Quote:
Originally Posted by talmy
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You can also see that the faster you go, the less the AC costs per mile. If you open windows, the drag will cost more at higher speeds. So there is probably a certain speed that represents the crossover point. I'd guess that in town the windows should be open and on an open highway they should be closed.
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This is mostly accurate given the recent data I've been seeing from my Graham miniscanner. In the case of the older 2001-03 Prius, at speeds below 42 miles per hour, the engine has to run to operate the compressor. So typical idle, engine air flow runs about 2 g/sec but with the compressor, 2.45 g/sec. The problem is we don't see as much 0 g/sec with the engine off. But that changes once the car is going over 42 miles per hour.
Over 42 miles per hour, the engine runs regardless of load. This means the 0.45 extra g/sec is pretty much just part of the ordinary operation. However, it also means that as the engine load increases with higher speeds, the AC can push the engine into inefficient modes. What this means is cruising at 65 miles per hour w/o AC is closer to cruising at 60 miles per hour with AC.
It is important to use air recirculate to prevent cooling hotter external air than internal. Also, set the internal temperature control to as warm as is comfortable . . . avoid seeing condensation on the exterior glass.
As for air drag effects from open windows, if they are partially open so as to get a breeze versus a full blast, the effects appear to be minimal although I don't have numbers, yet.
GOOD LUCK!
Bob Wilson