This paper takes the hydraulically driven VVT system used in most of the production models in the Chinese automoblie market as an example:
VVT System Schematic
The VVT system in the above figure mainly consists of a cam phaser and an oil control valve (OCV for short). The cam phaser is the actuator of the system, and the OCV is the controller of the system.
The engine management system(also commonly referred to as the ECU) looks up the MAP map based on the signals transmitted by the throttle opening sensor, the engine water temperature sensor, the rotational speed sensor, the air flow meter, etc., and calculates the required valve timing angle under various operating conditions of the engine. The time angle, that is, the target position. At the same time, EMS calculates the actual position of the camshaft based on the feedback signals from the crank position sensor and the cam position sensor. The EMS compares the target position with the actual position and sends an actuation signal to the OCV according to the EMS control strategy to change the position of the spool in the control valve, thereby changing the oil flow direction and flow rate in the oil passage as well as the signals such as advance, lag, and constant are fed back into the VVT phaser cavity by hydraulic pressure to realize relative rotation between the inner stator and the outer rotor of the phaser to adjust the timing angle of the camshaft, thereby the amount of intake air (exhaust) is adjusted, and the valve opening and closing time is reached.