porsche's Recipe for Green Chili
By Dan Edmunds, Director of Vehicle Testing
Date posted: 08-07-2007 EDMUNDS
Greenpeace's timing couldn't have been more laughable. We're gathered behind the security gates of porsche's Weissach technical center, part of a select group getting an in-depth look at running prototypes of the porsche Cayenne Hybrid.
Meanwhile, a few dozen kilometers away in Zuffenhausen, protesters are brandishing signs and screaming "Climate Pigs!" in the faces of bemused porsche executives. Apparently the pickets don't know that porsche sports cars are considerably more fuel-efficient than the cars they compete with. And they must not realize how close porsche engineers are to releasing hybrid technology into the market in not one, but two cars: the porsche Cayenne sport-utility and the porsche Panamera sedan.
For several years, porsche has been researching hybrid powertrains with VW and Audi, studying the technology with an eye toward improved fuel-efficiency and greenhouse gas reduction. In 2005, work began in earnest on a production-ready porsche Cayenne Hybrid.
Strong, Yet Simple
One of porsche's primary aims has been the creation of a so-called "full" or "strong" hybrid (the terminology is evolving with the technology), with enough capacity to propel the car electrically with the gasoline engine turned off. The porsche engineers have also sought to utilize as much of the original Cayenne as possible in order to preserve essential porsche driving characteristics, reduce complexity and keep development costs down.
To these ends, porsche started with the base-model Cayenne with its direct-injected, 290-horsepower 3.6-liter V6 and six-speed Tiptronic automatic transmission. The newly redesigned engine's narrow vee-angle of 10.6 degrees leaves sufficient room under the hood for porsche's sizable "Hybrid Manager" electronics module and power inverter.
The V6 engine sits in the standard location, but the transmission has been scooted back almost 5 inches. The resultant gap has been filled by a 51-hp (38 kW) three-phase electric motor-generator (MG), the output of which has been largely determined by the space available for the nickel-metal hydride (NI-MH) battery pack. A fully reinforced "crash box" containing 240 battery cells that put out a combined 288 volts DC is firmly mounted under the cargo floor where the spare tire usually sits.
The Critical Difference
This layout is conceptually similar to that of the 2007 Honda Civic Hybrid, which also sandwiches its MG between the internal combustion engine (ICE) and the transmission. The drawback to Honda's approach is that the electric motor and engine are always connected. Even when fuel to the engine is cut during electric-only operation, the crankshaft and pistons continue to spin, creating friction and sapping power.
porsche solves this by placing a computer-controlled "disengagement clutch" between the motor-generator and the V6 engine, allowing the ICE to be entirely disconnected and shut down during electric drive mode. When the engine is required to accelerate the car or recharge the battery, engagement shock is mitigated by careful control of the clutch and the automatic transmission's torque converter.
The porsche solution's only significant limitation is its inability to operate the electric motor and ICE at different rpm and blend them to maximize fuel economy. It also cannot propel the vehicle on electricity while the ICE charges batteries. To do these things, a Toyota-like system featuring a second MG and a continuously variable transmission is needed, and porsche rejected this approach for reasons of cost and complexity.
Riding Impressions
porsche didn't let us drive the Cayenne Hybrid at its Weissach proving ground, but we did go for a brief ride. Our driver told us that a fully charged battery can propel the Cayenne in electric mode up to about 30-35 mph. Indeed our ride stayed all-electric as we wound our way through the Weissach campus at low speed. When coasting, the ICE can remain off at speeds up to 75 mph.
Mashing the throttle results in a seamless leap of acceleration, and there's no perceptible delay as the gasoline engine fires up. Start/stop events at intersections and while coasting down hills are accomplished without a stutter, too. But since we weren't at the wheel ourselves, we suspect that the driver can sense momentary discontinuities in the operation of the powertrain and regenerative brakes that the passenger cannot. porsche didn't confirm or deny this, saying instead that more fine-tuning of the system is needed.
We were permitted to "drive" a Cayenne Hybrid on a dynamometer programmed to duplicate a small portion of the test loop used to determine European fuel-economy ratings. It was impossible to determine much in such a lab setting (don't touch the steering wheel!), but the benefits of staying in e-mode for as long as possible were easily seen. Driver 1 had a full charge and achieved 28 mpg, but Driver 2 started with only 50 percent in the electric "tank" and achieved only 20 mpg, as the engine was more often needed to propel the car and recharge batteries.
porsche engineers say that they have produced 24.4 mpg on the U.S. "FTP" city test (one of five tests used to determine EPA fuel-economy ratings), and this represents a 36 percent increase over the standard Cayenne V6. This increase suggests that the Cayenne Hybrid's city rating could rise from 14 mpg to 19 mpg. And porsche is still tweaking the system for better performance.
Secondary Systems
All hybrids must have regenerative braking capability, as this is the primary source of the electrical energy required to recharge the batteries. In the porsche hybrid system, the Cayenne's all-wheel-drive system is particularly advantageous, as the participation of all four wheels makes the recharge effort more fruitful and also provides an improved measure of dynamic stability during deceleration. The Cayenne's conventional braking system carries over intact, so performance in demanding real-world situations should be as good as ever. Making a Cayenne drivable when the engine is switched off naturally requires some careful modifications. The brakes retain their vacuum booster, so an electric vacuum pump is needed. porsche's standards for steering performance led hybrid engineers to reject electric-only steering assist, so another electric pump develops hydraulic assist for the Cayenne's standard ZF steering rack. The air-conditioning compressor and an oil circulation pump for the automatic transmission have been electrified as well. Low rolling resistance tires are being considered.
Coming Soon, but When?
Work continues on porsche's hybrid system, as the engineers seek improved durability and refinement. In its current state of development, the Cayenne Hybrid is packing about 250 pounds of additional weight. porsche is not ready to give us answers to our "When?" and "How much?" questions. It would only say that the Cayenne Hybrid will hit the market "before the end of the decade," adding that the "USA has the highest hybrid sales potential."
porsche continues working on a parallel path toward lithium-ion batteries for their hybrids, but admits we won't see results until after 2010. When lithium batteries are ready, a battery pack of the same size as the current one will provide 74 hp (55 kW) of electric propulsion — enabling another huge step forward in overall efficiency.
The porsche Cayenne Hybrid seems just about ready for prime time, but we have yet to see how much porsche-ness remains in the driving experience. Greenpeace's Climate Pig slur notwithstanding, porsche is dedicated to coming to market with the real thing.
Print | posted on Friday, August 17, 2007 6:35 PM