A/C works only when cooling??

<div class='quotetop'>QUOTE(DaveLeePrius @ Jan 29 2007, 11:09 AM) [snapback]382375[/snapback]</div>

I am pretty sure it works the way you think it should. That is, when you apply defrost, it runs the AC.

I also believe there is humidity sensor which then forces the AC on (as well as heat) in the winter as needed.

It's a pretty impressive system - there is a recent post, titled:

"Amazing climate control, The best I have ever used" that explains it pretty well - especially the link to the DENSO website.

 

The A/C compressor runs when needed for defrost, unless you turn it off from the MFD climate screen.

Tom

 

Apparently, I was also confused on this issue. When I have "Auto" selected in the Climate screen, the A/C button indicator lights up (goes green). I assumed this to mean that the AC is on, even if it's 50 outside and I have the desired temp set to 75, such that the air is warm (heated in the usual way) and dry (dehumidified by the AC). In fact this is what most people with allergies do in their cars since it also seems to cut down on particulate matter (don't know how, it seems that the filter would be main method of doing that). So my impression was, when the A/C button is lit, AC is on and the temperature of the incoming air is modulated by the heater. I have also observed, that with the outside air temp at 70, when lowering the desired temp from 75 to as low as it will go (in Auto mode) is NOT accompanied by the slight revving of engine that would indicate AC turning on, which would mean it was already ON when the temp was set to 75. I hope what I'm trying to say is coming across clearly. Correct me if I'm wrong. Thanks.

 

The A/C indicator on the screen only shows that the A/C compressor is "enabled". Annoyingly there's no indication of whether the compressor is actually running at any given instant, and if so at what speed.

The A/C compressor will be used automatically, as necessary, to maintain humidity, as long as you haven't disabled it with the button on the screen.

And you will never hear any engine pitch change due to the compressor starting and stopping - it's a variable-speed electric compressor. If the engine isn't running, you may be able to hear it. It is quite clearly audible if you lift the bonnet, or if it's running full pelt.

 

<div class='quotetop'>QUOTE(KMO @ Jan 30 2007, 12:40 PM) [snapback]382843[/snapback]</div>

That only works with your version of the Prius. In the U.S. they come only with a hood.

Tom

 

Here is an old post of Tony's that I saved because it was so well written and covers the subfject very well.

"Air conditioning, as a term, has been used by the auto industry for years to refer to the refirgeration of air. However the truth is that in most other parts of the world, and amongst those to do building HVAC for a living, air conditioning is both heating and cooling of air, along with air filtering and humidity control. True air conditioning indicates a fully controlled air environment.

Toyota's use of the term Air Conditioning in the Prius is technically correct, but many drivers confuse Toyota's use of the term with the common belief that Air Conditioning refers only to the cooling of cabin air.

The light on the dash indicates Auto A/C. If that light is on, the system will make all the decisions about air volume, air output, air source, compressor status and heat status based upon the temperature setpoint that is changed either at the steering wheel or on the MFD. The Prius system is fully automatic and behaves the same as nearly every other automatic climate control system I have ever used.

Now, with the light on, if you go into the MFD and start adjusting the system, the Auto A/C light will go off. That means that the system is not fully automatic anymore. Starting at Auto A/C, as soon as you manually adjust the fan speed, the system will not be able to adjust the fan speed automatically. That is why the Auto A/C light goes off. Not because the system is not going to run the A/C compressor, but because it is not fully automatic anymore. The same goes for air output, if you manually adjust that from the Auto A/C light, the system will not be able to choose the air output.

The temp setting does as it indicates. Its goal is to keep your cabin at the temp you indicate. It will heat as necessary and use the compressor as necessary to achieve these goals.

There is an A/C button on the MFD in the upper left corner. I call that the A/C defeat switch. If the yellow bar at the top of that soft button is on, it means that the automatic system has access to the A/C compressor. If you press the A/C button and the light goes out it means you have told the system that it may not use the A/C compressor. This light does not indicate when the vehicle is using the A/C compressor. You have no direct control over when or how the system uses the A/C compressor. You can adjust the cabin temp up or down, but that is the only control you have aside from the A/C defeat switch. I cannot stress this enough, use your A/C or lose it. The health of an A/C system is based upon it being used. Compressed refrigerant, as it goes through the system, carries with it refrigeration oil, which is mostly used to lubricate the compressor, but also keeps the o-rings and seals fresh and soft. If you intentionally choose to not use the A/C you are choosing to let the system dry out and that will require future expense to bring it alive again. The Prius battery also requires a "room temperature" environment for most efficient operation. If the interior of your cabin is hot after sitting out in the sun, the Prius uses the climate control system to bring the cabin temp down to a level which helps the battery operate efficiently. The battery does not run efficiently above 100 degrees and doesn't do that well when it is freezing out, so a plesant cabin temp is also plesant for the battery.

So, what I'm saying is that you should never defeat the A/C system as it does more than just keep you comfortable.

