What is the difference between the internal and external circulation of a car's air conditioning system?

[Daniel]

Car air conditioning internal and external circulation mainly differ in the following aspects: 1. Different states. Internal circulation mode closes the airflow channel between the inside and outside of the vehicle. Without the fan, there is no airflow circulation, and when the fan is on, the inhaled airflow only comes from inside the vehicle, forming an internal airflow circulation. External circulation mode uses the fan to draw air from outside the vehicle into the cabin, allowing air to flow between the outside and inside. The air blown by the fan comes from outside, and even without the fan, airflow is still drawn into the cabin while driving. 2. Different functions. Internal circulation primarily prevents external dust and harmful gases from entering the cabin promptly and effectively, such as when driving through smoke, dust, areas with strong odors, or in heavy traffic, blocking harmful exhaust emissions from vehicles ahead. External circulation allows for breathing fresh air from outside the vehicle.