This is copied from document 21CT-001, Technology Roadmap for the
21st Century Truck Program, A Government-Industry Research Partnership.
Driveline Losses
The driveline includes the transmission, drive shaft, differential, and wheel bearings. The driveline system
is a mature system achieving better than 95% efficiency at high-torque applications. At highway speeds,
the driveline efficiency approaches 98%. The efficiency drops under low-torque conditions because of
constant drag torque and viscous losses. The three areas of technical development that may lead to
improvements in efficiency include gear meshing, lubrication, and continuously variable transmissions
(CVTs).
Gear tooth profiles have evolved to a point where little benefit is anticipated from further research
activity. However, gear contact friction is an area where improvements are possible. Improvements would
include the reduction of surface roughness and the use of low-friction coatings, new gear materials, and
lubricants.
The second area of potential benefit is in the development of improved lubricants. Viscosity has long
been the key to reducing gear-tooth sliding friction and increasing durability, but viscosity is also a
contributor to reduced efficiency through viscous churning and damping.
The third area of potential benefit is the development of CVTs. To date, CVT technology has not been
widely accepted by the heavy truck industry because it has not been demonstrated that CVT design
concepts are practical in terms of cost or durability. However, newly emerging technologies are causing
manufacturers of heavy transmissions to rethink the possibility of offering CVT functionality to line-haul
trucks. This capability makes it possible for engines to operate at more nearly optimized speed/load
conditions as a means of improving efficiency. In addition, CVT functionality may represent an enabling
technology with respect to certain diesel aftertreatment technologies required to meet EPA regulations
beginning in 2007.
