meilers

This is my second Phoenix summer with the WRX and it is the time of the year when you have to run the air conditioning all the time (it is still 90 degrees at 10 PM, much of the time!). Just like last year, I have noticed a big drop in performance (especially on the highway) and a loss in gas mileage as well (20 to 30 miles per tank, down to 19 mpg from 22).

Now, I don't expect to get something for nothing, but in my previous cars I did not see such a drastic effect from turning on the A/C. Why is there a loss in power, as well as a loss on the fuel side? How exactly does the A/C compressor interact with the engine itself?

Obviously turning on the AC drains electrical power from the engine, which forces the generator to continuously charge the battery. Can anyone explain to me why there is such a severe power loss? Are there any strategies to reduce this load on the engine, such as an upgraded generator or lightened flywheel?

RoadSpike

its not just electrical. Here's a short an dirty explanation of how it interacts

First you click on your A/C and an electromagnitic clutch engages on a pulley attached to your A/C compressor in the engine compartment, you may have noticed a click that would be the clutch.

The compresser liquifys the r134 gases through brute compression and forces it into a nossle at the your inside end where it expands and performs that loving cooling effect we love. The excesses heat gathered by expanding gas is released at the cooler mounted in front of your radiator.

Tips: On a hot day crack your windows a lil and shade your car. When you startup the a/c run it on vent for a minute or so then switch to maximum a/c which is not vented. This will draw the relatively cooler air from outside (considering your car is prolly about 120F by now) into the drivers compartment. When you feel comfortable switch the a/c to a lower setting, this wont run the compressor as often.

Other than that... adapt to higher heat tolerances

Mulder

The compressor is a parasitic load on the engine since it is a belt-driven accessory. It's the load from the compressor itself that accounts for most of the power loss you feel when it's on, although the added electrical drain of the fans running and the compressor clutch is also a factor.
In the WRX the load of the AC is more noticeable than in other cars because you have a small-displacement, low compression engine. Therefore the load is most significant when off boost such as starting from a stop or accelerating from lower RPMs.
At or approaching WOT the compressor will cut out to reduce the load on the engine.
There isn't all that much you can do about it, other than keep the AC in efficient working order so the compressor doesn't work any harder than it has to. Make sure the system has the proper amount of charge.

meilers

Thanks for the info.

Not that I doubt your information, but if the A/C compressor is belt driven, how can it run when the engine isn't running and the car isn't moving? I turned on the car yesterday to roll up the windows, and it blew cool air at me for over a minute.

I've also been experimenting with turning the A/C "down" -- this has no effect. When you select anything besides 100% "on," the system just mixes warm outside air with the cooled air to get lukewarm air. When I turn the A/C down from 100%, I get a distinct "outside" smell in the vented air even when I have it set to recirculate. Turn it down to 50% (halfway between full cool and full heat) and the system not only lets in outside air, it shuts the A/C off entirely every few minutes (I can feel it lurch on the highway!) and then turns it back on. I'm pretty sure the power drain of starting and stopping the compressor negates any savings you might get from turning to this setting.

As for adapting to higher heat tolerances, that's a statement borne of ignorance of a Phoenix summer -- it isn't unusual for the air temp in your car to exceed 140 degrees after just 20 minutes in the sun. Turning on the A/C is survival, plain and simple. Rolling down all four windows would just result in me arriving to work as deaf as a stone and with my hair looking like this:

RoadSpike

ok look man I worked on A/C home units for years.

the a/c unit that underneath your dashboard has what we call an evperator coil. This little baby looks just like a fancy radiator but with one unique feature. It expands the liquid r134a (freon) that cools the unit. When you shut off your car you still has alot of liquid coolent in the line. That in turn gently continue to spray itself into the evaperator for a brief period in time. Besides that the unit itself basiccally is ice cold so it takes awhile for the whole thing to really heat back up again.

Far as the vented and non vented method goes Maximum A/C on the **** should put it into reciculation mode inside of outside vent. I wont gaurentee that you wont get outside air in simply becuase the lil panel that switches it isn't exactly air tight. It should help mildy.

Far as gas milage goes? well i intended for it to stop and start every few minutes...
Basically you use A/c your going to lose mpg period. Dont like it... tuff get used to the heat then like i do

Littleboyblue

I cant get as detailed as these guys, but I just drove to San Luis running the a/c and got 26 mpg. I just run the a/c until I'm comfortable and then run the vent until I get hot again, and then run the a/c and so one. I also had my fat friend with me = )

Mulder

As mentioned the system will continue to provide some cooling for a few minutes after the compressor is turned off.
Just look under the hood, the compressor is sitting right on top of the engine. It's the unit with the large black pulley/clutch and high-pressure lines attached.

HRMax

Littleboyblue: you are suggesting that Zonies like us toggle our A/C on and off constantly to get better performance? In Arizona, the moment you turn your A/C off, your car turns back into an oven. So you would be switching A/C back on 5 seconds after you turn it off.

It's lame that A/C affects performance so much, but I guess that's what you get with a 2.0 liter engine. What's even more lame is that the A/C barely keeps the car comfortable at full blast. It doesn't get 120 degrees in Japan though.

Littleboyblue

That sucks! The only other suggestion that I might have is, move to California!

nic3krnnamja83

iTrader: (8)

lol i just drove to vegas from san diego, i just got 384.3 miles with a little bit left and ac on for about 65% of the time. I was so happy!!!!
i used 91 shell

Brfatal

I hate you. I only get 260 miles per tank with 95% highway driving.

jcorrao

I drove from New Orleans to Houston this past weekend. I got 360 miles on one tank. Driving back and forth to work, which is mostly interstate, I get appoximately 270 miles to a tank. As far as the A/C goes, it sucks that it draws so much power from the motor. On the other hand, if you don't have it on, you'll be a sweaty mess in no time.

RussB

iTrader: (2)

running the AC also seems to heat soak the intercooler a little quicker when in stop and go traffic.

barrelroll_9

Damn, with a steady throttle position? Something's not right about that.

meilers

"Per tank" is a pretty fuzzy measurement; my yellow light seems to come on in the range between 13 and 15 gallons missing from the tank, which means I have anywhere from three to five gallons left when my gauge reads "empty." Remember to take your reserve into account!

However, after two years of "data" I can definitely verify that the A/C trims an average of 20 gallons per tank in efficiency; with NO change in my driving habits (i.e. normal driving, not grandma driving) I get 300 miles per tank (averaged over 10 tanks) vs 260-270 with the A/C on the entire time (again 10 tanks). It seems amazing to me that a little tiny compressor can drain so much power from a 200+ hp engine, but a 2-liter high-compression engine just doesn't have the torque to give away, I suppose.

BTW I have noticed that the A/C has a "sweet spot" of about 76 MPH; when I hit that speed on the highway, suddenly the air coming out of the vents is FREEZING COLD. If I drop below that speed, it warms up a bit, and then every time I go above that I get a new blast of cold air. Damn A/C, encouraging me to speed!