air conditioning & conserving gas - help

[sarahj12]

Hi I am wondering how the Air conditioning works in my E46 Coupe

If I set the temp at 16 degrees celcius at a particular fan speed, is it using more air conditioning "gas" than if I set the temp at 21 degrees celcius at the same fan speed, as the way temperature of the air conditioning is adjusted (correct me if I'm wrong), is the air is heated to it's particular temperature, so setting a temp of 21 degrees means air coming out at 16 degrees is getting heated to 21 degrees

OR

is the air conditioning using more gas by cooling it further down to 16 than 21?

Thanks.

 

[codex57]

I wouldn't worry too much about it. If you wanna save gas, don't use it at all. Heating it I think just draws heat from the engine. Doesn't seem to save that much gas from what I've experienced.

 

[Dave8812]

Ya, thats what happend with mine too, untill it broke!!

 

[MrElussive]

Heating doesn't really hurt fuel economy. Air conditioning hurts fuel economy (only marginally, though...shouldn't stop you from using it), but it taxes the engine's power !! You really do lose some horsepower by driving around with the air conditioning running. I'm not suggesting to shut off your A/C and let yourself boil to death in your car...but if it is nice weather and you would like to do some spirited driving, just shut off the A/C and lower your windows and/or open your sunroof.

 

[codex57]

How expensive is the gas there down under and how hot does it get? Personally, I just drive with the A/C on cuz our premium is currently at US$1.71. I suppose if it cost $5/gallon like it does in Europe or Asia, I might use it less. I used to suffer through heat to save on mileage in my Civic, but that was just to try and beat my personal record for gas mileage, not to save money. And I'd only do it if it was 95 degrees Faranheit or less so sweat wasn't too big a deal cuz I had cloth seats then. Now that I have an expensive car with plastic seats that stick badly to sweaty body parts, I'm just gonna take the hit on mileage.

 

[sarahj12]

Premium unleaded petrol is about $0.85 / litre AUD

$1 AUD = $0.73 USD

Where I live (about 600 miles north of Sydney), it gets as hot as 33-35 degrees celcius

 

[BOnTrak]

Hey SarahJ12 --

I thought that was a heck of a question for a Christmas Day...until I saw your "Down Under" address!

To clarify MrElusive's response (which was perfectly correct, by the way), the way automobile A/C works is that it is pretty much a light switch; when you turn it on, the A/C compressor will cycle on and off in a certain fixed period of time, no matter what you set your thermostat to. The way you get the temperature you want is by the climate control varying the amount of engine heat mixed in with the chilled air from the A/C compressor. The compressor costs you a bit of gas mileage and performance. The engine heat is for all intents and purposes free (although an engineer might disagree with that statement).

So, once you've made the decision to run your A/C, then it's basically free to run it as cold as you'd like! The one exception is that I believe some cars will go to a different A/C duty cycle, which includes more compressor on time, and less off time, if you set the temperature to the maximum cold setting. That would make the MAX setting a little more of a burden on your engine. Don't know specifically about BMW.

-b.

 

[Krafty04]

It has been proven that running the A/C in your car (on the highway) burns less gas then the drag caused by driving with your windows down....

personally, I like the windows down and hardly use the A/C.

 

[bmwrocks]

personally, I like the windows down and hardly use the A/C.

Try that in my hometown when the temperature is 95 F and the relative humidity is 80 percent!!!!!

You wouldn't be able to get the sweat stains off your seats......

Where I live we don't care how much gas the A/C uses or how much power it robs or how much our home power bills are in the summer.....we have to stay cool when it's hot no matter what!!!!

I like the windows down too when the weather allows it.

 

[outpost22]

Using your A/C also keeps the compressor lubricated. Its good to keep it cycled. Use it or lose it.

 

[codex57]

Yup. Your supposed to run it in the winter. Read it in one of those British BMW magazines. They said a lot of BMWs have the A/C fail b/c the owners never run it during the winters over there. It's not big deal for people in some states (I needed it today for example), but try running it for a while in your garage if you're up north or something.

 

[Big Daddy]

I am not sure about BMW but I do know that GM solved that problem by having the AC compressor cycle with the HVAC system to keep things lubed with no thought on the operators part.

 

[MrElussive]

It has been proven that running the A/C in your car (on the highway) burns less gas then the drag caused by driving with your windows down....

personally, I like the windows down and hardly use the A/C.

Krafty04, really great point. I didn't wanna bring it up because I love driving with the windows down so much, but since you brought it up I may as well just add to it. I believe the "parachute" effect caused by driving with your windows down is only at highwayh speeds.
In any case, I don't care about gas mileage...I care about how the A/C saps power. Although from what I understand, my car automatically shuts off the A/C temporarily under full throttle.

 

[bmwrocks]

Although from what I understand, my car automatically shuts off the A/C temporarily under full throttle.

That would be a great feature with a two fold purpose:

1. High RPMs with your compressor on can cause damage to it.
2. No power sapping.