Hybrid electric passenger cars may be hot, but a team of researchers at the University of Toledo is working to fine-tune a hydraulic-based hybrid system for refuse trucks, delivery trucks, and other heavy vehicles that make stop and go frequently.

Research into hybrid hydraulic propulsion systems is part of a $1 million partnership between the university and the U.S. Environmental Protection Agency. Walter Olson, PhD, a mechanical engineering professor leading the research, said such a system could double a vehicle's fuel efficiency while reducing emissions.

Olson, along with Mohammed Elahinia, an assistant mechanical engineering professor, and Thomas Stuart, an electrical engineering professor, will receive up to about $450,000 from the EPA for their work, said Nagi Naganathan, dean of the school's engineering college. The university will cover the rest of the program's $1 million budget.

The hybrid hydraulic propulsion system stores energy generated by braking by using it to compress gas in an accumulator instead of releasing it as heat. Olson said the system has the most benefit for heavy vehicles in which the hybrid electric systems found in cars are impractical.

The batteries used in hybrid electric engines cannot be charged or discharged quickly enough to take advantage of the energy available, Olson explained. "A battery is destroyed if you charge or discharge it too fast," he said.

In a hybrid hydraulic system, energy stored from braking would power a pump that forces fluid from one accumulator to another. When a driver depresses an accelerator pedal, fluid would reverse direction, powering a motor that supplements the power of the vehicle's engine.

Olson's research indicates that a hybrid hydraulic system would increase a typical four-wheel-drive sport utility vehicle's fuel economy from 17.2 mpg to 34.6 mpg. However, a vehicle's initial cost would increase anywhere from $500 to $3500. Olson estimates a vehicle could save $5000 in fuel costs over a vehicle's lifetime — even more for vehicles that make frequent stops.

Researchers also hope to reduce or eliminate the whine typically emitted by hydraulic system makes, Olson added. Other aims are increasing the amount of energy that can be stored and improving controls.