Question about HOW a car AC works... - Fuel Economy, Hypermiling, EcoModding News and Forum

hondaworkshop · 06-11-2009, 11:28 AM

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?

PaleMelanesian · 06-11-2009, 11:50 AM

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.

hondaworkshop · 06-11-2009, 12:03 PM

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.

PaleMelanesian · 06-11-2009, 12:07 PM

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.

Daox · 06-11-2009, 12:25 PM

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.

stevey_frac · 06-11-2009, 12:50 PM

Quote:

Originally Posted by Daox

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

Daox · 06-11-2009, 01:02 PM

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

theunchosen · 06-11-2009, 01:04 PM

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).

stevey_frac · 06-11-2009, 01:07 PM

Quote:

Originally Posted by Daox

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

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.

stevey_frac · 06-11-2009, 01:09 PM

Quote:

Originally Posted by theunchosen

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).

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.