In most internal combustion engines the engine cylinder intake and exhaust valves are opened and closed (at least for the most part) by cams in the engine.  This makes it relatively difficult and perhaps impossible to adjust the timings and/or amounts of engine valve openings to optimize those openings for various engine operating conditions such as changes in engine speed. 
It is known to include hydraulic lash adjusting mechanisms in the linkage between an engine cam and the engine cylinder valve controlled by that cam to make it possible to make relatively small adjustments in the valve strokes relative to the profile of the cam (see, for example, Rembold, et al.,  U.S. Pat.  No.  5,113,812 and Schmidt et al.,  U.S. Pat.  No.  5,325,825).  These lash adjustments may be used to provide additional valve openings when it is desired to convert the engine from positive power mode to compression release engine braking mode (see, for example, Cartledge, U.S. Pat.  No.  3,809,033 and Gobert et al.,  U.S. Pat.  No.  5,145,890).  Hydraulic circuitry may also be used to cause a part of the engine other than the cam which normally controls an engine valve to provide additional openings of the valve when it is desired to convert the engine from positive power mode to compression release engine braking mode (see, for example, Cummins, U.S. Pat.  No.  3,220,392, and Hu, U.S. Pat.  No.  5,379,737). 
Schechter, U.S. Pat.  No.  5,255,641, shows in FIG.  16 that an engine cam can be linked to an engine cylinder valve by a hydraulic circuit which includes a solenoid valve for selectively releasing hydraulic fluid from the hydraulic circuit.  Schechter points out that various phases of the engine cylinder valve lift versus the cam curve can be obtained by varying the solenoid voltage pulse timing and duration.  However, Schechter does not suggest that any lobe on the cam can be completely overridden in this way.  It may not be possible to convert an engine from positive power mode to compression release engine braking mode and vice versa without the ability to selectively completely override any lobe on an engine cam. 
Sickler, U.S. Pat.  No.  4,572,114, shows internal combustion engine cylinder valve control which essentially uses two substantially separate hydraulic circuits for controlling the motion of each engine cylinder valve.  One of these two hydraulic circuits controls selective decoupling of each engine cylinder valve from its normal cam-driven mechanical input.  The other hydraulic circuit provides alternative hydraulic inputs to the engine cylinder valve when the normal mechanical input is decoupled.  The control for these two hydraulic systems may be essentially mechanical and/or hydraulic as in FIG.  5, or it may be essentially electronic as shown in FIG.  7.  The two hydraulic circuits may have a common source of hydraulic fluid and they may have other cross-connections, but they are largely separate in operation and they each require a separate hydraulic connection (e.g., 136 and 212 in FIG.  5 or 258 and 212 in FIG.  7) to each cylinder valve operating mechanism. 
From the foregoing it will be seen that the known hydraulic modifications of cam control for engine cylinder valves tend to be either relatively limited in extent and purpose (e.g., as in FIG.  16 of the Schechter patent) or to require relatively complex hydraulic circuitry (e.g., as in the Sickler patent).