A lost motion assembly for a valve of an internal combustion engine includes a
cam, a cam piston which is spring biased to follow motion of the cam, a valve piston
operatively engaged with a poppet valve, and an auxiliary piston. The cam piston,
valve piston and auxiliary piston are in continuous contact with a fluid in an
enclosed control chamber having a fixed volume, such that rotation of the cam causes
displacement of the cam piston into the control chamber, which causes a corresponding
displacement of at least one of the valve piston and auxiliary piston while the
volume of fluid in the control chamber remains substantially constant.