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??
Does this mean I will get better gas mileage if I turn the radio off? :)
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).
>In a message dated 97-08-11 03:37:14 EDT, Jim Davis wrote:
>> How much additional horsepower do you think is required to compensate
>> from the load placed on the alternator from running an electric fan?
>Jim Davis, Jim Wallace, Adam Turner, and Shane Ingate:
>Whatever horse power is required to drive the fan from the engine, it will
>take anywhere from 1.2 to 2 times that horse power to drive the alternator to
>power it from an electric motor.
>Explaination? OK, first, consider the stock configuration, with the fan being
>driven by the engine. You have a crank pulley, a fan belt, a water pump, an
>alternator, and the fan, all pulling power from the engine. When you remove
>the fan, what changes? Only the fan - everything else stays the same. IOW,
>there is no efficency multiplier from just removing the fan.
>Now, put that same fan on an electric motor, and what happens? To move the
>same amount of air, the fan will require the same horsepower to spin as it
>did before. Where does that power come from? It comes from the conversion of
>electrical energy into mechanical energy by the motor. There is nothing in
>the world that is 100% efficient (if you could find two things that are, you
>could connect them together and have the worlds first perpetual motion
>machine). In addition to driving the fan, that motor also makes heat and
>noise - energy is required to do that. As a result, it takes at least an
>extra 10% electric energy (maybe even considerably more) for a given
>Where does the electrical energy come from? It comes from the alternator,
>which converts mechanical energy into electical energy, with the same kind of
>ineffeciency as the fan motor. Another 10% waste, at a minimum.
>Thus, the engine has to produce at least 20% more power to drive an
>electrically operated fan than it does for a mechanical fan.
>How much horse power? I don't know, but I can calculate it for an electric
>motor. The fans I have seen in my various catalogs draw from 4 to 20 amps. At
>12 volts, that equals 48 to 240 watts. Since there are 745.7 watts to one
>horse power, that figures to be less than 0.33 HP. (can you imagine how
>disconcerting it would be to a driver of one of our LBCs to have 18 HP
>kicking in and out while driving in traffic? It would be like having a small
>super-charger kicking in and out!)
>Does that mean there is no advantage to running an electric fan? Not at all.
>As you all pointed out, the benefit of an electric fan is gained when it is
>NOT running. The fan is only needed when you are running at low speeds. Even
>beyond that, though, there can be benefits from using one. It may be possible
>to position an electric fan in the air stream such that it will provide the
>same cooling as a mechanical fan mounted on the engine such that it has to
>move two-three times the air to be as effective. In my own TR5.0, for
>example, I am considering using an electric fan, because the mechanical fan
>sticks up over the radiator by about 1/4 of its circumference! I just don't
>want to use an electric fan because I am pushing the limits of the alternator
>as it is, but I may have to.
>If you do a lot of high speed driving, where the fan is off most of the time,
>fuel economy will be improved, but I doubt that the improvement will be very
>noticible. If most of your driving is done at low speeds, the mechanical fan
>would give better economy.
>As far as the merits of "sucker" fans vs "pusher" fans, most *experts* I have
>read give a 10 - 30 % advantage to the sucker configuration. I suppose this
>is because interupting the air flow before it goes through the radiator has
>more effect than interupting it after it has already gone through the core
>and provided its cooling. This is well beyond my area of understanding, but I
>would think it would have as much to do with the overall configuration as the
>location of the fan. I think I can visualize situations where it would be
>better to push than to pull. For instance, if you had a wide mouth shroud on
>the front, and the rear shroud had an opening just the size of the fan, I
>would think incoming air would be disturbed less by a fan than the exhaust
>air. But again, this is just a guess on my part.
>'71 TR6---------3000mile/year driver, fully restored
>'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
>'74 MGBGT---3000mile/year driver, original condition
>'68 MGBGT---organ donor for the '74