Understanding Tier 4 requirements: Ensuring cost-effective compliance

One of the main issues currently affecting the power generation industry is the applicability of Tier 4 certification versus Tier 4 compliance.


Figure 1: The photo shows a Tier 4 site at a large facility. Courtesy: MIRATECH Corp.U.S. Environmental Protection Agency Tier 4 Interim emission regulations went into effect Jan. 1, 2011. These federal regulations are generating widespread confusion, questions, and concerns throughout industries that rely on both emergency and nonemergency diesel engines.

The methodology and process to achieve certification versus compliance are not the same. Buying a Tier-4 certified engine may not be the only way to achieve Tier 4 tailpipe emission values. Commercially viable aftermarket and retrofit options are available to help customers meet these tailpipe emission requirements.

EPA Tier 4 emission limits

Tier 4 Interim and Final regulations make up the fourth phase of the EPA’s air quality regulations for nonroad diesel engines, which began going into effect in 1996. Tier 4 rules cover these primary regulated diesel engine exhaust pollutants:

  • Carbon monoxide (CO)
  • Hydrocarbons (HC)
  • Oxides of nitrogen (NOX)
  • Particulate matter (PM).


The focus is mainly on NOX and PM, both of which may cause serious health and environmental problems. EPA estimates that by 2030, controlling these emissions would annually prevent 12,000 premature deaths, 8,900 hospitalizations, and 1 million lost work days.

Tier 4 Interim emission limits require up to 90% reduction of NOX and up to 50% reduction of PM beyond Tier 3 emission limits. These Interim rules will be followed by even stricter Tier 4 Final emission limits: up to 88% reduction in NOX beyond Tier 4 Interim emission limits; further PM reductions for gensets ≥56 bkW ≤560 bkW; and 70% reduction in PM for gensets >900 bkW. The term “bkW” refers to mechanical brake horsepower. This article will focus on Tier 4 emission standards for large-size gensets >560 kW.

Both the Tier 4 Interim and Final regulations have a NOX requirement of 0.5 grams per brake horsepower per hour (gm/bhp/hr). For PM, the Tier 4 Interim is 0.075 gm/bhp/hr; Final is 0.022 gm/bhp/hr. These figures are commonly used in the diesel genset industry to specify emission limits for projects that wish to achieve Tier 4. However, there is much confusion about how to achieve these values.

There are two possible methods of achieving acceptable values. However, there is a significant difference between the processes used to obtain EPA certification to verify the published Tier 4 limits, and to obtain field compliance verification. First, a clarification of how the EPA defines an emergency genset is in order.

Emergency versus nonemergency gensets

Emergency gensets are exempt from the Tier 4 rules because these engines run infrequently and thus would not be among the major contributors to total NOX and PM emissions. Emergency engines built after January 1, 2011, can be sold meeting Tier 2 limits and do not—by EPA definition—require exhaust aftertreatment (Table 1).

Courtesy: MIRATECH Inc.

But the EPA definition of emergency is strict. To qualify for the exemption, an emergency genset engine may run only when the normal power source such as the local utility fails, except for tightly limited maintenance and testing. Gensets used in peak-shaving, revenue generation, and similar applications do not qualify for the EPA’s emergency genset exemption. There is no Tier 4 limit on the number of hours an emergency genset can run in an actual emergency. However, nonemergency run time is limited to a maximum of 100 hours per year for maintenance and testing. Therefore, many applications running low-use hours—for example, 101-200 hours per year—fall into the nonemergency category and must meet the newly enacted Tier 4 standards.

The current challenge that owners are facing is that factory-certified solutions available for the >560 kW range of gensets is extremely limited. This situation leaves them to seek aftermarket systems that will satisfy emissions requirements and allow for field compliance.

