Control device, exhaust gas purification system, and control method

A control device configured to direct an opening degree of a throttle valve disposed in an intake passage or an exhaust passage of an engine including a diesel oxidation catalyst (DOC) disposed in the exhaust passage and a diesel particulate filter (DPF) disposed downstream of the DOC in the exhaust passage, the control device comprising:

• an electronic control unit (ECU) including:
• a hysteresis occurring condition determination unit configured to determine whether a hysteresis occurring condition is met when the PDF is under forced regeneration, the hysteresis occurring condition being defined as a condition satisfying at least one of a first condition that a predetermined time elapses after an instructed opening degree of the throttle valve becomes equal to or smaller than a predetermined opening degree, or a second condition that a predetermined time elapses after an engine output of the engine becomes equal to or smaller than a predetermined output;
• and
• a hysteresis elimination execution unit configured to cause, if it is determined that the hysteresis occurring condition is met, the throttle valve to execute hysteresis elimination that involves for a moment increasing the opening degree of the throttle valve from the instructed opening degree and then returning the opening degree to the instructed opening degree during the forced regeneration of the DPF.