Andrew Brix, energy programs manager for the city of Ann Arbor, says the system will save $9,120 in fuel every year. By doubling the life of the brakes, the system will also save $3,300 a year in maintenance. Vince Duray, Eaton's chief engineer of the hydraulic launch assist, says that even without the government grant, the system should pay for itself within three years (although Duray uses more optimistic assumptions about savings from fuel consumption and brake repairs than the Ann Arbor officials).

Eaton has orders for 80 vehicles equipped with the technology. Though the initial application is garbage trucks, the company is eyeing other possibilities, like buses with a heavy start-and-stop route.

The EPA is also working on a more revolutionary type of hydraulic technology, called a "series" system. While the hydraulic launch assist used in the Ann Arbor trucks helps the vehicle start rolling, the series replaces the entire drive train, which means the engine is not connected to the wheels. Instead, the engine pressurizes the nitrogen tank, which pushes hydraulic fluid to drive the wheels. In this arrangement, the engine doesn't have to supply bursts of power for acceleration or driving up hills--this power comes from the nitrogen tank. As a result, the engine can be smaller and run at a steady, efficient rate. The system is 40 to 60 percent more efficient than conventional diesel trucks in city traffic.

Joseph Kovach, vice president of technology and innovation for Parker Hannifin Hydraulic Group Worldwide, which is developing the series hybrid, says his company is testing the system on delivery trucks, buses, and refuse trucks. The city of Miami will use it on a dozen refuse trucks beginning in September, and FedEx Ground expects it on some of its vans by spring 2011. While Eaton is also pursuing the series technology and thinks it has great potential, it's working to reduce the costs of this technology before it brings its version to market.