HP gains from switch to electric cooling fans? 2

[92xj]

One of those periodic fads has hit jeep land where everybody is converting from the stock belt-driven viscous cooling fans to electric cooling fans, claiming to gain HP and as much as 1.0 MPG fuel economy. I just can’t believe this – given the CAFE pressures on the automakers, I have to believe if this free lunch were available they would had jumped on it long ago.

Can you guys tell me what factors would come into play in evaluating such a conversion and measuring its results? And why it seems like nearly every maker (except with a few transverse-mounted engines) uses the belt-driven viscous fan?

Things that occur to me: (1) there has to be a power loss from converting mechanical motion (belt rotation) to electricity and then back to rotary motion through an electric fan (2) the load the alternator puts on the belt varies with the current requirements, so substituting an electric for belt driven fan eliminates the load from the fan, but replaces it with a higher load on the alternator (3) there could conceivably be a HP gain at peak to the extent the fan tries to draw more current than is available from the alternator and so instead draws some stored power from the battery (assuming that in the same circumstance the load of the belt-driven fan plus alternator would exceed the draw of the alternator at peak production), but the battery deficit would have to be made good later so there would be no MPG gain.

The only way this conversion could result in overall power/fuel economy gains would be if (1) the stock viscous fan produces more air flow than is needed in some circumstances, or (2) an electric fan is available with a more efficient design than the stock clutch fan.

On the latter point, efficient design, some argue that since the viscous fan is always turning to some extent, even when the engine is cold, it wastes engine power (I thought the fan’s drag when cold was very, very minimal), and since it never completely “locks up”, it wastes power at high RPMs.

Any thoughts?

 

[GregLocock]

Electric cooling fans do produce a net fuel economy gain, particularly at hot idle, when a viscous fan will be whirling away regardless, and at high speed cruise, when the electric fans will often switch off entirely.

You get a direct improvement in efficiency because the fan to shroud clearance is better controlled.

Your other observations are correct, but do not make up for the inherent advantages of an electric fan. 1 mpg seems a bit much. There is no significant CAFE pressure for 4wds, I believe.

Cheers

Greg Locock

 

[PEW]

The advantage comes from the fact that during forward motion there is already enough airflow through a properly designed cooling system without the need for a fan at all. The best thing to do, therefore, is to switch the fan off altogether and not waste any power driving it.

My little trials car has an 850cc engine (now modified) that was originally fitted with a small fixed 3 bladed fan. I was given advice that someone building a similar car had found another 4HP on the dyno by removing the fan from his engine. 4HP is 10% of the original output. Because of this, my original fan was never used; I decided from the outset to use an electric fan, wired through an adjustable thermostat fitted in the rad hose.

A trials car uses low gears and a lot of revs on the hills at relatively low road speed. From my own experience, the hottest time for the engine is actually just after a hill has been climbed. Even so, this time of year the fan hardly ever comes on but the temperature needle still stays in the normal sector of the gauge.

Another practical advantage in my car, which uses a pusher fan hidden in the radiator tunnel, is that there is no fan whirling in the engine bay to catch fingers etc. Not only that, there is no engine driven "propellor" to throw water and mud over the engine if crossing a deep puddle is needed, also useful for a 4 x 4 I would think.

 

[GregLocock]

4 HP sounds like the right number to me.

Cheers

Greg Locock

 

[Chumley]

Many of the radiator fans in newer cars are now electric.
A lot of the systems actually have TWO electric fan motors
or twin fans driven by a single multi speed motor.

Using an electric fan(s) allows more design flexibility for
the radiator location and shape. Thats' very helpful in
newer cars with cramped engine bays, lower hood lines, and
aerodynamic front end designs.

Higher performance NA cars as well as turbocharged cars
often use the electric fan as an after run cooling system
to lower under hood temperatures after engine shut down to
minimize cooking the under hood components. That's tough
to do with a mechanically driven fan. It's a real battery
killer though if you do a lot of short trip driving.

