Valve bouncing

VALVE BOUNCING

 VALVE BOUNCING

The valves are opened by the rocker arm pushing on the valve stem tip, but they are closed by two or sometimes three coil springs. Multiple springs are used, not only because of the additional force they provide but because

of a characteristic of the spring known as resonance. Valve opening is done with a series of impulses, and at a certain engine RPM, these impulses will occur at the resonant frequency of the spring. When this happens, the spring loses its effectiveness and allows the valve to float. To prevent this floating condition, two or more springs having different pitch, diameter, and wire size are used; and because of their different configuration, they have different resonant frequencies, so the engine can operate throughout its full range of RPM without valve float problems.
Valve bouncing is a phenomenon, which happens due to sudden release of compression energy of the spring at the moment of valve closure due to tappet clearance allowance. The valve would jump up and down on seat before closing shut finally. Spring surge (resonant vibration) at this moment also tends to help generate valve bouncing.
Valve bouncing can be reduced by:
(i) Increasing number of springs.
(ii) Improving spring material.
(iii) Using inner and outer springs, right and left handed.
(iv) Using hydraulic valve actuating gear.
(v) Using air spring.