kilometer magazine

celebrating european cars and motorcycles


km : Tech


3 November 2009

In a recent story about the 300SL Gullwing, we applauded Max Hoffman, the European car distribution and marketing pioneer, for his efforts in pushing manufacturers to produce stunning icons including the BMW 507, the Porsche 356 Speedster, and the aforementioned Merc. With him on his mind during this story, an exploration of the differences between BMW and Porsche hybrid technology, we couldn’t help but wonder what Hoffman might think of two enormously fast, enormously technological, yet also downright enormous crossovers from two of his favorite brands. It would certainly be one of those “Oh, brave new world” moments. But under the skin, are the superficially-alike Cayenne S Hybrid and the X6 ActiveHybrid really one idea with two forms?

Let’s start by looking inside the BMW two-mode system, which was co-developed with General Motors and Daimler. GM beat its partners in getting the technology out on the road in the Chevrolet Tahoe and Silverado, the GMC Yukon and Sierra, and the Cadillac Escalade, all of which show an impressive efficiency increase – especially in city driving – over their gas counterparts. Mercedes-Benz has chosen to introduce its version of the technology with a few upcoming S-class hybrid models, while BMW has picked the X6 to carry the hybrid torch. A 7-series hybrid has also been announced, but it uses a different, milder system and cannot run in full-electric mode.

With the odd intentions of building the world’s most powerful hybrid (they may also be at work on the world’s largest compact car, for all we know) BMW started the X6 hybrid project with its new twin-turbo V8 engine, contributing 407 hp. A two-mode hybrid transmission packs the punch of two electric motors, and while each produces about 90 hp, they aren’t used together and as such, maximum system output is 485 hp and a wonderful 575 lb-ft of torque. No, this isn’t a Prius-fighting fuel sipper, but it is a hybrid crossover that turns a 0-60 mph time of five and a half seconds.

Each of the X6’s two electric motors is tuned for a different job, though both are also capable of working as power generators when the vehicle is in motion but that engine isn’t in use. In addition to the motors, the anatomy of the two-mode transmission consists of three planetary gearsets and four multi-plate clutches. Though the design has much in common with a typical continuously variable transmission, the inclusion of gears combines with the electric motors to give the X6 seven individual gear ratios.

From a stop, the first electric motor does all the work of moving this big crossover by itself. It can continue working solo up to 37 mph, with a full charge of the 2.4-kWh nickel metal hydride battery sustaining emissions-free driving for as far as 1.6 miles, at a maximum output of 57 kW. With the X6 using run-flat tires, the hybrid’s engineers freed up space under the rear load floor to house the large battery pack. It has its own cooling system to maintain longevity and optimum performance, using a heat exchanger as well as a circuit connected to the cabin air conditioning system. The two can be used individually or together and a central control unit controls the flow automatically.

When the central control unit decides to flick the switch that brings the twin-turbo gas V8 to life, the second electric motor is activated and does the job of starting the engine. From there, it serves as a generator to provide a constant supply of electricity. That allows the first motor to aid acceleration, though the gas engine does much of the work in most conditions.

Those electric motors aren’t just the gas engine’s helpers, however. When it comes time to slow down, both engines go into generator mode to keep as much energy as possible in the system, as opposed to losing it through the heat and friction of braking. This function can provide up to 0.3 g of stopping force. In addition to generating electricity to aid acceleration later, this process also preserves the brake discs and pads as an added bonus.

BMW’s system can sometimes seem overwhelmingly complicated, but essentially the goal in designing this two-mode system was to bring a generator, electric motor, starter, and transmission into one easily interchangeable package. But of course, it sort of had to be that way with three different companies working on it for a whole range of different vehicles.

Porsche’s parallel hybrid system is much easier to visualize (and it’s also a bit more similar to BMW’s 7-series ActiveHybrid system.) Their engineers simply pulled the engine and transmission apart and slotted a clutch-controlled electric motor in between. But let’s start with those traditional elements on either side of the new goodies.

Like the Cayenne’s base-level V6, the Hybrid’s engine is borrowed from Audi, who was a development partner for the car itself and is also working on a hybrid version of its Q7, with which the Cayenne shares its platform. Looking for more power but no increase in fuel consumption, the Hybrid replaces the base V6 with the supercharged 3.0-liter currently offered in the new Audi S4 and the A6 3.0T. To earn the “S” designation that squeezes between “Cayenne” and “Hybrid” on the rear hatch, Porsche is using the higher-output, 333-hp, 324-lb-ft version from the S4. In addition to supercharging, the engine uses direct fuel injection.

