The U.S. Environmental Protection Agency and Chrysler Group have partnered to test a hydraulic hybrid system for minivans that could be cheaper than conventional hybrid systems and could save more gasoline. EPA and Chrysler are each spending $2 million (for a total of $4 million) on the project, and they expect to begin road testing next year.

Conventional hybrids save gas in part by using energy from braking to charge a battery. A hydraulic hybrid captures energy by using a hydraulic pump instead of conventional friction brakes to slow the vehicle. The pump forces fluid into a tank, compressing air that can then be used to help propel the vehicle.

Hydraulic systems are bulky and loud, which has limited their use to large vehicles such as garbage trucks. But they’re attractive because pumps and air storage tanks are cheap compared to batteries. Also, hydraulic hybrids have the potential for larger fuel economy improvements than battery hybrids because hydraulic systems can store energy very quickly, allowing them to capture more energy from braking.  While conventional battery hybrid systems for minivans might improve fuel economy by 25 percent compared to gas-powered cars, hydraulic systems could improve fuel economy by 30 percent to 35 percent under most conditions, says David Haugen, manager of the technology development group at EPA’s National Vehicle and Fuel Emissions Laboratory. In city driving, the improvement could be as much as 60 percent. Chris Cowland, Chrysler’s director responsible for advanced powertrains, says the 60 percent improvement is “way more significant than any other technology that we know of today.”

To make the system work in minivans, engineers will need to find ways to incorporate bulky air storage tanks—which take up more space than the car’s gas tank—without eating into cargo or passenger space, and without compromising safety. “Since we are working with an existing vehicle structure, we will have to modify some areas and reinforce others to maintain the strength of the current vehicle,” Cowland says. They’ll also need to minimize noise, likely by using some of the techniques automakers have used to muffle the sound of hydraulic steering systems.

Manufacturers are continuing to roll out hydraulic hybrid garbage trucks. Parker Hannifin, of Cleveland, Ohio, has partnered with Autocar, of Hagerstown, Indiana, to bring 11 hybrid refuse trucks to South Florida. Eaton, of Cleveland, Ohio, has more than 70 trucks in municipal fleet operations. Eaton says that although fuel economy varies by duty cycle, route, terrain, and driver, the hybrid hydraulic system in a refuse truck can save more than 1,000 gallons of fuel per year on average, reduce emissions by up to 20 percent, and, by reducing the use of friction brakes, extend brake life by up to 300 percent.