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Old 08-08-2007, 08:10 PM   #1
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a/c question

Okay my car doesn't seem to care where I set the temp control knob. When the a/c is on, I loose a healthy amount of MPG(say 10-15%) even if I use very conservative settings. Like low fan and dead center of temp control.

I have a variable displacement a/c compressor. Is there anyway to make this thing more efficient. My previous vehicle had a fixed displacement auto cycling system that seemed to be better suited for conservative use. I'd like to a little more comfortable without loosing 15% of my mileage for a few degrees cooler cabin.

He's a thread I posted for saturn specific info.
http://www.saturnfans.com/forums/sho...d.php?t=104359
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Old 08-08-2007, 08:12 PM   #2
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where do you live? dosent help but just wondering so i know how hot it is there
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Old 08-09-2007, 12:01 AM   #3
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Pulse and glide air - con ?. Turn it to maximum cold settings, and use it in bursts, with fans on 4, recirculating. Also, if doing long journeys, can you get a reflective sun shield to put in one of the rear side windows where the sun is shining?. And if you have a sunroof, close the cover for that to stop the sun shining through it?.
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Old 08-09-2007, 06:26 AM   #4
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I believe that when you turn the temperature dial up, you are probably just putting hot water through the heater core. So I doubt that that saves you anything, you know?
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Old 08-09-2007, 06:46 AM   #5
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Pulse and glide air - con ?. Turn it to maximum cold settings, and use it in bursts, with fans on 4, recirculating....
Not sure how well that would work. Thats what I did when I JUST joined this site. Now Im more hard-core and just sweat unless g/f is in car. I just got my ScanguageII to discover that my car idles at about .3-.5gph, but with the AC on it idles using about .6-.8gph essentially doubling my idle use of gas. Wheh I turn the AC off the gallons per hour go back down. However, yesterday when I was running late and could not arrive sweaty, I used AC on high on and off and noticed that gas used when gliding with AC was .9gph and it STAYED that way even when I shut off the AC while gliding. I was pissed. It was killing 200mpg glides and cutting my numbers in half. Sometimes after turning of the AC and turning the FAN on then off it might go back down to a regular netrual glide level. So there is a lot more testing to be done. But I rarely use AC anyway, so its not a heavy topic for me right now. Like I said, Ive learned to live without AC and just come out the car sweaty and stinky yet happy about my FE.

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Pulse and glide air - con ?. Turn it to maximum cold settings, and use it in bursts, with fans on 4, recirculating. Also, if doing long journeys, can you get a reflective sun shield to put in one of the rear side windows where the sun is shining?. And if you have a sunroof, close the cover for that to stop the sun shining through it?.
This is EXCELLENT advice. Also when you first drive off from a hot car. Drive the first few minutes with AKLL the windows down to get all the HOT STALE air out of the car. It is much easier for the AC to cool down fresh warm air from outside than to cool down stagnant HOT air thats been baking 'greenhousing' in your car since you parked it. Its kind of like trying to cool a hot oven with dried ice and a small fan. You would open the oven and let the hot air ESCAPE before and THEN try to cool it down. My two cents

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Old 08-09-2007, 07:12 AM   #6
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I believe that when you turn the temperature dial up, you are probably just putting hot water through the heater core. So I doubt that that saves you anything, you know?

In most cars, as you turn the temp dial up you are moving "doors" that mix cool air from the evaporator with hot air from the heater core. When you are on a warmer setting, there isn't as much cabin air (or warm air) flowing through the evaporator. When this occurs, the compressor isn't on as much.
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Old 08-09-2007, 07:18 AM   #7
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landspeeds post is the best, you want to minimize the reheating up of your cabin, so reflectors are a good idea.

I do stanley says and until I go above 35-40MPH (first 3-5 mins of my trip or so), I ride with my windows open (even if it's sweltering out) and fan on medium and not recirculate. I do this to get the stale even hotter air out of the vent ducts and my car as much as possible. As I cruise onto the interstate I close the windows on the off ramp and accelerate and merge, etc, I go as long as I can without AC but if it's just that bad I turn on the AC full with recirculate, that's where it will run the most efficient.

I guess I don't know for sure but I always though the compressor itself was really only on or off, there is no parital on and only draining 5-10% instead of the full 15%. If it's on and you tell it to maintain 70F I suspect it either cycles the AC or just adds some heat to the cold AC to balance it. This is why I do bursts, high AC until I'm almost cold, then turn it off and go as long as I can before I just cannot live without it again. I also turn it off if I'm going to accelerate or glide. I try to turn it off a good 5-10 secs before accelerating or gliding because I've found it can stay 'on' for a while. That is, it's not actively trying to cool down the cabin, but it must have some kind of cool down cycle before the compressor goes offline (I'm thinking maybe to prevent line freeze conditions in the condensor in super humid evironments?). I'm not sure, because sometimes during accelerating I can turn the AC off and instantly feel the increase in power, so sometimes it goes off quickly. Either way I maintain medium or so recirculating fan speed because there's a little bit more chill in the AC radiator even when off and because a fan at the then current cabin temp feels pretty good for a while.

I'm not sure if any of this helps, I know I feel bad whenever I use the AC, but sometimes I just can't live without it. Somehow I got one pretty good tank though, I really can't believe that tank so I hope to post a few more the same or better.

Some cars turn the AC on when you defrost but use full heat then too, to put out hot and dry air (AC to dry the air), this seems very inefficeint.

I'd say load balancing is the best thing to do, if you can add the tax on power and torque during your 'pulsing' or up a hill and still maintain high gear and max torque throttle then I don't think the penalty is to bad. If you ever have to downshift then the AC is hurting you a lot. And I think during glides or moderate throttle it hurts a lot too. At max torque (about 50-60% throttle for me) in top gear going up a hill I figure AC just makes it the 'same' as if I were going up an even steeper hill. Taking the loss when I've got my engine most efficient should cause the least impact.
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Old 08-09-2007, 07:20 AM   #8
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Originally Posted by mrmad View Post
In most cars, as you turn the temp dial up you are moving "doors" that mix cool air from the evaporator with hot air from the heater core. When you are on a warmer setting, there isn't as much cabin air (or warm air) flowing through the evaporator. When this occurs, the compressor isn't on as much.
So the compressor only works as hard as however fast you heat up the internal radiator (ie, blow cold air)?
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Old 08-09-2007, 07:55 AM   #9
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Originally Posted by itjstagame View Post
So the compressor only works as hard as however fast you heat up the internal radiator (ie, blow cold air)?

See this link

http://www.familycar.com/ac1.htm

If less warm air is going through the evaporator, the thermal expansion valve lets less refrigerant into the evaporator. I believe the comressor is pressure regulated, meaning if the expansion valve is closed, the pressure on the high side goes up and then the comressor shuts off to keep it from overpressurizing. This is why you hear the A/C system turing the compressor on and off while the system is running.
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Old 08-09-2007, 07:58 AM   #10
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Originally Posted by mrmad View Post
In most cars, as you turn the temp dial up you are moving "doors" that mix cool air from the evaporator with hot air from the heater core. When you are on a warmer setting, there isn't as much cabin air (or warm air) flowing through the evaporator. When this occurs, the compressor isn't on as much.
Okay, still, I doubt that adding hot air to the cabin is the most efficient way to keep it cool. KnowwhatImean?
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