Rearward of the electric motor, the Cayenne S Hybrid uses a new eight-speed transmission, a first for both the model and the brand. A new electric drive pump aids a conventional transmission oil pump in delivering smooth and quick gear shifts in both normal and electric drive modes. Top speed actually comes in sixth gear, while seventh and eighth exist only to enhance fuel economy at higher speeds.

Those extra gears are also vital to the Cayenne’s trump card. While the X6 is able to drive in all-electric mode up to 37 mph, Porsche has designed its system so that, when the vehicle is coasting, it’s able to run gas-free at speeds up to 86 mph. When accelerating, the Cayenne is good for a run up to around 30 mph on electricity. Under full tilt, it’ll sprint to 60 mph in 6.7 seconds. So no, it isn’t as quick as the BMW, but there is a trade-off. While BMW estimates the X6 is good for 9.9 liters of fuel consumption per 100 kilometers (23.8 mpg) in the European test cycle, Porsche predicts the Cayenne will undercut the 9.0-liter mark (26.1 mpg or better).

The electric motor slotted between the engine and transmission is a 52-hp, three-phase unit that also produces 221 lb-ft of torque. An automated clutch is responsible for connecting the electric and gas motors and can swap between gas and electric modes in just 300 milliseconds. Like the BMW’s central control unit that oversees the action, Porsche’s Hybrid Manager uses over 20,000 different parameters to determine the proper settings for the highest efficiency. When the electric motor isn’t at work, the PHM puts it to work as a power generator.

Again sharing something with the BMW, the Cayenne tucks that saved energy away in a NiMH battery hidden below the rear load floor. It has an output of 38 kW, weighs 154 pounds, and measures 13.7 x 24.9 x 11.5 inches. The use of that space obviously means that the Hybrid Cayenne goes without a spare tire.

Both the BMW and the Porsche also share a few elements that are simply essential to a hybrid operating on electric power. Ancillary components like the air conditioning pump and power steering pump are electrically driven to avoid a loss of pressure when the gas engine isn’t turning. And both, well, both should be quite expensive when they eventually go on sale next year.

Is it possible to say which approach is better? Not really, especially without defining just what “better” means. BMW set out to build the fastest hybrid in the world, and to that end, mission accomplished. But if the point of hybrids is to help save the baby seals and send those evil oil-hoarders to the poor house, we think maybe using a 4.4-liter twin-turbo V8 was maybe the wrong approach. It’s like saying “I’m going to plant a tree to help save the earth,” only you use a Komatsu D575A to dig the hole. BMW, you make fantastic inline-sixes. Don’t be afraid to use them. The Cayenne probably won’t win Green Car of the Year, either, but at least it is trying. However, marketing experts will surely defend BMW’s approach this way: A one-mpg gain from a 10-mpg vehicle is twice the percentage of a one-mpg gain from 20 mpg. From that approach, BMW’s biggest engine was the ideal candidate for the highest percentage gain from the addition of a hybrid powertrain.

There’s a bigger intention here though, we think. Hybrid powertrains are expensive to develop, and the technology is still very new. Regardless of the vehicle, the packaging, and the engine, neither Porsche nor BMW would likely be able to profit much from these cars. So rather than sell twice as many terribly slow but very efficient examples, make these hybrids image cars. Make them fast, so people develop a respect for hybrids. Put “S” badges and big wheels on to make them look aggressive. If there’s no short-term money to be made, these companies may as well deliver a few thrilling vehicles at a high premium rather than a bunch of dull ones for cheap. As the technology improves, there will be room for the mileage masters to fill the lower end of the price spectrum. We think that’s how this is supposed to work.

In the coming months, Kilometer will drive both of these vehicles and report more on how they drive, rather than just how they work. And maybe then we’ll see the light and fill these pages with praise. Until then, we’re skeptical but hopeful that there’s some true benefit for the masses coming from these projects. What would Max Hoffman think? Who knows, but we’re sure the wise businessman would at least applaud the brilliant marketing effort, if not the vehicles themselves.

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