Tier Emissions Ratings for Diesel Generators – What You Need to Know

Since the 1950s, the Environmental Protection Agency (EPA) has implemented regulations strategically to improve air quality by reducing harmful emissions. Diesel exhaust is one of these harmful emissions, and as such, the regulations have had an impact on on-road and off-road vehicles and engines, including non-road diesel engines and stationary diesel engine generator sets. But though this may sound as though it was a hardship for this industry, it in fact has led to engine improvements that reduce emissions and boost performance.

To make the process go smoothly, an implementation system was designed to allow all stakeholders — including scientists and manufacturers, among others — to develop the knowledge and technologies necessary to meet these regulatory goals. The implementation system is really more accurately described as a tier system put in place to accommodate the process. Here, we outline what you should know about the tier emissions ratings for diesel generators.

Fumes polluting air


What are Tier Levels?

There are four tiers in place to reduce harmful gases into the atmosphere, and each tier was built off the other to make compliance more manageable. Each tier level also has a certain standard with regard to harmful emissions and provides a cap at which point emissions must be limited. The standard progresses over the years in accordance with the tier level.

The intended and primary emissions to be reduced include:

  • Nitrogen oxides (NOx)
  • Hydrocarbons (HC)
  • Carbon monoxide (CO)
  • Particulate matter (PM)
  • Non-methane hydrocarbons (NMHC)

Currently, all engines (with a few exceptions – discussed later) are at the tier 4 level, meaning they have significantly reduced harmful emissions just as they have significantly improved engine performance. But it is still a work in progress, especially for diesel engines, both on and off the road.


How Do Tier Levels Relate to Generators?

Diesel engines were not the initial target when the EPA regulations were first published and implemented. But they are very much the subject of these regulations now. In 1998, non-road engine regulations were put into place as a 3-tiered system (with the 4th tier later added). Each tier involves a phase-in period over a number of years and in accordance with the engine’s horsepower rating. Below is an overview of these three tiers and how they relate to generators.


Tier 1 & Diesel Generator Emissions

Though Tier 1 standards were published in 1991, the implementation for diesel engines and generators did not begin until 1996 with the intention that all diesel engines and generators be in compliance by 2005. Initially, Tier 1 standards focused on diesel equipment under 37 kW (50 hp) with the phase-in period from 1996 to 2000. The below table identifies Tier 1 standards related to diesel engine power and harmful gases to be limited.

YearEngine Power (hp)COHCNMHC+NOxNOxPM
1996175 ≤ 3008.
300 ≤ 6008.
600 ≤ 7508.
1997100 ≤ 1756.9
199850 ≤ 1006.9
199925 ≤ 504.17.10.6
11 ≤
≥ 7508.



Tiers 2 & 3 Diesel Generator Emissions

Tier 2 standards were published in 1999 and Tier 3 standards were published shortly thereafter. These phases, however, did not begin until 2001 with all engines to be in compliance by 2008. These two levels required engines that were more sophisticated in design and advanced in performance in order to meet the criteria set out in the regulations. The below table identifies Tier 2 and 3 standards related to diesel engine power and harmful gases to be limited.

YearTierEngine Power (hp)COHCNMHC+NOxNOxPM
2001T2300 ≤ 6002.64.80.15
2002T2600 ≤ 7502.64.80.15
2003T2100 ≤ 1753.74.90.22
T2175 ≤ 3002.64.90.15
2004T225 ≤ 504.15.60.45
T250 ≤ 1003.75.60.3
T211 ≤
2006T2≥ 7502.64.80.15
T3175 ≤ 3002.63.0
T3300 ≤ 6002.63.0
T3600 ≤ 7502.63.0
2007T3100 ≤ 1753.73.0
2008T350 ≤ 1003.73.5



To note, sulfur content was not originally limited by environmental regulations in Tier 1 for non-road diesel fuels but was later mandated and required to be met by 2007 in Tiers 2 and 3.


Tier 4 – the Current Emissions Standard for Diesel Generators

Tier 4 is where we are at today and its objective is simple. All diesel generators are or should be at the level 4 tier. This is, of course, the most strict of all regulations. If you are environmentally conscious, then this level may meet much of your expectations. Here, the most advanced technologies are used to achieve the greatest reduction in emissions available today. The technologies used and needed for tier 4 level performance are those technologies that result in near-zero emission levels.

