That would be the “RECIRC” setting which recirculates the cool inside air instead of pulling in hot outside air. Most American cars use the term “MAX A/C” which just closes the vents to a recirculate position.
I understand your point, but would like to point out that how much fuel I put into my vehicle is more accurate than the 1/2 gallon (at best) accuracy of the fuel gage. Growing up, at every gas station we fueled up, my dad would use the reading on the pump, which is precise to within 1/1000 of a gallon. The odometer is precise to within a tenth of a mile. Both of these measurements, by law, must be as accurate as they are precise.
::: Sigh:::
think about this for just a second. Of course the pump is accurate. But how do you determine when the tank is full? I have had cars where you could add 2 gallons to the tank after the pump shut off the first time. Other times after the pump shut off I could only get 1 gallon in. So how full is full?
The dispenser is very accurate in telling you what was dispensed, but it does nothing to tell you how full your tank is.
Last week I stopped at a station that had a defective vapor recovery nozzle. The pump clicked off after 1 gallon. If I had stopped at that point and calculated my fuel mileage, I could say that my full sized Chevy Pickup with a 454 got 200 MPG! Obviously anyone with three brain cells to rub together knows that that truck could not get that type of mileage even if you pushed it out of an airplane.
Unless you have a calibrated container and a certified speedometer any fuel mileage calculation is just an estimate, and sometimes not a very good one at that.
I stand by my comments.
On a later episode, they found out that their driving speed of 45mph was right before the break-even point of 50mph, where the windows down and A/C uses the same amount of gas, and that faster than that the A/C uses less gas. They also realized that this explained their first experiment on the original show, where they used the gas mileage computer while driving at 55mph and discovered that the windows down used more gas.
Using trucks is kinda stupid, though. Trucks are not very aerodynamic. I understand that SUVs are pretty popular, but it would be itneresting to run a similar test with a car. I used to drive my cars with the windows down…not all the way down, mind you, because that’s just pointless torture, but about halfway, enough to get a good amount of air circulating through the car.
I consistently got 2 - 3 mpg more that way then when I used the A/C, and I was doing a lot of freeway driving (avg 60 - 70mph+)
That range of 4 to 15 hp seems a bit suspect, especially for a refrigeration system that is probably under 1 ton of capacity. For instance, I have a 9,000 BTUH window AC unit that plugs into a 120V 15A circuit and runs fine. Perhaps the cooling load on a car is a bit more than 1 ton, but 15 hp is a lot of power to a compressor (and in a car, the condensing fan basically runs for free).
In the pdf you link to, the cooling load for a car AC is estimated at anywhere from 1.2KW to 4.5KW. That 1.6 to 6 hp, which seems reasonable (the lower number was for 0% outside air, the higher for 100% OA). An evaporator fan running at 14.4v and 40 amps would be about another 0.75 hp, call it 1.5 hp for alternator losses. Your mechanic was off by about a factor of 2. Not too bad, but it was the 15 hp that made me think it was off.
With tank to tank comparisons, you are correct (though you might get some consistency if you use the same pump every time you refuel). If you track it long term though those variations won’t matter one bit and your calcualtions are as accurate as your odometer and fuel pump readings are. Put in 750 gallons and drive 15,000 miles over a year and you get 20 mpg. It won’t matter if you fill up only halfway most of the time, as long as you start and stop your measurements at the beginning and end of the year with the tank filled to the same level. I think the odometer is the limiting reading here, aren’t they usually only accurate to 1% or so?
Edited to add: Of course, over a year you are probably going to be using seasonal variations of gas blends. If you are a geek and are tracking this, you could maybe calculate 7 tank running averages and see how this measurement goes up and down over the year as the fuel formulation changes.
That’s all fine and good, and you are correct, but the OP is asking about the difference between running the AC on high and low.
I really don’t think a 30,000 mile test (15K with AC on high, 15K with it on low) is called for, do you? And how are you going to account for weather variations between the two years? If summer 09 is hotter than summer 08, just how do you integrate that into your results?
Isn’t that kind of like swatting a fly with a ICBM? Effective? yes. The best way to go about it? Probably not. Is it overkill for what we are talking about? For sure.
Depends on what “having it on a high setting” refers to. The high fan setting, or the higher AC setting? On Fords, you get 2 AC settings, the low setting brings in outside air and cools it, the high setting runs the fan faster and cools and recirculates the inside air.
As far as the air circulation fan speed, on at least some vehicles (Fords and Dodges I know), the circulation fan’s normal speed is High. The lower speeds are accomplished with a resistor pack that burns off extra electric current as heat, and slows the fan down. If the resistor pack fails (a not uncommon problem), the fan will only run on high.
Right. I had completely disregarded the OP. I wouldn’t try to analyze the problem by calculating mpg because I agree, there are too many variables. Rather I would determine the power requirements for the compressor and fan, which of course increase as the load increases.
I think there is something we have missed though, and that is the temperature setting in the car. We’ve been assuming that the temperature setting is at full cold in this discussion. There may be differences by car models, but I think that most cars start to blend hot and cold air when the temperature setting is between cold and hot. For instance, my truck has a sliding control for the temperature that ranges from cold to hot with a color band that is blue for the cool side and red for the hot side. When the setting is say, in the middle of the blue band and not at full cold, the control operates a damper which directs some air from the evaporator coil across the heating coil to temper the air*. That reheat represents a sensible load to the AC unit and will increase the power requirements of the system. It seems counterintuitive, but running the temerature warmer than full cold will use more power because of this.
I have somewhat older vehicles (1991 is the newest) and I’m pretty sure that’s how they work. Newer cars may use a different method to temper the air when full cold isn’t needed.
- this is a dehumidification process with reheat, which is used when you need to defrost the windshield. The AC kicks on, even in the middle of winter, to dehumidify the air.