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Old 08-06-2008, 09:55 PM   #1 (permalink)
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How to use your A/C more efficiently

OR... How to P&G your A/C
I split this into 3 sections since I'm verbose.
1. Reasons to use it
2. Theory behind a/c and how it works
3. Steps you need to take

1. This is an easy method of reducing your FE losses due to A/C while retaining some of its benefits. I have to use my A/C when I'm dressed up in a shirt and tie as well as when my wife and/or baby are in the car, and I use this to keep everyone, including my wallet, happy.

2. Basic air conditioner theory: Your air conditining is operated by an engine driven compressor, then two "radiators"; a high pressure, hot one that is warmer than the outside air, and a low pressure, cold one that is cooler than outside air. Thermodynamically speaking, when the compressor does work on vaporized refrigerant to compress it into a liquid, its temperature increases. This is an adiabatic process. The high pressure refrigerant travels from the compressor to the condenser, which is in front of your car's engine-cooling radiator. Since it's now very hot, it loses energy to the outside air flowing through it. After losing energy, it travels to the inside of the car, passes through an expansion valve that allows a small amount of refrigerant to pass through. The refrigerant then enters the evaporator core, an area of low pressure. The liquid refrigerant evaporates, absorbing energy from the air inside your car, and then returns to the compressor.
Your compressor does not run all the time your car is on; it also doesn't run all the time your a/c is turned on. Most cars' expansion valves are thermostatic; they work to maintain a constant temperature within the evaporator core. If the expansion valve only allows a small amount of refrigerant through, the pressure in the high side of the system doesn't drop. A sensor detects this and disengages the clutch on your a/c compressor.
Therefore, if you do what you can to keep the evaporator core cool, you can decrease the percent of time your compressor is running.

3. What to do: Two things will allow you to keep your evaporator core cool and your compressor clutched off. First, always use the recirculate function on your climate control. Already cool, dry air flowing through the evaporator won't warm it up as much as warming fresh air from outside.
If your car doesn't have a recirculate switch, chances are that it has a "normal a/c" and a "max a/c". Use the MAX! This seems counter-intuitive, but since the recirculating a/c works better, it's labeled as such, and it's also more efficient.
Second, once you get your car to a cool enough temperature, adjust the blower fan to the lowest speed setting. You still get cold air blowing on you, but the smaller volume of air that flows through the evaporator, the cooler it will stay.
Using these two steps, you can have your a/c and afford to eat for dinner too!

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Old 08-06-2008, 10:09 PM   #2 (permalink)
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Something I have wondered about this is Max A/C means the compressor does not work as hard, but it requires another fan to kick on (which is considerably louder than the normal fan, but probably irrelevant) which requires a larger load on the alternator.

So, which wins out? A more frequent but intermittent A/C compressor or a constant draw fan? I don't know the answer.
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Old 08-06-2008, 11:19 PM   #3 (permalink)
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The extra noise on max you here is a vent opening up right at the blower motor. Normally you can't hear it as bad since the vent is closed and it is pulling outside air. Look in the passenger side up under the dash when you turn it on max and you can probably see the vent open up and see the fan spinning.
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Old 08-07-2008, 07:48 AM   #4 (permalink)
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Quote:
Originally Posted by adrive7 View Post
Something I have wondered about this is Max A/C means the compressor does not work as hard, but it requires another fan to kick on (which is considerably louder than the normal fan, but probably irrelevant) which requires a larger load on the alternator.

So, which wins out? A more frequent but intermittent A/C compressor or a constant draw fan? I don't know the answer.
Coyote X is correct; the same fan is running, but with an interior intake open. This makes it more noisy and also blows harder since there's less resistance on the fan. I bet the blower motor uses the same amount of electricity on fresh or recirculating settings.
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Old 08-07-2008, 11:39 AM   #5 (permalink)
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Quote:
Originally Posted by gascort View Post
Coyote X is correct; the same fan is running, but with an interior intake open. This makes it more noisy and also blows harder since there's less resistance on the fan. I bet the blower motor uses the same amount of electricity on fresh or recirculating settings.
I was wondering about the blower energy usage between normal/max. On many vehicles the outside air intake is in the cowl area at the base of the windshield where there is an aerodynamic high pressure at highway speeds that might "assist" the air flow into the cabin. On Max or recirculate you're drawing air from cabin pressure and accelerating it back into cabin pressure.

OTOH most blowers are squirrel-cage fans that are very efficient at operating with suction on the intake, and on normal mode the blower is actually attempting to pressurize the cabin (unless it's very leaky or has a designed pressure relief) by cramming outside air inside. My truck has a pressure relief system on the rear cab wall that blows air out in front of the bed on normal mode so my blower doesn't blow harder on MAX it's just noisier.

I was interested to see this article since I have had my own thoughts on the subject, but come to find out everything you recommend I already do. I go one step further and actually turn off my compressor when I'm in MAX at low blower setting and have reached a comfortably cold temperature. My truck allows me to have the compressor on or off in any given setting but if someone's vehicle doesn't they could tap a switch in series with the refrigerant pressure switch circuit (the switch that detects the high-limit pressure between the condenser and the expansion valve that shuts off the compressor until the pressure drops) to manually turn off the compressor without being able to force it on. Which brings up one more point: if you want to be cool but not cold, do not turn up the temperature dial as it merely blends hot air into the conditioned air. Cycle the A/C on/off to make it warmer than the low-blower MAX A/C would normally make it to minimize compressor operation.