The Tier 4 standards, EPA certification path

The Tier 4 rules govern new diesel engines manufactured after Jan. 1 of this year. The standards moved NOX and PM tailpipe emission limits so low that the technology for achieving targeted reductions cannot currently be met with improved engine combustion alone. Manufacturers must develop aftertreatment technologies as part of the engine package to achieve targeted reductions “right out of the box.” Therefore, the manufacturer of record selling a new engine/genset must secure an EPA emissions certification that verifies the package meets Tier 4 requirements from the factory. In essence, the EPA certification ensures end users that the manufacturer of record has completed all necessary durability testing and development of the complete engine with an applicable aftertreatment system to meet the Tier 4 standard for its defined useful life. This requires extensive allocation of engineering resources, testing hours, and funds.

Figure 2: This illustration shows a cutaway drawing of typical Tier 4 Selective Catalytic Reduction Catalyst assembly. Courtesy: MIRATECH Corp.The certification testing and approval process is detailed by the EPA. One key aspect of how engines are tested to simulate real-world operation is via the ISO 1878 D2 test cycle. The test cycle is run over five steady-state points and has a weighted average. The weighted average values from the test runs are used to validate and certify tailpipe emissions to Tier 4 standards. Additional durability testing of the complete system is required by the OEM to validate the EPA useful life period of 8,000 hours run time or 10 years.

These increased engineering and development costs and extensive factory tests translate to higher end pricing to the customer. For operators who install these engines, the burden of compliance and record keeping in accordance with EPA guidelines requires following the manufacturers’ guidelines for proper installation, operation, and maintenance.

The Tier 4 standards equivalence, field compliance path

On this path to meet Tier 4 tailpipe limits, the engine dealer or packager works with the supplier of the aftertreatment system to develop a complete systems solution. This field compliance solution can meet the equivalent Tier 4 emission standards—with the aftertreatment provider carrying the burden of warranty and performance. With this option, the aftertreatment design can be custom fabricated for the site to meet Tier 4 level requirements (see Figure 1). This type of compliance solution has actually been available for many years.

An aftermarket aftertreatment solution can be used in various situations, which include:

  • Tier 2 emergency standby applications whereby projects or customers want to meet Tier 4 emission limits
  • Local air board requirements, or when the air permits request the genset to have Tier 4 emission limits
  • Current nonemergency applications where no factory-certified solution exists
  • Certain bi-fuel applications that do not have factory certification.


A complete aftertreatment system is designed to meet the 100% load point of provided engine output potential to emit emissions. The catalyst layout, volume, and emission reduction targets are based on these operational data to meet the Tier 4 limits.

End users can rest assured that emission limits are being met. Testing to confirm emission compliance to meet the Tier 4 numbers is performed after installation and start-up at the user’s location via third-party source testing.

While both paths achieve the same goal of meeting Tier 4 emission standards, new engines are Tier 4 certified. However, users don’t need to buy a new engine to become Tier 4 compliant. Actually, a compliant aftermarket aftertreatment solution can be more cost effective while achieving the same desired environmental results: meeting clean air targets.

Tips for ensuring compliance

To reduce engine output emission levels of a properly maintained and mechanically sound engine to Tier 4 standards, engine operators should consider three aftertreatment system components that must work together in close synchronization. Each component serves a specific emission reduction function.

  • Selective Catalytic Reduction for up to 99% reduction of NOX (see Figure 2)
  • Diesel Oxidation Catalyst for up to 99% reduction of HC and CO, and up to approximately 25% reduction in PM
  • Diesel Particulate Filter for up to 95% reduction of PM.


Look for solutions with proven reliability, such as integrated systems with components that work together and are designed for a specific operation. Expect top performance from equipment designed, manufactured, and installed by a reputable company

The decades-long process of achieving dramatic reduction of diesel emissions in the U.S. is approaching its final phase. The implementation of the Tier 4 Interim and Final rules offers potential environmental and health benefits.

As we move through this confusing, often challenging closing period, reliable information is the key to making sound decisions. When the long trek through the process of Tiers has reached its goal, we can all breathe a little easier.

McBryde is the Western U.S. and International Power Generation sales manager for MIRATECH Corp.


Clean Air Nonroad Diesel - Tier 4 Final Rule: www.epa.gov/nonroad-diesel/2004fr.htm

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