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
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.
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.
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
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.
On Wednesday, August 27, 2014 2:16:27 PM UTC-4, firstname.lastname@example.org 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
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
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
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.
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:
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
The fan problem was looked at years ago. The fan clutch was first, then the
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.
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.
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
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?
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.
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.
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.
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.
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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.