'Green' Porsche

Porsches are known for the sound of their rippling horsepower under the bonnet, but the company's new petrol-saving hybrid actually has the motor off or running on batteries more than half the time - leaving the driver with little more than the sound of the wheels on the road. Using a system developed with Volkswagen and Audi engineers, the prototype Cayenne S Hybrid relies on a combination of petrol, electric power and coasting to boost mileage from 14 mpg in the city to 24 mpg while reducing C02 emissions. Michael Leiters, project manager of the Cayenne product line in Stuttgart, called it a "special driving experience" after a recent test drive. Klaus-Gerhard Wolpert, director of the Cayenne Product Line for Porsche, says research anticipates overall hybrid sales in the US more than doubling by 2014 to 743,000 vehicles per year. The system’s unique feature is a hydraulic clutch between the engine and motor that disengages the engine so it can shut down under light loads. The electric motor then takes up the load until the engine restarts. Porsche engineers call the result “sailing”—for the quiet sensation of speed using only electric power. As in most hybrids, the electric motor also restarts the engine, and recharges the battery pack (believed to contain 1.2 kilowatt-hours of energy). Fitted into the former spare-tire well, the pack uses Sanyo nickel-metal-hydride cells (as do the Ford Escape Hybrid and Ford Fusion Hybrid). This is a simpler and less costly system than Toyota’s Hybrid Synergy Drive, but it comes with drawbacks, one being that the vehicle can’t use the engine to recharge the batteries while running on electric power at the same time. Porsche’s hybrid system is tuned to give lots of electric assist at highway speeds. This makes sense, since the large, heavy, tall SUV isn’t an ideal urban car, and may spend less time in city traffic than other hybrids. Porsche says the Cayenne S Hybrid runs up to 1.2 miles in all-electric mode. More remarkably, the company also says it can run on electricity at speeds up to 86 miles per hour—though that’s slightly deceptive, since it means the engine may shut down for a while at speeds that high, leaving the electric motor to propel the car. Still, traveling on battery power at speeds higher than any legal US limit is quite an accomplishment for a 5,000-pound SUV. As for on-road performance, the company claims 0 to 62 mph in 6.8 seconds with both the electric motor and the boosted engine providing power. On the European test cycle, the company recorded 26 miles per gallon, though US mileage may be slightly lower. But even the projected highway figure of 24 miles per gallon would be a marked improvement over the 2008 Cayenne S figures of 13 city and 19 highway mpg (with a much larger 4.8-liter V8).