Peterbilt and Eaton Corporation are jointly developing refuse trucks using Eaton’s parallel hydraulic hybrid system—Hydraulic Launch Assist (HLA). Peterbilt plans to build and evaluate a production version of the vehicle during the next year.
The hydraulic launch assist system uses regenerative braking to capture the energy otherwise lost in braking. Unlike its electric cousins which use regenerative braking to generate electricity to store in a battery for use with an electric motor, the hydraulic hybrid system recovers the energy in the form of pressurized hydraulic fluid.
The HLA system uses a reversible hydraulic pump/motor coupled to the drive shaft through a clutch and two accumulators. When a driver steps on the brake, the pump/motor forces hydraulic fluid out of a low-pressure accumulator into a high-pressure accumulator, increasing the pressure of nitrogen gas stored there to 5,000 psi.
During acceleration, the HLA system switches from pump mode to 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.
(The diagram at right is a conceptual sketch of the hydraulic launch assist configuration, from an EPA update on the technology. More on the EPA below.)
The hydraulic hybrid truck uses the hydraulic power for the intimal acceleration boost, then blends in the engine. This results in a significant reduction in fuel consumption and improved acceleration due to the high power density of hydraulics.
Eaton estimates that the HLA can provide a 25–35% percent improvement in fuel efficiency, with 25–35% reductions in emissions and some 50% reduction in brake wear.
With a version of HLA Eaton developed with Ford, the engineers found that approximately 80% of the initial kinetic energy was 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.”
The HLA system can provide high torque very quickly, even at very low speeds, and is well-suited for heavy vehicles that do a lot of stop-and-go driving—vehicles like a refuse truck or heavy delivery truck. Or, hmm, maybe even a large urban SUV.
The technology could have a significant impact on improving the operating costs of customers involved in stop-and-go applications, such as refuse. Hydraulic Launch Assist can be quickly tailored for maximum fuel economy or enhanced productivity through quicker acceleration and shorter cycle times. Additionally, the system increases brake life and reduces engine and transmission wear, potentially extending component life and lowering service costs. It is also more environmentally friendly by decreasing exhaust emissions and noise.—Peterbilt Chief Engineer Craig Brewster
Earlier this year, Ricardo Automotive presented to H-TUF the results of a simulation of fuel economy comparing hybrid electric systems to a hydraulic hybrid in a refuse collection vehicle. According to HTUF, Ricardo found significant potential for fuel economy improvement for all hybrid route vehicles where the duty cycle involves significant stop-and-go driving.
Using the assumptions in the Ricardo model, the hydraulic launch assist architecture provided the most consistent and highest average fuel economy improvement, improving fuel economy by roughly 48% in simulation over the measured driving cycle. Half the improvement came from the hydraulic launch assist plus the effect of engine downsizing. Idle stop and a near term “dual clutch” transmission also played a significant fuel-saving role.
Eaton is one of the leaders in working with hydraulic hybrid systems, but is not alone. Eaton worked with Ford and the EPA on the technology, and in 2002, Ford introduced an F-350 prototype using the HLA system.
The EPA continues to push forward with its hydraulic hybrid research. The agency has produced a test chassis (frame and powertrain, no skin) weighing 3,800 pounds that indicates that the hydraulic hybrid approach could triple the fuel economy of conventional vehicles (to up to 80 mpg)for a midsize sedan.
According to the EPA, it has made breakthroughs in designing hydraulic accumulators and pump/motors to be more efficient, smaller, and lighter for motor vehicle applications, which will also help improve fuel efficiency. The agency currently has cooperative research and development agreements with several private sector partners (such as Eaton) to further the development of hydraulic hybrid systems.
Permo-Drive, an Australian company, is another leader in hydraulic hybrid technology, and is working with its partner Dana Corporation on the FMTV (Family of Medium Tactical Vehicles) project for the US Army.