What is the Principle of BMW's Variable Valve Lift?
2 Answers
BMW's variable valve lift system primarily alters valve lift by incorporating components such as an eccentric shaft, servo motor, and intermediate pushrod into its valve train. When the electric motor operates, the worm gear mechanism drives the eccentric shaft to rotate, which then pushes the valve through the intermediate pushrod and rocker arm. The angle of the eccentric shaft's rotation varies, resulting in different valve lift magnitudes as the camshaft drives the valve via the intermediate pushrod and rocker arm, thereby achieving control over valve lift. Below are relevant details: Requirements of the variable valve lift system at low speeds: Engine demands for valve travel differ significantly across various speeds. At low speeds, if the valve travel is excessive, insufficient intake vacuum is generated. After fuel injection, the fuel cannot mix thoroughly with the incoming air, leading to low combustion efficiency, a significant reduction in low-speed torque, and increased emissions. Rolling friction: The contact surface between the roller-type valve tappet and the intermediate pushrod features an inclined platform where rolling friction occurs, minimizing mechanical losses. Additionally, the tappets and pushrods are categorized into different grades, with components of the same grade always installed on the same cylinder.
Having run a repair shop for ten years, I must say BMW's variable valve lift design is truly ingenious. It's called Valvetronic, and its core lies in using an electric motor to control the variation in intake valve opening depth. Traditional engines adjust throttle opening when you press the accelerator, whereas Valvetronic directly modulates valve lift to precisely manage air intake. Imagine this: the motor drives an eccentric shaft via a worm gear, altering the height of the intermediate pushrod in real time—like equipping each cylinder with a miniature lifting platform. This means during slow-moving traffic, the valves open just a slit to save fuel, while during hard acceleration, they open fully, allowing air to rush in like a floodgate release. First introduced in the E46 316i, it now works in tandem with twin-scroll turbos, significantly boosting low-end torque. However, this system demands high precision—if the camshaft sensor fails, the whole car turns into a tractor.
