On May 11, 2004, the Environmental Protection Agency (EPA) signed the final rule introducing Tier IV emission standards, which phased-in over the period of 2008-2015. The Tier IV standards requires that emissions of PM be reduced by approximately 90% and NOx by approximately 37%.
January 1, 2013 was the effective compliance date for the Tier IV Final emissions standard. To meet the new standard, transport refrigeration unit
(TRU) diesel engines required advanced emission-control technologies similar to those found on current truck and tractor engines.