Tier 4 emission standards began being phased-in in 2008. The regulations set out to reduce NOx, particulate matter, and hydrocarbon emissions substantially with respect to all off-road and on-road engines and generator sets. Carbon monoxide standards, however, remain relatively similar to those established in Tiers 2 and 3. The below chart provides an overview of what these emissions standards look like.

YearEngine Power (hp)CONMHCNMHC+NOxNOxPM
11 ≤
2011Generator sets > 1200 hp2.60.300.500.075
2011 – 2014175 ≤ 7502.60.140.300.015
2012 – 201475 ≤ 1753.70.140.300.015
201350 ≤ 753.73.50.022
2015All generator sets2.



With regard to the above table, however, there is a lot of deviation. These deviations are a result of multiple factors, like credits claimed by the manufacturer, certification as an alternative NOx limit, or participation with optional PM standards.


Who must comply with these standards?

Anyone manufacturing diesel engines, either for on-road or non-road purposes, including generator sets, must comply with these standards. That said, the rules do not apply to all types of power generators. Generally speaking, all non-road diesel engines, gensets and heavy equipment alike, rated at 56 kW and higher and are considered as prime generators (as opposed to standby) must comply with Tier 4 standards.


How do diesel engine companies meet the standards established by the EPA emission tier system?

Manufacturers, designers, and other relevant companies must improve the technologies and design of diesel generators so that they are in compliance with the law. Examples of what companies are doing to be in compliance materialize in a number of ways, including:

Engine Design Technologies

  • Fuel injection technology to improve injection timing to limit NOx emissions and injection pressure to limit PM emissions.
  • Exhaust gas recirculation to control NOx emissions.
  • Intake boosting for efficiency and PM reduction.
  • Intake temperature management technology to limit NOx and PM emissions.
  • Combustion chamber designs to measure and control PM emissions.

Fuel & Lubricant Technologies

  • Lubricating oil to reduce fuel consumption overall.
  • Alternative fuels to reduce carbon dioxide emissions.

Exhaust Add-ons and Aftertreatment Technologies

  • Selective Catalytic Reduction (SCR) to reduce NOx up to 50% in active systems.
  • Diesel Particulate Filter (DPF) to reduce emissions more than 90%.
  • Exhaust Gas Recirculation (EGR) to reduce emissions overall.
  • Diesel oxidation catalyst (DOC) to reduce high levels of HC and CO and moderate levels of PM as well as to enhance performance of SCR and DPF systems.


Emergency vs Non-Emergency Generator Standards

The emission standards for emergency generators differ from the standards of non-emergency generators. This is important to know for those companies who have on-site or may need emergency standby generators. The difference is founded on the basic reality that emergency generators are used for a temporary purposes and therefore not subject to the more stringent rules. Here’s what you should know.

As noted above, only prime generators need to adhere to Tier 4 standards. Standby (or emergency) gensets are only subject to Tiers 2 and 3, not Tier 4. As such, these machines used as standby only machines do not need to be updated if in compliance with Tier 2 or 3 standards. All our generators, whether for purchase or rental, meet the emission tier standards for their role.


Benefits of Tier 4 Generators

We noted at the beginning that though the tier-system seems to burden manufacturers and users of diesel engines and generators, it does offer benefits, too. Some of these benefits include:

  • Significant emission reduction.
  • Quieter and smoother operation.
  • Increased fuel efficiency.
  • Advanced technologies for overall optimized performance.
  • Minimal downtime for extended service and reduction in costs.

Overall, your Tier 4 generator benefits you from all angles: health, expenses, performance, efficiency, and productivity.


What Comes After Tier 4?

Now that we are at Tier 4, what’s next? Tier 5 is the natural response, but the EPA has done little to move towards it. That said, most generator manufacturers do not operate or market to the United States alone: Europe is well on its way to phasing in regulations that could be considered as Tier 5 standards. Stage 5 in Europe, however, is comparable to its Stage 4, so big changes are not expected; all to be expected is that companies keep moving forward with emission reductions and efficient productivity made possible through newer and more advanced technologies. Because of the similarities between Stages 4 and 5 in Europe, manufacturers do not have to worry about compliance so long as in compliance with Tier 4. And we at Worldwide Power Products are in compliance and intend to keep moving forward — always keeping our eyes on the future, today.