Engine Fans: Electric vs. Mechanical

jason thompson · **Posted:** Mon Jan 15, 2007 10:35 pm

unixxx wrote:

Alternators don't supply current unless it's needed. There needs to be a load for there to be a current draw. The alternator may be capable of supplying 100A, but if you're not running any accessories, then you're not actually taking that much power from the engine. The power required to turn the alternator is proportional to the load on the alternator. With an electric fan, you're adding a load to the alternator which is then transferred to the engine. Assuming identical air density, fin shape/size, and fan speed/efficiency, the same power is required to drive both a mechanical and electrical fan. Energy can't be created or destroyed; increased efficiency is the only way to achieve the same output with less input. 10% power is L.O.S.T. converting mechanical energy to electrical in the alternator then another x percent is L.O.S.T. converting electrical to mechanical in the fan. y percent is L.O.S.T. in the mechanical fan viscous clutch. If 10% + x > y, then the electrical fan is less efficient. I welcome constructive criticism, please feel free to tear my statements apart.

what you are saying would work but the horse power that is needed to turn the alternator at FULL load,wich is hard to get to, is going to way less then the horse power needed to turn the fan

alternators are not hard to turn compared to fans less rotating mass just to start

sleeve84028 · **Posted:** Mon Jan 15, 2007 10:39 pm

LiLredLibby wrote:

A mechanical fan pulls hot air from the radiator(with the hot tranny fluid in it)while your towing and heating up the tranny.An electrical fan pushes cool air across the a/c condensor to cool the freeon while it's in a liquid state at appr.250 to 300 degrees. That's why the electric fan cycles in sync. with the a/c compressor.....Make sence?

I'm suppling a picture of my fans

My electric fan only comes on when I'm sitting in traffic. My fan doesn't cycle on and off with the A/C compressor that I have noticed - and they are both on the engine comparment side of the radiator. It's not setup like a pusher / puller configuration.

My tranny cooler is seperate of the radiator... I can shoot a picture of that tomorrow, but I'd like to keep this topic on track with the fans.

LiLredLibby · **Posted:** Mon Jan 15, 2007 10:42 pm

Caravan....one electric fan in front of radiator turned on via engine temp. sensor at appr. 205 to 210 degrees.
another electric fan in front of a/c condensor turned on via a/c high press. switch at appr. 150 psi. Making any more sence? :wink:

unixxx · **Posted:** Mon Jan 15, 2007 10:53 pm

Assuming the mechanical and electrical fans are the same, both should require the same power to start and turn. If they both require 240W to turn, the mechanical fan will remove 240W, plus the inefficiency of the viscous clutch, from the engine. The electrical fan will remove 240W, plus the 10% inefficiency of the alternator (27W), from the engine. Just like the viscous clutch, the alternator is only going to decrease efficiency by adding another leak to the system. I guess you're saying that the viscous clutch is less efficient than the alternator, which I believe. But the electrical fan still has the added inefficiency of the electrical to mechanical conversion.

jason thompson wrote:

what you are saying would work but the horse power that is needed to turn the alternator at FULL load,wich is hard to get to, is going to way less then the horse power needed to turn the fan

alternators are not hard to turn compared to fans less rotating mass just to start

jason thompson · **Posted:** Mon Jan 15, 2007 11:02 pm

unixxx
you are putting way to much thought or effort in to this so I did a google search and found this

How much horsepower loss
will an alternator cost ?

As was stated in the answer to the first question, an alternator is a power converter. It converts mechanical power into electrical power. So it doesn't lose power, but rather changes its form. No electrical motor or generator is 100% efficient and this is true of alternators as well. Alternators are about 85% efficient with power being lost in the form of heat and wind resistance (cooling fans). Thus the real power loss of the alternator is relatively small.
How much horsepower does an alternator pull ? You can mathematically determine the horsepower cost at any given load. (Remember, alternators respond to load. If there is no load present the alternator is basically freewheeling.)

