With the gas prices affecting consumers paling in comparison to the pain operators of heavy-duty vehicle fleets feel, it is no surprise that that some of the biggest operators of these truck fleets have jumped on the hybrid vehicle bandwagon, according to a Thomson (News - Alert) News edge online report quoting the publication Design News.
From a fuel-economy standpoint, trucks have several inherent disadvantages, weight being the most critical. Fully loaded heavy-duty vehicles cover a weight range from 14,000 to more than 70,000 lb., the report noted. And on top of the weight, many of these vehicles have a fuel-burning duty cycle that requires continuous starts and stops.
The report noted that both Federal Express and UPS have added several dozen hybrid electric vehicles to their fleets over the past two years, both using a hybrid powertrain supplied by Eaton Corp.
Waste Management, the nations largest waste hauler, is likewise evaluating a variety of hybrid vehicle solutions for its refuse trucks, Lynn Brown, a company spokesperson, was quoted as saying in the report.
What may be surprising, though, is the kind of hybrid systems fleet operators like Fed Ex, UPS and Waste Management are considering for some of their heaviest vehicles. According to the report, these hybrids do not use the electric motors, batteries and wires of say, a Toyota Prius. Instead, they instead propel the vehicle with hydraulic pump-motors, high-pressure fluid lines and accumulators.
The report noted that the most radical and fuel efficient versions of these hydraulic hybrids eliminate the traditional mechanical drivetrain altogether. Instead, the vehicles use diesel engines driving a hydraulic pump-motor, in turn charging a high-pressure accumulator that drives a bent-axis pump-motor on the rear wheels to propel the vehicle. A low-pressure reservoir completes the hydraulic circuit, collecting the fluid before sending it back to the first pump-motor.
Like electric hybrids, hydraulic hybrids also provide regenerative braking capabilities. During braking events, of which there are many in a delivery vehicle or refuse truck, the pump-motor charges the high-pressure accumulator. The energy stored in the accumulator can be used to reduce the load on the diesel engine when the truck moves forward again. Or that energy could also allow limited bursts of engine-off propulsion for example, when operating a truck indoors.
Although hydraulics may seem like outmoded technology, hydraulic pump-motors and accumulators can provide a low-cost, reliable way to apply torque and store energy, the report noted. And hydraulics offers a significant power-density advantage over electrical systems.
Researchers at the U.S. Environmental Protection Agency’s Office of Transportation and Air Quality have developed a hybrid in conjunction with Eaton Corp., the Southwest Research Institute (SwRI (News - Alert)) and other partners, the report noted. Since June 2006, this system has been under evaluation on a UPS delivery truck in Detroit. The EPA has also worked on aspects of hydraulic hybrid design with Parker Hannifin under a separate cooperative research and development agreement, the report noted.
The SwRI has also been focusing on carbon-fiber-reinforced composite accumulators and its engineers designed the accumulators used for the EPA vehicle. Wendel says, in general, the weight of a composite accumulator is two-and-a-half times less than a conventional steel bladder-type accumulator and something more like 10-times less than a piston accumulator.
Aside from the cost and weight of the system itself, hydraulics have something else going for them power density, said Joe Kovach, Parker Hannifin’s vice president for hydraulics innovation. He explained that high-pressure accumulators and pump motors have at least 10 times the power density of batteries and electric motors. Ultracapacitors for such vehicles would be prohibitively expensive in these applications, Kovach was quoted as saying.
All that power density comes in handy in quickly generating the torque needed to overcome the high inertial loads associated with propelling a massive truck. Batteries have better energy density, but they can’t get that energy in and out fast enough for the stop-and-go applications where hydraulic hybrids make sense, Kovach added.
Spencer Chin is a contributing editor for TMCnet. To see more of his articles, please visit his columnist page.
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