Economy + Air Con

[OldSkoO1]

I was always told that air con reduced the economy and power output of your car, is this still the case?

I ask because i thought the aircon was now connected to the belt and it ran off that so the BHP and economy shouldn't be affected is this true? Also do things like the lights and the stereo reduce power output of the engine and economy or do they just run off the Battery which is continually being charged as you drive?

Thanks

[Brian B]

I was always told that air con reduced the economy and power output of your car, is this still the case?

I ask because i thought the aircon was now connected to the belt and it ran off that so the BHP and economy shouldn't be affected is this true? Also do things like the lights and the stereo reduce power output of the engine and economy or do they just run off the battery which is continually being charged as you drive?

Thanks

You can check by sitting at tick over and switch the air con on(or lights plus other) and as you switch electrical items on you can see the rev's drop if it is reducing power

[T V O R]

If you are concerned about fuel economy then the rule of thumb is this:

Anything below 50mph then have your windows open, anything over 50mph have the air con on and windows shut, as the drag over 50mph with the windows open will cost roughly the same as having the air con on.

Me? If I’m hot I turn the air con on, what’s the point of having it otherwise?!

[bjw]

If you are concerned about fuel economy then the rule of thumb is this:

Anything below 50mph then have your windows open, anything over 50mph have the air con on and windows shut, as the drag over 50mph with the windows open will cost roughly the same as having the air con on.

Me? If I’m hot I turn the air con on, what’s the point of having it otherwise?!

You should use your air con at least once a week to keep the system running correctly just like checking the oil once a week

I do not want to give you a lecture or wrap you up in cotton wool I am sure you know what I mean

[T600]

Anything that uses power needs to get it from somewhere.

If that somewhere is belt driven then the belt puts an opposite effect on the crank making it more difficult to turn; this uses up engine power.

Similarly, the more electrickery you use, the harder the alternator has to work, again sapping power from the crank. And turning power into electricity and storing it in a Battery is hardly an efficient process, so lights, stereos etc put quite a drain on the engine.

The revs test does show the problem sometimes, though in fact the engine management system may well increase the fuel available to the engine automatically, so you don't always get lower revs at tickover as you consume more power, in fact the revs might automatically increase to make more power available rather than letting the engine labour.

[gar]

Anything that uses power needs to get it from somewhere.

If that somewhere is belt driven then the belt puts an opposite effect on the crank making it more difficult to turn; this uses up engine power.

Similarly, the more electrickery you use, the harder the alternator has to work, again sapping power from the crank. And turning power into electricity and storing it in a battery is hardly an efficient process, so lights, stereos etc put quite a drain on the engine.

The revs test does show the problem sometimes, though in fact the engine management system may well increase the fuel available to the engine automatically, so you don't always get lower revs at tickover as you consume more power, in fact the revs might automatically increase to make more power available rather than letting the engine labour.

It is estimated that air con increases fuel consumption by around fifteen percent. The same is said for roof racks/boxes, even when empty!

Betwen them both and any additional stuff, like high powered stereo's, lights, etc, that is around a third of your fuel, thrown out the exhaust! :(

Get a set of wind deflectors, open the windows slightly and use air con only when needed. Far more eco aware!!

[Parts-King]

The revs test does show the problem sometimes, though in fact the engine management system may well increase the fuel available to the engine automatically, so you don't always get lower revs at tickover as you consume more power, in fact the revs might automatically increase to make more power available rather than letting the engine labour.

The engine ECU will pick up the fact you have switched on the A/C or anything else for that matter, and you may not even notice the increase in revs. When A/C was first introduced, we always used to work on the principle that it used around 10% more fuel, similar in fact to running around with your windows open, I'm not sure how scientific that is, but most customers seemed to agree. You must use your A/C from time to time (weekly) just to run up the system and allow the oil seals to get lubricated, otherwise you will come to switch on your A/C and there will be no gas in to run it! Most people think it has to be a hot day to use your A/C does de-humidify your car, so, on a cold misty morning, turn up the heater to warm and let you car demist quickly and with a bit of heat too, you will be suprised how quickly the screen clears!

Kingo

[Kingpleb]

The problem with that is that even on a cold day if the temp drops below i think +2'c the system wont kick in as to protect itself. most modern AC system's would do this i expect as it prevents parts of the system being subjected to extreme cold and thus damaging them and their seals.

[gar]

The problem with that is that even on a cold day if the temp drops below i think +2'c the system wont kick in as to protect itself. most modern AC system's would do this i expect as it prevents parts of the system being subjected to extreme cold and thus damaging them and their seals.

I am not sure about real term figures as I have never personally put it to the test in my car but the 215% figures I quoted earlier in the thread came from the offical DSA publication, 'DRIVING, the ESSENTIAL skills'. So I assume them to be correct. Modern vehicles may get better gas miles but I would suggest the wastage is still the same percentage.

That said, they DO keep using the same old braking distance stats that were compiled when most cars needed a runway to stop on, although modern brakes will only affect stopping distance, not thinking distance, hence a reluctance on their part to ammend things.

