How does AC temperature control work in my Toyota Camry?

[anorlunda]

When I use the air conditioner in my Toyota Camry, and I move the temperature control towards warmer:

1. Does it make the AC cycle on/off thus saving fuel?
2. Does it mix warm air with the cold, thus saving no fuel?

I couldn't find an answer to this on Toyota forums.

 

[jim mcnamara]

Maybe @Ranger Mike would know - at least where to look. We do have an automotive forum. Should we move this thread over there?

Edit: Wrong. I do not see the sub-forum...

 

[ChemAir]

The air conditioner compressor does cycle. There is a switch (usually pressure controlled) that starts and stops the compressor. When you move the mixing valve to the warm side, it does change how much air goes through the evaporator coil and the heating coil. If the airflow moving over the evaporator is higher, the compressor will run more to keep the refrigerant system at operational pressure. If it is lower, the compressor will not work as much.

There are tweaks to squeeze a little here and there to limit/help what is described above, so some vehicles will be better at AC operating fuel efficiency than others. You should save the most fuel (other conditions notwithstanding) if the compressor runs with less, or no load. Driving habits and maintenance should have a greater impact on miles per gallon these days than where you set the temperature on your AC system.

A couple of comments on this in this article under Minimizing Ancillary Losses https://en.wikipedia.org/wiki/Energy-efficient_driving

 

[Ranger Mike]

thanks Jim
not my best subject but here goes -
Today’s modern automotive Air Conditioner (A/C) has a vent output temperature of 40°F to 45°F (4°C to 7°C) measured after driving for 10 minutes at 45+ mph would be reasonable on a 85°F (29.4°C) day with 70% relative humidity. Measurement would be taken from a console vent using a precise stick & dial thermometer. At best you will have 40 degree F lower cabin temperature than outside.

Some Automobiles have A/C are good for over 50 degree F swing so really depends on the make and model. There are A/c cooling specs put out for each automobile, just so you know. There are temperature sensors in the cabin that cycle the A/C compressor system on and off as the driver desires.

When the A/C is operating, you have parasitic drag on the engine and fuel mileage suffers.
Forget about using fuel savings and A/C in the same sentence. These are diametrically opposed in MY OPINION.
Depending on the automobile you can have air ducts automatically mix fresh air / cabin air and combination to desired comfort levels or you manually dial in the dash board vents to direct the air current as you like.
Btw- the old window A/c in the house is only good for 20 degree F difference.

 

[anorlunda]

There are temperature sensors in the cabin that cycle the A/C compressor system on and off as the driver desires.

Thanks for the answer. I didn't know those numbers.

Sorry, I didn't mention that my car has no temperature sensors, so the knob makes it warmer/colder but the temperature is not necessarily constant.

The reason for my curiosity is that the same knob controls heat in winter when the AC is off. It has always been my understanding moving the knob simply moves a damper that changes the ratio of unheated air to heated air. Therefore, I wonder if it is not the same when cooling with the AC. Does the knob do different things with AC on or AC off?

Forget about using fuel savings and A/C in the same sentence. These are diametrically opposed in MY OPINION.

Maybe I can phrase my question better. If I use the AC to make the cabin temperature 20 degrees cooler than outside, does that use more gas than if I make the cabin temperature only 1 degree cooler than outside? (No temperature sensors, no closed loop control.)

 

[Ranger Mike]

any time the A/c compressor is working, the fuel mileage will be reduced. The compressor will be on more with the 20 degree requirement..therefore the 20 degree scenario will cause worse MPG

 

[substitute materials]

I have operated under the assumption that since there is no temperature sensor in my car either, it is simply mixing warm air to achieve the desired temperature, and thus not saving any fuel to move the temperature control towards warmer. Like Chemair said, the compressor does cycle, but this is pressure related. Changes to the blower fan setting will change how fast the evaporator absorbs heat, and thus how often the AC cycles. My solution is to run the AC at full cold, then turn it off and wait until I'm uncomfortable, and then turn it back on. My passengers are not always happy with this miserly approach.

 

[substitute materials]

Another option I like is to use when it is just slightly too warm is the defrost setting, which cycles the AC on a timer to provide dehumidification, in my old subaru outback its on for ~10 seconds then off for 10 seconds. This probably wears out the clutch on the AC compressor though...

 

[anorlunda]

My solution is to run the AC at full cold, then turn it off and wait until I'm uncomfortable, and then turn it back on. My passengers are not always happy with this miserly approach.

That's why I started this thread. I was tempted to do the same, but why torture myself and others if the logic is faulty.

Like Chemair said, the compressor does cycle, but this is pressure related. Changes to the blower fan setting will change how fast the evaporator absorbs heat, and thus how often the AC cycles.

That makes sense, but I'm still not sure if the question is answered. Let's say the AC provides 10 cfm of cold air (a made-up number). Moving the temperature control warmer might dump 5 cfm of the cold air and replace it with 5 cfm of warm air; thus leaving the AC heat load constant. I suspect this because the same control is used when heating or cooling, and in winter we simply dump the excess engine heat.

 

[substitute materials]

As far as I understand, the simple blue-to-red temperature knob is just controlling the flow of hot engine coolant into the passenger compartment heater core. That heater core and the AC evaporator are in the same airstream with the same blower, so when you turn on the AC and then turn the temperature knob towards red, you are cooling the air and then reheating it. So yes, I think the AC has to work just as hard as if you were running it at max. So intermittent operation would save gas. I'm sure there are different configurations out there though.

 

[russ_watters]

My impression is also that temperature control is maintained via mixing or reheat. The problem with cycling the compressor is that in a car it would cause rapid swings in temperature and humidity.

Have you ever noticed a strange smell when the AC's cooling is shut off, like the ozone smell after rain? I'm not sure what exactly causes that, but given that it is October and rainy, I've noticed when my car's automated climate control shuts off the compressor.

 

[jim hardy]

I can tell you that on my Ford Escort rotating the temperature knob moves a damper that mixes air from heater core with air from evaporator core.
and the manual says compressor cycling is normal, maybe as frequently as 20 seconds !.
Compressor cycle is i believe controlled by a suction side pressure switch not the computer, but it is sixteen years ago's technology.

I've noticed as outside temperature drops the compressor cycles increasingly often
which suggests to me the cooler temperature at the condenser makes it hold more refrigerant because it liquefies nearer the inlet
perhaps starving the evaporator?.. i need a measure of evaporator superheat...

Nowadays the computer is aware of compressor suction and discharge pressures as well as air temperatures.
So it could modulate dampers to hold desired temperature. Should be able to get at them from the Canbus port . ahh another someday project for Raspberry Pi and a Canbus to USB adapter..

not really any help, was I ?

old jim