UPS has started testing a fleet of diesel-hydraulic hybrid delivery vehicles, as an alternative to gas-electric hybrids. A few weeks ago I posted on fluid or gas accumulators as a possible future technology that does away with expensive batteries.

UPS Press Release

"In the series hydraulic hybrid diesel, a high-efficiency diesel engine is combined with a unique hydraulic propulsion system, replacing the conventional drivetrain and transmission. The vehicle uses hydraulic pumps and hydraulic storage tanks to store energy, similar to what is done with electric motors and batteries in hybrid electric vehicles. Fuel economy is increased in three ways: vehicle braking energy is recovered that normally is wasted; the engine is operated more efficiently; and the engine can be shut off when stopped or decelerating."