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I’m not sure about the logic that recirc saves gas. If the AC compressor is on then it’s drawing power from the engine whether the vent is on recirc or vent. I know it does keep the interior cooler but it also keeps in more stale air. If my car has been parked out in the sun I will roll down my windows and turn AC to low speed vent until the air coming out the vent begins to cool. Once it cools then I roll up my windows and the. Switch to recirc. The temperature inside the vehicle can be hotter than the air outside the vehicle so putting it on recirc would mean recirculating that hot air. Either way, how would recirc save on gas if the compressor is constantly running? Do some vehicles cycle the compressor off and on as needed?
Most, if not all modern vehicles cycle the compressor. As far as saving gas by using the recirculation feature, I doubt that it would make much difference, especially during short trips. My experience with recirculation mostly involves people who use it during cold weather, and then wonder why the car is fogging up.
If the inside air is near setpoint then it is much easier for the air conditioner to deal with vs. having to constantly cool and dehumidify hot outside air. Also, I'm guessing there might be some almost negligible difference on aerodynamics for having the fresh air vent closed vs. open.
Most, if not all modern vehicles cycle the compressor. As far as saving gas by using the recirculation feature, I doubt that it would make much difference, especially during short trips. My experience with recirculation mostly involves people who use it during cold weather, and then wonder why the car is fogging up.
My daily commutes are 15 to 20 minutes depending on traffic. I use to use recirc all spring and summer long (except for first two minutes), because I use to live without covered parking at home. Oh what a difference covered parking makes. I went longer without using the AC remaining comfortable in spring time temperatures. Unfortunately I don’t have an automatic climate control so I just turn the temperature dial to adjust. Lower two notches in the day and up two notches after dark.
Most, if not all modern vehicles cycle the compressor. As far as saving gas by using the recirculation feature, I doubt that it would make much difference, especially during short trips. My experience with recirculation mostly involves people who use it during cold weather, and then wonder why the car is fogging up.
More and more vehicles are moving to more complex systems that use variable displacement ac compressors. These do NOT cycle.
Instead, they can vary the piston displacement size depending on load. If there is high demand (such as pulling in hot air and trying to cool it down), the compressor will use 100% of the volume to provide the best cooling possible. As load lessens (like when cooling cabin air that has already been cooled), the pistons will displace less, which lessens load on the engine. Load is determined by interior and exterior temps as well as system pressures.
Using the recirc feature will cool the cabin down to temp more efficiently, so a system using a variable displacement compressor will have less load on it to hold that temp vs one having to constantly cool 90*+ air. Less demand on the compressor means less load on the engine. This is one of the reasons why you can forget you have the AC on, during a nice 50-60 degree day and not even notice because the load on the engine is very minimal that the average person would never notice.
Even an older system using a compressor that cycles will gain some efficiency by using the recirc feature. All the systems monitor pressures and cycle/adjust displacement based on the pressures they see. You will see different pressures depending on if 90*+ air is passing over the evap, or if 70 degree air is passing over it. The efficiency of the older cycling compressor will not be as good as the modern variable displacement compressor, but there will be still some gain as the cycling intervals will be different than if it was a 95 degree humid day.
SO yes, the article is generally correct while failing to actually mention the technical reasons why. Your AC system does work less in recirc mode once cabin is at desired temp.
Last edited by BostonMike7; 05-09-2022 at 08:57 AM..
If the inside air is near setpoint then it is much easier for the air conditioner to deal with vs. having to constantly cool and dehumidify hot outside air. Also, I'm guessing there might be some almost negligible difference on aerodynamics for having the fresh air vent closed vs. open.
Therefore, recirc for the win!
Agreed, that makes sense - it's a lot less work to take inside air @74 degrees and cool it to 70 degrees than it is to do the same for @ 95 degrees outside air..
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Originally Posted by OutdoorLover
Agreed, that makes sense - it's a lot less work to take inside air @74 degrees and cool it to 70 degrees than it is to do the same for @ 95 degrees outside air..
This might actually make a difference in a car with a little 3-4 cylinder engine, but with a bigger V6 or a V8 I would expect it to be an insignificant difference in MPG. When you first get into a car that's been sitting in the sun you would be better off using 85 degree outside air than to recirculate the 104 degree inside air. Definitely use recirculating when in a wildfire area where it's smoky.
40 years ago, only the really high end cars had A/c systems that cycled the compressor as a function of temperature control. These cars usually had numeric set points (similar to a home thermostat) that you could set your desired cabin temperature at. The vast majority of us mere mortals had simpler slide-lever controls where you could select from heat at one end and cool at the other, (and the range in between) and all you were actually controlling was a mixing damper that allowed air circulating through your heater core to mix with cold air passing through the cooling coil.
Modern vehicles with computer control systems to maximize gas mileage, bring compressor cycling to the masses. Obviously re-cooling inside cabin air 10 degrees will require less work for the compressor than cooling outside air 20+ degrees...so recirculate will cause the compressor to run less often
BUT AN EVEN BIGGER BENEFIT of "Recirculate" is that the humidity has already been largely removed from cabin air, compared to outside air which could be 80-90% relative humidity. The phase-change process removing that water vapor from the air in the form of "condensation" requires additional work from your compressor.
So, there is an appreciable benefit to reducing that workload on your compressor, through the use of your "recirc" function.
How much that shows up on your fuel guage, is IMO questionable. I personally suspect that factors such as wind drag and slope of the surface you are driving on will have far more significant impact on your mileage, to the extent that meaningfully measuring the result of the recirculate control would be impossible outside of a laboratory
To me it makes no sense to recirculate the air in the cabin on AC mode, unless the air temperature in the cab-during the summer-is lower than the ambient vent temperature outside the cab, and even so I just ignore it. Now, those who can't afford burning a small amount of fuel can always open the windows to cool-off when driving a the lower speeds in the city. Once you are driving at highway speeds, close the windows and use the AC. Just keep in mind that the compressors comes on automatically whenever you place the airflow lever to defrost, regardless if you turn the AC on or not.
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