Amps x Volts = Watts
Watts / 745.7 (one HP) = Electrical HP Produced by the Alternator
HP x 15% Efficiency Loss = HP Loss
HP + HP Loss = Total HP Used

Example:
57A x 14.9V = 849.3 Watts
849.3 Watts / 745.7 = 1.14 HP
1.14 HP x 15% = 0.17 HP
1.14 + 0.17 = 1.31 HP Total

now knowing this and knowing that an electric fan draws only about 1/2 or less power to run than the above equasion we know that the horse power loss will be about 1/2 a HP I know that a mech fan will draw more than that and you do too

unixxx · **Posted:** Mon Jan 15, 2007 11:28 pm

Thanks for throwing the equations out there; they make things easier to follow. My new question is then, why does a mechanical fan consume more horsepower? Is it because the viscous clutch isn't very efficient? I don't see any other opportunities for loss in a mechanical engine driven fan.

jason thompson · **Posted:** Mon Jan 15, 2007 11:46 pm

unixxx wrote:

Thanks for throwing the equations out there; they make things easier to follow. My new question is then, why does a mechanical fan consume more horsepower? Is it because the viscous clutch isn't very efficient? I don't see any other opportunities for loss in a mechanical engine driven fan.

wind resistance
it takes mechanical HP to turn a mechanical fan wich comes right off the front of the engine the only gearing is through the belt drive so you could change the pully and slow down the fan to gain HP but loose cooling or speed up the fan and loose HP
a 230HP engine sounds like alot but realy it is not that strong with out gearing you loose alot more when you dont have gears to help out
also remember that your max HP is at a high rpm 5000+ I think at 1500 it is like 100 and little things rob it easy

as stated in the last artical the alternator makes or produces energy by using mechanical energy more efficiently so you can get more out of it with less loss of mechanical power

The reason that an electric fan takes so much current to run is because it has a motor on it that prduces 1/3-1/2 a HP but through gearing and speed it uses that HP better than the mech. fan

it can all be looked at from a point of CFM at RPM your mech fan makes say 2000cfm at 2500rpm
the electric fan makes say 2800cfm at 5000rpm but so what it will deliver more cfm at idel than your mech fan wich is only turning at 600rpm at idel
rpm=cfm kinda

RFCRD · **Posted:** Mon Jan 15, 2007 11:54 pm

unixxx wrote:

RFCRD, from what I've read, Jeep Libertys with the 42RLE transmission have an auxiliary transmission cooler bolted in front of the engine radiator. Libertys with the 45RFE transmission have a cooler built into the engine radiator. Liberty CRDs with the 545RFE have a combination transmission cooler and A/C condenser bolted in front of the engine radiator. It seems your observation is correct.

LiLredLibby, on my 05 Renegade, both the electrical and the mechanical fan are on the engine side of the radiator. Both are pulling relative to the radiators.

And don't forget the CRD has a charged air cooler sandwiched inbetween the radiator and A/C condensor/tranny cooler combo. Quite the plumbing nightmare shoe-horned into that space. There appears to be multiple systems controlling the electric fan operation on this beast.

unixxx · **Posted:** Tue Jan 16, 2007 12:31 am

So, at higher RPMs, an electrical motor is more efficient at driving a fan than a gasoline motor. Also, at lower RPMs, the ability of an electrical fan to maintain airflow results in better engine cooling. So at both high and low RPMs, the electrical fan preserves the most horsepower.

In favor of the mechanical fan, at higher RPMs, a viscous clutch counteracts decreasing fan efficiency by slowing the mechanical fan.

Sound about right?

sleeve84028 · **Posted:** Tue Jan 16, 2007 8:41 am

Alright - we have been able to establish that an electric fan is superior to a waterpump driven unit in many ways. Now, I'd like to wonder about the libertys design with the two stacked fan setup - one electrical and one mechanical as there are two questions that I have:

1) Is it possible that the extra mechanical fan is only a fail safe device - in case of emergency?
2) Does having the extra fan contribute to a lack, or loss, of airflow across the radiator fins as the engine bay is more cluttered?

This picture isn't the best - but if you look closely you can see the radiator fins, the plastic bracing for the e-fan, plus the blades of the mech-fan

from that picture, it sure seems like the air would flow past the radiator with less effort if that extra fan wasn't there... Or is the mech-fan there simply to create a low pressure area behind the radiator surface to help move the air when the electric fan is on? (did that last sentence make sense?)

Jeepjeepster · **Posted:** Tue Jan 16, 2007 10:56 am

This is one heck is a discussion here!

The good thing about the mechanical fan is it moves air ALL the time. If you are offroading, the electric fan seems to kick on and off, it doesnt stay on all the time. Thats one reason that I unplug my A/C compressor and turn the A/C on inside. That will cause the fan to stay on all and time and the A/C will not be kicking in(since its unplugged).

I do not believe the air would flow better if the mechanical fan wasnt there.
That would be true if the fan was locked and didnt move when it wasnt engaged, but it spins even if the clutch is not engaged.

