Tiers of a generator: Emissions regulations for diesel gensets

Manufacturers face the next phase of EPA emissions limits for diesel engines and gensets.

By Jack Smith, Managing Editor March 16, 2010

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Most people who have been stuck behind an older truck or bus in slow, heavy traffic on a two-lane road would likely understand the need to reduce emissions from diesel engines. The “brown L.A. haze” to which the early Jimmy Buffett song “Come Monday” refers can be attributed, in part, to diesel exhaust.

Since the mid-1950s, the U.S. government has been aware of the need to control atmospheric pollutants. The evolution of regulations governing emissions from diesel engines have forced manufacturers to improve not only how much pollution goes into the atmosphere, but engine performance as well.

Diesel-powered generators were not targeted originally. However, they could not escape regulatory oversight forever. The U.S. Environmental Protection Agency now includes stationary and non-road mobile diesel engines used in gensets in its increasingly stringent regulations governing all diesel engines. The emissions the EPA focuses on primarily include oxides of nitrogen (NOx), hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM), which includes any visible smoke and soot. Almost 40 years of phased emissions regulations has reduced NOx and PM emissions from diesel engines by nearly 85%. The latest phase, known as Tier 4i (“i” is for interim), takes effect on Jan. 1, 2011, for engines rated above 130 bkW. The emission regulation phases will culminate in what is known as Tier 4 Final , which will take effect in 2013-2015, depending on engine horsepower.

Phased emissions tiers

The EPA has implemented emissions reductions in phases to allow scientists, manufacturers, and the market to develop and absorb the technologies, costs, and working knowledge needed for compliance. This doesn’t mean that emissions reductions have been driven totally by regulatory forces—diesel customers and the public exposed to diesel emissions want cleaner engines, too.

The first federal legislation involving air pollution was the Air Pollution Control Act of 1955, which essentially provided funds for federal air pollution research. The Clean Air Act of 1963 authorized research into techniques for monitoring and controlling air pollution. In 1967, the Air Quality Act was adopted, which initiated enforcement proceedings that—for the first time—enabled the federal government to conduct extensive ambient monitoring studies as well as stationary source inspections.

The next federal legislative milestone was the enactment of the Clean Air Act of 1970 (1970 CAA), which substantially expanded the government’s enforcement authority and authorized the development of comprehensive federal and state regulations to limit emissions from both stationary (industrial) and mobile (motor vehicle) sources. The 1970 CAA was adopted at approximately the same time as the National Environmental Policy Act that established the EPA. The EPA was created on May 2, 1971, to implement the various requirements included in 1970 CAA. Amendments to 1970 CAA came in 1977 and 1990.

Tier 1 was the first phase. Some of the amendments that came in 1990 substantially increased the authority and responsibility of the federal government and authorized the issuance of stationary source operating permits.

EPA’s ongoing push to reduce emissions from new non-road mobile diesel engines came about in 1994. This definition of non-road mobile diesel engines included agricultural equipment such as tractors, construction equipment such as bulldozers, and industrial equipment such as diesel-powered generators used in mobile applications. This established Tier 1 —the first set of emission standards for all new non-road mobile diesel engines of all horsepower categories except for those used in marine vessels and locomotives.

Tier 2 emission standards were adopted in 1998. These regulations were more stringent and addressed NOx, carbon monoxide, unburned hydrocarbons, and particulate matter emitted from new non-road diesel engines. Tier 2 emission standards were phased in from 2001 to 2005 for all engine sizes. Tier 3 was implemented between 2006 and 2008, and restricted exhaust emissions even further for engines of 50 to 750 hp.

Until 2006, EPA regulations governing emissions from diesel engines affected only mobile units, regardless of whether they were on-road or non-road. However, on July 11, 2006, the EPA finalized the New Source Performance Standards (NSPS), which means that—for the first time—most horsepower ranges of new stationary diesel engines have to meet the same emission standards imposed on non-road mobile units and the associated tier levels. This NSPS final ruling became effective on Jan. 1, 2007.

