I don't know why I'm getting into this discussion. I'd
really prefer to stay out of the electric cooling fan
discussion, but would like to correct one of the
misunderstandings I've seen -- that being that there
is "free excess electricity" from the alternator or
generator.
An alternator or generator both get harder to turn
when they are generating more electrical power.
Kinderlehrer's wrote:
> Ok, now I'm very confused. The output of the alternator
> is dependent on the rotation speed of the armature, ie RPM
> of the engine. Unlike an air conditioner, there is no
> clutch mechanism that engages and disenges, so if I put
> an electric fan in the car, then I have to go faster to
> get more horsepower to the fan motor??
That's almost right, however, it is the "peak" output
of the alternator that is dependent on rotation speed.
Even if the RPM is constant, the loading on the alternator
varies according to how much power is being used. This
really isn't any different from the way the gasoline engine
in the car works!
Let's use the gasoline engine as an analogy:
For example, just because the engine is turning 3000 RPM
doesn't mean it is always consuming the same amount of
fuel at 3000RPM. If this were the case, driving up a
steep hill at 60MPH in a TR6 (3000rpm in 4th) would use
the same amount of fuel (and energy) as driving on a
flat surface at 60MPH in a TR6 (3000rpm in 4th), as would
driving down a steep hill at 60MPH (3000rpm in 4th).
I know the above seems kind of hard to believe, but is
really true. I have seen an exhibit at the Boston
Museum of Science a few years ago that had a hand-crank
generator with a few light bulbs hooked on to it, with
switches turning the light bulbs. You could turn the
crank with one hand, and it turned quite freely with
the light bulbs all turned off. Each time a light
bulb was switched on, the crank got harder and harder
to turn. With all 4 light bulbs in the experiment, one
really had to push hard on the crank to keep it turning.
It easily demonstrated in real life that the power just
doesn't come "for free."
> Does this mean I will get better gas mileage if I turn
> the radio off? :)
Yes, it does! However, even for a 240watt stereo system,
only about 0.33HP would be used. This difference is
quite insignificant, and would never be noticeable. The
same is true for your lights, wipers, etc...
> I guess I'm lucky because I have a TR3 with a generator
> which is far less than 100% efficient. It keeps on
> turning even after the battery is charged up and it
> isn't really needed, so I can use those wasted revs
> to drive my electric fan (when I get one).
As mentioned above, this isn't the case; the more load
one puts on the generator, the harder the engine has
to work to turn the generator.
However, getting back to the original question of
electric vs. mechanical fans... Dan Masters mentioned
that the electrical fans use about 240 watts max (0.33HP).
Could it be that the mechanical fan at high rpm uses
much more power from the engine? This may be the case if
the electrical fans are turning at much lower speeds.
I don't know for sure, but am just speculating. (I
guess I'm just tossing more fuel on the fire...)
--ken
'70 & '74 TR6 Daily Drivers
--
Kenneth B. Streeter | EMAIL: streeter@sanders.com
Sanders, PTP2-A001 |
PO Box 868 | Voice: (603) 885-9604
Nashua, NH 03061 | Fax: (603) 885-0631
|