The Detroit Diesel Allison Series 92 two-stroke cycle diesel engines have undergone a number of changes within the past five years aimed at improving fuel economy and reducing gaseous emissions. These changes include the use of aftercoolers, higher efficiency turbochargers, reduced area intake ports, camshafts for increased expansion stroke, low friction cylinder kits and bypass blowers. Fuel economy has improved approximately 14%. Future changes to lower engine speeds, increase the blower bypass area, improve combustion efficiency, utilize advanced charge air cooling, reduce mechanical friction, increase turbocharger efficiency, and incorporate electronics will improve fuel economy further. This paper describes the changes the Series 92 has undergone and the changes planned for the future to further improve fuel economy.