In a case where an internal combustion engine is executing an all-cylinder operation to operate all cylinders, an air-fuel ratio estimation part of an ECU 1 estimates an air-fuel ratio of each of the cylinders by using a first observer. On the other hand, in a case where the internal combustion engine is executing a cylinder-cut operation to rest a part of the cylinders and to operate other of the cylinders, the air-fuel ratio estimation part does not estimate the air-fuel ratio of each of the cylinders by using the first observer.

In a case where an internal combustion engine is executing an all-cylinder operation to operate all cylinders, an air-fuel ratio estimation part of an ECU 1 estimates an air-fuel ratio of each of the cylinders by using a first observer. On the other hand, in a case where the internal combustion engine is executing a cylinder-cut operation to rest a part of the cylinders and to operate other of the cylinders, the air-fuel ratio estimation part does not estimate the air-fuel ratio of each of the cylinders by using the first observer.

An internal combustion engine includes cylinders that are divided into a first cylinder group and a second cylinder group, a cylinder reduction mechanism that holds intake valves and exhaust valves of the first cylinder group in closed states so as to establish a reduced-cylinder state. When the engine is stopped in the reduced-cylinder state, the electronic control unit provided in the engine starts the engine by ignition, by executing fuel injection and ignition in an expansion-stroke cylinder. When the first cylinder group includes an exhaust-stroke cylinder, the engine is started by ignition through fuel injection and ignition in the expansion-stroke cylinder, after a piston is moved in a reverse direction through fuel injection and ignition in the exhaust-stroke cylinder. When the first cylinder group does not include the exhaust-stroke cylinder, the engine is started by ignition, through fuel injection and ignition in the expansion-stroke cylinder and an intake-stroke cylinder.

An internal combustion engine includes cylinders that are divided into a first cylinder group and a second cylinder group, a cylinder reduction mechanism that holds intake valves and exhaust valves of the first cylinder group in closed states so as to establish a reduced-cylinder state. When the engine is stopped in the reduced-cylinder state, the electronic control unit provided in the engine starts the engine by ignition, by executing fuel injection and ignition in an expansion-stroke cylinder. When the first cylinder group includes an exhaust-stroke cylinder, the engine is started by ignition through fuel injection and ignition in the expansion-stroke cylinder, after a piston is moved in a reverse direction through fuel injection and ignition in the exhaust-stroke cylinder. When the first cylinder group does not include the exhaust-stroke cylinder, the engine is started by ignition, through fuel injection and ignition in the expansion-stroke cylinder and an intake-stroke cylinder.

Methods and systems are provided for learning fuel injector error for cylinder groups during a deceleration fuel shut-off (DFSO), where all cylinders of an engine are deactivated, sequentially firing each cylinder of a cylinder group, each cylinder fueled via consecutive first and second fuel pulses of differing fuel pulse width from an injector. Based on a lambda deviation between the first and second pulses, a fuel error for the injector and an air-fuel ratio imbalance for each cylinder is learned. Alternatively or additionally, a difference in crankshaft acceleration between the first and second pulses relative to the expected deviation may be used to learn torque error, and adjust fuel injector error and air-ratio imbalance for each cylinder.

Methods and systems are provided for learning fuel injector error for cylinder groups during a deceleration fuel shut-off (DFSO), where all cylinders of an engine are deactivated, sequentially firing each cylinder of a cylinder group, each cylinder fueled via consecutive first and second fuel pulses of differing fuel pulse width from an injector. Based on a lambda deviation between the first and second pulses, a fuel error for the injector and an air-fuel ratio imbalance for each cylinder is learned. Alternatively or additionally, a difference in crankshaft acceleration between the first and second pulses relative to the expected deviation may be used to learn torque error, and adjust fuel injector error and air-ratio imbalance for each cylinder.

Systems and methods for operating an engine with deactivating and non-deactivating valves are presented. In one example, a position of one or more volumetric efficiency control devices is changed in response to a request to deactivate one or more engine cylinders while at the same time the engine central throttle is adjusted. Spark timing may also be adjusted if engine air flow deviates from a desired engine air flow.

Systems and methods for operating an engine with deactivating and non-deactivating valves are presented. In one example, a position of one or more volumetric efficiency control devices is changed in response to a request to deactivate one or more engine cylinders while at the same time the engine central throttle is adjusted. Spark timing may also be adjusted if engine air flow deviates from a desired engine air flow.

Systems and methods for operating an engine with a plurality of deactivating valves for deactivating engine cylinders are presented. In one example, a plurality of different cylinder operating modes may be available to operate the engine. Engine fuel consumption in each of the plurality of different cylinder operating modes is estimated and the engine may be operated in the cylinder mode that provides greatest fuel efficiency.

Systems and methods for operating an engine with a plurality of deactivating valves for deactivating engine cylinders are presented. In one example, a plurality of different cylinder operating modes may be available to operate the engine. Engine fuel consumption in each of the plurality of different cylinder operating modes is estimated and the engine may be operated in the cylinder mode that provides greatest fuel efficiency.