1.  Field of the Invention
The invention relates to an internal combustion engine having solenoid-operated valves, and in particular to such an engine having solenoid-operated valves, which is operable in a partial operating mode with at least one of the cylinders being deactivated by inhibiting combustion therein under cylinder deactivation control.  The invention also relates to a control method for controlling such an internal combustion engine. 
2.  Description of Related Art
Internal combustion engines capable of performing cylinder deactivation control under which a selected one or more of cylinders is/are deactivated or disabled are known in the art, and one example of such engines is disclosed in Japanese Patent Laid-Open Publication No.  7-279697.  In this type of engine, fuel injection and ignition are stopped with respect to one or more cylinders selected from a plurality of cylinders of the engine, and cylinder deactivation control is performed by keeping an intake valve and an exhaust valve in their closed positions.  Here, "deactivating a cylinder" means inhibiting any of suction of an air/fuel mixture, combustion, and exhaust of combustion gas, while allowing reciprocating motion of the piston.  In the following description, cylinders that are deactivated or disabled under cylinder deactivation control will be called "inactive cylinders" or "deactivated cylinders" when appropriate, and cylinders in which combustion takes place even when the engine is in a partial operating mode are called "active cylinders" or "activated cylinders."  With the cylinder deactivation control performed, the fuel injection is inhibited in the inactive cylinders, and pumping loss can be reduced, with a result of improved fuel efficiency. 
To perform cylinder deactivation control, only a particular cylinder or cylinders may be deactivated or disabled, or the cylinders of the engine may be successively deactivated in a certain order. 
In the case where particular cylinders are selected as inactive cylinders, gas contained in the combustion chamber of each inactive cylinder slowly leaks into the crankcase through a sliding surface of the piston, and the pressure within the inactive cylinder becomes lower than the pressure of active cylinders.  Consequently, torque variations arise from a difference between the pressure within the active cylinders and the pressure within the inactive cylinders, resulting in increased vibration of the engine. 
Where the cylinders of the engine are successively deactivated in a certain order, there arises almost no reduction in the pressure within the currently deactivated cylinder due to the gas leakage as described above.  When the piston of the inactive cylinder reaches the top dead center, therefore, burnt gas contained in the combustion chamber is compressed so that the pressure within the inactive cylinder increases to substantially the same level as that achieved upon combustion.  Here, the sum of the cylinder pressures in the engine as a whole differs depending upon whether or not the timing of an increase in the pressure within the inactive cylinder coincides with the timing of an increase in the cylinder pressure due to combustion in an active cylinder.  Accordingly, even if the cylinders are successively deactivated one after another, the engine still suffers from increased vibration due to variations in the output torque of the engine.