The Hydraulic Hybrid: Expanding Options for Green Driving

A new type of ‘green vehicle’ is being tested for future use in both fleet vehicles and consumer cars. A novel system called “hydraulic hybrid” has so far been tested in a Peterbilt garbage truck that increases the fuel economy an estimated 30 percent and reduces carbon dioxide emissions by 40 percent or more. Developed at EPA’s National Vehicle and Fuel Emissions Laboratory (NVFEL) in Ann Arbor, Michigan, the Hydraulic Launch Assist (HLA) system essentially operates in the same manner as a gasoline-electric hybrid.

Like gas-electric versions, HLA works by recovering a portion of the energy normally lost as heat by the vehicle’s brakes. But instead of a battery pack, a hydraulic system uses pistons to capture the wasted energy by compressing nitrogen gas stored in a tank. When the nitrogen is allowed to expand, it pushes a piston in a cylinder filled with hydraulic fluid that helps the diesel engine turn the rear wheels.

An aggressive variation of the HLA design has been tested in two UPS parcel delivery trucks. Using a series-type hybrid design, the V6 diesel engines do not power the vehicles, but serve solely to actuate the hydraulic systems’ pumps. It’s estimated that these vehicles could see fuel economy improvements of as much as 60 percent and a reduction of greenhouse emissions of 40 percent.

The engineers at the EPA’s Michigan laboratory have built and tested many prototypes over the years, including a diesel-hydraulic passenger car that delivered more than 80 miles per gallon. In 2004 the EPA displayed a hydraulic hybrid Ford Expedition SUV with a small diesel engine replacing the large displacement gasoline V8. It is unknown if this new type of hybrid will ever become mainstreamed, but it is another glimpse of hope in the race to create a super fuel efficient vehicle.