With gas prices reaching record highs recently, consumers have already endured pain at the pumps, but it’s really just a twinge compared to the pain operators of heavy-duty vehicle fleets feel. The trucks that deliver our packages and haul our garbage have a couple of strikes against them from a fuel-economy standpoint. Weight is the big one. Fully loaded heavy-duty vehicles cover a weight range from 14,000 to more than 70,000 lb And on top of the weight, many of these vehicles have a fuel-burning duty cycle that requires continuous starts and stops. So it should come as no surprise to anyone that some of the biggest operators of these truck fleets have jumped on the hybrid vehicle bandwagon. 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 nation’s largest waste hauler, is likewise evaluating a variety of hybrid vehicle solutions for its refuse trucks, according to Lynn Brown, a company spokesperson.

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. These hybrids don’t use the electric motors, batteries and wires the way the Toyota Prius does. They instead propel the vehicle with a combination of hydraulic pump-motors, high-pressure fluid lines and accumulators.

The most radical – and fuel efficient – versions of these hydraulic hybrids eliminate the traditional mechanical drivetrain altogether. In the vehicles, diesel engines drive a hydraulic pump-motor, which in turn charges a high-pressure accumulator. That accumulator 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.

To consumers and even some engineers, hydraulics may seem like outmoded technology in an increasingly electronic world. Yet hydraulic pump-motors and accumulators can provide a low-cost, reliable way to apply torque and store energy — which is exactly what hybrid vehicles require. And hydraulics offer a significant power-density advantage over electrical systems, at least for now. “It looks like hydraulics will make a lot of sense, at least for the heavy end of the heavy-duty truck spectrum,” says John DeCicco, a Ph.D. mechanical engineer who is a senior automotive strategies fellow for Environmental Defense.

Hydraulic Propulsion Styles

Hydraulic hybrid systems currently come in three main variants, all of which can still be considered developmental. Researchers at the U.S. Environmental Protection Agency’s Office of Transportation and Air Quality (OTAQ) have developed a hybrid in conjunction with Eaton Corp., the Southwest Research Institute (SwRI) and other partners. 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.

Both Eaton Corp. and Parker Hannifin also developed their own proprietary hydraulic hybrid systems. Eaton’s is a parallel system that uses the hydraulics for launch assist but still gets most of its propulsive power from a mechanical drive train. Over the last year and a half, Parker Hannifin has developed a new hydraulic hybrid design with some duty-cycle input from Waste Management. Joe Kovach, a Ph.D. mechanical engineer and Parker Hannifin’s vice president for innovation in the hydraulics division, says his company will build prototype refuse trucks that incorporate this new hydraulic hybrid system later this year. Similar systems for larger delivery trucks are also in the works.

All of the different systems promise significant gains in fuel economy and emissions reductions. The biggest gains, though, will likely come from full-series hybrids — or those that don’t rely on a mechanical drive train. According to John Kargul, OTAQ’s director of technology transfer, the EPA estimates parallel hybrids will produce fuel economy improvements in the 20 to 40 percent range. The agency’s modeling of full-series hybrids predicts they’ll offer a fuel economy boost of 40 to 80 percent.

In the case of the UPS truck, the EPA’s modeling and dyno testing predict 60 to 70 percent more mpg than a similarly sized vehicle with a conventional drive train, Kargul says. The EPA just started to analyze the fuel economy data from the UPS truck’s real-world performance on the streets of Detroit. Kargul says it’s too early to release the results publicly but adds, “UPS has been very happy with the early data.”

Parker’s estimates, meanwhile, place the potential fuel economy improvements associated with hydraulic hybrid drive trains at 30 to 70 percent, depending on the vehicle’s duty cycle, the specifics of the drive train design and the engine management strategies.

The main difference between EPA’s UPS truck and Parker’s in-house hybrid design comes down to the use of a secondary mechanical direct drive system for the real wheels under certain driving conditions. In the EPA’s full-series hybrid, only the hydraulic system connects the diesel engine and rear wheels. So all of the propulsion comes from the hydraulic system at all times. Parker, by contrast, augments the hydraulic drive train with a mechanical direct drive system that connects the engine to the rear wheels during highway driving, meaning steady state speeds about 50 mph or so.

