Patent Publication Number: US-6901746-B2

Title: Exhaust gas deNOx apparatus for engine

Description:
RELATED APPLICATION 
   This application is a divisional application of application Ser. No. 10/043,311 filed on Jan. 14, 2002, now U.S. Pat. No. 6,681,564. 

   TECHNICAL FIELD 
   The present invention relates to an exhaust gas deNOx apparatus for an engine, and particularly, it relates to an exhaust gas deNOx apparatus for a diesel engine. 
   BACKGROUND ART 
   Various contrivances are conventionally proposed regarding an exhaust gas deNOx apparatus for a diesel engine, and one disclosed in Japanese Patent Application Laid-open No. 7-279718 is cited as an example thereof. 
   According to Japanese Patent Application Laid-open No. 7-279718, an NOx adsorber is placed in an exhaust passage of an engine, and an exhaust gas recirculating (EGR) circuit having an EGR control valve for controlling an amount of flowing exhaust gas is provided. The NOx adsorber absorbs NOx in a normal condition, and it releases NOx when an air fuel ratio inside a combustion chamber becomes rich, and reduces NOx by CO and HC. When the air fuel ratio inside the combustion chamber is to be made rich, the EGR control valve is opened to allow exhaust gas to recirculate, an amount of intake air is decreased, and injection fuel is increased to thereby make the air fuel ratio rich. At the same time, a fuel injection amount is further increased so that engine output torque does not change. 
   However, in the above-described constitution, a large amount of NOx is contained in exhaust gas because the EGR control valve is closed during normal operation, and therefore it is necessary to increase the size of the NOx adsorber, which makes a space area larger in the case of a large-sized engine, thus causing the constitutional disadvantage, and also causing the disadvantage of cost being high. In addition, when fuel is increased under a heavy load, there arises the fear that inner pressure of cylinders and exhaust gas temperature are increased and reliability and durability of the engine are impaired. 
   SUMMARY OF THE INVENTION 
   The present invention is made in view of the above-described disadvantages, and its object is to provide an exhaust gas deNOx apparatus for an engine at low cost, which can reduce an NOx adsorber catalyst in size without impairing reliability and durability of an engine. 
   In order to attain the above object, an exhaust gas deNOx apparatus for an engine according to the present invention is an exhaust gas deNOx apparatus for an engine comprising
     an NOx adsorber catalyst in an exhaust pipe line of an engine, for adsorbing NOx when an air fuel ratio of an exhaust gas flowing therein is in a lean state and releasing NOx when the air fuel ratio of the exhaust gas flowing therein is in a rich state, and   an exhaust gas recirculaing circuit for mixing the exhaust gas into intake air, and has the constitution provided with exhaust gas recirculating amount control means for recirculating a predetermined amount of exhaust gas for reducing NOx when an adsorbed NOx accumulation amount is not more than a predetermined value, and recirculating the predetermined amount or more of exhaust gas to bring the air fuel ratio into a rich state when the adsorbed NOx accumulation amount exceeds the predetermined value and the NOx is to be released.   

   According to the above constitution, a predetermined amount (for example, 10% to 15% of the exhaust gas amount) of exhaust gas can be recirculated when the air fuel ratio of the exhaust gas of the engine is in a lean state, and therefore NOx in the exhaust gas can be reduced in a normal lean-burn state. In addition, since the exhaust gas recirculating amount control means is provided, the exhaust gas recirculating amount can be adjusted to the amount that can optimize emission, and the NOx adsorber catalyst can be made compact, thus reducing a space area to make the engine compact. 
   Further, in the exhaust gas deNOx apparatus for the engine,
     the exhaust gas recirculating amount control means has the constitution in which it is able to adjust an exhaust gas recirculating amount, which is fed when the air fuel ratio of the exhaust gas flowing into the NOx adsorber catalyst is brought into a rich state, so that an excess air ratio is more than 1.0 and is about 1.3 or less.   

   According to the above constitution, the exhaust gas recirculating amount control means that can make the exhaust gas recirculating amount have the excess air ratio of 1.0 to about 1.3 is provided. As a result, the air fuel ratio of the exhaust gas is brought into a rich state, whereby NOx can be released and reduced, a large amount of fuel as a reducer does not have to be sprayed, and fuel consumption rate can be prevented from changing for the worse. The excess air ratio is “1” when the air fuel ratio is a theoretical mixture ratio. The rich state means the state in which the air fuel ratio is near the theoretical mixture ratio, and the lean state corresponds to the state in which the air fuel ratio is larger than in the rich state, but the excess air ratio corresponding to each state differs according to the kind of the engine. When the above-described exhaust gas recirculating amount is to be adjusted, the oxygen concentration in the exhaust gas is converted into air with the oxygen concentration of about 20%, the converted air is regarded as part of the intake air amount and fresh air is added thereto, which is made the total air amount. 
