# Review of Advancement in Variable Valve Actuation of Internal Combustion Engines
## 4 Discrete VVL (DVVL) and Associated VVA Systems  
A discrete VVL (DVVL) system includes a cam profile switching mechanism to activate one of two or three cam profiles or lobes, and it becomes a cam-based VVA (or DVVL + VVT) system when a VVT mechanism is further incorporated. The applications of DVVL and DVVL + VVT systems include, but are not limited to, Honda, Audi, Subaru, Proton, Yamaha, Mitsubishi, Nissan, Porsche, Toyota, Honda, and Audi, as shown in Table 1.  
In 1989, Honda launched, in Integra, the world's first commercial DVVL system in a motor vehicle engine called the variable valve timing and lift electronic control (VTEC) system [10; 19; 54]. VTEC uses two (or occasionally three) intake camshaft profiles, switched via hydraulically actuated rocker arm locking pins. In the system, the timing variation is fixed in the cam profiles and not independent of the lift variation. Honda then launched, in 2001, a cam-based VVA system called intelligent VTEC (i-VTEC) in high-output DOHC four-cylinder engines by adding continuous intake cam phasing (timing) to the traditional VTEC. VTEC controls are still limited to distinct low- and high-RPM profiles, but the intake camshaft is now capable of advancing between 25\({}^{\circ}\) and 50\({}^{\circ}\), depending upon engine configuration [55].  
In 1992, Mitsubishi launched the world's first cam-based VVA system, a DVVL + VVT system called the Mitsubishi innovative valve timing electronic control system (MIVEC). It has low-lift and high-lift cam profiles for low-speed and high-speed engine modes, respectively, which are switched via a locking pin mechanism. The low-lift cams and rocker arms, used to drive separate intake valves, are situated on two sides of a centrally located high-lift cam. Each intake valve is operated by a low-lift cam and rocker arm, while a T-lever between them engages the high-lift cam [15]. The VVT-i system from Toyota has a similar switching mechanism [18].  
In 1999, Toyota launched the variable valve timing and lift intelligent system (VVTL-i or VVT-iL). The Toyota VVTL-i concept, including its a lift variation system via rocker arm locking pins, is similar to the Honda i-VTEC concept. Each cam has two lobes, one designed for lower-speed operation and another designed for high-speed operation, with higher lift and longer duration.  
In 1999, Porsche launched its VarioCam Plus, a DVVL + VVT system on the intake side. The two-lobe valve-lift function is performed by electro-hydraulically controlled switchable tappets. Each of these 12 tappets consists of concentric lifters which can be locked together by a pin. The inner lifter and the outer ring element are actuated by a small cam lobe and a pair of larger-profile lobes, respectively. The timing of each valve is seamlessly varied by an electro-hydraulic rotary vane cam phaser [1,4]  
In 2006, Audi launched, in the 1.8 L TFSI engine, the Audi valve lift system (AVS) [11]. It uses sliding electro-magnetic sleeves on the camshaft to vary the lift of the valves in two stages depending on load and engine speed. The system, thus, increases torque while also reducing fuel consumption. Two versions of the AVS system are available: (1) in the V6 engines in which AVS is used, it acts on the intake valves, regulating the amount of intake air so that the throttle can remain wide open for free breathing even at part load, thus reducing throttle losses and improving efficiency; (2) in the latest-generation 2.0 TFSI, the AVS varies the lift of the exhaust valves, thus reducing flushing losses in the combustion chamber and ensuring the optimal flow of the exhaust gas to the turbocharger.