The Italian Junkyard

Thoughts, ideas, criticism about cars. Interesting news and facts from the world of the automobile. Events in Italy and Modena. What you can find elsewhere, filtered through the eyes of a discerning enthusiast. Design, style, everything on the chopping block. Nobody is safe anymore.

Friday, February 5, 2010

Inside Line put a BMW ActiveHybrid X6 on the dyno, IL and Autoblog wonder on figures lower than declared. The Italian Junkyard tries to explain why.

Lately I'm just writing the comments I'm about to post on and other sites here, rather than on the original page. Whatever.

First of all, the image, courtesy of BMW, seems to be wrong, as it shows both the two electric motors inside of the transmission as on the ActiveHybrid X6 and the electric motor in place of the flywheel from the BMW ActiveHybrid 7 as well.

Point in case, InsideLine dynoed a BMW ActiveHybrid X6, and got 368 bhp as a result.
While the engine should be worth about 480 bhp (WRONG), with the given losses, you'd expect the car to give similar figures to other 480 bhp cars.
So, since the Nissan GT-R and the Porsche 997 Turbo both have 480 bhp, and both gave about 405 bhp on the very same dyno, they were expecting about 30-40 additonal bhp from the BMW.

Well, no.
Make the jump to know more about it.

Why did I say that the engine doesn't develop 480 bhp, as stated both on autoblog and on IL?
Because that's actually the maximum system power, which is quite different from what the engine can express. Basically that "system" is made of the internal combustion engine, and the hybrid transmission from the Glocal Hybrid Cooperation between General Motors, Daimler and BMW.
The hybrid transmission itself contain two electric motors.
Having studied hybrids for a while for the thesis, I have a couple of things to say.

The particular system adopted by the X6 deploys its max power at lower speeds or heavy loads, while at higher speeds the system acts more like a conventional car with just a bit of additional power from the electric motors when required.
Regardless of how that it technically works, it's the electronics to decide what is going to happen, who is going to propel the car or in which percentage both the engine and the electric motors are going to work together.
So in the end, when you put an ActiveHybrid X6 on the dyno, it's the ECU to decide what's going to happen, regardless of your foot being riveted to the floor.
The system of the BMW X6 is tuned so to deliver the best performance, whit a decent mileage. It isn't made to be green, not mainly, but it still takes into account the efficiency, so when you're on the dyno and you press the throttle, the system elaborate what you're doing and come up with the conclusion "you're asking" a certain kind of performance. At this point it starts calculating which mode could be the most efficient to achieve what you asked, given the system got it right or even accepted your order (first thing that comes to mind is the brake pedal being able to switch off the accelerator pedal...TOYOTA!!!).

So, those 480 bhp are just the best you can get, but no one said under which conditions you're going to get it. Most of the time we use more power than what we need and in the wrong way, and that's precisely here that an hybrid car can make the difference, especially when equipped with CVT or automatic gearboxes. Just look at the mileage for the upcoming Honda CR-Z if equipped with the manual transmission, much lower than with the CVT.

The effect is even larger when looking at the torque, as electric motors can provide plenty of it, but it's something you don't decide, the ECUs do.

So in the end, to answer both Inside Line and, the BMW ActiveHybrid X6 gave back lower numbers than expected because you weren't able to ask to the car its max system power. That doesn't exclude that BMW declared more power (or even less) than what ti actually is, but it explain the discrepancy.

Heck, at least I can say this degree is useful in some way...

Photo Copyright: BMW Group


Anonymous said...

Seeing as it would be impossible for the BMW to output that quoted number even after drivetrain loss, it should have never been written in the first place.

Damiano Garro said...

It wouldn't be impossible to output that figure, just that number isn't "simply" function of how much (and how) you press the throttle.
The system receive your input and data about the situation (speed, acceleration, cornering, temperature, info about the previous combustion cycles, etc etc) and it interprets the whole situation, applying an output that according to its calculations is the most efficient or performing.

It's like on race bikes, when the ECU directly cuts the power on lower gears or at the start, knowing the best acceleration time wouldn't be possible with all the power because of spinning or difficult in handling the power.

If you ask me, I think this whole 2-Mode hybrid powertrain is a bit pointless.
It doesn't save much fuel compared to other systems, and its so expensive people would just buy a standard and better handling (lighter) petrol version.
In this case probably the X6 M (which is even more pointless if you ask me).

Eventually, cars are not made to be tested on a dyno.

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