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Car air-conditioning systems do not run for free”. In fact, in hot parts of the world, they can account for up to 30% of fuel consumption.
Even in Switzerland, with its temperate climate, the use of air-conditioning systems is responsible for about 5% of total fuel usage, rising to about 10% in urban traffic, as shown by a new Swiss study.
Furthermore, two-thirds of the additional fuel usage could be saved if air-conditioning systems were simply turned off when the air temperature falls below 18C.
Car air-conditioning systems require energy to compress the cooling agent, and the greater the degree of cooling required, the more fuel they use.
Little known, however, is the fact that these systems also use fuel when the outside air temperature is cooler than in the vehicle.
For this reason, the Swiss have been investigating in detail the fuel consumption of six modern cars – both diesel and petrol models – with their air-conditioning systems switched on and off under varying ambient temperatures and humidity.
The study, the results of which have just been published in the scientific journal, Environmental Science and Technology, shows that the fuel consumption of the test vehicles with air-conditioning systems in operation increases with rising ambient air temperature and humidity, reaching a value of some 18% on a typical Swiss summer day with an air temperature of 27C.
Temperature-wise, that’s not much different to a good summer’s day in the southern half of the UK.
In addition, the researchers noted that the air-conditioning systems in cars with automatic transmissions only turn themselves off when the external temperature drops below 5C, when the cooling system could ice-up.
This occurs because air-conditioning systems not only cool the air before blowing it into the vehicle interior, but also dry it so as to avoid causing condensation on the front windscreen when it rains, among other reasons.
This is, of course, perfectly sensible and important for safe driving, but only when the air humidity is high, and not all the time, as is currently the case.
It is known from physiological studies that the average driver feels comfortably warm when the air temperature around the head is 23C.
This means that if the outside temperature is below 18C, the car’s air-conditioning system could be turned off without any loss of comfort.
For the petrol-engined vehicles investigated, this would represent a saving of some two-thirds of the additional fuel usage, which is, after all, 3.6% of the total consumption.
With the diesel-fuelled vehicles tested, the additional consumption due to the use of air-conditioning, particularly for urban driving, is somewhat lower (2.7%).