My guess would be that more fuel savings comes from the
improved aerodynamics rather than the net HP improvement
but both are a fuel saving factor.

Chumley

 

[partizan]

92xj, you state that electric cooling fans are beginning to become popular in jeep circles. Electric cooling fans have been somewhat popular on extreme 4x4's for a while because of the ability to manually turn the fan off before entering deep water or mud so as to avoid damage to the fan blades, radiator, etc.

 

[92xj]

Partizan, the clutch-to-electric conversion has been popular before, but there seems to be a fresh wave in the recent few months – or maybe it’s just a function of the market segment I’m in touch with. In the previous waves, they usually seemed to be followed by waves of un-conversions as the electrics failed to keep the engine sufficiently cool. I think this latest wave might have been triggered by the increased availability of high-CFM aftermarket fans.

As you say, there are other reasons for an electric besides the HP gain, the big one being the ability to turn it off in water/mud. Also, some like to have the manual control so you can do things such as “pre-cool”, i.e., turn it on in advance of a high temp usage period, or “post-cool” after turning the engine off (as Chumley mentioned). These are all good and I’m not questioning that, just the claims of more HP and more fuel economy. (Incidentally, HP and fuel economy are not paramount concerns in the off-roading world, many would be willing to give up some here to achieve better cooling.)

But it sounds like the consensus here is that there is a possible gain. I’ve read some of you guys other posts in the archives here, so your opinion carries weight with me – now help me understand why. First, to what extent am I right that there’s a power loss converting belt motion to electricity and then back to fan motion? Is it just a 1-2% loss or is it significant?

Second, how does a clutch fan work during what Greg calls a “hot idle” and “high speed cruise”? I’ve been making the assumption (admittedly dangerous) that the clutch fan was well designed to run only as needed. I guess you’re saying that since the clutch fan is viscosity-driven and the engine is hot, the fan is causing significant drag during these periods when the air flow really isn’t needed? And that the clutch fan doesn’t cut out at high speed cruise? (And why wouldn’t you need the air flow at hot idle? Isn’t that the classic stop-and-go traffic scenario for overheating?)

Third, greg, can you expand on “direct improvement in efficiency because the fan to shroud clearance is better controlled” – I don’t follow that. Are you saying (like I think Chumley was saying) that you can better match fan size to radiator shape?

The radiator on these jeeps is 11-inches high by 32 inches long, and the stock setup is a clutch fan covering about one half of the radiator and an aux electric fan covering the other half that comes on as needed. However, one of the popular conversions is to put a 16-inch electric in place of the clutch fan, with a large shroud with an 11x16 opening. What does this do to the flow when the lower 5 inches of fan is deflected upward by the shroud? And what is the efficiency difference between cooling one half of the radiator a lot and nothing for the other half (one fan running) versus having both halves cooled a moderate amount (two fans running).

And, now the final question (Greg, I gather from your other posts that you work for one of the makers, or maybe a major supplier?) – with all that is to be gained, why didn’t Chrysler put in electric fans OEM? Maybe I put too much faith in the car companies, but I always tend to be skeptical of huge performance boosts in modern cars with aftermarket additions. I think SUVs are considered trucks rather than cars for CAFE but isn’t there a mandate on truck fleet MPG as well? And even if there isn’t, it must be a major selling point.

Thanks for the knowledgeable replies, you guys are great.

 

[ivymike]

Third, greg, can you expand on “direct improvement in efficiency because the fan to shroud clearance is better controlled” – I don’t follow that. Are you saying (like I think Chumley was saying) that you can better match fan size to radiator shape?

If the fan shroud and fan are always in the same positions w/respect to each other (ie single subassembly) then you don't need to give the fan extra clearance to take up tolerances. Also, if they're mounted relatively rigidly to each other, you don't need to allow clearance for engine-radiator relative motion. Having less clearance between the fan and the shroud means that there will be less airflow squeezing past the tips of the fan blades, etc., and the fan will perform better.

