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Old 03-08-2021, 10:21 PM   #1 (permalink)
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Radiator fan power draw on highway?

Hello everyone, I am wondering if anyone has tested to see how much power electric radiator fans draw while driving down the highway compared to while stopped.

The reason I am asking this is because a lot of cars (mine included) unnecessarily run the radiator fans whenever the AC compressor is running, even at higher speeds where there is plenty of natural airflow to cool the radiator and condenser without the help of the fans. This potentially wastes a significant amount of power and runs the battery down faster than necessary for those of us who disable our alternators.

As an example, my car has dual electric radiator fans that draw around 23 amps while on. 23A X 14V = 322 watts. Assuming a standard alternator is 50% efficient, that's a draw of ~644W from the engine, which is equal to about 7/8 of a horsepower. Assuming the AC compressor runs at a 50% duty cycle on the highway (which is often pretty close to accurate on my car depending on weather and fan speed), that averages out to a constant load on the engine of about 3/8 of a horsepower. Although not crazy high, that's definitely high enough to be worth figuring out a way to eliminate. People do much crazier things for much less gains!

I have a few ideas of how to automatically disable the fans at high speeds, but I would like to know what the power draw is at higher speeds before implementing such a system because I suspect that the draw may be less while driving at high speeds since the air being forced through the fans would theoretically reduce the load on the motor by helping turn it like a windmill, but I am not sure how much the draw would actually change.

I am going to be testing the power draw at different speeds when I have time and will report back, but for now I am wondering if anyone else has thought about this or done any testing.

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Old 03-09-2021, 10:42 AM   #2 (permalink)
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My understanding is the fans are driven on by a temperature sensor somewhere in or near the peak temperature spot in the cooling system. Ditto for multi-speed. When you have the A/C on you are dumping hot air into the cooling systems input effectively making it a very hot summer day. My golf reduces engine power if it gets too hot after measuring intake temps. If you think there's a benefit to killing the fan, just disconnect one at the fanhousing power connector and measure the change. Those connections are made to come apart.

On the other hand the F250 fan mostly idles,BUT it has 8 gallons of coolant and is overcooled for most operations below 1/2 throttle
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Old 03-09-2021, 11:19 AM   #3 (permalink)
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Quote:
Originally Posted by EcoCivic View Post
I suspect that the draw may be less while driving at high speeds since the air being forced through the fans would theoretically reduce the load on the motor by helping turn it like a windmill
I would have to agree with this. Even if it's a series-wound motor - with no set top speed - it will still lighten the load. How much is going to depend on how much the air is slowing down going through the radiator and pressure differences...the only answer, as you are already intent on finding, is to test it.
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Old 03-09-2021, 11:46 AM   #4 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
My understanding is the fans are driven on by a temperature sensor somewhere in or near the peak temperature spot in the cooling system. Ditto for multi-speed. When you have the A/C on you are dumping hot air into the cooling systems input effectively making it a very hot summer day. My golf reduces engine power if it gets too hot after measuring intake temps. If you think there's a benefit to killing the fan, just disconnect one at the fanhousing power connector and measure the change. Those connections are made to come apart.

On the other hand the F250 fan mostly idles,BUT it has 8 gallons of coolant and is overcooled for most operations below 1/2 throttle
Thanks. On my car the fans are activated by a temp sensor in the engine block when the coolant reaches 207 degrees or whenever the AC compressor is engaged regardless of coolant temp or speed to cool off the condenser. Problem is that the fans don't need to be running while driving down the highway as there is already plenty of natural airflow to cool the radiator and condenser.
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Old 03-09-2021, 11:56 AM   #5 (permalink)
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Originally Posted by Stubby79 View Post
I would have to agree with this. Even if it's a series-wound motor - with no set top speed - it will still lighten the load. How much is going to depend on how much the air is slowing down going through the radiator and pressure differences...the only answer, as you are already intent on finding, is to test it.
Thanks. I think the only accurate way to test this will be to cut and temporarily extend one of the fan power wires into the cabin so I can get my amp clamp around it and monitor the amp draw at different speeds, then multiply by 2. Since I moved my battery to the trunk and I ran the positive cable behind the trim in the cabin my initial plan was to unplug the alternator and monitor total current draw from the battery at different speeds and work out the difference, but I decided that the fans need full, consistent alternator voltage for the test to be valid since their current draw will vary depending on the voltage supplied. I will report the results as soon as I have time to test it
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Old 03-09-2021, 04:27 PM   #6 (permalink)
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On thr highway, if it's an electric fan it should be 0.
Sounds like you need a switch to disable the fan above a certain speed.
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Old 03-10-2021, 05:26 AM   #7 (permalink)
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A couple thoughts.

One, the faster an electric motor spins the less electricity it consumes. So if it is turning on at highway speeds there is a very likely chance it will use less electricity than at a stop.

Another thought is that more airflow through the condenser could, in theory, help the compressor use less energy to compress the refrigerant. So now not only do you need to actually check and measure the power comsumption of the fan, it would be a good idea to measure the new power consumption of the compressor. Of course there are many factors that could play a part in the results.
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Old 03-10-2021, 07:21 AM   #8 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
A couple thoughts.

One, the faster an electric motor spins the less electricity it consumes. So if it is turning on at highway speeds there is a very likely chance it will use less electricity than at a stop.

Another thought is that more airflow through the condenser could, in theory, help the compressor use less energy to compress the refrigerant. So now not only do you need to actually check and measure the power comsumption of the fan, it would be a good idea to measure the new power consumption of the compressor. Of course there are many factors that could play a part in the results.
That is a great point, I have thought about that as well. Poor airflow through the condenser definitely would decrease the system's efficiency and performance. I have no way to measure the compressor's power usage, but what I could do is set up a thermocouple to measure the temp of the liquid line with the fans on and off at different speeds. This would tell me at which speed running the fans no longer does any good. Measuring head pressure would do the same thing, but measuring temp would be easier in my case.
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Old 03-10-2021, 10:15 AM   #9 (permalink)
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Quote:
EcoCivic

I have a few ideas of how to automatically disable the fans at high speeds, but I would like to know what the power draw is at higher speeds before implementing such a system because I suspect that the draw may be less while driving at high speeds since the air being forced through the fans would theoretically reduce the load on the motor by helping turn it like a windmill, but I am not sure how much the draw would actually change.
No need to disable the fans.

If you want to know if your electric fan is running just do what I did. Connect a 12V LED running parallel to the ground wire coming from the fan. Ground the other end of the LED and place it in a area where you can see it on the dash or instrument panel. The LED (fan) will come on when the engine gets hot, the a/c is on, the defroster is on and possibly when the re-circulation button is pushed also.

The LED will also come on (dimly) when you're driving fast enough for the fan to free spin and generate enough power to light the diode.


Hope this helps.




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Old 03-10-2021, 04:31 PM   #10 (permalink)
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highway

Above 40-mph, ram-air should be sufficient for all engine cooling needs.
I drove 100-miles with no fan whatsoever in my grandad's Dodge pickup. All highway.

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