Concentration determination device and method

A concentration determination device of a purification system,

• the purification system comprising:
• a selective catalytic reduction catalyst arranged in an exhaust pipe of an internal combustion engine to purify NOx in exhaust gas of the internal combustion engine;
• a reducing agent supply unit arranged to supply urea or ammonia as a reducing agent to the selective catalytic reduction catalyst;
• an ammonia sensor arranged to detect, as a downstream ammonia concentration, an ammonia concentration of the exhaust gas flowing out of the selective catalytic reduction catalyst;
• and
• a NOx sensor arranged to detect, as a downstream NOx concentration, a NOx concentration of the exhaust gas flowing out of the selective catalytic reduction catalyst,
• the concentration determination device being configured to determine a value of the downstream ammonia concentration based on a detection result of the ammonia sensor, the concentration determination device comprising:
• a supply control section that controls the reducing agent supply unit to supply the reducing agent to the selective catalytic reduction catalyst in a fuel-cut state where fuel supply to the internal combustion engine is stopped;
• a supply correction section that corrects the determined value of the downstream ammonia concentration based on a detection result of the NOx sensor and an oxygen concentration of the exhaust gas after the supply of the reducing agent to the selective catalytic reduction catalyst with the control of the reducing agent supply unit by the supply control section.