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Old 08-07-2007, 05:46 PM   #1
lovemysan
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Default a/c, how does it work?

On my previous cars I would use the a/c very conservatively. I would set the cabin temp around 80f. This would allow me to be comfortable and still get decent mileage. On my saturn it doesn't seem to matter where the temp is set the compressor doesn't ever kick off. Running at 75mph with a/c on I was getting 39mpg. I tried several different a/c settings and could not improve the mileage.

Can someone tell me why the compressor doesn't cycle on and off. And if there is a way to improve this.

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Old 08-07-2007, 05:56 PM   #2
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Default Re: a/c, how does it work?

I wouldn't be complaining about 39 mpg at 75 mph with the AC running.

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Old 08-07-2007, 05:59 PM   #3
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Default Re: a/c, how does it work?

Quote:
Originally Posted by lovemysan View Post
On my previous cars I would use the a/c very conservatively. I would set the cabin temp around 80f. This would allow me to be comfortable and still get decent mileage. On my saturn it doesn't seem to matter where the temp is set the compressor doesn't ever kick off. Running at 75mph with a/c on I was getting 39mpg. I tried several different a/c settings and could not improve the mileage.

Can someone tell me why the compressor doesn't cycle on and off. And if there is a way to improve this.
The compressor only cycles off when at wide open throttle. There is no temperature setting, this isn't your grandmother's Lincoln, it's a Saturn. Once turned on, the a/c is on, unless you floor it, and then it turns off for acceleration. The only "settings" you can control, as I'm sure you can see, is the blend between hot/cold air, and how high the fan is set, plus whether the source is recirculated air, or fresh outside air. Running 39 mpg's with the air on is phenomenal, and you shouldn't complain.

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Old 08-07-2007, 06:07 PM   #4
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Default Re: a/c, how does it work?

If I'm not mistaken, our a/c compressors are the variable displacement types that can stay running constantly but use less engine power for better fuel economy as you just posted. I find nothing negative about having your cake and eating it with 75 mph speeds, a/c use, and 39 mpg!?

The improved compressors will someday run w/o being able to turn them off because of internal modifications that vary the output according to certain design pressures to allow greater efficiency compared to the original compressors. If you had an old compressor you wouldn't even approach 30 mpg and would feel the power robbing drag of it at any speed, let alone speeding along at 75mph. As in life there are compromises and to increase mpgs you'll have to bite the bullet, either run w/o a/c or lower your speed.

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Old 08-07-2007, 07:57 PM   #5
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Default Re: a/c, how does it work?

Quote:
Originally Posted by fdryer View Post
If I'm not mistaken, our a/c compressors are the variable displacement types that can stay running constantly but use less engine power for better fuel economy as you just posted. I find nothing negative about having your cake and eating it with 75 mph speeds, a/c use, and 39 mpg!?
To my knowledge they are not. They are cheaper fixed displacement units and they have a low pressure cutoff that act not only as a safety but will cycle compressor when heat load is low enough and also prevent it from running at all in really cold weather. R134A system also usually have a high pressure cutout too because unlike R12, R134 pressure can spike wildly when temps get high enough (pressure curve is not linear)

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Old 08-08-2007, 12:19 AM   #6
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Default Re: a/c, how does it work?

A/C compressor does NOT go on and off based on load. High/low pressure sensor is a system protector.

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Old 08-08-2007, 05:27 AM   #7
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Default Re: a/c, how does it work?

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Originally Posted by madpogue View Post
A/C compressor does NOT go on and off based on load. High/low pressure sensor is a system protector.

No so, I have a few big GM vehicles and on moderate days the compressors does cycle when cooling. This happens because there is more cooling capacity available in expanding refrigerant than needed and the cooler the evaporater gets, the lower the pressure gets because vapor pressure of refrigerant drops with temperature. When this happens if it is not cycled evaporator can freeze with time and the low pressure switch does this as well as prevents operation when low on refrigerant. This setup is cheaper to build than using variable compressors or variable expansion valves. (your home window A/C mostly still has a variable expansion valve)

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Old 08-08-2007, 09:22 AM   #8
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Default Re: a/c, how does it work?

