ATLANTA — As part of a public-private partnership to increase the commercial availability and use of alternative fuel vehicles, UPS announced its first purchases of hydraulic hybrid vehicles.

The technology, originally developed in a federal laboratory of the Environmental Protection Agency (EPA), stores energy by compressing hydraulic fluid under pressure in a large chamber. UPS was asked to road-test the technology two years ago and says it is the first delivery company to place an order for hydraulic hybrid vehicles.

"There is no question that hydraulic hybrids, although little known to the public, are ready for prime time use on the streets of America," said David Abney, UPS’s chief operating officer. "We are not declaring hydraulic hybrids a panacea for our energy woes, but this technology certainly is as promising as anything we’ve seen to date."

Disclosing the results of its road testing on Detroit routes for the first time, UPS and the EPA said the prototype vehicle had achieved a 45 percent to 50 percent improvement in fuel economy compared to conventional diesel delivery trucks.

UPS believes similar fuel economy improvements and a 30 percent reduction in CO2 are achievable in daily, real-world use. The EPA believes the technology can perform equally well in other applications such as shuttle and transit buses and refuse pick-up trucks.

UPS will deploy the first two of the new HHVs in Minneapolis during the first quarter of 2009. Eaton, which helped develop and refine the vehicle’s hydraulic hybrid power system, will monitor the vehicle’s fuel economy performance and emissions in the Minneapolis area. The additional five HHVs will be deployed later in 2009 and early 2010.

With a diesel "series" hydraulic hybrid of the type being purchased by UPS, a high-efficiency diesel engine is combined with a unique hydraulic propulsion system, replacing the conventional drivetrain and transmission. The vehicle uses hydraulic pumps and hydraulic storage tanks to capture and store energy, similar to what is done with electric motors and batteries in a hybrid electric vehicle. In this case, the diesel engine is used to periodically recharge pressure in the hydraulic propulsion system. Fuel economy is increased in three ways: vehicle braking energy is recovered that normally is wasted; the engine is operated more efficiently, and the engine can be shut off when stopped or decelerating.

UPS’s current "green fleet" totals more than 1,600 low-carbon vehicles, including all-electric, hybrid electric, compressed natural gas (CNG), liquefied natural gas (LNG) and propane-powered trucks.

In addition to the hydraulic hybrid, UPS has road-tested hydrogen fuel cell delivery trucks. UPS has a long history of using alternative fuel vehicles, starting in the 1930s with a fleet of electric trucks in New York City.