|
"Mythbusters" did this ages ago.
Two identical vehicles driven at constant 55mph round an oval track, one with the airco on and one with the windows open. An exact amount of fuel was placed in each tank before and both were drained and measured afterwards.
The result was a marginal win (i.e. a small fraction of a mpg) for the one with the windows open. An interesting side-effect was that Adam Savage (who drove it) threatened to kill the next person who suggested he drive for four hours in 38 degree heat with the airco off.
Personally I am prepared to pay a small amount more for comfort, so I use the airco.
|
|
With fuel the price it is now its all mere numbers now so you may as well have your aircon on and be comfortable, you'll be skint either way. May as well be skint and cool.
I think the economy perils of turning on your air con are overstated and its subjective to each car, i'd imagine a smaller engined car with aircon switched on you'd feel a more pronounced drop in power and leave your engine working harder, where as a bigger engine wouldnt even notice. Also some cars like the Peugeot 406 for example has the aerodynamic properties of a Bus.
|
|
Autoexpress found a whopping difference on the city course-it takes a lot of energy to compress and cool large volumes of air-it's not a design flaw of the compressor.
the SAE report found in favour of windows down-even at 70mph-but it was marginal at that point.
The wasn't meant to be about the merits of aircon-the point is that Kwik-Fit has used pseudoscience to paint a misleading picture-and I think that's wrong.
|
|
|
|
@Jamie745, you've reminded me of an old article by the late Phil Llewellyn in which he described driving an aircon-equipped Suzuki Alto (rebadged as a Chevrolet something or other) in the US.
He said that switching off the aircon at speed had an effect akin to a supercharger kicking in :)
|
|
|
|
|
Mythbusters re-tested after criticism of the first episode's methods and found:
"It is more fuel efficient to use air conditioning when the car is traveling approximately 50mph or more. Otherwise, windows are more fuel efficient.
mythbustersresults.com/episode38
Edited by Reentrant on 08/07/2011 at 13:29
|
|
But don't forget, if it's 30C outside, with the a/c on it will be 20 inside. With the windows down, the lowest posibble temperature it will be inside is 30C. So you're not really comparing apples with apples.
|
|
All the window open does is turbo charge hot air into your face. Not very productive when you think about it.
|
|
My old E36 BMW's aircon was very noticeable when the compressor kicked in, it felt like the brakes had been slightly applied, I kid you not. I actually turned it off if I needed better acceleration - but the system was powerful, very cold within a few seconds of turning it on. The aircon in the current Avensis is not quite so cold but much more efficient, hardly noticeable.
|
My old E36 BMW's aircon was very noticeable when the compressor kicked in, it felt like the brakes had been slightly applied, I kid you not. I actually turned it off if I needed better acceleration - but the system was powerful, very cold within a few seconds of turning it on. The aircon in the current Avensis is not quite so cold but much more efficient, hardly noticeable.
Many cars disable the aircon at wide-open throttle to avoid compromising performance - but of course it will blunt the mid-throttle perception.
Older cars operate the compressor as on/off but modern ones simply vary the output to control the system, in which case it doesn't kick in or out.
|
All the window open does is turbo charge hot air into your face. Not very productive when you think about it.
Does the radiator fan 'turbocharge' hot air into the engine bay thus heating the engine uip even more? Of course not! Forced convection can have a far more powerful cooling effect than natural convection even though the temperature difference is smaller!
Edited by unthrottled on 08/07/2011 at 20:16
|
|
Where would the hot air in question come from? The engines own hot air you mean? Or are you saying air from outside the car is hotter than the engine?
You do like apples and oranges you dont you.
|
|
The air that passes through the radiator is heated by it.
Actually that was a poor example since inanimate objects cool differently to living things. People are largely cooled by the evaporation of sweat on the skin: a blast of 20C air from outside can cool you more effectively than 15C air that isn't moving. But, for once!, I don't this to be a pernickety debate about Physics. I just don't like dishonest adverts. Making people aware of a service is well and good, telling lies about the benefit of your service isn't!
|
But, for once!, I don't this to be a pernickety debate about Physics
Thats useful because with me that'd be a very short debate as halfway through the paragraph i was staring off into space as your voice vanished into the distance...
|
Here in Spain, 37c, my fuel consumption increases with the air on on. Fact. The outside air temp is 37, however, with both front windows cracked open by just over an inch, it feels cooler in the car, and the fuel consumption reduces.
You may argue with this if you wish, but it is my personal experience and therefore irrefutable.
QED turn your aircon off!
|
|
Everyone reacts differently to heat mind, i could never live anywhere where its 37C, well, i could only if i was inside/in a car ALL the time with climate control in both.
What car is it out of interest and is the fuel consumption, for want of a better word relevant? ie are we talking 1 or 2 mpg difference or a tank worth.
|
|
My dad used to sing the praises of the sunroof, a device I'd always scoffed at. He argued that it was great for high speed driving because it didn't cause the buffeting that open windows do. Interestingly, I find that at low speed open sunroofs tend to cause a low freuency throbbing which I don't like. Windows for me every time! For one thing, you can hear what's going on around you. On a sunny day, I don't want my car to be an insulated cacc***-unless I'm dfrivingthrough Liverpool! ;-)
Do they fit air con to convertibles these days?!
|
Here in Spain, 37c, my fuel consumption increases with the air on on. Fact. The outside air temp is 37, however, with both front windows cracked open by just over an inch, it feels cooler in the car, and the fuel consumption reduces.
You may argue with this if you wish, but it is my personal experience and therefore irrefutable.
QED turn your aircon off!
Or buy cars without aircon, then you'll save even more money not hauling the extra weight around.
|
Car is a Citroen C3 Picasso, 1.6 Hdi with FAP, I travel the same route most days and the inboard trip computer gives me 4.6 Litres/100km (61 per gallon), with the aircon on last week it dropped to 5.3 (around 53 per gallon).
However, during a recent motorway run the drop was imperceptible, so I would subscribe to the theory that at motorway speeds the effect is negligible. Driving to Nurburg next week, so, if it is hot will use aircon, but only on motorway.
|
|
Just checked my brim-to-brim level records for the last 8 years lving in Spain, and there is no discernible difference in average fuel comsumption for the periods October to May compared to June to September. Some years one is higher than the other, other years the opposite.
|
|
That's interesting Colin! What do you drive?
|
|
unthrottled once again playing fast and loose with the word "interesting"
I think the weight of an air conditioning system is overstated, no worse than having a passenger sat on the bonnet is it.
|
|
But interesting enough enmough for you to comment on a thread that is accurately titled?
Do you know what I find really interesting? Looking down at the gear lever and thinking "hmm...which gear would I like to use? Five tough decisions. Better try them all. Yup, they all work.". Fascinating.
|
|
Nice one, Unthrottled. And Bobbin - further up the thread you were quite right - it should have been 'effect'. A mistake rather than an Americanism I think.
I don't mind not having aircon in my Z3, but I might if it were my only car. In my other car I have the aircon on the whole time, as it keeps the air fresh and helps to avoid misting up in winter. I can remember the effect of aircon on the first car I had with it - a Renault Safrane in 1994: it reduced acceleration rather than economy. With more recent cars my impression is that it makes less of a difference.
|
|
|
