What does automotive on-demand four-wheel drive mean?
3 Answers
On-demand four-wheel drive refers to a system that activates four-wheel drive when appropriate, while operating as a two-wheel drive system under other conditions. Advantages of on-demand four-wheel drive: It is suitable for models with a front transverse engine and front-wheel drive platform, enabling many SUVs or four-wheel drive sedans based on this platform to be equipped with a four-wheel drive system. Compared to rear-wheel drive platforms, front-wheel drive platforms are more conducive to expanding interior space, offer higher transmission efficiency, and produce less noise from the transmission system. Disadvantages of on-demand four-wheel drive: Most on-demand four-wheel drive systems are limited by structural flaws when transferring power between the front and rear axles, making it impossible to deliver more than 50% of the power to the rear axle. Vehicle models using on-demand four-wheel drive: Globally, there are roughly two main branches of vehicles employing on-demand four-wheel drive technology: one is represented by European models using the four-wheel drive system provided by Sweden's HALDEX, such as Volkswagen's Tiguan, Passat R36, Golf R20, etc.; the other branch is represented by Japanese models using the four-wheel drive system provided by Japan's JECKT, like Toyota's RAV4 and Highlander, among others.
The part-time four-wheel drive system means the car usually runs on front wheels only, and automatically switches to four-wheel drive when it detects slippage. For someone like me driving an urban SUV, the most common scenario is slippery roads on rainy days. The system kicks in in the blink of an eye, keeping the wheels firmly planted, which is particularly reassuring. It's much more fuel-efficient than traditional full-time four-wheel drive, with fuel consumption about 15% lower. The electronic control unit constantly monitors wheel speed differences, and immediately redistributes power to the rear wheels when front wheel spin is detected. However, don't expect it to handle hardcore off-roading—for conditions like deserts or deep mud, you'll still need a professional selectable four-wheel drive system. Nowadays, mainstream SUVs like the Honda CR-V and Toyota RAV4 are equipped with this system, making it easy even for novice drivers to handle.
Having driven several vehicles with on-demand AWD systems, I find them most suitable for daily commutes. During traffic jams, you barely notice it working, yet it instantly stabilizes the body when taking elevated turns in rainy conditions. This system employs a multi-plate clutch to distribute power, featuring simple structure and lower costs – typically making these vehicles ¥20,000-30,000 cheaper than full-time 4WD models. I remember one icy winter when a colleague's sedan got stuck spinning its wheels, while my car automatically locked all four wheels and crawled out steadily. Though adequate for snowy roads, continuous steep slopes still require proper off-road tires due to torque distribution limitations.
