Exhaust monitoring apparatus and exhaust monitoring method

An exhaust monitoring apparatus in an exhaust system, the exhaust system including an exhaust purification device disposed in an exhaust pipe of an engine and configured to remove nitrogen oxides contained in exhaust gas, and a nitrogen oxide sensor disposed downstream of the exhaust purification device in the exhaust pipe and configured to measure a concentration of nitrogen oxides contained in exhaust gas that has passed through the exhaust purification device, wherein

• the exhaust monitoring apparatus is configured to acquire a downstream concentration that is an output value of the nitrogen oxide sensor, and
• the exhaust monitoring apparatus comprises processing circuitry configured to:
• when the downstream concentration is less than or equal to an upper limit value calculated based on a purification efficiency of the exhaust purification device, output the downstream concentration as a value indicating the concentration of the nitrogen oxides contained in the exhaust gas that has passed through the exhaust purification device;
• and
• when the downstream concentration is greater than the upper limit value, output a processed concentration as the value indicating the concentration of the nitrogen oxides contained in the exhaust gas that has passed through the exhaust purification device, the processed concentration being obtained by replacing the downstream concentration with the upper limit value.

An exhaust monitoring method for an exhaust system, the exhaust system including an exhaust purification device disposed in an exhaust pipe of an engine and configured to remove nitrogen oxides contained in exhaust gas, and a nitrogen oxide sensor disposed downstream of the exhaust purification device in the exhaust pipe and configured to measure a concentration of nitrogen oxides contained in exhaust gas that has passed through the exhaust purification device, the exhaust monitoring method comprising:

• acquiring an upstream concentration that is a concentration of nitrogen oxides contained in exhaust gas flowing into the exhaust purification device;
• acquiring a downstream concentration that is an output value of the nitrogen oxide sensor;
• when the downstream concentration is less than or equal to an upper limit value calculated based on the upstream concentration, outputting the downstream concentration as a value indicating the concentration of the nitrogen oxides contained in the exhaust gas that has passed through the exhaust purification device;
• and
• when the downstream concentration is greater than the upper limit value, outputting a processed concentration as the value indicating the concentration of the nitrogen oxides contained in the exhaust gas that has passed through the exhaust purification device, the processed concentration being obtained by replacing the downstream concentration with the upper limit value.