re: How is the alternator used by a BMW?

[Luis Marques <marques@xxxxxx>]

Brian M Kennedy <kennedy@domain.elided> wrote:
>
> In a number of recent posts, the alternator operation has been
> mentioned, discussed, or questioned....

 <<< long description of alternator behavior >>>

You are mostly correct in your understanding of the operation of the
alternator.  It's not an on-off switch.  Yes, it gets harder to spin as
the load (current draw) increases.  Yes, the battery starts to supply
current when the alternator can't keep up with the load, and yes, the
battery keeps sucking power once the load is reduced as it recharges. 
The only thing I would add is that the alternator/regulator has an
internal resistance, so as the load increases, the output voltage will
decrease slowly until it reaches its limits, then the voltage begins to
drop faster and the battery starts to take over the duties of supplying
additional electrical power.  Just because an alternator is rated at 90
amps doesn't mean that it can't supply more than 90 amps.  It just gets
hotter and more inefficient until the battery starts supplying
additional current.  Depending on how they rate alternators this could
be anywhere form 80 to 110 amperes.

About the A/C cutoff at WOT, I have noticed that on the hot 110 deg.
summer we had here in Dallas, whenever I started from a stand still at a
red light and used a little more throttle than mid-throttle ;) (although
not going much over 4k rpm) the A/C on my 328i got warm for a few
seconds, which leads me to believe that at least at lower speeds there
is some sort of A/C compressor cutoff mechanism w/timer for WOT on this
particular model.  I don't know if it still cuts off at higher speeds. 


- --- those not interested in E*ram talk can skip the rest of the post ---


About the infamous e*ram, unless you have your A/C at max, your 500W
high beams on, your rear window defroster on and your megawatt stereo
blasting at 130 dB spl, etc, etc, if it only draws 30 A of current, then
the alternator will be powering it since it has enough capacity.  30A at
14 volts is roughly 420 watts, which is about 1/2 horsepower.  So
depending on the efficiency of the alternator, the power lost driving
the alternator while the e*ram is running is about 1 hp (assuming about
50% alternator efficiency).

If the E*ram is capable of a 5hp increase on a 200hp engine (that's a
2.5% increase) then it means that the manifold pressure is being
increased by roughly the same percentage, or ~0.35 psi, plus 0.05 psi
more to account for the increased alternator drag.  So for the E*ram to
work it must be capable of boosting the intake pressure by at least 0.1
psi (for a net gain of 0) to about 0.40 psi (for a 5 hp gain in a 200 hp
engine).  Just food for thought (and assuming it only draws 30A of
current).  I'll let somebody else decide whether it really is capable of
these kind of boost pressures or if a hampster-powered unit would work
better.

Luis Marques
'97 328i

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