Abstract:
The purpose of the present invention is to provide an exhaust purification device that is capable of adding an appropriate amount of urea solution without the need to directly measure the NOx emissions amount using an NOx measuring means. The exhaust purification device uses urea solution as a reducing agent for reducing nitrogen oxides within exhaust, and is provided with a temperature sensor, a humidity sensor, and a control device. A map is stored in the control device, and in said map the real NOx emissions amount for each rotation speed and each load of an engine occurring at a predetermined air temperature and a predetermined absolute humidity are converted using a correction formula into a reference NOx emissions amount for each rotation speed and each load of the engine while in a standard state. Using the map, the control device calculates a reference NOx emissions amount that corresponds to the rotation speed detected by a rotation speed sensor and to the load detected by a load sensor, uses the correction formula to convert the reference NOx emissions amount into the real NOx emissions amount occurring at an air temperature and an absolute humidity, and calculates a urea solution addition amount.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an exhaust purification apparatus. Especially, the present invention relates to a urea solution injection device for a ship. 
       BACKGROUND ART 
       [0002]    Conventionally, an exhaust purification apparatus is known in which a selective reducing type NOx catalyst (SCR catalyst) is arranged inside an exhaust pipe and NOx (nitrogen oxide) is reduced into nitrogen and water with ammonia as a reducing agent for decreasing the NOx in exhaust gas discharged from an internal combustion engine. 
         [0003]    A urea solution is supplied from a urea solution injection nozzle arranged inside an exhaust pipe to exhaust gas, and ammonia is generated from the urea solution by heat of the exhaust gas so as to reduce NOx into nitrogen and water. 
         [0004]    In the exhaust purification apparatus, when an addition amount of the urea solution against a NOx discharge amount is insufficient, NOx cannot be decreased to a target purification rate (denitration shortage). When the addition amount of the urea solution against the NOx discharge amount is excessive, NOx in the exhaust gas is decreased more than the target purification rate (over-denitration) and ammonia slip that ammonia exceeding a theoretical equivalent is discharged to the atmosphere occurs. Then, there is a configuration that a NOx sensor is provided in an exhaust pipe and control is performed so as to add the urea solution of a suitable amount against the NOx discharge amount. For example, it is like the Patent Literature 1. 
         [0005]    However, in the NOx sensor of the exhaust purification apparatus described in the Patent Literature 1, accurate measurement of the NOx discharge amount may not be performed because of interference of ammonia. It is disadvantageous that the NOx sensor in the internal combustion engine which is operated for 24 hours such as an engine for a ship requires frequent maintenance work because of a short life of the NOx sensor. 
       PRIOR ART REFERENCE 
     Patent Literature 
       [0006]    Patent Literature 1: the Japanese Patent Laid Open Gazette 2008-157136 
       DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
       [0007]    The present invention is provided in consideration of the problems as mentioned above, and the purpose of the invention is to provide an exhaust purification apparatus which can add a urea solution of a suitable amount without measuring directly a NOx discharge amount with a NOx measurement means. 
       Means for Solving the Problems 
       [0008]    The problems to be solved by the present invention have been described above, and subsequently, the means of solving the problems will be described below. 
         [0009]    According to the present invention, an urea solution injection device of an exhaust purification device in which an urea solution is added as a reducing agent to exhaust gas of an internal combustion engine so as to reduce nitrogen oxide in the exhaust gas, includes a temperature sensor detecting a temperature of atmosphere, a humidity sensor detecting an absolute humidity or a relative humidity of the atmosphere, and a control device calculating an addition amount of the urea solution. A map, which converts an actual NOx discharge amount of the internal combustion engine driven with each rotation speed and each load under predetermined temperature and absolute humidity of the atmosphere into the standard NOx discharge amount of the internal combustion engine with each rotation speed and each load under standard conditions with a correction formula is stored in the control device. A rotation speed detection means detecting the rotation speed of the internal combustion engine and a load detection means detecting the load of the internal combustion engine are connected to the control device. The standard NOx discharge amount corresponding to the rotation speed detected by the rotation speed detection means and the load detected by the load detection means is calculated with the map, the standard NOx discharge amount is converted into the actual NOx discharge amount of the rotation speed and the load under the temperature of the atmosphere detected by the temperature sensor and the absolute humidity of the atmosphere detected by the humidity sensor with the correction formula by inverse operation, and the addition amount of the urea solution is calculated based on the actual NOx discharge amount. 
