# Review of Advancement in Variable Valve Actuation of Internal Combustion Engines
## 1 Introduction
### Classification of Valve Actuation Systems
#### 1.3.1 Cam-Based Valve Actuation Systems  
A cam-based valvetrain system is based upon the traditional cam-system to drive the engine intake and exhaust valves with limited control over valve timing and/or lift, and it is now widely adopted in many new production engines. Cam-based systems include the following:  
* **Variable valve timing:** Only the valve timing is independently controlled while the valve lift and duration remain the same. The VVT systems are also called cam (or camshaft) phasers. VVT  
Figure 2: Regular valve timing diagram for naturally aspirated engines, defining valve lift, timing, and duration.  
systems are further classified into hydraulic (HVVT), mechanical (MVVT), and electrical (EVVT) types based on their respective cam phasing actuator designs. Many production VVT systems are HVVTs, using a device known as a variator that allows continuous adjustment of the cam timing, and EVVTs are getting popular for improving system response time at low temperature or engine start-up. However, the duration and lift cannot be adjusted.
* **Variable valve duration (VVD):** Only the valve duration is independently controlled.
* **Variable valve lift (VVL):** Only the valve lift is independently controlled. VVL systems further include discrete VVL (DVVL) and continuous VVL (CVVL) designs. A DVVL system includes a cam profile switching mechanism to activate one of two or three cam profiles or lobes, and a CVVL system includes a mechanism capable of continuous variation of the life profiles. In most, if not all, VVL systems, the lobes and mechanisms are designed such that the valve duration increases with the valve lift, which is a fixed relationship and not an independent control of the valve duration, although it serves the needs of a normal combustion. These VVL systems by themselves are, therefore, not classified as VVT, VVD, or VVA systems.
* **Cam-Based Variable Valve Actuation (VVA):** Cam-based VVA systems include (1) the VVL + VVT type, which is a combination of VVL (either DVVL or CVVL) and VVT, and (2) the lost-motion type (LMVVA).  
Major cam-based valve actuation systems in production engines are listed in Table 1. Some of them are discussed in more detail in the later sections.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Classification** & **Company** & **System** & **Ind. Timing** & **Ind. Lift** & **Ind. Duration** & **Introd. Year,** \\  & & & & & **Comments \& Refs** \\ \hline HVVT & Nissan & VTC/NVCS & 2-stage & & 1987 \\ \hline HVVT & Toyota & VVT-i & Cont, Int & & 1996 \\ \hline HVVT & Mazda & S-VT & Variable, Int & 1998 \\ \hline HVVT & Ford & Ti-VCT & 2-stage, Both & 2011 \\ \hline HVVT & Alfa Romeo & VCT & 2-stage, Int & & 1980, lst VVT, \\ \hline HVVT & BMW & Single VANOS & 2-stage \& Cont, Int & 1992, [9] \\ \hline HVVT & BMW & Double VANOS & Cont, Both & 1996, [9] \\ \hline HVVT & Ford & VCT & 2-stage, Int & \\ \hline HVVT & GM & DCVCP & Cont, Both & \\ \hline HVVT & Hyundai & CVVT & Cont, Both & \\ \hline HVVT & Hyundai & VTVT & Variable, Both & \\ \hline HVVT & Daihatsu & DVVT & Cont, Int & \\ \hline HVVT & Ducati & DVT & Cont, Both & \\ \hline HVVT & Nissan & CVVTCS/CVTC & Cont & \\ \hline HVVT & Subaru & AVCS & VVT & \\ \hline HVVT & Toyota & Dual VVT-i & Cont, Both & \\ \hline HVVT & Toyota & VVT & 2-stage & \\ \hline MVVT & Porsche & VariCam & Cont, Int & 1992, 1 st Cont VVT, \\  & & & & [7,9] \\ \hline EVVT & Toyota & VVT-iE & Cont, Both & & 2007, electric Int, \\  & & & & hydraulic Exh \\ \hline VVD & MG Rover & VVC & & Cont, Int & 1993, eccentric \\ \hline VVD & Hyundai & CVVD & Cont, Int & 2019, eccentric \\  & & & & mechanism \\ \hline DVVL & Honda & VTEC & 2- \& 3-lobe, & 1989, [10] \\  & & & & Int & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Major cam-based valve actuation systems in production engines.