   Further, in the exhaust gas deNOx apparatus for the engine, 
   the exhaust gas recirculating amount control means comprises
     at least any one of an intake air throttle valve free to be opened and closed, which is provided in a pipe line of an intake pipe of the engine, and an exhaust gas throttle valve free to be opened and closed, which is provided in a pipe line of an exhaust pipe of the engine, and a controller, and   the controller outputs a control signal to decrease degree of opening of either one of the intake air throttle valve or the exhaust gas throttle valve when the air fuel ratio of the exhaust gas is to be brought into a rich state.   

   According to the above constitution, by decreasing the degree of opening of the throttle valve provided in the exhaust gas pipe line or the intake air pipe line, the supply air pressure can be easily lowered more sharply than the exhaust gas pressure. As a result, a large amount of exhaust gas is recirculated and thereby the air fuel ratio of the exhaust gas can be made rich, which makes it possible to reduce a space area and cost with the simple structure. 
   Further, in the exhaust gas deNOx apparatus for the engine,
     the exhaust gas recirculating circuit comprises a first exhaust gas recirculating circuit and at least one second exhaust gas recirculating circuit provided in parallel with the first exhaust gas recirculating circuit,   the exhaust gas recirculating amount control means comprises a second recirculating circuit adjusting valve free to be opened and closed, which is provided in the second exhaust gas recirculating circuit,   NOx amount detection means for detecting an NOx amount, and   a controller for calculating an NOx accumulation amount in the NOx adsorber catalyst based on the NOx amount outputted from the NOx amount detection means and determining whether the calculated NOx accumulation amount is a predetermined value or less, or not, and   when the calculated NOx accumulation amount exceeds the predetermined value, the controller outputs a control signal to open the second recirculating circuit adjusting valve to make the air fuel ratio of the exhaust gas of the engine rich.   

   According to the above constitution, the exhaust gas is recirculated only in the first exhaust gas recirculating circuit when the air fuel ratio is lean. On the other hand, when the air fuel ratio is made rich, the second exhaust gas recirculating circuit is also used, and therefore a large amount of exhaust gas can be recirculated with less resistance. As a result, the performance is improved, and the second recirculating circuit adjusting valve is made an ON-OFF valve with a simple structure, thus making it possible to reduce cost. 
   In the exhaust gas deNOx apparatus for the engine,
     the exhaust gas recirculating circuit comprises a third exhaust gas recirculating circuit provided with a plurality of recirculating circuit adjusting valves in parallel,   the exhaust gas recirculating amount control means comprises the plurality of recirculating circuit adjusting valves,   NOx amount detection means for detecting an NOx amount, and   a controller for calculating an NOx accumulation amount in the NOx adsorber catalyst based on the NOx amount outputted from the NOx amount detection means and determining whether the calculated NOx accumulation amount is a predetermined value or less, or not, and   when the calculated NOx accumulation amount exceeds a predetermined value, the controller outputs a control signal to increase a total opening area of the plurality of recirculating circuit adjusting valves to be larger than that in a lean state and makes the air fuel ratio of the exhaust gas of the engine rich.   

   According to the above constitution, with only one exhaust gas recirculating circuit, fine adjustment is facilitated by making the first recirculating circuit adjusting valve compact when the air fuel ratio is lean, and thus emission can be optimized. When the air fuel ratio is to be made rich, the second recirculating circuit adjusting valve is opened to make the total opening area larger, whereby a large amount of exhaust gas can be recirculated. In addition, an ON-OFF valve can be used as the second recirculating circuit adjusting valve, which is simple in structure, and the engine can be constituted to be compact, which can make the cost lower. 
   Further, in the exhaust gas deNOx apparatus for the engine,
     a turbocharger is provided at the engine, and   the exhaust gas recirculating amount control means comprises an air bleed valve free to be opened and closed, which is provided at an outlet port of a compressor of the turbocharger,   and extracts supply air to an outside, and   a controller for outputting a control signal to open the air bleed valve when the air fuel ratio of the exhaust gas of the engine is to be made rich.   

   According to the above constitution, by opening the air bleed valve provided at the outlet port of the compressor of the turbocharger, the supply air pressure can be more sufficiently reduced than the exhaust gas pressure. Accordingly, a large amount of exhaust gas can be recirculated. 
   Furthermore, in the exhaust deNOx apparatus for the engine,
     the exhaust gas recirculating amount control means comprises fuel supply means for supplying fuel into at least one of cylinders of the engine and an exhaust pipe of the engine,   when the air fuel ratio of the exhaust gas is brought into a rich state, the fuel supply means supplies fuel as an adjustment amount to provide an air fuel ratio that allows the NOx adsorber catalyst to release and reduce NOx.   

   According to the above constitution, when the air fuel ratio is to be made rich, fuel is supplied to adjust an insufficient amount. As a result, the air fuel ratio can be surely controlled to be near the theoretical air fuel ratio. Further, since a large amount of recirculating exhaust gas is circulated, supply amount of fuel can be reduced, which is economical and can prevent the inner pressure of the cylinders and exhaust gas temperature from excessively rising, and thus the air fuel ratio can be made rich without reducing reliability and durability of the engine. 