         [0010]    According to the present invention, the addition amount is calculated in consideration of a target purification rate and a concentration of the urea solution. 
       Effect of the Invention 
       [0011]    The present invention brings the following effects. 
         [0012]    According to the present invention, the NOx discharge amount based on the characteristic of the internal combustion engine can be calculated while considering the temperature of the atmosphere and the absolute humidity of the atmosphere which influence the NOx discharge amount greatly. Accordingly, the urea solution of the suitable amount can be added without measuring directly the NOx discharge amount with a NOx sensor. 
         [0013]    According to the present invention, the addition amount of the urea solution can be adjusted corresponding to the operating condition. Accordingly, the urea solution or ammonia of the suitable amount can be added without measuring directly the NOx discharge amount with a NOx sensor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a drawing of an exhaust purification apparatus according to an embodiment of the present invention. 
           [0015]      FIG. 2  is a drawing partially in section of a urea solution injection nozzle of the exhaust purification apparatus according to the embodiment of the present invention. 
           [0016]      FIG. 3  is a flow chart of control processes of an addition amount of a urea solution according to the first embodiment of the present invention. 
           [0017]      FIG. 4  is a graph of a relation between an actual NOx discharge amount and the addition amount of the urea solution. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    An explanation will be given on an exhaust purification apparatus  1  according to an embodiment of the present invention referring to  FIGS. 1 and 2 . In this embodiment, an “upstream side” means an upstream side in a flow direction of fluid, and a “downstream side” means a downstream side in the flow direction of the fluid. The exhaust apparatus is not limited to this embodiment and may alternatively be an air-less type apparatus which does not use pressurized air. 
         [0019]    As shown in  FIG. 1 , the exhaust purification apparatus  1  purifies exhaust gas discharged from an engine  20 . The exhaust purification apparatus  1  is provided in an exhaust pipe  21  of the engine  20 . The exhaust purification apparatus  1  has a urea solution injection nozzle  2 , a pressurized air supply pump (compressor)  6 , a pressurized air valve  8 , a urea solution supply pump  9 , a switching valve  11 , a control device  14 , a temperature sensor  12 , a humidity sensor  13 , a first supply flow path  15 , a second supply flow path  16 , a NOx catalyst  19  and the like. 
         [0020]    The urea solution injection nozzle  2  supplies a urea solution to an inside of the exhaust pipe  21 . The urea solution injection nozzle  2  includes a tubular member, and one of sides (lower side) thereof is inserted into the inside of the exhaust pipe  21  from the outside. The urea solution injection nozzle  2  has a double pipe  3 , a liquid nozzle  4 , an air nozzle  5  and the like (see  FIG. 2 ). 
         [0021]    The pressurized air supply pump (compressor)  6  supplies pressurized air. The pressurized air supply pump  6  pressurizes (compresses) air and supplies the air. The pressurized air supply pump  6  supplies the air to an air tank  7  when a pressure of the air tank  7  becomes lower than a predetermined pressure, and stops when the pressure of the air tank  7  reaches the predetermined pressure. In this embodiment, the pressurized air supply pump  6  is not limited and may be a member which can maintain the pressure of the air tank  7 . 
