Audi valvelift

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Audi Valvelift

Audi's Valvelift system made its debut in the company's 2.8-liter direct injection V6 and is expected to be expanded for use in many other members of the 90-degree V6 / V8 family. The Valvelift system itself is a cam-changing type VVT, but as Audi's V6 / V8 engines are already equipped with cam-phasing VVT, I classify it as the combination type VVT here.

Compare with Honda's or Toyota's mechanism, Audi's seems to be simpler and more efficient. It does the variable lift without using complex intermediate parts (e.g. hydraulic-operated lockable rocker arms), so it saves space and weight while reduces frictional loss and, theoretically, improves revvability. How can Audi do that? the answer is: in Valvelift system, the cam pieces can slide in longitudinal direction to change the actuating cams.



Each intake valve can be actuated by a fast cam (11mm lift) or a slow cam (5.7mm in one intake valve and 2mm in another in order to create swirl in the air flow for better fuel mixing at low speed). The two cams are mounted on a single cam piece. Which cam acts on the roller cam follower depends on the longitudinal position of cam piece. This is controlled by a pair of metal pins incorporated at the cam cover. There is a spiral groove rolled into the camshaft. When one metal pin is lowered, it engages the spiral groove on the camshaft and pushes the cam piece by 7mm in longitudinal direction. A spring-loaded locker will lock the cam piece in the new position. In this way, the operating cams are changed from one set to another set.

To revert to another cam, another metal pin presses against a reverse spiral groove and moves the cam piece back to the original position. The cam piece is locked by the spring-loaded locker again. The change from one cam set to another takes one combustion cycle, or two engine revolutions. As Audi reprogrammed the ignition and electronic throttle to smoothen the transition between the two cam sets, it can be hardly detectable.

Theoretically, the Valvelift system should deliver better power than Toyota's VVTL-i and Honda's i-VTEC, but in the 2.8-liter V6 its priority is put on fuel economy. We shall see whether Audi will use its advantage in its performance engines in the future.

Advantage: Continuous VVT improves torque delivery across the whole rev range; Variable lift and duration lift high rev power.
Disadvantage: More complex and expensive
Who use it: Audi 2.8 V6
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