What's that car that just breezed past? It's a Hybrid Air — an experimental vehicle that French automaker PSA Peugeot Citroen has been trumpeting as an exemplar of energy efficiency. While some skeptics wonder whether the Hybrid Air is truly a breakthrough technology, the Peugeot and Citroen research cars powered by it were some of the more intriguing models on display at the Geneva auto show in March.
Peugeot says a compact model like a Citroen C3 equipped with the technology — which combines hydraulic drive with a conventional gasoline engine — will get about 81 mpg in city driving. That would be significantly better than existing gasoline-electric hybrids, such as the Toyota Prius, can achieve in stop-and-go traffic.
PSA Peugeot Citroen, the second-biggest carmaker in Europe after Volkswagen, says it plans to begin rolling out Hybrid Air cars by 2015 or 2016.
Like a Prius, the Hybrid Air system recovers energy each time the driver brakes or decelerates. But instead of capturing the kinetic energy of the slowing car with a generator that charges a battery, as in the Prius, the Hybrid Air system uses a reversible hydraulic pump.
The pump compresses nitrogen gas in what looks like an oversize scuba tank that also contains hydraulic fluid. The next time the driver presses the accelerator, the compressed gas pushes the hydraulic fluid, in the manner of a syringe, through a gearbox to turn the wheels.
The amount of energy stored in the nitrogen tank is small — equivalent to about 5 teaspoons of gasoline. While that is enough to power the car only a few hundred yards until the gasoline engine takes over again, when repeated over the course of a day of city driving, those extra teaspoons of energy add up to a big improvement in mileage, Peugeot says.
The idea of using so-called hybrid hydraulics to power a car has been around for years. Peugeot prefers to call its system "hybrid air" technology because the energy is stored by compressing nitrogen gas rather than pressurizing hydraulic fluid. In the U.S., Chrysler and Ford have each studied the approach, and the Environmental Protection Agency has encouraged the research.
UPS has added several dozen hybrid hydraulic delivery vans to its alternative-fuel fleet. Other companies are applying the technology to garbage trucks, which — like UPS vans — are big, make frequent stops and can benefit from recovering energy otherwise wasted in heat generated by the brakes.
"The logic of an electric hybrid is completely different," says Andres Yarce, one of Peugeot's project leaders. With an electric hybrid, "you let the vehicle run for a few kilometers, have the engine shut off, then run silently on an electric motor. It took time for people to grasp that the Hybrid Air works differently but gets the same results."
When the car is ready for the market, Peugeot plans to price it at about $26,000. Peugeot says it can undercut existing hybrids on price because its car does not require an expensive battery and electric motor, like a Prius, although the Hybrid Air does employ a standard car battery.
Karim Mokaddem, another project leader, acknowledges that bringing the Hybrid Air to market would require overcoming a "key challenge" in adapting the hydraulic parts, which were used mainly in applications like elevators, tractors and aircraft, and producing them in volume.
Market acceptance may present a bigger hurdle than the technology challenges. Ferdinand Dudenhoffer, director of the Center for Automotive Research at the University of Duisburg-Essen in Duisburg, Germany, says that the Peugeot idea "looked interesting" but that he was skeptical whether it could overcome the big head start that hybrid-electric vehicles have in the market.
Even if Hybrid Air proves to be a superior technology, Peugeot could have trouble getting traction unless it can show "a tremendous cost advantage," Dudenhoffer says.
"We already have one company that is successful in the car market with hybrids," he says. "It's called Toyota."