Ford Motor Company and Eaton Corp. recently introduced a regenerative braking system called the hydraulic launch assist (HLA). The system is designed to improve fuel economy in large trucks during city driving by an estimated 25 to 35%. The HLA system featured on the Ford's new F350 Tonka concept truck recovers energy normally lost during deceleration and converts it to hydraulic pressure in an accumulator, where it is available as a source of energy during the vehicle's next acceleration.

The HLA system consists of a reversible hydraulic pump/motor from Eaton's Fluid Power Group (Eden Prairie, MN) coupled to the drive shaft through a clutch and two accumulators. As the driver steps on the brake, the pump/motor forces hydraulic fluid out of a low-pressure accumulator and into a high-pressure accumulator, increasing the pressure of nitrogen gas stored there to 5,000 psi.

During acceleration, the HLA system switches from the pump mode to the motor mode, the nitrogen gas forces the hydraulic fluid back into the low-pressure accumulator, and the pump/motor applies torque to the driveshaft through the clutch. If quick acceleration is required, the F-350's diesel engine works with the HLA system.

Eighty percent of the initial kinetic energy is returned to the vehicle. "The system stores approximately 380 kJ of energy," says Brad Bohlmann, a mechanical engineer and business development coordinator in Advanced Technology at Eaton's Fluid Power Group. "With that much energy, we can accelerate a 10,000-lb vehicle from a dead stop to between 25 and 30 miles per hour with no assistance from the vehicle's combustion engine."

"Ford thinks that both electric and hydraulic regenerative systems have a future," says John Brevick, a Ford mechanical engineer working on the HLA system. "But for heavy vehicles like our 10,000-lb F-350 trucks, hydraulics are better at capturing lost energy than electric systems."

Eaton's Bohlmann explains that the real advantage of hydraulics is in its power density. "Hydraulics is capable of transferring energy very quickly," says Bohlmann. "The rate of energy transfer is in a hybrid electric system of a similar size is much lower."

Bohlmann adds that the specific vocation or driving cycle of the vehicle determines whether a hybrid hydraulic or hybrid electric system is best suited for the vehicle. "Think of hydraulics as a sprinter, providing fast bursts of power," he says. "An electric hybrid is more like a marathon runner."

"The challenge now is to make use of the regenerative braking system seamless to the customer," says Ford's Brevick. "We still have room for improvements and feel the HLA system has a lot of potential."

As for the future, Ford, Eaton, and the U.S. Environmental Protection Agency (EPA) are jointly conducting research on hydraulic hybrid vehicles, including internal combustion engines that would produce hydraulic pressure to drive the vehicle. Eaton signed both a technology license agreement and a cooperative research and development agreement with the EPA for the development of future generation systems.