Technical solutions are described for an emissions control system for a motor vehicle including an internal combustion engine. The emissions control system includes a selective catalytic reduction (SCR) device, an NOx sensor, and a controller for ammonia slip detection. The ammonia slip detection includes comparing an NOx measurement from the NOx sensor with a predicted NOx value. In response to the NOx measurement exceeding the predicted NOx value by a threshold value, the threshold value being calibrated to a first predetermined value, the threshold value is calibrated to a second predetermined value, a timer is initiated to a predetermined duration, and during the predetermined duration of the timer, in response to a second NOx measurement from the NOx sensor exceeding the predicted NOx value by the threshold value set to the second predetermined value, a reductant dosing rate of the SCR device is adapted according to the second predetermined value.

Technical solutions are described for an emissions control system for a motor vehicle including an internal combustion engine. The emissions control system includes a selective catalytic reduction (SCR) device, an NOx sensor, and a controller for ammonia slip detection. The ammonia slip detection includes comparing an NOx measurement from the NOx sensor with a predicted NOx value. In response to the NOx measurement exceeding the predicted NOx value by a threshold value, the threshold value being calibrated to a first predetermined value, the threshold value is calibrated to a second predetermined value, a timer is initiated to a predetermined duration, and during the predetermined duration of the timer, in response to a second NOx measurement from the NOx sensor exceeding the predicted NOx value by the threshold value set to the second predetermined value, a reductant dosing rate of the SCR device is adapted according to the second predetermined value.

An amount of ammonia flowing out from an SCR catalyst is reduced at the time of carrying out rich spike control to reduce an amount of NOx stored in an NSR catalyst. The NSR catalyst and the SCR catalyst are arranged in order in an exhaust passage of an internal combustion engine which is able to be operated at a lean air fuel ratio, wherein a target air fuel ratio of exhaust gas flowing into the NSR catalyst during the rich spike control is made higher within the range of a rich air fuel ratio, in the case where the outflow of ammonia from the SCR catalyst in accompany with the rich spike control is estimated or detected, than in the case where it is not estimated or detected.

A method for evaluating a sensor signal of an exhaust NOx sensor, which is disposed downstream of a three-way catalytic converter in an exhaust system of a spark ignition internal combustion engine. An ammonia factor is modeled downstream of the three-way catalytic converter using an ammonia formation model. A NOx emission is modeled in the exhaust system downstream of the three-way catalytic converter using a NOx model. The modeled ammonia emissions and the modeled NOx emission are separated by a separation algorithm using the sensor signal of the exhaust NOx sensor. The separation algorithm provides quantitative information about the tailpipe ammonia emissions and the tailpipe NOx emissions of the spark ignition internal combustion engine. An engine control unit and an internal combustion engine for carrying out such a method are also provided.

An abnormality diagnosis apparatus includes: an exhaust gas purification apparatus arranged in an exhaust passage of an internal combustion engine and including a selective catalytic reduction catalyst; a supply apparatus supplying an additive such as ammonia to the exhaust gas purification apparatus; an EGR apparatus recirculating a part of exhaust gas from the exhaust passage at a downstream side of a position of supplying the additive to an intake passage; calculating means for calculating a NO.sub.x inflow amount into the exhaust gas purification apparatus using a parameter indicating an operating state of the internal combustion engine; diagnosing means for diagnosing an abnormality in the exhaust gas purification apparatus using the calculated NO.sub.x inflow amount as a parameter; and correcting means for, when a part of the exhaust gas is recirculated, increasingly correcting the calculated NO.sub.x inflow amount in accordance with an amount of the additive to be recirculated together with exhaust gas.

An amount of ammonia flowing out from an SCR catalyst is reduced at the time of carrying out rich spike control to reduce an amount of NOx stored in an NSR catalyst. The NSR catalyst and the SCR catalyst are arranged in order in an exhaust passage of an internal combustion engine which is able to be operated at a lean air fuel ratio, wherein a target air fuel ratio of exhaust gas flowing into the NSR catalyst during the rich spike control is made higher within the range of a rich air fuel ratio, in the case where the outflow of ammonia from the SCR catalyst in accompany with the rich spike control is estimated or detected, than in the case where it is not estimated or detected.