When it’s raining I would often turn the car AC on to cut down on the humidity, even if the temperature is relatively cool. I know the engine drives the compressor in the AC so there’s a hit to fuel economy. I assume that the AC has to work harder when it’s hot outside vs cool, though I’m not clear on the mechanisms, and that recirculating inside air is more efficient. So the question is, how much does the ambient temperature affect fuel economy when using the AC?
A second related question, which I imagine may depend on the car, is: how much does changing the temperature setting on the AC affect fuel economy? In other words, if it’s 75 outside and I set the AC to 75, would that have a lower impact on mileage than if I had set the AC to 65?
I’m sure it would assuming that the compressor switches on and off based on the thermostat settings.
FYI, I’ve tested my AC to the extend that I know that running it on the interstate costs me about 2 miles per gallon when I’m doing 70-75. But I haven’t tried anything related to turning the temp up or down – and the dial isn’t calibrated to degrees, so it’s just guesswork as to what the actual temp setting is.
In some cars (sorry, don’t know which manufacturers) the AC compressor stays on continuously. Temperature changes are made by adding more heat to the air mix. In those cars, the ambient air temperature makes no difference in how hard the AC is working.
On every car I’ve ever owned, the a/c compressor works until the coolant reaches a certain pressure and then it cycles off until the pressure drops to a certain point and then repeats. The cycle on and off times are irrespective of whatever temp setting you’ve set inside the car.
When the ambient temperature is higher, the pressure in the condenser is higher, and the compressor had to do more work to pump the refrigerant.
Setting thermostat to a cooler temperature causes more work one way or the other.
The clutch switch on Ford AC is on the suction side, and stops the compressor below some pressure which corresponds to about 35-40 degree evaporator temperature, so it doesn’t ice up. Some cars use thermostatic expansion valves for this purpose.
AC hurts the gas mileage the most with a small engine. With a big V8, pumping losses are huge, so the extra load just offsets some of the pumping losses, and it may be difficult to determine how much effect the AC had on mileage.
It’s certainly more effective in achieving colder air coming out of the ducts. However, it can be counterproductive when trying to reduce humidity as it allows the moisture from breath and insensible perspiration to build up in the vehicle.
A side note: on many cars, leaving the AC off and opening the car windows instead causes a worse hit to the gas mileage than using the AC, because of the increased wind resistance for the car with the windows open.
I’m sure this has been covered ad infinitum (The Mythbusters did it twice I think), but I find this most often highly unlikely. Any vehicle (short of a yard-high, aerodynamic supercar) is basically a brick pushing the air out of its way. Whether the windows are open or closed is going to make almost no difference. Running the AC puts a larger load on the engine and will always cause more fuel usage than that.
Not sure if you meant you do both, but you do NOT want to set the AC to recirculate when using it to dehumidify/defog as this will severely impede its ability to do so.
Hmm, does this apply even if it’s raining? I would have expected it to be easier to dehumidify recirculated air since whatever moisture is being added has got to be less than the humidity outside. Is it due to the way the AC works?
Just some… SOME variables: Each car has a power-to-weight ratio, specific gear ratio, power curves, shift points… shift habits/programs, transmission and throttle settings (on some cars: sport, econ, sport+). Some transmission and engine management systems are adaptive to your driving style – some are, but suck at it. Some aren’t. Some cars have more low-end torque that can take the AC drag in low-speed driving, some get by on horsepower and REVS and use more gear changes… and some cars are JUST SMARTER in how they sort out all the conditions to provide power and shift for economy… and how all this is married into a package means:
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No one here can tell you what kind of hit YOUR car with YOU driving will take when running under certain conditions, such as with AC running**. Altitude matters. Local fuel matters. Local weather matters. The variables are mind boggling.
YOU need to do enough tests, while eliminating enough variables to draw a conclusion.
According to Click and Clack, open windows and no AC are best, fuel mileage wise, at speeds under 45 mph. Above that, air resistance and drag works in and it’s more economical to crank up the windows and turn on the AC.
Moisture is added by the respiration of the occupants in the car. The human body is a humidity ‘engine’, ever time you exhale you’re adding moisture to the car’s cabin air. If there are two or more occupants this is significant. Even when it’s humid & rainy outside the cabin air is still often more easily dehumidified by constantly bringing in that outside air (and dehumidifying it) rather than recirculating the same small amount within the cabin. Can vary depending on car interior size, number of passengers etc. but this is why many modern cars automatically disable ‘recirculate’ (and enable the AC) when set to ‘defog’.
Hmmmmm. Along these lines, My old man always said that running the a/c in a car on normal settings in the summer was more efficient than having windows open due to drag. He was specifically referencing being on the highway. I always believed that, and he was assuming a normal car, not a truck or SUV, etc in saying so. Was his medulla oblongata wrong?
Look, I haven’t read every post but here’s my evidence.
I drive 167 kms to work every day and have not noticed any difference between winter and summer driving: at all! Years ago, with carburated engines I would notice a 25% drop in winter fuel consumption, but not any more.
So you can take this as another anecdote that states there’s no perceivable difference between air conditioning and no air conditioning. I would have noticed the difference in three years of highway driving, but I haven’t.
This is false. Humidity from occupants is miniscule compared to the normal water content of atmospheric air. When you turn on defrost the air is recirculated by default and you can’t turn it off.
I’ve tested 3 cars (Kia Spectra5, Nissan Altima, and Hyundia Elantra) using an ODBII scanner while on a flat road. On those three vehicles, having any combination of windows open was much more fuel efficient than using AC.
I know this was done to some degree of accuracy by the Myth busters.
But, I’m amazed that the fed, some college or university, or some car company hasn’t tested this to a nat’s ass. I understand it would change from car to car, but with all the focus on global warming, fossil fuel usage and the like, why isn’t this available?
