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A NOx catalyst for selectively reducing NOx in exhaust gas by adsorbing ammonia is provided at an exhaust system of an engine, ammonia or urea water is supplied to the NOx catalyst by reducing agent supplying means, a consumption amount of ammonia adsorbed to the NOx catalyst is derived by consumpti

A NOx catalyst for selectively reducing NOx in exhaust gas by adsorbing ammonia is provided at an exhaust system of an engine, ammonia or urea water is supplied to the NOx catalyst by reducing agent supplying means, a consumption amount of ammonia adsorbed to the NOx catalyst is derived by consumption amount deriving means based on an exhaust amount of NOx exhausted from the engine detected or estimated by NOx exhaust amount deriving means and an actual NOx cleaning rate by the NOx catalyst derived by actual NOx cleaning rate deriving means, an actual adsorption amount of ammonia adsorbed to the NOx catalyst is derived in accordance with an addition amount of ammonia and the consumption amount of ammonia by adsorption amount deriving means and the reducing agent supplying means is controlled by controlling means in accordance with the actual adsorption amount.

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1. A NOx cleaning apparatus of an internal combustion engine, comprising:a NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent;a reducing agent supplying unit for supplying the reducing agent to the

1. A NOx cleaning apparatus of an internal combustion engine, comprising:a NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent;a reducing agent supplying unit for supplying the reducing agent to the NOx catalyst;a NOx exhaust amount deriving unit for one of detecting and estimating an exhaust amount of NOx exhausted from the internal combustion engine;an actual NOx cleaning rate deriving unit for deriving an actual NOx cleaning rate by the NOx catalyst;a consumption amount deriving unit for deriving a consumption amount of the reducing agent absorbed to the NOx catalyst based on the NOx exhaust amount detected or estimated by the NOx exhaust amount deriving unit and the actual NOx cleaning rate derived by the actual NOx cleaning rate deriving unit;an adsorption amount deriving unit for deriving an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with a sum of a difference of an addition amount of the reducing agent by the reducing agent supplying unit and the consumption amount of the reducing agent derived by the consumption amount deriving unit and a derived value of the adsorption amount at a preceding time by the adsorption amount deriving unit; anda controlling unit for controlling the reducing agent supplying unit in accordance with the actual adsorption amount derived by the adsorption amount deriving unit. 2. The NOx cleaning apparatus of an internal combustion engine according to claim 1, wherein the actual NOx cleaning rate deriving unit derives the actual NOx cleaning rate from a difference or a ratio of an upstream side NOx concentration detected by a preceding NOx sensor provided at the exhaust system on an upstream side of the NOx catalyst and a downstream side NOx concentration detected by a succeeding NOx sensor provided at the exhaust system on a downstream side of the NOx catalyst. 3. The NOx cleaning apparatus of an internal combustion engine according to claim 1, wherein the reducing agent supplied by the reducing agent supplying unit is ammonia or urea water. 4. The NOx cleaning apparatus of an internal combustion engine according to claim 3, wherein the controlling unit controls the reducing agent supplying unit such that the actual adsorption amount falls at a vicinity of a limit amount at which ammonia slip is not brought about at the NOx catalyst. 5. A NOx cleaning method of an internal combustion engine having,NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent, anda reducing agent supplying unit for supplying the reducing agent to the NOx catalyst, said method comprising:(a) a step of detecting or estimating an exhaust amount of NOx exhausted from the internal combustion engine;(b) a step of deriving an actual NOx cleaning rate by the NOx catalyst;(c) a step of deriving a consumption amount of the reducing agent adsorbed to the NOx catalyst based on the detected or estimated exhaust amount of NOx detected or estimated in the step (a) and the actual NOx cleaning rate derived in the step (b);(d) a step of deriving an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with a sum of a difference of an addition amount of the reducing agent by the reducing agent supplying unit and the consumption amount of the reducing agent derived in the step (c) and a derived value of the adsorption amount at a preceding time in the step (d); and(e) a step of controlling the reducing agent supplying unit in accordance with the actual adsorption amount derived in the step (d). 6. The NOx cleaning method of an internal combustion engine according to claim 5, wherein the step (b)_derives the actual NOx cleaning rate from a difference or a ratio of an upstream side NOx concentration detected in the exhaust system on an upstream side of the NOx catalyst and a downstream side NOx concentration detected in the exhaust system on a downstream side of the NOx catalyst. 7. The NOx cleaning method of an internal combustion engine according to claim 5, wherein the reducing agent supplied by the reducing agent supplying unit is ammonia or urea water. 8. The NOx cleaning method of an internal combustion engine according to claim 7, wherein the step (f) controls the reducing agent supplying unit such that the actual adsorption amount falls at a vicinity of a limit amount at which ammonia slip is not brought about at the NOx catalyst. 