Additionally, the compressor is not on or off, as in most cars, but is variable speed and runs at speeds necessary to provide the appropriate amount of cooling to the system. In the summer, as the cabin cools down the compressor will slow down. This is a significantly more efficient A/C design than in conventional vehicles in which the A/C compressor is engaged or not engaged and nothing in between. You should also be aware that the A/C compressor is electric and as such does not load down the engine with a high demand pully accessory. Instead all of the energy used to run the compressor comes from the HV battery and electrical system.

I have discovered that I lose only a couple of MPG in the summer with the A/C, if that much. As such I do not consider it a sufficient loss in MPG to choose to go without A/C. And considering the additional benefits of running the A/C (ciruclation of oils, battery cooling), there is no question that the auto A/C system that Toyota put on the Prius is there for a reason and is as integrated into the vehicle's operations as the HSD system."

 

<div class='quotetop'>QUOTE(DaveLeePrius @ Jan 30 2007, 10:52 AM) [snapback]382886[/snapback]</div>

changing the temperature setpoint will not turn off the AC Auto light on the dash - and the system will be in full automatic to achieve that temperature (turn on / off ac compressor, turn on/off heater, speed up / slow down fan... )

However, changing the fan speed will turn off the AC Auto light - because the fan will no longer be in automatic mode - and will remain at whatever speed setting you selected.

Sometimes you do that when the car is too hot or too cold and you just got in it... and you might notice the fan going to fast (and its annoying you)... so your reaction is to slow the fan down a bit.

That's ok to do, but once the system gets near a normal point, you might enjoy it better by pressing the AC Auto button on the MFD and selecting a temperature that maintains the cabin comfortable for you. You will notice the fan slow down and the cabin will be just right...

Hope that helps... John.

 

<div class='quotetop'>QUOTE(DaveLeePrius @ Jan 30 2007, 10:52 AM) [snapback]382886[/snapback]</div>

Dave, if you change the temp with the buttons on the wheel it will adjust the fan speed according to the temp you have set. If you get into your car after leaving it setting in the sun, the climate control system will try to reach the temp setting as fast as possible. It will automatically go to recirc and high fan speed to do this. If you increase the ttemp fron the wheel the fan speed will reduce too. Most changes made fron the MFD will turn off the Auto AC. I just use the wheel controls.

 

Great write up. Thanks a lot!

 

I assume that the basic control computer is silmilar to the Lexus which will operate like this:

Leave it in Auto and it will run the compressor to cool and / or dry the air into the cabin. When the temperature drops to a certain point (I have seen two different set points in two different models), the compressor will not run.

This is for two reasons. First, when the air is cold, it is already dry. Cooler air cannot carry as much moisture so running the compressor would be a waste. Second, if the air coming by the evaperator is too cool, it will not be able to "boil" the liquid refrigerant fast enough. The compressor is designed to compress the freon gas back into a liquid and those of you that remember physics know that you cannot compress a liquid. If liquid freon gets into the compressor because it didn't "boil", it will destroy the compressor.

Now don't all panic and manually hit the A/C button to shut off your compressor... The car is smart enough to prevent this.

 

<div class='quotetop'>QUOTE(NeilKW @ Jan 30 2007, 07:50 PM) [snapback]383065[/snapback]</div>

Actually, I believe you can in fluid dynamics, but we usually assume it's incompressible for the simplicity of calculations.

If I'm wrong someone will call me on it in short order ...

 

Dumb question, but can someone please explain when to use recirc and when not to? And to add to that, is the autoAC smart enough to switch that for you?

 

<div class='quotetop'>QUOTE(Silver Phoenix @ Jan 31 2007, 11:44 AM) [snapback]383430[/snapback]</div>

the car does do the recirculation part on its own (I am not 100% sure, but believe this applies only when it is trying to cool the car - not heat it).

Recirculation is great for minimizing odors (you are going by a dairy farm), for minimizing dust and to cool the car faster.

When you press the Recirculate button, a flap on the outside air intake closes (or at least closes most of the way), such that the air that the fan is sucking on is coming from inside the car (usually, from under the glove box for most cars). On a hot day, when you are trying to cool the car, the assumption is that the air inside the car is cool already... so, it is a little easier to cool that air further than it is to take the hot outside air and cooling it all the way down to the desired temperature.

In the heating cycle, it is generally better to not recirculate - as you will build up humidity in the car and fog up your windows (when it's raining - have you ever seen cars around you with passenger windows all fogged up? - that's because they have the system completely shut off or it is in recirculate.)

By the way, if you are in recirculate - you won't run out of oxygen - the manufacturer always makes sure there is some fresh air coming into the car.

 

<div class='quotetop'>QUOTE(NeilKW @ Jan 30 2007, 07:50 PM) [snapback]383065[/snapback]</div>

Denso stopped using freon gas a long time ago. They are using more environmentally friendly gas instead.