Radiator fan power draw on highway?
 

Hello everyone, I am wondering if anyone has tested to see how much power electric radiator fans draw while driving down the highway compared to while stopped.

The reason I am asking this is because a lot of cars (mine included) unnecessarily run the radiator fans whenever the AC compressor is running, even at higher speeds where there is plenty of natural airflow to cool the radiator and condenser without the help of the fans. This potentially wastes a significant amount of power and runs the battery down faster than necessary for those of us who disable our alternators.

As an example, my car has dual electric radiator fans that draw around 23 amps while on. 23A X 14V = 322 watts. Assuming a standard alternator is 50% efficient, that's a draw of ~644W from the engine, which is equal to about 7/8 of a horsepower. Assuming the AC compressor runs at a 50% duty cycle on the highway (which is often pretty close to accurate on my car depending on weather and fan speed), that averages out to a constant load on the engine of about 3/8 of a horsepower. Although not crazy high, that's definitely high enough to be worth figuring out a way to eliminate. People do much crazier things for much less gains!

I have a few ideas of how to automatically disable the fans at high speeds, but I would like to know what the power draw is at higher speeds before implementing such a system because I suspect that the draw may be less while driving at high speeds since the air being forced through the fans would theoretically reduce the load on the motor by helping turn it like a windmill, but I am not sure how much the draw would actually change.

I am going to be testing the power draw at different speeds when I have time and will report back, but for now I am wondering if anyone else has thought about this or done any testing.

My understanding is the fans are driven on by a temperature sensor somewhere in or near the peak temperature spot in the cooling system. Ditto for multi-speed. When you have the A/C on you are dumping hot air into the cooling systems input effectively making it a very hot summer day. My golf reduces engine power if it gets too hot after measuring intake temps. If you think there's a benefit to killing the fan, just disconnect one at the fanhousing power connector and measure the change. Those connections are made to come apart.

On the other hand the F250 fan mostly idles,BUT it has 8 gallons of coolant and is overcooled for most operations below 1/2 throttle

I would have to agree with this. Even if it's a series-wound motor - with no set top speed - it will still lighten the load. How much is going to depend on how much the air is slowing down going through the radiator and pressure differences...the only answer, as you are already intent on finding, is to test it.

Thanks. On my car the fans are activated by a temp sensor in the engine block when the coolant reaches 207 degrees or whenever the AC compressor is engaged regardless of coolant temp or speed to cool off the condenser. Problem is that the fans don't need to be running while driving down the highway as there is already plenty of natural airflow to cool the radiator and condenser.

Thanks. I think the only accurate way to test this will be to cut and temporarily extend one of the fan power wires into the cabin so I can get my amp clamp around it and monitor the amp draw at different speeds, then multiply by 2. Since I moved my battery to the trunk and I ran the positive cable behind the trim in the cabin my initial plan was to unplug the alternator and monitor total current draw from the battery at different speeds and work out the difference, but I decided that the fans need full, consistent alternator voltage for the test to be valid since their current draw will vary depending on the voltage supplied. I will report the results as soon as I have time to test it

On thr highway, if it's an electric fan it should be 0.
Sounds like you need a switch to disable the fan above a certain speed.

A couple thoughts.

One, the faster an electric motor spins the less electricity it consumes. So if it is turning on at highway speeds there is a very likely chance it will use less electricity than at a stop.

Another thought is that more airflow through the condenser could, in theory, help the compressor use less energy to compress the refrigerant. So now not only do you need to actually check and measure the power comsumption of the fan, it would be a good idea to measure the new power consumption of the compressor. Of course there are many factors that could play a part in the results.

That is a great point, I have thought about that as well. Poor airflow through the condenser definitely would decrease the system's efficiency and performance. I have no way to measure the compressor's power usage, but what I could do is set up a thermocouple to measure the temp of the liquid line with the fans on and off at different speeds. This would tell me at which speed running the fans no longer does any good. Measuring head pressure would do the same thing, but measuring temp would be easier in my case.

No need to disable the fans.

If you want to know if your electric fan is running just do what I did. Connect a 12V LED running parallel to the ground wire coming from the fan. Ground the other end of the LED and place it in a area where you can see it on the dash or instrument panel. The LED (fan) will come on when the engine gets hot, the a/c is on, the defroster is on and possibly when the re-circulation button is pushed also.

The LED will also come on (dimly) when you're driving fast enough for the fan to free spin and generate enough power to light the diode.


Hope this helps.


