OT: Car radiator fans and HP

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I got into a spirited discussion with some gear heads in a BMW forum. The BMWs, like many other cars, use a plastic mechanical belt-driven fan as the primary cooling fan for the radiator. Like many other cars, they have an aux electric fan to assist when needed. These guys are convinced that the mechanical fan is using so much power to run, that by removing it and switching to an electric fan, they will get a noticeable increase in performance and fuel economy.
I say that small plastic mechanical driven fan is similar in power requirements to other small fans that we are familiar with. A window fan for example which is maybe 1/6hp. Even a furnace blower motor is typically 1/3 - 1/2HP. But they are claiming they will get 5HP in savings. And keep in mind, you still have to power the replacement electric fan via the alternator and that is going to use power. It also involved inefficiency in going from mechanical power to electric and back via the alternator/motor. There seems to be a variety of companies selling electric fans to people who believe that the mechanical fans use 5, even 15HP and that they can then get that increased HP out of their cars. I could not find a single study, test, etc that either showed this worked or that even established how many HP that mechanical fan uses.
I think you probably could save some power, just that the existing plastic fan is likely fractional HP and I don't believe you'd feel or see the difference in performance. The main benefit to the electric fan is that it only has to run when really needed, while the mechanical fan spins all the time. But even the mechanical has factors that mitigate that. Those fans have a temperature sensitive hydraulic clutch. When it's cool, it doesn't fully engage, so the fan is not rotating an engine speed. When it gets hotter the clutch starts to lock up, so that the fan then runs faster. Also, I think a factor those folks are ignoring is that the fan spends most of it's time at road speed, with air being rammed into it by the movement of the car. That would greatly reduce the power needed to turn it. Windmills, for example don't consume power, they generate it.
I'd be interested in any thought you folks might have as to how much HP the mechanical fan uses and if this is a worthwhile endeavor.
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On Wed, 27 Aug 2014 05:52:05 -0700 (PDT), trader_4
My guess would be less than 1/10 hp but I'm more inclined to zero savings for the reasons you write below.

I agree with that completely.
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CRNG wrote:

Hi, When I was towing fiver, I replaced belt driven OEM clutch fan assembly with a light weight thermo. controlled electric fan and it worked better. But using electric power, extra however small charging load, has to be considered. Any how, car designers are even fiddling with the shape of side view mirrors to minimize air friction to save gas, a fraction of pint? But every small thing adds up I believe. Also raving guys like electric fans with light weight blades.
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A fan takes more power as the rpm goes up. It goes up with the cube of the rpm. From searching the internet a car fan may use about 1/10 of a Hp while iddling but as much as 4 to 6 Hp at 4000 rpm. Going up to 6000 to 8000 in a racing engine will cost even more HP.
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On Wed, 27 Aug 2014 14:02:00 -0400, "Ralph Mowery"

That is why they put the fluid clutch on them sometime in the 60s. The fluid link can only transmit a certain amount of power towards the fan blade.
The real driver for the electric fan is the transverse mounted engines on FWD cars and the manufacturers just expanded that across their product lines. I imagine there is some incremental fuel saving but it is not a big one. It is just another case of increased complexity with little benefit.
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On Wednesday, August 27, 2014 2:16:27 PM UTC-4, snipped-for-privacy@aol.com wrote:

I agree. IDK what the max speed is of the fan, but I suspect it never gets up to 4000 RPM, unless that hyrdraulic clutch seizes up. I'd be interested in seeing where that 4 to 6 hp number came from, if Ralph has it. I couldn't find anything credible, just people winging numbers.

One of my arguments against it being likely that it consumes 5 hp was that auto manufacturers the world over have been desperately seeking to increase fuel economy for 30+ years. If there really is 5 hp of easy, low hanging fruit, you would think the conversion to electric fans would have happened a long time ago. The fact that it's only ocurring recently, suggests that it may be for other reasons and that any gains in reducing wasted HP, probably aren't big. But if you're designing the car from scratch, a little here, a little there, it all adds up.

