ATLANTA- UPS is testing a new hydraulic hybrid system for some of its trucks, according to Parker Hannifin Corp., the motion and technologies company supplying the systems.

The trucks use a Freightliner custom chassis and hydraulic hybrid system from Parker, and are being financed by the Department of Energy's Clean Cities program. The deal calls for 638 vehicles.

The hydraulic drive system is purported to be more efficient than traditional hybrid systems because there is less energy lost between the engine and the wheels.

Specially designed for stop-and-go vehicles, the system captures energy as the vehicle brakes. When the vehicle is restarted, the hydraulic system puts the vehicle in motion. When the captured energy is depleted, then the engine kicks in.

The developers say this method captures 70 percent of energy used from braking, compared to just 20-25 percent for typical electric hybrid models.

The vehicle should show a 50-70 percent improvement in fuel economy in stop-and-go situations, compared to typical delivery vehicles, according to a Parker press release. Also, because the engine is not running during its initial acceleration, it should reduce emissions at depots and delivery points.