“We’ve built hybrid systems both ways — with and without the driveshaft,” Kovach says. In fact, Parker, over the years, has been involved in the full spectrum of hydraulic hybrids. One of its European divisions has supplied parallel hybrid systems that provide hydraulic launch assist, but get most of their propulsive power from a conventional drive train. “We still have about 20 buses using a system like this,” he says. And in 1991, Parker helped develop a refuse truck that was a pure series hydraulic with no mechanical drive train.

Why put a secondary mechanical drive on this latest hybrid system? Kovach acknowledges the direct drive components add some complexity — in terms of mechanical components and controls — that would be avoided by a pure series hybrid design. But he argues the direct drive system offers far more efficiency during highway driving — more than enough to offset the complexity penalty.

Glenn Wendel, a principal engineer at the SwRI, which helped develop parts of the EPA system, estimates some hydraulic hybrid systems can see their efficiency dip as low as 75 percent during highway driving, far less than the 90-plus percent efficiencies they get during city driving. A traditional geared transmission and drive train would also offer efficiencies upwards of 90 percent on the highway. “That’s why you won’t see hydraulic hybrid passenger cars or 18-wheelers,” Wendel says.

The EPA has opted to offset the efficiency loss during highway driving differently than Parker. “We’ve redesigned the pump-motors to get more efficiency than we could with an off-the-shelf model,” Kargul says. For example, EPA researchers created a pump-motor design with higher displacement angles than most off-the-shelf bent-axis pump-motors — the type used to drive the rear wheels. “Our pumps displacement angles in the 45-50 percent range in the same package size as off-the-shelf pumps with displacement angles in the 20 to 25 percent range,” says Kargul, noting bent-axis pump-motors run most efficiently with higher displacements.

So which of these two series hybrid strategies will win out? “It’s really too early to tell,” says Wendel.

Why Hydraulics Make Sense

Yet, if ever there was a time for hydraulic hybrids, it is now. “The stars have finally aligned,” says Kovach. One reason is high fuel costs. Parker’s cost analysis of hydraulic hybrids systems suggests they offer a two-to-three year return-on-investment standpoint when fuel prices exceed $2/gallon. And because they use a proven, low-cost technology, some observers believe hydraulic hybrids are expected to be relatively inexpensive compared to electric hybrid for heavy-duty trucks. Kargul cites an incremental cost target of $7,000 for its hybrid power train at production volumes.

The capabilities of the hydraulic components have also gotten better in recent years. The bent-axis pump-motors used in these systems have become smaller and more efficient over the years. Kargul says he can hold the pump-motor barrel from the UPS demo in one hand, yet it delivers 300 HP from a fully charged accumulator and about 150 HP with the accumulator empty.

And an even more important change involves the accumulators used in these high-pressure hydraulic systems. Kovach recalls when Parker dipped its toe into hydraulic hybrid design in 1991, the metal accumulators for a refuse truck weighed 3,000 lb. “Now we’re using the space-age composites used on planes and our accumulators weigh only 300-400 lb,” he says.

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, though, hydraulics have something else going for them — power density. “Nothing out there beats hydraulics when it comes to power density,” says Kovach, who explains high-pressure accumulators and pump motors have at least 10 times the power density of batteries and electric motors. Ultra capacitors, in theory, can get pretty close from a power density standpoint. “But they’re still way too expensive in these applications,” says Kovach, who estimates an ultra capacitor of the size needed for these heavy-duty vehicles would cost around 50 times as much as an accumulator that does the same or better job. (See chart for power density and cost data).

Hydraulics suppliers also have an easy route to bumping up their power density — by increasing system pressures. The EPA’s demo for UPS runs at 5,000 psi, mostly because the engineers who built it couldn’t at the time find fluid line connectors rated for higher pressures. “We’ve since found higher pressure connectors and the pump-motors are designed for pressures up to 7,000 psi,” says Kargul. Consequently, the EPA’s next generation urban delivery van may run at that higher pressure. And since power density scales with operating pressures in these systems, that same pump-motor Kargul holds in his hand goes from 330 HP to 510 HP.

Kovach makes the same point and says Parker could boost system pressures — and thus, power density — by two or even three times using existing hydraulics technology. “We already have systems running on America’s Cup boats at 12,000 psi,” he says.

