If you live in Baltimore or Atlanta and are expecting a package, check out that UPS truck that shows up in front of your house. It might be an extra-special delivery.

UPS's fleet of iconic brown delivery vans is getting a little bit greener with the addition of some new hybrid trucks. But these hybrids won't run on batteries. They're hydraulic hybrids, and they're the result of more than six years of research.

After testing out a few hydraulic hybrid vans back in 2009, the company is ready to debut 40 of them in the Baltimore and Atlanta markets. Even if the demonstrator fleet is small, it's a good chance for those working on the somewhat obscure technology to showcase their work in a real-world application.

Though they remain relatively unknown to the general public, hydraulic hybrids aren't that exotic. In fact, they work on a similar principle as battery-electric hybrid vehicles, recapturing energy usually lost to braking. Instead of storing that energy in a battery, however, hydraulic hybrids use a high-pressure accumulator that's capable of running the vehicle with the engine off.

The technology is especially useful in heavy commercial vehicles, which use lots of energy during braking and make frequent stops. Already, school buses and city buses have had accumulators installed, and Chrysler is working with the EPA on a hydraulic hybrid minivan.

In the case of UPS' hydraulic hybrids, the delivery vans will gain about 90 minutes of engine-off time on most routes, which will reduce emissions in addition to improving fuel economy. The company says the new trucks will reduce CO2 emissions up to 30 percent over traditional diesel delivery vans, and will improve fuel economy by up to 35 percent. That can add up to significant savings for a company whose profits depend on gas prices.

"As early adopters of this technology, we are very pleased with the significant fuel economy and emission reductions that come from the HHVs," said Mike Britt, UPS' director of alternative fuel vehicle engineering.

The vans themselves were developed by Freightliner and Parker Hannifin, use fuel-efficient diesel engines, and were supported in part by funding from the US Department of Energy's Clean Cities initiative.

Photo, Image: UPS