James wrote:
>
> Brad.Kahler@141.com wrote:
> > > >Engineering books state a belt driven fan takes away 6-9% enginepower.
> >
> > What does the engineering book say about the extra horsepower needed
> > to keep the battery fully charged ?!!?
>
> The fan uses 6 to 9% @7.5% of the total HHP, assuming a 100 BHP car,
> that's 7.5BHP. If you workthat out in kW as a rough gide I'll use a
> conversion of 2.2 kW per BHP. Thats 16.5kW at peak revs.
Okay, time to back up a bit... one hp equals 746 watts. Therefore the
conversion from hp to watts is 1 hp=0.746kW. So, by the reciprocal, 1
kW=1.34 hp.
> The electric fan for Michael's GT6 drew a current of 10 Amps. 10A * 14V
> = 140W. Assuming an altinator efficiancy of 50% the extra load on the
> engin if the fan is on all the time is 280W. This is reasonable because
> this is probebly what the mechanical fan draws a idle. Even if I got
> the conversion wrong and its 2.2BHP per kW, it is still 280W when
> compared to 3400W.
The rough engineering book figure of 6-9% is likely a little meaningless
in this context, because it depends upon belt losses, which are
variable, depending upon design and speed, and fan design, which has an
optimum efficiency at a particular speed. I doubt that any of us knows
what that speed is and the loss of efficiency at different speeds. (!)
However, let's just say a big fan on a small engine might tilt that
percentage. <g> At any rate, let's say the charging system is 50%
efficient, and that the belt losses are, nominally, about 15%. Not
considered in the above is the efficiency losses in the fan motor,
nominally about the same as the alternator, 50%. Let's say that the fan
is, for argument's sake, 100% efficient. Still, this works out to 50% x
85% x 50%=approx. 21% efficiency. For a 10A/14V input, efficient cooling
costs 280w/.21=1.33kW, or 1.8 hp. Still not too bad compared with
belt-driven fans. This is one of the reasons they're common today, as
well as for other reasons. Do maintenance costs go up because of the
additional complexity? Yes.
> You also have to take into account that the figure is for max RPM's
> the higher your engin revs (or the grater the difference between idle
> and max RPM's) the higher the %.
This is very true--the energy required to move anything is based on a
very simple principle--forces go up by the square of the
speed--therefore, the energy required to keep something moving at twice
the speed requires four times the energy than at the previous speed.
> The other thig is the drain at lower
> revs is probebly more than at maximum.
Do you mean the drain on the electrical system with an electrical fan?
If so, that's true, because the alternator is running at a much slower
rpm, producing lower output, and the current draw of the electric fan is
constant. If you mean the hp requirements of an engine-driven fan, then,
no, for the reasons stated above. As a percentage of total power
available at idle, yes, perhaps, but in an absolute sense--total amount
of power required, no.
> You don't get that much air
> through the radiator so the air the fan is sucking will be a lower
> pressure. Hence like a blocked vacume cleaner the motor spins faster,
> using less power. This is probebly why when my frend did a test he came
> up with 15 to 20%. At lower RPM's the fan will have to move higher
> pressure air, so the load/rpm's will be higher.
Well, an explanation for what you're saying above, as I gather--at idle,
and the car at rest, pressure on the forward side of the radiator is
approximately atmospheric. At speed, depending upon duct design, the
effective pressure on the forward side of the radiator is slightly
higher. But pressure is not directly correlated to velocity--this
depends upon the resistance the radiator creates. At idle, with the car
standing still, the pressure is 1.0 atm incoming. The fan must create a
pressure drop behind the radiator sufficient to move air through the
radiator. At, say, 60 mph, the pressure at the incoming side of the
radiator might be 1.1-1.2 atm, but the velocity through the radiator is
approx. 50-60 mph, depending upon the resistance of the radiator to
flow. What provides the heat exchange is the _weight_ of the air passing
through the radiator. All such measurements are expressed in BTUs/lb,
and have to be calculated with the same or equivalent units. At a static
barometric pressure, higher velocity means more air going through the
radiator, therefore a greater weight of air, and increased heat
exchange.
> The next thing to remember is when the car is traveling at speed it
> dosent need the fan, and so you will more than likly find that it is
> only used 5% of the time. On a hot day in a trafic jam my car with a
> BMC 'A' serise engin came on 25% of the time. You will also find that
> the cooling fan is only used when you don't have lights, heated rear
> windscreen etc. on, because your not ushaly get stuck in traffic with
> the engin overheating in that situation.
I can certainly see situations (not necessarily in GB (!)) where there
might be considerable current draw from other circuits such as lights,
at night, for example, when the temperatures are still high. Common in
summer, where I live.
All this still furthers the following obvious point--a badly-maintained
cooling system needs help. If that help comes in the form of an
auxiliary electric fan, there are other demands on the car's systems.
Another consideration here is fairly obvious--for as long as engines
were longitudinally mounted, they used crank-driven fans--simple, cheap
and cost-effective. When car design gradually evolved to
transverse-mounted, front-wheel drive arrangements, crank-driven fans
weren't an option--not enough cooling at low speeds. Had Triumph been
building knockoffs of the Austin Mini in the `70s, they would have
rapidly embarked on an electric fan program, and had that research
information and vendor support filtered down to other models, it
probably would have been incorporated. Long after the fact of Triumph's
demise, all we have here are aftermarket approximations of what might
have been properly engineered. (!)
Cheers.
--
My other Triumph runs, but....
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