Piaggio VVT for SOHC Scooters
Despite the constant warnings that the end is near for internal combustion and that electric power will take over in the imminent future, there’s still a substantial amount of development work going into piston engines as manufacturers make them ever more efficient. Variable valve timing is just one of the avenues that’s been largely untapped by most motorcycle companies but now Piaggio is developing a VVT setup specifically for small, low-cost engines.
The company’s design, revealed in a detailed patent application, is intended to alter the intake timing on a single-cylinder, single-overhead-camshaft engine, for use in scooters. That places particular pressure on the design, as profit margins in that part of the market are tight, making simplicity and a reduced component count particularly important.
At first glance, Piaggio’s SOHC VVT system looks almost identical to the design Yamaha uses in its “VVA” (Variable Valve Actuation) engines, fitted to a variety of bikes including the YZF-R125 and MT-125 as well as the Aerox 155 scooter. But there’s a subtle difference in the design that should give the Piaggio system an important advantage.
Like the Yamaha VVA system, the Piaggio design uses two profiles for the intake cam lobes, acting on a forked rocker arm that transmits the motion of one or the other cam lobe to two intake valves. Also following in the Yamaha’s footsteps, the system uses a pin in an extension above the main part of the rocker arm to connect or disconnect the part of the rocker that follows the “high rpm” cam lobe, with an actuator to insert the pin and a spring to disengage it when the second lobe isn’t needed.
But this is where there’s a key difference that separates the capabilities of the two systems. Yamaha’s VVA has two parts to its rocker arm. One is a relatively conventional rocker that acts on the shorter-duration, lower-lift “low rpm” cam lobe. That’s a single piece running from the camshaft end to the valve end, pivoting on the rocker shaft in the middle. The second part is an element of the rocker that’s moved by the high-lift, long-duration cam lobe intended to maximize power at higher rpm. It’s 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 to its rocker arm. The central, Y-shaped part pivots on the rocker shaft and has an extension above it to accommodate the locking pin. It’s flanked by two arms that pivot on the rocker shaft and extend toward the camshaft, one for the “high rpm” lobe, the other for the “low rpm” lobe. The locking pin, controlled by an actuator much like Yamaha’s, slides from side to side in the extension above the rocker shaft to connect the one or other of the cam lobes to the Y-shaped arm that operates the valves.
In operation, that gives the Piaggio system a distinct advantage, because it means the valve timing can be altered as well as the lift and duration. The Yamaha system can only switch between a low-lift, short-duration lobe and a high-lift, long-duration one. That switchover lifts the other rocker away from the gentler cam lobe but doesn’t disconnect it from the valves. In contrast, the Piaggio design completely disconnects the low-rpm part of the rocker from the high-rpm part when it’s actuated, so as well as having a different profile the high-rpm cam lobe can have different timing (the timing is retarded to keep the intake open later in the cycle at high revs).
The patent illustrates the system on what appears to be the engine from the Vespa GTS 300, which is also used on other models including the Piaggio MP3.
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