Beginning with diesel engines manufactured on or after January 1, 2010, the engines are required to meet lowered NOx standards. All of the heavy duty engine (Class 7-8 trucks) manufactures except for Navistar International continuing to manufacture engines after this date have chosen to utilize SCR. This Includes Detroit Diesel (DD13, DD15, and DD16 models), Cummins (ISX line), PACCAR, and Volvo/Mack. These engines require the periodical addition of Diesel Exhaust Fluid (DEF, a urea solution) to enable the process.