   In the exhaust gas deNOx apparatus for the engine,
     the exhaust gas recirculating amount control means includes load detection means for detecting a load of the engine, and a controller, and   the controller inputs a detection signal from the load detection means, and when the detected load is a predetermined value or less, it brings the air fuel ratio of the exhaust gas into a rich state.   

   According to the above constitution, when the engine is operated under a lighter load than a predetermined load, that is, in a light load range, the means for making the air fuel ratio rich is operated. Since the engine is under a light load, added fuel can be reduced, which is economical and can prevent the inner pressure of the cylinders and the exhaust gas temperature from excessively rising and the like, and the frequency of addition of fuel in a high load range is decreased, thus making it possible to improve reliability and durability of the engine. 
   Further, in the exhaust gas deNOx apparatus for the engine,
     the exhaust gas recirculating amount control means comprises a variable turbocharger provided at the engine, for making a degree of opening of a turbine passage variable, and a controller, and   when the air fuel ratio is to be brought into a rich state, the controller outputs a control signal to decrease the degree of opening of the turbine passage.   

   According to the above constitution, the degree of opening of the turbine passage is made small, for example, the degree of opening is made extremely small, whereby the exhaust gas pressure at the exhaust gas turbine inlet port can be made higher than the supply air pressure at the compressor outlet port of the variable turbocharger. Then, this pressure difference between the supply air and exhaust gas is utilized to allow a required amount of exhaust gas to flow into the supply air. Accordingly, with use of the variable turbocharger, the same effects as the exhaust gas throttle valve can be also obtained. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a first embodiment of the present invention; 
       FIG. 2  is a flowchart of a deNOx operation process of the exhaust gas deNOx apparatus of the first embodiment; 
       FIG. 3  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a second embodiment of the present invention; 
       FIG. 4  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a third embodiment of the present invention; 
       FIG. 5  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a fourth embodiment of the present invention; 
       FIG. 6  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a fifth embodiment of the present invention; 
       FIG. 7  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a sixth embodiment of the present invention; 
       FIG. 8  is a flowchart of a deNOx process of the exhaust gas deNOx apparatus of the sixth embodiment; 
       FIG. 9  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a modification example of the fourth embodiment; 
       FIG. 10  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a seventh embodiment of the present invention; 
       FIG. 11  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of an eighth embodiment of the present invention; 
       FIG. 12  is a conceptual diagram of an exhaust gas deNOx apparatus for an engine of a ninth embodiment of the present invention; and 
       FIG. 13  is a diagram showing relationship between discharged NOx amount, and fuel injection rate and engine speed according to a tenth embodiment of the present invention. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Preferred embodiments of an exhaust gas deNOx apparatus for an engine according to the present invention will be explained in detail below with reference to the drawings. 
     FIG. 1  is a conceptual diagram of an exhaust gas deNOx apparatus of a first embodiment. An engine  1  includes a turbocharger  2 , and the turbocharger  2  is constituted by an exhaust turbine  2   a  and a compressor  2   b . The exhaust turbine  2   a  is attached to an exhaust manifold  7 , and an exhaust pipe  8  is attached at an exhaust port. A deNOx catalyst  9  is fitted in the exhaust pipe  8 , and a tail pipe  8   a  is attached at an outlet port thereof. An intake pipe  3  is attached at an intake port of the compressor  2   b  connected to the exhaust turbine  2   a , and the intake pipe  3  is provided with an intake air throttle valve  21  that can adjust an opening area of the intake pipe  3 . An air supply pipe  4  is attached at an exhaust port of the compressor  2   b  to be connected to an intake manifold  5 , and an inter cooler  6  is fitted in the air supply pipe  4 . 
   A downstream side of the inter cooler  6  of the air supply pipe  4  and an upstream side of the exhaust turbine  2   a  of the exhaust manifold  7  are connected by an exhaust gas recirculating circuit  10 . A recirculating circuit adjusting valve (hereinafter called an EGR valve)  24  that can adjust an opening area of the exhaust gas recirculating circuit  10  and a recirculating gas cooler  11  are fitted onto the exhaust gas recirculating circuit  10 . The exhaust pipe  8  is provided with an NOx sensor  30  for detecting an amount of NOx and an O 2  sensor  31  for detecting an oxygen amount. A controller  32  connects the NOx sensor  30 , the O 2  sensor  31 , the intake air throttle valve  21  and the EGR valve  24 . The controller  32  inputs a detection signal from each of the aforementioned sensors and outputs a control signal to the intake air throttle valve  21  and the EGR valve  24  after performing predetermined computation, thus constituting exhaust gas recirculating amount control means  20 . 
