What does the internal and external air circulation in a car mean?

[DiFrancisco]

Car air conditioning internal and external circulation means: 1. Internal circulation allows air to circulate only inside the car, driven by the blower to cycle the air within the vehicle. 2. External circulation connects the inside of the car with the outside. While driving, airflow automatically enters the car using wind pressure, and when stationary, outside air can still enter the blower through the air ducts and be blown into the car. There are two situations when deciding whether to use internal or external circulation in car air conditioning: 1. When the outside air quality is poor, to prevent harmful gases from entering the cabin, it is necessary to set it to internal circulation. 2. During long highway drives, to prevent a decrease in oxygen levels inside the car, it is advisable to alternate between internal and external circulation.

[SanAlexandra]

Having driven for over a decade, I use the air recirculation and fresh air modes quite frequently. Simply put, the recirculation mode closes all windows and only circulates the air inside the car, just like turning on the AC with doors and windows shut. I use this mode in traffic jams or when passing by areas with unpleasant odors, as it prevents exhaust fumes or roadside barbecue smells from entering the car. The fresh air mode does the opposite, specifically drawing outside air into the car. I use this mode in suburban areas with good air quality, as it keeps the cabin air flowing and feels particularly refreshing. However, using recirculation for more than 20 minutes can make the cabin feel stuffy—just press the button with an arrow on the dashboard to switch modes. Remember, in winter, if the windows fog up, switching to fresh air mode and blowing for a while will clear the fog quickly.

[McLiam]

My child has allergies, so I've done extensive research on air circulation modes. The recirculation mode can block 95% of external pollutants, which is especially useful when passing through construction sites with dust or areas with high pollen counts. However, prolonged use can cause windows to fog up easily due to increased carbon dioxide concentration. The fresh air mode is perfect for highway commutes when you want to avoid noise while still getting ventilation - it's like having a small window open. Last month during a long trip, I forgot to switch to fresh air mode, and when we stopped at a service area, the whole car smelled of sweat. Now I've developed a habit: using fresh air mode on highways and pressing the recirculation button when entering the city. Although I change the cabin air filter every six months, using these modes correctly can extend the filter's lifespan.

[VanRiver]

New drivers might think the air conditioning switch is sufficient, and I used to think the same. The recirculation mode means the AC only processes the air inside the car. On a hot summer day, getting into a sun-baked car feels like stepping into a steamer—turning on the AC with recirculation cools it down super fast, usually within five minutes. The fresh air mode is mainly for ventilation; on rainy days, you can dehumidify without opening the windows to prevent fogging. Last week, my colleague went for a barbecue, and the car got filled with smoke smell—I suggested turning on fresh air mode for ten minutes, and it cleared right up. Both functions have physical buttons: older cars have them as knobs on the dashboard, while newer models usually integrate them into the touchscreen. A crucial reminder: always use recirculation mode in highway tunnels—last year, a truck’s exhaust fumes leaked into a car and nearly caused an accident.

[Mabel]

These two car AC functions affect fuel consumption. Using recirculation mode when cooling saves fuel, as it doesn't need to cool additional hot outside air. Tests show using fresh air mode in summer consumes 0.3L/100km more than recirculation. But in winter, defrosting requires fresh air mode - using engine waste heat to clear fog is more efficient than blowing hot air. I discovered this trick when my dog got carsick: pets get less nauseous with fresh air mode. A mechanic told me AC filters turn completely black when only using recirculation long-term, costing 50 yuan more than fresh air mode filters. Now I use a combination: first 10 minutes recirculation to cool down, then switch to fresh air for ventilation.

[GarrettLynn]

Recently, while helping my father-in-law choose a car, I noticed some design differences. In electric vehicles, aggressively using the air conditioning with alternating between internal and external circulation can reduce the range by up to 50 kilometers, whereas this isn't as noticeable in gasoline cars. High-end cars come with an automatic mode that intelligently switches between the two, but basic versions require manual adjustment. I've observed that heavy smokers tend to prefer internal circulation to prevent smoke from escaping, while pregnant owners mostly use external circulation for better ventilation. For modified cars, take note: when installing ambient lighting, ensure the wiring doesn't block the air vents. A friend of mine experienced a 50% reduction in external airflow after modifying his car. Rainy day defogging test results: external circulation directed at the windshield works fastest, while using internal circulation at maximum fan speed can actually cause water buildup.