:turtle:

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highway
 

Above 40-mph, ram-air should be sufficient for all engine cooling needs.
I drove 100-miles with no fan whatsoever in my grandad's Dodge pickup. All highway.

Update- I extended one of the fan power wires into the car temporarily so I could measure current draw, but I was not able to because my alternator died (while stuck in a bad traffic jam in the middle of nowhere 160 miles from home. Good times). Needless to say I was more concerned with getting home before the battery died than the power draw of the fans. I could test the draw at different speeds now, but doing so at less than normal voltage is pointless. Testing will resume once I replace the alternator.

So I finally replaced the alternator and went to check the draw but for some reason my amp clamp won't work, I think the batteries may be dead. But either way I feel pretty confident that the draw is significant because the voltage momentarily drops below 13V when the fans come on even when driving 70 and the blower fan speed momentarily drops as well, which tells me the draw is pretty significant. And with the alternator off, forcing the fans on while driving down the highway drops the voltage by a couple tenths of a volt. I will report back when I have a number.

I finally have a solution though. What I did is I replaced the 2 position fan override switch I had with a 3 way switch so I can select off, auto, and on as I see fit instead of just on and auto. Normally the switch stays in auto (the middle), but I turn it off when on the highway. Not sure about any fuel savings, but I feel confident that the electrical load was reduced because I no longer experience that random voltage drop when driving down the highway as the AC compressor/fans come on. If anyone wants a diagram of how I did this let me know. I'm not thrilled with the solution of having a manual switch to shut the fans off since it's one more thing to think about and remember, but life goes on.

Of course it's going to sag. Motors have huge starting currents relative to running current. All inductors exhibit the same characteristic including the A/C clutch. Look at the current after the fan is up to speed.

You're right. But my thought was since the fans should have already been spinning from the wind blowing through them, there shouldn't have been much of a drop on startup. The AC compressor clutch may be the same way, but it only draws a couple of amps instead of 20+ amps and there is no noticeable voltage drop when it comes on as long as the fans are off.

They may be spinning but I bet it isn't more than 1/2 the rated rpm.

I was finally able to get a measurement. At idle with the alternator off, I see a draw of about 16 amps for both fans. At 70 MPH I see a draw of about 9 amps from the fans and there is definitely a spike in current draw when they start up, but there appears to be less of a spike than when idling. This seems to indicate that the fans do spin while driving down the highway, but at less than full speed. Not sure how the alternator being on affects this yet, but there are definitely power savings to be had from shutting the fans off on the highway. Although the reduction in alternator drag would be impossible to see on my MPG by itself, it will significantly save my battery when driving around with the alternator off and possibly extend the life of my alternator as well since it won't be working as hard.

The main wear point to an alternator is the friction contact to the commutator by the brushes which wont be reduced by load, followed by bearing drag. There is some arc erosion, but it seems insignificant with a wise choice of brush carbon composition

The fans will run at whatever their predetermined speed is, typically there are two speed settings. The current draw is due to the different amount of work the fans are doing.

As the fans are being assisted by the flow of air forced through the radiator, it reduces the power requirement for any given fan speed.

I didn't read the entire thread, just cruising through so I apologize if I missed it. One possible fix for keeping the A/C from running the fans would be to have a secondary relay that interrupts the circuit below a set speed. Or even a separate fan controller at a different temperature monitoring the hot side of the refrigerant.

I didn't know that, thanks for the information. It just seemed like less load and heat would be better for the alternator's longevity, but it sounds like any differences would likely be negligible.

My car has a very basic 1 speed fan setup that runs at full speed whenever the AC compressor is engaged or whenever the coolant temp gets to 207 degrees.

I did add a secondary relay to shut the fans off with a manual switch above around 50 MPH, but it would be nice to have some type of automatic system so I don't have to think about it and so there's no chance of me forgetting to turn the fans back on, but I'm not sure how I would do that.

The Dakota Digital fan controller has an option to shut the fans off above the speed the user selects. They recommend disabling the fans above 35 MPH. But that setup would also cost $230 for the controller and the separate module and it's hard to justify spending so much on something that would make so little difference, even though it would be cool and fun to play with.

Never tried, BUT, a bolt on electric hot water heater controller can be had for $20 and is decently adjustable from about 100f, and should handle the switching loads

If the goal is extending the duration of leaving your alternator off, perhaps money spent on providing additional energy would be better?

It's one of those situations where you've created a "solution" that requires more solutions.