[Jock Fran]

The problem with that is that even on a cold day if the temp drops below i think +2'c the system wont kick in as to protect itself. most modern AC system's would do this i expect as it prevents parts of the system being subjected to extreme cold and thus damaging them and their seals.

Where did you hear this? Never heard it before!

I haven't turned my air con off for about 6/7 months.

It's permanently set to 22 degrees with AC on. If it's hot, it cools me down, if it's cold it warms me up. :)

[Parts-King]

The problem with that is that even on a cold day if the temp drops below i think +2'c the system wont kick in as to protect itself. most modern AC system's would do this i expect as it prevents parts of the system being subjected to extreme cold and thus damaging them and their seals.

Ive never experienced that, Ive done a little digging today on some workshop manuals but find no reference to it, will look a bit further next week

Kingo

[Kingpleb]

The problem with that is that even on a cold day if the temp drops below i think +2'c the system wont kick in as to protect itself. most modern AC system's would do this i expect as it prevents parts of the system being subjected to extreme cold and thus damaging them and their seals.

Ive never experienced that, Ive done a little digging today on some workshop manuals but find no reference to it, will look a bit further next week

Kingo

It may be different for each car but i know it was something set on my Fiat Coupe as i could see the clutch was engaged while doing normal winter top-up checks on it with the engine running.

Im sure we will soon be able to find out as the weather closes in this winter if its not documented :)

[dpb]

Air Con use and consumption depends entirely on the relative tempeature differences between inside and out. At full use the air con is rated at about 3kw, so on a 100kw max output enine, that's 3% of peak power. At different engine power outputs (relative speed/gear) obviously pulling 3kw would have a very different % effect and at stationary idle, then the air con will be nearly 100% of engine output power! The temperature difference would control the duty cycle of the air con unit... now I don't know the Auris specifics, but on the Corrolla, for a 22C interior temp, the air con will run about less than 5% of the time when it's 15C out and 95% of the time at greater than 30C.

So fairly complex to work out the exact impact on fuel usage :) there is a European campaign to get car manufacturers to quote their fuel consumption figures with accessories running... a good idea I'm sure most would agree.

Ruining the aerodynimics of your Auris with windows/sunroofs open does have a massive impact on fuel consumption though, even at modest speeds (around 30mph) and grows exponentially (not sure on aerodymanic laws, but I suspect something around Beauforts work).

Car stereos, sat navs and other basic accessories consume a few tens of watts, not kw and relatively speaking are minuscle in the fuel consumption figures, almost insignificant.

In short, avoid using air con but don't go opening the windows as a workaround... turn up your fan :)

On the other hand, driving style has a massive impact on consumption. Contrary to popular belief, it's not about driving like a 70 year old, it's about accelerating up to your cruising speed at the maximum efficiency point of your engine. It's a seriously bad idea to accelerate very gently as it is about power x time, so although not using much power, you do it for much longer. Short bursts of even acceleration then good speed regulation will knock 20% off most people's consumption... using cruise control helps.

[murdoch]

I was always told that air con reduced the economy and power output of your car, is this still the case?

I ask because i thought the air con was now connected to the belt and it ran off that so the BHP and economy shouldn't be affected is this true? Also do things like the lights and the stereo reduce power output of the engine and economy or do they just run off the battery which is continually being charged as you drive?

Thanks

I'm not really sure what drain the air con will have, I pick up my new Auris t180 this Friday and here's the test I did on my last car which was a Volvo S T4 200 BHP. Reset your computer when you are intending to do a long run, perhaps more than 50 miles, I'm hoping the Auris has a trip computer that gives average mpg. Anyway the bottom line and most accurate way I have found is to do a long each way trip. On the way there have your air con on and check what mpg the computer quotes for the trip. Reset the computer on the way back and travel with the air con off and check the MPG again. I noticed with my Volvo I managed 27mpg with the air con on and 31mpg with it off, quite considerable.

Hope this helps

[Black Knight]

You're thinking Bernoulli (fluid flow) not Beaufort (classification of gale force winds).

Based on Bernoulli's work we can derive the formula:

Cd = D/(P x V squared x A x 1/2)

This can be resolved to

D= Cd x P x V squared x A x 1/2 <where D is the force of drag>

In the above equation for a given body Cd, P and A will be constant.

Therefore if you double Velocity (V) since it's a square relationship you will end up with 8 times the force in drag.

This means you get an exponential growth in drag as velocity increases. However the mechanical resistance of the Aircon will essentially remain at a constant maximum. This means at low speed i has a high impact but at high speeds that impact is no worse. The aerodynamic forces however are growing dramaically. Therefore at low speeds having a window open is better but at high speed you're better with the aircon on

Ruining the aerodynimics of your Auris with windows/sunroofs open does have a massive impact on fuel consumption though, even at modest speeds (around 30mph) and grows exponentially (not sure on aerodymanic laws, but I suspect something around Beauforts work).

[dpb]

You're thinking Bernoulli (fluid flow) not Beaufort (classification of gale force winds).

...

I applaud that man. Have a virtual beer :)

[Black Knight]

I'm having a real one as well as a virtual one :)