         [0022]    The pressurized air valve  8  opens and closes a flow path of the pressurized air. The pressurized air valve  8  is provided in the second supply flow path  16 . The pressurized air valve  8  includes an electromagnetic valve and a solenoid thereof is connected to the control device  14 . The pressurized air valve  8  can be switched to a position V and a position W by sliding a spool. When the pressurized air valve  8  is at the position V, the second supply flow path  16  is closed. Then, the pressurized air is not supplied to the urea solution injection nozzle  2 . When the pressurized air valve  8  is at the position W, the second supply flow path  16  is opened. Then, the pressurized air is supplied to the urea solution injection nozzle  2 . The pressurized air valve  8  is not limited thereto and may alternatively be held at the position V or the position W by a driving motor or the like. 
         [0023]    The urea solution supply pump  9  supplies a urea solution. The urea solution supply pump  9  is provided in the first supply flow path  15 . The urea solution supply pump  9  supplies the urea solution in a urea solution tank  10  via the first supply flow path  15  to the urea solution injection nozzle  2  at a predetermined flow rate. In this embodiment, the urea solution supply pump  9  is not limited and may be a member which can supply the urea solution at the predetermined flow rate. 
         [0024]    The switching valve  11  switches a flow path of the urea solution. The switching valve  11  is provided at the downstream side of the urea solution supply pump  9  in the first supply flow path  15 . A drain pot  17  is connected via a flow path  15   a  to the switching valve  11 . The switching valve  11  includes an electromagnetic valve and a solenoid thereof is connected to the control device  14 . The switching valve  11  can be switched to a position X and a position Y by sliding a spool. 
         [0025]    When the switching valve  11  is at the position X, the first supply flow path  15  is closed and the urea solution injection nozzle  2  is communicated with the drain pot  17 . Then, the urea solution is not supplied to the urea solution injection nozzle  2 , and the urea solution in the first supply flow path  15  and the urea solution injection nozzle  2  at the downstream side of the switching valve  11  is discharged to the drain pot  17 . 
         [0026]    When the switching valve  11  is at the position Y, the first supply flow path  15  is opened. Then, the urea solution is supplied to the urea solution injection nozzle  2 . 
         [0027]    The temperature sensor  12  detects a temperature T of the atmosphere. The temperature sensor  12  is arranged at a position such as an engine room of a ship at which the temperature T of the atmosphere sucked by the engine  20  can be detected. This embodiment is not limited thereto, and any means is available if it can detect the temperature T of the atmosphere and transmit a detection signal of the temperature to the control device  14 . 
         [0028]    The humidity sensor  13  detects an absolute humidity H of the atmosphere. The humidity sensor  13  is arranged at a position such as the engine room of the ship at which the absolute humidity H of the atmosphere sucked by the engine  20  can be detected. This embodiment is not limited thereto, and any means is available if it can detect the absolute humidity H and transmit a detection signal of the absolute humidity to the control device  14 . For example, it may alternatively be configured that a relative humidity is detected and a detection signal thereof is transmitted to the control device  14  so as to calculate the absolute humidity H based on the temperature T of the atmosphere. 
         [0029]    The control device  14  controls the urea solution supply pump  9 , the switching valve  11 , the pressurized air valve  8  and the like. Various programs and data for controlling the urea solution supply pump  9 , the switching valve  11 , the pressurized air valve  8  and the like are stored in the control device  14 . The control device  14  may be configured by connecting a CPU, a ROM, a RAM, a HDD and the like by a bus, or may alternatively be configured by a one-chip LSI or the like. The control device  14  may be configured integrally with an ECU  22  which controls the engine  20 . 
         [0030]    A map M is stored in the control device  14 . The map M converts an actual NOx discharge amount, which is an amount of NOx included in exhaust gas of the engine  20  driven with each rotation speed and each load under predetermined temperature and absolute humidity of the atmosphere, into a standard NOx discharge amount Ns, which is an amount of NOx with each rotation speed and each load under standard conditions (for example, 10.71 g/kg 25° C.), with a correction formula F which is known or a measured formula. Concretely, the actual NOx discharge amount of the engine  20  with optional rotation speed and load under the predetermined temperature and absolute humidity of the atmosphere is measured under each driving condition. Then, the actual NOx discharge amount is converted into the standard NOx discharge amount Ns under the standard conditions with the correction formula F based on temperature and absolute humidity of the atmosphere at the time of the measurement. The map M of the standard NOx discharge amount Ns made as the above is stored. In addition, the correction formula F is stored in the control device  14 . 
