When the engine is in the high speed range, a working stroke of the four-stroke engine takes only a few thousandths of a second. Such a short time often causes insufficient engine intake and exhaust, which affects the efficiency of the engine. Therefore, it is necessary to compensate for the lack of intake air and exhaust air through the early opening and closing of the valve. In this case, a moment when the intake and exhaust valves are simultaneously opened is inevitable, and the valve phase is called "valve overlap angle".
The angle of valve overlap often has a large impact on engine performance, so how large is this angle? We know that the higher the engine speed, the shorter the absolute time left for inhaling and exhausting in one working cycle per cylinder. Therefore, to achieve higher charging efficiency, it is necessary to extend the intake and exhaust time of the engine. Obviously, the higher the speed, the greater the required valve overlap angle. However, under low-speed conditions, excessive valve overlap angle will cause excessive exhaust gas to rush into the intake end, and the intake air will decrease, and the airflow in the cylinder will be disordered. At this time, the ECU will also have difficulty in air-fuel ratio. Precise control is performed, resulting in unstable idle speed and low low speed torque. Conversely, if the valve train is optimized for low speed conditions, the engine will not be able to achieve high peak power at high speeds.
Therefore, in order to solve this problem, the valve timing can be adjusted according to the engine speed and working conditions. The ideal intake and exhaust efficiency can be obtained at both high and low speeds. This is the original intention of variable valve timing technology development.