NOW MAKE FUEL EXPENSIVE ENOUGH FOR THE TRANSITION TO BE WORTHWHILE:

High-Pressure Hybrids: Fuel-Efficient Hydraulic Vehicles Come of Age: Once relegated to the sidelines due to bulky components and complicated controls, these systems are benefiting from lightweight materials and ecofriendly fluid formulas (Christopher Johnston, 8/11/10, Scientific American)

A hydraulic system replaces all of the electronic components of an gas-electric hybrid (starter motor, generators, controllers, batteries, etcetera) with three parts: a small diesel motor–powered pump, a hydraulic motor and an accumulator. (Hydraulic-powered vehicles also contain an electric battery to power secondary equipment such as lights or a radio.)

The diesel pump pushes hydraulic fluid into the accumulator to reach pressures of up to 385 kilograms per square centimeter. By pressing down on the accelerator, the driver releases that pressurized fluid from the accumulator to then drive the hydraulic motor.

The best analogy might be a person operating a handheld air pump to pressurize a balloon, says Joe Kovach, group vice president of technology and innovation for Cleveland-based Parker Hannifin Corp.'s Hydraulics Group. In this comparison the person equals the pump, which drives air (technically a fluid) into the balloon, and the balloon represents the accumulator. The energy that can be harnessed comes from the force of the air released from the balloon.

"We think it's quite possibly the most cost-effective technology to achieve high fuel efficiency with low carbon emissions," says John Kargul, director of technology transfer with the U.S. Environmental Protection Agency's (EPA) Office of Transportation and Air Quality in Ann Arbor, Mich. Since the early 1990s Kargul and his research team have been testing hybrid hydraulic systems as a core drive-train technology for roughly 20 different sizes of vehicles—from 1,500-kilogram passenger cars to 18,000-kilogram commercial trucks. They also partner with automotive manufacturers including Ford Motor Co., Parker Hannifin and Eaton Corp. as well as delivery companies UPS and FedEx to design, build and field-test prototype vehicles.