Quote:
Originally Posted by 97coupe View Post
I have a few big GM vehicles and on moderate days the compressors does cycle when cooling.
And this has what to do with our Saturns, exactly?

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Old 08-08-2007, 09:45 AM   #9
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Default Re: a/c, how does it work?

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Originally Posted by madpogue View Post
And this has what to do with our Saturns, exactly?
GM A/C's is what. GM did not reinvent the A/C for a Saturn.

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Old 08-08-2007, 10:17 AM   #10
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Default Re: a/c, how does it work?

GM didn't "reinvent" anything, since non-cycling A/C was already around. Where is the documentation that every GM car, including Saturns, uses cycling A/C? Using the "my other GM cars..." thinking, I had a GM car with a two-speed automatic tranny. Therefore....

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Old 08-08-2007, 12:19 PM   #11
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Default Re: a/c, how does it work?

Quote:
Originally Posted by fdryer View Post
If I'm not mistaken, our a/c compressors are the variable displacement types that can stay running constantly but use less engine power for better fuel economy as you just posted. I find nothing negative about having your cake and eating it with 75 mph speeds, a/c use, and 39 mpg!?
Lets just say I'm accustomed to more than 39mpg. For the time being I'm manually cycling the compressor or just rolling the windows down. My nissan pickup had a basic system and it cycled on/off based on load. I'm now wondering if there is a way to retrofit my current a/c to a similar method of control. Seems kinda silly to put an inefficient a/c setup in an economy car. I'm looking to hit 45-46mpg using a/c lightly at 70-75mph.

Thanks for the information guys!

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Old 08-08-2007, 12:30 PM   #12
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Default Re: a/c, how does it work?

I hate you !? I'm only squeezing 21 in the city and possibly (I forget) 28 on the highway with my L300. I'm impressed that anyone could easily achieve 39 mpg with a/c and 75 mph. As I stated previously, but not sure of, some Saturns (mine) have the newer variable displacement compressors to help with fuel economy and place lower loads upon the engine and I think yours would be one of them too. The Saturn techs would have to confirm this though.

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Old 08-08-2007, 12:48 PM   #13
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Default Re: a/c, how does it work?

I was wondering the same thing. Yeah, it's a non-cycling, variable-displacement system, but how is that more "inefficient" than a cycling system? Wouldn't it simply put less load on the serp belt when there's less of a load? Is that less efficient than cycling on and off all the time? Or is it more efficient?

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Old 08-08-2007, 01:26 PM   #14
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Default Re: a/c, how does it work?

Recalling from memory of my FSM, the vdc uses pressure within the compressor to regulate displacement of refrigerant. The text further explained that by varying the displacement more or less refrigerant is being pumped depending upon demand of the thermal expansion valve. Since the txv controls pressure to the evaporator coil that effectively regulates temperatures to above freezing for maximum heat exchange, refrigerant must build up behind the txv backing up to the compressor. If the compressor can sense too much head pressure then Im guessing the internal pressure regulator changes the swashplate to output less refrigerant. There's a video from one of the compressor manufacturers that shows an elegant cutaway of the variable displacement compressor as it changes displacement. This is far more efficient than the old fixed displacement compressors. Most vehicles are already using them but some seem to think that progress hasn't taken place. I was certainly surprised to find this out but understand the improvement in fuel economy and less power loading on the engine so I think the op has one in his car to be able to achieve the high mpgs going 75 mph.

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Old 08-08-2007, 03:35 PM   #15
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Default Re: a/c, how does it work?

