What does the internal and external circulation of a car mean?

[LeNathan]

Car air conditioning internal circulation: Internal circulation is like the air in a sealed container, which is extracted, processed, and then sent back into the container. Since the air intake for internal circulation is inside the car, the air conditioning system only circulates the air inside the vehicle, preventing outside air from entering and inside air from escaping. Correct methods for using internal and external circulation: 1. During traffic jams: Use internal circulation for the air conditioning, as external circulation would draw exhaust fumes from outside into the car. 2. General air conditioning cooling: Use internal circulation to quickly lower the temperature inside the car. Once the temperature stabilizes, switch to external circulation appropriately to exchange air with the outside. 3. Air conditioning cooling after outdoor exposure: First use external circulation and open the windows to speed up heat dissipation. After a few minutes, close the windows, switch to internal circulation, and turn on the cooling. 4. Air conditioning heating: Do not rush to turn on the air conditioning when starting a cold engine. Wait for the engine temperature to rise before turning on the heater with internal circulation. Once the temperature stabilizes, switch to external circulation appropriately to exchange air with the outside. 5. Rainy or foggy days: These conditions can easily cause window fogging. To prevent this, simply press the front window defogging button on the air conditioning, lower the temperature slightly, and increase the fan speed.