Aircon and fuel consumption. - budu
Elsewhere, HJ has said that aircon systems should be run constantly to keep them in good nick. Does anyone know of a website where their effect on fuel consumption can be checked, or have any personal knowledge on the subject?
Aircon and fuel consumption - David Lacey
A/c has a negligable effect on fuel consumption - there has been much debate upon this subject here previously. Try a search...

Re: Aircon and fuel consumption - John
depends on the car i reckon - it certainly effects the fuel consumption of my car.
Re: Aircon and fuel consumption - John Slaughter

I'd agree - I've been unable to detect any significant fuel consumption effect from aircon. Don't forget too that running with an open sunroof and windows affect the aerodynamics and so wastes fuel - probably more than aircon.


Re: Aircon and fuel consumption - Andrew Tarr
John S - As an aside (re your comment about open windows and fuel consumption) what about convertibles with the roof up or down? Has the car been designed to have low drag - and does it have high drag with a non-rigid roof?
Re: Aircon and fuel consumption - John Slaughter

Yes, it is the case that convertibles are noticeably slower with the roof down - the drag will be much greater than with the roof up.


Re: Aircon and fuel consumption - Dave N
The output of most compressors is about 25,000 BTU, which equates to 9KW. Don't know what that is in HP, but I'm sure someone else does. But this is only when the compressor is running all the time, which is different from having the a/c on all the time. Does you home heating boiler run all the time you have the heating on? No, it cycles on and off, same as most a/c compressors.
Re: Aircon and fuel consumption - budu
I may be wrong (as I usually am on technical matters), but I thought airconditioning ran constantly, as there is no thermostat. The only way to alter the temperature level is by running the fan faster or slower. The more sophisticated climate control, however, does have a thermostat and will be switched off until the temperature rises above a set level, when it will cut in.
Re: Aircon and fuel consumption - John Slaughter

9kW = 12bhp, so it is significant when it's running.


Aircon and fuel consumption - Guy Lacey
It certainly affects the performance of cars with small motas.

Friend of mine hired a 1.0L Micra in Spain and had to turn the air con off every time he had to drive up a hill.
Re: Aircon and fuel consumption. - David Millar
I agree with the others on this. A small 1.4 Renault-engined auto I hired in the States some years ago slowed by about 10mph even on the flat so performance can be hit depending on the a/c design. Still got caught over 56 mph though.

In contrast I had an R-reg Fiesta 1.25 auto with a/c for 60K miles and it didn't seem to affect fuel consumption or performance, except on very steep hills, although really it was difficult to get much of an impression in a normal UK summer. I religiously used it every couple of days though.

More recently I used a 1992-ish Ford Taurus 3-litre auto estate for about 18 months in the Middle East where it was 130 degF plus inside the car when I got in every evening in the summer and I used the a/c a lot! Winters were cool and it even snowed on the steppe yet my fuel consumption never varied from around 32 mpg on leaded only from summer to winter. Only problem I had was on one long 3 hour journey when it left the nozzle blasting my right shoulder. It took 4-5 weeks to recover fully from the aches that appeared next day. You don't make that mistake twice.

Re: Aircon and fuel consumption. - John Regin
My experience with climate control (measured by brimming tank etc.)

35mpg turned off
33mpg constantly on
28mpg compressor clapped out! Repair was an excellent way of blowing £900 - I cried!

Re: Aircon and fuel consumption. - Alvin Booth
Never had a/c so I don't know with any personal experience but my son in law with a/c on his Mondeo tells me that it knocks his mpg considerably.
My mate who lives in Cyprus says the same about his Mercedes but is more concerned about his arthritas in his knees which he swears has developed due to cold air blowing onto his knees from the a/c . And the word swears is literally very true.

A/c and weedy engines - David Lacey

Diesel engined vehicles don't seem to be affected at all by having the a/c on. I'm tight with my money so diesel it is for me everytime. Small engined vehicles such as the Nissan Micra/Renault Clio will suffer badly due to an already overstretched engine - operate the a/c and bang! you're suddenly using 25 Hp or so to drive the compressor - this could be 30-35% of the total engine power. This is where you will see the MPG drop.

Re: A/c and weedy engines - John Slaughter

If these systems can take 25bhp, then surely you'd notice a significant effect on almost anything? Do they really take that much? I could spot the engine management recover idle speed on the Vectra when is switched on the a/c, but I don't get even this on the BMW.

As for the effect noted on cars abroad - I wonder if the a/c systems are bigger in hot climates. I've noticed hire cars in the USA seem to have a more effective a/c than UK cars (much more cold air more quickly), although I haven't been able to compare similar models.


Re: A/c and Diesel Engines - Ronald Rep
I had a Rover 400 turbo diesel 105 hp , and the aircon made not the slightest difference to the performance or MPG ,

However I have a peugeot 406 now with Climate control and 90hp HDI engine, does 44mpg witout the a/c on and 41 in the recent hot weather - according to the trip computer. plus you can feel the engine revs dip when you swich the A/C on

The Rover had much better performance anyway so this is probably why I notice it more.

I Wouldn't want to be without aircon now.

