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Hydraulic hybrids on the horizon

President Bush recently spoke about an initiative to develop better gasoline-electric hybrid vehicles. Prior to that, Consumer Reports magazine reported that these hybrids can pose a dilemma for car buyers. On one hand, hybrids do use less gasoline, and they produce fewer emissions. On the other, hybrids cost several thousand dollars more than their all-gasoline counterparts. According to Consumer Reports, "In our analyses, none of the six hybrids we have tested recovered its price premium in the first five years and 75,000 miles of ownership. Nor did any when the analysis was extended to ten years and 150,000 miles."

It sounds as if these gas-electric hybrids have a long way to go. So rather than trying to develop a better gas-electric hybrid, perhaps attention should be shifted to a different technology altogether — say a gas-hydraulic hybrid.

In the last year or so, we've reported periodically on the progress of a couple new designs of hydraulic motors and pumps that may hold promise for more efficient hydraulic drives — especially hybrid drives that use accumulators to store and recover braking energy.

One of these unconventional drives is called the Hydristor (, which operates much like a variable-displacement vane pump or motor. However, several design variations in the Hydristor all but eliminate sliding friction that plagues conventional vane pumps and motors, according to inventor Tom Kasmer.

The other unconventional design is the Sanderson Mechanism, which uses a trio of pistons nestled in a cylinder block to drive (or be driven by) a rotating shaft. According to Robert Sanderson, inventor, the success of the design is attributed to a type of universal joint that eliminates side loading when converting the reciprocating motion of pistons to shaft rotation, or vice versa.

Variable displacement is a necessity for building a continuously variable transmission (CVT) — a key element of hybrid drives. Of course, CVTs using hydraulics have been around for decades. We know them as hydrostatic transmissions (HSTs), which are widely used in mobile equipment. Skid-steer loaders probably are the most common and visible example of the impressive performance and versatility of HSTs.

More recently, we have reported on three projects underway to develop hydraulic hybrid transmissions. Our first report, in the October 2005 issue, describes a joint venture between the EPA, Eaton Hydraulics, UPS, and other organizations to develop and test a hybrid HST for a package delivery vehicle. Just last month, we reported on a joint program between the EPA and the University of Toledo to develop another version of a hybrid HST. Still another hybrid HST program was just announced by Parker Hannifin. In all three cases, the vehicles will rely on hydraulic braking to store deceleration energy as pressurized fluid in accumulators. On command, this energy can be used instantly to accelerate the vehicle.

So whether it's with a tried-and-true HST, or with one of the new unconventional designs, hydraulics may be in the driver's seat in our quest to conserve energy.