Electric vs. belt driven cooling fan

[ttoyoda]

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.

Wow. I agree.There was a time when fans had 5 or 6 blades..

[Gregte]

I finally measured the load that my original belt driven fan puts on the motor of my 1994 2.2L GMC Sonoma. I mounted the fan (viscous fluid clutch type) onto my 1 horse electric table saw motor. I set the motor on a steel bench in such a manner that it was free to roll and used a spring scale to measure force to measure ft. lbs. of torque and thus calculate HP. The motor turned at 3450 RPM which equates to the GMC's crankshaft turning at 2500 RPM due to the pulley sizes involved. The speed of the unengaged fan does not matter as I am only concerned with the torque load on the motor itself.

It turns out that the fan load measured 0.19 horsepower. I then heated the bi-metal coil on the clutch to engage the fan. At this point my setup was too unstable to use my spring scale for making a measurement so I used my best judgement for feeling the pounds of twist on the motor and came up with 5 pounds. This calculated to be 0.996 HP.

It should be noted that the clutch virtually never engages on this fan in my normal driving, so therefore I am not including it's power load in my calculation below.

Assuming that the fan requires 0.19 HP to cruise down the highway at 55 (no tach so I don't know if that's 2500 RPM but seems it must be close) and assuming that it takes perhaps 10 HP to cruise 55 (just a WAG) and also assuming 34 MPG at this speed (quite easily achievable) I wonder if this means that the fan would cost about 2 percent of my fuel to turn it since 0.19 HP is about 2% of 10 HP.

If the above assumptions are true then that fan was costing me about 0.68 MPG.

I drive about 10,000 miles/year. At 34 MPG and $4 gas it must cost me $1176 per year.

Removing the belt driven fan would then be saving me $23.53 per year. I paid $85 or so for the fan w/thermostat so it will take 3.6 years to brake even with $4 gasoline.

Also, my new electric fan uses almost zero power since it only comes on for a very short time, only on very hot days, as I climb my 1/2 mile hill in 1st gear coming home. Therefore I consider its energy use too insignificant to matter. It only uses about 85 watts when it is turning and that is brief.

With the information I have now I would not have installed the electric fan. It was fun however so all is not in vain. It makes me a bit suspicious of some claims that I have read of people saving 3-5 MPG by replacing a mechanical clutch fan with an electric fan.

Or, maybe all of my measurements are screwed!!

[ttoyoda]

Or, maybe all of my measurements are screwed!!

Well if you want share your raw data we can check all your math at least
Did you take a pic of your setup?
Do you still have the measurement of the lever arm etc?

[Gregte]

Originally Posted by ttoyoda

Well if you want share your raw data we can check all your math at least
Did you take a pic of your setup?
Do you still have the measurement of the lever arm etc?

The lever arm used was a bracket on the motor housing that measured 3.5 inches from motor shaft center. I measured 1 pound of force pulling perpendicularly from the radius between shaft and the point on the bracket when I attached a fish scale. This tells me the force in ft. lbs. was 0.2916.

I used 3450 RPM X 0.2916 / 5252 to get 0.19 HP.

No pics, but just imagine a basically cylindrical motor laying on a steel table top. It is free to roll. However, when it tries to roll it cannot because I have a fish scale attached to a bracket on the motor. I see to it that the scale is pulling perpendicular to a radius which extends from the motor shaft to the scale's attaching point. This radius is 3.5 inches long. Therefore, with the motor and fan running, I divide the fish scales reading (1 pound) by 3.428 (3.428 is 12 inches/ 3.5 inch radius) to get the actual foot pounds of torque which is 0.2917 ft. lbs.

I assume the motor speed is 3450 because it's plate says it is. The actual RPM of the fan (it's clutch is not engaged, but it still spins) is irrelevant. Only the torque load seen by the motor and the motor's RPM are needed.

Anyway, I gotta get to work now lest I have even bigger problems ;-)
Later

[ttoyoda]

In the attached sketch
Is this the setup? Assume that the things close to being in line or at 90 degrees to each other are exactly so. The fan has been omitted for clarity.
Is dimension DIST-AA and DIST-BB 3.5 inches, or close enough?

[Gregte]

Originally Posted by ttoyoda

In the attached sketch
Is this the setup? Assume that the things close to being in line or at 90 degrees to each other are exactly so. The fan has been omitted for clarity.
Is dimension DIST-AA and DIST-BB 3.5 inches, or close enough?

Yes, that is it exactly and the AA and BB are indeed 3.5 inches.

The motor sits on the table on its very smooth and round surface.

[ttoyoda]

To solve a statics problem, the sum of all the forces AND torques must be zero.
So there must be a force to offset or balance the force on the spring scale. This force is the red arrow marked REACTION FORCE on the diagram. This force must be equal in magnitude to the spring scale force but opposite in direction.

If we choose the center of the motor shaft as the pivot point for the torque calculations it makes things easy, because the lever arm length is the same for both the spring scale and the reaction force. Adding the two values gives a result that is twice what was measured on the spring scale. Alternately we could have chosen the point at which the motor touches the table as the zero point, in this case the spring scale force would be as measured but the lever arm would have doubled in length. Both solution methods give the same result. So either way, the torque on the fan/motor is twice what was initially calculated.

Statics problems are rich in opportunities to make errors. Hopefully I have not made one here.

[ebacherville]

here is a idea for electric fans, I had a old chevy vega that i put a big b lock in , well needless to say a mechanical fan would not work for space reasons.. so i put in a electric fan and wired it directly to the coil wire so it was always on when i was running the motor as my radiator was rather small for the motor so i wanted to stay on top of the heat issues..

any ways when id shut the car off in a slight wind blowing and the car faced into the wind it would stay running as the fan was being spun by the slight wind and powering the ignition cloil.. the coolest part is that when not windy and faced into the wind, it was very easy to time the fan spinning down, so I could shut the car off, get out and walk a few yards away and snap my fingers just as it shut off.. people thought it was funny , like the Fonz on the jukebox..

However you easly could make that electric fan generate power when its not used, with a simple electric circuit.. to run it throw a relay to feed it power , when not running the other side of the relay feeds back intot he electric system and youd just needa (is it a diod?) to make a "one way valve" that if there is more than 14 volts comming back from the fan it charges the system.. that fan is always spinning when running down the freeway , recapturing some of it seems like a decent idea.

[dremd]

I have no #'s for my supra fan swap (mechanical - electric- mechanical) but i don't think that the fuel differance was much. However the hot high rpm performance was very noticeably worse.
However even 4 e fans (1 16" 1 12" 2 6") with 50 amps peak draw were woefully in-adequate cooling under track conditions.

[Gregte]

Originally Posted by ttoyoda

To solve a statics problem, the sum of all the forces AND torques must be zero.

Thanks for that information! I would not have figured that out myself but after reading what you wrote I do understand what you are saying and it does make sense.

I had originally concluded that it would take 3.6 years of fuel savings to pay for my electric fan. Your insight reduces this to 1.8 years. And this is all at $4 gas. If gas doubles in the meantime (hey, it might) my mod becomes even more worth the doing. In all honesty, I think with me it is more a matter of foolish pride than of the actual fuel savings although saving money, no matter how little, is always good.