AC/fan setting vs. mpg

Heavy use of the cooling will lower mileage a bit more than "normal use". By "heavy use" I mean you set the temp. to 60F and it's 100F outside in full sunlight. For example. Normal use results in about 1-2 MPG loss according to most here who have posted about it. "Normal use" would be, for example, you set the temp to 75F and it's 85F outside.
Fan speed should have little effect.

-I- recommend you use the "auto" function and let the car set the fan speed etc. while you just set the target temp., but feel free to experiment.

 

The second option is true only in cold weather. The others choices are always false.

 

All I can think: using AC, with any settings, is making a significant dent in your mpg.

I'd say when running AC, use auto, and "control" fan speed by setting temp high enough to avoid racing fan.

 

Yes, use of auto is recommended. If you can get by without A/C, turn it off and just use the fan.

From my (very rough) measurements a few years back, the A/C compressor will use up to 1500 watts, or 1350 in Eco mode. The fan will use up to ~250 watts at full blast. If you turn the temperature down and the fan way down too, you're still using 1500 watts for the compressor, and maybe only 50 watts in the fan. On the other hand, if you turn the temperature higher, the compressor will run at a lower power (say maybe 750 watts) - at that point, you can run the fan full blast and it's still using less power than if the temperature is turned way down.

Using auto will balance both fan speed and compressor speed to keep them relatively efficient while getting you comfortable. Just set the temperature however you like to make you comfortable. (For me, that sometimes means I start out with a high temperature, like 78 or 80, and then as it reaches that temperature, I turn the temperature down more, to keep the cold air coming at a moderate rate so that I stay comfortable.)

I skipped voting because it isn't specifying whether it's talking about heating or cooling, which makes a rather big difference.

 

I watch AC watts sometimes on ScanGuage. If you've got the Mode in Heat/Defog AC'll come on surreptitiously, regardless of your AC button being on or off, typically dances between 0 and 200 watts. If you've expressly push the AC button on, it'll run a bit higher, up to 500 watts I think, been a while since I watched it.

 

LDB I am in Dallas and I find going without AC to be a fantasy for me as well. But my Prius seems to have a very effective AC and putting the temp on 76 or 75 while on auto seems to work fine for me. MPG (indicated as I still don't have a full tank fill to compare to) seems to be just over 50. Of course, it isn't 110 outside at this point in the year either...we will see what happens next August

 

Is there any difference in surreptitious AC power between that "Heat/Defog" mode and actually pushing the front defog button?

 

I can check that. I believe I might have, but I'm a bit fuzzy on it. Best of my recollection, pushing the front defog, it went immediately to something like 500 watts, and stayed there. OTOH, setting Mode to Heat/Defog, and AC button off, it bounced around, at values between 0 and 200. In increments of 50. Sometimes, it stated constantly at 0, I believe below some ambient temp threshold, somewhere below 5C as I recall.

 

Since the topic of the defroster has come up in this thread, has anyone measured how much pull the rear defroster has? In other words, how many watts does it take to run the electric rear defroster? I am guessing Gen III will be more than Gen II since Gen III also has wires on the rear side windows too.

 

I don't know, but you could easily find out by removing the relevant fuse and connecting an ammeter across the fuse terminals with the defroster turned on. The area of the back window covered by the defroster wires is not exactly generous.

 

CR94, I checked the front defog watt draw tonight: pushed the button, and in a couple of seconds it jumped to 1000, then down to 600, then 850, a second or two at each level. ScanGauge has a bit of lag too. I shut it off then.

 

I'd have to think on the options more.

This much I can say. Manual fan speed set higher than needed will draw more air across the coils. This means more cold or hot air. As such, depending on other factors, your compressor or engine must run more often to keep the system properly "charged" to do the job of cooling or heating.

At highway speeds, the heat would be a non-factor since the ICE is always running enough to supply ample heat. In EV mode, you'd put more of a drain on the battery pack...requiring the ICE to kick in sooner than normal.

 

As you say, higher fan speed means more air moving and more heated or cooled air

The air supply temperature and heat transfer efficiency however will also change and the result may not always be what you expect. Some of the effects are:

• Heat transfer to/from cabin supply air is a bit greater due to greater turbulence at the heat transfer surface and a greater temperature differential between the heating/cooling source and the air leaving the coil
  
• Somewhat more cooling or heating is available so cabin temperature can change faster.
• If dehumidification is occurring, it may reduce
  
• Air in the cabin does not have to be quite as cool for the same cooling effect on the occupants.
• Fan power use increases faster than air flow increase (square law: Double the flow, 4 times the fan power use) CORRECTION [Cube law: Double the flow, 8 times the power]
• The fan and a/c air movement get noisier
• Compressor may run longer or not (It's variable speed)
• Overall cooling may get more efficient or not

Exactly what happens and any effect on mpg will depend on the conditions, the change in fan speed and Toyota's control algorithms.

 

It's even worse than than that. Doubling flow requires doubling fan speed, which requires 4 times the torque to drive the fan, which in turn requires 8 times the power to drive the fan. Electrical power to drive the motor wouldn't follow that pattern exactly, since the motor efficiency is not constant, but varies with speed and load.

 

Yes, you are right. Resistance follows the square law. Power follows a cube law.

If anyone following this wants more detail. Read this http://www.nyb.com/Catalog/Letters/EL-02.pdf or google "Fan Laws"

 

Maximum flow isn't always effective. The evaporation coil can only absorb X heat per second. The faster air flows across the coil, the less chilling of the air takes place. Often, my dad's set AC fan speed to somewhere less than maximum to get better quality cooling. Max flow is best when you want to purge the cabin air as fast as possible and opening the windows isn't a practical choice.