Valve mechanisms

A valve mechanism comprising a valve having a valve stem which is located for axial movement in a valve guide and a valve head adapted, at one extreme of movement of the valve, to locate against and close a valve seat, a valve lever is pivotally mounted at one end and is attached adjacent the other end to the end of the valve stem remote from the valve head, said valve lever defines a track, a drive pin engages in said track, said drive pin being mounted at one end of a first link, the other end of the first link being pivotally mounted at a fixed pivot, so as to constrain the drive pin to move in a fixed arcuate path, a second link is pivotally mounted at one end about a movable pivot, the first and second links being interconnected by an intermediate link pivotally attached to said first and second links at positions separated from their pivots, a drive link being pivotally connected at one end to the second link or intermediate link at or adjacent the interconnection of said second link and intermediate link and at the other end to a crank, so that rotation of the crank will cause the linkage to oscillate and the drive pin to perform a reciprocating motion along its arcuate path, the track in the valve lever having a first portion which is engaged by the drive pin when the valve is closed and coincides with the arcuate path of the drive pin and a second portion which diverges from the path of the drive pin, so that engagement of the second portion by the drive pin will cause the valve lever to move opening and closing the valve, means being provided for moving the pivot of the second link to vary the timing and duration of opening of the valve.