Question about HOW a car AC works...
 

I love my 2000 Civic's AC system... its is still super frosty after 9 years.

I have a question about exactly HOW it works. There is a round dial for adjusting the temperature from coldest to hottest. Does that dial simply adjust how much of the air is passed through the heater core, or is it smart enough to also control the duty-cycle of the AC unit? I'd like to think turning it to 3/4 cool means the AC is running less, and not just fighting the heater core.

Hope that makes sense... anybody know?

Many newer systems do control the duty cycle. Old ones would just mix in some heat from the heater core to moderate the cold temperature - not ideal. I'm not exactly sure how ours work. Do you have a scangauge or something? You could watch the GPH as you adjust the temperature and see if it changes, or if it cycles up and down.

I do have a Scangague II. I wonder if the engine water temp will decrease when I turn the knob from coldest to the first tick from coldest...

I figured the best case scenario is the AC temp knob controls a thermostat, and when the cabin temp reaches a certain level it kicks on the pump until it cools back down.

Maybe I'll listen to the servo's while the cars off and see if I can figure it out.

I know my 2004 Odyssey does have a thermostat like that. You set the temperature you want and it does whatever is needed to get there.

Our civics on the other hand... I think they might cycle the compressor on and off, but i'm not sure.

I'm also not sure what is used on newer cars. I'm pretty sure my Paseo doesn't cycle according to temp set (on the very rare occasion when I do use it). So, I always keep my temperature set to full cold and adjust cabin temps with the fan speed and vent positions. I do the same with the Matrix.

I do this as well.

A modern A/C will cycle. It does that naturally, however it dosen't cycle based on the temperature you set in the cabin as far as i know. Rather it cycles based on the pressures the A/C system sees. If your driving on the highway, and don't have a grill block however, the highside pressure won't get high enough to tell the compressor to take a break I don't think. This isn't something that gets tested in the EPA test, therefore there is little incentive to have a smart A/C. However, many A/C's will cycle off during heavy aceleration.

What would be damned cool would be a 2-stage compressor. 1 for light cooling needs, 1 for texas.

-Steve

I believe the 2008 EPA testing does turn on A/C, but I'm sure on the specifics of it.

The newer "climate control" systems have thermostats. If it doesn't have "climate control" it doesn't monitor interior car temps at all. It could be 4 degrees F in there and it would still try and pump what it thinks is cold air in.

They won't be fighting the heater core. Even really really old systems don't do that. If you set it to heat the air flows by the engine. If you set it to cold it blocks off that loop so it doesn't preheat the air. You are fighting the external temps and your AC runs full tilt in anything not climate control. Best bet is to run it full-tilt until it reaches the not very comfortable chilly point, cut AC and recirculate, kick AC back on as it gets to the warm part of the comfort but leave it on recirculate(you are no longer cooling from external temps of say 80-90 but interior temps of 70-80).

This is a higher user input form of climate control thats gives you the same efficiency(slightly more climate control will not automatically recycle air unless you tell it to).

The new EPA testing does turn it on, however, they'll just run it straight cold, with no benefit to use a partial A/C on type scheme I believe.

So, you are saying there is another loop, seperate from the heater core, used to provide heat from the engine to the air when the A/C is turned on, but is set all the way to coldest?

Either I'm missing something, or that's a whole lot of unnecessary complexity... Be a lot easier, and just as efficient to run a small amount of air to through the heater core, and the rest over the A/C condensor.

I guess my real question here is, if I have a dial with 30 clicks between hottest and coldest, what does the first click from coldest do to make it not as cold?

I THINK some of it runs through the heater core, and some past the condensor. However, i don't know.

-Steve

With most a/c set ups the air is first sent over the a/c condenser to make it dry and also cold and then if needed it is sent through the heater matrix / core to add whatever heat is deemed necessary.

Many drivers want both warm air and dry air in a humid environment to defog windows etc.

Also auto a/c compressors are notoriously inefficient due to the operating rev range they are needed to work over.
If they were even close to the efficiency levels in household a/c units they would not last at speeds much above idle.

Pete.

I think the original question as I understand is valid and representative of healthy curiosity, but I think the answer could be more tailored to comprehensive understanding if there is a secondary reason behind asking.

Peter7307 is correct (except air is cooled at the evaporator in the heater box; the condenser is up by the radiator and gets HOT). Cars with A/C have two heat exchangers in the HVAC box; an evaporator and a heater core. From there I'm aware of two different ways to control temperature that may be used separately and/or together. Some vehicles have a shutoff valve in the coolant line feeding the heater core that will stop the flow of hot coolant when no warming of the air is desired. Other vehicles have a "blend door" that controls the flow of air between the blower and the vents and allows varying proportions of air to flow over the heater core by changing position of a flap in the ductwork. AFAIK both could be easily used together.

The cycling of the A/C compressor is based on the refrigerant pressure. When running fully cold with a low blower speed the evaporator will get very cold as there is little air blowing over it to warm it up. When this happens the refrigerant regains less pressure after passing through the evaporator, decreasing the pressure on the low side of the compressor. Some systems have a low-pressure cycle switch that shuts off the compressor to prevent freeze-up until the low-side pressure increases (which will happen as the evaporator warms). I'm not sure if many vehicles do this, but one could detect an increase in high side pressure that will correspond with the same events as the decrease in low-side pressure and control the compressor in that way.

To improve fuel economy while using air conditioning the goal should be to maintain the coldest evaporator possible. Operating the ventilation system in recirculation mode allows the A/C to chill already cooler air from inside the car to keep the car cool while operating the compressor less. Using as low a blower speed as you are comfortable with helps as well. At night, in the shade, or when it isn't too hot outside you may notice that in recirc mode at low fan speed it still gets too cold. Rather than cooling outside air or turning up the temperature to warm the air with engine coolant you should turn off the compressor if your vehicle allows you independent control of the compressor (my truck has a snowflake button). When it begins to warm up again you can turn it back on.

Modern cars cycle based on pressure of both high and low sides. They have a high side pressure switch, and a low side pressure switch. The low side pressure switch will not engage if the pressure is too low, and the high side pressure switch will not engage if the pressure is to high.

This also prevents the A/C from running if the coolant has leaked out!

-Steve

MechEngVT,
Yes you are correct.
The evaporator in the heater box is where the cooling takes place not the condenser at the front of the vehicle.

Got those two mixed up.

Pete.