Quote:
Originally Posted by madpogue View Post
GM didn't "reinvent" anything, since non-cycling A/C was already around. Where is the documentation that every GM car, including Saturns, uses cycling A/C? Using the "my other GM cars..." thinking, I had a GM car with a two-speed automatic tranny. Therefore....
I really hate to burst your bubble but GM has been using "cycling" A/C's since the late 70's when they did away with using variable expansion valves to cut costs. Orifice is fixed and when heat load is low enough that evaporator gets cold enough the pressure drops the systems cycles out. If the low pressure cutout switch (which is adjustable BTW on most models) is set too low the evaporator can freeze up (I have a R12 car that does that sometimes if blower is set too low) but with R134 it does not happen as much because it has a lower heat content than R12 and a higher boiling point temp too. Most car A/C R134 system have little reserve capacity by design and do not tend to cycle out as much. With a fixed orifice they kinda depend on pressure curve of refrigerant to control flow and cooling too. As evaporator get colder the return pressure is lower and the high side pressure drops which take a little less HP to run and lower pressure also lower expansion rate or flow across valve/orifice.

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Old 08-08-2007, 05:05 PM   #16
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Default Re: a/c, how does it work?

SL2 auto. AC = 28 mpg at 65 mph. I "hate" you!

ick

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Old 08-08-2007, 05:19 PM   #17
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Default Re: a/c, how does it work?

My AC cycles... I turn it on full blast, when my car gets cold, I turn it off. When it warms up again, I turn it on full blast...

This is only on the really hot days. Otherwise I always have my sunroof open, and roll my windows down when I'm toodling through town.

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Old 08-08-2007, 09:38 PM   #18
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Default Re: a/c, how does it work?

Quote:
Originally Posted by 97coupe View Post
I really hate to burst your bubble but GM has been using "cycling" A/C's since the late 70's when they did away with using variable expansion valves to cut costs. Orifice is fixed and when heat load is low enough that evaporator gets cold enough the pressure drops the systems cycles out. If the low pressure cutout switch (which is adjustable BTW on most models) is set too low the evaporator can freeze up (I have a R12 car that does that sometimes if blower is set too low) but with R134 it does not happen as much because it has a lower heat content than R12 and a higher boiling point temp too. Most car A/C R134 system have little reserve capacity by design and do not tend to cycle out as much. With a fixed orifice they kinda depend on pressure curve of refrigerant to control flow and cooling too. As evaporator get colder the return pressure is lower and the high side pressure drops which take a little less HP to run and lower pressure also lower expansion rate or flow across valve/orifice.
From the FSM: "The Saturn compressor is a variable displacement rotary vane type pump....The compressor has an internal control valve that allows it to change its pumping capacity or displacement, reacting to heat loads on the air conditioning system."

Where/how do you adjust the low pressure switch? I couldn't find anything in the FSM, but then again, the index sux.

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Old 08-08-2007, 09:49 PM   #19
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Default Re: a/c, how does it work?

Quote:
Originally Posted by fdryer View Post
Recalling from memory of my FSM, the vdc uses pressure within the compressor to regulate displacement of refrigerant. The text further explained that by varying the displacement more or less refrigerant is being pumped depending upon demand of the thermal expansion valve. Since the txv controls pressure to the evaporator coil that effectively regulates temperatures to above freezing for maximum heat exchange, refrigerant must build up behind the txv backing up to the compressor. If the compressor can sense too much head pressure then Im guessing the internal pressure regulator changes the swashplate to output less refrigerant. There's a video from one of the compressor manufacturers that shows an elegant cutaway of the variable displacement compressor as it changes displacement. This is far more efficient than the old fixed displacement compressors. Most vehicles are already using them but some seem to think that progress hasn't taken place. I was certainly surprised to find this out but understand the improvement in fuel economy and less power loading on the engine so I think the op has one in his car to be able to achieve the high mpgs going 75 mph.
So I assume that I have a variable displacement compressor and that the displacement is controlled by the evaporator pressure. I guess there's not a good way to control the displacement myself.

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Old 08-08-2007, 10:28 PM   #20
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Default Re: a/c, how does it work?

you will not be able to do a better job than the current setup is doing.

if you want better millage buy a TDI Jetta, a Prius or just drive slower.

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