 

[Chumley]

92xj: I'll save Greg the effort.

"You get a direct improvement in efficiency because the fan
to shroud clearance is better controlled."

Most mechanically driven fans are mounted on a driven
pulley which is mounted on the engine when the engine is
longitudinally mounted. The radiator is usually mounted
on the body front crossmember. The engine (and fan) move
around an inch or so either way off their original center
line when torque/over run is applied to the engine mounts,
causing the fan to move within its shroud. The fan to
shroud clearance must allow for that movement. Not so when
an electric fan motor(s) is mounted directly on the fan
shroud which is directly mounted to the radiator.

Electric fan blades can also be designed weaker because
they are subject to more constant speeds and more gentle
accelerations established by the electric motor that drives
them rather than the regular accelerations/decelerations
(based on engine rpm) that mechanical fans must tolerate.

More than once, I've seen mechanically driven unshrouded
metal fan blades separate, cut radiator hoses, and then
poke holes through steel hoods durning an over rev on
missed shifts or during a sudden traction loss.

Chumley

 

[Andy330hp]

As for the efficiency questions:

I think Greg was the one I heard this from in fact...

IF you assume the same blade design, then yes it is less efficient to belt drive an alternator which converts mechanical energy to electric energy which powers a motor with an efficiency of it's own, versus directly belt-driving a fan. I do not know the percentages, Greg will. BUT:

A large portion of the time, with an electric setup, you are not expending any(!!) energy becuase the fans are turned off. Going down the highway for example. Expending zero energy is the most efficient of all! There's your economy. You hinted at it in your initial post. Mechanical fans are sized for maximum requirements, so a huge percentage of the time they are moving way more air than required. Electric fans can take advantage of this by shutting down.

 

[shabba]

Although there is little more to add with regard to 92xj's original question, except perhaps that no-one has mentioned possible reliability concerns, I thought it was worth mentioning that some automotive suppliers are looking seriously at moving more and more ancilliarys away from the FEAD in favour of remote electrical mounting.

I believe this is mainly driven by the trend towards hybrid and stop/start systems becoming more fashionable, however I have seen that a few are looking seriously at driving the water pump electrically on 'standard' engines. The thinking behind this is that obviously an IC engine requires cooling across a wide rev range, but specifically in low speed stop/go traffic situations. This means the water pump has to be sized in order to satisfy the cooling requirements of the engine at the minimum speed. Also, as the pump is invariably of a centrifugal design, the power absorbed increases roughly with the cube of the speed increase (affinity laws) and therefore much power is wasted at higher engine speeds.
Hence it is beneficial to have a smaller pump running at a (more) constant speed. I'm sure this is also the case with fans, and the use of multi-speed fans or sets of two fans will increase efficiency further.
This principle has also been suggested for oil pumps and such, however, the effect of reliability here has made this a risky application.

In short, if you can control the timing and speed of an ancilliary device independant from that of the engine and in relation to demand/requirements, you will invariably reap the rewards of improved fuel economy, power, noise, versatility (wrt location), and in some cases weight.

I will try to find some links to information on the internet and post them here when I do. For the minute, as I can't find any, you'll have to take my word for it!

Discuss.

 

[SBBlue]

I have a 1975 Chevy PU with a 350 engine in it.

Several years ago I started wondering about the issue of rigid fans vs. clutch fans vs. electric fans.

I decided to experiment, and took off the fan entirely.

In two years I haven't been able to notice any difference at all in the engine temps. The only time I had heating problems was when I had a leaking radiator hose. My driving is typically in the city, but includes periodic trips on the freeway.

I will confess that I haven't tried pulling a full load up a steep hill at 60 mph when it's 110 degs outside, though.

 

[GregLocock]

Just a few loose ends then!

