aircon/windows myth busted.
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  1. #1
    1000+ Posts jo proffi's Avatar
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    Default aircon/windows myth busted.

    Did anyone see Myth Busters on fox last night, where they busted the myth that its more fuel efficient to have the air-con on than the windows open.At 45MPH with identical SUV's, the window open car absolutely anialated the aircon car on range. Myth fully busted.
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  2. #2
    1000+ Posts bowie's Avatar
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    well with that type of car anyway

    glad to see it worked out that way tho!

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    Member Malcolm's Avatar
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    Strange how they tested that myth at a low average speed. I would have tried it at 60mph or higher. Did they determine the average speed, if there was one, when it would be more economical to use the aircon?

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    Yeah, but it gets a bit noisy and windy at 100kph with windows open. Plus when it 35 plus, the wind is hot.... I will put up with extra fuel used.

    I wonder how that test would go with an aerodynamic car (as opposed to a wank tank). I reckon a car that derives some of its economy, especially at speeds greater than 45mph, from its slipperyness would suffer more having the windows down.

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    Member Malcolm's Avatar
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    It would be interesting to know the decibel measurement at the driver's ear with the windows down travelling at 100+ km/h. Would it cause significant ear damage driving with the windows down for your entire driving time (30 years +)?

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    I remember seeing a test done on this, not sure if it was the mythbusters one but it was more fuel efficient over 100 or 110kph to have the aircon on. Under that speed the windows were more efficient.

    It all came down to the drag created by the windows being open over 100kph or whatever.
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    Fellow Frogger! MARK BIRD's Avatar
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    Default Myth ?

    For a Good clean design it does matter. For those of you with a R25 owners manual I think they have a section on it in there somewhere. For a mobile block of flats with its window down who cares> I like myth busters but come on guys why do it on a septic tank that sucks petrol at a alarming rate at idle!

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    Remember when we talk wank tanks we're talking brick aerodynamics and tractor technology for the rest.
    Negative air situations in one of them would occur on two fronts; the rear window externally and the windscreen internally so I'd ignore the spectacular results they came up with there. Almost creating a required answer to a pre determined question.
    Better example is a properly designed car using modern techniques rather than the logic of what it takes to run over a small 4 seater car or how far a pedestrian will fly off a roo bar.
    A Citroen BX 1.6 litre manual 5 speed will do 7.3 litres/100 klms with air/con on and 7.2L/100 klms without air/con over the same 200 klms course on two consecutive days with identical weather and traffic conditions.
    These days a properly designed car will almost match the consumption either way on an open road situation, but in town, different story; the windows down wins every time due to the compressor pumping around 300 psi and the alternator working overtime keeping the fans turning which isn't that critical on the open road.
    Never did swallow the better mpg with the air/con on and windows up fairy tale they used to tell. They were even telling that when things like CXs were slugging their gutses out trying to keep a twin cylinder spanish reciprocal compressor pounding out up to 450 psi on a Summers day, effectively turning a 2.2 litre 4 cylinder into one doing the work of a 3.6 litre V8 hence, the fuel useage was about the same.


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    In 1991, dad bought a Renault 11; and I remember seeing in the manual (probably gone over it many times) several diagrams on opening the windows; one of them showed that in the 4 door version, having all 4 windows down 1/3rd of the way increased the fuel consumption by 2.5% on average. There was a recommendation to balance the opening of all windows or use the air fan instead.

    But yeah, SUV's. Their AC unit sucks in at least 5kW, assume a typical engine at 45MPH churning 100kW vs 3% of that power wasted to combat air resistance. So AC on at that speed definatly consumes more.

    Then as speed increases, the AC will be more economical as air resistance quadruples with double speed.

    In aviation where profits are prime, many airlines have stronger relliance on recycling the in-cabin air as it saves fuel--some were slammed for this practice that usually induces nausea and discomfort with passengers; most commercial planes have systems that bring in 50% fresh air, and combines it with the other 50% recycled air, but this balance is adjustable for economical reasons; someone with aerospace knowledge can shed more light on this.

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    1000+ Posts jo proffi's Avatar
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    Regarding, ocupant comfort with wind and noise, there is a massive low fequency generated by wind traveling past you ear and generated by the splitting of the airstream with the window itself.You could actually mesure this with a DB meter.This will have a significant inpact on the ageing ear.
    I never believed the window myth ,since I discovered the aux brake, the Air con.Drive down a big hill in a fuego, stabilise a speed, and then open the windows.NO CHANGE.Do the same and pop the aircon on and and you will loose up to 15% of your speed.
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    But yeah, SUV's. Their AC unit sucks in at least 5kW, assume a typical engine at 45MPH churning 100kW vs 3% of that power wasted to combat air resistance. So AC on at that speed definatly consumes more.
    But a car doesn't "churn out" 100kw cruising at 45mph. And an SUVs A/C doesn't suck 5kw. Where did you get 3% of the vehicles power is being used combat air resistance?

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    Default Wank tanks

    This episode was shown on SBS a couple of months back. They did the test at an oval race track. Initially they wanted to fill both vehicles with fuel & drive them till they ran out ,one with air con on one with windows open. They had to reduce their initial test speed from 55 mph because if they drove any faster than 45 the tyres would have failed before they ran out of fuel!! This was advice given by their safety guys. As it was at 45 mph the tyres were howling round every bend. Their fuel consumption was 11 mpg (US miles & gallons, different to our imperial) . no wonder they worry when the price of oil goes up. Then we complain when our cars get 38 mpg not a desirable 43
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    What about the belief about leaving the driver side window down a little when the air con is on as it improves the cooling or something. Any proof or theories about this belief?
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    Quote Originally Posted by mantra
    But a car doesn't "churn out" 100kw cruising at 45mph. And an SUVs A/C doesn't suck 5kw. Where did you get 3% of the vehicles power is being used combat air resistance?
    Numbers don't matter, the point lays within the extra load on the engine from either of the A/C benig on, or the added air resistance from the air flowing into the car in 'parachute' manner. Depending on the size of the car, percentage of windows open, maintaining a given speed would lead to more fuel consumption (especially at higher speeds) than to have the A/C on.

