In an internal combustion engine, exhaust gas components are generally purified by an exhaust gas purifying catalyst provided in an exhaust passage. The exhaust gas purifying catalyst efficiently purifies the exhaust gas components when the air-fuel ratio of the air-fuel mixture that is burned in the internal combustion engine is within a predetermined range. Therefore, a sensor, which outputs a signal corresponding to the oxygen concentration in the exhaust gas, is provided in the exhaust passage. The actual air-fuel ratio of the air-fuel mixture is detected based on the signal output from the sensor. Then, air-fuel ratio control that feedback controls the fuel injection amount is executed such that the detected actual air-fuel ratio becomes equal to a target air-fuel ratio.
The feedback control is executed when a predetermined execution condition is met, for example, when the temperature of the internal combustion engine has become sufficiently high after starting operation of the internal combustion engine. When the execution condition is not met, prediction control is executed based on the operating condition of the internal combustion engine to adjust the fuel injection amount regardless of the output signal from the sensor.
Patent Document 1 discloses a device applied to an internal combustion engine provided with a variable valve lift mechanism, which changes the lift amount (more specifically, the maximum lift amount) of an intake valve. In such a device, the variable valve lift mechanism is controlled to make the actual lift amount equal to a desired lift amount. Accordingly, the intake valve is selectively opened and closed such that the lift amount of the intake valve becomes a value appropriate for the current operating condition of the engine.
The lift amount of the intake valve slightly differs among engines due to, for example, assembly errors. Thus, the passage area of a communication portion between an intake passage and a combustion chamber in the internal combustion engine slightly differs from the reference area. Also, when deposit collects on the intake valve as the engine is operated, the passage area is changed, increasing the difference between the passage area and the reference area. The difference of the passage area from the reference area reduces the adjusting accuracy of the intake amount, that is, the adjusting accuracy of the air-fuel ratio of the air-fuel mixture. In the internal combustion engine in which the above-mentioned feedback control is executed, the variation of the air-fuel ratio due to the difference between the passage area and the reference area is basically compensated for through the feedback control, thereby avoiding reduction in the adjusting accuracy of the air-fuel ratio.
However, in the internal combustion engine provided with the variable valve lift mechanism, when the passage area differs from the reference area, the variation of the air-fuel ratio due to the difference differs depending on the operating manner of the variable valve lift mechanism. Thus, when the operating manner of the variable valve lift mechanism is changed frequently so as to be appropriate for the current engine operating condition, the variation of the air-fuel ratio is also frequently changed in accordance with the change. When the feedback control is executed based simply on the signal from the sensor as in the device of Patent Document 1, it may not be possible to follow the frequent change in the operating manner of the variable valve lift mechanism. As a result, it may be impossible to accurately compensate for the variation of the air-fuel ratio accompanying the change in the operating manner of the variable valve lift mechanism.
In particular, since the feedback control is not executed when the execution condition is not met, if the fuel injection amount adjusted by the prediction control deviates from the amount corresponding to the passage area, the actual air-fuel ratio deviates from the target air-fuel ratio, causing the exhaust gas property to become constantly poor.
Such deterioration of the exhaust gas property can be suppressed by the following learning process. That is, during the execution of the feedback control, the constant deviation amount between the correction amount set in the lift amount region used when the execution condition is not met and the reference value is learned as a learned value. Then, the prediction control based on the learned value is executed when the feedback control is not executed.
However, if the learned value is unnecessarily reset due to, for example, replacement of the battery, the state in which the exhaust gas property is poor continues thereafter for a long period of time until the learned value is updated to an appropriate value.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-263015