In a message dated 3/14/99 4:58:56 AM Eastern Standard Time,
randallyoung@earthlink.net writes:
> I agree entirely with your conclusions, but have a few minor points :
>
> > The purely mechanical losses would, I think, not be much more than the
> losses
> > from a simple idler pulley, and would be very small.
>
> Don't forget, the alternator has it's own fan which also takes power.
> Unfortunately, there's no practical way to turn it off and on.
Randall,
True, but I still think it would be rather small. I have no hard data, so i'm
only guessing -- it may be more than I think.
> > Right out of the starting
> > gate, your ignition system is seeing a 14% reduction in power.
>
> True with points-type ignitions. However, most electronic ignitions are
> "constant power" and will simply draw more current at lower voltages.
This may be true, but only to a point. Below a certain voltage, I would think
that even an electronic ignition would fail to provide a full spark. You're
right, though, that would buy you some time before the battery went.
> > If you were to be running at the 24hrs of Le Mans, you
> > would not last the duration unless you swapped your battery out for a
> fresh
> > one at each pit stop. For normal street use, it just doesn't seem like a
> good
> > idea.
>
> Well, the point is you use a switch that disables the alternator only at
> full power. Even at LeMans, you don't keep the throttle wide ALL the
> time <g>
I'm not a racer, but I thought except for blipping the throttle for double
clutching and downshifting, the gas pedal was operated as an on off switch -
you're either full on the throttle or full off, no in between (up to the
engine redline on the longer straights). Same for the brakes. You're either
stopping as hard as you can or going as fast as you can, but no in between!
> > Now, let's talk about electrical vs mechanical fans. First off, we have
to
> > make some assumptions:
> >
> > 1) The fan blades (and fan installation details -- shrouding, etc) used
on
> > both the mechanical and the electrical fans are identical. We have to
make
> > this assumption so we are comparing apples to apples, rather than apples
> to
> > doughnuts. If the fans are not identical, the differences in fan
> efficiencies
> > would confuse the issue. If this is not true, we could always swap blades
(
> and
> > redesign the installation) so that it is true.
>
> A simplifying assumption that is clearly NOT true. The mechanical fan
> has to operate over a wide range, and survive being bolted to the
> crankshaft. The electric fan can be optimized for a single speed and be
> much thinner, etc.
You're probably right, but I wonder if the difference between that assumption
and reality is that great. There are two things I miss about being retired -
the xerox machine and the ability to walk down the hall and ask an engineer
from another discipline questions such as this. It was nice to have
mechanical, civil, hydraulic, HVAC, and thermo engineers available for free
consultation.
> > 2) At very high speed (and corresponding high engine speed), the
> mechanical
> > fan may draw more power than an electrical fan; however, because neither
> is
> > required, the electrical fan will be off, so the mechanical fan will draw
> > significantly more power.
>
> Your "very high speed" is actually about 40 mph <g>
40 mph is about 1900 rpm. I seem to recall reading somewhere in an ad for
electric fans that they ran at around 1800 rpm. Don't know for sure.
> > There are too many unknowns to make a definitive conclusion, but in
> general,
> > from a horsepower standpoint, it might be concluded that an electrical
fan
> is
> > beneficial if you do a lot of driving at higher speeds, while a
mechanical
> fan
> > might be better if you do mostly low speed driving. If you spend a lot of
> time
> > idling, the electric fan may again be the better choice -- not because of
> > power or economy concerns, but because of better cooling.
>
> Very few of us PLAN to drive mostly below 40 mph <g>
I don't know about that! I know a lot of LBC owners that rarely go very fast.
Of course, those are definitely not the ones that have any interest in such
topics as this. Electric fans? Blasphemy!!!!
> I think you overlooked the part about lower underhood temperatures when
> leaving the starting line er, I mean traffic light.
>
> My conclusions : Disabling the alternator produces no noticeable
> increase and isn't worth the hassle. The electric fan produces a just
> barely discernable difference but is worthwhile just to avoid
> overheating.
>
> Of course, if you are racing where every .01 second counts, then go for
> it !
Agree with all three.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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