Abstract: An engine system includes: a hydraulic valve stop mechanism configured to switch states of an intake valve and exhaust valve of a same stopped cylinder; a hydraulic pressure changing device configured to change hydraulic pressure supplied to the valve stop mechanism; and a valve control portion configured to control the hydraulic pressure changing device. When a return from a reduced-cylinder operation to an all-cylinder operation is requested, and an engine revolution is less than a reference revolution, the hydraulic pressure is changed such that opening of one of the valves of the stopped cylinder able to restart at an earlier stage is first restarted. When the return is requested, and the engine revolution is not less than the reference revolution, the hydraulic pressure is changed such that opening of the exhaust valve of the stopped cylinder is restarted before opening of the intake valve.

Abstract: Provided are a valve stop mechanism capable of switching intake valves and exhaust valves of deactivated cylinders between an openable/closable state and a closed state, and an engine speed control unit which controls the engine speed. The engine speed control unit controls the engine speed in such a manner that the amount of change in the engine speed with respect to time is reduced, as compared with a case in which a specific condition is not satisfied, when the specific condition that switching by the valve stop mechanism is not completed, and that connection between an engine and a power transmission unit is released is satisfied after issuance of a switching request from one of a reduced-cylinder operation and an all-cylinder operation to the other thereof.

Abstract: In response to a switching demand from an all-cylinder operation to a reduced-cylinder operation, the amount of air to be taken into each of cylinders 2A to 2D is made larger than the amount of air during the all-cylinder operation in a normal state. Preparatory control of changing the ignition timing of ignition device to a timing on the retard side with respect to the ignition timing during the all-cylinder operation in the normal state is executed. After termination of the preparatory control, the reduced-cylinder operation is started. During execution of the preparatory control, the valve closing timing of an exhaust valve 9 is changed to a timing on the advance side with respect to the valve closing timing thereof during the all-cylinder operation in the normal state so as to reduce the amount of internal EGR gas in at least a part of an operating range C.

Abstract: When a difference between a target intake air amount during the reduced-cylinder operation, which is greater than a target intake air amount during the all-cylinder operation in a normal state, and an actual intake air amount is greater than an allowable deficient intake air amount, a preparatory control for increasing the actual intake air amount and shifting an ignition timing toward a retard side with respect to an ignition timing during the all-cylinder operation in the normal state. Then, at a time when the difference between the target intake air amount and the actual intake air amount becomes equal to or less than the allowable deficient intake air amount, the reduced-cylinder operation is started, wherein the allowable deficient intake air amount is set to a larger value when a current speed stage is a high speed stage than when it is a low speed stage.

Abstract: An engine system includes: a hydraulic valve stop mechanism configured to switch states of an intake valve and exhaust valve of a same stopped cylinder; a hydraulic pressure changing device configured to change hydraulic pressure supplied to the valve stop mechanism; and a valve control portion configured to control the hydraulic pressure changing device. When a return from a reduced-cylinder operation to an all-cylinder operation is requested, and an engine revolution is less than a reference revolution, the hydraulic pressure is changed such that opening of one of the valves of the stopped cylinder able to restart at an earlier stage is first restarted. When the return is requested, and the engine revolution is not less than the reference revolution, the hydraulic pressure is changed such that opening of the exhaust valve of the stopped cylinder is restarted before opening of the intake valve.

Abstract: Provided are a valve stop mechanism capable of switching intake valves and exhaust valves of deactivated cylinders between an openable/closable state and a closed state, and an engine speed control unit which controls the engine speed. The engine speed control unit controls the engine speed in such a manner that the amount of change in the engine speed with respect to time is reduced, as compared with a case in which a specific condition is not satisfied, when the specific condition that switching by the valve stop mechanism is not completed, and that connection between an engine and a power transmission unit is released is satisfied after issuance of a switching request from one of a reduced-cylinder operation and an all-cylinder operation to the other thereof.

Abstract: When a difference between a target intake air amount during the reduced-cylinder operation, which is greater than a target intake air amount during the all-cylinder operation in a normal state, and an actual intake air amount is greater than an allowable deficient intake air amount, a preparatory control for increasing the actual intake air amount and shifting an ignition timing toward a retard side with respect to an ignition timing during the all-cylinder operation in the normal state. Then, at a time when the difference between the target intake air amount and the actual intake air amount becomes equal to or less than the allowable deficient intake air amount, the reduced-cylinder operation is started, wherein the allowable deficient intake air amount is set to a larger value when a current speed stage is a high speed stage than when it is a low speed stage.

Abstract: In response to a switching demand from an all-cylinder operation to a reduced-cylinder operation, the amount of air to be taken into each of cylinders 2A to 2D is made larger than the amount of air during the ail-cylinder operation in a normal state. Preparatory control of changing the ignition timing of ignition device to a timing on the retard side with respect to the ignition timing during the all-cylinder operation in the normal state is executed. After termination of the preparatory control, the reduced-cylinder operation is started. During execution of the preparatory control, the valve closing timing of an exhaust valve 9 is changed to a timing on the advance side with respect to the valve closing timing thereof during the all-cylinder operation in the normal state so as to reduce the amount of internal EGR gas in at least a part of an operating range C.