Speaking of which.... The BMW X5 I have uses both a mechanical fan and an aux electric fan. BMW decided to make the aux fan, variable speed, controlled via a PWM signal. A regular on/off wasn't good enough. Some nice side benefits of that are:
1 - You now have an electronic circuit board in the fan motor, which sits in front of the radiator, in one of the worst possible environments, ie high heat, rain, etc.
2 - In their wisdom, it has a 3 wire connector, 12v, GND, and PWM signal, and the 12V is always hot, even with the car off. So, one common failure mode is the battery winds up dead overnight and you can't figure out why.
3 - Per the above, unless you have the puter that BMW has that can hook up to the car and send commands to the system, a mechanic can't simply apply 12V to the fan and figure out if it spins. Similarly, mechanic also can't see if it's getting the PWM control signal or not, without a scope.....
And they did this for? Only thing that makes sense is either to save a wee bit of power, by not running the fan at full speed on the occasions it's needed. Or else to make it harder to fix the car, unless you're the dealer.
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On Wed, 27 Aug 2014 13:03:04 -0700 (PDT), trader_4

I am not sure about that. My 83 Firebird had an electric fan.
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> I agree.

You can search Google for it. Here is one place tha places the HP loss at about 6 % of the total at several engine rpms. Not sure how fast the fan would actully turn as there is usually a pully that changes the speed. Also doing some research shows the water pump to take about 5 HP.
http://www.carcraft.com/techarticles/ccrp_0311_drivetrain_power_loss/tests.html
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On Wednesday, August 27, 2014 4:47:35 PM UTC-4, Ralph Mowery wrote:

That test was done with an aftermarket fan of some kind and it's not clear if it has a hydraulic clutch or not, but that would make a big difference. With a hydraulic clutch it's speed/power would be limited, where without it would not and it would consume a lot more power.
What I can'r reconcile is folks like that saying it takes 16hp to run a small radiator fan with what we know about how many hp it takes to run similar small fans. We know from everyday experience how many HP it typically takes to move air with small fans like that. Here for example is a fan manufacturer that has belt driven vent fans of various sizes, with CFM, and the associated HP:
http://www.powellhvac.com/Bull/AVBU664.pdf
The first two digits of the part # is the diameter of the fan. So, with a 24" fractional HP fan, you can move 4000 to 6000 CFM. A 5 hp fan has blades that are 3 or 4 ft and moves 20,000 to 40,000 CFM. The folks doing these conversions are talking about an electric fan that moves 2000 CFM being a typical size they use, so I would expect that the existing mechanical fans are probably in that range.
The problem I'm having is reconciling how a small, plastic radiator fan, with a hyrdraulic clutch can be taking 5 to 22 hp. It's an order of magnitude more energy than one would expect. Same thing for the 5 hp number for the small water pump. A swimming pool pump that's pushing water 50 ft moves 75 GPM with a 1 hp motor. It's hard for me to reconcile that with needing a 5hp water pump in a car.
If fans were sucking 16hp and water pumps 5, wouldn't you think these would have been targeted and solved decades ago to meet mileage standards? I mean auto manufacturers have done a lot of things that are harder to save .1mpg, it's hard for me to believe they have a 5 to 16hp fan that could be replaced with a fractional hp electric one and they didn't do it a long time ago.
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The fan problem was looked at years ago. The fan clutch was first, then the electric motor.
Think about it for a while, the cooling system has to be designed for the worse case. High air temperature and the car not moving as stuck in traffic. You have to have a large enough fan to do that. I had a car at one time that even had an electric fan that blew toward the motor , probably to keep the carburator from vapor locking. It often cane on about 20 seconds after I had cut the engine off. With the power going up at the cube of the fan speed, when you start at an engine speed of 500 to 800 rpm and then go to 6000 to 8000 that is a lot of difference to cube. Most of the time while driving the engine will be going around 2000 to 3000 rpm in many of the cars. I have not given it much thought, but say the fan speeds up 3 times, that would make the fan hp go up 27 times. So if it was only .1 hp at iddle, that would be over 2.5 hp at normal driving rpm. Feel free to correct this thinking as I did not give it much thought.
When a car is moving the fan is probably not needed at all. That is where the electric fan comes in. It most likely takes a fraction of an HP to cool of the car , so when it is moving, it can be cut off. The fan belt driven fan has to speed up when the engine speeds up.
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On Wednesday, August 27, 2014 6:28:25 PM UTC-4, Ralph Mowery wrote:

