A/C and Heater

I think what is confusing people is when you put it in "Auto" the A/C light usually comes on. This DOES NOT mean the compressor is on, just that it -could- run if required (to lower the -inside- temp for example ). A/C will run if the -inside- temp gets higher than the set temp or if you turn on "defrost". It will not run if the outside temp gets high, only if the inside temp gets higher than the set temp. The system can become "confused" if the set temp and the outside temp are within a few degrees of each other. "Confused" means -you- may not be sure what is happening. The A/C system isn't smart enough to get "confused" in our sense of meaning.

 

If I've got AC off and hit Auto, the AC light does not go on. And AFAIK the AC is never running, it definitely doesn't feel like it.

do not know if it's a different build or whatever, it's Canadian Touring model, manufactured Aug 09.

 

Mine comes on even in the "dead of winter". Perhaps the system firmware was changed in the 2012 refresh.

 

Could be. I like it the way ours behaves, fwiw.

 

It increases the cooling rate needed but reduces the cooling rate available.

If the temperature difference between the occupied space and the ambient air increases, more energy is required to achieve the same amount of cooling.

If the a/c compressor is already running full out, it will produce less and less cooling as the ambient temperature rises.

The reason for this is that for the a/c system to get rid of heat, the hot refrigerant gas leaving the compressor has to be at a high enough pressure and temperature that it can transfer heat to, and be condensed (liquified) by, outdoor air passing through the condensing coil. The higher the outdoor temperature, the higher the necessary refrigerant pressure and temperature , the harder the compressor has to work and the greater the power needed.

 

As I said, I'm no engineer, but if it has no affect on the hybrid battery then why is the AC, along with fuel efficiency, affected by being in Eco mode?

 

I've only needed to turn ECO off a few times in summer when temps go over 40 deg C. (104 F) Usually ECO A/C is sufficient.

 

Yeah, I think the behavior might've changed in the refresh of the Gen III; prior, pressing Auto would not turn on A/C if it was off last time it was used, but the newer ones will turn on A/C.

Another thing that may not be obvious; even if the A/C light is off, if the mode is set to defrost+feet, it will turn on the A/C. There's no feedback that this is happening and no way to disable it (short of switching to another vent mode), but if you have some sort of an OBD gauge, you can see that the compressor is active.

 

In Eco mode, the AC works at a slightly lower capacity, thereby reducing power needed for the AC and ultimately increasing MPG. That is the only reason. Eco is to squeeze out that last little bit of MPG you like to have.

 

A fourth gen wish list item: to be able to have all the ECO adjustments, without the gas pedal remapping.

 

I was curious how much the A/C compressor ran when the temp was low so I added the PIDs to Torque and this is what I see when driving in ECO mode EV until above 62 MPH, but that was trumped by low temp and ICE coming on to warm the engine fluids. I also always leave climate in auto mode and as I mentioned previously the A/C light stays on even when the temp is low and I can sense heat coming into the cabin. It looks like the A/C compressor runs fairly often to regulate the evaporator fin just above freezing. IIRC I had the cabin air set at 74 so I am guessing that the A/C was reducing the humidity in the cabin.

I have not found a PID for a humidity sensor, not certain if one exists that I can read. I have checked several other logs this week and the trend is the same, so I think for now the A/C light on means the compressor is running as it needs to maintain lower humidity or lower temps. The blower, which I assume is the fan was also quite boring in this case as it did not change much since the cabin temp was already where it needed to be. I have seen it vary the speed quite a bit more when the cabin and temp setpoint are significantly different.

I have never noticed that the re-cycle/fresh air has automatically changed when in auto climate to date either. It might be changing when I am not looking as this is another PID I cannot locate.

 

100 degrees yesterday afternoon. The Battery temperature at rest was still warm (117F) from both drive into town and resting in the heat. Those darn love bugs swarming around cars so I left windas shut. I just cant stand them crawling around me as I drive! I turn my A/C down on automatic outside air first few miles to push air in towards battery fan. I then run it cold Auto and that battery drops to 93F for rest of trip. The lil fan does a good job.. Keep it clean! Well worth the time and no money!

 

That solar fan seemed to be a good idea, only on the tech level? And dropped in 2016.

 

Hmm, I've just finished reading this thread from the beginning, and there are a lot of ideas about how the heat/AC work, and some of them contradict each other, and some seem to be closer to the mark. So, for a little recap of the best info....

The A/C compressor is not just on/off, it is variable power. It is very variable power, as you can see if you program in the PID for A/C compressor watts. It can modulate from over 2500 watts running full out (when it sounds like a little airplane under the hood) down to 250 watts or less (hardly audible) when just loafing in mild conditions ... and of course, when even less cooling is needed, it can cycle between very-low-power and off.

The A/C "amplifier" (ecu) chooses the compressor power in interaction with lots of other things, like the inside, outside, and set temperatures, amount of sun falling on the solar sensor, and the fan speed. Lowering the fan speed manually won't result in a situation where the compressor runs full out and wastes power; the ecu will reduce the compressor power to suit the reduced amount of coolable air moved by the fan. The A/C watts PID will show this. It is unavoidable, because if the compressor did continue at high power with the airflow reduced, it would cool the evap coil below freezing, and the coil would ice up.

The ecu does also control an air-mix door that affects temperature by how much of the airstream goes through the coils and how much goes around.

For anyone interested, the behavior of the heat and AC in all modes of operation is explained and diagrammed in great detail in the New Car Features Manual on techinfo.

It's no use looking for simple rules explaining exactly how the ecu drives its various outputs based on all its various sensor inputs. The basics are commonsensical, but the details are actually worked out by an artificial neural network that runs on the heat/AC ecu, and is trained to produce results that humans in testing typically found comfortable. The New Car Features Manual goes deeper into this. Using a neural net to control a heater seems like a bit of overengineering to me, but also fun in a geeky way.

Anyone who's had a machine-learning class will remember that neural nets can be trained to perform impressive feats, while being really hard to "see into" and understand just how they reached their conclusions ... not unlike the many non-artificial neural nets you talk with every day....

-Chap

 

That would make sense. What too often actually happens in that circumstance is cycling between very low power and (very unwelcome!) heat.

 

Great post, and a lot of useful info.

One thing I found missing is the customisation options that some posters seem to have enabled/disabled. I found them on Carista website, but have not tried them myself yet:

 

I believe that Eco mode will slow down the fan and compressor. Although I generally always run in Eco mode, on really hot and humid days I turn it off to boost cooling.

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