Hydraulic hybrid systems are time-tested technology in the world of heavy equipment like farm tractors and bulldozers. Now the technology is making its way into delivery and garbage trucks as well – and smaller vehicles could be down the road.

Eaton Corp. is one company moving into the field. Starting next year, UPS trucks using Eaton’s new series hydraulic hybrid system will hit the road in Minneapolis – two in early 2009, and seven total by 2010.

Hydraulic hybrid systems use pressurized fluid, instead of electricity, to power vehicles. Both types of hybrids cut down on fuel use by shutting down engines at times to use their alternative power source to move the vehicle.

But hydraulic hybrids are particularly good candidates for trucks, buses and other vehicles that start and stop a lot, according to the U.S. Environmental Protection Agency, which has been testing them for years.

Hydraulic hybrids can capture about 70 percent of the kinetic energy from their regenerative braking systems in their high-pressure hydraulic accumulators – the systems that store and release hydraulic energy. That’s better than electric hybrids, which can capture about 25 percent of that energy in batteries now available, the director of EPA’s National Vehicle and Fuel Emissions Laboratory said.

It will be interesting to see how quickly this technology can be implemented by the government and the private sector if these tests produce positive results.