My goal is to reduce the overall power use for slightly improved MPG when the alternator is on and more range/less battery drain when it is off. Not running the fans when they aren't needed would go a long way towards saving battery power. I'm not particularly concerned about running my battery dead as I have an Optima Yellowtop and I don't usually drive far enough from home for that to be a concern, but using less of the battery's capacity will save a few cents on my power bill and should help to prolong its life, which is a big consideration when a comparable replacement AGM deep cycle battery is nearly $300. My battery is going on 8 years old and I hope to not have to replace it for a while.

Adding a secondary battery would also add significant weight and take up more space in my trunk, so reducing power consumption is a better solution than adding more batteries even ignoring cost.

I believe there is a way to rebuild an alternator to be far more efficient, fyi. I would think the easiest way to reduce electrical load is to just unplug one of the fans, no?

Thanks for letting me know, I never heard of that. I just know that alternators are generally only about 50% efficient. Do you have a link to any information on that?

Unplugging one of the fans wouldn't work, I live in the south and we see 100+ degree days here. The engine cools fine with 1 fan, but the AC doesn't work well at idle and low speeds with only half of the condenser getting cooled off and the compressor drags the idle down noticeably more than with 2 fans. The extra power the compressor draws from the higher head pressure would almost certainly outweigh any efficiency benefit from the power savings as the compressor uses much more power than the fans.

I wonder how you could make an alternator more efficient...

I'm trying to remember how they usually work. If I remember correctly the voltage regulator shunts current through the rotor which cause an alternating current in the stator (or is it vise versa?) In any case, if you could use thicker wire without reducing the number of windings, that would help.

Now if the rotor's current is controlled by means of resistance you could instead control current by means of pulse width modulation instead to help increase efficiency. A permanent magnet rotor could be even more efficient, but then you'd have to think of another way of controlling voltage and current.

The rectifier is another point of energy loss. Using more efficient diodes may help. But more efficient diodes may not tolerate as much current. So lowering max current may be necessary.

Lowering voltage can also help by not charging the battery up to the point that it's self-discharge also raises. Although in theory your alternator could be more efficient if it ran at higher voltages/less current (more turns) and feed a higher voltage battery. Of course then there'd be DC to DC losses from the higher voltage down to 12V unless you completely rewired everything to use the higher voltage, including modding the ECU and any other place where voltage is stepped down for use in electronic chips and also your have to build your own ignition coils. But all this would probably not be worth the trouble.

Also, I don't remember if the rotor is wye wound or delta wound. A wye wound rotor would be more efficient, but the voltage and current would be different without changes in wire thickness, length and number of turns.

Another area of loss is the housing and rotor construction. You want them (or at least the housing) to be made out of very thing lamanates that are electrically isolated from each other made of an iron alloy with a very low hysteresis.

Of course if you have a lot of stop and go traffic, maybe making the alternator come on only while decelerating would help.

https://ecomodder.com/forum/showthre...tor-38719.html

There have been several threads over the years

Thanks, that is an interesting idea. As concerned as manufacturers supposedly are about fuel efficiency, I wonder why they don't use higher efficiency alternators such as permanent magnet. There must be a reason, I just don't know what it is. Perhaps they wouldn't be reliable in such a harsh environment or something. I remember reading that there are ways to build alternators with 80-90% efficiency, but their downsides such as reduced output and reduced reliability in harsh conditions make them unsuitable for automotive use. I'm sure cost has something to do with it also, but there must be another reason.

If an alternator drags down the fuel economy by 10%, half of which is inefficiency, if you made it 99.9% efficient then you'd only increase overall efficiency by about 5%.

I do question whether alternator technology really hasn't improved over several decades. Are modern alternators really only 50% efficient?

As far as the battery goes the lead acid battery is tried and tested. You can make a lithium battery work. But now it needs thermal management, both cooling and heating, plus electrical management because they need to be balanced and if you leave your lights on you don't want a lithium ion battery discharged below it's threshold. Not to mention lithium ion can't be used in a 12V system, so you'd have to use another voltage and you couldn't use common 12V parts.

info
 

'Improving Alternator Efficiency Measurably Reduces Fuel Costs', by Mike Bradfield, MSME, Remy Inc. ( DELCO REMY ), 2008
* starting with fuel to the engine at 100%
* the engine is 40% efficient
* the serpentine belt/pulley is 98% efficient
* the alternator is 55% efficient
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From AMCA / ASHRAE:
* the electric radiator fan motor is 92% efficient
* The airfoil shaped cooling fan is 90% efficient
-------------------------------------------------------------------------------------
From the gas tank to the air actually moved by the electric fan, total efficiency is 17.3 % ( 82.7% of all the energy is lost )
------------------------------------------------------------------------------------
If the fan is running at all, at 40-mph or above, something's wrong.

