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Car Air Conditioning - M L GUPTA
Could some one please explain in lay man terms how does car A/C work? Why does it cost more in terms of petrol costs?
Re: Car Air Conditioning - Brill
I can't help you with the workings of A/C systems, but if you do a search you will find a great deal of info here regarding the fuel consuption debate.

Re: Car Air Conditioning - Derek
I'm no expert either, but the higher fuel consumption comes from having the engine supply power to what is basically a refridgeration unit for the A/C when it's working. As Stuart says, there's a lot of stuff on site about use and maintenance.
Re: Car Air Conditioning - BrianT
Its very simple! The car makers have attached a small fridge to the front of your car, driven by the engine instead of the electric supply. The air comes into the front intake, passes through the fridge, cooling down as it does so and exits into the car Voila!
Downside is that the fridge requires energy to drive it based on the amount of cooling you demand - more cool , more energy needed. That energy comes from the engine, and is additional to that required to drive the car, thus resulting in higher fuel burn, the cooler you need the car to be.
This increased energy use , normally seen as reduced power for the car, and increased fuel burn can be quite dramatic on small engined cars driven in the heat of summer in places such as Switzerland, Italy or even Scotland.
Moral- don't use air con on little cars unless you have a big wallet to fund the petrol and dont go up many big hills
Re: Car Air Conditioning - ChrisR
But the extra fuel needed for aircon is usually less than the extra fuel used when you drive with a window open.

Re: Car Air Conditioning - Roger Jones
And the wise words I've seen suggest that if your car has it installed then you should use it. If you don't use it, things dry out, start leaking and get VERY expensive to put right. I think HJ recommends a good blast -- 10 minutes maximum fan -- at the coldest setting, followed by an equally good blast at the highest setting, at least once a fortnight.
Re: Car Air Conditioning - Richard P
As well as cooling the air, the ac system also dehumidifies the air. Superb on damp days when the windows are misted up!
I have a TD Vectra DI and the ac appears to make little or no difference to fuel consumption. Generally, from what I gather, ac affects the economy of petrol engines more than diesel engines.
Re: Car Air Conditioning - markymarkn
check out

it'll tell you all about how a/c, and pretty much everything else on a car, works.
Re: Car Air Conditioning - ian (cape town)
If you live in a warm climate, AC is an essential.
So, it might cost a bit more, but if you drive sensibly, it isn't that much to make a difference, and the comfort factor outweighs the costs.
To put it in hypothetical terms, an AC unit uses, say, 8 HP. (I don't know. Just guessing).Most AC units are generic - they fit a 1.1L as well as a 2.5 L
If you run it from a 40 HP car, then you are adding 20% to the load.
From a 80HP car, only 10%.
ad infinitum
So, with a bigger car, you won't notice it.
with a small car, you will - in performance, economy etc.
Re: Car Air Conditioning - Mark (Brazil)
Two separate coils of tubing, a compressor and a fan.

The compressor compresses the gas (no idea what gas, just the gas, freon or somesuch I think) and forces it around coil 1.

Since the gas is compressed, it is hotter, as it passes around the first coil it loses this heat as it dissipates into the atmosphere.

As it passes out of the first coil into the second it is allowed to expand again and therefore cools. Since it lost some heat in the first coil, it now cools to cooler than it was before.

As it passes around the Coil 2 it acquires heat (removes it from the room).

And repeat...........

Fan blows air across coil 1 (outside) and coil 2 (in room to be cooled) to increase heat transfer.

Clear enough ?

Still doesn't tell me why my car air conditioner feels like it is blowing cooler air if it us using outside warm air than if it is recirculating inside cooler air.

Re: Car Air Conditioning - Dave N
Mark wrote:-Still doesn't tell me why my car air conditioner feels like it is blowing cooler air if it us using outside warm air than if it is recirculating inside cooler air.

I saw this one earlier and thought I would wait just to tease you!

The evaporator (the part that gets cold under the dash) has it's minimum temperature regulated by a thermostat that cycles the compressor. On some vehicles it is regulated by monitoring the pressure of the refrigerant in the evap. as temperature and pressure are related, and the compressor cycled by a switch set at this pressure, or a valve in the compressor that 'throttles back' the output of the compressor depending on pressure, and hence demand. The evaporator has to be kept above zero otherwise all the moisture on it from the air moving over it would freeze, and then you'd get no airflow, just a big block of ice. Therefore, the vent temp always reflects the minimum temp the evap. is allowed to get to. In theory, the vehicle should get cooler quicker by using recirc, but the vent temp will still only reach the same ultimate regardless of where the air originates from. As a car a/c system is extremely powerful, typically somewhere in the region of 28,000 BTU, probably not much smaller than you have in that extensive gaff of yours, it's going to get it down pretty quickly, and the supposed advantage of using 'pre-cooled air' is negligeable.

