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Old 01-21-2007, 12:44 PM   #1
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Lightbulb Strategically turning on AC?

Last summer I was driving a big 1991 Ford Thunderbird LX with the 5.0 HO engine. Even with that powerful V8 the air conditioning would dampen acceleration noticeably in city driving.

Throughout summer of 2006 (since I was trying to conserve fuel while delivering pizza) I would turn off the AC every time I needed to accelerate and only turn it on when holding a steady speed or slowing down.

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I never did comprehensive calculations of mpg during those times, but I wonder if it makes any difference with FE? Something to think about if you use AC often.
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Old 01-21-2007, 01:20 PM   #2
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If you are in town, then whenever you are coasting downhill, and are going to have to slow down, that is a good time to put the A/C on - in fact anywhere where you need to lose speed. Just keep the engine at e.g. 1500rpm using the gears, with the AC on, thereby utilising 'free' energy as it will just go into the brakes anyway, later. The question is whether you can get enough cooling from this alone.

A good time to use the AC may be when driving in neutral, although haven't yet had the chance to quantify this. (And my cars aircon is not working at present).


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Old 01-21-2007, 01:37 PM   #3
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landspeed -

Downhill + in gear + AC on to lose speed and get some free AC? Sounds cool (literally and figuratively). I will try it. What I have noticed is that when I am idling and I turn on the AC, the ScanGauge shows a load increase of about +10. I would guess that this would follow an inverse displacement/cylinder rule something like this :

1.0 3 Cylinder => +13.5 load
1.9L 4 Cylinder => +10 Load (me)
3.8L 8 Cylinder => +5 load

On a Saturn-only note, Saturns suffer from an engine overheat problem when idling in traffic. The coolant temp can go to 220 degrees F before the electric fan will kick in to bring it back down to (my) normal temp of 190 degrees F. However, there is a trick. If I turn on the AC for about 5 seconds only, the electric fan will also turn on and stay on for about 2 minutes.

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Old 01-21-2007, 01:38 PM   #4
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Maybe a vacuum switch on the intake manifold could automate the A/C cutout: when you're accelerating, the vacuum will always be low.
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein
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Old 01-21-2007, 03:03 PM   #5
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I was pretty sure that most cars already had a vaccum switch that kills the a/c when the engine is lugging/wide open throttle, but maybe that is just the cars who's repair manuals I make a habbit of reading (you'd be suprized), but I do know that alot of cars that come with factory a/c have some sort of a/c cut out, but it's most likely all in how far you have to open up the throttle befor it cuts out, but I don't see why you couldn't alter it to give less a/c.
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Old 01-21-2007, 04:23 PM   #6
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No question that A/C increases drag on the engine.

I don't like to switch it on and off so quickly, I'm afraid it will shorten compressor life.

What I usually do, however, is turn the A/C on just after going up a hill or accelerating, and if I'm going to turn it off I will wait until after I slow down or go down a hill.

I also like to leave the compressor off for at least 3 minutes before turning it on again. (I'm not sure that is still necessary with R134a however)

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Old 01-21-2007, 05:00 PM   #7
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Some A/C compressors (mine for one) have a variable displacement. It doesn't cycle on/off to maintain some temp, but modulates the throughput to match the setting. Low engine speeds mean more displacement for the same cooling as a low compressor displacement at a higher RPM.
It's installed in a diesel (no throttle, so no manifold vacuum to sense) with a 'drive-by-wire' electronic accelerator pedal. Pressing the pedal rapidly, even if not necessarily to the floor, will cause the electronics to cut the compressor completely. Once the fuel delivery sensors determine that the torque requested isn't that close to the maximum available, it re-engages the A/C compressor clutch. I imagine that red light to red light downtown city use would have my A/C switching on and off every 30 to 45 seconds. I'd hope that the designers had considered that driving possibility and developed a lifespan appropriately.
But then again, the dew point has to be 80F+ for me to use it at all.
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Old 01-22-2007, 04:33 AM   #8
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I go down hill and turn up the fan and found that they injected more gas even with my foot off the pedal - the xB increased speed decelerated less and the SG showed more fuel use. Turning it to low speed and recirculate once the evaporator got cooled off showed very little power use - good old variable compressor technology works great. Don't worry about turning it on and off - it's not an electric motor driven pump so no problems starting up. Unless a Hybrid!!

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