Aprilia’s V4 to get VVT: Firm take out variable valve timing patent for future RSV4 superbike

Suzuki, BMW, Kawasaki, Ducati and Yamaha have all taken steps into variable valve timing and now Piaggio are developing their own setup for a future Aprilia RSV4.

Suzuki’s VVT design, used in the GSX-R1000, stands apart from the rest as a purely mechanical design, with no electronic or hydraulic elements.

Patent documents show Piaggio are following this route, but with a simpler, lower-cost design that also promises to have operating advantages over its Japanese rival.

Suzuki’s impetus to develop mechanical VVT was MotoGP. In the blue ribband series, electronic and hydraulic variable valve systems are banned, leaving a loophole for anyone who can develop a mechanical system.

Does Aprilia’s RS-GP also use VVT? That’s something only the firm’s mechanics and designers are likely to know, but the passive system shown in the patent would be legal in the series.

Suzuki’s system uses a dozen weighted ball bearings sitting inside slightly spiralled, radial channels carved into the cam sprocket.

These mesh into matching channels, angled slightly differently and machined into a disc mounted onto the intake camshaft via a spline. As revs rise the balls move outwards due to centrifugal force, changing the angle of the camshaft in relation to its sprocket and advancing the timing.

The Piaggio/Aprilia system uses the same theory, but with a twist that reduces friction and makes it cheaper to make and easier to modify the VVT’s behaviour.

It also uses weighted ball bearings and centrifugal force, but instead of a dozen of them there are just three.

The balls sit in radial channels machined into the inlet cam sprocket and a matching drive plate mounted on the camshaft, with those channels sitting at slightly different angles on the two halves to alter the cam timing as the balls move outwards.

With only three balls there’s less friction than with the Suzuki system, and the design requires less complex machining.

There’s a thin disc between the cam sprocket and the camshaft-mounted drive plate, with slots for the balls and three W-shaped springs which also fit into indents in the drive plate.

As the camshaft turns faster and the balls move outwards, turning the camshaft by a few degrees in relation to its sprocket, the three springs get compressed. When the revs drop, the springs overcome the centrifugal force, returning the cam timing to its original state.

The Piaggio/Aprilia system can be tuned by changing the springs or the cut-outs in the third, central disc to alter the VVT’s behaviour and what revs are required to activate the system.

Why VVT matters now

Variable valve timing has become more significant than ever with the arrival of Euro5 which places stricter limits on emissions.

High-performance motorcycle engines need lots of valve overlap (when both the exhaust and inlet valves are open simultaneously) to work efficiently at high revs, but at low revs the same amount of valve overlap gives enough time for unburnt fuel to get into the exhaust before the exhaust valve closes.

Simple, inlet camshaft phasing VVT means that the inlet valves can open later at low rpm, reducing emissions and improving torque and efficiency, but earlier at high revs, giving the overlap needed to make lots of power.

How Aprilia’s VVT system works

Chain-driven: The engine shown in the Piaggio/Aprilia patent uses a chain to drive the inlet camshaft, which in turn drives the exhaust camshaft via a gear. It’s the same setup that the Aprilia RSV4 uses and suggests that’s the engine this system is targeting.

Phasing system: The inlet cam phasing system (marked 1 in the drawings) is mounted inside the sprocket (2) that connects the inlet to the exhaust camshaft.

Spinning up: The cam phaser (1) is made up of the sprocket (2), driven by the cam chain, and the drive disc (3) connected to the camshaft. 

On the slide: A disc (4) sits between the sprocket and drive disc, carrying three balls (5) and three springs (6) in shaped cut-outs. These balls move outwards as revs rise, sliding along skewed slots in the sprocket and the drive disc to the cam timing at higher rpm.

Spring back: The three springs (6) push to bring the sprocket and the drive disc back to the low-rev, retarded timing position, overcoming the centrifugal force acting on the balls at lower rpm.