Electrical Efficiency-

Here's the numbers Andy-

Alternators are around 40% efficient, motors are 60%, and we replaced one engine driven fan that consumed up to 5 hp with a 400W rated motor fan. So, at worst the electric fan is using 2 hp at the belt.

In practice it spends very little time at high speed (1%>>). The rest of the time it is at 1/2 power(~1%),1/4 (~10%) or off. When we first installed it it had PWM drive, ie we could match the speed exactly to the cooling load. This was great fun to play with (I was the calibrator on this) but useless in practice, most of the time in reality the fan is just turning over to keep the air going the right way. What I did was to use the engine coolant temperature to drive the pulse width - and at idle the fan would just kick over. A very odd thing to see.

Shroud clearance

Chumley's right (as usual)

Your engine is flexibly mounted. Typically it will move by about 0.6 inch in each direction (can be more higher up). The accuracy within which the engine is placed relative to the radiator as installed is probably no better than 1/2" (certainly 6mm would be a big ask)

So you HAVE to have 1.1 inch radial clearance between the fan and the shroud if you have an engine mounted fan.

Typical clearance in an electric fan assy would be 3mm.

I don't know what effect that has on efficiency in actual numbers, but tip clearance is a much discussed topic when you are designing them.

I don't know anything much about how viscous fans operate, I've never had to play with them. In hot idle our electrofan runs at full speed for 30 seconds max, then drops to half speed. I am pretty sure that viscous fans are not that responsive.

Reasons not to fit an electrofan at OEM - cost, reliability, electrical load. If the OEM isn't pushed for fuel economy (in particular) on that model, why bother? If you /are/ pushed for fuel economy then the fairly trivial on-cost for electro fans (20-40 bucks at a guess) is a cheap way of buying mpg. 1 mpg is a BIG challenge in the world of fuel consumption, we'd probably have to spend several hundred dollars to get that.



Cheers

Greg Locock

 

[92xj]

Thanks to all, very informative. Also dangerous, from a time consumption point of view. You got me started now, I've been googling on various terms people mentioned and finding all sorts of interesting stuff. Electric water pumps, dispensing with the whole FEAD approach (thanks shabba, post those links if you find them, I found an interesting although probably pretty basic paper here

http://www.infineon.com/cmc_upload/0/000/011/088/APEC2000.pdf),

42-volt electrical systems, maybe a direct-drive generator.... Off-road guys love to customize their vehicles, and this looks like a whole 'nother place to start customizing

I'm going to keep looking into the fan issue, I'm stubborn so it will take me a while to become convinced. Ya'll have persuaded me on a few points and opened my eyes to some others, but there are still a few nagging questions I need to research more - I rarely post but I lurk here fairly often so any other comments or related subjects would be most welcome.

 

[turbododge]

I messed with the electric fans on the twin turbo Dodge for a long time (over 10 years) before going to the viscous clutch, engine driven, setup I have now. I won't be going back to electrics, especially with the turbos. Good electrics pull at least 30+ amps to cool a V8 with a normal size radiator that was designed for a driven fan. If you don't upgrade to a very big output at idle alternator, you will lose voltage at idle, heat up more, lose more voltage etc, until the ignition shuts down and you sit. If the car was originally designed for electrics, they seem to have much larger radiators, and do much better. My radiator is only 16 by 26 inches fin area.

The one major thing that I noticed as soon as I took off the electric fans, was that I was not chasing my mixture settings very much anymore. With the electric fans, when you sit still, but are not up to fan on temp, the underhood temp goes way up. This can very adversely affect you mixtures and jetting requirements until you have moved enough to cool the engine compartment back down. When I changed to the viscous fan, my max engine compartment temp dropped by almost 100 degrees F.

There have been a lot of shootout tests on electrics vs viscous vs flex fans vs fixed fans and they show that the viscous fans give up very little (1 to 2 hp) to electrics on power as long as they don't run the fans off of a battery only during the tests. I have felt no loss of power, and actually notice an improvement from stoplights because of the cooler underhood/intercooler temps.