationary emergency standby engines installed on or after Jan. 1, 2013, but narrows the applicability of this emission standard to those engines rated greater than or equal to 175 bhp and located within 50 meters of sensitive receptors, such as residences, daycare centers, and hospitals. The Tier 4 PM emission limit does typically require aftertreatment for most engine sizes. Emergency standby engines located at or within 100 meters of a school would still be required to meet the current NOX and PM emission limits. The proposed amendment also delays aftertreatment based Tier 4 PM emission limits until July 1, 2015, for certain engine sizes. At the time this article was written, the proposed amendment to the rule had not been accepted (see Figure 4). 

Figure 4: This map indicates the air districts in California. For more information about California air districts as well as a resource directory of district contacts, refer to the California ARB website at www.arb.ca.gov. Courtesy: California Air ResourcCalifornia is not the only state with additional control boards that are more stringent than the EPA’s. New York and Texas are among those states that have more rigorous regulations in place, and more states are beginning to adopt additional PM and NOX standards. It is important for engineers to understand the local requirements that apply to their specific application when designing stationary diesel engine power systems. 

Compliant power systems

As the Tier 4F deadline approaches, additional emergency power system design and specification concepts should be considered to ensure they comply with the requirements. Providing emergency power to a facility may no longer consist of a simple engine generator. Because of these increasingly stringent regulations, the system now must be a complete EPA-certified power system. Additional design aspects, such as space planning, additional components, and overall cost, must be considered very early in the project. 

As previously discussed, the key to understanding the appropriate rules and regulations is to fully investigate the site location and all applicable local regulations. These regulations exist to specifically control PM, hydrocarbons (HCs), carbon monoxide (CO), and NOX. PM is the solid and liquid particles found in the exhaust of diesel-fueled engines from unburned carbon and may be controlled by optimizing the combustion temperature and improving combustion efficiency. HC is essentially unburned fuel, which contributes to ozone and smog production and may be controlled by improving combustion efficiency. CO is a colorless, odorless gas resulting from the incomplete combustion of hydrocarbon fuels and may also be controlled by improving combustion efficiency. NOX is a combustion by-product that combines in the atmosphere to create ozone and smog and may be controlled by reducing the combustion temperature inside the engine cylinder.

The existing technologies available to meet Tier 4F emission requirements in controlling these pollutants either address emissions within the engine cylinder or are aftertreatment approaches. While in-cylinder design improvements and precise combustion control have helped to reduce primary pollutants, these technologies can do only so much considering the mechanical limits of the engine. Therefore, the generator manufacturer and the specifier must often consider aftertreatment technologies to comply with the latest requirements. Currently, the three major aftertreatment technologies are selective catalytic reduction (SCR), diesel oxidation catalyst (DOC), and diesel particulate filter (DPF) (see Figure 5). 

Figure 5: This drawing illustrates how currently available aftertreatment technologies may be applied to an emergency power system. Courtesy: Cummins Power Generation Inc.SCR: An SCR system uses catalytic reduction to reduce levels of NOX in diesel exhaust. It is called selective because it reduces the levels of NOX by injecting nitrogen-containing compounds, such as ammonia or urea, into the exhaust stream as a reducing agent—or reductant—within a catalytic chamber. The reductant reacts with NOX to convert pollutants into nitrogen, water vapor, and CO2. Published tests show that SCR technology alone can achieve NOX reductions of greater than 75%. 

SCR systems require high operating temperatures, so, depending on ambient conditions and the demand load, achieving optimum temperature could take 10-15 min—or even longer in colder climates. SCR systems are typically not suited for emergency standby engines because their startup periods and typically short operation sessions result in exhaust temperatures that are too cool for NOX reduction to occur. The demand load is also a key factor to consider in the design. A lightly loaded engine that is not operated for long periods of time would not reap the full NOX reduction potential of an SCR system. 

DOC: A DOC is a flow-through device where exhaust gases are brought into contact with materials that oxidize unburned HCs to reduce emissions. The device uses a chemical process to break down pollutants and turn them into less harmful components. DOCs are normally coated with a catalyst designed to trigger a chemical reaction to reduce particulate matter. DOCs reduce pollution odor and may reduce PM emissions by 20% to 40%. 