         [0031]    The control device  14  is connected to the solenoid of the pressurized air valve  8  and can control opening and closing of the pressurized air valve  8 . 
         [0032]    The control device  14  is connected to a driving motor of the urea solution supply pump  9  and can control an operation state of the urea solution supply pump  9 . Namely, by controlling the operation state of the urea solution supply pump  9 , the control device  14  can change optionally an addition amount Q of the urea solution added to the exhaust gas. The control device  14  is connected to the solenoid of the switching valve  11  and can control opening and closing of the switching valve  11 . 
         [0033]    The control device  14  is connected to the temperature sensor  12  and can obtain a signal of the temperature T of the atmosphere detected by the temperature sensor  12 . The control device  14  is connected to the humidity sensor  13  and can obtain a signal of the absolute humidity H of the atmosphere detected by the humidity sensor  13 . A relative humidity can be detected and a detection signal thereof can be transmitted to the control device  14  so as to calculate the absolute humidity with the control device  14  using the temperature T of the atmosphere. 
         [0034]    The control device  14  is connected to the ECU  22  and can obtain various kinds of information about the engine  20  obtained by the ECU  22 . Concretely, the control device  14  can obtain a rotation speed R of the engine  20 , which is detected by a rotation speed sensor  20   a  of the engine  20 , via the ECU  22 . The control device  14  can obtain an output of a dynamo  23 , driven by the engine  20 , detected by a load sensor  23   a  of the dynamo  23  as a load L of the engine  20  via the ECU  22 . The load L is not limited to the detection value of the load sensor  23   a  and may alternatively be calculated from a rack position, a fuel injection amount or an actual rotation speed. The control device  14  may obtain various kinds of information about the engine  20  directly not via the ECU  22 . 
         [0035]    The control device  14  is connected to an input device (not shown) and can obtain a signal about a target purification rate inputted from the input device and concentration of the urea solution. Alternatively, information about the target purification rate and the concentration of the urea solution can be inputted and defined previously. 
         [0036]    The NOx catalyst  19  promotes deoxidization reaction of NOx. The NOx catalyst  19  is arranged inside the exhaust pipe  21  and at the downstream side of the urea solution injection nozzle  2 . The NOx catalyst  19  is configured honeycomb like and promotes reaction that ammonia generated by thermal hydrolysis of the urea solution reduces NOx included in the exhaust gas into nitrogen and water. 
         [0037]    Next, an explanation will be given on the urea solution injection nozzle  2  of internal mixing type concretely referring to  FIG. 2 . The type of the urea solution injection nozzle  2  is not limited to this embodiment and an external mixing type urea solution injection nozzle may alternatively be used. A fluid nozzle used for an air-less type exhaust purification apparatus which does not use pressurized air may alternatively be used. 
         [0038]    As shown in  FIG. 2 , the urea solution injection nozzle  2  has the double pipe  3 , the liquid nozzle  4 , the air nozzle  5 , and the like. 
         [0039]    The double pipe  3  is a main component of the urea solution injection nozzle  2  and constitutes the flow path of the urea solution and the flow path of the pressurized air. One of sides of the double pipe  3  is arranged inside the exhaust pipe  21  and the other side (upstream side) thereof is arranged outside the exhaust pipe  21 . The downstream end of the double pipe  3  is arranged upstream the NOx catalyst  19  arranged inside the exhaust pipe  21 . 