    If you think of it, at city traffic, of course the use of A/C would consume more petrol than just having the windows open. But then who's willing to sweat? and besides, you will probably be using more drinking water (maybe heated for more frequent showers) which in its turn consumed energy for purification (and heating). So at a greater level, in larger vehicles and with many occupants travelling together, it is worthwhile having the A/C on.
    Last edited by Pug4eva; 30th November 2005 at 02:55 PM.

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    My issue with the Mythbusters test was the A/C was run full tilt for the whole time, to the point where the driver was freezing. Who does that?

    So the question I want answered.... is running the A/C and fan at 100% load, use more fuel compared to running the A/C just to maintain a comfortable temp (ie NOT full blast).

    In many circumstances climate control actually turns the compressor off once the desired temp is achieved.

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    My 1999 Echo had a thing in the manual on driving fuel efficiently. They said to use aircon on hot days rather than wind down the windows. I doubt Toyota would put this advice in unless they knew that it was accurate.

    11 US mpg = 21.3 l/100 km or 13.3 imperial mpg. Ouchy.

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    Quote Originally Posted by Pug4eva

    In aviation where profits are prime, many airlines have stronger reliance on recycling the in-cabin air as it saves fuel--some were slammed for this practise that usually induces nausea and discomfort with passengers; most commercial planes have systems that bring in 50% fresh air, and combines it with the other 50% recycled air, but this balance is adjustable for economical reasons; someone with aerospace knowledge can shed more light on this.
    This is why Airlines were so keen on banning smoking on flights- they can decrease the amount of fresh air that must be pressurised (using energy) into the cabin.

    No doubt their insurance went down a bit too with a reduced risk of fire.

    I think that one of the features of the new generation of passenger jets is that they maintain a lower cabin altitude than current ones.

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    Quote Originally Posted by sdabel
    This is why Airlines were so keen on banning smoking on flights- they can decrease the amount of fresh air that must be pressurised (using energy) into the cabin.

    No doubt their insurance went down a bit too with a reduced risk of fire.

    I think that one of the features of the new generation of passenger jets is that they maintain a lower cabin altitude than current ones.

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    Quote Originally Posted by vanderaj
    11 US mpg = 21.3 l/100 km or 13.3 imperial mpg. Ouchy.
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    "At its most efficient rate (on a freeway) the Hummer slurps 21 litres of fuel for every 100 kilometres travelled - more than twice the average family sedan. Around town, that figure climbs above 30 litres per 100km - or more than triple the average car."

    Then again if you can't afford to run it, you can't afford to buy it.

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  20. #20
    1000+ Posts jo proffi's Avatar
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    Quote Originally Posted by vanderaj
    My 1999 Echo had a thing in the manual on driving fuel efficiently. They said to use aircon on hot days rather than wind down the windows. I doubt Toyota would put this advice in unless they knew that it was accurate.

    11 US mpg = 21.3 l/100 km or 13.3 imperial mpg. Ouchy.

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  21. #21
    1000+ Posts robmac's Avatar
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    Default Aircon

    Quote Originally Posted by jo proffi
    Did anyone see Myth Busters on fox last night, where they busted the myth that its more fuel efficient to have the air-con on than the windows open.At 45MPH with identical SUV's, the window open car absolutely anialated the aircon car on range. Myth fully busted.
    Jo

    ....But it is a lot more comfortable to have they air on.....

  22. #22
    1000+ Posts jo proffi's Avatar
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    Quote Originally Posted by robmac
    ....But it is a lot more comfortable to have they air on.....
    Not when you chain smoke.
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  23. #23
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    Quote Originally Posted by XTC
    Hum dinger

    The Hummer has conquered war zones but can the world's most politically incorrect vehicle master Sydney? JOSHUA DOWLING tests the beast of Baghdad on the battlefields of Balmain.
    http://www.drive.com.au/editorial/article.aspx?id=10718

    "At its most efficient rate (on a freeway) the Hummer slurps 21 litres of fuel for every 100 kilometres travelled - more than twice the average family sedan. Around town, that figure climbs above 30 litres per 100km - or more than triple the average car."

    Then again if you can't afford to run it, you can't afford to buy it.

    - xTc -
    I read that article in the paper last friday. They had a photo of the Hummer H2 driving next to an original Mini. Just a slight difference in size I don't know how you could live with fuel consumption figures like that
    Speaking of high fuel consumption, according to the Motor article where they drove the Bugatti Veyron, "at full noise it can suck the 100L tank dry in 20 minutes" Motor said that equates to 100L/100km but by my calculations at 400km/h it would travel 133km in 20mins giving 75L/100km so I assume by "full noise" they mean hard acceleration for 20 mins. Either way it's an insane amount of fuel.
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    Budding Architect ???? pugrambo's Avatar
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    on a good trip i get 38mpg from the 306 without air and 36mpg with air

    hardly worth worrying about for the sake of comfort (is that possible in a GTi-6 ??)
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  25. #25
    sans witticism SLC206's Avatar
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    Quite frankly, screw that.

    I don't care if it's fuel efficient or not. Why drive down a freeway with the windows open?
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