Concentration calculation apparatus, concentration calculation system, and concentration calculation method

A concentration calculation method for calculating a downstream ammonia concentration of a purification system, the purification system comprising a selective reduction catalyst disposed in an exhaust pipe of an internal combustion engine so as to purify exhaust gas discharged from the internal combustion engine and containing NOx;

• a reducing agent supply section for supplying a reducing agent to the selective reduction catalyst;
• a filter section disposed in the exhaust pipe so as to remove particulate matter from the exhaust gas;
• a first sensor for detecting a downstream ammonia concentration which is the concentration of ammonia contained in the exhaust gas having passed through the selective reduction catalyst and the filter section;
• and a second sensor for detecting a specific component concentration which is the concentration of a specific component which is contained in the exhaust gas having passed through the selective reduction catalyst and the filter section and which differs from ammonia, the concentration calculation method calculating the downstream ammonia concentration based on the first sensor output value which is an output of the first sensor, wherein
• the second sensor has an output value which changes not only with a change in the specific component concentration but also with a change in the downstream ammonia concentration;
• the concentration calculation method comprises a correction value computation step of computing a correction value for correcting the first sensor output value or the downstream ammonia concentration, through use of the first sensor output value and the second sensor output value during a regeneration process for the filter section;
• the concentration calculation method calculates the downstream ammonia concentration based on the first sensor output value and the correction value;
• the first sensor is an ammonia sensor;
• and
• the second sensor is an NOx sensor.