Gas mileage and AC question

[dxkj]

Co-worker and I had a discussion about this, but I have no clue about the inner workings of AC


1) If you have the AC on, and put your fan to 1 2 3 4 or 5 (fan speeds) does the AC use up the same amount of energy/reduce gas mileage as much at speed 1 as it does at speed 5? IE If you have ac on, is it any less efficient to have it cranked?


2) Would you get better gas mileage by cranking up the ac on 5, and then turning it off when you are cold, and then warming up slowly, and then cranking it again


Would probably be 1 minute on , 2 minutes off

 

[Saint Nick]

i don't believe so. i don't believe it changes how cold the A/C gets, it only controls fan speed.

 

[dxkj]

Originally posted by: jndietz
i don't believe so. i don't believe it changes how cold the A/C gets, it only controls fan speed.

My argument was that fan on high would cause the ac to warm up (liquid whatever), thus requiring it to use more energy to cool it down again.

 

[Budmantom]

That's a good question, does that car use more gas when the ac is cranked all the way up?



Tom

 

[nakedfrog]

We've had these threads before... no, it doesn't use the same amount of energy on every setting, IIRC.

 

[iamwiz82]

A 12V signal is sent to the fan with a resistor that slows down the fan speed, so no, it won't matter.

 

[FoBoT]

it doesn't matter

if you are hot , turn the A/C up more
if you are cold, turn the A/C down some


/thread

 

[radioouman]

The engine uses more gas when the outside temperature is hotter because the compressor gets harder to turn. Also, it has to keep the compressor turned on more to get the desire amount of cooling.
Fan speed has nothing to do with it.

And generally, you get better gas mileage with your windows rolled up and the A/C on than with the windows down.

 

[dxkj]

Originally posted by: FoBoT
it doesn't matter

if you are hot , turn the A/C up more
if you are cold, turn the A/C down some


/thread

stupid response... but i guess thats to be expected



I think cranking the AC on high for 1 minute and turning it off for a few is better for gas mileage, but what do i know

 

[dxkj]

Originally posted by: radioouman
The engine uses more gas when the outside temperature is hotter because the compressor gets harder to turn. Also, it has to keep the compressor turned on more to get the desire amount of cooling.
Fan speed has nothing to do with it.

And generally, you get better gas mileage with your windows rolled up and the A/C on than with the windows down.

That I have heard.... but luckily its too hot out to even consider


So if fan speed has nothing to do with it, I guess I go with the crank it up, turn it off, crank it up, turn it off method

 

[nakedfrog]

Originally posted by: radioouman
And generally, you get better gas mileage with your windows rolled up and the A/C on than with the windows down.

I forget who it was, but someone here at ATOT tested that on a long road trip, and found it to be false.

 

Originally posted by: dxkj
I think cranking the AC on high for 1 minute and turning it off for a few is better for gas mileage, but what do i know

The extreme wear and tear this would place on your A/C compressor would outweigh any real gas mileage benefits.

Which would you rather do? Replace your A/C compressor at 35,000 mi., or get gas a couple more times a year?

 

[JohnCU]

I'm confused. Are you telling me that, when you turn up the fan to it's max power, it's pulling more current, which is working the battery harder, which is working the alternator harder, and it's not contributing to gas mileage? Not even .000001%?

i don't know how cars work that well, so correct me if i am wrong.

 

[dxkj]

Originally posted by: JohnCU
I'm confused. Are you telling me that, when you turn up the fan to it's max power, it's pulling more current, which is working the battery harder, which is working the alternator harder, and it's not contributing to gas mileage? Not even .000001%?

i don't know how cars work that well, so correct me if i am wrong.

No..... Im saying that if you turn the fan up higher, it is forcing more air past , cooling more air off, and making the compressor work harder to keep it cool

 

[radioouman]

Originally posted by: jumpr

Originally posted by: dxkj
I think cranking the AC on high for 1 minute and turning it off for a few is better for gas mileage, but what do i know

The extreme wear and tear this would place on your A/C compressor would outweigh any real gas mileage benefits.

