New hybrids proclaim their German engineering

I'm a big fan of "German engineering"--that combined focus on power and precision that distinguishes the better automobiles designed or manufactured in Germany.

At Frankfurt's International Motor Show, BMW will be showing off two new hybrid cars intended to deliver the full promise of German engineering. This is no small thing because most hybrid cars to date have been lightly built and somewhat underpowered in order to improve fuel economy.

The two new BMWs are more like previous offerings from that company: big, solid cars with lots of power to maintain performance in spite of the weight. They're also real cars, not just prototypes.

Fortunately, I don't need to describe these new cars here; there's a great article by Antuan Goodwin over on CNET's Car Tech blog that does a fine job of that (see "BMW unveiling two big hybrid models at Frankfurt"). What I would like to do instead is to drill down into their respective powertrains, which represent two different solutions to high-performance hybrid design, using images provided by BMW.

The ActiveHybrid X6, due to go on sale in the U.S. later this year, represents one end of the spectrum: higher-power electric motors and a larger battery pack. As the first image shows, the new X6 model has a twin-turbo V8 gas engine with 400 horsepower. Though this is a reasonably efficient engine for its size, it certainly wasn't chosen primarily for its fuel economy.

BMW's ActiveHybrid X6 uses a large NiMH battery pack and a new transmission with two integrated electric motors to augment its 400-horsepower gasoline engine.

(Credit: BMW)

The large NiMH (nickel metal hydride) battery pack is also visible in this view, mounted above the rear drivetrain components and below the floor of the luggage compartment. This battery pack holds 2.4 kWh (kilowatt-hours) of energy; BMW also specifies a "nominal" 1.4 kWh figure, but doesn't explain the difference between total capacity and nominal capacity. I suspect the difference may be related to improving battery lifespan, or perhaps provide some extra storage to ensure that electric braking assist (regenerative braking) is always available. BMW does say that this braking mode can generate as much as 50 kilowatts of power and 0.3 g of deceleration force.

In the following view of the X6's transmission, the electric motors are shown built right into a new transmission that BMW describes as an "electric continuously variable transmission"--the electric motors and three planetary gearsets work together to provide the equivalent of seven gear ratios.

The transmission in the ActiveHybrid X6 has two integrated electric motors totalling 177 horsepower.

(Credit: BMW)

Interestingly, with ratings of 91 and 86 horsepower respectively, the two electric motors could provide a total of 177 horsepower, but the vehicle isn't designed to operate that way. BMW specifies a maximum system output for the ActiveHybrid X6 of 480 horsepower, reflecting a maximum contribution from the electric motors of around 80 horsepower.

Part of the issue here is that the battery pack is rated at a maximum output of 57 kW, roughly 76 horsepower, so there isn't enough electrical power to drive both motors at full power. I expect there are also some issues related to heat and torque, but BMW hasn't offered a full explanation of this limitation.

It's also interesting to note that while the new X6 supports a pure electric drive operating mode, it's only good up to 37 mph and 1.6 miles. This figure is well below the energy capacity of the battery pack, probably reflecting more limitations imposed in the name of long-term reliability.

The ActiveHybrid 7, a hybrid version of BMW's 7-series luxury sedan due out in the spring of 2010, takes a very different approach. It has only a small electric motor positioned between the engine and transmission, and it comes with only a small lithium-ion battery pack. The following figure shows that the battery pack is located to the side of the trunk, leaving plenty of space for golf bags. (BMW says that four standard bags will still fit, which I suppose is a critical specification for 7-series customers.)

The ActiveHybrid 7 is powered by a 440-horsepower gasoline engine and a small 20-horsepower electric motor with a lithium-ion battery pack.

(Credit: BMW)

Zooming into the transmission in the following picture, we can see the pancake-style electric motor in front of what appears to be a fairly conventional 8-speed automatic transmission. In its press release, BMW does emphasize that this is a new transmission design "specifically tailored to the demands of hybrid technology," but the release doesn't explain how this gearbox differs from the 8-speed automatics on previous BMW cars.

The ActiveHybrid 7's electric motor is tucked away between the gas engine and a conventional 8-speed automatic transmission.

(Credit: BMW)

The new 7's electric motor produces a mere 15 kilowatts, roughly 20 horsepower. Together with the gas engine, the vehicle's maximum output is rated at 455 horsepower. The electric motor also functions as a starter motor for the gas engine and a generator to charge the two batteries on the car: a conventional 12V lead-acid battery and the 120V lithium-ion pack in the trunk.