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 says.

Hydraulic Hybrid Applications

Identifying those applications can still be a bit tricky. In general terms, though, the advocates of hydraulic technology are pushing it for applications where the truck is both massive and prone to frequent starts and stops.

In terms of vehicle weight ratings, the EPA deems hydraulic hybrids suitable for Class 3 work trucks right up to the heaviest Class 8 trucks, based on the fuel economy gains and costs it has measured in its test vehicles. Parker’s Kovach believes the hydraulics start to shine at Class 5 and up. “Class 3 and 4 may be better served by electric hybrids. Class 5 could go either electric or hydraulic,” says Kovach. “But in Class 6, 7 and 8 with starts and stops, no technology will come close to hydraulics for a long time.”

But there’s more to duty cycle than the frequency of the stops. Jeff Carpenter, chief engineer for Eaton Corp.’s hybrid power systems group, points out the distance between stops as another duty cycle factor that can favor one type of hybrid over another. “What’s the distance between stops? If it’s short and the vehicle is heavy, hydraulics may make sense. It’s, say 1,000 yards or more, maybe electric storage makes more sense,” Carpenter says. Put differently, he argues some trucks will get the maximum benefit from the power density advantage provided by hydraulics while others will get more benefit from the superior energy storage offered by batteries. “Both hydraulic and electric systems have advantages and disadvantages that depend on how the customer will use the vehicle,” he says.

It’s likewise difficult to determine which type of hybrid has the efficiency edge. Carpenter points out, electric and hydraulic hybrids can both achieve overall efficiencies over 90 percent — when used in the right kind of duty cycles. “They may be a percentage point or two difference but nothing too significant,” he says.

But how they derive that efficiency can help determine the best fit for each technology. Kargul maintains hydraulic hybrids are much more efficient at recovering braking energy. He says a hydraulic hybrid with a properly sized accumulator ends up returning about 70 percent of the braking energy to the wheels while a typical electric hybrid returns something around 21 percent. The reason, he says, has to do with the time it takes to charge today’s batteries. Kargul says the energy from a truck’s braking would cause even a modern lithium ion battery to exceed its charging limits, so the batteries only recover what they can handle. With more batteries, a hybrid could accept as much energy as the hydraulic systems. “But then you have weight, cost and packaging issues. Designing hybrids involves a compromise between how much regen energy you want to recapture versus these other factors,” he says.

Making the choice between electric and hydraulic systems even less clear cut is the fact both already overlap within a given duty cycles and weight class. Consider that Eaton’s own hybrid electric power trains and the demo hydraulic power train it built with the EPA go into UPS vehicles that overlap in terms of weight and duty cycle. Carpenter points out, however, the electric version is fully commercial and now in it’s second generation. UPS now has 50 of them in the 16,000 to 24,000 lb range. The hydraulic version remains a demo, though a promising one.

The choice probably won’t become clear for some time. “The verdict is still out,” says Kargul. With only 150 Class 6 electric hybrids and one similarly sized hydraulic hybrid on the road, there isn’t enough data, in Kargul’s view, to get a sense of which technology will be the most cost efficient. “The bottom line is that the big fleet owners will continue to look at any hybrid technologies that can significantly lower their cost,” Kargul says. “You’re seeing that right now at both UPS and at FedEx.”

Over the long haul, though, even advocates of the hydraulic hybrids believe electric systems may prevail. Environmental Defense’s DeCicco argues electric propulsion will at some point become so common it’s adoption will become inevitable for commercial, rather than purely technical, reasons. “The competition between hydraulic and electric hybrids in some ways start to look like the competition between BetaMax and VHS,” he says.

And Kovach, who develops fluid power technology for a living, doesn’t think hydraulic hybrids will be the best choice for heavy-duty trucks forever. “Ultracapacitors will get better,” he says. “At some point, they may offer a more cost-effective solution than hydraulics. But that won’t happen for a long time, especially since we can continue to increase our pressures and power densities. Hydraulics can help us save lots of fuel today without the need for any new technology.”

		UPS has been testing a hydraulic hybrid version of a Class 6 delivery vehicle in Michigan for the past year. The company has also put 50 hybrid electric delivery vehicles into service.
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