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Old 01-14-2008, 12:53 AM   #11 (permalink)
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Think in terms of % of size
the alternator on any vehicle requires ~ 1hp to operate, even when electrically idle
300hp - 1hp = 0.33%
49hp - 1hp = 2.00%

Bigger difference in the smaller engine.

I'll shut-up now. S.


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Old 01-14-2008, 01:07 AM   #12 (permalink)
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I don't think peak hp is where it matters.

It takes approximately 6 hp to drive my car @ 40 mph on a level road. Regardless of whether that 6 hp is coming from a 3-pot capable of 49 max. hp or a V8 capable of 300, it still only needs to burn enough fuel to make 6 hp. So if you remove the hypothetical 1 hp burden of the charging system, you've just reduced power requirements by 1/6th for both motors.

The fuel savings of doing that won't scale exactly of course, due to the extra power required to overcome internal friction in the V8, and their different BSFC profiles & how they respond to changing load. But the effect definitely won't be proportional to the peak power of each mill.
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Old 01-14-2008, 10:45 AM   #13 (permalink)
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Originally Posted by MetroMPG View Post
There are significant caveats, however.
Ah, the truth in advertising...

Nice write-up, BTW.
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Old 01-14-2008, 11:40 AM   #14 (permalink)
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From a (now old) press rel: http://www.bmwreport.com/bmwmininews...w-5-series.asp

Brake-energy regeneration is new to the 5 Series, which recycles previously lost energy by using a smart alternator and special battery. Whereas a conventional alternator constantly draws energy from the engine, the smart alternator only does so when the battery needs it — as well, engine energy generated while braking or descending a hill goes to charge the battery. This set-up achieves an overall three-percent improvement in fuel economy.
The 1-series is getting this too.

Any sufficiently advanced technology is indistinguishable from magic. - Clarke's Third Law
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