Why a massive radiator fan in pickup? 6

[knowlittle]

I get useful repair information for my 20 year old Ford pickup from an F150 forum. New Ford pickups (Chevy pickups as well) drop clutch fan and come with electrical radiator fan, and some forum members adopt the setup in their older vehicle. According to the tribal knowledge there, the dual fan setup draws as much as 60 A (not surge current). 60A x 14V = more than 1 HP. I have a shop fan on a 1/2 HP motor and know how much air it blows. I cannot imagine how much air a 1 HP fan can blow.

Assuming a car radiator is designed to provide sufficient cooling from natural air flow while the car is moving, electrical fan seems to kick in only when car is stopped at traffic lights. Someone used a small fan from a Toyota sedan (after removing clutch fan) without any heat related issue. What is the basis of installing a massive cooling fan?

 

[IRstuff]

One anecdote does not make a theory. Car companies are not in the habit of arbitrarily over-designing, as that reduces profits.

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[EdStainless]

Look up Engine Masters on youtube and search for their cooling fan episode (#20).
A stock cooling fan on an engine consumed nearly 30hp at 5500rpm, you can see the power drain at anything over 3000rpm.
It even cuts off almost 20 lb-ft of torque at 3800rpm.
This why electric fans are the way to go.

Why are they large? Because of the resistance of the radiator and air flow required.
Think of worst case, 115F outside temp, long grade, low forward speed, full throttle.
That is a lot of cooling load.

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[GregLocock]

Before electric cooling fans the fan operated all the time. As you say, one main requirement is when stationary, when the engine is at idle so the fan has to be big to move enough air. An electric fan can operate at full power at idle, a mechanically driven one cannot.

"Someone used a small fan from a Toyota sedan (after removing clutch fan) without any heat related issue."

And did they test it like the manufacturer did? No, of course not.

Cheers

Greg Locock


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[BrianPetersen]

Bear in mind the following:

(1) The cooling fan, no matter how it is driven, including the old mechanically driven ones, has to pull enough air through the radiator with the engine idling with the transmission in "drive" and the air conditioning on full blast in summer in Arizona. That is probably the worst-case condition, or close to it.

(2) The old school mechanically driven fans were no exception to that requirement, but if they were a fixed drive ratio to the engine, they of course draw much more air than the engine probably needs when the engine is running under load.

(3) When the engine is actually running under load, the truck is generally moving through the air, and probably no longer needs fan operation at all.

(4) The power taken to operate a fixed-geometry fan varies with the 3rd power (cube) of the speed of the fan. If it pulled 30 hp at 5500 engine rpm then it uses 0.06 horsepower (46 watts, just over 3 amps on a charging circuit that runs at 14 volts) at 700 engine rpm. Of course the electric motor isn't 100% efficient and we're making some simplifying assumptions so it might be more than this in reality.

(5) That fan might not necessarily have fixed geometry and it might not necessarily have a fixed drive ratio to the engine. Fan blades intentionally designed to flex under load, and mechanical/thermostatic fan clutches that partially disengage at higher engine speed, might mean the real power vs RPM relationship isn't the simple cubed relationship described above.

(6) The electric fan in my VW wasn't a fixed-speed fan. It has some sort of speed control in it (probably pulse-width modulation of its power supply) and the ECU controls the fan speed in response to some strategy that the manufacturer has presumably thought about to minimize the noise and power consumption.

You can debunk the 60-amp "tribal knowledge" rumor very easily. Find the fuse that controls the power supply to the fan, and find out how big that fuse is. 60 amps? No way.

 

[BrianPetersen]

Another small thing. A physically large fan drawing a large volume of air slowly over as much area of the radiator as possible will be more effective and draw less power and be quieter than a physically small fan spinning fast but only drawing air through a portion of the radiator. Just because the fan is physically large doesn't necessarily mean that it has a huge motor operating it.

I have a motorcycle with an undersized cooling fan like that, which only covers a corner of the radiator. I pretty much can't ride it in traffic.

 

[dgallup]

Don't know about an F150 but the fuse to my Corvettes PWM fan drive is 60 amps. If anything, a pickup of similar power might need more cooling air as it's likely to see higher loads at lower speeds. Remember, these things have to be sized for the worst case application, not tooling around empty the way the majority of pickups are driven today.

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[GregLocock]

Typical electric cooling fan is 200-300W, maybe they have a surge allowance.