The regulation applicable to both stationary and non-road mobile applications has two phase-in periods: Tier 4 Interim and Tier 4 Final. The Tier 4 Interim period is intended to enable a gradual phase-in to final regulations for manufacturers. The Interim emissions regulations will be phased in by engine power category, followed by Tier 4 Final standards. Tier 4 Interim begins in 2011-2012; Tier 4 Final begins in the 2013-2015 time period. Tier 4 Interim emissions regulations became effective for engines below 56 bkW in 2008, and will become effective in 2011 for diesel powered generator sets rated 130 bkW and higher. Tier 4 Interim becomes effective for the 56 to 130 bkW range in 2012. Diesel engines that comply with Tier 4 Interim regulations will require an additional 50% reduction in NOx emissions at Tier 4 Final.

Emission reduction technologies

Diesel-powered generator sets are a popular choice for standby and emergency power systems, as well as peaking and load shedding, because of their reliability, low lifecycle cost, high efficiency, ready availability, ease of installation, operational flexibility, and high-quality electrical performance. However, increasingly stringent emissions regulations continue to push manufacturers to develop effective technologies to overcome these obstacles.

One of the biggest emission-reduction challenges involves the trade-off between NOx and PM emissions. Techniques that are effective in decreasing NOx tend to increase PM, and those that reduce PM tend to increase NOx. Both NOx and PM are linked by combustion temperatures. Increasing temperatures within the cylinders reduces PM and increases NOx; decreasing temperatures reverses the effect. The goal is to optimize the combustion cycle to reduce both pollutants as much as possible, then employ external after-treatment technologies as necessary.

The available technologies aimed at reducing genset emissions are either internal or external to the diesel engine. Internal methods affect or modify combustion parameters and include electronic engine controls, injection systems, the physics of the combustion chamber, turbocharging, and Exhaust Gas Recirculation (EGR). Electronic controls regulate fuel quantity, injection timing, and turbocharger boost pressure, while compensating for load, temperature, and barometric pressure.

Retarding the injection timing while increasing injection pressure is effective in reducing NOx without significantly increasing PM or HC production. Higher injection pressures also improve fuel atomization and combustion chamber penetration, which improve fuel economy while reducing PM. Turbocharging is used on most medium and large gensets to boost power, improve combustion efficiency, and reduce emissions. A turbocharger increases charge air density, which increases power output.

EGR reduces NOx by recycling a portion of the inert exhaust gases with incoming engine air, which reduces combustion temperatures. However, EGR also reduces power output and fuel efficiency, and increases PM. The power, efficiency, and PM effects can be compensated for by optimizing the injection and turbocharging technologies.

External after-treatment technologies include oxidation catalyst, diesel particulate filter (DPF), and selective catalytic reduction (SCR). Oxidation catalysts can reduce HC and CO by more than 90% and remove 20% to 25% of PM from the exhaust stream. Oxidation catalysts are effective for equipment in indoor applications, confined spaces, or highly populated areas. Unlike some after-treatment options, oxidation catalysts can be used regardless of the level of sulfur in the fuel.

A DPF can reduce HC, CO, and PM by 90%, but it can only be used with ultra-low sulfur fuel (less than 15 ppm sulfur). Observe caution when specifying DPF technology: Filters are sensitive to duty cycle and must be designed for the specific application. An improperly applied DPF can damage both the filter and the engine.

SCR technology can reduce NOx by up to 95% in many stationary engine applications. When used with an oxidation catalyst, the combination can reduce CO and non-methane, non-ethane HC by as much as 90%. An SCR system injects aqueous urea into the exhaust stream just ahead of the catalyst. The urea reacts with NOx in contact with the catalyst and is converted into nitrogen gas (N2) and water (H2O).

How regulations affect ENGINE manufacturers

Because of the NSPS final rule, most stationary diesel engines must meet emission requirements of Tiers 2, 3, or 4, depending on power range. Beginning in 2011, non-emergency diesel engines less than 10 liters per cylinder and greater than 175 hp will be required to meet Tier 4 regulations. The term “non-emergency” is very important in this context. Essentially, if an installation is classified as emergency, the genset must not run unless the primary electrical power source is not available. However, owner/operators are allowed to run their emergency-classified gensets up to 100 h per year for maintenance and testing. There is no current limit for run time of emergency units.

