Does air pass through the A/C filter when the car A/C is on internal circulation?

A netizen sent over a question asking if the air conditioning air passes through the air conditioning filter when the car's air conditioning system is in internal circulation mode. My answer to him was: the vast majority of automobile air conditioning systems, whether in internal or external circulation mode, the air goes through the air conditioning filter. There are only a very few models where the air passes through the A/C filter on the outer cycle, but doesn't go through the A/C filter on the inner cycle.

Let's take a look at the direction of air circulation flow in a car's air conditioning system. Below is a sketch of an automotive air conditioning system air distribution unit. Let's first look at the outer circulation mode. In the external circulation mode, valve 1 is flipped upward, blocking the indoor air inlet and allowing outside air to enter. After the outside air enters, it is first filtered through the air conditioning filter, then flows through the air conditioning evaporator or the warm air water tank, and out of the air outlet in the center console to regulate the indoor air temperature.

If adjusted to the internal circulation mode, valve 1 is flipped downward, blocking the outdoor air inlet, prohibiting the entry of outside air, and air can only be sucked from inside the car. As you can see from the diagram, the air inside the car is also first filtered through the air conditioning filter, then flows through the air conditioning evaporator or the warm air tank, and out of the air outlet on the center console to regulate the indoor air temperature.

So, whether the air conditioning is internal or external circulation, it has to go through the air conditioning filter. Nowadays, the default when the car air conditioning is turned on is the external cycle, and the internal cycle needs to be turned on manually. Some automatic air conditioners will automatically switch to internal circulation in case of rapid cooling, reverse gear, etc., and then switch to external circulation when the set temperature is reached, which ensures the freshness of the air inside the car.

The exceptions are also available, such as the old Santana, Jetta, etc., their air conditioning filter is installed in the front windshield on the lower right side, from where the outside air into the cab; if you switch to the internal cycle, the internal duct baffle directly to the entrance blocked, the air is only circulated inside the cab, this time, it does not go through the air conditioning filter. Similarly, the air conditioning system of the truck, their air inlet in the front of the front of the vehicle, there is only an air conditioning filter, when switching to the internal cycle, this mouth is blocked by the baffle, the air will not pass through the air conditioning filter.