DPF: A DPF is a device designed to physically capture PM, removing it from the exhaust stream. The filter typically consists of alternately blocked channels that force the exhaust gas to flow through the channel walls where the PM are captured and a chemical reaction occurs. Because a filter can eventually fill up, there must be a means of burning off, or removing accumulated particulate matter. A process called passive regeneration automatically removes excess PM by burning or oxidizing it on the filter when exhaust temperatures are adequate. By burning off trapped material, the filter is cleaned or regenerated. Not all DPFs are designed to regenerate this way. Therefore, a disposable filter is an alternative. When the backpressure limits are approached, the filter is removed and cleaned or discarded. To ensure proper operation, filter systems must be designed for the particular application. 

One of the biggest challenges with aftertreatment—and emission reduction as a whole—is the delicate trade-off between NOX and PM reduction. Both NOX and PM are linked to combustion temperatures. NOX formation is more prevalent at high temperatures and excess oxygen, while PM occurs more at lower temperatures. Techniques that effectively lower the in-cylinder temperature to reduce NOX increase the production of soot and PM. Increasing in-cylinder temperature has the reverse effect of reducing PM, but increases NOX. The goal is to balance the two to provide the optimal combustion cycle that reduces both pollutants at their mutual sweet spot, and then use aftertreatment technologies only as necessary (see Figure 6). 

Figure 6: Because of the trade-offs associated with controlling NOX and PM, obtaining the optimum combustion cycle helps reduce both pollutants effectively. Courtesy: ccrd partnersAnother caveat to consider is that if a genset is field-retrofitted with an aftertreatment system, as opposed to the system being installed as a complete package from the manufacturer, it may be compliant with emission regulations but not EPA certified. After the equipment is sold, it is not possible to certify it to a current standard as required by EPA. An engine can only be EPA certified by the manufacturer during manufacturing. Compliance usually requires testing by a local entity, but being EPA compliant and EPA certified is not the same thing. 

Design considerations

It is evident that an aftertreatment system will increase the installed cost of an emergency power system. However, the engineer as well as the owner must also carefully consider the following:

  • An SCR and its associated reductant tank require additional space for installation
  • In-cylinder treatment to cool the exhaust gas before it is recirculated requires additional heat to be removed from the facility—especially for indoor gensets, where more fan horsepower may be required to ventilate the space and more exterior wall louvers may be required to remove the exhaust
  • Some aftertreatment devices may add back pressure to the system, which would require detailed engineering calculations associated with increasing exhaust stacks
  • The additional structural impact of the added weight of these devices must be considered
  • Operational funds must be available to support the use of alternate fuels, oils, and catalytic consumables
  • Combinations of DPF and SCR generally require the use of ultra-low sulfur diesel, so the facility’s fuel supplier must be considered. In addition, this type of fuel is less stable and more susceptible to microorganism growth in the tank, so fuel maintenance systems should be addressed. 

Adapting to constantly evolving emissions regulations can be confusing. However, if properly applied, implementing Tier 4 final rules offers potential environmental and health benefits. Consequently, the industry has developed cleaner and greener energy solutions, which reduce the health and environmental impact of technologies intended to make our lives safer and more productive. 


“The Impact of Tier 4 Emission Regulations on the Power Generation Industry,” by Natekar and Menzel (Cummins Power Generation), 2010

Amendments to the Airborne Toxic Control Measure for Stationary Compression Ignition Engines – Final regulation Order: www.arb.ca.gov/regact/2010/atcm2010/atcm2010.htm

“Understanding Emission Compliance: The New Design/Specify Caveat”, by Chris Rasmussen, 2011

“Tiers of a Generator: Emissions Regulations for Diesel Gensets”, by Jack Smith, 2010

Proposed Amended Rule 1470 – Requirements for Stationary Diesel-Fueled Internal Combustion and Other Compression Ignition Engines: http://www.aqmd.gov/rules/proposed.html#1470

Airborne Toxic Control Measure for Stationary Compression Ignition Engines – Frequently Asked Questions: www.arb.ca.gov/diesel/ag/documents/faq020708.pdf

“Understanding Tier 4 Requirements: Ensuring Cost-Effective Compliance”, by Scott McBryde, 2011

EPA NCDC Technologies Diesel retrofit Devices: www.epa.gov/cleandiesel/technologies/retrofits.htm


Danna Jensen is a senior associate and senior electrical engineer at ccrd partners with 12 years of experience designing emergency power systems with special focus on healthcare facilities. Jessica Navarro is a senior associate and senior electrical engineer at ccrd partners with 12 years of experience specifying generators throughout the country for healthcare facilities including many in California.

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