“Rather than write completely new governing regulations, the EPA wrote an NSPS for stationary applications, and then aligned us with the mobile non-road regulations for certification purposes,” said Allen Gillette, senior vice president of engineering at Generac, Waukesha, Wis. “The stationary emergency units will be able to remain at a Tier 3 or a Tier 2 level because they are exempt. In most cases, what that means to Generac is we will have a certified product for emergency standby use—that we will have an engine in that generator certified to model year 2011, as a stationary emergency provision. This provision would allow it to remain as ‘no aftertreatment required.’”

Although meeting EPA regulations presented challenges to engine manufacturers, it also offered opportunities to improve diesel engine performance and efficiency. “The most talked about benefit for this next generation of engines is modestly improved fuel consumption—less fuel consumed per brake kW,” said Tim Cresswell, Tier 4 product definition manager, Caterpillar, Griffin, Ga. He added, “Engine manufacturers could take the opportunity to optimize their lineup in terms of power density. Manufacturers have the opportunity to tune the power to get the best available output from that engine.”

Most engines still have to meet increasingly stringent emissions standards. “The regulations imply the use of aftertreatment,” said Cresswell. “They are designed to—or intended to—push manufacturers toward using aftertreatment. That allows manufacturers to relax slightly the engine settings and rely on the aftertreatment to a degree to recapture some of the fuel consumption that was lost in earlier tiers when the cleansing of the engine was being done just by restricting its settings.

“I think that the objective is to keep performance the same in each emission tier. It’s more difficult to restrict what engines are allowed to do in terms of its combustion system,” Cresswell said. “There has been a tendency for performance to fall back into earlier tiers. I think in Tier 4 interim, the industry will see performance maintained in terms of load acceptance and ambient capabilities.”

Diesel engine manufacturers also bear the burden of testing their diesel engines and certifying them according to EPA guidelines. The EPA expects that the engine manufacturer will certify the engine and aftertreatment as a complete system. “There’s a considerable amount of testing to be done,” said Cresswell. “The EPA mandates testing on the emissions useful life for the overall system, which means the length of time over which it must conform to the regulated limits. That’s 8,000 operating hours for a standby type of installation. In order to prove that a system will last that long, the EPA demands that the engine manufacturers carry out what’s called a deterioration factor test, where they run an accelerated test of the overall engine after-treatment system to ensure that it will maintain performance. For that reason, the combination of engine type and aftertreatment is certified at the factory. It doesn’t really allow for the retrofitting of gensets in the field.”

Owner/operator responsibilities

As the industry adjusts to the transition into Tier 4, owner/operators should understand how the regulations affect them. Manufacturers must make gensets that comply with the appropriate regulations. However, owner/operators have a great deal of responsibility to understand how the regulations affect genset availability, how installations are classified, and even record-keeping and maintenance requirements.

“There will be a time lag in availability, higher initial costs, higher operating costs, and higher maintenance costs for new gensets,” said Roddy Yates, senior product manager-generators, Baldor, Fort Smith, Ark. “Depending on local laws, existing gensets will typically have a grandfathered operating clause that will run out before the normal life expectancy of the engine and genset. Older used gensets may only have scrap value.”

Before they select equipment or technology packages from a manufacturer, owner/operators must understand and be very clear about how their installation is classified. “The regulations provide for a set of alternative standards for emergency installations,” said Cresswell. “It is considered emergency if the genset runs only when the normal source is not available. The EPA expects that the genset will sit idle until the power fails, and will then start up maybe cushioned by the UPS system. An owner/operator has to decide whether or not his installation falls within that emergency categorization. If it is a true emergency installation, then it doesn’t have to go to Tier 4 at all.”

While the National Electric Code (NEC) is specific regarding definitions of emergency standby power, the EPA is less specific about installations that fall under this classification. “If you think about data centers where storm-avoidance running is quite prevalent, that potentially would not be an emergency installation, so they would have to comply with Tier 4,” Cresswell said. “If you think about an installation that has dual utility feeds, for example, where if one feed were lost for some reason, typically, the installation will start running, even though one of the feeds is still available. That potentially falls outside the ’emergency’ definition.”

