Exhaust purification apparatus for engine

An exhaust purification apparatus for an engine comprising:

• an ammonia selective reduction NOx catalyst interposed in an exhaust passage of an engine for selectively reducing NOx contained in exhaust gas by using ammonia as a reducing agent;
• a urea-water supply device that supplies urea-water into exhaust gas existing upstream of the ammonia selective reduction-type NOx catalyst;
• and
• a control device includes an algorithm, the algorithm when implemented in the control device controls the urea-water supply device to supply the urea-water intermittently according to a predetermined supply duration time and a predetermined supply suspension time when the urea-water from the urea-water supply device is required for the purpose of providing ammonia to the ammonia selective reduction NOx catalyst,
• when the urea-water is supplied from the urea-water supply device, the control device controls the urea water supply device by executing the algorithm, which subsequently performs the following step calculates a first judgment value by adding an additional value corresponding to a formation amount of a solid formed from the urea-water and subtracting a subtraction value corresponding to an annihilation amount of the solid in each predetermined cycle, using a predetermined lower limit value as an initial value, determines that the supply duration time has lapsed when the first judgment value is increased up to a predetermined upper limit value, and then stops the urea-water supply from the urea-water supply device, and
• when the urea-water supply from the urea-water supply device is temporarily suspended, the control device executes the algorithm, which subsequently performs the following step calculates a second judgment value by subtracting the subtraction value in the each predetermined period, using the upper limit value as an initial value, determines that the supply suspension time has lapsed when the second judgment value is decreased to the lower limit value, and then starts the urea-water supply from the urea-water supply device.