You could look at the mechanical fan as being a fail safe. It would surely keep you cooler offroad if the electric fan failed and you didnt have any other fan. :wink:

Edwin · **Posted:** Tue Jan 16, 2007 12:43 pm

RFCRD wrote:

sleeve84028 wrote:

you need to be doing about 65+ for the air flow to overtake the fan.

What about on 55MPH country 2-lane roads / highways?

Read where on most transverse engine cars (electric fan), the fan is about useless over 40 mph with a normal load in moderate climates. If you have enough load or it's hot enough to get the H2O temp up it might kick in at highway speeds.

The CRD has both fans. The only time I hear the electric fan running is while stopped in traffic with the A/C on.

I have both fans on my '04 gas KJ.

unixxx · **Posted:** Tue Jan 16, 2007 1:10 pm

When we were off-roading a few months ago, another member's electric fan failed. The mechanical fan is probably a good failsafe.

sleeve84028 · **Posted:** Tue Jan 16, 2007 1:14 pm

I forgot to add this in as well... I had an old car and I converted the cooling fan from Mechanical to Electric - it did work much better while sitting in stop and go traffic but it also quit working one day. I had to drive home without any sort of cooling fan and was pretty far away from home. I was lucky enough to be able to use highways for a majority of the journey but the interesting thing is that I needed to be going at least 65MPH in a constant state to see my temps drop and no one could be in front of me. Going 40MPH in a city didn't provide enough airflow over the heat exchanger probably because of the stop and go traffic and the temps did rise almost to the point of a boil over... Thank goodness for a fantastic heater in those old mustangs

Jeger · **Posted:** Tue Jan 16, 2007 2:26 pm

sleeve84028 wrote:

I forgot to add this in as well... I had an old car and I converted the cooling fan from Mechanical to Electric - it did work much better while sitting in stop and go traffic but it also quit working one day. I had to drive home without any sort of cooling fan and was pretty far away from home. I was lucky enough to be able to use highways for a majority of the journey but the interesting thing is that I needed to be going at least 65MPH in a constant state to see my temps drop and no one could be in front of me. Going 40MPH in a city didn't provide enough airflow over the heat exchanger probably because of the stop and go traffic and the temps did rise almost to the point of a boil over... Thank goodness for a fantastic heater in those old mustangs

Reminds me of a broken fan belt on my samurai, the heater trick does help, but in the summertime it really sucks. I wish I would have had a dual fan setup that day. Its the only way to fly.

Gris · **Posted:** Tue Jan 16, 2007 2:51 pm

I'm the member Unixxx is talking about that.

3 times i've had to park or leave early because the electric fan got gummed up. I can't even say it's because my fan was old and dirty. After the first time, we stripped the fan and cleaned everything, then lubed it up. It ran fine in traffic during the summer months. Next time I went off-road I had to park because it was overheating shortly after we left the start. I got most of the way home just fine with highway driving but it started overheating when we hit some construction. Luckily it was on a long down slope, so I just turned the engine off and coasted down the hill.

When I got home I took the brand new fan out of my wife's '06 liberty that had never seen mud in it's life. I swapped the fans and sent her off to the dealer to get it fixed under warranty. The very next time out to the trails, it started overheating again by the time the day was over. Driving home, even with traffic, i had no problem at all. I cleaned it out when I got home, but there really wasn't THAT MUCH mud on it. And it's not like the off-roading we do involves a lot of mud. For the most part Paragon and Rausch Creek are rock parks, not mud parks.

I'm really not satisfied with the electrical fan in the cooling system at all. If I ever find a wrecked KJ that had the towing package, i'm ripping the mechanical out and installing it.

As a premtive measure, I now unhook the AC compressor and turn the AC on to force the fan to stay on. I figure it will throw the mud off before it has a chance to slow down and stop the crappy electrical motor.

Jeepjeepster · **Posted:** Tue Jan 16, 2007 2:54 pm

sleeve84028 wrote:

I forgot to add this in as well... I had an old car and I converted the cooling fan from Mechanical to Electric - it did work much better while sitting in stop and go traffic but it also quit working one day. I had to drive home without any sort of cooling fan and was pretty far away from home. I was lucky enough to be able to use highways for a majority of the journey but the interesting thing is that I needed to be going at least 65MPH in a constant state to see my temps drop and no one could be in front of me. Going 40MPH in a city didn't provide enough airflow over the heat exchanger probably because of the stop and go traffic and the temps did rise almost to the point of a boil over... Thank goodness for a fantastic heater in those old mustangs

Haha, guess you couldnt draft on the way home? :lol:

Needed to stay in the clean air.

Engine Fans: Electric vs. Mechanical

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