What does 4WD mean on a car?

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4WD stands for "four-wheel drive," which means all four wheels have driving force. Four-wheel drive is mainly used in hardcore off-road vehicles and is now also applied to urban SUVs. Many mid-to-high-end sedans and luxury sports cars also adopt four-wheel drive. 4WD includes three forms: full-time four-wheel drive, part-time four-wheel drive, and on-demand four-wheel drive. Part-time four-wheel drive: This is the most common type of four-wheel-drive system. Part-time four-wheel drive does not have a central differential. During turns, the speed difference between the front and rear drive shafts can only be offset by one tire slipping. Vehicles equipped with part-time four-wheel drive cannot be driven on roads with good traction. When the road surface has good traction, it is difficult for the car to slip while driving. Therefore, part-time four-wheel drive cannot be used on roads with good traction. Representative model: Jeep Wrangler. Technical features: High reliability, manual operation required for mode switching, relatively slow response speed, and cannot be used on paved roads. Full-time four-wheel drive: This means all four wheels are always driving wheels, whether driving straight or turning. Full-time four-wheel drive adds a central differential to the part-time four-wheel-drive system, allowing the speed difference between the front and rear drive shafts to be smoothly resolved. Additionally, it generally allows adjustment of the driving force distribution ratio between the front and rear wheels based on driving conditions. Representative model: Mercedes-Benz G-Class. Technical features: Fast response speed and strong off-road capability. On-demand four-wheel drive: This refers to a drive system that automatically switches to two-wheel drive when four-wheel drive is not needed and to four-wheel drive when it is required. The main difference between on-demand four-wheel drive and part-time four-wheel drive is that the switching is done automatically by the system without manual control. Since on-demand four-wheel drive typically operates in two-wheel drive mode, the non-driving wheels only engage when the driving wheels slip, requiring a reaction time. The length of this reaction time reflects the actual off-road capability of the four-wheel-drive system. Representative model: BMW X5. Technical features: More intelligent mode switching compared to part-time four-wheel drive.