Researchers at the U.S. Environmental Protection Agency (EPA) are
developing a hydraulic-hybrid drive system for vehicles. The system uses
hydraulic pressure to recapture energy lost through braking and then
releases the pressure during acceleration. The EPA believes that
hydraulic hybrids can boost fuel mileage by 55 percent. Current
gas-electric hybrids have raised mileage by about 30-40 percent.
Hydraulic hybrids are also less expensive, so the payback period is
A gasoline-powered Ford Expedition averages 13 mpg in the city and
20 miles per gallon on the highway. A hydraulic hybrid, one of the
prototypes the EPA is working on, would average 32 mpg in city driving
and 22 mpg on the highway. Traditionally, vehicles get better mileage on
the highway, but the added weight of the hydraulic system lowers
highway fuel mileage.