Cheers

Greg Locock


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[EdStainless]

dg, the fuse is 60A? I wonder what the actual load is. Or what else is on that circuit.
I look at some online and couldn't find any that actually drew more than 25A

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[romke]

a car cooling system is designed in such a way that it will keep the engine within the optimal temperature range as much as possible. that means the use of less circulating cooling fluid just after startup and gradually feeding more fluid via a thermostat to the radiator to dissipate heat when the engine tends to go beyond the foreseen optimal temperature. as long as the thermostat controlled fluid stream stays within the foreseen temperature range, no fan is needed. as soon as the temperature rises beyond the foreseen temperature and thus the radiator is fully operating not being able to dissipate all the heat needed, a fan can help due to the fact that the air flow through the radiator is increased. I once had a Peugeot 605 turbo where the fans hardly ever operated and when, fan 1 was first activated at half speed, subsequently at full speed, then fab 2 kicked in at half speed and finally, if necessary, fan 2 was also operated at full speed. that only happened in very hot weather, after high speed driving on a motorway, ending up in a very slow moving traffic jam. the fan(s) then ran for a few minutes only and when the traffic jam persisted they occasionally kicked in again when required.

what all this means is that regulating a cooling system can be quite complicated and that depending on engine design, engine load, ambient temperature and airflow through the radiator all play a role and that the cooling capacity required not simply varies with engine load but also with the momentarily possible heat dissipation through the radiator. when after prolonged high speed driving you end up in a traffic jam or at a traffic light there still be quite a large amount of heat transfer from the engine hardware to the cooling fluid, although the cooling capacity is virtually non existent due to the fact that there is no airflow at all....a situation possibly worse then when lugging up a hill with a heavy trailer in a ambient temperature of 40 degrees C....

from all this it can be seen that using a fixed fan all the time is a waste of energy - what is needed is a controlled fan that is able to sustain sufficient airflow even when standing still when the engine just ran on full power. the cause of this is the fact that heat produced is not under all conditions in sync with the engine load and the cooling capacity of the system.

 

[LionelHutz]

The engine speed varying means the mechanical fan can't be build to draw ~1hp and only ~1hp. The power draw of the mechanical will be similar to the electric fan ~1hp draw when the engine is at idle with the truck in gear. But, the power draw of the mechanical will increase significantly as the engine engine speed increases. The mechanical fan also can't be fully turned off when it's not needed. So, sure you can argue that a mechanical being direct driven is more efficient, but it will ONLY be more efficient in a very small engine rpm range and ONLY when cooling is required.

I have yet to see a fan system that requires 60A when operating. 35-40A is more typical. Having a 60A fuse pretty much proves that fan don't draw 60A when operating.

 

[Compositepro]

Fan clutches are fluid couplings, so they do not simply run at engines speed. They run at closer to constant torque. Many are also thermostatically controlled by a bimetal valve. One of the main reasons for electric fans is that on front wheel drive cars the engines are transversely mounted, and belt drive is not practical.

 

[EdStainless]

Another big reason to go electric is the cost of fluid or bimetallic clutches. They are not cheep.



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[LionelHutz]

Compositepro - Yes, most cars built in the last 50 plus years with mechanical fans have clutches. Still, I've never seen one that doesn't sound louder and have more airflow when the engine is revved above idle. Of course, even if the clutch made the fan assembly into a perfect constant torque load, the power draw from the engine would still increase linearly with rpm. Then, multiple people have tested them and found they are not perfect constant torque loads and that the torque transferred to the fan does increase with rpm meaning the power draw is more than a linear function with rpm.

 

[knowlittle]

Thank you for educating me, especially about the worst case. I will pass the info so that no one will replace the huge clutch fan with a small electrical fan.

I got curious myself and checked the owner manual of 2010 Ford pickup. Two electrical fan relays, 50A x2 with max tow package, 40A x2 without. I commonly see 30A x2 fuses running 8A x2 fans in sedans. So tribal knowledge goes down the drain.
Thank you again.

 

[BrianPetersen]

You can go ahead and replace the clutch fan with an electric one ... just not an undersized one!

 

[LionelHutz]

The motor typically used in such a fan is typically brushed and has a very low impedance at zero speed. So, the fuses do need to be well over-sized to avoid blowing the fuses during the starting current surge. A 30A fuse running a 8A fan doesn't surprise me. The wiring is likely around 12 gauge and the relay is likely 30A rated. So, using a 30A fuse for protection is quite fine since the fuse is only for short circuit protection.

 

[JayMaechtlen]

The other thing about trucks- they may be hauling a load (maybe camper + boat on trailer?) up a long grade - and may have a tailwind.
Essentially a high load for substantial periods of time - easily 30 minutes plus.
Effective speed of outside air into the radiator = pretty slow!
It's for those cases the extra heavy duty packages are made.


Jay Maechtlen

http://www.laserpubs.com/techcomm