It is good information, but the true differences are difficult to assess, as many segments are averaged together, so one cannot calculate the standard deviations for each group. It does seem that on 9/7 there was less head wind.....
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)?
Vehicles with lower fuel economy are less sensitive to differences, because 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
Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5.
The behavior of the Scan Gauge is a little flaky in regards to measuring fuel consumption. I recalibrate it at every fuel stop and it can vary by 10% or more. Since the calibration changed between test sessions, you can't compare the absolute milage figures between sessions. Also, gradually changing terrain, elevation, wind and different test speeds complicate the situation. That is why I looked at relative fuel economy between the sessions. Below is the individual session data in case you or someone else wants to do more sophisticated analysis. Just keep the above in mind.
Test 1 8/31 76 mph
A/C Window m/g miles on closed 18.2 10.0 off closed 19.5 10.0 off wide opn 20.2 10.0 off closed 21.2 10.0 on closed 17.4 10.0 off closed 18.9 10.0 off wide opn 18.7 10.0 on closed 18.8 10.5 off closed 19.7 10.0 off F3 R3* 19.3 10.0 on closed 19.1 10.0 off R6 18.5 10.0
Test 2 9/4 75 mph
A/C Window m/g miles off closed 21.0 10.0 off R4 20.0 10.0 off closed 22.5 8.7 on closed 20.9 10.0 off closed 22.4 11.4 off F2 R4 22.4 10.0 off F2 R4 21.4 11.4 off closed 21.7 10.0 off closed 21.0 10.4
Test 3 9/5 75 mph
A/C Window m/g miles off closed 19.6 10.0 off R4 20.5 10.0 on closed 18.5 7.8 off closed 20.0 10.0 on closed 19.2 10.2 off R4 20.8 10.1 off closed 21.7 10.0 on closed 20.1 10.0
Test 4 9/5 73 mph flatland
A/C Window m/g miles off closed 21.6 10.0 off F3 R4 21.1 10.2 on closed 21.2 10.0 off closed 22.5 10.0 off F2 R4 22.4 10.9 on closed 20.6 11.6 off closed 21.4 10.0 off F3 R4 21.7 10.0 on closed 20.6 10.0 off closed 22.8 13.7 off F3 R4 21.4 10.1 on closed 20.3 10.5 off closed 21.6 12.0 off F3 R4 21.8 11.0 on closed 21.4 10.0 off closed 23.4 13.0
Running the heater? Isn't this just waste heat anyway?
The raw number would be lower, but the percentages should be similar.
I guess it has to do with the amount of juice needed to run the fan.......????
The Prius doesn't normally run the engine when the car is moving slowly or stopped, so the waste heat often falls short. The engine has to run more just to make heat.
Mike
I think the discrepancy comes from working with the reciprocal of what we 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.
My bad on the dates. 7.2% loss between air on and air off with windows closed, with almost identical standard deviations with decent sample sizes.
It looks like with windows partially open is just about the same as them closed (.4% difference probably insignificant due to standard test error) until you get to the 6" opening in the rear (though there is only
1 sample), which is just about the same as with them wide open (again, small sample size). 6" to wide open the same as with the air on and closed.Probably could use more "open window" numbers just to verify, but it makes sense if you look at some simple HVAC calculations for orifice flow....
A 14" diameter that has about 154 sq/in of area (semi close guess to a car window, I haven't actually measured one) with a round edge discharge co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and almost 100K CFM at 20 psi (sharp edge drops numbers by 40%). I'm not sure about the effects of the angle of the flow on the opening and I haven't measured the air pressure entering a car through an open window moving at 70 mph, but I'm sure it would be quite high (and then multiple the CFM x 4!) That is a lot of drag......
I'll take the 7.2% loss without the wind howling in my ears!
Thanks for the clarifications. Good real world application.
The net effect will be determined by environmental conditions and choice. My preferences are usually determined by speed of travel and temperature/humidity conditions.
"L Alpert" wrote
Ditto.
What Gordon found appears to be consistent with some study (or a summary of a study) I read not too long ago. Tom and Ray of "Car Talk" may have cited it at their web site, if memory serves. Namely, even when moving at highway speeds, not using the A/C and leaving windows open for some cooling is best.
Plenty of studies on the net on this. E.g.
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
The effect on the amount of time the engine runs can be pretty radical. The 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
Amen.
Amazing. That's not something they advertise.......
Nope - and I wouldn't if I were them, either. I think it will be 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
The old air-cooled Bugs didn't have an excess of heat either, so one option 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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