         [0040]    The double pipe  3  includes an outer pipe  3   b  and an inner pipe  3   a  arranged inside the outer pipe  3   b . A urea solution flow path  3   c  which is a flow path of the urea solution is configured in the inner pipe  3   a . A gas flow path  3   d  which is a flow path of the pressurized air is configured in a space between the inner pipe  3   a  and the outer pipe  3   b . In a middle part of an outer side of the outer pipe  3   b , a connection part (not shown) which can be connected watertightly to the exhaust pipe  21  is configured. In downstream ends of the inner pipe  3   a  and the outer pipe  3   b , a female screw part  3   e  and a male screw part  3   f  are formed respectively. In an upstream end of the double pipe  3 , a urea solution supply port  3   g  communicated with the urea solution flow path  3   c  and a gas supply port  3   h  communicated with the gas flow path  3   d  are configured. 
         [0041]    The liquid nozzle  4  injects the urea solution. The liquid nozzle  4  is formed by a substantially cylindrical member and arranged downstream the double pipe  3 . One of ends (downstream end) of the liquid nozzle  4  is formed conically around the axis. At a center of the end, a projection part  4   a  which is substantially cylindrical is formed so as to be projected axially. In the other end (upstream end) of the liquid nozzle  4 , a male screw part  4   b  is formed so as to be projected axially. Furthermore, in an axial center part of the liquid nozzle  4 , a urea solution flow path  4   c  is formed so as to penetrate axially the whole liquid nozzle  4  from the male screw part  4   b  to the projection part  4   a . A middle part of the urea solution flow path  4   c  is contracted diametrically so that an inner diameter of a downstream end of the urea solution flow path  4   c  is formed smaller than an inner diameter of an upstream end of the urea solution flow path  4   c.    
         [0042]    The male screw part  4   b  of the liquid nozzle  4  is screwed to the female screw part  3   e  of the double pipe  3 . Accordingly, the double pipe  3  is connected to the liquid nozzle  4  and the urea solution flow path  4   c  is communicated with the urea solution flow path  3   c  of the double pipe  3 . Then, the urea solution can be supplied from the urea solution flow path  3   c  of the double pipe  3  to the urea solution flow path  4   c.    
         [0043]    The air nozzle  5  injects the urea solution which is atomized. The air nozzle  5  is formed by a substantially cylindrical member. The air nozzle  5  is arranged downstream the liquid nozzle  4  so that one of ends (upstream end) of the air nozzle  5  touches the downstream end of the double pipe  3 . A flange part  5   a  is firmed in a side surface of an upstream end of the air nozzle  5 . In an axial part of the air nozzle  5 , a hole which has a substantially conical diametrical contracted part contracted diametrically from a middle part toward the other side (downstream side) is formed penetratingly from the upstream end to the downstream end. An inner diameter of an upstream end of the hole is formed enough for the pressurized air to pass therethrough even if a downstream end of the liquid nozzle  4  is inserted into the upstream end of the hole. In an axial center part of a diametrical contracted side end of the diametrical contracted part, a mixing flow path  5   c  of the urea solution is formed. In a downstream end of the air nozzle  5 , an injection port  5   e  which is an opening of the mixing flow path  5   c  is formed. 
         [0044]    The air nozzle  5  is connected to the double pipe  3  by a nut  5   b . The downstream end of the liquid nozzle  4  is inserted into the hole of the upstream side of the air nozzle  5  (the mixing flow path  5   c ). At this time, a space is formed between the hole of the air nozzle  5  and the liquid nozzle  4 . The space is communicated as a gas flow path  5   d  with the gas flow path  3   d  of the double pipe  3  and the mixing flow path  5   c . Accordingly, the urea solution is supplied from the urea solution flow path  4   c  of the liquid nozzle  4  to the mixing flow path  5   c , and the pressurized air is supplied from the gas flow path  5   d  to the mixing flow path  5   c . Namely, the injection port  5   e  can inject the urea solution by screwing the air nozzle  5  to the double pipe  3 . 