9. An NOx cleaning apparatus of an internal combustion engine, comprising:a NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent;a reducing agent supplying unit for supplying the reducing agent to the NOx catalyst;a NOx exhaust amount deriving unit for one of detecting and estimating an exhaust amount of NOx exhausted from the internal combustion engine;an actual NOx cleaning rate deriving unit for deriving an actual NOx cleaning rate by the NOx catalyst;a consumption amount deriving unit for deriving a consumption amount of the reducing agent absorbed to the NOx catalyst based on the NOx exhaust amount detected or estimated by the NOx exhaust amount deriving unit and the actual NOx cleaning rate derived by the actual NOx cleaning rate deriving unit;an adsorption amount deriving unit for deriving an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with the consumption amount of the reducing agent derived by the consumption amount deriving unit and an addition amount of the reducing agent by the reducing agent supplying unit;a target adsorption amount setting unit for setting a target adsorption amount of the reducing agent with respect to the NOx catalyst in accordance with one of a catalyst temperature of the NOx catalyst and a parameter correlated with the catalyst temperature; anda controlling unit for controlling the reducing agent supplying unit in accordance with a result of comparing the actual adsorption amount derived by the adsorption amount deriving unit and the target adsorption amount set by the target adsorption amount setting unit. 10. An NOx cleaning apparatus of an internal combustion engine having,a NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent, anda reducing agent supplying unit for supplying the reducing agent to the NOx catalyst, said method comprising:(a) a step of detecting or estimating an exhaust amount of NOx exhausted from the internal combustion engine;(b) a step of deriving an actual NOx cleaning rate by the NOx catalyst;(c) a step of deriving a consumption amount of the reducing agent adsorbed to the NOx catalyst based on the detected or estimated exhaust amount of NOx detected or estimated in the step (a) and the actual NOx cleaning rate derived in the step (b);(d) a step of deriving an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with a sum of a difference of an addition amount of the reducing agent by the reducing agent supplying unit and the consumption amount of the reducing agent derived in the step (c) and a derived value of the adsorption amount at a preceding time in the step (d);(e) a step_of setting a target adsorption amount of the reducing agent with respect to the NOx catalyst in accordance with a catalyst temperature of the NOx catalyst or a parameter correlated with the catalyst temperature; and(f) a step of controlling the reducing agent supplying unit in accordance with a result of comparing the actual adsorption amount derived in the step (d) and the target adsorption amount set in the step (e). 11. An NOx cleaning apparatus of an internal combustion engine, comprising:a NOx catalyst provided at an exhaust syst em of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent;a reducing agent supplying unit for supplying the reducing agent to the NOx catalyst;a NOx exhaust amount deriving unit for one of detecting and estimating an exhaust amount of NOx exhausted from the internal combustion engine;an actual NOx cleaning rate deriving unit for deriving an actual NOx cleaning rate by the NOx catalyst;a consumption amount deriving unit for deriving a consumption amount of the reducing agent absorbed to the NOx catalyst based on the NOx exhaust amount detected or estimated by the NOx exhaust amount deriving unit and the actual NOx cleaning rate derived by the actual NOx cleaning rate deriving unit;an adsorption amount deriving unit for deriving an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with the consumption amount of the reducing agent derived by the consumption amount deriving unit and an addition amount of the reducing agent by the reducing agent supplying unit;a reference NOx cleaning rate deriving unit for deriving a reference NOx cleaning rate based on the actual adsorption amount derived by the adsorption amount deriving unit and one of a catalyst temperature of the NOx catalyst and a parameter correlated with the catalyst temperature; anda correcting unit for correcting the actual adsorption amount derived by the adsorption amount deriving unit in accordance with a result of comparing the reference NOx cleaning rate derived by the reference NOx cleaning rate deriving unit and the actual NOx cleaning rate derived by the actual NOx cleaning rate deriving unit;a controlling unit for controlling the reducing agent supplying unit in accordance with the actual adsorption amount corrected by the correcting unit. 12. An NOx cleaning method of an internal combustion engine having,a NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent, anda reducing agent supplying unit for supplying the reducing agent to the NOx catalyst, said method comprising:(a) a step of detecting or estimating an exhaust amount of NOx exhausted from the internal combustion engine;(b) a step of deriving an actual NOx cleaning rate by the NOx catalyst;(c) a step of deriving a consumption amount of the reducing agent adsorbed to the NOx catalyst based on the detected or estimated exhaust amount of NOx detected or estimated in the step (a) and the actual NOx cleaning rate derived in the step (b);(d) a step of deriving an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with a sum of a difference of an addition amount of the reducing agent by the reducing agent supplying unit and the consumption amount of the reducing agent derived in the step (c) and a derived value of the adsorption amount at a preceding time in the step (d);(e) a step of deriving a reference NOx cleaning rate based on the actual adsorption amount derived in the step (d) and a catalyst temperature of the NOx catalyst or a parameter correlated with the catalyst temperature; and(f) a step of correcting the actual adsorption amount derived in the step (d) in accordance with a result of comparing the reference NOx cleaning rate derived in the step (e) and the actual NOx cleaning rate derived in the step (b);(g) a step of controlling the reducing agent supplying unit in accordance with the actual adsorption amount corrected by the step (f).