Dan Masters writes:
>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.
>
>2) Given the above, we have to assume that the blades on both fans require
the
>same power and move the same amount of air if they are turning at the same
>speed.
>
>3) The alternator is 50% efficient. This may not be the correct value, but
it
>will do for the analysis, especially since we don't know the actual value.
>This value can be adjusted as desired, skewing the end results up or down
>accordingly.
>
>4) The electrical motor driving the fan is also 50% efficient. It takes
twice
>as much electrical energy in as the mechanical energy out. The same
arguments
>as in 3) also apply here.
>
>Now, there are some facts to be considered:
>
>1) An electrical fan is pretty much a single speed device. Speed may
change
>somewhat as car speed, and air speed through the radiator, changes, but
the
>change should be small.
>
>2) The mechanical fan speed is always a constant percentage of the engine
>speed. Since the fan is driven by the crankshaft on a TR6, the percentage
is
>100% - the fan turns at engine speed. On most American cars, the fan turns
a
>bit faster than the crank.
>
>3) Power required to spin the fan blades increases exponentially (I think
that
>is right - correct me if I'm wrong) as fan speed increase.
Close. The power required is a cube function of the speed. This is
because
the pressure drop through the radiator is a square function of the rate of
air flow. So to calculate power you multiply the air flow X the pressure
differential. If you will, the air flow is analogous to current and the
pressure differential is analogous to voltage.
>4) At higher car speeds, air flow through the radiator from vehicle
velocity
>is sufficient to cool the car without a fan.
>
>5) At low speed, and especially at idle, the fan is the only real source
of
>air flow through the radiator, and is mandatory.
>
>6) Any fan produces more air flow at higher speeds than at low speeds.
>
>Considering all the above, we can draw some conclusions:
>
>1) With the engine turning at an RPM such that the mechanical fan speed is
the
>same as the electrical fan speed, it will take four times as much power
from
>the engine to operate the electrical fan than it will to operate the
>mechanical fan -- a 50% loss through the alternator and another 50% loss
>through the fan motor. (I can't be certain at all, but I would guess that
this
>speed is somewhere in the range of engine speed needed for driving in stop
and
>go traffic, as this is probably the condition for the most stringent
cooling
>demands) How much power are we talking about? If the electrical fan draws
9
>amps (a reasonable number, based on the fans available), the HP drain on
the
>engine will be 0.35HP, and the equivalent HP drain for the mechanical fan
at
>this speed will be 0.35/4 = 0.09.
OK. Using this "feinstance" if the mechanical fan uses 0.09 HP at, say,
1000
RPM then at 5000 RPM (in a competitive situation) the fan uses:
0.09 HP X (5000/1000)^^3 or 0.09 X 125 = 11.25 HP!
>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.
>
>3) Even at lower speeds, the electrical fan can be shut off when not
needed;
>the mechanical fan cannot (excluding some of the clutch type, or the "flex
>blade" types -- these are not "turned off," but the power requirements to
>drive them are greatly reduced at times). Any time the electrical fan is
off,
>it will, of course, draw less power then a mechanical fan which is still
>running.
>
>4) At idle, the mechanical fan speed may not produce enough air flow to
>provide sufficient cooling.
On most cars the mechanical fan is sized to provide adequate cooling at
idle so at any speed at all it is overkill. Of course on LBC's the fan is
sized for adequate cooling at idle at 50 degrees F (typical summertime
temperatures in England.) ;-)
>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.
>
>Real world numbers may vary from above, but not by an order of magnitude.
>Power gains from an electrical fan on a TR6 will not be as great as on an
>American car, because the fan turns at a lower speed for a given RPM. I've
>seen claims of horsepower gains as much as 20 HP from switching to an
>electrical fan. These claims may be true on a high performance, high RPM
>engine, using a very powerful fan, but I don't think we will see any where
>near these numbers in a TR6.
>
>Correct me if I'm wrong (yeah, right, like I have to ask!).
>
>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
A very good disertation, Dan. Please accept my modifications in the spirit
they were meant. (That is from a busybody know it all! ;-) )
Another point is that the electric fan draws power from the engine mostly
when it is idling and we don't miss the HP anyway!
The reason most cars of the era had belt driven fans is because that type
of fan is cheap. The reason most modern cars have electric fans today is
because the motors are mounded sideways and there are no reasonable
alternatives.
Dave Massey
St. Louis, MO USA
57 TR3
71 TR6
80 TR8
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