         [0045]    According to the above, the urea solution injection nozzle  2  has the liquid nozzle  4  which injects the urea solution toward one of the sides (downstream side) and the air nozzle  5 , and injects the urea solution toward the NOx catalyst  19 . In this embodiment, in the urea solution injection nozzle  2 , the urea solution flow path  4   c , the gas flow path  5   d , and the mixing flow path  5   c  are configured by the liquid nozzle  4  and the air nozzle  5 . However, the configuration is not limited thereto and the urea solution flow path  4   c , the gas flow path  5   d , and the mixing flow path  5   c  may be configured respectively. 
         [0046]    An explanation will be given on an operation mode of the pressurized air valve  8  and the switching valve  11  referring to  FIG. 1 . 
         [0047]    As shown in  FIG. 1 , the air tank  7  is connected to the gas supply port  3   h  of the urea solution injection nozzle  2  via the pressurized air valve  8  by the second supply flow path  16 . 
         [0048]    As mentioned above, normally, the pressurized air valve  8  is held at the position V. In this case, since the second supply flow path  16  is closed, the pressurized air is not supplied to the gas supply port  3   h  of the urea solution injection nozzle  2 . 
         [0049]    When the control device  14  energizes the solenoid of the pressurized air valve  8 , the pressurized air valve  8  is switched from the position V to the position W. In this case, since the second supply flow path  16  is opened, the pressurized air is supplied to the gas supply port  3   h  of the urea solution injection nozzle  2 . 
         [0050]    When the control device  14  stops the energization to the solenoid of the pressurized air valve  8 , the pressurized air valve  8  is switched to the position V. In this case, since the second supply flow path  16  is closed, the pressurized air is not supplied to the gas supply port  3   h  of the urea solution injection nozzle  2 . 
         [0051]    As shown in  FIG. 1 , the urea solution tank  10  is connected to the urea solution supply port  3   g  of the urea solution injection nozzle  2  via the urea solution supply pump  9  and the switching valve  11  by the first supply flow path  15 . 
         [0052]    As mentioned above, normally, the switching valve  11  is held at the position X. In this case, since the first supply flow path  15  is closed, the urea solution is not supplied to the urea solution supply port  3   g  of the urea solution injection nozzle  2 . The urea solution supply port  3   g  of the urea solution injection nozzle  2  is atmosphere-opened in the drain pot  17  via the flow path  15   a.    
         [0053]    When the control device  14  energizes the solenoid of the switching valve  11 , the switching valve  11  is switched to the position Y. In this case, since the first supply flow path  15  is opened, the urea solution is supplied to the urea solution supply port  3   g  of the urea solution injection nozzle  2 . Since the communication with the drain pot  17  is cut off, the urea solution supply port  3   g  of the urea solution injection nozzle  2  is not atmosphere-opened. 
         [0054]    When the control device  14  stops the energization to the solenoid of the switching valve  11 , the switching valve  11  is switched to the position X. In this case, since the first supply flow path  15  is closed, the urea solution is not supplied to the urea solution supply port  3   g  of the urea solution injection nozzle  2 . Since the communication with the drain pot  17  is permitted, the urea solution supply port  3   g  of the urea solution injection nozzle  2  is atmosphere-opened in the drain pot  17 . 
         [0055]    An explanation will be given on an operation mode of the urea solution injection nozzle  2  referring to  FIGS. 1 and 2 . 
         [0056]    As shown in  FIG. 1 , when the supply (injection) of the urea solution to the inside of the exhaust pipe  21  is started, the control device  14  switches the switching valve  11  to the position Y so that the urea solution is supplied to the urea solution supply port  3   g  of the urea solution injection nozzle  2  (the double pipe  3 ). The urea solution is injected from the projection part  4   a  of the liquid nozzle  4  to the mixing flow path  5   c  of the air nozzle  5  via the urea solution flow path  3   c  of the double pipe  3  and the urea solution flow path  4   c  of the liquid nozzle  4 . 
