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SAE International

Standards

Does Turning Off Air Conditioning in Your Car and Opening the Windows Save Fuel?

That depends

Published: Tuesday, August 18, 2015 - 11:04

(SAE: Warrendale, PA) -- Consumers continue to be concerned about the fuel consumption of their vehicles, yet many are unaware of the effect that accessories have on fuel consumption. Vehicle air conditioning (A/C) is one of the highest energy-consuming accessories and has become standard equipment for vehicles.

Members of the SAE International Interior Climate Control Standards Committee authored an article discussing automobile fuel consumption and A/C. The choice of how you use (or don’t use) the A/C involves your and your passengers’ comfort. There are many factors that effect how much fuel is used, including the specific vehicle fuel use (MPG rating) for the type of driving, as well as temperature and humidity conditions.

The energy required to provide comfort in a vehicle depends on many factors, such as the load on the A/C system, which is a function of the fan setting, the outside ambient temperature and humidity, and the speed of the vehicle, among other variables. The fuel consumption of the A/C will vary greatly depending on the weather conditions and also on traffic patterns typical of a given city.

So can you save fuel by not running the A/C? That depends.

Windows open vs. operating the A/C

The use of the A/C system can reduce driver fatigue, a major safety issue, and may result in reduced energy requirements when comparing A/C operation vs. open windows in a vehicle. Keep in mind that rolling the windows down effects the vehicle’s drag, and increased drag increases vehicle fuel consumption.

Figures 1 and 2 look at four different vehicle operating conditions—maximum cooling system load (outside air, high fan); minimal cooling system load (recirculated air, low fan); and A/C system off with vehicle windows open—and compare the fuel consumption under those conditions to running the car with the A/C system off and windows closed (minimal A/C load and vehicle drag). When the cabin becomes comfortable, and is not operating at full cooling capacity, there may be reduced energy requirements using the A/C when compared to the windows open with the A/C off. Obtaining comfort by operating with closed windows, A/C on, low fan speed, and the selection of recirculated air may result in the use of less fuel.

Figure 1: Fuel use in MPG

Vehicle fuel consumption

This comparison of two luxury four-door sedans (vehicle “A”and “B”) and a large SUV (vehicle “I”) was conducted on a test track with weather conditions of a sunny 90° F (32° C) day with 20-percent relative humidly. This resulted in the same fuel use for vehicles “I” and “B.” The effect of operating the vehicle with the windows open or closed can change the drag and fuel consumed. At 50 MPH, vehicle “A” used more fuel with windows open and the A/C off than it did with A/C on, recirculation, and low blower speed.


Figure 2: Percent fuel use

Depending upon many factors, the energy requirements when operating or shutting off the A/C system and opening the vehicle’s windows could be very similar.

Click here (then scroll down) to learn more about this article.

 

 

 

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About The Author

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SAE International

SAE International with headquarters in Troy Michigan, is a global association of more than 138,000 engineers and related technical experts in the aerospace, automotive, and commercial-vehicle industries. SAE International’s core competencies are life-long learning and voluntary consensus standards development. SAE International’s charitable arm is the SAE Foundation, which supports many programs including A World in Motion, SAE’s curriculum program that brings science, technology, engineering, and mathematics (STEM) to life in the classroom, and the Collegiate Design Series.

Comments

Word usage, and more

the word effect is a noun, the word affect is the verb.  Effect was used several times as a verb in this article.  http://web.ku.edu/~edit/affect.html 

In another comment, the upside down graphs are harder to comprehend, why was mpg used in lieu of amount of fuel? 

It has been know for a long time that driving with the windows down uses more fuel than using the airconditioner.  I guess you are trying to say that if the AC unit is run at high cooling and fan it uses more fuel and may be about even to driving with the windows down.  Do many run low fan, recirculate, in lieu of high fan and no recirculate?  I usually run recirculate when I wnat to cool down in a hurry.

The noise level increases considerably with the windows down.  If the temperature outside is cooler than your mid-90's example, then a mix might be used.  At highway speeds, there rarely is anyone driving with the windows down due to the noise.  Way back when I was a child, AC units wern't in every vehicle, and we had to run with windows down.