The General Electric ES44AC (or simply, "GEVO") is a energy-efficient, 4,400 hp locomotive which was first introduced into full production in 2005, with the first ES44AC prototype built in November 2002. The ES44AC is the direct successor of the AC4400CW, which could not be produced domestically after 2004. Like EMD, GE had to essentially design a new locomotive to comply with EPA Tier 2 standards. Unlike EMD, GE had to give its new locomotive a new power source; the 7FDL16 prime mover was replaced with a more efficient prime mover, the GEVO12. Only a handful of production units operate with this engine. Most newer units have a completely reworked 7FDL-16 prime mover.