On the contrary, when the engine speed is low, the intake flow rate is low, and the flow inertia energy is also small. If the intake valve is opened too early, because the piston is moving upward to exhaust at this time, it is easy to squeeze fresh air out of the cylinder, causing the intake air to be reversed. Less, the engine works erratically. Therefore, there is no fixed valve superposition angle setting that can make the engine output perfectly at high and low speeds. If there is no variable valve timing technology, the engine can only choose the optimal fixed according to the needs of its matching model. Valve stacking angle. For example, the engine of the racing car generally adopts a smaller valve overlap angle to facilitate the power output at high speed. Ordinary civilian cars use a moderate valve stacking angle, while taking into account the power output at high and low speeds, but will lose a lot of power at low and high speeds. The variable valve timing technology is to solve this contradiction by realizing the variable valve superposition angle through technical means.