A new kind of hybrid technology for heavy-duty vehicles uses hydraulics, rather than batteries, to store energy. Trucks using the system promise to be 30 percent more fuel-efficient than conventional vehicles. The system could also cost less than similarly performing hybrid technology that requires batteries.

The city of Ann Arbor, MI, which is rolling out four garbage trucks that use the technology, is among the first in North America to use such trucks. It was also among the first to receive funding for them through the U.S. Department of Energy’s Clean Cities Program, which is intended to help local governments reduce fuel consumption. The technology could help cities and trucking companies meet the new fuel-economy regulations for heavy-duty vehicles that are now being drafted by the U.S. Environmental Protection Agency.

The technology was developed at the EPA National Vehicle and Fuel Emissions Laboratory in Ann Arbor. The EPA partnered with the Cleveland, OH-based industrial giant Eaton to bring it to market. Christopher Grundler, deputy director of the EPA’s office of transportation and air quality, says the technology is a promising way to dramatically improve fuel economy on large trucks. “The beauty of this is, you don’t need battery technology, which is still expensive,” he says.

The technology, called hydraulic launch assist, is different from the electric hybrid technology currently used on some passenger cars and buses. Those systems use an electric-motor generator, along with batteries, to store energy that would otherwise be wasted–for example, the energy that dissipates during braking.

The hydraulic system uses a pump motor and a tank, called an accumulator, that stores energy by using automatic transmission fluid to compress nitrogen gas. When the driver lets off the accelerator, the wheels drive a hydraulic pump, which pumps hydraulic fluid to compress nitrogen gas and slow down the vehicle. When the driver accelerates, the high-pressure gas pushes the hydraulic fluid back through the pump motor, and the resulting energy drives the wheels.

This regenerative braking system captures up to 75 percent of the energy that would otherwise be wasted as heat during stopping and starting. This energy is then used to get the vehicle moving again. So it’s ideal for vehicles like garbage trucks, which regularly stop and start. The system is so heavy that it’s not efficient for cars, but it’s perfect for trucks.

Hybrid electric technologies already exist for buses, but hydraulic hybrids offer additional advantages. One potential one is cost: including the cost of the system and the savings from maintenance, a hydraulic system would likely cost cities and companies less money than both conventional trucks and current electric hybrids.

Because Eaton’s technology requires the driver to merely ease off the accelerator to hydraulically stop the vehicle (as opposed to applying the brakes), the new system will likely mean huge savings in brake repairs. A traditional refuse truck needs replacement brakes several times a year, at about $2,000 per brake job. Such brake jobs would be needed less than once a year with the hydraulic regenerative braking system.

Battery-based hybrids also reduce brake repairs, but to a lesser extent because their battery-based regenerative braking systems don’t slow the vehicles down as much. Heavy-duty electric hybrids only capture about 25 percent of the energy during braking because the amount of battery storage to capture more would cost too much to be practical. Since hydraulic hybrids store more energy–capturing up to 75 percent of the energy during braking– they’re more efficient than battery-based hybrids.

The new trucks will each cost $40,000 more than their conventional diesel counterparts. Ann Arbor is receiving grant money to offset the difference–$156,000 in federal stimulus money from the Clean Energy Coalition, a nonprofit focused on expanding the use of clean energy technology in Michigan. (The money is a portion of a grant that the Clean Energy Coalition received from the DOE.)

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.