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Suppose a very hot summer day, driving on a long straight highway at 130 km/h. What is most fuel efficient solution :
One uses fuel for running AC, other uses fuel from bad aerodynamics due to open windows.
This may vary depending on car model and AC model, yet is there a way to know which case is most fuel efficient in a general way?
It depends on several factors:
The SAE did an experiment, the full results are paywalled, but there's some useful detail in the summary:
On-road and laboratory experiments with a 2009 Ford Explorer and a 2009 Toyota Corolla were conducted to assess the fuel consumption penalty associated with air conditioner (A/C) use at idle and highway cruise conditions. Vehicle data were acquired on-road and on a chassis dynamometer. Data were gathered for various A/C settings and with the A/C off and the windows open. At steady speeds between 64.4 and 113 kph (40 and 70 mph), both vehicles consumed more fuel with the A/C on at maximum cooling load (compressor at 100% duty cycle) than when driving with the windows down. The Explorer maintained this trend beyond 113 kph (70 mph), while the Corolla fuel consumption with the windows down matched that of running the A/C at 121 kph (75 mph), and exceeded it at 129 kph (80 mph). The incremental fuel consumption rate penalty due to air conditioner use was nearly constant with a slight trend of increasing consumption with increasing vehicle (and compressor) speed. A lower fuel penalty due to A/C operation is observed at idle for both vehicles, likely due to the low compressor speed at this operating point, although the percentage increase due to A/C use is highest at idle.
So at maximum AC it was more efficient to have the windows open at lower speeds, and it differed at higher speeds depending on the vehicle. At a lower compressor load the trade-off speed will be lower, again depending on the vehicle.
Mythbusters did an episode which replicated this finding, but there were some issues in the test method which make the result not quite scientific enough to use.
So the short answer is it's probably more efficient to have the windows down, unless you are going pretty fast in an aerodynamic vehicle. There are many factors in this, it's impossible to say without testing in specific vehicles.
Keeping a constant speed using cruise control with windows shut and ac on because once the cabin is down to temperature it requires little to keep it there and the ac will modulate its power needs.
Driving with windows open will increase the drag and that will only reduce if you either reduce speed or close the windows.
Fuel efficiency shouldn't be your only consideration. Rolled down windows will expose you to noise levels well above 85 decibels which can cause permanent hearing damage with prolonged exposure.
British scientists tested everything from a Mazda to an Aston Martin in non-rush hour traffic going 50, 60 and 70 MPH.
They found that regardless of speed or model, when the windows were down the driver was exposed to a nearly constant 89 decibels of noise.
https://helpingmehear.com/drive-with-the-windows-open-your-hearing-may-be-at-risk/
Suppose a very hot summer day, driving on a long straight highway at 130 km/h. What is most fuel efficient solution :
- Driving with AC on and closed windows?
- Driving with AC off and four full opened windows?
Of several choices, those are the worst. Why are you confining your selection to those? Also, running windows-open will make you deaf.
One uses fuel for running AC, other uses fuel from bad aerodynamics due to open windows.
That is why they are both lousy choices. Here is your better choice.
We are at the best choice available given the speed you want to go. This should improve your fuel economy significantly over the other two.
Your best choice, however, is
It matters more than you think. At 130 kph, the vast majority of your fuel is being spent to overcome aerodynamic drag.
We could make a quantitative comparison using some ballpark figures.
Power consumption of your car's air conditioner depends on many factors, including its settings and the environment. A fair assumption of average consumption is approximately 3 kW (which might be as much as a factor of 2 off). This number can be compared with The increase of air drag power losses by opening windows.
Air drag power losses can approximately be calculated as P = ½ ρ C A v3, with ρ the air density, C the vehicle air drag coefficient, A the vehicle frontal surface area and v the vehicle velocity. We have ρ = 1.225 k/m³ and v = 130/3.6 km/h = 36.111 m/s. A and C largely depend on the car type. For a modern car (sedan) we have C A ≈ 0.3×1.8 = 0.54 and for an MPV/SUV C·A ≈ 0.45×3 = 1.35. We find P = 15.7 kW and P = 38.9 kW for respectively the sedan and MPV. The next question is: how do these figures increase if windows are opened? A study has shown that power losses increase with ca. 8% and 20% for sedans and SUVs respectively. Notably, this difference in percent seems to scale inversely linear to C A, suggesting that the increased losses are not so much a function of the car's shape. This makes sense: car windows have approximately the same size in both big and small cars, and opening them does not change your car's frontal shape. It follows that the increased losses can be roughly described by Δ P = ½ ρ 0.108 v3, for which we find 3.1 kW (for 130 km/h), approximately the same as the 3 kW figure of the air conditioner!
Be reminded that various of the above figures are very rough, including the 3kW figure of the air conditioner. As such, we cannot come to an obvious conclusion. However, also note that air drag increases cubically with speed. It will likely be safe to say that, for speeds under 100 km/h (ca. halve the drag) open windows will win, and for speed above 160 km/h (ca. double the drag) the air conditioner will win. Anyway, as others have mentioned, for higher speeds (>80 km/h), other issues will become more prominent, such as noise, for which closed windows might nevertheless be preferred.
If you want to save fuel, forget window vs. AC, the obvious solution is: drive 100km/h rather than 130km/h.
Although the difference seems like not that much (merely going 25% slower), you need 2.2 times as much energy to drive 130 instead of 100 km/h. Velocity is the single biggest deciding factor in the equation, as it's raised to the third power. Anything raised to the third power isn't very interesting with small numbers, but becomes crazy as numbers get big.
