How does A/C work on the Prius?

We've had a few A/C threads lately. What I haven't seen--though I could have missed it--is a description of how A/C works in the Prius, and what the most energy-efficient way of using it is.

OK, OK, I know from the discussion and life generally that Prius A/C works like most other A/C in the world, with a compressor and a cooling fluid. I also figure that you use more energy the cooler you try to get the car.

The questions that I'm asking are these:

Is it most energy-efficient just to turn the thing onto Auto and let the computers decide how to cool it?

Or, as I suspect, is it more efficient to turn the fan down a bit from the full blast you get on Auto and cool the car more slowly, if you can stand it? The reason I suspect this is that in Eco mode the fan doesn't run as fast as in Normal.

When the A/C is not on Auto, is the system still trying to cool the cabin to your target temperature? If not, what's it trying to do?

When the fan is on High, is the compressor working harder, or just the fan?

And if I've asked too many questions, is there any place to go to get the whole story of how the Prius climate control system works and how to use it best?


BTW, Little Rock, Ark, hit the highest temperature ever recorded here, 114 F yesterday.
For GrumpyCabbie, Socrates, and all the other non-USers, that's 45C.

It is a combination of what you want for comfort versus economy. If you want to cool off in a hurry, turn on the AC, put the temperature at LOW, which is the lowest setting, and then put the the Prius in Power mode. This will allow the AC to work at the highest power to achieve the comfort you desire. If you place the Prius in the ECO mode, it will still try to achieve the comfort you want, but it will take longer due to your choice of economy being first. When the fan is at higher levels, it is due to the difference in the current temp and the temp you want (either warmer or cooler), as well as whether you are in ECO or Power mode.

It will take some practice, but you will eventually learn what setting and what mode to place your Prius in to give you the comfort you desire and how fast.

BTW, the software will calculate the temperature difference for what you desire and will use the compressor and fan as necessary. I just set the temperature through experience and let the Prius do its thing. Works very well actually.

Kgall, I live in NLR and I felt that 114 yesterday. So far the most efficient settings for me have been ECO+AUTO+RECIRCULATE and I try to stay within 10-15 degrees of ambient temp. That really shot that to hell yesterday though. The temperature was so rediculous I had it on 77 most of the day. Any higher and the humidity is a killer.

I'd like for someone to test the idea that running the fan slower will reduce the mpg hit in the Geniii. In the Genii it didn't matter what the fan speed was. The 10mpg drop was consistent if ambient temp was much higher than your desired setting.

When ambient temp was nearly equal to the desired temp, the mpg drops was only 1-2mpg.

Can someone please test this on the geniii?

If trying to cool down an overheated car, say that's been parked in the sun, I like to start out in Auto, with AC button depressed, and with the temp set high enough that the fan doesn't race, goes about halfway up the fan speed range. Typically that's around 27-28 centigrade. Leave it on fresh intake.

Then as the interior temp starts dropping and the fan speed accordingly drops also, I'll move the target temp down, just until the fan speed again climbs up to medium. I'll repeat the process till the temp is down to around 24, or maybe 23.5, which seems pretty comfortable, with stabilized AC.

I'm still not clear what happens if you don't expressly push the AC button. I think sometimes the computer is using AC, but it doesn't seem as positive/agressive.

The best way to do it would be to run in auto with the temperature set as high as you can stand it, starting at the highest setting below MAX. The temperature control seems to be what controls the compressor's power, and when you're in Auto, the fan speed will be adjusted to match. So when the fan is slower, you know the compressor is using less power. As the car reaches the set point, decrease the set temperature to keep the fan speed high enough to keep you comfortable. Eventually, you'll reach a temperature where you're comfortable and the fan and compressor will reduce power automatically to maintain the temperature. In Eco mode, the maximum compressor power and fan speed are limited; this is to reduce the power draw of the compressor, and if the compressor isn't running full-speed, there's not much gained by running the fan full-speed either.

Yes, it's still trying to get to the set temperature. Auto controls two things, which can each be auto while the other is not; fan speed and air outlets. If you are in auto, changing either of these will leave the other still in Auto, although the Auto light goes out. In any case, even when both are manually adjusted, the car will heat or cool the air as appropriate to reach the target temperature. If you set the temperature to the minimum and the fan speed to the lowest, you will still run the air conditioner's compressor at full, but very little air will move through the system. This means the air will get quite cold, but it's a very inefficient way to run the system, since the fan takes relatively little power when compared to the compressor. (It does take a somewhat noticeable amount of power at the 2 or 3 highest settings, but it'll still be dwarfed by the compressor.)

Depends; if the fan is on Auto, then the compressor and fan will ramp up and down together. If the fan is on manual, then the compressor still runs as hard as it needs to to maintain the set temperature.

Somebody noted this from their ScanGauge in one of the other recent threads; at the minimum temperature setting, even when the fan is on low, the compressor is still running at high.

As far as I'm aware, it will only use A/C if the A/C button is lit, or if you are using the defogger switch. Otherwise, it will only use the fan. I occasionally use it just for the fan, since the weather is often pretty mild in my area.

