Particulate matter and NOx, two significant constituents of diesel engine emissions, have often been considered as two sides of a seesaw. On the one hand, high temperatures and excess oxygen are conducive to the formation of NOx. So lowering the in-cylinder temperatures and oxygen content reduces NOx; however, it also increases the production of soot, thanks to lower fuel conversion efficiency. Decreasing both these constituents at the same time has challenged engine manufacturers to develop innovative and alternative solutions.
Currently, Tier 4 emissions standard is the strictest EPA emissions requirement for off-highway diesel engines. For most of the power range, the final version of Tier 4 is phased in after an interim period. Such interim requirements are less stringent on one of the main components of diesel exhaust emissions, either NOx or PM. Since 2008, the Tier 4 requirement has been in effect for engines below 25 mechanical horsepower, while the Tier 4i requirement has been in effect for engines in the 25–48 HP range. For engines that are greater than 48 HP, Tier 4i is required in 2011 or 2012 depending on the power range, and Tier 4 final takes effect in 2013– 2015, depending on the power range.
The Tier 4 emissions initial regulations govern most diesel engines used in power generation, industrial applications, oil and gas applications, mining operations, and mobile equipment. However, generators used in EPA-defined emergency stationary applications with an engine rating at greater than 49 HP are exempt from this new standard and are allowed to stay at 2010 emissions tier levels when the regulations change in 2011 or 2012. These generators provide power only in the event of a disruption of the normal power source. Generators used for peak shaving or as the normal source of power, on the other hand, need to comply with the Tier 4 interim or Tier 4 final emissions limits because they do not operate solely when utility power is lost. It should be noted that the duty rating of a generator (standby, prime, or continuous) does not correlate to the EPA’s emissions certification requirement; the application type itself is the determining factor. The purpose of the duty rating is to help indicate the generator’s capability. Any power duty rating may be applied to an EPA-defined “emergency stationary application” for as long as it is only used per the EPA’s guidelines of an emergency situation or within the defined testing and maintenance run time requirements.