When turning on the air conditioner, which operation is more fuel efficient, increasing the temperature or decreasing the wind speed?

Seeing that it is getting hotter every day, we can talk in summer. It's the season of high appearance rate of automobile air conditioning. Every year, I receive some questions about how to use car air conditioners to save fuel. Among them, temperature setting and wind speed setting have the greatest influence on fuel consumption. This question is actually very interesting, and it also has a certain guiding role for our daily car use. Let's start with the conclusion: the influence of wind speed on fuel consumption is greater than temperature. Let's talk about it in detail.

Principle of automobile air conditioning

The picture above is the schematic diagram of the automobile air conditioning system, and we only need to know a rough idea. A compressor compresses a gaseous refrigerant into a high-temperature and high-pressure gaseous state, and then reaches a condenser, where a cooling fan cools the refrigerant and the refrigerant becomes a liquid state. After drying, the liquid refrigerant comes to the evaporator in the cockpit, where it evaporates and absorbs a lot of heat, resulting in a low temperature in the evaporation box. Then the fan drives the airflow through the evaporation box, and the airflow is cooled and sent to the cockpit.

Among them, the compressor is directly driven by the engine, which will lead to an increase in oil consumption. Secondly, the condenser cooling fan, air supply fan and compressor electromagnetic clutch also consume electricity when running. Although it does not directly consume engine oil, it will increase the load of the generator, so it will also increase fuel consumption.

Influence of temperature regulation function on fuel consumption

The temperature adjustment function of automobile air conditioner only controls the air outlet temperature, not the interior temperature. In other words, it will try to keep the outlet temperature at the set value, and then continue to run. Even if the temperature inside the car is lower than the set temperature, it will not stop.

There are two ways to adjust the temperature of automobile air outlet: mixing temperature adjustment and compressor temperature adjustment, and the specific implementation methods are as follows:

Hot air flow temperature regulation

This way is realized by hot air tank. The evaporator of this air conditioning system is next to the heater box, and the air passes through the evaporator first and then the heater box. The cooled air is reheated by the warm air tank to control the temperature of the air outlet. More common in some manual air conditioning systems. For example, in the manual air conditioning panel shown above, the knob on the left is the temperature adjustment knob. This knob controls the flow valve of the heating box.

The above picture shows the flow valve of the warm air tank, which is controlled by the temperature adjustment knob. When we turn the temperature adjustment knob to the end in the blue direction, the water valve is completely closed, there is no coolant circulating in the heater box, and the air outlet temperature reaches the lowest. If the screw is turned in the opposite direction, the flow valve will gradually open, the coolant flow will gradually increase, and the temperature of the warm air box will gradually increase, which will have a better effect on heating the cold air and the outlet temperature will also increase.

Air door temperature regulation

In this system, there is no flow valve in the warm air tank and there is always coolant circulating in the warm air tank. It's just that the heating box is blocked by the damper. We actually adjust the temperature at the opening angle of the damper. The higher the temperature is adjusted, the greater the opening angle of the air door, so that more cold air cooled by the evaporator is heated by the warm air tank, and the air outlet will not be too cold. The lower the temperature adjustment, the smaller the throttle opening, the less cold air passing through the warm air box, and the lower the outlet temperature. If the temperature is adjusted to the lowest, the damper will completely block the warm air trough, and the cold air will be blown out directly without passing through the warm air trough, so that the air outlet temperature will reach the lowest.

These two temperature control systems have no effect on fuel consumption whether you adjust the temperature high or low, because they do not adjust the temperature by adjusting the operation of the compressor. Even if you turn the temperature to the highest, the compressor is still working hard, but the system heats the airflow for you. To some extent, the higher the temperature is adjusted, the more fuel is consumed. Because the low temperature generated by the compressor is not fully utilized.

Compressor control temperature

Some advanced cars use variable displacement compressors, which are very similar to household variable frequency air conditioners. If the temperature is raised a little, the compressor will run at a lower power, and the temperature of the evaporation box will not be so low. When the temperature is adjusted to the lowest, the compressor runs at full load and the refrigeration effect is the best.

Influence of wind speed on fuel consumption

I think wind speed has a greater impact on fuel consumption than temperature setting. We can analyze from these two points:

1, wind speed can indirectly affect the compressor work.

When the air conditioner is turned on, the fan drives the airflow to pass through the evaporator, and the airflow exchanges heat with the evaporator. The heat of the airflow is absorbed by the evaporator, and the temperature of the airflow decreases and the temperature of the evaporator increases. The higher the wind speed, the more air flow through the evaporator and the more intense the heat exchange. Under the premise of constant compressor power, the lower the air volume, the lower the evaporator temperature.

The temperature of the evaporator can determine the running state of the compressor. Because when the evaporator is very cold, there will be a lot of condensed water on the surface, and when the temperature of the evaporator is too low, these condensed water will easily frost. Once the condenser is frosted, the airflow cannot pass, which will lead to no wind at the air outlet of the air conditioner. Therefore, there is a temperature sensor on the evaporator of the air conditioner to detect the evaporator temperature. When it is close to the frosting temperature, the compressor will be turned off.

The lower the air volume, the easier it is for the evaporator to keep the low temperature and get close to the frosting temperature, which leads to the shutdown of the compressor and naturally reduces the oil consumption of the compressor.

2. The influence of fan power consumption on fuel consumption

The higher the air volume adjustment, the higher the fan speed and the greater the power consumption. And the condenser cooling fan will also work when the air conditioner is running. The power of the air-conditioning fan of a general family car is between 100-200 watts, and the power of the cooling fan is basically the same. Although the sum of the two is only a few hundred watts, the impact on fuel consumption is definitely there, but the impact is different.

Therefore, it is not difficult to draw the conclusion that:

When using a constant displacement compressor, it is most fuel-efficient to adjust the vehicle temperature and wind speed to the lowest.

The higher the temperature adjustment of vehicles using variable displacement compressors, the more fuel-efficient they are.

However, this is only a theoretical analysis. It is hard to say how much oil can be saved in actual use. But it can definitely be redeemed.

I don't recommend sacrificing comfort to save fuel. If the cooling effect is not good after you adjust the air volume to the minimum, you might as well turn off the air conditioner and open the window. If you really want to save fuel, you should work hard from other aspects, such as cleaning the condenser and evaporator, improving the heat exchange efficiency, planning the route in advance before going out, and trying to avoid congestion. Concentrate on driving, try to avoid sudden acceleration and sudden braking, and learn to control the speed with the throttle. These details are in place to ensure that fuel consumption will be lower and lower.

This article comes from car home, the author of the car manufacturer, and does not represent car home's position.