Why EPA Emissions Standards for Diesel Generators Matter

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Beginning in 1994, the Environmental Protection Agency (EPA) set in place a tiered emissions standard for nonroad compression-ignition (diesel) engines. The standards went through four stages, called tiers, which tightened the standards for these diesel engines periodically until 2015. All stationary diesel engines currently produced (with some exceptions) must comply with final Tier 4 (T4F) regulations.

The EPA enacted these standards primarily to combat two pollutants, particulate matter (PM) and nitrogen oxides (NOx). These pollutants contribute to smog and affect the health of the lungs and heart. Before 1994, nonroad diesel engines had been unregulated.

The regulations resulted in a total reduction of 95% of emissions from nonroad diesel engines over a 20 year span. Over the period, 99% of NOx emissions were reduced from the same engine category.

Why Should You Care About Diesel Generator Emissions?

As a facility manager or as someone simply looking to purchase a generator, you might wonder whether these regulations meaningfully affect you or whether they’re simply a concern for manufacturers. The answer to this is, yes, you should be concerned because the regulations do affect you.

During their tiered rollout of new emissions standards, the EPA allowed manufactures to sell generators they had previously built that did not comply with stricter standards, had the new standards rolled out within two years of when the manufacturer built the engine. However, these generators had to be installed within two years of when they were made for the engine to fall under previous emissions regulations. After those two years, the generators had to meet the current standards.

Regulation Compliance

This means that if you purchase a generator built prior to Jan. 2015, it might not comply with T4F regulations. So, even if the generator is unused, you will have to retrofit it to bring it up to code if it doesn’t meet it. The reason this is, is because the EPA doesn’t so much consider the date an engine was built as when it was installed. Now that it has been more than two years since the T4F regulations came into effect (2015), all installed generators must meet the latest requirements. Generators installed before T4F regulations began are judged by older standards.

We should note that T4F regulations only cover generators used as primary power sources. Diesel generators used solely for emergency power have less stringent requirements.

Diesel generators with horsepower (hp) between 49 and 173 horsepower must only meet 2012 regulation standards. Those with or more than 174hp must only meet the 2010 Tier 3 regulations.

The EPA lets these generators emissions slide because they run so infrequently that they cannot add meaningful levels of PM and NOx to the atmosphere.

Maintenance and Care

Another reason you should care about the new regulations is because they have affected the performance and maintenance required for diesels. Less tolerance for light loads is one such way regulations have affected diesels.

Diesels already have problems handling light loads, as they are at risk for wet stacking when they operate in such conditions. (Wet stacking means the diesel engine does not burning all the fuel being pumped into its cylinders. It will eventually damage an engine.) Now, as a result of selective catalytic reduction (SCR) components, which were introduced to help engines comply with NOx regulations, diesels are even more touchy while running under light loads.

Diesel engines also require diesel exhaust fluid (DEF) be maintained. DEF is a solution that’s sprayed into the exhaust to further reduce NOx emissions.

Current Emissions Regulations

The Electronic Code of Federal Regulations lists the following requirements for nonroad diesel engines. All numbers are listed in grams per kilowatt-hour (g/kW-hr).

Maximum engine power Application PM NOx NMHC NOx + NMHC CO
<19kW All 0.40     7.5 6.6
19kW to <56kW All 0.03     4.7 5.0
56kW to <130kW All 0.02 0.40 0.19   5.0
130kW to <560kW All 0.02 0.40 0.19   3.5
  Generator Sets 0.03 0.67 0.19   3.5
560kW+ All except generator sets 0.04 3.5 0.19   3.5


To verify that your generators meet federal requirements, the EPA may come to your facility and “inspect and monitor any aspect of engine or equipment test procedures or test-related activities” (§1068.20(b)(2)). If the EPA did not certify your diesel engine at the factory — and instead it is only “compliant” or “verified” (meaning the manufacturer tested it but that their claims were not verified) — you might have to have an EPA agent certify your generator’s compliance at your location.

For generators that meet the EPA’s T4F regulations, contact Midwest Generator Solutions in Indianapolis, Indiana. We serve the entire Midwest and provide of Generac, Kohler, Cummins Onan, and Briggs & Stratton generators.


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