What exactly is Tier 4?
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 other harmful toxins from new, non-road diesel engines. As part of this clean-air initiative, the EPA established acceptable levels of emissions in diesel engines. Standards established through this law are designed to progressively reduce emissions over time to achieve a steady rate of air quality improvement without unreasonable economic disruption.

What is the difference between Tier 4 interim and Tier 4 final?
Tier 4 interim, or Tier 4i, refers to the emissions standard that became effective on Jan. 1, 2011, for all new, off-highway diesel engines, including those used in rental generators. The Tier 4i standard significantly reduced the limit on NOx and PM emissions, compared to the EPA’s previous Tier requirements.

Tier 4 Final, or Tier 4F, refers to the emissions standard that became effective on Jan. 1, 2014. Requiring a significant reduction in PM, Tier 4F represents the highest level of clean air regulations proposed to date. Cummins plans to offer a full line of Tier 4F-certified generators

How do Tier 4 regulations apply to rental power generators?
Within the off-highway classification in the EPA's regulations, rental generators qualify as "mobile" applications. The EPA defines "mobile" as any application that changes location at least once within a 12-month period. Because rental power generators are designed to be portable, the EPA rules state that all diesel engines are now required to meet Tier 4 for mobile, non-road use or unlimited run-time stationary use.

There seem to be a lot of different ways to interpret the Tier 4 standard. What are the differences among certified, compliant, verified and capable?
Before we talk about the differences in these classifications, it’s important to realize first that Cummins provides fully EPA-certified Tier 4 generator sets.

Certified — The engine, including aftertreatment, is tested by the manufacturer and certified by the EPA. It therefore does not require additional testing once installed. Rental generators are required to be certified because they qualify as mobile off-highway per the EPA’s classifications.

Compliant — The manufacturer has tested and internally validated compliance with the Tier 4 standard, but once installed, the unit must be tested by the EPA to verify compliance. This may also be referred to as “verified.” It’s important to understand that in this case, the OEM has not engaged with the EPA for certification; site verification is not a substitute for official EPA certification. Only allowable in some circumstances. 

Will I be required to recertify my existing rental generators?
No. The EPA only requires new rental generators bought on or after Jan. 1, 2014, for 150–300 kW and Jan. 1, 2015, for 60–150 kW and > 300 kW products to meet Tier 4F requirements.

What is TPEM?
Known as the Transition Program for Equipment Manufacturers, (TPEM) is a program established by the EPA that allows mobile equipment manufacturers to gradually comply with the latest emissions regulations. TPEM may be used to transition either from T3 (T2 in the case of an engine > 751hp) to Tier 4 Final or transition from Tier 4 interim to Tier 4 Final - not both. It's applicable for up to seven years from the regulation's effective date.

TPEM imposes a limit on the number of annual non-road mobile equipment allowed to be sold per equipment manufacturer.

What are the EPA’s regulatory differences between mobile and stationary applications?
By default, all mobile applications are required to comply with Tier 4F regulations. Therefore, rental power generators must meet Tier 4F emissions standards.

Tier 4 can be certified as mobile, stationary or both. Cummins certifies as both for Tier 4 engines. Emergency engines are all stationary and subject to non-emergency use hour restrictions.

However, the EPA does allow for an emergency operations clause for stationary generator sets, and the EPA does not impose  a limit on the number of hours that a generator may operate in emergency situations  Nevertheless, the EPA does limit operators to run their emergency generator sets 100 hours per year for maintenance and exercise purposes. Stationary generators used for applications such as peak shaving, rate curtailment and prime/continuous operations are required to meet Tier 4 standards  Cummins recommends Tier 4-certified generator sets for any customer seeking maximum operational flexibility throughout the lifetime of the generator.