Which would you rather do? Replace your A/C compressor at 35,000 mi., or get gas a couple more times a year?

THat would not cause additional wear and tear on the compressor. The compressor has a clutch which is engaged and disengaged all the time while driving. This keeps your compressor from freezing up when it is 60 degrees out, etc. Also, when going up a hill, your compressor may disengage to allow more engine power to go to the wheels rather than drive the compressor.

 

[JohnCU]

der I can't read.

 

[C'DaleRider]

Originally posted by: radioouman
The engine uses more gas when the outside temperature is hotter because the compressor gets harder to turn. Also, it has to keep the compressor turned on more to get the desire amount of cooling.
Fan speed has nothing to do with it.

And generally, you get better gas mileage with your windows rolled up and the A/C on than with the windows down.

I don't know where you got the information for that last statement, but you are quite misinformed as it is pure and utter BS!

Not only does the A/C compressor put an additional load on the engine as it cycles, using more gas, but the increased electrical usage by the compressor and fan makes the alternator drag more, also increasing gas usage. And as long as you're driving along the highway, I suppose the increase in heat generation from the engine would have little effect except that many cars automatically turn on their cooling fans when the A/C compressor is engaged, so that also would increase electrical usage and burn a bit mroe gas.

Actually, Mythbusters took care of that myth on Nov. 10, 2004. They wanted to see if the computer model programs that predicted cars ran more efficiently with the A/C on vs. driving with the windows down and no A/C on was true. So, they used two identical vehicles, filled with indentical loads of gasoline and drove both on the same oval track at the same time at the same speed. The only difference between the two was one vehicle was using its A/C, one had its windows open. MYTH BUSTED!!!

The result? The open window vehicle completed over 30% more distance than the A/C-using vehicle did.......the A/C made that vehicle consume its load of gas much faster than the vehcile not using its A/C. Of course, anyone in the South could have predicted that.....gas mileage suffers terribly when using the A/C and that's from experience, not from some inaccurately conceived and executed computer modelling program.

 

[0roo0roo]

from what i remember its just the fan setting. the compressor or whatever always works at the same rate. air con doesn't really have settings..it cycles on and off at different rates. like the fridge. phase change is phase change i bet. i remember some older cars had a econo ac mode. all it did was cycle it on and off every X minutes. course doing this too rapidly would probably put bad wear on the system, so don't go nutz.

 

[C'DaleRider]

Originally posted by: radioouman

Originally posted by: jumpr

Originally posted by: dxkj
I think cranking the AC on high for 1 minute and turning it off for a few is better for gas mileage, but what do i know

The extreme wear and tear this would place on your A/C compressor would outweigh any real gas mileage benefits.

Which would you rather do? Replace your A/C compressor at 35,000 mi., or get gas a couple more times a year?

THat would not cause additional wear and tear on the compressor. The compressor has a clutch which is engaged and disengaged all the time while driving. This keeps your compressor from freezing up when it is 60 degrees out, etc. Also, when going up a hill, your compressor may disengage to allow more engine power to go to the wheels rather than drive the compressor.

While the last is quite true that the compressor has a cut-off switch for WOT operation of the car (WOT=Wide Open Throttle), there are a growing number of cars that now have Variable Displacement compressors, compressors that, when turned on, are always engaged and use an internally variable set of impellers to control how much the compressor is being used to work the refrigerant. So, they esssentially have no clutch but are always dragging the engine a bit.


Also remember, when you're using your defog/defrost setting on the temp control panel, your A/C compressor also engages.

 

[radioouman]

Originally posted by: C'DaleRider

Originally posted by: radioouman
The engine uses more gas when the outside temperature is hotter because the compressor gets harder to turn. Also, it has to keep the compressor turned on more to get the desire amount of cooling.
Fan speed has nothing to do with it.

And generally, you get better gas mileage with your windows rolled up and the A/C on than with the windows down.