It would seem to me that if a 50% efficient alternator drops MPG by 10% and you could improve that efficiency to 99.9%, the alternator would then use only half the power from the engine and it would only drop the MPG by 5%. Could be wrong though.

Obviously making an alternator 99.9% efficient wouldn't be possible, but if the efficiency could be improved from, say, 50% to 80-90%, that could still show a measurable fuel savings and be worth doing depending on what the drawbacks are. My theory is since at least to the best of my knowledge the EPA's MPG test cycle doesn't include accessory use, the gains on the test cycle wouldn't be significant enough to be worth the cost or potential downsides.

Of course in the real world the efficiency of an alternator would become more important the more load it's under. It's not too uncommon for an alternator to be under a 700W load on more modern vehicles (50A X 14V). If the alternator is only 50% efficient then 700W of power is being wasted as heat, which I would consider pretty significant as that would require almost 1 extra HP from the engine to overcome the inefficiency.

Mechman alternators use a 6 phase design that they claim is more efficient than a standard 3 phase alternator by somewhere around 10-20% if I remember correctly. I put one of those on my buddy's Bronco because he has electric fans and a big sound system and it does run noticeably cooler than the stock alternator, but I can't say for certain that it runs cooler because it's more efficient as it probably has a better cooling fan also.

Thanks for the correction! I think I meant to put 75% and then put 99.9%.

alternator efficiencies
 

From online:
* typical efficiency........................................ ............ 55% - 60%
* high-efficiency........................................ ............... 68% - 75%
* excited rotor type.............................................. .... up to 96%

So does that mean that simply switching the roles the stator and the rotor play would increase efficiency? In other words, put the field in the stator housing and excite the rotor instead?

There was a thread a long time ago over here. A guy with an SUV type american car (greenish if i recall) was looking at alternative diodes for the rectification circuit.

Pretty sure to switch the stator and field requires rewinding at least the stator with smaller diameter wire to get more amp turns. Could also need a different winding pattern, but my motor theory is a bit soft here.

Well that didn't take long, I already forgot to turn the fans back on. I noticed my AC wasn't cooling very well as I was pulling out of a grocery store parking lot and realized I forgot to re enable the fans. As soon as I turned the fans back on the AC started to cool. Hopefully I didn't damage my AC system, it seems to still work well so I think it's alright.

My system has a high pressure cutoff switch on the receiver dryer and the compressor has an over temp cutoff switch on it. I'm not sure if either activated or not, but either way I can't imagine it would be good for the system to overheat to the point of tripping a safety switch. Hopefully forgetting to turn the fans back on is a mistake I don't make again.

Worst thing you'll do is cause a leak and then the low pressure shutoff (which works REALLY well) shuts down the A/C to prevent system damage from lack of lubricant.

If this occurs often then I suggest a different operational mode: system default is on, manual override to turn off, can be done with a cheap $5 car relay set up to latch open

Yeah creating a leak isn't something I want to do. I completely rebuilt my AC system a few years ago and I don't want to damage it. I'm not sure what pressure the high pressure cutoff activates at, but I would think it would be significantly below the maximum pressure the system can actually withstand.

My fan switch's middle position is auto with down being off and up being on. I set it up with a normally closed relay that the switch activates when off to disable the fans. The problem isn't that the system malfunctioned, I just forgot to flip the switch back to auto when I got off the highway. That's why my preference was for the system to be automatic, but it doesn't seem that there's a practical way to do that, so the switch will have to due for now until I figure something else out.

One idea I came up with is to install an indicator light that comes on when the fan switch is off somewhere I would see it like by my Scangauge to remind me that the fans are off so I hopefully don't forget to turn them back on again.

Logically it's easy to do this automatically using common stuff. Attach an A/C thermostat, preferably old style manual, and use the ac side which operates backwards: when its hot the system switches on. Bit of experimentation to do the high point setting, but a pair of needle nose pliers works wonders and it will sink 20 amps through the mercury switch and it's adjustable through 50 degrees. The other direction is the hot water heater controller on the radiator I mentioned but it's a bit insensitive and somewhat non repeatable.