Additional point: When starting your car after it has sat in the sun lower the windows with the A/C on normal mode to pull the interior temp down. Outside air is cooler than cabin air at this point so normal is more efficient, plus it pushes the superheated cabin air out the windows. This might only take a minute or less before you feel cold air at the vents, at which point you can switch to MAX and roll the windows up.
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Old 08-07-2008, 03:33 PM   #6 (permalink)
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Quote:
Originally Posted by MechEngVT View Post
Which brings up one more point: if you want to be cool but not cold, do not turn up the temperature dial as it merely blends hot air into the conditioned air. Cycle the A/C on/off to make it warmer than the low-blower MAX A/C would normally make it to minimize compressor operation.
Excellent point: If your temp selector is at all off of the coldest setting, your heater control valve allows hot coolant into your heater core. Bad!!!

My wife's car has electric dual zone climate control. I hate it. Sometimes it gets colder than it was trying to get, so it turns on the heater to compensate, or vice versa in the winter it will turn on the a/c.
My wife and I always fight because in the winter she likes it warm, so she turns up the passenger side temperature. This makes the heat run on the right and the a/c run on the left - super inefficient and freezes the driver! I tell her to use a blanket!

Also, good point on open window usage in the first minute or two. Another thing to add is to use a reflective sun shade (I do even in the winter to keep the steering wheel cool and keep stuff from melting/fading) and to park facing South or toward wherever the sun will be if you have the option. Combining the reflective sun shade used properly and a cracked window or sunroof can drop the temp inside by 30F, and the temp of the steering wheel/seats by 50F.
After I remove the trim pieces that my roof rack used to fit in and fill with Bondo, I'm painting my whole roof a more reflective color. Probably white, but maybe, after seeing Johnny Mullet's transmission, ..... Kentucky Chrome
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Old 08-07-2008, 09:24 PM   #7 (permalink)
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The "Kentucky Chrome" looks real nice when it is first applied, but after time it dulls and fades to an almost white color. I would go with white.
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Old 08-07-2008, 10:36 PM   #8 (permalink)
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Quote:
Originally Posted by Johnny Mullet View Post
The "Kentucky Chrome" looks real nice when it is first applied, but after time it dulls and fades to an almost white color. I would go with white.
Haha, around here, we call it Brocton Chrome, as in Brocton, NY-- the most ass-backwards of ass-backwards rural hell holes.

To add to the discussion, I had really good results pulsing the AC on recirculate last weekend-- I got 37mpg, my best tank yet, despite a lot of highway miles. Of course, I've also been neutral coasting more, and EOC'ing slightly more, to that helps too. I also did a lot of idling while trying to revive and jump my girlfriend's Chrysler (it's as big as a whale!), so I probably could have done better, actually.

I run the AC on recirc until it's comfortably cool, then turn it off, with the fan still running. When I start to get warm again, I turn it back on.
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Old 12-03-2009, 02:11 PM   #9 (permalink)
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two thoughts:
the first I have done successfully, the second just an idea
1) on my Toyota Land Cruiser (I know, I know, but I live in rural costa rica, a metro would be a zero mpg vehicle for me) there is an auxiliary fan on the front that runs when the AC is running, to help cool the condenser up front. Toyota includes a "medium" pressure switch on the AC system that is normally open, but closes when the pressure is above a given level. I don't have the number handy, but it is at the middle of the AC pressure spectrum. In any event, tapping the fan control relay into this switch means that instead of cycling the fan w/ the compressor or the AC-on button, now the fan turns on when the pressure in the condenser builds up, and turns off when the condenser is cooler/lower pressure. I don't think that this saves an awful lot of energy-- but it does mean that the compressor seems to cycle less often, which saves some energy, and that the fan only runs when needed (ie: at higher speeds the airflow is sufficient to cool the condenser and the fan doesn't run).

2) Here's the mod I haven't tried (yet) but plan to do-- my A/C blows cold. Very cold. Colder than I want. On LOW in the evening I get uncomfortable. I don't want to adjust the temp in the cab as that just puts engine heat in to fight the cooling and is wasteful. So I'm looking at adding some sort of system in to fool the system into shutting down the compressor with a little less pressure in the system. I think that putting in a switch and a resistor should give me the option of "low setting" on the AC. Or adding a potentiometer can make the level of coldness variable. Anyone who knows more than I do about how the thermistors and pressure sensors work... please pipe in!
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Old 12-10-2009, 09:15 PM   #10 (permalink)
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Sandcruiser "I don't want to adjust the temp in the cab as that just puts engine heat in to fight the cooling and is wasteful. "

I totally agree and wish that someone would find a way to make a variable output compressor that would only work hard enough to get to the temperature that you want instead of hard enough to freeze a Popcicle in Phoenix in August, then diluting that hard worked cold air with hot water from the cooling system.

I just cycle between A/C and vent and try to spend as long as possible in the vent setting to reduce the engine load.

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