Electric vs. belt driven cooling fan
 

I just got done replacing the belt driven cooling fan on my GMC Sonoma with an electric fan, for the purpose of trying to save fuel. But after doing this I figured out a way to measure (guesstimate actually) the amount of fuel savings I might expect.

The electric fan draws 6 amps at 14 volts (84 watts). If one horsepower is 745 watts then the electric fan uses 0.11 horsepower. My electric fan seems to blow about as much air as the original mechanical fan. This suggests that the original fan was probably using about the same amount of power, which would be about 1% of the total cruising speed power used by the vehicle if cruising power used is around 11 HP.

Note, almost no energy is consumed by my electric fan because it is almost never running. In other words, this pickup rarely needs a fan at all under the conditions which I use it. So this is saying that I have removed the 0.11 horsepower robbing mechanical fan, with almost zero added energy deficit of the new fan.

I realize my method of measuring the energy expense of the original belt driven fan is a bit crude but I could not think of any better way to calculate it.

My question is, what are the opinions of others on how much fuel might be saved using an electric fan vs. mechanical, and how did you come up with your figures?

The only things to consider are:

* Your truck probably came with a fluid clutch fan. While it was still pulling energy, I doubt it was "0.11HP" all the time.

* Your electric fan gets its energy via the alternator. The 0.11HP it consumes is magnified by whatever your electric system efficiency is (i.e. 0.11HP may actually be 0.15HP).

You will definately see better mileage (no idea how much, though). I just wonder how long the fan will take to start making you money. My guess is that you won't turn a profit for a few months, but it should help in the long term.

I'm just anal, but I find it valuable to calculate overall costs. We've all heard of the guy driving 15 miles for gas 5c cheaper. :p

- LostCause

I did not even realize that there were any vehicles made in the last 15 years that did not have an electric fan. However, here is a link that says 16 to 41 hp used for big trucks.
A comparison of viscous drives and on/off fan drives for diesel engine cooling | Diesel Progress North American Edition | Find Articles at BNET
I think what you did is worth while. If you had the opportunity, you could measure manifold vacuum at a particular speed on a particular stretch of road with the old vs new cooling fan. Then, with the electric fan in place, go back to that road, and speed up till your manifold vacuum is the same as it was with the non-electric fan.

Now you know (more or less, not exactly) how much faster you can go and use the same amount of gas. If you do a roll down test of the truck you can calculate .cd, and from that calculate the power it takes to move it at different speeds. From that you get the power the old fan took, and with a guess at engine efficiency, you can back out the value of MPG.

But in my case, the electric fan is almost using no power as I mentioned. This is because it almost never comes on. In fact, I drove around with no fan at all for three days before I got the electric fan installed and the temp gage never rose above its normal range.

I drive about 10,000 miles/year and I calculated, using my 0.11 HP figure, that it will increase my mileage by 0.33 mpg and thus take 6.6 years to pay for itself @ $4/gal and the 33 mpg that I get. The fan w/T-stat cost $80.

I guess I would not have done this mod had I done this calculating before buying the new fan, but that's water under the bridge now.

Anyway, thanks for the input.

Thanks for the link.

Gee, if a truck fan consumes a minimum of 16 HP then my little pickup fan may consume a whole 1 HP perhaps instead of the 0.11 HP I was calculating. But, I wonder if my clutch actually ever engaged. The reason is, as I mentioned earlier, I have driven around a few days without any fan at all and had no temp problem at all. My new electric fan only comes on near the end of the 1/2 mile hill I must climb in 1st gear to get to my house. Otherwise the normal driving airspeed provides adequate airflow.

I drive a school bus and I can feel the loss of power when its fan clutch engages. It is not real pronounced but it is definite.

Measuring manifold vacuum as you suggest would be a good idea but it is too much work to remove and reinstall fans so I guess I will pass.

One thing that I thought of that might be beneficial is that the electric fan may allow the engine to warm up faster when first started since it does not blow any cold air over the motor, since it is off until needed. The mechanical fan blew air constantly, even though it has a fluid clutch, and that must have had some cooling affect on a motor that is trying to get warmed up.

Thanks for the input.

That's a good point (warm-up times of fluid clutch vs. electric).

Gregte, are you tracking your fuel consumption? Just curious. (Feel free to enter your truck in the garage: Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com)

Well maybe 8 hp, but I would guess more than 1.
I thought the fluid clutches spun the fan all the time, but sometimes they spun it more and sometimes less? On the schoolbus (ha I used to drive one when I was in high school) does the fan have a clutch like an AC compressor does? Where it is locked up hard OR completely free?
The other way you could test the fan HP is to mount that old fan on an electric motor that you have set up as a dyno (you let the motor pivot on an arm, and measure the force the arm pushes with using a scale. This force * the length of th arm is torque. Then multiply by rpm to get hp) Then run the motor at diffrent speeds and you will get the curve of fan power VS rpm.

The bus I drive (Navistar T444E) has a viscous fluid clutch just like my GMC Sonoma had. It is always spinning but faster when engaged.

Now this sounds like a good idea! I may even consider doing it if I don't run into too much trouble mounting the fan onto an electric motor.

The fan on my GMC just looks terribly inefficient at moving air. My attached picture doesn't make it look quite so bad as it does in person.

That's exactly how some eddy-current dynamometers work. Just make sure that you include the distance from the center of the armature axis to the outside of the motor can when you measure your arm length. It's also important that this arm is the ONLY way you have to keeping the motor can from rotating. Most motors have a pinch clamp bolt where the bracket attaches to the motor can or housing, so you would need to freely loosen that up or mount it on bearings so that all the counter-rotational force of the housing goes through the load cell on the arm.

I was thinking in terms of mounting the motor on a plywood square, which would have a narrow triangular support under the middle, under the center of the armature as you say. Then the support arm would be adjusted so it is level. With a narrow or sharp (knife edge) support the accuracy could be quite good.