And of course the biggest weapon, when you first get to your car, if the interior is hotter than the outside temperature, open all the windows and run the AC full blast to initially blow out the hot air and cool down the ductwork. Then roll up the windows, set on Auto and let the car do its thing. If you want a more aggressive result, put the car on Power mode instead of ECO. As you play with this you will find what works best for you and what difference in economy as a result.

We made sure to order a white one (picked it up yesterday) As a test try putting your hand on top of different colored cars. The white ones are way cooler than anything else. It was 100 degrees and humid in Ocala yesterday...put 60 miles on it with the AC on 76 and the car in ECO mode. Stayed comfortable and got great mileage. Of course acceleration seems somewhat reduced in ECO.

What I have seen with my ScanGauge monitoring AC watts is that most of the time the compressor watts and fan speed will decrease together as the cabin temp approaches setpoint. In these instances, mpg does seem to be relative to compressor watts, and thus fan speed, when they track each other.

In this extremely hot weather of late, 100+F, I have noticed that the compressor watts and fan speed will begin to decrease together but then the system has trouble maintaining cabin temp. In this case, I have noticed the compressor watts increase without increasing fan speed. Without a ScanGauge it would be hard to notice this, other than noticing the change in temp of the vent air.

I don't think PWR mode has any effect on AC. From everything I've read, only ECO mode affects AC.

I agree. I use Normal mode and it has no affect on the AC.

An interesting read from Toyota's New Car Features...

View attachment 31641

Thanks for that document! I almost thought it was a spoof when I read

and the description:

And then:

Interesting. Will take a while to read through that.

One thought: it goes into detail as to what setting ECO mode does to AC behaviour. It would be kind of nice if just those changes could be in effect, ie: you could get the ECO AC behaviour without the changes to gas pedal response.

Thanks to all.

So, what I hear is that Prius A/C (and I bet a lot of other auto A/C systems) are unlike what I think of as the basic home A/C system with thermostat:
the compressor and fan are ON until they get the house down to the target temperature, and then they turn OFF and stay off until the house gets hotter again (unless you are in FAN ON mode, when the fan stays on constantly).
Presumably, the Prius system works the way it does in AUTO, with variable compressor speeds and variable fan speeds because it is more efficient, and the increased efficiency outweighs the complexity issues.

So, that suggests to me that there is an idea out there that might make someone who knows more about A/C and neural networks than I do millions: put this type of control into new home A/C systems.

Or is someone already doing that?

(Mendel--I know a guy who was working on "neural networks" in computers about 20 years ago. But it sounded cool to me. I'm sorry he's gone on to other things.)

Actually Trane makes a variable speed scroll type compressor that runs at whatever percent speed it needs to run to keep the temp.

I'd guess that it wouldn't make as much sense in a home system. In a car you always need air moving, since there's so little space, and the sun is often shining in making it very warm very quickly. (Start with a comfortable car on a hot, sunny day, then turn off the A/C. You'll probably want it back on in less than a minute.) Some cars do cycle the A/C on and off while leaving the fan on (you could hear it on my dad's old Honda Accord), but a variable-speed compressor is more comfortable because the vent temperature and humidity stays the same.

In a house, however, there's a lot of air hanging around, and there's a lot more thermal mass keeping it a constant temperature, even when the sun is shining in. Because of this, it's acceptable to cycle the system on and off, since the temperature variation from doing so likely won't be very noticeable. Variable-speed compressors are usually most efficient at or near their peak power - so the most efficient thing to do is to cycle it on and off, and never use it at anything other than the most efficient point. And since that's the case, you might as well not have a variable-speed compressor at all, since a single-speed one is probably cheaper and even more efficient.

A variable speed compressor can provide better comfort in a house too.

A variable speed compressor and variable air flow help in residential systems. They aren't common because of economics. Americans like an up front bargain, even if it costs more in the long run. We like "free" cell phones, which we pay for with a two year contract. We do the same with most of our purchases.

A single speed system with bang-bang control is cheaper to make. A prospective buyer looks at the price and capacity, and decides to buy the cheap system. As energy gets more expensive, we may see a change in this pattern.

Tom

Thanks again.
I understand that large commercial buildings (e.g., skyscrapers and big-box stores) have very complex computerized control systems for HVAC (heating, ventilating and A/C), and have had for some time. Am I right?
I don't know if this is paired with complex hardware such as variable speed compressors.

Variable speed compressors can definitely make things more comfortable even in a house - but they're not absolutely necessary like they are in a car, and are probably less efficient than the single-speed variety.

And yes, large buildings have very complex systems, and they use systems that can vary the load. They spend quite a bit of time perfecting them at my office, where there's a separate system spaced about every 30 or 40 feet or so, and thermostats all over. Keeping it the right temperature isn't an easy problem. Twice in recent history, a relay-ish sort of thing has broken in the system above me, causing it to put out full heat (and probably trying to cancel it with full cooling simultaneously). So my area gets a little warm. That temperature spills over into the surrounding areas served by a different system; they see the warmer temperature, so they increase their cooling to make up for it a bit. But then that cool air spills into the next region away from me, causing them to get heating... So the whole floor becomes an alternating pattern of warm and cool zones all because of one broken actuator.