The latter is a small pack storing only 400 watt-hours of energy--that's about like eight average laptop batteries. BMW doesn't mention whether the new 7 can run solely on the electric motor, but I doubt it; 20 horsepower probably isn't enough for that. Certainly the range would be negligible given the low battery capacity.

Instead, BMW describes the value of the electric motor in terms of two uses: first, it's powerful enough to restart the gas engine almost instantly (in less than one rotation of the crankshaft) so the engine can be shut down at stoplights, just as the Toyota Prius does. Second, the electric motor provides supplemental power while the gas engine is running, thus improving overall fuel efficiency. Since the battery can be charged by recovering braking energy, the power from the electric motor is often free.

BMW says the hybrid systems in the ActiveHybrid 7 were developed jointly with Daimler, maker of Mercedes automobiles. This partnership also led to some Mercedes models, and indeed, Mercedes has also introduced hybrids with powertrains similar to that of the ActiveHybrid X6. (Mercedes is announcing a new S500 hybrid in Frankfurt but didn't provide such nice pictures, so I didn't include it in this post.)

Both of these approaches will need to be developed substantially before they can reduce the total cost of owning and operating a motor vehicle. But it's clear that BMW, having waited this long to get into the hybrid car business, is giving its customers two very different choices. What the company does in the future will probably depend on how its customers respond.

[bimmin]

How did this article not mention the most exciting car BMW is showing at the Frankfort autoshow?? 62.6 mpg and a 0-60 in less than 5 seconds!

http://www.e90post.com/forums/showthread.php?t=296845

[Peter N. Glaskowsky]

Yes, that was pretty interesting. It's a concept car, however, and BMW didn't release pictures of the innards, so I didn't think it was really part of the same story.

Thanks for the comment!

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[hexjones]

The Germans are the only people capable of such engineering. (yes, this is sarcasm)

[xdre]

LOL

Well to be fair, GM does have a number of German engineers who could have worked on it! Although it does sound more impressive when you call it "ActiveHybrid" instead of "Two-Mode Hybrid".

http://www.automobilemag.com/features/awards/0802_gm_two_mode_hybrid_technology_of_the_year/index.html

[Peter N. Glaskowsky]

xdre,

Thanks for finding that. I mentioned the cooperation among BMW, DaimlerChrysler, and GM in a blog post here back in 2007, along with that "two-mode hybrid" label, but BMW didn't mention GM's participation in the work even though it did mention Daimler's, and I didn't want to just assume this was the same effort.

http://news.cnet.com/8301-13512_3-9755100-23.html

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[xdre]

Peter,

Very cool that you replied! I'd hate to speculate as to why BMW didn't mention GM, but the Global Hybrid Whatsit collaboration has (very recently) been disbanded. BMW and Daimler are still working on hybrids together, so perhaps that's a possible explanation?

[Peter N. Glaskowsky]

xdre,

Yes, it's probably something like that. I haven't been able to find any press releases or news stories addressing that issue in particular, though. If we're lucky, maybe someone will find this article and provide an explanation in a comment.

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[relativityboy]

Some cut and paste at the bottom of the article went awry.

[Peter N. Glaskowsky]

Sorry, could you explain in more detail? I don't see the problem.

[SactoGuy018]

Actually, of more interest is Mercedes-Benz showing off their BLUefficiency turbodiesel models, Unlike their past smaller-displacement turbodiesel efforts, the new BLUefficiency models are designed to meet even US emission regulations, so don't be surprised we see the C250 CDI BLUefficiency and MLK250 CDI BLUefficiency models in the US market by spring 2010.

[carlhage]

Several of the questions above are explained by comparison with the Toyota system (Prius, etc.) which also uses 2 motors and a planetary gear in lieu of a transmission with shifted gears. In the Prius the electric main motor is 40kW with a 20kW battery. When the engine is on full power but the car is at low speed, the other motor is a generator feeding the extra energy to the main electric motor. From the BMW diagram, the engine is 300kW, total 357kW, so battery is 57kW by deduction. The extra motor ratings are due to the power shift from the generator-motor in lieu of gear shift.

Most batteries have a rated max kWh, but are used only at 25-75% to keep long life-- hence the 1.4kWh nominal vs 2.4 rated.

But do you really need 480hp?

[Peter N. Glaskowsky]

Thanks for the explanation.

I've had a 286-hp car and a 333-hp car, and neither one was fast enough. (Especially on autobahns and race tracks. :-) Reviewers seem to think the 1,001 hp of the Bugatti Veyron is enough, but of course they don't get to drive the car long enough to become habituated. I'd love the chance to find out for myself...

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