There are now four different tier ratings for diesel generators. What do they all mean and how do you know what tier rating you need? Hopefully we can demystify diesel generator tier ratings for you.

In 1994 the EPA met with major diesel engine manufacturers and the California Air Research Board to produce guidelines that would reduce the emissions from diesel generators. Government studies researching the impact of diesel emissions on air quality had discovered that diesel emissions of NOx, carbon monoxide (CO), hydrocarbons, and particulate matter had a huge impact on poor air quality in the United States. These tier ratings were designed to indicate the amount of emissions a given engine put out – and the intent was to slowly allow the industry to phase in higher rated tier engines.

Based on a given systems’ engine horsepower rating, generators are rated from Tier 1 to 4; most non-emergency diesel engine generators were required to arrive at Tier 4 by 2012.

Tier 1 was aimed at diesel engines in moving vehicles and was fully implemented by 1996.

Tiers 2 and 3 brought stationary diesel engines into compliance in 2000 and ran through 2008. By 2007 all new diesel gensets had to meet these requirements.

Tier 4 is the newest and strictest of emissions requirements. By the end of 2017 all new prime gensets will conform to these requirements, regardless of their size.

There is one major exemption to these rules. Emergency generators – which typically have very low hours and are used less frequently than continuous generators – were exempt from Tier 4 regulations and only required Tier 2 and 3.

That’s the first difference – if you’re using a generator for full time use, the EPA requires that it is Tier 4 rated. If you’re only using it for emergency use, Tiers 2 or 3 are fine. The EPA defines an emergency power generator as “a generator whose sole function is to provide back-up power when electric power from the local utility is interrupted.” These emergency generators should not be run for more than a total of 200 to 500 hours per year.

Your local municipality or State may have further guidelines and regulations that you should check with. For example, the State of California has much more rigid standards than other States.

To achieve the new Tier ratings, engine manufacturers altered the composition of the diesel fuel and the engine design. To reduce exhaust pollutants the sulfur content of diesel fuel was lowered and electronic engine controls were used to more efficiently operate the engines.

What about older generators? All older sets are grandfathered in. They must be in compliance with state and local air quality standards but do not necessarily need to meet Tier ratings.

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