What do the external circulation and internal circulation of the air conditioning mean?

[DonovanRose]

Internal circulation means that the air only circulates inside the car, driven by the blower to cycle the air within the vehicle; while external circulation connects the inside of the car with the outside. When the vehicle is moving, airflow uses wind pressure to automatically draw in outside air. When stationary, outside air can also enter the blower through the air ducts and then be blown into the car. The usage scenarios for internal and external circulation are as follows: Usage scenarios for external circulation: When the external environment is very good, using the external circulation mode can fill the car with fresh air, improving the air quality inside the car. However, when driving in congested areas, try to avoid using the external circulation mode. Usage scenarios for internal circulation: During daily driving, the system is mostly in internal circulation mode, meaning the air exchange between the inside and outside of the car is minimized. Most of the air comes from inside the car and is continuously reused. In summer, this can quickly lower the temperature inside the car, while in winter, it helps maintain warmth and a constant temperature.

[StLouis]

The recirculation and fresh air buttons are quite common on car air conditioning systems, something I've learned from years of driving. The fresh air mode brings outside air into the cabin, like opening a small window for ventilation, while recirculation only circulates the existing air inside the car without exchanging with the outside. In summer, I immediately turn on recirculation when getting in the car because it allows the AC to cool down faster - especially when it's unbearably hot in the city, it's much more efficient than fresh air mode. During winter or foggy conditions, I switch to fresh air to prevent windshield fogging and maintain ventilation to avoid stuffiness. In heavy traffic with polluted air, I also use recirculation to avoid exhaust fumes. Cars with automatic climate control can intelligently switch modes with the AUTO button, but manual control offers more flexibility. Mastering these functions makes driving safer and more comfortable.

[LeMadeline]

I didn't fully understand car air conditioning until I bought my own vehicle: The external circulation brings outside air into the cabin, essentially opening an air passage to let you breathe fresh air; while internal circulation blocks external intake and only recirculates the air inside. I tend to use external circulation more often during long highway drives—it ventilates the car and makes the interior feel less stuffy. But on smoggy days or when passing through industrial areas, I immediately switch to internal circulation to block out pollutants. External circulation is particularly useful during rainy seasons as it quickly clears fogged-up windows. Automatic systems adjust modes based on temperature and humidity, but when manually operating, I prefer to switch modes every half hour or so to maintain good airflow and save a bit of fuel.

[Sarah]

As a car enthusiast, I find it very practical to understand the difference between air recirculation and fresh air modes: Recirculation mode circulates only the interior air inside the car, which is ideal for rapid cooling or heating; Fresh air mode introduces outside air, helping with dehumidification and ventilation. In dusty areas, I avoid using fresh air mode for too long to prevent filter clogging; while in clean environments, fresh air mode enhances driving comfort. Automatic AC systems determine the mode automatically, but when manually controlling, I typically use recirculation mode for the first five minutes after startup to cool faster, then switch to fresh air mode to maintain airflow. Simply put, it's wise to toggle between modes based on road conditions.