I don't know where you got the information for that last statement, but you are quite misinformed as it is pure and utter BS!

Not only does the A/C compressor put an additional load on the engine as it cycles, using more gas, but the increased electrical usage by the compressor and fan makes the alternator drag more, also increasing gas usage. And as long as you're driving along the highway, I suppose the increase in heat generation from the engine would have little effect except that many cars automatically turn on their cooling fans when the A/C compressor is engaged, so that also would increase electrical usage and burn a bit mroe gas.

Actually, Mythbusters took care of that myth on Nov. 10, 2004. They wanted to see if the computer model programs that predicted cars ran more efficiently with the A/C on vs. driving with the windows down and no A/C on was true. So, they used two identical vehicles, filled with indentical loads of gasoline and drove both on the same oval track at the same time at the same speed. The only difference between the two was one vehicle was using its A/C, one had its windows open. MYTH BUSTED!!!

The result? The open window vehicle completed over 30% more distance than the A/C-using vehicle did.......the A/C made that vehicle consume its load of gas much faster than the vehcile not using its A/C. Of course, anyone in the South could have predicted that.....gas mileage suffers terribly when using the A/C and that's from experience, not from some inaccurately conceived and executed computer modelling program.

Hmmmmm.
That was something that I read in the newspaper a couple years ago. Never tested it myself, and never saw any information to refute it. Well, like you said, myth busted!

Also, there isn't much current drawn by the compressor. There is a small amount to engage the clutch (solenoid), small amount to turn the blower fan, and if your car has an electric cooling fan, then that is generally running more often when the A/C is on. However, if you are on the highway, your fan typically isn't running. Therefore, the additional load that the alternator compensates for is negligible. Driving with your headlights on would draw more current.

 

[radioouman]

Originally posted by: C'DaleRider

Originally posted by: radioouman

Originally posted by: jumpr

Originally posted by: dxkj
I think cranking the AC on high for 1 minute and turning it off for a few is better for gas mileage, but what do i know

The extreme wear and tear this would place on your A/C compressor would outweigh any real gas mileage benefits.

Which would you rather do? Replace your A/C compressor at 35,000 mi., or get gas a couple more times a year?

THat would not cause additional wear and tear on the compressor. The compressor has a clutch which is engaged and disengaged all the time while driving. This keeps your compressor from freezing up when it is 60 degrees out, etc. Also, when going up a hill, your compressor may disengage to allow more engine power to go to the wheels rather than drive the compressor.

While the last is quite true that the compressor has a cut-off switch for WOT operation of the car (WOT=Wide Open Throttle), there are a growing number of cars that now have Variable Displacement compressors, compressors that, when turned on, are always engaged and use an internally variable set of impellers to control how much the compressor is being used to work the refrigerant. So, they esssentially have no clutch but are always dragging the engine a bit.


Also remember, when you're using your defog/defrost setting on the temp control panel, your A/C compressor also engages.

I didn't know about variable displacement compressors. That is neat!

 

[DainBramaged]

Originally posted by: FoBoT
it doesn't matter

if you are hot , turn the A/C up more
if you are cold, turn the A/C down some


/thread

Yes...the air compressor is either on or off.

 

[0roo0roo]

as for wear and tear..all i know is it was on car talk with those click and clack brothers..and they said it was a bad idea.

 

[JulesMaximus]

Originally posted by: FoBoT
it doesn't matter

if you are hot , turn the A/C up more
if you are cold, turn the A/C down some


/thread

QFT

I turn it on if I'm hot and usually leave it on low for long trips unless I'm in the desert and it's 110 in the shade. My comfort is more important to me than saving a buck or two at every fill up. A/C is on in my car almost all the time during the summer months.

 

[woowoo]

If you run the fan on low the compressor will not engage as often. (Less gas used)

If you run the fan on high, the compressor will engage more often (More gas used)

The compressor will cycle when the head pressure gets low.

The fan blowing over the evap coil will lower the head pressure.

Higher fan speed = more gas used.