   Here, the NOx sensor  30  for detecting a NOx amount has the constitution including a detecting member for detecting a NOx content and a measuring member for measuring an exhaust gas flow rate. Accordingly, the NOx sensor  30  can detect an NOx amount by performing calculation in the controller  32  based on the NOx concentration and the exhaust gas flow rate. It should be noted that the NOx sensor  30  may include only the NOx concentration detecting member. In this case, the exhaust gas flow rate is obtained according to an ordinary calculation method based on an operation condition of the engine  1  or the like. The O 2  sensor  31  for detecting an oxygen amount has the constitution including a detecting member for detecting oxygen content and a measuring member for measuring an exhaust gas flow rate. Accordingly, it can detect an oxygen amount by performing calculation in the controller  32  based on the oxygen concentration and the exhaust gas flow rate. It should be noted that the O 2  sensor  31  may include only the oxygen concentration detecting member. In this case, the exhaust gas flow rate is obtained from exhaust gas flow rate measurement date of the NOx sensor  30 , or an ordinary calculation method based on an operation condition of the engine  1 . The NOx sensor  30  is an example of the “NOx amount detection means” for detecting an NOx amount, and a constitution of a tenth embodiment that will be described later may be suitable as the NOx amount detection means. 
   An operation of the engine  1  will be explained below. The compressor  2   b  takes in intake air from the intake pipe  3  when driven with the exhaust turbine  2   a , and the compressed supply air is fed by pressure to the intake manifold  5  via the air supply pipe  4 . The supply air compressed during that time is cooled by the inter cooler  6  and is supplied in high density to the intake manifold  5  of the engine  1 . Combustion takes place in the engine  1 , and discharged exhaust gas is fed from the exhaust manifold  7  to the exhaust turbine  2   a  to drive it. Thereafter, it passes through the NOx adsorber catalyst  9  from the exhaust pipe  8 , and is discharged to an outside via the tail pipe  8   a . During a normal operation, operation is performed with supply air in a lean state, but in the present invention, a predetermined amount (for example, 10% to 15% of the exhaust gas amount) of exhaust gas is always recirculated to reduce NOx in the exhaust gas. 
   During a normal operation, discharged NOx is adsorbed in the NOx adsorber catalyst  9 . When NOx accumulation amount in the NOx adsorber catalyst  9  reaches a predetermined amount, an air fuel ratio of the exhaust gas is made rich, whereby NOx is released from the NOx adsorber catalyst  9  and reduced. When the air fuel ratio of the exhaust gas is to be made rich, the controller  32  inputs a detection signal from the NOx sensor  30  and performs predetermined computation. After performing the computation, the controller  32  outputs a control signal to the intake air throttle valve  21  and the EGR valve  24  to throttle back the intake air throttle valve  21  and open the EGR valve  24 . The amount of the exhaust gas recirculated at this time is, for example, 50% to 75% of the exhaust gas amount. As a result, combustion is carried out in the engine in a state near a theoretical mixture ratio, and the NOx adsorber catalyst  9  releases NOx and converts it. 
   An exhaust gas deNOx operation process in the first embodiment will be described in detail below based on a flowchart shown in FIG.  2 . 
   In step  101 , the NOx sensor  30  detects the NOx amount, and based thereon, the controller  32  calculates the NOx accumulation amount in the NOx adsorber catalyst  9 . In step  102 , the controller  32  determines whether or not the NOx accumulation amount reaches an absorption limit amount of the NOx adsorber catalyst  9 , and in the case of NO, a command is given to return to the situation before step  101 . In the case of YES in step  102 , the controller  32  calculates a throttling amount of the intake air throttle valve  21  necessary to make the air fuel ratio of the exhaust gas rich in step  103 . In step  104 , the controller  32  outputs a control signal to the intake air throttle valve  21  and adjusts the throttling amount based on the calculated value. 
   In step  105 , the controller  32  calculates degree of opening of the EGR valve  24  to make the air fuel ratio of the exhaust gas rich. In step  106 , the controller  32  outputs a control signal to the EGR valve  24  and adjusts the degree of opening based on the calculated value. In step  107 , the controller  32  calculates an amount of air and the air fuel ratio. In step  108 , O 2  sensor  31  detects oxygen concentration and outputs the detected value to the controller  32 . In step  109 , the controller  32  corrects the air fuel ratio. In step  110 , a timer that is set at an exhaust gas recirculating time Tdef necessary to reduce NOx completely, which is previously specified, is started. In step  111 , the engine performs combustion at a theoretical mixture ratio. 
   In step  112 , the controller  32  determines whether or not the exhaust gas recirculating time T exceeds Tdef or not, and in the case of NO, it gives a command to return to the situation before step  110 . In the case of YES in step  112 , the controller  32  opens the intake air throttle valve  21  to make the air fuel ratio of the exhaust gas lean in step  113 . In step  114 , the controller  32  calculates degree of opening of the EGR valve  24  necessary to recirculate a predetermined amount of exhaust gas (for example, 10% to 15% of the amount of the exhaust gas) in the state of a lean air fuel ratio. In step  115 , the controller  32  outputs a control signal to the EGR valve  24 , and adjusts the degree of opening based on the calculated value. In step  116 , the engine performs normal lean-burn combustion (a predetermined amount of exhaust recirculation is carried out), and a command is given to return to the situation before step  101 . 
   Since the exhaust deNOx apparatus of the first embodiment has the constitution and operational method as described in the above, the following effects are obtained. 
   The EGR valve  24  is always adjusted to have an area enough to recirculate a small amount of exhaust gas (for example, 10% to 15%) being opened in an ordinary lean-burn state. Thus, the discharge amount of NOx is reduced. Accordingly, the NOx adsorber catalyst  9  can be made small, the engine can be made compact, and the cost can be reduced. 
