Gas mileage and AC question

[spidey07]

well this isn't really so much a car question as it is a HVAC question.

And going on that steady cooling without rapid swings in temperature are more efficient than running full blast and letting temps swing around.

So yes - full blast and off is less efficient (requires more energy hence poorer gas mileage) than steady cooling, hopefully with some kind of thermostat.

I'm sure there has to be a web page that explains it better than this. Take the car out of the picture, you are only talking about engergy and cooling.

 

[Kenazo]

Originally posted by: dxkj

Originally posted by: jndietz
i don't believe so. i don't believe it changes how cold the A/C gets, it only controls fan speed.

My argument was that fan on high would cause the ac to warm up (liquid whatever), thus requiring it to use more energy to cool it down again.

I think that would be a fair assumption, but the effect you're talking about would probably be quite minimal.

 

[C'DaleRider]

Chrysler, for one, has had them installed in their vehicles since the late 1980's. Supposedly saves a tad of gas and makes the compressor last longer. Less shock to the system with all the on-and-off cycling the typical A/C compressor does. The variable displacement compressor does have a clutch, but it engages once when turned on and stays engaged until the A/C is turned off.

The basic compressor mechanism is a variable angle wobble plate with six axially oriented cylinders, at least in older Mopars with the VCD. It's supposed to match the car's A/C demands under all conditions without the clutch cycling. The wobble plate instead increases or decreases its angle by using the crankcase-suction pressure differential. This effectively increases or decreases the displacement of the compressor.

When the heat load at the evaporator decreases, the suction pressure decreases. The result is a small wobble plate angle and therefore, a small piston displacement for the small heat load. When the heat load increases at the evaporator, the suction pressure increases and gives a larger wobble plate angle and therefore, a large piston displacement in the compressor.

Now you have to contrast that with the more familiar fixed displacement compressor. Every time it engages, it is operating at full capacity, either on or off. The control for it is in the temp setting you've chosen to maintain inside the vehicle vs. the outside ambient temperature. The cycling is then maintained as the inside thermostat trips the compressor into its ON cycle to get the inside air as cool as you want, and when it achieves that, turns off. So the compressor either gives the engine a no-load or full-load situation, nothing inbetween.