Thread: Aircon/ventilation D2

Originally Posted by ladas

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In terms of ecconomy of using AC - you increase drag on the vehicle and thus increase fuel consumtion - more - when you open a window - than you do when you switch the AC on.

Food for thought.

It's not quite that simple.
I'm not sure how it works in vehicles with the aerodynamics of a brick, but in vehicles shaped like a well worn bar of soap, it goes something like this.

Below about 80kph it is more economical to open the windows.
Above about 80kph it is more economical to close the windows and turn on the A/C.

The extra drag created by having the windows open doesn't exceed the extra power needed to run the compressor until you go over about 80kph.

Sorry but I have to disagree, whilst speed is relevant the slower you go the more drag caused by an open window

- the power required to run the compressor aproximates to 7% +/-

The compressor pulls the same relational power whether at 40 km or 90 km

The head pressure of a compressor is directionally relative to the speed of the compressor to a point.

so, in theory the faster you go the more power it needs.

............but the compressor will pull more power - relatively - at a very low speed - due to the fuild dynamics and the friction elements which are constant.

So if at 1000 rpm the head pressure is 200 psi, which will equate to approx 5 hp - the fluid dynamics - or static restistance of the refrigerant being used will add another say 0.5 hp and the friction element will add another say 0.50 hp of drag - so at 1000 rpm you have 6hp of drag.

at 2000 rpm - the head pressure will rise to 225 - so the HP draw will be 5.55 but the fixed elements (fluid dynamics and friction) remain static. so a total of 6.55 - so it argues against the direct pressure relationship. but its still relative.

The effect of a window open can reduce power greater than 12% - regardless of speed , but it is temp/humidity/density/wind direction relative

You do not reduce pressure inside the vehicle by going slower - in fact you increase it.

The faster you go the pressure outside the vehicle is reduce - and thus the inside of the vehicle is at a + pressure, and will in effect reduce drag.

Just look at basic physics X amount of air having to move over a large area, and increase speed = pressure drop

The lower the speed of the vehicle the closer the internal and external pressures are, and thus the more drag is induced.

If you smoke just do a very simple experiment - drive around at 40 km/h with the window open with a cigarette alight - or something else that gives off smoke - at 40 km/h most of the smoke remains in the vehicle, because the pressures are near equal.

When you increase speed - watch the smoke - it will start to move towards the window and out of it, because the pressure outside is being reduced by speed.

Air exiting will not increase drag, it is only the free movement of air from outside to in does this.

sorry I have rattled on about this - but I have spent weeks/months working on this type of experiments.

Originally Posted by vnx205

It's not quite that simple.
I'm not sure how it works in vehicles with the aerodynamics of a brick, but in vehicles shaped like a well worn bar of soap, it goes something like this.

Below about 80kph it is more economical to open the windows.
Above about 80kph it is more economical to close the windows and turn on the A/C.

The extra drag created by having the windows open doesn't exceed the extra power needed to run the compressor until you go over about 80kph.

Hi Allan
Myth Busters Busted that one !!!!!
The airconed vehicle used more fuel,not substantially though.They were very thorough in their testing.
Mike thats how I do mine.Mines BLACK so it gets hot when parked and get hot air,thats why I just hit the econ button(which is compressor ON/OFF) to cool the aircon system then when I get cold air econ again to switch the compressor off.
In winter when its colder outside the vehicle climate control will keep the vent temp at your set temp with the econ set on(no compressor),ie will regulate the hot air to your temp set.
Andrew