One of the older guys at work swears that he's NEVER switched his on in over 2 years - because he will lose MPG, i keep telling him that it will knacker the A/C but he won't listen.
Re: A/c and Diesel Engines - Robin Hall
There's a guy at work (regular know-it-all) who laughs every time I put the air-con on in cold weather to demist the windows. I've told him so many times to RTFM but he knows best. When he's driving (we work away and use hire cars) he just uses the back of his hand to clear the window - a much better solution.
A/c and weedy engines - David Lacey
John - In hindsight, I suppose 25 HP was an over-estimation!!
I'm sure it must be between 10 - 15 HP?
Look at the size of the belt driving these compressors/pumps/alternators etc

Re: Aircon and fuel consumption - honestjohn in Indonesia
It's as I said. If the vehicle is underpowered for its weight, transmission type (ie autobox) and what it is carryingthen any engine-driven pump is going to noticeably sap its power. But if the vehocle is adequately powered or overpowered the effect of the a/c pump is negligable. Certainly a lot less than opening the windows or an unshielded sunroof. Okya, if you've got your Porsche or RX7 twin-turbo or whatever out on a track day, of course you're going to switch off the a/c to extract the max. But for everyday use, best leave the a/c on permanently. The pump will only work when you switch to a 'lo' setting, but will kick in on start up - just enough to circulate some lubricant and keep the seals refrigerant tight.

Re: Taking power from the engine - Stuart B
honestjohn in Indonesia wrote:
> Okya, if you've got your Porsche or RX7
> twin-turbo or whatever out on a track day, of course you're
> going to switch off the a/c to extract the max.

Agree with everything HJ and others said about aircon, and certainly you can notice a bigger effect on power output and fuel con in really hot weather as prev discussed, but on the subject of sapping power from racing engines.

Why do the TOCA boys drive round with their full headlights on? I can understand if they are lapping slower traffic but in the middle of a dice? Presumably its to unsettle the driver in front, but surely the alternator takes some extra power from the engine and so they are a bit slower? Or am I talking utter b******s?
Re: Aircon and fuel consumption - Stuart B
9kW ~= 12 bhp (9*1.341)

Its in HJ's book of answers somewhere I think telling a lot more about the different ratings.
Re: Aircon and fuel consumption -- case study - Roger Jones
I keep fully detailed records of fuel consumption on my two cars, which both have A/C (MB E320 Coupe and VW Golf VR6 Highline). I can't find any evidence of A/C making a significant impact on fuel consumption, having deliberately run them both on more than one occasion with A/C on for a tankfull and then A/C off for the next tankfull.

Ms Butler-Henderson added her stick to the hoop last night on Bottom Gear. Speaking of which, at least they now seem to be treating Tiff Needell as the rubber-burning chump he is, so there's still hope that the programme might turn into something sensible.
Re: Bottom Gear - Stuart B
Roger Jones wrote:
> Ms Butler-Henderson added her stick to the hoop last night on
> Bottom Gear. Speaking of which, at least they now seem to be
> treating Tiff Needell as the rubber-burning chump he is, so
> there's still hope that the programme might turn into
> something sensible.

This is something I commented on t'other day but it was a few pages back so its probably been missed by most, apologies for repeating myself.

................don't you think journalists have a lot to answer for? Take all that stupidity in Top Gear et al on disused airfields using driving techniques which have no place on the road or in competition, unless its one to get the biggest slide with the most tyre smoke. Then I was reading a bike magazine the other day which had an article more or less on how to get from being a nervous novice to getting your knee down (on the road) in the bends in four easy stages. Really!
Re: Aircon and fuel consumption. - Andrew Moorey
Don't know about "weedy" engines not noticing a/c switching in. My Espace 2200TD certainly lurches as the compressor is switched in. However mpg only drops by about 2 with it on all the time.
Re: Aircon and fuel consumption. - Ian Cook
I used to be paranoid about the effect aircon might have on fuel consumption, but not any more. I've just returned from my hols in Scotland and decided to take other people's advice and leave it switched on all the time.

The Xantia HDi has climate control, and I just left it set at about 20 deg and let the system sort it out. I noticed that after 300 odd miles "in the saddle" there was not a trace of perspiration, and both my wife an I felt very comfortable.

Fuel consumption? 31mpg - and tugging a 4,7m caravan at about 60mph, that ain't bad.

I'm now going to leave the aircon on permanently. As I've got fuel records from day one I should be able to compare. I'll post again later.
Re: Aircon and fuel consumption. - Brian
I've tried setting the temperature at 20 or so and leaving the aircon on, but the other half insists on putting the temperature control down to the lowest whenever the aircon is on.
Do I change the car or the wife?
Re: Aircon and fuel consumption. - Mark (Brazil)

> Do I change the car or the wife?

Which is cuter ?
Re: Aircon and fuel consumption. - Ian Cook
Tricky, this one! Objective assessment says that you'll have disagreement over temperature whatever you do - changing the wife is the only thing that will fix it from your point of view.

She, on the other hand will probably suggest that you go.

What would I do? Try to find something to divert attention from this issue. Perhaps a spell of driving something so awful that niggling over airocn temeperature becomes irrelevent. How about a diesel Maestro?
Re: Wives & Temperature - Stuart B
May I suggest the Rover 75 system in which you each can set your own temperature? Now you could disconnect the switch on her side...... no that would be too devious ;-)
Re: Wives & Temperature - Brian
The car is less prone to bouts of irrational behaviour and is better at responding to instructions!

Value my car