Variable Valve Actuation (VVA)

Hannu Jääskeläinen

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Abstract: Variable valve actuation (VVA) technologies are used to add flexibility to the engine’s valve train by enabling variable valve event timing, duration and/or lift. The main types of VVA technologies include valve timing control (VTC), variable valve lift (VVL) and camless valve trains.

Classification of VVA Technology

Variable Valve Actuation (VVA) or Valve-Event Modulation (VEM) are general terms that can be used to describe a range of technologies used to add flexibility to the engine’s valve train by enabling variable valve event timing, duration and/or lift. Table 1 summarizes one classification of common VVA technology for light-duty applications as well as their capabilities [2274][4568].

Table 1
General classification of VVA technology
TypeValve lift characteristicsPhaseLiftEventDeacti­vationConti­nuous controlEngine perfor­manceInstal­lationCost
With camValve timing controlyesnononoyesLowGoodLow
Cam switchingpartialyesyesyespartial
Variable valve event and lift controlpartialyesyesyesyes
Variable valve durationpartialnoyesnoyes
With­out camHydraulic or electromagnetic driveyesyesyesyesyesHighPoorHigh
* Additional functions made possible by combining with valve timing control.

VVA systems can be classified as camshaft based systems or camless systems. Camless systems offer the most flexibility in valve lift and timing but are prone to increased risk of catastrophic failure if the valve in the lifted position can interfere with the piston. Camshaft based systems are less flexible in that the valve event must occur within the confines of the cam profile. They are however quite robust and interference between the piston and valve can generally be avoided even during system malfunctions. With the exception of some large low speed two-stroke engines, all current production VVA systems are camshaft based.

Camshaft based systems can be further classified according to their function. Systems that provide Valve Timing Control (VTC)—also referred to as Variable Valve Timing (VVT) systems—are the most basic camshaft based VVA technology and simply change the timing of the valve events without significantly altering the lift.

Variable valve event and lift control, often referred to as Variable Valve Lift (VVL), can provide a discrete or continuous range of lift and/or duration control between two limits. In some cases, it can allow modest adjustments in phasing. Camshaft based VVL technologies can be combined with VTC to enable variable lift and timing control.