Driving in summer, I can cool my car by using air conditioning or opening the windows. One drains the battery, the other makes the car less aerodynamic. Which is more efficient?
Graham Jones
Bridgham, Norfolk, UK
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It would be much better to use the air conditioning rather than open the windows.
As far as I know, all car air conditioning systems are run directly from the engine, via a drive belt such as the one used for the alternator, water pump and power steering. A small amount of electricity will be used for the control mechanism, air blower and the like, but the electrical power comes from the alternator, so the battery isn’t drained.
Early systems (typically in US cars of 60 years ago) operated the coolant compressor at maximum cooling and there would be manual or automatic mechanisms to mix the very cold air with warmer air from the heating set-up. These systems would take a significant proportion of the available engine power and vehicle performance suffered noticeably as a result.
More modern versions are rather more efficient, but in the UK, some entry-level car air conditioning systems still operate the compressor at maximum cooling. Most modern cars, however, are more sophisticated and use a variable displacement compressor. A control system ensures that only just enough power is taken from the engine to maintain the desired inside temperature.
The cooling mechanism dries the incoming air. This helps with screen demisting, and I think many modern cars rely on using the air conditioning to provide demisting. So the air conditioning is actually a safety aid.
By contrast, if you open a window and drive faster than a walking pace, the air turbulence creates very significant drag. Open rear windows in particular make a lot of noise and generate buffeting that can be very distressing for the driver. So opening the windows is generally a very bad idea.
The issue becomes more relevant for electric vehicles. I have seen estimates of air conditioning reducing their range by about 10 per cent. But a more relevant figure is that using the heating reduces range by about 15 per cent, and this will be particularly relevant in cold weather when screen demisting or de-icing is required and the cold reduces battery capacity.
Martin van Raay
Culemborg, The Netherlands
A 2004 study that below 45 miles per hour (72 kilometres per hour) it is more efficient to open the windows, as over this speed the aerodynamics increases the fuel consumption of the car more than the power the air conditioning draws.
Mind you, you don’t have to open all the windows all the way to get the cooling effect of the wind blowing over your skin. So the cut-off speed might actually be higher than 45 mph. Also, on a sunny day, use a windshield sun shade, so your parked car heats up less.
I use an ice/frost shield with an outer layer of aluminium. It keeps ice from my windshield in winter and keeps the heat out in summer. If you live in the Netherlands, you might know the following joke: “This car does not have airco; it has ARKO!”, where ARKO stands for “alle ramen kunnen open” (“all windows can open”).
Guy Cox
Sydney, Australia
First of all, unless Penny, who posed the question, is driving an electric car, air conditioning doesn’t drain her battery – it is powered by a belt from the engine. So it will, of course, increase her fuel consumption. How much it does this depends on the type of car she is driving – less if it runs on diesel, since such cars are more fuel efficient.
On the motorway, it will pretty much always be better to shut the windows and use the air conditioning. And she probably couldn’t stand driving at motorway speeds with the windows open anyway.
Given that extreme heat is rare in Cumbria in the UK where she lives, she would probably do better by opening windows when meandering through quiet country lanes. In between – well, she would have to test for herself.
Dennis Doren
Lake Mills, Wisconsin, US
The dilemma about use of air conditioning or open windows was scientifically answered by the TV programme some years ago. It measured fuel consumption to gauge the impact of these options. The finding was that open windows were more efficient than air conditioning when the car was travelling at or below about 40 mph (64 km/h), but air conditioning was more efficient than open windows above that velocity.
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