# Review of Advancement in Variable Valve Actuation of Internal Combustion Engines
## 8 Electro-Hydraulic VVA (EHVVA) Systems  
In EHVVA systems, primary actuators are hydraulic actuators, such as a piston-cylinder mechanism, controlled by electro-hydraulic valves. Compared with an EMVVA, an EHVVA generally has higher power density but lower efficiency. The hydraulic fluid has high bulk modulus suitable for snubbing in the valve seating process, and its viscosity is highly sensitive to temperature, becoming too viscous for proper function at lower temperature. Some major EHVVA systems are listed below, which are also listed in Table 2 for comparison. * Sturman Industries developed the hydraulic valve actuation (HVA) system. It includes two digital two-way pilot valves, a proportional valve, a hydraulic actuator with boost and drive pistons, and a position sensor necessary for closed-loop lift control [35; 77]. The actuator is returned either hydraulically or by a return spring. It offers full control in valve timing, duration, and lift, and it was used in an experimental 15 L natural gas engine and as universal research modules for various research programs [78]. Its necessary use of a position sensor may incur high cost and reliability concerns for mass production. * Lotus and Eaton jointly developed the active valve train (AVT) system. It includes one digital three-way pilot valve, one servo valve, one return spring, and a hydraulic actuator integrated with a position sensor needed for closed-loop lift control [79]. Like Sturman's HVA, the AVT system offers full control in valve timing, duration, and lift. It may also have cost and reliability issues associated with the position sensor. * AVL and Bosch developed the electro-hydraulic valvetrain system (EHVS) system [80]. It includes two digital main valves, a hydraulic actuator with a two-stage differential piston drive, a pilot-controlled variable snubber for seating control, and no return spring. It uses an open-loop control and, thus, has no need for a position sensor, which offers substantial cost and reliability benefits but presents concerns in lift calibration and accuracy. * Gongda Power developed the Gongda-VVA-2 (GD-VVA-2). It includes one digital three-way pilot valve, one digital three-way main valve, an actuator with one lift-control sleeve, two-step seating control, open-loop two-step lift control, and no position sensor [81; 82; 83; 84]. The two-step lift control provides robust and accurate position control, which is delineated mechanically by the lift-control sleeve, without the need for an expensive and unreliable position sensor. It also has a two-level hydraulic damping mechanism for effective valve seating speed control over a wider temperature range. The two-step lift control does present certain functional compromise, which can be compensated for by its infinitely variable timing capability inherent in this and other EHVVAs. One GD-VV-2 prototype system passed 1000 h of durability testing on a test bench. There is also a proposal to incorporate some CVVL mechanisms into the base GD-VV-2 design, resulting in a full VVA system, still without the need for a position sensor for each engine valve [85]. There are many other studies on EHVVA systems. One major effort is to minimize the energy consumption by the VVA system itself by using some kind of pendulum mechanism, similar to the compression spring pendulum used in the EMVVA system. Some examples are as follows: