You wrote:
>Dave:
>In my original post, I said "for the same cooling." That applies to
whatever
>RPM the engine is operating at. If it takes 21.6 HP to operate the
mechanical
>fan at 4800 RPM, it will take at least 26 HP to operate an electric fan
with
>the same air flow.
Ok, I see your point. Yes you are very right. Horsre power is horse power
and
what ever type of fan you use, it takes horse power to move air and that
horse power
is coming from the engine - either directly from the engine in the case of
the engine
driven fan or via the motor - alternator combination in the case of the
electric fan.
BTW Your 26 HP is quite optomistic. Probably more like 80 HP. These small
motors
and generators have rather poor efficiency.
>My original point, however, is still valid. Many people are of the opinion
>that it does not take engine power to operate electrical accessories.
People
>have responded to this thread with just that concept. I remember quite
well
>in the energy crisis of the '70s, that people were asked to turn on their
>lights to remind others to conserve energy. The idea being that electrical
>power was free in a car. Not so! The whole point of my original post was
to
>point this out.
Yes, true! Try this experiment. Loosen up the fan belt and see how your
battery runs
down. Thats because it takes power to turn the alternator/generator and
without a tight
belt you can't transmit power to it.
>For anyone contemplating a swap to an electric fan, it would be a good
idea
>to have all the facts. Anyone making the swap to gain 18 HP, especially at
>normal driving speeds, will be disappointed. Likewise, at normal driving
>speeds there won't be a dramatic increase in economy. Due to the sporting
>nature of our cars, they are most often driven around windy, twisty,
roads,
>at a relatively slow speed. An electric fan would be on quite a bit under
>these conditions.
This is where the dichotomy comes in. The engine driven fan provides
adequate cooling
when the engine is turning approx 1200 RPM. Anytime you drive faster than
that the fan is
pumping more air than you need at a cost of horse power and fuel
consumption (albeit
a rather trivial amount.) Also since the ram-air effect of driving will
force cooling air through
the radiator there is a speed above which you need no fan at all. Easily
accomplished with
an electric fan but not so with an engine driven fan. And I think this
speed is more like 12 - 15
MPH. But anybody who thinks they are going to get free 18 HP are mistaken,
as you say. But
racers and autocrossers, who routinely run at these stratospheric speeds
will gain significant
improvements.
>Dan Masters,
>Alcoa, TN
You raise a good point though. The power required to operate the
electrical devices has to
come from somewhere and that somewhere is the engine. The electrical load
is fed from
the generator and the generator places a mechanical load on the engine
which has to burn
more gas to trun it. There is no free lunch.
Dave Massey
Somewhere in the USA, on the digest. Sorry about the delay in responding.
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