   When the NOx absorption amount in the NOx adsorber catalyst  9  increases and reaches the limit and the air fuel ratio of the exhaust gas needs to be made rich, the opening area of the intake air throttle valve  21  is reduced, and the degree of opening of the EGR valve  24  is increased. Thus, the amount of intake air is reduced, the inner pressure of the air supply pipe  4  is reduced, and a large amount (for example, 50% to 75% of the amount of exhaust gas) of recirculating gas flows into the air supply pipe, whereby the air fuel ratio of the exhaust gas can be made rich. Specifically, the air fuel ratio of the exhaust gas is made rich with recirculating gas, and release and reduction of NOx are performed, thus causing no excessive rise in the inner pressure of the cylinders or in the exhaust gas temperature, whereby reliability and durability of the engine are not impaired. 
     FIG. 3  is a conceptual diagram of an exhaust gas deNOx apparatus of a second embodiment. The same components as in the first embodiment are given the same numerals and symbols, the explanation thereof will be omitted, and only the parts different therefrom will be explained. In  FIG. 3 , in the second embodiment, instead of the intake air throttle valve  21 , the exhaust pipe  8  is provided with an exhaust gas throttle valve  22 , which is connected to the controller  32  to constitute exhaust gas recirculating amount control means  20   a . When the air fuel ratio of the exhaust gas is made rich, the controller  32  outputs a control signal to throttle back the exhaust gas throttle valve  22  and open the EGR valve. As a result, the pressure on the exhaust side rises, and the difference from the air supply pressure is increased. The operation and effects are the same as the first embodiment, and therefore the explanation will be omitted. 
     FIG. 4  is a conceptual diagram of an exhaust gas deNOx apparatus of a third embodiment. The same components as in the first embodiment are given the same numerals and symbols, the explanation thereof will be Omitted, and only the parts different therefrom will be explained. In  FIG. 4 , the intake air throttle valve  21  is eliminated in the third embodiment, but the air supply pipe  4  and the tail pipe  8   a  are connected by an air bleed pipe  12 , and the air bleed pipe  12  is provided with an air bleed valve  23 . The air bleed valve  23  is connected to the controller  32  to constitute exhaust gas recirculating amount control means  20   b . When the exhaust gas is made rich, the controller  32  outputs a control signal to open the air bleed valve  23  to release part of the supply air outside. As a result, the amount of air supply is decreased and the exhaust gas becomes rich, thus making it possible to sufficiently reduce the air supply pressure as compared with the exhaust gas pressure and recirculate a large quantity of exhaust gas. 
     FIG. 5  is a conceptual diagram of an exhaust gas deNOx apparatus of a fourth embodiment. The same components as in the second embodiment are given the same numerals and symbols, the explanation thereof will be omitted, and only the parts different therefrom will be explained. In  FIG. 5 , a second exhaust gas recirculating circuit  14  with a larger passage area is provided in parallel with a first exhaust gas recirculating circuit  13  with a smaller passage area. The first exhaust gas recirculating circuit  13  is provided with a small-sized first recirculating circuit adjusting valve (hereinafter, called a first EGR valve)  25 , and a large-sized second recirculating circuit adjusting valve (hereinafter, called a second EGR valve)  26  is fitted in the second exhaust gas recirculating circuit  14 , which are connected to the controller  32  to constitute exhaust gas recirculating amount control means  20   c . During a normal lean-burn operation, the controller  32  outputs a control signal to close the second EGR valve  26  and open the first EGR valve  25  a little to recirculate a small amount of exhaust gas as describe above. On this occasion, since the first EGR valve  25  is compact, fine adjustment is facilitated, thus making it possible to improve performance. When the air fuel ratio of the exhaust gas is to be made rich, the controller  32  outputs a control signal to open the second EGR valve to recirculate a large amount of exhaust gas. Since the second EGR valve  26  may be an ON-OFF valve, the constitution becomes simple. Further, since the passage area of the second exhaust gas recirculating circuit  14  is large, resistance is very small, thus making it possible to smoothly recirculate a large amount of exhaust gas. 
   Though  FIG. 5  shows an example in which the first EGR valve  25  and the second EGR valve  26  are fitted, but the first EGR valve  25  of the exhaust gas recirculating amount control means  20   c  may be omitted to make it exhaust gas recirculating amount control means  20   e  as shown in FIG.  9 . In the case of this exhaust gas recirculating amount control means  20   e , the controller  32  outputs a control signal during a normal lean-burn operation to close the second EGR valve  26  and recirculate a small amount of exhaust gas only in the first exhaust gas recirculating circuit  13 . On the other hand, when the air fuel ratio of the exhaust gas is made rich, the controller  32  outputs a control signal to open the second EGR valve  26 , whereby a large amount of exhaust gas can be recirculated by means of the second exhaust gas recirculating circuit  14  with a large passage area and the first exhaust gas recirculating circuit  13 . 
