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Porsche Cayenne Hybrid, Cayenne Diesel, and a gratuitous Grateful Dead reference

September 1, 2009 | 11:43 am

ICayenne S Hybrid2f you hooked the engineers at Porsche’s Weissach research center up to a polygraph and lubricated them with a liter or two of weissbier, they’d probably tell you they are not all that enthusiastic about hybrid technology. The Cayenne is a good case in point. According to a briefing last week by Cayenne project engineer Michael Leiters, the Cayenne S Hybrid – which will come to the U.S. next year – will get about 33% better fuel economy than the V8-powered Cayenne S, or about 24 mpg.

But as I noted to Dr. Leiters, that figure will surely go down, perhaps by a lot, in cold weather, which hybrid batteries (nickel metal-hydride, in the case of the Cayenne S Hybrid) detest. The charming Dr. Leiters simply shrugged. The consensus around the Porsche table is that, even though hybrid technology may not be the optimum efficiency solution, in the U.S. it’s an imperative part of image building.

Cayenne Diesel profile2 Meanwhile, the diesel Cayenne – which accounted for a stunning 60% of Porsche’s volume in January-February – gets 28% better fuel economy than the V8, while offering boot-in-the-butt torque of 406 pound-feet, 0 to 60 mph acceleration of just over 8 seconds and a 133 mph top speed. It’s sorted, ready, and far less vulnerable to cold-weather declines in performance. I’m actually driving a Cayenne Diesel this week (look for the review soon) and it’s a locked-down cinch of a car. Fantastisch. Porsche would love … love… to sell the Cayenne diesel in the States but, according to Leiters, “We need more market demand for this car.”

Cayenne Hybrid Chassis DiagramThat leaves us with the Cayenne S Hybrid, which I tested for a few miles last week in neighborhoods around Bel-Air. Going its own way on hybrid architecture – no surprise – Porsche has created a north-south version of a parallel hybrid system, sort of like Honda’s Integrated Motor Assist (IMA). The engine is an Audi-sourced 3.0-liter DFI V6 (333 hp). The engine’s flywheel connects to a disengagement clutch, which is associated with a 38kW (52 hp) toroid permanent-magnet electric motor/generator. Downstream of the hybrid unit is the lockup torque converter and eight-speed automatic transmission. The nickel metal-hydride battery pack under the load floor in back is rated at 288 Volts and 38kW max discharge. The all-electric speed threshold is 32 mph.

Put it all together and the hard numbers are these: 374 hp (gas/electric combined); 406 pound-feet of torque; 0 to 60 mph in a plenty-freaking-quick 5.5 seconds; and a top speed of 149 mph. Leiters noted that the new Panamera super sedan was pre-engineered to accommodate the hybrid system, so it would be “relatively easy” to offer a Panamera Hybrid. Consider it a fait accompli.

All the engine accessories – power steering, air conditioning, brakes – are electrically powered so the Cayenne S Hybrid has uncompromised stop/start functionality. The Panamera’ stop/start system actually shuts down the air conditioning if cabin conditions permit.

In my brief test drive the Cayenne S Hybrid felt unremarkable except for its unusual freewheeling behavior. Because of the architecture, the system is able to decouple the engine completely and do what Porsche calls “sailing,” which is to say, freewheel without any engine compression or electric motor drag. This coasting behavior is possible up to speeds of 86 mph.

And that’s fine. However, given that the Cayenne S Hybrid weighs around 5,500 pounds, when this thing starts going downhill, holy hell, look out! It’s like Casey Jones and the wreck of the old 97.

Porsche may need to dial that feature back a bit before production.

 -- Dan Neil