The air conditioning of the car is today an almost essential element in the car. Its function is to cool and maintain an optimal temperature in the passenger compartment. How does this system work and how much does it spend? Will we save more fuel if we open the windows?
Air conditioning systems have evolved and improved over the years. Now this technology is more advanced and precise and even allows you to adjust the temperature by zones (two-zone, three-zone or up to four or five zones). However, its basic principle remains the same: take in hot air, cool it and introduce it into the passenger compartment.
This process requires energy consumption. These systems carry out work (with friction, resistance, losses) and consume energy to do it. In the car, this consumption translates into fuel consumption (gasoline or diesel) if it operates with a heat engine, or electricity or gas in other mechanics.
This is how a car air conditioning works
The air conditioning of a car is a cold machine that, thanks to various mechanical elements and a gas that changes state, manages to cool the air and, at the same time, filter and dehumidify it. Or, in other words, it manages to extract the heat from the air that enters from the outside, or the air from the passenger compartment when the recirculation mode is activated.
An air conditioner works by the mechanical compression of a substance that changes state, from gas to liquid, and from liquid to gas. This substance is the refrigerant gas in the circuit, which performs a cycle that repeats continuously when the air conditioning is activated. The system consists of the following elements:
Compressor : sucks in the refrigerant gas and compresses it, putting it under pressure, thus raising its temperature.
Condenser : compressed gas passes into this device, which is basically a coil radiator, condenses and turns into a liquid state. In addition, this radiator is in contact with the outside air and the resulting liquid is hotter than the air itself, thus giving it heat.
Expansion valve : the compressed liquid becomes much more volume in an instant (it is decompressed), it changes to a gaseous state and cools more.
Evaporator : the cold gas passes through another radiator, in contact with the air, goes into the passenger compartment and collects its heat. The air is cooled and blown into the passenger compartment with the fan through the air outlet nozzles. And the gas starts the cycle back to the compressor.
Car air conditioning and energy consumption
The greatest consumption of air conditioning falls on the use of the compressor. The compressor is coupled to the rotation of the engine, by means of a belt, and when it works it does so by means of the force produced by the car's propeller. Therefore, the task falls on the mechanics itself, which, by having to help the compressor, consumes more energy.
So how much does consumption increase when the air conditioning is operating? It depends on the work that we ask of you at all times. The warmer the air in the passenger compartment, the warmer the air outside and the lower the temperature that we seek to have, the greater the workload of the system and, therefore, the energy consumption will also be higher.
It is considered that consumption can rise between 5% and 20%, in the worst case. This can be in practice between a couple of tenths and a liter per 100 kilometers at most. Although, to be more exact, in the case of air conditioning, it would be necessary to talk about consumption per hour, depending on the time the compressor is operating.
The usual thing in a modern air conditioning system is that the compressor turns off and disengages automatically as soon as it detects that it is not necessary to continue cooling the air, to turn on again when it is. The less time the compressor is running, the lower the consumption. If the initial temperature inside the car is 40 ºC and we want to lower the temperature to 25 ºC, we will need less time than if we want to lower it to 18 ºC.
Sometimes, the use of air conditioning can cause a slight decrease in the power of the vehicle that the driver perceives. It is estimated that the air conditioning subtracts approximately between 2 and 15 hp from the engine. This is more appreciated in cars with small propellant and low delivery, while in 110 or 120 hp engines it is not very noticeable.
In modern cars, when the accelerator is fully depressed, the compressor automatically shuts off so as not to put the driver in trouble. And in the same way, cars that are equipped with stop / start systems, and as long as the compressor is not electric, it turns off when the propeller does the same in a stop.
What spends more: putting on the air conditioning or keeping the window open?
Another common question is whether the air conditioning spends more or less than opening the windows. And again, it depends. Over the years, different tests have been carried out in the laboratory, on rollers and with a fan to simulate aerodynamic resistance, as well as on the track. And the bottom line is that it depends on the speed at which the car travels.
Although the air conditioning will consume practically the same regardless of the speed at which we drive, the same does not happen when we drive with the windows down. In short, the more aerodynamic a car is, the lower its resistance to the wind and, therefore, it will consume less.
Opening the windows worsens the aerodynamics of the car, since air enters the cabin and slows it down, similar to what happens when we use a parachute. This means that the engine has to exert more effort to move the wheels in order to keep the car moving, so it is forced to consume more fuel.
It should also be taken into account that the effect of aerodynamics on fuel consumption varies as a function of speed. If this is low and we have the windows lowered, the resistance increases rather little and, therefore, the consumption hardly increases. But if we circulate at high speed, the aerodynamic resistance will increase notably and, consequently, so will the consumption.
From between 80 and 90 km / h, having the windows down consumes more than if we go with the air conditioning activated. This means that by city we could save the air conditioning and open the windows, but not on highways: driving at 120 km / h we will always consume less with the air conditioning on and with the windows closed.
What we can do, however, is to help the air conditioning to reduce its impact on fuel consumption as much as possible. For example, trying to park the vehicle in a garage or in the shade. In this way the passenger compartment will not be excessively hot and the air conditioning will have to work little, just enough to maintain the temperature.
If this is not possible and the car is in full sun, we can ventilate the vehicle and then drive with the windows down for a few minutes, while we turn on the air conditioning. In this way, we will help the hot air to go by natural convection current and we will allow the air conditioner to cool the interior of the car more effectively.