Methods and systems are provided to determine air-fuel imbalance of cylinders in a variable displacement engine. In one example, the method may include during a cylinder deactivation event, sequentially deactivating each cylinder of a cylinder group including two or more cylinders and estimating a lambda deviation for each cylinder following the sequential deactivation of each cylinder of the cylinder group; and learning an air error for each cylinder based on the estimated lambda deviation.

An exhaust purification system comprising an exhaust purification catalyst, a downstream side air-fuel ratio sensor, and a control device performing air-fuel ratio control for controlling an air-fuel ratio of exhaust gas and abnormality diagnosis control for diagnosing the downstream side air-fuel ratio sensor. In the air-fuel ratio control, the control device alternately and repeatedly switches the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst between a rich air-fuel ratio and a lean air-fuel ratio. In the abnormality diagnosis control, the control device judges that the downstream side air-fuel ratio sensor has become abnormal when the air-fuel ratio of the exhaust gas is made the rich air-fuel ratio by the air-fuel control and the output air-fuel ratio of the downstream side air-fuel ratio sensor changes from an air-fuel ratio richer than a predetermined lean judged air-fuel ratio to an lean air-fuel ratio.

An abnormality diagnosis system of an air-fuel ratio sensor acquires a blowby gas flow ratio showing a ratio of the flow of blowby gas to the flow of gas to a combustion chamber and an output current of an air-fuel ratio sensor during fuel cut control in which an internal combustion engine stops the feed of fuel to the combustion chamber and at a plurality of points of time of different flows of blowby gas passing through a blowby gas passage and flowing to the downstream side of a throttle valve in the intake passage, calculate an output current of the air-fuel ratio sensor corresponding to a blowby gas flow ratio smaller than the blowby gas flow ratios acquired at the plurality of points of time, based on the acquired blowby gas flow ratio and output current, and judge the air-fuel ratio sensor for abnormality based on the calculated output current.

A method is disclosed for detecting a malfunction of an oxygen sensor in the exhaust gas system of an internal combustion engine having several cylinders. The cylinders are operated at the same air-fuel ratio and the resultant first output signal of the oxygen sensor is monitored. The cylinders are operated at varying air-fuel ratios and the resultant second output signal of the oxygen sensor is monitored. The first and second output signals are compared to determine whether the oxygen sensor has malfunctioned.

An abnormality determination device includes an injection instructor that realizes a rich state of an air-fuel ratio in an exhaust gas by sending an instruction of (i) stopping an application of bias to a plus terminal and (ii) adjusting a fuel injection amount from an injector, and an abnormality determiner that distinctively determines abnormality of, i.e., in terms of which one of, the plus terminal or a minus terminal having a sky failure or a ground failure by detecting an electromotive force of an air-fuel ratio sensor when the air-fuel ratio in the exhaust gas is in the rich state according to the instruction of the injection instructor.

To keep medium purification efficiency at a high level and prevent deterioration of emission performance. An aspect of the present invention includes: a downstream equivalence ratio calculation unit that calculates a catalyst downstream exhaust gas equivalence ratio by using a catalyst statistical model that receives at least a detection value of an air-fuel ratio sensor on an upstream side of a catalyst and outputs a catalyst downstream exhaust gas equivalence ratio; an oxygen output calculation unit that calculates an output value of an oxygen sensor by using an oxygen sensor statistical model that receives the catalyst downstream exhaust gas equivalence ratio and outputs an output value of the oxygen sensor on the downstream side of the catalyst; a downstream equivalence ratio correction unit that corrects the catalyst downstream exhaust gas equivalence ratio calculated by the downstream equivalence ratio calculation unit based on a calculation result of the oxygen output calculation unit and the detection value of the oxygen sensor; and an air-fuel ratio control unit that controls an air-fuel ratio of an air-fuel mixture of an internal combustion engine based on the corrected catalyst downstream exhaust gas equivalence ratio and air-fuel ratio target value.

A method for planning a vehicle diagnosis in a vehicle includes: estimation of an operating characteristic of the vehicle on a route to be traveled by the vehicle; and planning of the vehicle diagnosis based on a probability that the estimated operating characteristic of the vehicle will correspond to an operating characteristic suitable for the vehicle diagnosis.

In automatic stopping of the internal combustion engine, an electronic control unit controls a heater so that an element temperature of the air-fuel ratio sensor becomes a first temperature. The first temperature is a temperature that is below an activation temperature range in which the air-fuel ratio sensor is activated, and is also below a desorption temperature range in which HC components adsorbed on the air-fuel ratio sensor desorb therefrom. In a case where, after the element temperature of the air-fuel ratio sensor becomes the first temperature, it is estimated that the amount of HC components adsorbed on the air-fuel ratio sensor increases to exceed a first predetermined amount, the electronic control unit controls the heater so that the element temperature of the air-fuel ratio sensor becomes a second temperature. The second temperature is a temperature included in the desorption temperature range.

A signal processing apparatus for a gas sensor is applied to a gas sensor that is disposed on an exhaust passage of an engine to detect a concentration of a specific component in exhaust gas flowing through the exhaust passage. The signal processing apparatus includes a filtering means that attenuates exhaust pulsation noise included in a detection signal of the gas sensor, and a filter characteristic setting means that variably sets filter characteristics of the filtering means based on engine speed.

A control unit (6) estimates an output value of a PM sensor (S2) located at a downstream side of a DPF used as a reference filter, and detects whether the estimated output value exceeds a predetermined value (S3). When the estimated output value exceeds the predetermined value (YES in S3), the control unit detects an output value of the PM sensor (S4), and a heater heats the PM sensor (S5). The control unit detects an output value of the PM sensor (S6) after the PM sensor is heated, and calculates a change ratio of the output values of the PM sensor before and after heating (S7). The control unit estimates an average particle size of PM based on the calculated change ratio (S8), and detects whether the DPF has failed based on a comparison result of a corrected output value of the PM sensor with a threshold value.