Did a search on this topic, could not really find anything too exact.

I've had a long history of driving my cars with just the air fan on, not with the a/c. I guess its always been something of a comfort level with me, I just seem to think/feel that it keeps the airflow in my cabin fresh and what not. I was just wondering if anyone knows if it has any effect on my vehicles performance at all? I mean I know using a/c will affect mpg's but how bout just running the fan? Does using that little bit of electricity sap engine power? I've been too lazy to run a real-life full tank test so I thought I'd ask around.

P.S. Funny enough, when I get below 10miles on my fuel range meter on my '05 MCS, I have this incessant fear that if I don't turn my fan off I'll run out of gas quicker.

 

Running the fan will not affect you MPG because it's just electric. If using the A/C, the engine needs to turn the compressor -> more force needed to get the car going -> more MPG.

 

Theoretically, running the fan will reduce mileage as the alternator will have to work harder and thus put more drag on the engine. However, the difference will be so slight as to be unnoticeable.

 

I do this all the time as well, but mostly because my car is livelier without the drag of the A/C compressor, I've noticed virtually no difference in the mileage. You shouldn't run your tank down to the last drop anyway, you never know what you might suck up out of the dregs into your pump or filter.

 

I always find this statement funny. If you were to pull (most) in tank fuel pumps, you would realize that the above statement really makes no sense. The fuel is always sucking fuel from the bottom of the tank, regardless of the fuel level. So wether there is a full tank or bone dry tank, it makes no difference unless you assume the debris is floating on the surface...not likely. Also, the pump sits in a closed "well" within the fuel tank (in most BMW's, I have never pulled the MINI's pump). This "well" has a small hole at the bottom to let fuel in and out very slowly to allow the fuel in the well to equalize with the fuel in the tank (or surrounding the well). the purpose of the well is to avoid fuel gauge fluctuations as the fuel slooshes around in the tank. So, assume low fuel level causes an increase in the disturbance of the debris in the in the bottom of the tank (why would it be there anyway?), it is still not very likely that any more significant amount of debris would make into the well than it would normally from the constant motion of the fuel within the tank.

Another myth is that low fuel may starve the pump of lubrication and cause early bearing failure. This is not the case because the fuel pump sit high in the tank anyway. The fuel passing through the pump is what lubricates and cools the pump, not the fuel that surrounds the pump.

So, do not worry about driving with low fuel, no harm. Just do not ever run out of fuel as you will definately damage the pump in doing so.

 

I guess I'm old school, when the pickup point was not in the bottom of the tank and fuel pumps were mounted outside the tank - like on my '69 XK-E.....

But your points are valid, the crap would normally be in suspension from sloshing around the tank.........

 

I run mine down to [- - -] all the time. (range till empty)

 

You ask a very interesting question. The answer is yes! All electrical loads contribute to a greater parasitic load on the engine; the effect is cummulative. When the charging system is required to produce more current, more effort is required from the engine to turn the alternator as the magnetic fields inside the alternator becomes stronger to generate more output. During times of heavy electrical loads i.e. wipers, rear defog heat/ac headlights, fog lights, driving light, stereo blasting you will definitely loose a few horses and MPG will suffer!

 

True--but, Click and Clack, the tapett brothers from Car Talk, said running the radio and other electric items (not the A/C) would only be a minimum drain on the engine and reflect in a very small loss in MPG--sorry, they didn't give a number. But you are right, the result is cummulative and will accummulate.

 

Well,

Thanks for the input. I guess I can say that running the fan only has a very slight effect on overall performance. However, the more its combined with other electronic devices(lights, wipers, etc.) the more it effects performance. So while its detriment is barely noticeable I guess there was something to my nagging suspicion.

 

Yeah thats what I do because I hate the loss of power with the AC running. One thing I did notice which was kind of shocking is if you open your hood, being in a quiet garage, and flick the fan on from completely off and you can hear the engine taking some form of load. I was surprised I didn't think it would be noticeable in any form (tho I highly doubt anything could be felt powerwise)

 

I was always under the impression that the altenator was a constant loss due to the fact that it's not clutch regulated like the a/c compressor. So wouldn't you be able to run everything and not lose additional power as it's already running ,charging the battery?

 

It's true that the alternator isn't clutch-controlled, but the greater the current being drawn from the alternator, the harder it is to turn. This increases the load placed on the engine by the alternator.

 

At the end of the day, power can neither be created or destroyed; the best we can hope to do is convert one form of energy into another (this step is never 100% efficient). The alternator accomplishes this task by converting mechanical energy into electrical energy; the more electrical energy required the more mechanical effort will be required.

 

[FONT=Calibri][SIZE=3]The alternator is a constant load does not make the engine run any harder as you increase the load. The alternator has two parts the stator (which is fixed) and the rotor which spins free in ball bearing and this is what the engine spins. As one increase the load by adding lights, radio, air condition the initial increase in current demand is draw from the battery. Then the alternator produces enough current to maintain the battery charge as the load increase. [/SIZE][/FONT]

 

And when the alternator's current output increases to handle the additional electrical load, it becomes harder for the engine to turn it.

Here's a relevant YouTube video. It's pretty cheesy, but starting around the 1:10 mark, the demonstrator shows how it requires more force to turn the generator as more electrical load is placed on it. Granted, it's a generator and not an alternator, but the principle is the same (generating current by moving a rotor inside a fixed stator).

 

You mention the mechanical operation but left out the part that cannot be seen: the electro-magnetic phenomenon that occurs as the rotating mass turns!......Like I said before....it is not possible to set up a charging system and then extract an unlimited amount of energy from it without requiring more effort to turn the alternator........if this was true then a single system would be able to power however many loads for the same effort......such a miracle system does not exist..........................

 

Gotcha!