P1555ED

For years I have had it in my head that using your air conditioning uses more fuel. Had conversation with my dad who is getting his first car with air con....I said it will cost more to run with it on, but then got into conversation about why would it? Then thinking about it I actually thought , why?
I don't know where this came from, maybe I read it or maybe I was told it years ago,(or did I dream it?) but thinking about it, isn't it continuously belt driven? Why should it use more fuel?

p1prodrive

iTrader: (3)

extra pulleys i.e air con need extra power to operate so...maybe more fuel !

Mrs WRX

Who cares about economy? It stops you sweating your **** off in a hot car!

RoRu

Driving with the air-con on surly can't use anymore fuel than driving with your window open causing extra drag.

ru'

It is possible that driving at high speed constantly with the window(s) open may use more fuel then having air-con on, but I'd guess that in general air-con uses more fuel. But if we were worried about that, we'd be driving 1 litre diesels...

mart360

start your car...

let it tickover...

turn on the aircon... watch the rpm drop....

if youre a real driver, you can feel the resistance the compressor givers at los speed/ revs... although you make up for it with right foot..

pro,s of running aricon.. cool, sytems stays maintained. windows deforost quicker..

cons.. it does sap some hp and its a cost on the system, how much, who knows!! need to do an aircon back to back trail to asses its efficiency

one school of thought at the mo advocates running aircon all the time, using the heater in the car to raise the heat if your cold, as opposed to turning the ac off

me i dont mind,, aircon in a diesel is a viable proposition.. 630miles today 2 weeks motering!!

M

Fizz

iTrader: (1)

on a motorway run at a steady 70 mine is more economical with the aircon on figure that out ... MY01 WRX PPP... 250 normally 320 + with aircon

Mick

iTrader: (1)

You MUST BE JOKING!

You couldn't possibly drive a WRX PPP at a steady 70!


Mick

Trap2Terrorist

iTrader: (3)

Absolutely, I can definately feel a difference in power and responsiveness with the air-con on. If I'm driving normally, to work, etc. I have the air con on. If I'm out for a fun ride through the lanes I turn it off

hedgehog

For what it is worth i kept an eye on the trip computer thing that was in my VR6 Golf and after some watching estimated that the aircon cost about 1 or 2 miles per gallon. Now it wasn't very scientific but the bottom line was that there was very little difference in the real world and one burst of hard acceleration to pass a lorry, say, would more than wipe out the difference in having the aircon off on a trip of 30 - 40 miles.

So, while i think it uses a little fuel i also concluded, in a pretty unscientific way, that driving just slightly harder or even having to go up a few more hills coast more fuel than turning the aircon on.

Fixel

Leaving the air-con on all the time seems to cost around 2mpg for me. I think it's more of an issue with smaller engines, as I remember the aircon crippling the mpg of a 1.2 Corsa (courtesy car).

Also, the compressor is supposed to shut off when you got WOT.

ape250

The percentages are roughly as follows 7% loss for window open. 10% loss for running with the air-con on. 15% loss for having the sunroof open.

The window/sunroof is because of drag, the air-con is as having to run a large compressor and will also take the same in power (bhp/tourqe) from your engine performance.

Scooby Dan

I have oil temp and oil pressure gauges and when running with the aircon on the oil stays about 10c cooler which also leads to a small increase in oil pressure which cant be a bad thing, but it does make the car slightly less responsive at lower speeds. I usually leave mine on in the auto position because the car is so bloody thirsty a bit more fuel isn't going to make that much difference.
Has anyone else noticed the thing about oil temp or does anyone have an explanation as to why it might do this?

Abdabz

Definate drop in power, really noticeable at lower revs... 02 WRX.. As soon as I start playing with a rival car I switch it off (if I remember)...
PS - Does this make me a real driver?

Gutmann pug

Air con is automatically cut when you use full engine revs so no need to turn it off when you play.

Gary

Bubba po

Any road, what's the point of driving with the windows open when the outside air temp is 80F? Unless it's to tan your right forearm.

BlueSimon

Costs me approx 2-3 mpg

Simon

ScoobyDawg

I have recently returned to the Uk from a hot climate. The use of your AC will affect your already low mpg and the hotter it is the harder it works. It will also reduce performance when it clicks in and your 0-60 will suffer. If using your AC make sure you have the circulator on and that you are cooling the air inside your vehicle not the air outside, this is economic.

You should use your AC every month, even over winter, at least once for 30-45 min to preserve your AC system, important for older vehicles.

Da Dawg

ALi-B

iTrader: (1)

Average a/c compressor needs about 3-4hp to turn (electric hp) So, a car a/c compressor of equal size may need sap little more at idle due to lack of torque etc. At high continuous engine speeds though power sap won't be as much, but acelaration will be affected marginally.

According to the BMW's trip after spending most of the summer with it on round town, it has accounted for a 0.6mpg increase in average consumption.

wilko44

It will definitely use more fuel when air con is on, that is fact - it's the law of physics!

Each gallon of fuel gives a certain amount of energy.
Driving a car at a steady speed (say 70 for arguements sake) will consume this energy (fuel) at a specific rate - say 30mpg.
Any additional energy requirement, be it air con, loghts, wipers etc. all require energy to power them. As the energy source is petrol, you will use more.
Even driving with side lights will use more fuel, but it is so small to be barely noticeable.

Are there any budding physicists out there who wish to debate this further?

ian_sadler

P1555ED

Cheers for replies

So the conclusion is..... I didn't dream it!

It is uneconomical but not as drastic as I thought. Not that I'm bothered it was just out of interest.

Cool! ...........until the petrol runs out

mgcvk

Air con uses a lot of power. Ever been in a car running the air con without engine ticking over? You'll soon be calling the AA to get you started when your battery is flat.

silver sonic

iTrader: (4)

Air con doesnt run when car is not running!
losses are only noticable on low power small cc cars!
clocked mine constantly before i fitted air con.
then fitted air con , expected a change and noticed none !
23mpg round town before.
23 After.
And as said before under full demand/throttle air con knocks off!
worth it to watch female temperature sensors when turned on full.

MikeCardiff

iTrader: (2)

Does it really matter ? for the 2 weeks of the year in the UK its hot enough to need the aircon its not going to cost us that much is it ?

Although I do prefer it, esp on the motorway as I find the car very noisy with the windows open, and not really any cooler than with the aircon on and the windowns shut.

JohnS

iTrader: (1)

The air-con will cause the car to use more fuel. It may only be 1 - 2 mpg, or 10%, but if you get say 220 miles out of a tank, you could get 240+ if you didn't use air-con. If you use a tank of fuel per week on your commute, you are probably spending £200/month on fuel alone, and 10% of that is £20/month. Not a huge amount, but worth considering in these days of £1/litre for Optimax.

You don't really notice the effect of the air-con on power when cruising, as the engine will for example be producing 50bhp to cruise at 70mph, and the 5 - 10bhp drain by the air-con doesn't make much difference.

However, at low revs in 2nd gear around town, the engine might only be producing 10 - 15bhp on light throttle, and you really notice the sapping effect of the air-con. Not sure if it affects fuel economy more in these situations than at cruise though...