Piaggio – VVT system for small and low-cost engines

Despite constant warnings that the end of internal combustion is near and that electric power will take over in the near future, there is still a substantial amount of development work on combustion engines as manufacturers make them increasingly efficient. Variable valve regulation is just one of the ways that has not been explored by most motorcycle companies, but now Piaggio is developing a VVT configuration specifically for small and low-cost engines. The company’s project, revealed in a detailed patent application, aims to change the intake timing in a single-cylinder engine with a single overhead camshaft, for use in scooters. This puts special pressure on the design, as profit margins in that part of the market are tight, making simplicity and a reduced number of components particularly important. At first glance, Piaggio’s SOHC VVT system looks almost identical to the design that Yamaha uses in its “VVA” (Variable Valve Actuation) engines, found in a variety of motorcycles including the YZF-R125 and MT-125, as well as the Aerox 155 scooter. But there is a subtle difference in the design that should give the Piaggio system a significant advantage.Just like Yamaha’s VVA system, Piaggio’s design uses two profiles for the intake cam segments, acting on a bifurcated oscillating shaft that transmits the movement of one cam segment or the other to two intake valves. Following Yamaha’s lead, the system also uses a pin on an extension above the main part of the oscillating arm shaft to connect or disconnect the part of the oscillating shaft that follows the “high-speed” cam segment, with an actuator to insert the pin and a spring to disconnect it when the second segment is not needed.

But here is where a fundamental difference exists that separates the capabilities of the two systems. Yamaha’s VVA has two parts in its oscillating arm. One is a relatively conventional oscillating arm that acts on the cam segment of shorter duration and lower lift at “low speeds.” It is a single piece that goes from the camshaft end to the valve end, rotating on the middle oscillating arm shaft. The second part is an element of the oscillating arm that is moved by the high lift and long duration cam segment, intended to maximize power at higher speeds. It is this section that can be connected or disconnected by the locking pin, moved by the actuator inside the cylinder head.

In contrast, the Piaggio system has three elements in its swingarm. The central part, in a Y shape, rotates on the axis and has an extension above to accommodate the locking pin. It is flanked by two arms that rotate on the axis and extend towards the camshaft, one for the “high rotation” segment and the other for the “low rotation” segment. The locking pin, controlled by an actuator very similar to Yamaha’s, slides from side to side in the extension above the axis to connect one or the other of the cam segments to the Y-shaped arm that operates the valves.

In operation, this gives the Piaggio system a distinct advantage, because it means that valve timing can be altered, as well as lift and duration. The Yamaha system can only switch between a low lift and short duration sector and a high lift and long duration sector. This change pushes the other away from the smoother cam segment, but does not disconnect it from the valves. In contrast, the Piaggio design completely disconnects the low rotation part of the swingarm from the high rotation part when activated, so that, in addition to having a different profile, the high rotation cam segment can have a different timing (the timing is delayed to keep the intake open later in the cycle at high speeds).

The patent illustrates the system in what appears to be the engine of the Vespa GTS 300, which is also used in other models, including the Piaggio MP3.