         [0057]    In this state, the control device  14  switches the pressurized air valve  8  to the position W so that the pressurized air is supplied to the gas supply port  3   h  of the urea solution injection nozzle  2  (the double pipe  3 ). As shown in  FIG. 2 , the pressurized air is injected at a predetermined pressure via the gas flow path  3   d  of the double pipe  3  and the gas flow path  5   d  of the air nozzle  5  to the mixing flow path  5   c  of the air nozzle  5 . As a result, the urea solution collides with the pressurized air inside the mixing flow path  5   c  of the air nozzle  5  and is atomized, and then injected via the injection port  5   e  of the air nozzle  5 . 
         [0058]    As shown in  FIG. 1 , when the supply (injection) of the urea solution to the inside of the exhaust pipe  21  is stopped, the control device  14  switches the switching valve  11  to the position X so that the supply of the urea solution to the urea solution supply port  3   g  of the urea solution injection nozzle  2  (the double pipe  3 ) is stopped. Accordingly, the urea solution supply port  3   g  of the double pipe  3  is atmosphere-opened via the first supply flow path  15  and the switching valve  11 . 
         [0059]    An explanation will be given on a mode of calculation of the addition amount of the urea solution referring to  FIG. 3 . 
         [0060]    The control device  14  obtains the signal of the temperature T of the atmosphere from the temperature sensor  12  and obtains the signal of the absolute humidity H of the atmosphere from the humidity sensor  13 . The control device  14  obtains the signal of the rotation speed R of the engine  20  from the rotation speed sensor  20   a  and obtains the signal of the load L of the engine  20  from the load sensor  23   a . The control device  14  calculates an actual NOx discharge amount Nr based on the obtained information and controls the operation state of the urea solution supply pump  9  (see  FIG. 1 ). 
         [0061]    As shown in  FIG. 3 , the control device  14  controls the operation state of the urea solution supply pump  9  with below steps. 
         [0062]    Firstly, at a step S 110 , the control device  14  obtains the signal of the temperature T of the atmosphere from the temperature sensor  12  and obtains the signal of the absolute humidity H of the atmosphere from the humidity sensor  13 . The control device  14  obtains the signal of the rotation speed R of the engine  20  from the rotation speed sensor  20   a  and obtains the signal of the load L of the engine  20  from the load sensor  23   a.    
         [0063]    At a step S 120 , the control device  14  calculates the standard NOx discharge amount Ns, which is an amount of NOx discharged from the engine  20  driven with the rotation speed R and the signal of the load L under standard conditions, from the signal of the rotation speed R and the signal of the load L of the engine  20  with the map M. 
         [0064]    At a step S 130 , the control device  14  calculates an actual NOx discharge amount Nr, which is an amount of NOx discharged from the engine  20  driven with the rotation speed R and the signal of the load L under the temperature T and the absolute humidity H of the atmosphere, from the standard NOx discharge amount Ns corresponding to the rotation speed R and the signal of the load L calculated at the step S 120  and the signal of the temperature T and the signal of the absolute humidity H of the atmosphere with the correction formula F by inverse operation. Namely, the actual NOx discharge amount Nr which is necessary to set the amount of NOx discharged from the engine  20 , which is driven with the rotation speed R and the signal of the load L under the temperature T and the absolute humidity H of the atmosphere, to the standard NOx discharge amount Ns is calculated with the correction formula F. 
         [0065]    At a step S 140 , the control device  14  determines the addition amount Q of the urea solution, which is necessary to reduce the actual NOx discharge amount Nr from the target purification rate and the concentration of the urea solution which are set optionally. 
         [0066]    At a step S 150 , the control device  14  controls the operation state of the urea solution supply pump  9  so as to supply the addition amount Q of the urea solution in the urea solution supply pump  9  to the exhaust gas. Subsequently, the control device  14  returns to the step S 110 . 