     FIG. 6  is a conceptual diagram of an exhaust gas deNOx apparatus of a fifth embodiment, which shows a constitution of an EGR valve part. Specifically, the two exhaust gas recirculating circuits being the first exhaust gas recirculating circuit  13  and the second exhaust gas recirculating circuit  14  in the fourth embodiment are replaced with one third exhaust gas recirculating circuit  15  with a large passage area. A first EGR valve  25  with a small diameter and a second EGR valve  26  with a large diameter are provided in parallel inside the third exhaust gas recirculating circuit  15  and are respectively connected to the controller  32 . Further, the recirculating gas cooler  11  in the fourth embodiment is omitted, but it may be provided as necessary. In the case of an ordinary lean-burn operation, the second EGR valve  26  is closed and the first EGR valve  25  is opened to finely adjust the circuit area, and when the air fuel ratio of the exhaust gas is to be made rich, the second EGR valve  26  is opened. Since the apparatus in the fifth embodiment has only one exhaust gas recirculating circuit, the constitution is simple and compact, thus making it possible to reduce the cost. 
     FIG. 7  is a conceptual diagram of an exhaust deNOx apparatus of a sixth embodiment. The same components as in the first embodiment are given the same numerals and symbols, the explanation thereof will be omitted, and only the parts different therefrom will be explained. A fuel injection pump  34  of the engine  1  is provided with a fuel injection amount sensor  41  and an engine speed sensor  42 , which are connected respectively to the controller  32  to constitute load detection means  40 . Further, a fuel injection nozzle  35  is attached to the exhaust pipe  8 , and the fuel injection pump  34  and the fuel injection nozzle  35  are connected to the controller  32  to constitute the fuel supply means, and the intake air throttle valve  21  and the EGR valve  24  are added to constitute exhaust gas recirculating amount control means  20   d . When the air fuel ratio of the exhaust gas is to be made rich, the intake air throttle valve  21  is closed by a predetermined amount, and the EGR valve  24  is opened to perform exhaust gas recirculation. At the same time, as an adjustment amount for providing the air fuel ratio that makes it possible to release and reduce NOx, the controller  32  outputs a control signal to increase an injection amount of the fuel injection pump  34  and inject fuel into the exhaust pipe  8  from the fuel injection nozzle  35 . When the air fuel ratio is made rich by the above fuel supply means, fuel is supplied to adjust an insufficient amount. As a result, the air fuel ratio can be surely controlled to be near the theoretical air fuel ratio. 
   An operation process of the exhaust gas deNOx apparatus of the sixth embodiment will be described in detail below based on a flowchart shown in FIG.  8 . 
   In step  201 , an engine speed and a fuel injection rate are detected by the engine speed sensor  42  and the fuel injection rate sensor  41 . In step  202 , the controller  32  calculates a load of the engine from the engine speed and the fuel injection rate, and determines whether the load on the engine is a predetermined load (for example, brake mean effective pressure 6 kg/cm 2 ) or higher, or not. In the case of YES in step  202 , a command is given to proceed to step  203 , where the NOx sensor  30  detects the NOx amount and the controller  32  calculates the NOx accumulation amount in the NOx adsorber catalyst  9 . In step  204 , the controller  32  determines whether the NOx accumulation amount reaches the absorption limit amount of the NOx adsorber catalyst  9 , or not, and in the case of NO, a command is given to return to the situation before step  202 . In the case of YES in step  204 , a command is given to proceed to step  207 . 
   In the case of NO in step  202 , a command is given to proceed to step  205 , where the NOx sensor  30  detects the NOx amount, and the controller  32  calculates the NOx accumulation amount of the NOx adsorber catalyst  9 . In step  206 , the controller  32  determines whether the NOx accumulation amount exceeds the specified absorption amount (for example, 50% to 70% of the absorption limit amount) of the NOx adsorber catalyst  9 , or not, and in the case of NO, a command is given to return to the situation before step  202 . In the case of YES in step  206 , a command is given to proceed to step  207 , where the controller  32  calculates a throttle amount of the intake air throttle valve  21  necessary to make the air fuel ratio of the exhaust gas rich, and outputs a control signal to the intake air throttle valve  21  to adjust the throttle amount. 
   In step  208 , the controller  32  calculates the degree of opening of the EGR valve  24  necessary to make the air fuel ratio of the exhaust gas rich, and outputs a control signal to the EGR valve  24  to adjust the degree of opening. In step  209 , the controller  32  calculates the amount of air and the air fuel ratio. In step  210 , the O 2  sensor  31  detects the oxygen concentration, and outputs the detection value to the controller  32 . In step  211 , the controller  32  corrects the air fuel ratio. In step  212 , the controller  32  calculates the injection rate of a reducer (fuel in this embodiment) and an injecting time Tdef as an adjustment amount to provide the air fuel ratio for the exhaust gas necessary to reduce the total amount of NOx. 
   In step  213 , a timer is started. In step  214 , the controller  32  outputs a control signal to the fuel injection pump  34  and the fuel injection nozzle  35 , and injects fuel based on the calculated value. In step  215 , the engine performs combustion in the condition near the theoretical mixture ratio. In step  216 , the controller  32  determines whether a fuel injection time T exceeds Tdef or not, and in the case of NO, a command is given to the situation before step  213 . In the case of YES in step  216 , the controller  32  opens the intake air throttle valve  21  to make the air fuel ratio of the exhaust gas lean in step  217 . 
