A/C fan speed effect on MPG?

[overst33r]

Coming back from a trip a few states away, the low fuel light came on on the hwy, shortly after, the car started feeling hot. I noticed that it was blowing significantly less cold air than before. The A/C was still on, but barely coming out of the vents, even on the highest fan speed setting.

When we filled up with gas again, the car returned to normal operation. Turns out the car retards the fan when the low fuel light comes on.

My question is, how much effect does the highest fan speed have vs the lowest one? I'm assuming it's less than just shutting off the A/C... In that case, why didn't the car just shut off the A/C if it wanted to save gas? Maybe to maintain at least some comfort?

Car is a 2008 Acura TL-S. FWIW, managed 29mpg on the way back from TN to FL. I'm sure that's probably at least 10% higher than actual, but not bad.

 

[LTC8K6]

Turns out the car retards the fan when the low fuel light comes on.

Sounds odd to me. You'd shut off the compressor if you wanted to save fuel, not the blower motor.

 

[overst33r]

Sounds odd to me. You'd shut off the compressor if you wanted to save fuel, not the blower motor.

That's what I thought. Just found it odd... :\

 

[jaydee]

I noticed that it was blowing significantly less cold air than before.

It probably cut some of the load off the A/C, just like it cut some of the load off the blower. If the A/C load was still maxed out, the air probably would have been colder at low fan speed.

 

[LTC8K6]

Good point. If the air was warm, then it probably did shut down the compressor.

 

[KIAman]

Although I am making a wild assumption, I think this is normal behavior and it makes sense. A/C is almost considered a critical occupant component these days. Failed A/C systems could cause bodily harm to passengers so it makes sense to attempt to keep it up at all times.

Turning down the A/C fan has the effect of lessening the heat exchange rate which makes it easier for the compressor to re-compress the expanded refrigerant. Basically, the less heat introduced into the refrigerant via cooling the interior by whatever means (lower fan speed for example), means the less mechanical energy required extract the heat away. Also, this lessens the load on your electrical system which leads to the alternator not requiring as much mechanical energy.

Less mechanical energy required means less siphoning of mechanical energy of the engine which leads to better mileage.

 

[T2urtle]

i've always wondered too... the main thing is the compressor is on so there is already a LOAD on the system so obviously MPG gets lowered. But i dont think fan speeds should effect anything. the system should be a 12v system, and the fan selector switch is a resistor holding back the power. ideally 12v is always going to it, just pulling out numbers it uses 3v on low, 6v 2nd setting and 12v high setting. i dont think having the AC on low or high can really add up to anything more then .5 MPG (if that at all) since 12v are always going in.

only thing i can think of is your AC system when into ECON mode because it reached the temp you wanted it to. And when you went to fill up you opened the door letting hot air in so when you got back in to start her up the AC system realized its not at the temp you wanted to it went to full blast to get there.

 

[KIAman]

i've always wondered too... the main thing is the compressor is on so there is already a LOAD on the system so obviously MPG gets lowered. But i dont think fan speeds should effect anything. the system should be a 12v system, and the fan selector switch is a resistor holding back the power. ideally 12v is always going to it, just pulling out numbers it uses 3v on low, 6v 2nd setting and 12v high setting. i dont think having the AC on low or high can really add up to anything more then .5 MPG (if that at all) since 12v are always going in.

only thing i can think of is your AC system when into ECON mode because it reached the temp you wanted it to. And when you went to fill up you opened the door letting hot air in so when you got back in to start her up the AC system realized its not at the temp you wanted to it went to full blast to get there.

Voltage is measure of potential, not energy. The equation is V=IR so given same V (volt) but increased R (resistance - to slow down the fan), then I (amps aka current) must proportionally go down.

 

[soydios]

Voltage is measure of potential, not energy. The equation is V=IR so given same V (volt) but increased R (resistance - to slow down the fan), then I (amps aka current) must proportionally go down.

Lower voltage over the same resistance equals lower current. Power = volts * current = (current^2) * resistance = (voltage^2) / resistance, so lower voltage and lower current equals lower power equals lower fan speed.

 

[Meghan54]

Your compressor got cut off which is why it blew hot air. That's a function of you getting your low fuel light turned on....you had, according to the vehicle's computer, a couple of gallons of gas left, so to maximize this, it cut the compressor off to save fuel. Turning the fan to fast or slow had no impact at all in the amount of fuel you'd be burning...its load is insignificant as compared to the load the compressor puts on the engine.

Very few thing make your A/C blow hot.....compressor not running and very low Freon levels. Since your A/C returned to normal function after you filled up with gas, it was the former, not the latter.

Rather simple.

 

[StageLeft]

Failed A/C systems could cause bodily harm to passengers

??

 

[angry hampster]

??

.

Letting r134a into the cabin perhaps?

 

[Meghan54]

.

Letting r134a into the cabin perhaps?

No, he must be driving one of those hermetically sealed cars with windows that don't roll down.

But even with vehicles that have automatic temp controlled cabins, you can turn it off and turn off your A/C and *gasp* roll down your windows, just like people used to do back in the "olden days".

 

[marvdmartian]

I know I've driven work trucks that will cut off the A/C blower fan when you are accelerating heavy. Once you let off the gas, it cuts back on again.

Likely the same idea. :hmm:

 

[LTC8K6]

I know I've driven work trucks that will cut off the A/C blower fan when you are accelerating heavy. Once you let off the gas, it cuts back on again.

Likely the same idea. :hmm:

Pretty much all modern vehicles do that. This has been true for a while since my 95 Taurus does it.

 

[BUTCH1]

Pretty much all modern vehicles do that. This has been true for a while since my 95 Taurus does it.

Yup, the control module will de-energize the compressor clutch at WOT, which is a good thing, especially if you driving a '95 Escort, I used to call my AC switch an extra E-brake!..

 

[jaydee]

... But i dont think fan speeds should effect anything. the system should be a 12v system, and the fan selector switch is a resistor holding back the power. ideally 12v is always going to it, just pulling out numbers it uses 3v on low, 6v 2nd setting and 12v high setting. i dont think having the AC on low or high can really add up to anything more then .5 MPG (if that at all) since 12v are always going in.

Hopefully, by 2008 Acura wasn't still using the resistor array to regulate blower speeds (as been done for ages). I don't have access to that sort of information for Honda, but I do for GM and know that they've moved on to a more efficient pulse-width modulation method.