Summary:
If under 50 MPH, roll down windows.
If over 50 MPH, roll up windows and activate AC.
A/C or not, if you are making a long distance road trip, the very best ways to increase MPG are to make certain your tires are properly inflated and to use a speed control.
I don’t agree with this. If you switch to normal, and the outside air is warm, the compressor has to work much harder to remove the heat from it than from already cold, re-circulated air. I find it hard to believe that the “normal” setting would ever get cooler, unless your outside air is already cool and dry.
The “Max” setting also turns the fan to high speed in some cars. In many cases the compressor has a fixed capacity - it is on or off (except those systems with a variable displacement system as Rick noted). THe more air you force across the evaporator with a constant compressor capacity, the warmer that air is going to be. Reducing the air flow will result in cooler air, albeit less of it. Recirc in some cars is a separate option. In others, it is only available at the MAX setting. In general, once the car is cool, recirc will give you colder air as well as a (slightly) lower load on the compressor, but as Rickalso pointed out, it fosters the growth of some nass-asty crap on your evaporator coil and drain system. If your drain gets clogged up with mold, mildew and slime and then mosquitos start breeding in your evaporator drain pan, you will wish you used recirc less often.
To the OP, on a Hyundai Accent (like my daughter’s), which does not have a variable displacement compressor, leaving the temp setting at coldest in recirc mode and setting the fan control at the lowest setting to maintain a comfortable temperature will result in the lowest energy use, but you will never notice the difference at the gas pump. Sliding the temperature control to a warmer setting simply passes some air across the heating coil. It has no effect on the compressor.
I don’t think you understand that car A/Cs are not like home A/Cs. In most cars the temp setting only affects how much air that does not flow across the heat exchanger (i.e. raw outside air) is blended with the air that is cooled. The compressor cycles on & off independently of the temp setting.
ARRGH!!! THAT WAS NOT MY QUESTION!
Ahem. Carry on. Thanks folks.
In sum, as I not understand it:
Temp setting at coldest. Fan setting at lowest. Don’t expect any miracles.
I’m not quite sure if I have this part right. Maybe Rick can enlighten me: At low engine speeds, the A/C increases engine load more than at high revs.
Whatever the engine speed is, turning on the air will increase the revs slightly so that the alternator is producing more juice.
If that increase is, say, 100 rpm, at 4000rpm that’s only an extra 2.5%. At 2000 rpm that’s an extra 5%, meaning the net effect on mileage will be greater. Right?
Fuel consumption isn’t linear with ANYTHING.
It’s closer to linear with power produced than with anything else.
Your car may stay at the same RPM going down the highway with the AC turned on… you’ll just be opening the throttle further and using more gas. This is especially likely if you have a stick or a lock-up torque converter.
Ignoring automatic transmissions and CVTs, your engine RPM is determined by the vehicle speed and the gear selected. It may be performing more work (and consuming more gas) depending on the other loads on the engine, but the RPMs won’t change.
Maybe it’s something that is unique to GM vehicles, but my Silverado and my girlfriend’s car both do the same thing - after max has been on for a while, switching to norm results in an immediate and definite drop in temperature of the air coming out of the vents. shrug
However, the interior temp does cool off faster when you start off in max when the vehicle has been parked. It just doesn’t get as cool as it does when you switch to norm.
YMMV (no pun intended)
I used to run my a/c on recycle all the time. However the manuals say not to continually recycle because it doesn’t allow the moisture in the cab to excape. Two hondas and a current nissan’s manuals all say this. What sux is when you pull up at a light behind some smokey-ass 1978 rusted out POS and your a/c is on “fresh” and sucks that pollution right into your cab…cough cough. I guess they expect you to make a jump for the recycle button. As for the temp setting…the compresser cycles…the warmer you set the temp the more the compresser stays off…you can feel the compresser turning on and off with the sound and the sudden leach of power. A warmer temp would keep the compresser off more and use less gas.
I used to run my a/c on recycle all the time. However the manuals say not to continually recycle because it doesn’t allow the moisture in the cab to excape. Two hondas and a current nissan’s manuals all say this. What sux is when you pull up at a light behind some smokey-ass 1978 rusted out POS and your a/c is on “fresh” and sucks that pollution right into your cab…cough cough. I guess they expect you to make a jump for the recycle button. As for the temp setting…the compresser cycles…the warmer you set the temp the more the compresser stays off…you can feel the compresser turning on and off with the sound and the sudden leach of power. A warmer temp setting would keep the compresser off more and use less gas.
I will try.
Not sure just what you mean by this statement. If you are saying that the compressor takes 5 hp at idle and 3 hp at 3,000 RPM, I would say no. I think that we can assume that the compressor takes no less, and maybe a bit more hp at speed then it does at idle (High side pressures go up with engine speed) If however you are saying as a percentage of engine output the percentage of the engine output required to drive the compressor is greater at idle then at speed, I completly agree.
Well in a word, no. On some cars there is an idle boost when the A/C is kicked on. Note the words some and idle. This is done for a couple of reasons, one is stabize the idle and prevent it from falling too low, the other reason is to increase the airflow over the condensor to improve A/C performance. Once you are off idle, I am not aware of any car that does an engine speed “bump”. If the engine speed went up with A/C engagement, then the car would speed up everytime the A/C engaged. This would probably piss of the drivers to no end.
See above. Engine speed does not increase once you are off idle, and probably won’t then if you drive a modern car. On my brand of cars, I think the last car we had that had an idle bump for A/C was 1985. I am guessing that other car makers have also done away with this. Computer controls are much more sophisticated then they were 20-25 years ago when idle compensation solonoids were common.
Now let’s try your cmparsion again from a different viewpoint.
Let’s say at idle your car requires
::: Rick reaches around and pulls a number out of his ass:::
five horsepower to drive the alternator, load the auto transmission spin the power steering, water pump and everything else. Furthermore let’s say it takes 5 horsepower to run the A/C That is 50% of the output of the engine.
Now let’s head up a steep hill at 80 mph, 75% throttle the engine now requires
::: Rick reaches around again:::
150 horsepower to run the car. The A/C still only needs 5 hp. Percentage wise the compressor is using a much smaller part of the total engine output.
Hmmmm. You’re actually scary close on your estimate there.
I have a metric I call the 679 test for a car.
I consider a car 100% adequately powered if it can maintain 79 MPH on a 6 degree uphill grade.
A Camry-sized car takes right about 143 or so horspower to maintain 79 MPH indefinitely on a 6 degree hill.
Of course you’ve also been in the car game since before I was able to read and write…
It’s all in the ass. mine is calibrated you see. Part of ISO 9000 requirements. I have to send my ass in once a year for calibration. No Mexican food or Chili that week.
As far as the difference between you answer and mine my car is heavier, has a higher CD, and ah, ah, HEADWIND!
Yeah, that’s the ticket, my car is fighting a headwind.
So I was right about the decreased relative load, except that it was against the power output rather than engine revs (which made perfect sense when I realized obviously an increase in revs would also make the car go faster :smack: )
Well, hell, no need for a cop-out.
I know you did your calculations in some obscure model of mid-80s Volvo, so it’s all good.
Don’t feel too bad, your basic assumption about RPM vs MPG wasn’t too far off, if speed is held constant.
Check out the charts here:
http://metrompg.com/posts/rpm-mpg.htm
Calculations? What are these calculations you speak of?
[Treasure of Sierra Madre] Calculations, we don’t need no stinkin calculations[/TSM]
Seriously, I pulled that number out of my ass.
Nice to know I was close.