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156 
DESMODROMIC VALVE MECHANISM
Page 114
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'THE use of springs for closing valves I has its limits and for high-speed engines, proposals have often been made to open and close the valves by positive action. A scheme of this nature is disclosed in patent No. 861,369. (F. Porsche, Porsche-Strasse, Stuttgart-Zuffenhansen, Germany.)
The drawing shows the scheme diagrammatically. A cam-shaft (1) is linked by a push-rod to a cam (2), pivoted at (3). A rocker (4), pivoted at (5), has at one end two faces which contact the cam. The upper face (6) is for valve opening, whilst the lower face (7) is for valve closing. The other end of the rocker has a cylindrical bore into which the ball-shaped head (8) of the valve stem fits.
It will be seen that rotation of the camshaft rocks the cam (3) and through it the rocker (4) thus opening and closing the valve.
Both the cam and the rocker pivot pins are located in a lever (9) which is pivoted on the cylinder head at the end nearest the valve. The free end of this lever is adjustable vertically, being connected to a hydraulic piston (10). A centrifugal governor (11) acting on a lever, controls the supply of fluid—directing it either above or below the piston.
In operation, as the engine speed increases, so the end of the lever is raised, and the layout is such that raising the
lever increases the stroke of the valve and in addition advances it slightly.
The cam and rocker pivots are eccentrically mounted to enable the working clearance to be adjusted.