What you're missing here is that the hydraulic clutch decouples the fan speed from the engine speed. If the car goes from 800 RPM to 8000, the fan isn't going to be spinning 10 times as fast. Just being around those fans, observing them, that;s obvious.
They typically have a thermostatic component too, so that when it's cool, the clutch has max slip. As the temp rises, the clutch starts to slip less. But for a given temp, I think it still slips more at 6000 RPM than it does at 1000 RPM, limiting the speed gain. You do have a point that whatever the speed increase, the power goes up as the cube.... But again, I wonder what the effect of the air ramming in has? You would think that would have a lowering effect on the power required too.
Most of the time while

I agree. And that would represent the biggest potential savings.
That is where

It speeds up to some extent with engine RPM, but again I think the hydraulic clutch greatly limits the speed range. I'm not arguing that there isn't some gain to be had, it's just that I find those 5 to 20 hp numbers to run a fan hard to believe.
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On Wed, 27 Aug 2014 14:52:56 -0700 (PDT), trader_4

I imagine there is some efficiency problem trying to find a fan that provides a useful amount of air at low RPM and can still do 4000 efficiently. I know electric fan blades have to be matched to the motor or they will quickly burn the motor up. BTDT The fluid coupler would blunt that quite a bit but then the fluid is going to get hot. Notice they have cooling fins on them.
BTW the other night when the power was off I was thinking about one of those electric radiator fans. I wonder what they draw?
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On Wed, 27 Aug 2014 18:28:25 -0400, "Ralph Mowery"

My wife had an 88 SSE with the infamous Bosch anti lock brakes. They used an electric pump that would overheat and trip out ... no brakes. I ended up putting in a 4" fan, just to cool that pump motor. It was a decent work around until we could dump the car.
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On Wednesday, August 27, 2014 7:12:12 PM UTC-4, snipped-for-privacy@aol.com wrote:

Every car I've ever had, the fans either electric or mechanical, always blew towards the motor.... It would be terribly inefficient if they blew the other way.
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Sure the hydraulic clutch will cut the speed and power draw. The numbers I am using are for fans that are direct drive like they were way before the clutches. I doubt that in those days there was very little though given to gas milage or emissions and such.
Just as way back houses were not insulated and a small house would have an oil furnace of 100,000 but or more in the area I live in. Oil was under 20 cents per gallon. Now with the nornal ammount of insulation being installed, it takes much less to be warmer.
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On Wednesday, August 27, 2014 7:38:52 PM UTC-4, Ralph Mowery wrote:

The issue I brought up is people are doing these fan conversions to cars *today* that have smaller, plastic fans, hydraulic clutches, etc. And they are using the alleged 5 to 15hp numbers to justify it.
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wrote:

This fan was not the one on the radiator, but a much smaller one mounted on the firewall. Think it was the 1981 Datsun that had that extra fan.
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wrote:

At anything over about 20 MPH most vehicles will cool adequately without a fan unless you are working them hard like pulling a trailer. I don't think the fan was on for more than 20 minutes driving the loaded PT cruiser from Kitchener to PEI and back with 4 adults and a load of luggage.
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On Wed, 27 Aug 2014 19:40:41 -0400, "Ralph Mowery"

240Z had them for sure. There were a few other models too - I'm sure at least one high output American beast.
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wrote:

The fan in my PT cruiser went out. It caused the head pressure in the A/C to go sky high at idle and the AC would cut out. When it cut back in after the pressure dropped back to perhaps 300 psi it would almost stall the engine. That was the only reason I noticed the fan had stopped working until the weather got real hot. Then the gauge would climb more then normal but it never actually overheated. That's completely unlike the old cars I have with V8. No fan in them and it's a boil over.
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