How much does the air conditioning in your car?

You have arrived the heat, and in Spain we notice but based on well. Take a trip in mid – afternoon, or back to the car after having left it parked in the sun, you can assume little more than a faint heat that can accumulate inside the car, like a greenhouse.

Today there are very few cars not equipped with air conditioning, or any variant technologically similarly, call it what you call: manual climate control, automatic climate control, electronic climate control, dual zone climate control, Trizone, cuatrizone … Essentially we always refer a cold machine. Let’s see briefly how it works, and above all, how much you spend.

Take hot air, cool it and put it in the passenger compartment

At this point no one should be surprised if we say that the air conditioning consumes energy, or in other words, in a petrol or diesel car consumes (or gas, or whatever you use your car to get around). They are whims of physics and thermodynamics in particular, but to do a job (with frictions, resistances, losses, etc.) have no choice but to consume energy.

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Air conditioning or climate control car, no longer a cold machine, thanks to various mechanical elements and a gas changes state, gets cool air, while filter it and de-humidification. Or rather, gets withdraw the heat to the air, either the air inside the cabin itself, when the air conditioner is in recirculation mode (sucks the air inside and re-boost) or outside air, in normal mode.

An air conditioner works by mechanical compression of a substance which changes state from gas to liquid (and liquid to gas), or in other words, the refrigerant circuit. A cyclical process that is repeated and repeated and repeated while the air conditioner is turned on is performed. The key elements, and easily summarized, are:

• The compressor refrigerant gas sucking and compresses, i.e. the pressurized, so that its temperature rises.
• The condenser, the compressed gas passes this element, which is basically a radiator, or essentially a coil, condenses, hence its name, and becomes liquid. This radiator is in contact with the outside air. The liquid is warmer than the air and gives off heat.
• Expansion valve, the compressed fluid passes in an instant to have much more volume (decompressed) changes to gas state and is further cooled.
• Evaporator, the cold gas passes through another radiator in contact with the air going into the cabin, and picks up the heat. The air cools and is driven by the fan through the ventilation grates of the dashboard. The gas cycle begins again to the compressor.

How much consumes?

Well, compressing a gas trouble and that work rests with the compressor. The compressor is coupled to the motor rotation by belt, and when to do work is done by the force of the car engine. But of course this work ends up falling on the engine itself, and if you have to do more work, more effort, as if you were moving alone without the ballast of the compressor, then consume more.

Therefore, the AC motor consumption increases. How much? It depends. And what does it depend? For the work we ask in every moment. It is generally considered that consumption can rise between 5% and 20% (in the worst case). This can be in practice between a couple of tenths and one liter per hundred, at most (usually are only a few tenths).

In fact to be more correct, for air conditioning we should speak rather of consumption at the time, by the time the compressor is running.

The truth is that the hotter the air inside the car, the warmer the air outside the car, and the lower the set point temperature, i.e. the temperature that we program ourselves at the controls of the air conditioning or climate control (i.e. very cold), more work needs to make the air conditioning and the higher the energy consumption.

Normal with a modern air conditioning is turned off the compressor and automatically disengaging as you no longer need to continue cooling the air, and turned back on when needed. The less time the compressor is running, less consumption. If the initial temperature inside the car is 40 degrees Celsius, and we want to lower the temperature to 25 degrees, obviously need less time than if we go down to 18 degrees.

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As the air conditioning compressor, when operating, overloading a little car engine, it is normal also being noticed a slight decrease in performance because the engine does not turn only to spin the wheels, but also to rotate the compressor having to do the work of compressing the gas.

So while the engine continues to have the same power to the wheels can get you a little less. It is usually considered that the air conditioning takes between 2 and 15 CV approximately, depending on each system, each car and the effort that is making the compressor. Usually this is most noticeable in small and underpowered engines, but from 110 or 120 hp, is little note.

In modern cars when the accelerator demanding maximum acceleration pedal is pressed, usually temporarily disconnect the compressor automatically, so as not to embarrass the driver. In cars stop-start, unless the compressor is electric, it turns off when the engine stops in an arrest.

Air conditioning or window

Since air conditioning consumes energy and increases fuel consumption, would not it be interesting not turn it on and roll down the window to run the air and the passenger compartment to cool naturally? Yes, and no, it depends.

Over the years there have been different laboratory tests on rollers and fan to simulate drag, and also in circuit, and has been shown to depend on the speed at which the car moves.

The air conditioning more or less will consume almost the same circulators the speed at which circulators, but the window no. A car has a specific aerodynamics. The more streamlined the car, experiences less resistance to forward through the air, and therefore less consumed.

By lowering the window it is that we get worse the aerodynamics of the car or in other words, “goes into the air and slows us”, similarly to as a braking parachute. This greater resistance means that the engine has to do more work, that is, more effort, to do what he was doing, move the wheels and the car moving forward, so you have to consume more.

What happens is that the effect of aerodynamics on consumption is exponential; it varies with the square of the speed. At low speed the window down slightly increases drag, and consumption also increased slightly, but high speed greatly increases the resistance, and then does increase consumption.

In general it has been found that from about 80 or 90 km / h the effect of the window is much worse. So city you can go with the windows down, consuming a little less than if we did with the air conditioning, but highway, 120 km / h, it is better to shut carry, because it consumes less using the air conditioning.

As in many other things, go with the window down or with the air conditioning set is also a matter of personal taste.

A good tip when we returned to the car and we found it as an oven, is to lower the windows slightly during the first minutes (a crack), while turn on the air conditioning for the hot air to go down stream natural convection (hot air rises, cold air falls), and help to make it more effective cold air that drives the air conditioning. After a few minutes the close, and ready.