   In step  218 , the controller  32  calculates the degree of opening of the EGR valve necessary to recirculate a predetermined amount of exhaust gas (for example, 10% to 15% of the amount of exhaust gas) in a lean state of the air fuel ratio of the exhaust gas, and adjusts the EGR valve  24  based on the calculated value. In step  219 , the engine performs normal lean-burn combustion (recirculation of a predetermined amount of exhaust gas is performed), and a command is given to return to the situation before step  201 . 
   The exhaust gas deNOx apparatus of the sixth embodiment of the present invention has the constitution and the operation process as described above, and therefore the following effects can be obtained. 
   As in the first embodiment, the recirculating circuit adjusting valve  24  is always adjusted so that an area enough to recirculate a few percent of the exhaust gas amount is opened in an ordinary lean-burn state. As a result, the exhaust amount of NOx is reduced, and the NOx adsorber catalyst  9  can be made small, thus making it possible to make the engine  1  compact and reduce cost. Further, in the sixth embodiment, the fuel as an adjusting amount is designed to be injected so that the air fuel ratio of the exhaust gas becomes such an air fuel ratio as to allow NOx to be released and reduced, and therefore the air fuel ratio of supply air can be surely controlled to be close to the theoretical air fuel ratio. 
   Further, when the engine  1  is operated under light load, combustion at the theoretical mixture ratio is performed when the accumulation amount in the NOx adsorber catalyst  9  reaches 50% to 70% of the absorption limit amount. Since the engine  1  is under a light load, addition of the fuel can be reduced, which is economical and can prevent excessive rise in inner pressure of the cylinders  1   a  provided at the engine  1  and the temperature of the exhaust gas and the like. Further, the frequency of addition of fuel in a heavy load area is reduced, and reliability and durability of the engine  1  can be improved. 
   Though in the sixth embodiment, the added fuel is injected into the cylinders  1   a  and the exhaust pipe  8 , but it may be suitable that fuel is injected into either the cylinders  1   a  or the exhaust pipe  8 . 
     FIG. 10  is a conceptual diagram of an exhaust gas deNOx apparatus of a seventh embodiment, and is an example in which the third embodiment, a modification of the fourth embodiment and the sixth embodiment are partially combined. Specifically, as compared with  FIG. 7  of the sixth embodiment, the air bleed pipe  12  connecting the air supply pipe  4  and the tail pipe  8   a , and the air bleed valve  23  provided at the air bleed pipe  12  are further included and the bleed valve  23  and the controller  32  are connected, as shown in  FIG. 4  (the third embodiment). Further, as compared with  FIG. 7 , the exhaust gas throttle valve  22  is proved at the exhaust pipe  8  and the exhaust gas throttle valve  22  is connected to the controller  32 , as shown in  FIG. 9  (the modification example of the fourth embodiment). 
   Further, as compared with  FIG. 7 , the exhaust gas recirculating circuit  10  for connecting the down stream side of the inter cooler  6  and the upstream side of the exhaust turbine  2   a , and the EGR valve  24  fitted onto the exhaust gas recirculating circuit  10  and the recirculating gas cooler  11  are eliminated. Instead, as shown in  FIG. 9 , the first exhaust gas recirculating circuit  13  with a small passage area and the second exhaust gas recirculating circuit  14  with a large passage area are provided in parallel, and the recirculating gas cooler  11  is fitted onto the first exhaust gas recirculating circuit  13 , while the second EGR valve  26  is fitted onto the second exhaust gas recirculating circuit  14 . The NOx sensor  30 , the O 2  sensor  31 , the air bleed valve  23 , the intake air throttle valve  21 , the exhaust gas throttle valve  22 , the aforementioned fuel supply means  33 , the second EGR valve  26  and the controller  32  constitute exhaust gas recirculating amount control means  20   f.    
   In the seventh embodiment described as above, each element of the exhaust gas recirculating amount control means  20   f  basically operates as described in the aforementioned third embodiment, the modification example of the fourth embodiment and the sixth embodiment. However, regarding the operation of the air bleed valve  23 , the intake air throttle valve  21  and the exhaust gas throttle valve  22 , only any one of them may be controlled, or both the air bleeding valve  23  and the exhaust gas throttle valve  22  may be controlled. In the seventh embodiment like this, the same operational effects as in the aforementioned corresponding embodiments can be obtained. In  FIG. 10 , the engine  1  includes the turbocharger  2 , but when the air bleed valve  23  is not operated, the turbocharger  2  may be eliminated or may not be operated. The O 2  sensor  31  may be omitted if it is not necessary. 
