Hydraulic Hybrid Cars: No Batteries Required

April 28, 2008

What if there was a hybrid vehicle technology that could at least double the gas mileage of passenger cars, SUVs and light trucks? What if it could slash emissions by 50 percent or more? And what if it could challenge our usual mileage expectations by offering better fuel consumption in the city than on the highway?

Even better from an engineering and manufacturing standpoint, what if this hybrid technology didn’t rely on expensive, heavy, bulky battery technologies that aren’t really ready for prime time and instead used a time-tested energy storage method?

Well, there is just such a hybrid-vehicle technology, and it’s one based entirely on hydraulic components rather than electric ones.

In many ways, these hydraulic hybrids conceptually resemble their electric hybrid cousins. Only in this case, energy storage takes place not in a battery but in high-pressure hydraulic accumulators usually charged in excess of 3,000 psi. The best of these accumulators have power densities of roughly 500 kW/kg, according to Jim O’Brien, founder and chief technology officer for Hybra Drive Systems , a start-up focusing on the development of hydraulic power trains.

Design concepts for hydraulic hybrids vary, but typically the car’s diesel or gas engine powers a hydraulic pump motor, which charges that high-pressure accumulator. The accumulator, in turn, drives one or more additional pump motors connected to the wheels. A second lower pressure accumulator typically completes the hydraulic circuit. Depending on the design, there may be one pump motor to drive a pair of wheels through a differential or one pump motor per wheel for an all-wheel-drive version with independent torque control. During braking, the pump motors on the wheels reverse themselves, re-charging the accumulator and capturing energy that would otherwise be lost to heat.

The hydraulic hybrids now under development can communicate with modern engines and do have some electronic controls. Yet in their purest form, they don’t really need any electronics to function. Hybra Drive, for example, has shoehorned a prototype hydraulic power train into a 1968 Volkswagen Beetle. “The only electric thing on that car is the spark plug,” says O’Brien.

Hydraulic hybrid vehicles aren’t exactly news to everyone. Operators of truck fleets have recently taken an interest. FedEx, UPS and Waste Management have been evaluating hydraulic hybrids developed by fluid power players such as Parker Hannifin and Eaton Corp . The U.S. Environmental Protection Agency has also been involved with the design and testing of these vehicles and even showed a hydraulically driven UPS truck at the International Fluid Power Exposition , which devoted two different technical sessions to hydraulic hybrids recently in Las Vegas.

What is new about these vehicles is that a growing chorus of academic researchers, start-up companies and fluid power suppliers are starting to evaluate the suitability of hydraulic hybrid technology for smaller vehicles such as passenger cars and SUVs. None of them claim hydraulic hybrids are going to kill the Prius and its ilk anytime soon. “Electric hybrids have a tremendous amount of momentum right now,” says Simon Baseley, director of engineering strategy and program management for Bosch Rexroth

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