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. 

Yes. All maintenance, testing and non-emergency uses count toward the 50-hour yearly limit.

Storm avoidance is an effective tool for large stationary genset operators to mitigate potential risks of incoming storms, rolling blackouts or other unforeseen natural disasters. Only generator sets that are EPA non-road certified meeting Tier 4 emissions levels are permitted to operate with no limits for such circumstances. Those seeking best available technology or lowest emissions possible may opt for an EPA Tier 4 certification.

Essentially, large, stationary gensets must be Tier 4-certified for all non-emergency applications. Rate curtailment, where the operator elects to run during a utility’s peak demand period in exchange for rate reduction, and prime power, where the operator elects to run the genset as a primary source of power, are common non-emergency uses. Non-emergency operation is only legal when using a generator set equipped with a non-road certified engine meeting Tier 4 emissions levels.

Before we talk about the differences among these classifications, it’s important to realize first that Cummins provides fully EPA certified Tier 4 products.

Certified — Means that the complete generator set system (engine and exhaust aftertreatment) has been tested by the EPA on-site at the manufacturer’s location, and therefore does not require additional testing once installed. Cummins has a comprehensive, full line of Tier 4 certified high-horsepower stationary gensets.

Compliant — Means the manufacturer has tested and internally validated compliance with 4i or 4, but has not been certified to the EPA's regulatory standards. This may also be referred to as “verified.” It’s important to understand that in this case the manufacturer has not engaged with the EPA for certification; site verification is not a substitute for official EPA Tier 4 certification.