A question about efficient use of A/C in the Prime

The AC is fully variable speed. The heat pump is the AC.

 

This brings up an interesting question I forgot to ask a while back.

I could swear I read somewhere in the manual, that to save gas, one should put the FAN speed as low as possible.
This seemed ridiculous to me (whereas putting a lower TEMP setting would of course indeed result in a drop of MPG from the energy used to cool the car more). But just the speed of the fan itself shouldn't cause an MPG drop, should it?

In all the other cars I've had, the speed of the fan made no difference. If the AC unit was cooling, it would draw energy from the engine, but the speed of the fan (low, med or high) didn't make a difference.

Say you have the A/C or Heat completely off in ANY car -- but just turn the Fan by itself (NO A/C or heat). That's not going to pull energy from the engine, it's just going to draw a tiny bit of energy from the 12V Battery, no different than a ceiling light would. It won't impact the MPG anymore than the ceiling light would. Now, turn on the Cooling, and it sure will. But just the fan by itself?

Is it different on the Prime? Where just the fan itself (not heating or cooling) impacts the MPG? I could swear I read something to that effect somewhere in the manual (as an "tip" aside somewhere) that to save MPG, put the FAN (not TEMP or Cooling) on a lower setting. Am I dreaming? Will putting on the ceiling light also make a dent in the MPG? Makes no sense to me..

 

It's already saying 100 degrees when I get into my car here in Florida. I run max all the time.

 

I set temp to 80 when outside was 85. It felt like I had it max cold. Pdog808 is on to something questioning if Prime's heat pump behaves differently than the conventional AC.

With 2014 PIP, I used to set the temp to Low to prevent ICE from coming on. Needless to say, with Prime, you don't need to do that.

What's more confusing is that the Eco guidance at power-off tells me to set the temp higher when I have the temp set to Low (max cold, from PIP habit) with AC turned off!

The fan speed setting without AC does not appear to make any difference according to MFD. My energy consumption stays at 1% regardless of fan speed setting while AC turned off.

Prime's heat pump appears to cool cabin quicker than my 2013 Leaf's with less MPG penalty as expected.

 

Yeah, it did, and it does in the Prime too.

Keeping the air cool will draw more power if you are cooling a larger volume of air. On conventional cars a low fan setting means the compressor will run maybe 10% of the time while a high setting might make it run as much as 100% of the time. On the Prime the fan setting will drive how fast the compressor needs to be spun and thus how much power it draws.

 

Perhaps it is time for us to use our cars to measure:

• max windows down + AC off - fixed speed on given route and temperature
• windows up + max AC on - fixed speed on same route and temperature

Do the experiment and then we can compare it to other citable sources. But we also need to think long and hard about a math model. For example, important variables:

1. speed - what is the threshold speed for windows drag
2. heat/temperature/solar radiance difference - determines the cycling time and load

Bob Wilson

 

I did some experiments on another car.

I found that driving a hot car with the windows down cools it far more quickly than the AC does. This is because of extremely large mass flow and high delta T (i.e. 90F outside, 160F in the car). In three minutes the car interior is down to within 20F of ambient.

Once there, rolling up the windows and using AC is just as fast or faster at cooling the car.

On that car I did a drag test AC on versus windows rolled down. Crossover point was about 50mph.

 

I keep sun shade up on front and door windows. Before I get into the car in hot temp, I air out the car by opening all doors and hatch.

 

I use my key fob function to open all the windows before I get into the car

 

Oh, great idea. Is this something you had the dealer enable?

 

Nope, I did it myself

 

Can you come to Minnesota and do mine?

 

sure, I'm on my way starting May 30th on US 2. I will be in Bemiji Best western inn on the 31st

 

I'd be tempted to take you up on it - but I'll be in Paris.

 

I have always wished that my car's climate control worked like my house thermostat. At home I can set it to either heating or cooling. During summer if I have it set to 78 for example, if the indoor temperature drops overnight below 78 the heater doesn't come on, which is exactly how I want it to work. Heating it to 78 would be a waste of energy. By allowing it to stay below 78 this helps when the outside temperature begins to rise again because the air conditioner won't have to come on as soon. Now if we had unseasonable cold weather and I wanted the heater to come on then I just flip the switch. But I don't want it to do that on it's own, and it doesn't. But my car does if I'm not careful with the temperature settings.

If it was using 100% recirculation it might not matter, but it draws in a certain percentage of outside air to keep the cabin air from getting stale. At higher fan speed I would think you will be drawing in more outside air, which would make the AC work harder, which would affect MPG, although I have no idea how much. Even when you have the AC set to recirculate mode you are still drawing in a certain percentage of outside air, although not as much as in regular mode. Recirculate mode is more efficient but it can make the cabin air seem a bit stale. Recirculate is essentially the same thing as the old "Max" setting on car air conditioning systems, used mainly for quick cool down when the cabin is hot.

I wonder if there are different combinations of the four windows open, closed, partially open that would have less turbulence and thus less impact on MPG. Of course for your proposed test you need to keep it standardized so windows have to be full down. But maybe someone on their own could experiment with different window settings.

Another one of those old school advices that I first heard decades ago and still follow, that if the cabin is hotter than the outside air, to drive for the first few minutes with windows down and AC off to blow the heat out, then roll up the windows and turn on AC. That's what I always do, and usually the first mile or two you aren't going at highway speeds anyway.