Hydraulic Regenerative Braking Boosts Fuel Economy
July 25, 2002
A new braking system called hydraulic launch assist will make stop-and-go driving for large trucks more efficient by recapturing energy that would normally be lost as heat.
The HLA system is composed of a reversible hydraulic pump/motor and two accumulators. When the driver steps on the brake, the pump/motor propels hydraulic fluid out of the low-pressure accumulator and into the high-pressure one, intensifying the pressure of nitrogen gas stored there to 5,000 psi. During acceleration, the retained energy is released back as the nitrogen gas forces the hydraulic fluid back into the low-pressure accumulator, and the pump/motor transfers torque through the clutch to the drive shaft. For quick acceleration, the energy generated by the HLA system combines with engine power.
Eighty percent of the initial kinetic energy is recaptured. "The system stores approximately 380 kJ of energy," says Brad Bohlmann, mechanical engineer and advanced technology business development coordinator 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 at work 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." Garbage trucks are also good candidates for the HLA system because they often run for less than 100 ft. between stops. According to Bohlmann, the driving cycle must be considered to determine the best system for a vehicle. While power density is a major strength of hydraulicsmaking these systems comparable to a "sprinter"an electric hybrid system can be likened to a "marathon runner," he says. "Hydraulics is capable of transferring energy very quickly," says Bohlmann. "The rate of energy transfer in a hybrid electric system of a similar size is much lower."
The HLA system adds weight and cost to the truck. The systemwhich includes the pump/motor, control systems, two cylindrical accumulators and 5 gallons of fluidweighs approximately 450 lb. and is placed under the body on one side of the frame. It is expected to tack on about $2,000 to vehicles' price tags and reduce fuel bills by 25%.
As for future plans, Ford, Eaton and the U.S. Environmental Protection Agency (EPA) are working together on hydraulic hybrid vehicle research. Future systems include internal combustion engines that would generate hydraulic pressure to run the vehicle.
Sources: F-350 Tonka: Hydraulic Regenerative Braking
Ward's Auto World, Feb. 1, 2002
Hydraulic Regenerative Braking Improves Large-Truck Fuel Economy
Bruce Wiebusch, Regional Editor
Design News, June 17, 2002