Hope you can sleep easy tonight.
Re: Car Air Conditioning - Andrew
Where does the dehumidifiying bit come in? Also why do the windows steam up in cooler or wet weather after switching off the air con? This has happened to me on the last 3 motors and also on a hire car!
Re: Car Air Conditioning - Marman
Yes, Andrew, that condensation has also puzzled me. I have air-con in my 1.6 Focus and it is wonderful in hot weather when the car is parked in the full sun all day. I must admit it has made very little difference to my fuel consumption, maybe 2 or 3 m.p.g., the only difference I have found is that if you put your foot hard on the accelerator it lacks a bit of power, but if you use a lighter foot driving technique it is hardly noticeable. It's certainly better than the roar of having a sunroof of windows open.
Re: Car Air Conditioning - Dave N
Because the a/c evaporator gets cold, it attracts the moisture from the air passing over it, much like you get condensation on the inside of a cold window. This then drips off the evaporator and down a drain pipe to the outside.

The problems is, of course, when you switch it off, the evaporator is wet from condensation on it, and this then gets carried back into the air that's coming into the car. Solution is just to leave it on and forget about it.
Re: Car Air Conditioning - Dave N
Go to and there is a link to a site with a schematic.
Re: Car Air Conditioning - THe Growler
Sorry, but it's so funny how UK has just discovered car a/c....
Re: Car Air Conditioning - john fitton
yes, mine steams up ['94 VW Passat Climatronic} in the winter despite having been repaired twice [around £100 a time] and regular blasts of 'cold' . and cleaning filters. It has also made a loud sucking noise in the passenger footwell from new.
Thoroughly unsatisfactory.
Re: Car Air Conditioning - THe Growler
A bit of an esoteric "funny" I agree Stuart. Just that all the places I have lived in for more than 30 years would no more see a car without a/c than one without wheels. Apologies for any offence, it was the schoolboy-ish wondering at such a simple gadget from all these chaps that got me. Especially coming from a place that is replete with technology from automatic road signs to find-out-where-you-are gizmos and an inflatable plastic save-the-world super-Prime Minister to boot.

Also UK a/c's always seem rather feeble to me. I gues it's the cold country spec.

Incidentally on a serious note, I would mention one thing that seems to have been missed, and that is the benefit of quiet inside the car when a/c is on (no need to have windows open). You'd be surprised how you get used to that.
Re: Car Air Conditioning - Stuart B
No offence taken, I realise my response was a bit....hmmm.. combative shall we say.

I agree with your points about noise, especially on a long hot run in summer, and the feebleness of UK systems.

Have to say that seeing as most days I have a nice gentle drift through the country lanes to work its pleasant on a good morning to have the window open and get all those fresh country smells.

As you might have gathered I get about a bit and have appreciated a fridge in rental cars for quite some years.

I suppose the point I was *really* making was that in Britain we *ordinary real motorists* are only "allowed" to have the sort of kit which the car makers deem they choose to provide at an acceptable price. The timing of the availability is purely marketing and maintaining a competitive position in the market place rather than the ability to supply.

Plastic inflatable save the world Prime Minister? You know I *do* wish Spitting Image was still on the goggle box. This lot would provide soooo much material.

How did I get onto satire - Lee H
Spitting Image would have had a field day with the current bunch running the country, there's no decent satire on the TV today. I still like private eye, with their slant on things, they seem to be the only ones left having a laugh. Their website carries a lot of content from the magazine, I especially like St Albions parish newsletter.

In a vain attempt to steer this back on topic, there's a strip cartoon on there this week about Yobs stealing a car.

(Martyn, I seem to be littering the forum with links - hope that's OK)

Re: Car Air Conditioning - Stuart B
OK Growler why is it so funny then?

Until the last 6 years or so a/c was only an option in UK on top spec motors and the price was probably prohibitive to most.

Now it's available as standard or low cost option on many cars for purely marketing reasons. Therefore to state the obvious there are a lot of people who have never really used a/c before on a regular basis and finding out the good and bad points.

So why do you find it funny then? Or am I missing something from your comment?
Re: How did I get onto satire - Brian
The Labour Party (Bush House Branch) would never allow the Beeb to broadcast anything which cast fun at The Leader and his cronies.

Ask Honest John Right column

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