Variable valve lift

Variable valve lift is a technology which is increasingly used in automotive piston engines. It varies the height that a valve opens, in order to improve performance, fuel economy or emissions.

When used in conjunction with variable valve timing, variable valve lift can potentially offer infinite control over the intake and exhaust valve timing.

History

In September 1975, General Motors (GM) patented a system intended to vary valve lift. GM was interested in throttling the intake valves in order to reduce emissions. This was done by minimizing the amount of lift at low load to keep the intake velocity higher, thereby atomizing the intake charge. GM encountered problems running at very low lift, and abandoned the project.

Production versions

In 2001, BMW introduced the Valvetronic system. The Valvetronic system can continuously and precisely vary intake valve lift, and in addition, the independent Double VANOS system can concurrently vary the timing for both the intake and exhaust valves. The precise control the system has over the intake valves allows for the intake charge to be controlled entirely by the intake valves, eliminating the need for a throttle valve and greatly reducing pumping loss. The reduction of pumping loss accounts for a 10-15% increase in power output and fuel economy.^[1]

Nissan introduced its Variable Valve Event and Lift (VVEL) in 2007 as the world's second CVVL system. The first application was on the VQ37VHR V6 engine of Skyline Coupe (Infiniti G37). Nissan's system is more compact than BMW’s, involving fewer parts and less energy loss, and is therefore more adaptable to high-performance engines.^[2] The VVEL does not use a conventional intake camshaft. Each valve is actuated by a cam which is pivoted on - but not fixed to - the camshaft. While conventional camshafts feature fixed rotating lobes, the cam in VVEL swings up and down; this is why it does not need a symmetric profile. Its movement is driven by the camshaft via a series of components. VVEL varies valve lift by the eccentric control shaft inside the rocker arm. By rotating the eccentric control shaft, the position of rocker arm is shifted, changing the swing angle of cam. The swing angle of cam determines the degree of valve lift.^[2]

Toyota introduced its Valvematic technology in 2008. Valvematic employs an intermediate shaft to achieve continuous variable valve lift. The intermediate shaft has an actuating member for each cylinder.^[3] Each actuating member is made of two finger followers on either side of a roller bearing member. The followers can rotate in relation to the roller member by means of internal gear threads and an electric motor attached to the end of the intermediate shaft. The gear threads of the roller member and finger followers are in opposing directions. This means when the shaft swivels, the roller member and finger followers will move in opposing directions, moving either apart or closer together. In this way, the axle angle between them can be varied infinitely by the electric motor. The intake valve is actuated by camshaft via the intermediate shaft.^[3] Specifically, the camshaft acts on the roller member of intermediate shaft, transferring the movement to both finger followers, then towards the roller rocker arms and ultimately to the intake valves. When the angle of the finger followers is narrow with respect to the roller member, valve lift is low. As the angle increases, so does valve lift. In this way, Valvematic can vary valve lift by adjusting the angle of the finger followers in relation to the roller bearing member.

See also

• Valvetronic (BMW)
• Valvematic (Toyota)
• Variable valve timing

References