Hydraulic hybrid systems are time-tested technology in the world of heavy equipment like farm tractors and bulldozers. Now the technology is making its way into delivery and garbage trucks as well – and smaller vehicles could be down the road.

Eaton Corp. is one company moving into the field. Starting next year, UPS trucks  using Eaton's new series hydraulic hybrid system will hit the road in Minneapolis – two in early 2009, and seven total by 2010.

Hydraulic hybrid systems use pressurized fluid, instead of electricity, to power vehicles. Both types of hybrids cut down on fuel use by shutting down engines at times to use their alternative power source to move the vehicle.

But hydraulic hybrids are particularly good candidates for trucks, buses and other vehicles that start and stop a lot, according to the U.S. Environmental Protection Agency, which has been testing them for years.

Hydraulic hybrids can capture about 70 percent of the kinetic energy from their regenerative braking systems in their high-pressure hydraulic accumulators – the systems that store and release hydraulic energy. That's better than electric hybrids, which can capture about 25 percent of that energy in batteries now available, the director of EPA's National Vehicle and Fuel Emissions Laboratory said.

The difference has to do with the power density, or ability to deliver power on demand, of hydraulic pressure accumulators versus hybrid electric vehicle batteries now available. (Power density is different than energy density, which is a measure of how much energy per kilogramstoragesystems can hold.)

Because of this advantage, the EPA has judged hydraulic hybrid systems to be a good bet for enhanced fuel economy. Parallel hydraulic hybrid systems – which use hydraulic power to drive mechanical drive train and transmissions, much as hybrids like the Toyota Prius use electricity to drive a conventional drive train – are now in trucks being tested by UPS, FedEx Corp. and trash-hauling giant Waste Management. Eaton and Cleveland, Ohio-based Parker-Hannifin Corp. make systems for vehicles like these.

But Cleveland, Ohio-based Eaton's new series hydraulic hybrid system in UPS trucks goes a step further – it does away with the mechanical drive train and transmission entirely. Instead, it uses a fossil-fueled engine to drive a hydraulic pump motor, which either provides energy directly to a hydraulic drive assembly that propels the truck or gets stored in the truck's high-pressure accumulator. 

Such hydrostatic transmission systems have been in use in off-highway heavy vehicles for more than 50 years, since the offer full torque at low speeds, said Brad Bohlmann, business development manager with Eaton's Fluid Power Group.

"There's no mechanical connection between the engine and the drive wheels. It's all done with fluid power," he said. That allows the engine to run at speeds that are most efficient, rather than ramping up in lower gears as happens with a conventional transmission, which is where "a lot of inefficiencies occur," Bohlmann said.

The EPA says those features could make series hydraulic hybrids even better than their parallel hydraulic hybrid predecessors, offering potential fuel economy improvements of 40 percent to 80 percent, versus the potential 20 percent to 40 percent improvements from parallel hybrids.

Over an 18-month trial period, the UPS trucks with Eaton's series hydraulic hybrid systems have demonstrated up to 50 percent improvement in fuel efficiency, Bohlmann said.

Eaton isn't the only company working on series hydraulic hybrid systems. Parker-Hannifin Corp. announced in December 2007 that it was testing its series hydraulic hybrid system in FedEx trucks (hat tip to Green Car Congress).

There are still some advantages for hybrid electric vehicles, namely long-distance, steady-speed situations, Bohlmann said.

"Hybrid electrics in general provide smaller amounts of power for longer durations of time, and hybrid hydraulics provide much greater amounts of power for shorter periods of time," he said. Eaton isn't missing out – it has made hybrid-electric systems in use by UPS, FedEx, and for city buses in Guangzhou, China.

Still, when it comes to hydraulic hybrid systems, "we're very much targeted on smaller vehicles," Bohlmann said, though he wouldn't give any more details on how Eaton might be aiming for that market.

Deerfield, Mich.-based startup Hybra-Drive Systems is also aiming at the smaller vehicle market – if you can call a Hummer a small vehicle. The company retrofitted a Hummer H1 with a series hydraulic hybrid in partnership with Denver, Colo.-based Gates Corp., and has a research and development deal with the U.S. Army.

In May, Hybra-Drive was selected for a U.S. Department of Energy grant to build three series hydraulic hybrid drive trucks for testing with UPS, FedEx and Purolator.

Of course, just like with electric hybrids, what the payback in fuel savings might be for hydraulic hybrid systems depends on the price of fuel.

Bohlmann declined to discuss the extra costs associated with Eaton's system in the UPS trucks now being tested. The EPA, however, stated in April that Eaton's systems added only about 15 percent to the cost of the UPS trucks they were in, meaning that, "At current fuel prices, this technology will pay for itself in two to three years."

Of course, that was back when diesel fuel was selling for a nationwide average price of more than $4 a gallon. As of Dec. 19, that average price had fallen to $2.52 a gallon, according to AAA.