As auto makers struggle to design more efficient batteries — key components in hybrid-electric vehicles — hydraulic hybrid technology is getting a second look.

Hydraulic systems are “showing efficiency numbers better than you get with batteries — a lot better,” says David Hermance, executive engineer-environmental engineering at Toyota Technical Center, U.S.A., Inc.

“If, in volume production, those prove out to be reasonable numbers, then maybe you start looking seriously at how would I package it?”

Packaging has been the bane of hydraulic hybrid technology because, to date, such systems require large cylinders to store the required energy. For this reason, heavy-duty vehicles have been the testing ground for hydraulic systems.

Recently, China Daily reported a fleet of 50 hydraulic hybrid buses were ready to hit the streets of Beijing. Last year, the Environmental Protection Agency agreed to work on a hydraulic hybrid UPS truck with Eaton Corp., which previously had outfitted a garbage truck with such a system.

“They work really best as garbage trucks because that duty cycle really, really, really has lots of high-power pulses and lots of high-power discharges during acceleration,” Hermance says. “And the hydraulic systems are really good at that. There are challenges with packaging hydraulic systems in a small vehicle. Will we see them in smaller vehicles? Maybe.”

As Toyota marches toward its 2010 goal of putting one million hybrids on the road annually, it is doing “a huge amount of work” on “a wide variety” of hybrids, Hermance says during an interview at the SAE World Congress.

Among them is a system that uses lithium-ion batteries instead of the currently popular nickel-metal hydride.