     FIG. 11  is a conceptual diagram of an exhaust gas deNOx apparatus of an eighth embodiment, and is an example in which the third embodiment, the fifth embodiment and the sixth embodiment are partially combined, but it can be said to be the modification example of the seventh embodiment. Specifically, as compared with  FIG. 10  of the seventh embodiment, the first exhaust gas recirculating circuit  13 , the second exhaust gas recirculating circuit  14 , the recirculating gas cooler  11  and the second EGR valve  26  are eliminated. Instead, the third exhaust recirculating circuit  15  with a large passage area in  FIG. 6  (the fifth embodiment), the first EGR valve  25  with a small diameter and the second EGR valve  26  with a large diameter provided in parallel in the third exhaust gas recirculating circuit  15  are used. The first EGR valve  25  and the second EGR valve  26 , and the controller  32  are connected. The NOx sensor  30 , O 2  sensor  31 , the air bleed valve  23 , the intake air throttle valve  21 , the exhaust gas throttle valve  22 , the aforementioned fuel supply means  33 , a plurality of EGR valves  25  and  26 , and the controller  32  constitute exhaust gas recirculating amount control means  20   g.    
   In the above eighth embodiment, similarly to the seventh embodiment, the same operational effects as in the aforementioned corresponding embodiments can be also obtained. In  FIG. 11 , the engine  1  includes the turbocharger  2 , but when the air bleed valve  23  is not operated, the turbocharger  2  may be eliminated or may not be operated. The O 2  sensor  31  may be omitted if it is not necessary. Further,  FIG. 11  shows the two EGR valves  25  and  26 , but a plurality of EGR valves including three or more may be used. Further, the recirculating gas cooler  11  that is used in the other embodiments is omitted, but it may be provided as necessary. 
     FIG. 12  is a conceptual diagram of an exhaust gas deNOx apparatus of a ninth embodiment, and is a modification example of the seventh embodiment. Specifically, as compared with  FIG. 10  of the seventh embodiment, the turbocharger  2  is changed to a variable turbocharger  50 , and the exhaust gas throttle valve  22  provided at the exhaust pipe  8  is eliminated. An ordinary variable turbocharger can be used as the variable turbocharger  50 , which is constituted by an exhaust turbine  50   a  and a compressor  50   b . Degree of opening of a turbine passage  51   a  in the exhaust turbine  50   a  is controlled to be variable according to a control signal from the controller  32 . Specifically, exhaust gas recirculating amount control means  20   h  of the ninth embodiment includes the variable turbocharger  50  instead of the exhaust gas throttle valve  22  in the seventh embodiment. 
   According to the above constitution, by making the degree of opening of the turbine passage  51   a  small (for example, extremely small), exhaust gas pressure at an inlet port of the exhaust gas turbine  50   a  can be made higher than supply air pressure at an outlet port of the compressor  50   b . When the air fuel ratio of the exhaust gas is brought to a rich state, this pressure difference between the supply air and exhaust gas is utilized, whereby a required amount of exhaust gas can be made to flow into the supply air. As a result, with use of the variable turbocharger  50 , the same effects as with the exhaust gas throttle valve  22  of the seventh embodiment can be also obtained. The O 2  sensor  31  may be omitted if it is not necessary. 
   Next, a tenth embodiment will be explained. The tenth embodiment is related to another example of the “NOx amount detection means”, and is applicable to the constitution including the load detection means  40  having the fuel injection rate sensor  41  and the engine speed sensor  42 , and the controller  32 . Specifically, the tenth embodiment is an example with the sixth embodiment (FIG.  7 ), the seventh embodiment (FIG.  10 ), the eighth embodiment ( FIG. 11 ) and the ninth embodiment ( FIG. 12 ) as the basis. 
     FIG. 13  shows relationship between the NOx amount discharged per unit horse power per unit time, the fuel injection rate (vertical axis) and the engine speed (horizontal axis), which is obtained from the engine test of the engine  1 . Each curve in  FIG. 13  shows the same NOx amount. In the tenth embodiment, the above  FIG. 13  is stored in the controller  32  as “a map”. Signals from the engine speed sensor  42  and the fuel injection rate sensor  41  are inputted, and when the engine speed and fuel injection rate at that time are known, the NOx amount discharged per unit horse power per unit time is known. In the tenth embodiment, the load detection means  40  constituted by the fuel injection amount sensor  41  and the engine speed sensor  42  and the map are made the “NOx amount detection means”. Hereinafter, “the engine speed and the fuel injection amount at that time” is called an “operation point”. 
   The controller  32  measures how many hours the engine  1  is being operated for at various operation points by means of a timer (not shown) or the like, and the NOx amount discharged as the result of the engine  1  being operated at each operation point is integrated. From the integration, the NOx amount discharged from the engine  1  is estimated as the NOx accumulation amount of the NOx adsorber catalyst  9 . In the above embodiment, the same operational effects can be obtained as in the case in which the NOx sensor  30  is used.  FIG. 13  is one example of a map showing the NOx amount per horse power per unit time discharged from the engine  1 , and it changes depending on the various conditions such as the presence or absence of the turbocharger  2 , and a fuel injection timing by the fuel injection nozzle  35 . 
   In the above-described first, second, fourth, fifth, sixth and tenth embodiment, the engine  1  includes the turbocharger  2 , but the turbocharger  2  may be eliminated.