It’s a Hybrid Air — an experimental vehicle that the French automaker PSA Peugeot Citroën has been trumpeting as an exemplar of energy efficiency. While some skeptics wonder if the Hybrid Air is truly a breakthrough technology, the Peugeot and Citroën research cars powered by it may prove to be some of the more intriguing models on display this week at the Geneva motor show. The show opens on Tuesday for two days of press previews and runs through March 17.

Peugeot says a compact model like a Citroën C3 equipped with the technology, which combines hydraulic drive with a conventional gasoline engine, will get about 81 miles per gallon in city driving. That would be significantly better than existing gasoline-electric hybrids like the Toyota Prius can achieve in stop-and-go traffic.

PSA Peugeot Citroën, 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 five teaspoons of gasoline. While that is only enough to power the car 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 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 United States, Chrysler and Ford Motor have each studied the approach, and the Environmental Protection Agency has encouraged the research.

The United Parcel Service has added several dozen hybrid hydraulic delivery vans to its alternative-fuel fleet. Other companies are applying the technology to garbage trucks, which, like U.P.S. vans, are big, make frequent stops and can benefit from recovering energy otherwise wasted in heat generated by the brakes.

The Indian automaker Tata has promised to produce a car powered solely by compressed air, although that uses a different technology, developed by Motor Development International.

The 200-member Hybrid Air team, led by Karim Mokaddem, a Peugeot engineer, appears to be moving the fastest of any at a global automaker to bring this alternative hybrid technology to production.

“The logic of an electric hybrid is completely different,” Andrés Yarce, another of Peugeot’s project leaders, said at the company’s technical center in Carrières-sous-Poissy, near Paris. 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,” Mr. Yarce said. “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.

Mr. Mokaddem said that the pricing was meant to make the Hybrid Air a viable option in emerging markets like China and India, where most gasoline-electric hybrids were expensive and too complex for local service and repair operations.

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. The hydraulic system also adds about 220 pounds to the weight of a conventional Citroën or Peugeot. And because of the heat generated by the energy transfer process, the engineers have had to adapt the car’s cooling system.

The most obvious difference between the prototype Hybrid Air and an ordinary car is the presence of two pressure tanks and a special gearbox that manages the energy handoffs between the hydraulics and the 1.2-liter standard gasoline engine. The system’s designers say the setup left them room to keep a standard-size trunk and gasoline tank.

The accumulator, or nitrogen pressure tank, is about four feet long, with a volume of 20 liters, or about five gallons, and a maximum pressure of about 3,600 pounds per square inch. Any rupture in a pressurized steel tank could send metal flying, of course, but the design team says it has shielded against that by installing the tank under the floorboards and adding emergency release valves. They also note that nitrogen gas, which constitutes nearly 80 percent of the air we breathe, is not flammable.

Article source: http://www.nytimes.com/2013/03/03/automobiles/compressing-gas-for-a-cheaper-simpler-hybrid.html?partner=rss&emc=rss