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      02-27-2013, 05:07 PM   #74
jdong
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Drives: 2013 BMW ActiveHybrid 3
Join Date: Dec 2012
Location: NorCal

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Quote:
Originally Posted by BlackJetE90 View Post
One problem is when people start quoting cars like the Prius when talking about the i8. Like in this video, it is already being compared to the 911 or M3. The i8 is being sold as a sports car. People drive their 911's and M3's hard, they are not eco-driving them like a Prius.

Now your argument makes 100% sense when talking about the i3.
Dude, please read what I wrote again... I was talking about the Prius and AH3/Volt's power management strategy, not about their sporty performance. The basic power management strategy in a hybrid is the same whether it's for a Prius or for a high-end hybrid sports car... It's about maintaining enough reserve power for boost when the user needs it. I used to work on hybrid electric power architecture for high-end high-power applications, so I do know what I'm talking about. Also, note that the turbo-3 in the i8 is good for 220HP, which is well over double that of the Prius, not to mention you get the turbocharged flat torque profile. The i8 is not even remotely similar in performance to a Prius. Even if you take out the electric boost.

I didn't say you'd get 100% of the rated power 100% of the time. In fact I said the exact opposite. I said the car can only continuously give you peak output for a limited amount of time, but in the real world it will aggressively recharge the pack when you're asking for less power, so it's ready for the next time you floor it.


I don't know how you drive your 911 or M3, even an "aggressive" driver is going to have a hard time depleting the battery on an i8. You're going to hit some limit before long -- whether it's the felony speeding limit, or the maximum safe speed for the track you're on (e.g. enter a corner? the car's gonna charge while you let off on the gas), or the governed speed limiter that prevents the drivetrain/tires from flying apart.

Let's do some hypothetical calculations. The i8 is powered by a 96kW(129HP) electric motor and a 220HP (164kW) turbo-3. So, even in full gasoline mode, you have a 220HP, 3300lb car. That's not all that wimpy. If you are flooring it fully, a 7.2kWhr battery pack can provide 96kW for 4 minutes 30 seconds. The car is going to reach its governed top speed well within a minute, so you can't even apply full electric power for all that long. It's unclear from public information what the generator is in the i8. Assuming it's a parallel hybrid using the traction motor as the generator, it will be able to recharge the battery fully in 4 minutes 30 seconds as well.

Of course the above numbers are theoretical back of the envelope calculations, but the real figures are in the same ballpark. The electric machine only provides 1/3 of the total system power, and its role in instantaneous boost only requires a few seconds of electric charging (via load shifting) and its role in continuous assist can be sustained for much longer than one can be foreseen to need!
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