         [0067]    Accordingly, as shown in  FIG. 4 , in the case in which the addition amount Q of the urea solution is Q 1  while the actual NOx discharge amount Nr is Nr 1 , when the actual NOx discharge amount Nr is decreased to Nr 2 ′, NOx is decreased more than the target purification rate (a two-dot chain line in  FIG. 4 ) (over-denitration) (see A′). Then, by the above control, the decrease of the actual NOx discharge amount Nr to Nr 2 ′ is calculated, and the urea solution supply pump  9  is controlled so as to set the addition amount Q of the urea solution to a suitable addition amount Q 2 ′. Furthermore, when the actual NOx discharge amount Nr is decreased to Nr 2 , NOx is decreased more than the target purification rate (over-denitration) and ammonia exceeding a theoretical equivalent (a dashed line in  FIG. 4 ) and remaining is discharged outside the exhaust pipe  21  (ammonia slip). Then, by the above control, the decrease of the actual NOx discharge amount Nr to Nr 2  is calculated, and the urea solution supply pump  9  is controlled so as to set the addition amount Q of the urea solution to a suitable addition amount Q 2 . When the actual NOx discharge amount Nr is increased to Nr 3 , denitration shortage that NOx cannot be decreased to the target purification rate occurs (see a point B). Then, by the above control, the increase of the actual NOx discharge amount Nr to Nr 3  is calculated, and the urea solution supply pump  9  is controlled so as to set the addition amount Q of the urea solution to a suitable addition amount Q 3 . 
         [0068]    As the above, the exhaust purification apparatus  1 , in which ammonia is added as a reducing agent to the exhaust gas of the engine  20  which is an internal combustion engine so as to reduce nitrogen oxide in the exhaust gas, has the temperature sensor  12  detecting a temperature T of the atmosphere, the humidity sensor  13  detecting the absolute humidity H or the relative humidity of the atmosphere, and the control device  14  calculating the addition amount of the urea solution. The map M, which converts the actual NOx discharge amount Nr of the engine  20  driven with each rotation speed and each load under predetermined temperature and absolute humidity of the atmosphere into the standard NOx discharge amount Ns of the engine  20  with each rotation speed and each load under the standard conditions with the correction formula F is stored in the control device  14 . The rotation speed sensor  20   a  which is a rotation speed detection means detecting the rotation speed of the engine  20  and the load sensor  23   a  of the dynamo  23  which is a load detection means detecting the load of the engine  20  are connected to the control device  14 . The standard NOx discharge amount Ns corresponding to the rotation speed R detected by the rotation speed sensor  20   a  and the load L detected by the load sensor  23   a  is calculated with the map M. The standard NOx discharge amount Ns is converted into the actual NOx discharge amount Nr of the rotation speed R and the load L under the temperature T of the atmosphere detected by the temperature sensor  12  and the absolute humidity H of the atmosphere detected by the humidity sensor  13  with the correction formula F by inverse operation. The addition amount Q of the urea solution is calculated based on the actual NOx discharge amount Nr. 
         [0069]    According to the configuration, the actual NOx discharge amount Nr based on the characteristic of the engine  20  can be calculated while considering the temperature T of the atmosphere and the absolute humidity H of the atmosphere which influence the NOx discharge amount greatly. Accordingly, the urea solution of the suitable amount can be added without measuring directly the actual NOx discharge amount Nr with a NOx sensor. 
         [0070]    The addition amount Q is calculated in consideration of the target purification rate and the concentration of the urea solution. 
         [0071]    According to the configuration, the addition amount Q can be adjusted corresponding to the operating condition. Accordingly, the urea solution of the suitable amount can be added without measuring directly the actual NOx discharge amount Nr with a NOx sensor. 
       INDUSTRIAL APPLICABILITY 
       [0072]    The present invention can be used especially for an exhaust purification apparatus for a ship. 
       DESCRIPTION OF NOTATIONS 
       [0000]    
       
         
           
               1  exhaust purification apparatus 
               12  temperature sensor 
               13  humidity sensor 
               14  control device 
               20  engine 
               20   a  rotation speed sensor 
               23   a  load sensor 
             R rotation speed 
             W load 
             Ns standard NOx discharge amount 
             Nr actual NOx discharge amount 
             Q addition amount