Influence of window opening vs. A/C use on fuel economy

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Gordon McGrew wrote:


I thought the compressor worked less when the Vent is not always bringing in Warm air from outside, I wonder what difference that would have made. Personnally though, I worry about having it in Re-Circulate mode on a long trip and not getting enought Oxygen and staying alert. Bg
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On 7 Sep 2006 14:03:30 -0700, snipped-for-privacy@hotmail.com wrote:

I think it brings in some fresh air even in recirc mode.
Regarding your point on the compressor; This car does not have a thermostat but I do sometimes sense that the compressor is shutting off sometimes. Not so much that I can say for sure. Does anyone know if this car cycles the compressor?
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System pressure cycles the compressor, by nature of its design.
Spdloader
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A/C is essentially a "minutes per gallon" question while driving represents more of a "miles per gallon" cost. I think it's certain that at low speeds the efficiency is better with the windows open and A/C off while at high speeds the opposite is true. Where that changeover point is undoubtedly varies widely from model to model, and the "high speed" regime may start above the speed limit for many cars.
When the Mythbusters ran their test they used SUVs which drank a lot of gasoline anyway and probably didn't suffer much when the windows were open. In addition, the speed was so low A/C would be a clear loser.
Mike
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Michael Pardee wrote:

I don't believe the original gas mileage matters, only the differences.
Maybe one can assume that windows open would be less of an effect on a vehicle with more mass and available torque (unless the vehicle allowed a higher volume of air)?
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the losses are already pretty high. In the Toyota Prius fora people are shocked to find that running the heater can decrease the in-town fuel economy 10 mpg... the finer the edge the more quickly it dulls.
Mike
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On Sat, 9 Sep 2006 16:41:34 -0700, "Michael Pardee"

Running the heater? Isn't this just waste heat anyway?
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Gordon McGrew wrote:

I guess it has to do with the amount of juice needed to run the fan.......????
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wrote:

car has a display for the mpg over 5 minute intervals, so the first winter we had it I decided to see just how much the heater would drag it down. I opened the windows and turned the heater on full while driving in town. The previous five minute bars had been something like 40 or 45, while with the heater on full it dropped to 25! Even at 75 mph with the A/C on full and five adults in the car it never went below 30 in real life.
Mike
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Michael Pardee wrote:

Amazing. That's not something they advertise.......
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increasingly common as we see more efficient cars, and especially if electric cars make inroads into everyday use. We all know it takes a lot of energy to make heat but in cars we assume there's always an excess of heat.
Mike
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was a gasoline fueled cab heater. That doesn't sound real economical, but I'll bet it's more economical than having to fire up the engine to get heat. It doesn't sound real safe either, but if you maintained it it wasn't too bad.
Earle
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stopped, so the waste heat often falls short. The engine has to run more just to make heat.
Mike
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Michael Pardee wrote:

The raw number would be lower, but the percentages should be similar.
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really want to measure: fuel per increment. For example, suppose it requires 5 gallons of gas to move the car 100 miles with the windows rolled up and A/C off. That is 20 mpg. If the window drag at some speed consumes 0.2 gallons in 100 miles at that speed the economy drops to 19.2 mpg for a loss of nearly 5%. If the drag is the same in a vehicle that requires 2 gallons to move the car 100 miles (50 mpg) the window drag drops it to 45.5 mpg, a nearly 10% hit. I'm not a big fan of the "liters per 100km" measurement but it works a lot better than mpg here.
So, here's an example of how it goes - theoretically - with the window and A/C. Assume (for illustration) that at 50 mph the window drag consumes 0.1 gallon per hundred miles. Similarly, assume the A/C consumes 0.1 gallon per hour. For our hundred mile trip that would mean the windows would use 0.1 gallon and the A/C would use 0.2 gallons at 50 mph. If we increase the speed to 100 mph the window drag, increasing with the square of the speed, becomes 0.4 gallons for the one hour the trip takes while the A/C loss drops to 0.1 gallon for the hour instead of 0.2 gallons for two hours.
For different vehicles the numbers would change; the A/C losses are higher for a larger vehicle with more glass and the window drag will certainly vary with body style. In the end, all that will change is the speed at which the A/C is more economical than windows down.
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Michael Pardee wrote:

The net effect will be determined by environmental conditions and choice. My preferences are usually determined by speed of travel and temperature/humidity conditions. <90 and lower humidity while driving on back roads at <50 mph would be open windows for me for the most part (general rule of thumb). >90 would most likely be AC all the time.
Highway speeds would either be AC or vent, depending on the preferred comfort level at that specific time. I am not very concerned about the differences in fuel economy, only in comfort level (although it is always nice to know.....).
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That's undoubtedly the bottom line. I used to live in Phoenix, and the windows don't open wide enough to make 117 F comfortable!
Mike
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Michael Pardee wrote:

Amen.
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