Submitted 9 months ago by scytale@lemm.ee to [deleted]
On a hot day and traveling in a car, which method of keeping the occupants comfortable with the A/C is more energy and fuel efficient?
Set the thermostat to a lower temperature and keep fan speed to the lowest setting
Set the thermostat to a slightly higher temperature and compensate by setting the fan to a higher speed so you can feel cool enough with the breeze
theoretically the most fuel efficient would be not having it on at all.
But it really depends on how vehicle refrigerant phase change loops work. I’m not a mechanic nor am i familiar so hopefully someone else can chime in?
There are a few outcomes here depending on the cycle. If it uses some sort of refrigerant flow restriction that is variable, then theoretically the thermostat setting will have an effect. (i would assume this is a case these days) However the effect it will have is not going to be linear, given that its a fluid pump cycle. Though it’s powered directly by the engine (most likely?) so that probably won’t matter at all, engines are horribly inefficient to begin with.
if it’s not using some sort of variable valve to control restriction and thus cooling potential, it’s doing some other shenanigans and running the phase change at a set point, though it would have to introduce some sort of external variable there in order to compensate so probably not.
I would probably set the ac to a low temp, and then the fan to a low speed, because it takes advantage of physics better that way (the greater the difference in temperature, the more effectively you can transfer heat away from it) since you’re running a low fan speed, you’re gonna need a high temperature gradient to make the best use of that. There is also the inverse, but i doubt that would be better tbh.
Though i think realistically, it just won’t matter enough for you to bother. Like you’ll probably see a bigger difference in usage by taking a different route somewhere, or idling longer. There are a number of other factors that can impact fuel efficiency.
There is some optimal amount of thermostat vs fan speed. Whether you’re better with a lower thermostat and higher fan speed or vice versa depends on which side of this optimum you’re on. We as random internet strangers have no way to find this.
It depends on a large number of factors. Largely it doesn’t matter.
What I personally do is start my car with a low fan speed, let the evaporator cool down, then start cranking up the fan speed so I feel the cooler air. When it’s really hot out my AC can’t keep up in my car, and blasting the AC essentially guarantees that I’ll never feel cold air. I’ll just have a cooler ambient temperature, but the air blowing at me (that I’m feeling) is warmer so I feel hotter.
IDontHavePantsOn@lemm.ee 9 months ago
A7thStone@lemmy.world 9 months ago
A vehicle AC compressor does cycle. There is a clutch on the pulley attached to the compressed that engages the compressor or lets the pulley free wheel.
ostsjoe@lemmy.world 9 months ago
Also the alternator absolutely takes more power from the engine in proportion to how much energy it’s putting out.
IDontHavePantsOn@lemm.ee 9 months ago
I just accidentally deleted my original comment.
Heres I what I deleted.
There’s a few things a bunch of other commenters are getting wrong, at least from my understanding.
The AC compressor is run by a belt from the engine. Unlike a home AC, a vehicles AC is either on or off. There is no cycling. The temp control is just changing how much outside air comes in, or how much heat is drawn from the engine.
The fan speed is not going to effect anything at all. The alternator spins to generate electricity to recharge the battery, but the amount of energy required to spin it doesn’t change based on how much electricity is being drawn.
Tamping your AC down in any way, whether by lowering fan speed, or increasing the temperature is less efficient. Use it at it’s coldest temp and highest fan speed if you’re going to use it at all.
Edit: I almost forgot to add that AC efficiency is going to be more closely tied to your driving speed and outside air temperature. Driving faster makes your AC more efficient (not necessarily your whole car). More air passing over the evaporators means more heat released from your AC to the outside, which makes colder air in your car. The colder the air outdoors, the more heat the AC can release to the outdoors, which means colder air in your car.
As for EVs, I have entirely no clue, but I would assume it’s a similar situation.
Edit: I may be entirely wrong. I’m not sure about anything anymore. I know a guy who knows all about these things, let me get him down here tomorrow and we can go from there.
IDontHavePantsOn@lemm.ee 9 months ago
I got my guy down here and he told me I was right and wrong. I’ll give the details tomorrow but I can barley work my phone.