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What's the most efficient way to run the AC in my car?

In my 2007 Subaru I can set the temperature to 65 F and the temperature will reach and stay at a comfy 65 degrees. But I imagine the A/C compressor works in an "on/off" sort of way and that the temperature is being maintained by mixing cold air with warm air. Therefore gas is going toward running the compressor to create cold air that isn't being used. Is this a correct understanding of the system? And if it is, does that mean it would be more efficient to run the AC on full blast, turn it off when it's cold enough, and repeat? Or is the difference in gas consumption negligible and not worth thinking about?

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You haven't said how old your car is... Old cars did indeed function as you describe with the AC compressor cycling on and off. It was either compressing, or it wasn't. You had to size the compressor for the worst case cooling need (i.e. relatively large) and when the compressor kicked in, it was quite noticeable (i.e. the engine would jerk as the compressor clutch closed). Of course, this is not very efficient, and it is really hard on the engine and compressor to operate in this manner. But those were the old days....

Nowadays, cars have variable displacement compressors. For high cooling needs, the compressor can work with a large amount of refrigerant, and then scale back to compress less for maintaining temp. So, when the clutch closes and the displacement is small, there is no jerking or sudden load on the engine or compressor. Some variable compressors don't even use a clutch. Basically, the compressor is controlled so that it does the minimum amount of work necessary to deliver the needed cooling. You won't improve on that through manual intervention IMHO.

  • Sorry, added make and year of vehicle to the question. I didn't know modern compressors worked that way. Thanks! – Josh Johnson Nov 19 '14 at 18:00
  • Yeah, I think your car has a clutchless variable displacement compressor. – bobfandango Nov 19 '14 at 18:12
  • When you say "nowadays" ... aren't you really talking about cars that are around a couple of years old or newer? It's my understanding this technology is very new, at least when applied to the automotive industry. Also, I'd bet not all new cars are using it. – Pᴀᴜʟsᴛᴇʀ2 Nov 19 '14 at 18:33
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    They have been around since at least the 90s. My old 96 Nissan Maxima had a variable displacement compressor. I'm not sure exactly when they became basically a defacto standard. I imagine there are still some cars that don't use them (GM still makes a freakin 2 valve pushrod engine!!). But most modern cars I think all use them. Small engines cannot deal with the sudden load of a full displacement compressor kicking in.... – bobfandango Nov 19 '14 at 19:19
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    It seems like this technology would only work on a vehicle that has a thermostat type climate control (where you set the desired temperature). On the type that just has a hot/cold knob, how would the compressor know how hard to work? – Nate Eldredge Nov 23 '14 at 5:12

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