Cresswell said that owner/operators are allowed to run an emergency installation up to 100 h per year for maintenance and testing, and must keep legal records of when they run. Logs must reference the non-resettable hour meter on the genset. Records must be kept if and when the generator runs because of a qualifying emergency event. “Ultimately, it’s the responsibility of the owner/operator to choose the right thing, based on the information available. Absolutely have a good dialogue with your local air board or whoever sets the local standards,” he said.

Maintenance requirements

Obviously, owner/operators are responsible for maintaining their gensets. While good maintenance has always been among end-user best practices, the emissions regulations now imply that maintenance will be part of compliance. Although the regulations are not explicit, the EPA requires a diesel genset to remain in compliance throughout its defined useful life, and implies that normal maintenance is the only way to do that.

“In general, the expectation of the EPA—and ultimately the states and the local districts—is that you are running equipment per your permit, you are maintaining it, and you have good records,” said Gillette. “For example, you wouldn’t want them to discover that your hour meter has been removed from the equipment and that you don’t have fuel purchase records. By granting you a permit for a stationary emergency generator installation, they expect that you will operate it a certain way, and that you will have records available. If an auditor asks for this information, you will have it.”

Gillette explained that air cleaners, oil, and fuel filters should be inspected or changed regularly. Diesel fuel can degrade over time—from water or with biomatter. For that reason, Gillette recommends periodic fuel analysis as well. “If you haven’t run the genset, or you haven’t changed the fuel for five years, it may plug the filter when you go to operate it,” he said.

Although routine maintenance will remain largely unchanged, generally, some of the consumables required will likely change. For example, a new grade of lubricating oil will be required for Tier 4 engines. “The industry is moving from what’s classified as ‘CH’ grade to ‘CJ’ grade,” Cresswell said. “The principal difference is that CJ is a low ash oil. A number of the engine manufacturers use a diesel particulate filter on the exhaust. This filter traps the soot—the particulate emissions of the engine. But it also traps oil that is burned as part of the blow-by. Whereas soot can be oxidized out of particulate filters, the ash can’t. The more ash you produce from your oil, the sooner you will have to physically empty or service your particulate filter.”

Design considerations

In addition to emissions regulations, permits, and specifications from owner/operators, design engineers preparing to work on a Tier 4 project need to be aware of equipment differences imposed to accommodate these regulations—primarily aftertreatment. Larger gensets that typically use SCR need to have a urea supply tank, as they always have. Designers are still responsible for supplying the tank but should be aware of potential increases in space requirements as well as tank location issues.

There are thermal management and building foundation issues to consider as well. “Designers need to be aware of the additional heat loading that might take place in the engine room,” said Cresswell. “The physical size and the weight of the additional components would have an impact on the foundations. Designers would need to make the appropriate calculations there.”

On smaller gensets that use particulate filters, some manufacturers use a high temperature system to regenerate the filters by oxidizing the soot. Temperatures on some of these systems could potentially reach 1112 F, which Cresswell said is nearly twice the temperature of existing systems. However, this is only a periodic increase while the soot is being oxidized from the particulate filter. Although this operation is cyclic, designers must factor the impact of these higher temperatures into their designs.


The first of the Tier 4i regulations took effect in 2008; more take effect in 2011. Owners and engineers need to prepare accordingly for the next generation of gensets. Although their diesel engines will have lower exhaust emissions, in some cases, gensets will be larger and heavier, requiring increased foundation reinforcement. Some will also run hotter at times, requiring a higher level of facility thermal management.

Owner/operators must understand how their installations will be classified before the genset specification is written. They must also be diligent regarding genset maintenance, as well as keeping adequate records of both maintenance and run time events. Owner/operators must be aware of changes in genset consumables such as fuels, oils, and catalytic requirements.

Although EPA regulations have presented challenges to the diesel engine industry, overcoming these challenges has led to cleaner air and better performing equipment. And for that, we can all breathe easier.