Valve operating control system for internal combustion engine

A valve operating control system for an internal combustion engine having a low-speed cam and a high-speed cam, comprising

• an oil pump;
• a cam switching type, first valve operating characteristic changing mechanism;
• a first hydraulic pressure control valve, wherein hydraulic pressure is supplied from said oil pump through said first hydraulic pressure control valve to said first valve operating characteristic changing mechanism;
• a cam-phase changing type, second valve operating characteristic changing mechanism;
• a second hydraulic pressure control valve, wherein the hydraulic pressure is supplied from said oil pump through said second hydraulic pressure control valve to said second valve operating characteristic changing mechanism;
• wherein said first valve operating characteristic changing mechanism selects said low-speed cam to establish a low-speed valve timing, when no hydraulic pressure is supplied from said first hydraulic pressure control valve, and selects said high-speed cam to establish a high-speed valve timing, when the hydraulic pressure is supplied,
• wherein said second valve operating characteristic changing mechanism includes an advancing chamber and a retarding chamber, said second valve operating characteristic changing mechanism changing the cam phase, when the hydraulic pressure is supplied selectively to said advancing chamber or said retarding chamber,
• and wherein when said first valve operating characteristic changing mechanism establishes the high-speed valve timing and said second valve operating characteristic changing mechanism sets the cam phase in a most-displaced basic position, said second hydraulic pressure control valve closes both of said advancing chamber and said retarding chamber, and is maintained in a neutral position in which it cuts off hydraulic pressure supplied from said oil pump.