Failure determination device for emission control apparatus of internal combustion engine

A failure determination device for an emission control apparatus of an internal combustion engine, the failure determination device comprising:

• a first supply valve that is provided in an exhaust conduit of the internal combustion engine to supply a reducing agent into the exhaust conduit;
• a first selective reduction NOx catalyst that is provided downstream of the first supply valve in the exhaust conduit to selectively reduce NOx with the reducing agent adsorbed to the first selective reduction NOx catalyst;
• a second supply valve that is provided downstream of the first selective reduction NOx catalyst in the exhaust conduit to supply the reducing agent into the exhaust conduit;
• a second selective reduction NOx catalyst that is provided downstream of the second supply valve in the exhaust conduit to reduce NOx with the reducing agent adsorbed to the second selective reduction NOx catalyst;
• a NOx sensor that is configured to detect a NOx concentration in an exhaust emission flowing out of the second selective reduction NOx catalyst;
• and
• a controller that is configured to determine a supply amount of the reducing agent based on an amount of NOx flowing into the first selective reduction NOx catalyst and give an identical instruction to operate the first supply valve and the second supply valve, wherein
• the controller gives an instruction to the first supply valve and the second supply valve such as to make a supply amount of the reducing agent from the first supply valve and the second supply valve larger than the supply amount of the reducing agent determined based on the amount of NOx flowing into the first selective reduction NOx catalyst, and determines whether the first supply valve is abnormal or the second supply valve is abnormal, based on a NOx concentration detected by the NOx sensor after elapse of a first specified time duration since a certain instruction time point when the instruction is given.