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Tier 4 refers to a set of emissions requirements established by the EPA to reduce emissions of particulate matter (PM), oxides of nitrogen (NOx) and air toxics from new, non-road diesel engines. As part of this clean air initiative, the EPA proposed New Source Performance Standards (NSPS) to define the acceptable levels of emissions in large stationary generator sets. Standards set forth by NSPS are intended to regulate national emissions and are designed to be progressively tightened over time to achieve a steady rate of air quality improvement without unreasonable economic disruption.

Tier 4i, or interim, refers to the NSPS emissions standard that became effective on Jan. 1, 2011, for all new, high-horsepower diesel generator engines. The Tier 4i standard significantly cuts NOx emissions, and expands operational flexibility to also include non-emergency use with the achievement of Tier 4i certification.

Tier 4, often referred to as "Final", refers to the NSPS emissions standard that became effective on all large stationary generator sets (gensets) in 2015 replacing Tier 4i. Requiring a significant reduction in PM, Tier 4 represents the highest level of clean air regulations proposed to date. The EPA began issuing Tier 4 certification in January 2014. Cummins Power Generation was the first generator set manufacturer to apply for and receive Tier 4 certification.

Allowable emissions are dependent upon the size of the generator set's equipped engine.


The EPA defines “stationary emergency applications” as those in which the generator set operates only during periods of an outage of the normal utility power supply (with the exception of limited-duration operation for testing and maintenance). All other uses, such as prime power, rate curtailment and storm avoidance constitute non-emergency use.

The EPA does not impose a limit on the number of hours that a generator may operate in emergency situations. However, the EPA does limit operators to run their emergency gensets 50 hours per year for maintenance and exercise purposes. 

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