Abstract:
An emission control system, includes: an exhaust pipe portion, adapted to guide exhaust gas from an engine to outside; a catalyst, accommodated in the exhaust pipe portion; an additive injection valve, provided at the exhaust pipe portion located at upstream side of the catalyst, and configured to inject additives to be supplied to the catalyst toward a flow of the exhaust gas in the exhaust pipe portion. The exhaust pipe portion has a flow path section located at upstream side of the catalyst, which intersects an injection flow of the additives injected by the additive injection valve, and the flow path section is made to have substantially the same shape as an injection region of the injection flow which confronts the exhaust gas.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    For purification of exhaust emissions from a car (a vehicle) having a diesel engine, an emission control system is used in which a adsorption type NOx catalyst, a selective reduction type NOx catalyst and a diesel particulate filter are combined together to prevent the emission of NOx (oxides of nitrogen) and PM (particulate matter) which are contained in exhaust emissions from a diesel engine to the atmosphere 
         [0002]    An emission control system like this adopts a construction in which a catalyst referred to as a pre-stage catalyst such as an oxidation catalyst, a NOx trap catalyst or a selective reduction type NOx catalyst is provided in an interior of an exhaust pipe portion which expels exhaust gases discharged from the engine to the outside thereof, and a fuel injection valve (such as one for adding a reducing agent) which injects a fuel required for reaction of the catalyst to an upstream side of the catalyst, for example, the oxidation catalyst. 
         [0003]    In the emission control system, for efficient reaction of the pre-stage catalyst, it becomes important that the injected fuel is sufficiently mixed with the exhaust gases before the fuel flows into the pre-stage catalyst. 
         [0004]    For this to happen, a sufficient fuel spray travel needs to be secured in a segment from the fuel adding valve to the catalyst. 
         [0005]    However, as it is required that a location where a catalyst is installed is secured in the vicinity of an exhaust side of an engine in order to meet the recent tendency of enhancing the exhaust emission purifying efficiency at the time of cold start, it becomes difficult to ensure such a sufficient fuel spray travel. An example of an engine adopting the aforesaid construction is disclosed in JP-A-2005-127260. 
         [0006]    Namely, in order to make full use of the catalyst installation design described above, as is disclosed in JP-A-2005-127260, a fuel injection valve has to be provided at a part of an exhaust pipe which lies directly upstream of the catalyst, that is, for example, at a bent portion of the exhaust pipe. To make this happen, it becomes difficult to ensure a long enough fuel spray travel for mixture of fuel with exhaust gases between the fuel injection valve and the catalyst. 
         [0007]    From the reasons regarding the installation of the fuel injection valve and the catalyst, the situation in which the fuel spray travel for mixture of fuel with exhaust gases becomes difficult to be ensured is seen not only the emission control system in which the bent portion is provided in the exhaust pipe but also in an emission control system in which no such bent portion is provided. 
         [0008]    To cope with this, as is disclosed in JP-A-2004-44483, an emission control system is proposed which has a construction in which a fuel injection valve is disposed in a position lying far apart from an exhaust pipe portion so that fuel is injected from the position which lies far away from the flow of exhaust gases. 
         [0009]    However, even with this proposed emission control system, from the limitations on the installation of the fuel injection valve and the catalyst, which are similar to the aforesaid one, there still remains the situation in which the fuel spray travel for mixture of fuel with exhaust gases is difficult to be ensured. 
         [0010]    Due to this, in an emission control system, it remains difficult to mix injected fuel with exhaust gases sufficiently. This has caused a problem that uniformly atomized fuel is difficult to be supplied to a catalyst and hence the catalyst cannot fulfill its function. 
       SUMMARY 
       [0011]    It is therefore an object of the invention to provide an emission control system which enables a sufficient mixture of a reducing agent with exhaust gases even though a fuel spray travel necessary for the aforesaid mixture is not ensured between an additive injection valve and a catalyst. 
         [0012]    In order to achieve the object, according to the invention, there is provided an emission control system, comprising; 
         [0013]    an exhaust pipe portion, adapted to guide exhaust gas from an engine to outside; 
         [0014]    a catalyst accommodated in the exhaust pipe portion; 
         [0015]    an additive injection valve, provided at the exhaust pipe portion located at upstream side of the catalyst, and configured to inject additives to be supplied to the catalyst toward a flow of the exhaust gas in the exhaust pipe portion, wherein 
         [0016]    the exhaust pipe portion has a flow path section located at upstream side of the catalyst, which intersects an injection flow of the additives injected by the additive injection valve, and 
         [0017]    the flow path section is made to have substantially the same shape as an injection region of the injection flow which confronts the exhaust gas. 
         [0018]    The flow path section may be an exhaust gas induction portion that is formed at the exhaust pipe portion located at upstream side of the catalyst, which intersects the injection flow of the additives injected by the additive injection valve, and that has a shape being identical with an injection region of the injection flow as viewed from a direction which intersects the flow of the exhaust gas. 
         [0019]    The exhaust gas induction portion may have a shape that expands towards a leading end of the injection flow and that is shaped in such a manner that an upstream side of the injection flow is thin and a downstream side of the injection flow is thick. 
         [0020]    The exhaust pipe portion located at upstream side of the catalyst may be formed in such a manner as to intersect the injection flow of the additives at an acute angle and to confront the catalyst. The exhaust gas induction portion may be formed from an upstream part of the exhaust pipe portion which intersects the injection flow of the additives. 
         [0021]    The exhaust pipe portion located at upstream side of the catalyst may be formed in such a manner as to intersect the injection flow of the additives in a substantially normal direction and to confront the catalyst. The exhaust gas induction portion may be formed from an upstream part of the exhaust pipe portion which intersects the injection flow of the additives. 
         [0022]    A release portion where a contact with the injection flow of the additives which is deflected as a result of collision with the exhaust gas is avoided, may be formed at a downstream part of the exhaust pipe portion which intersects the injection flow of the additives. 
         [0023]    The exhaust gas induction portion may be formed in such a manner as to keep an area of a flow path from an upstream side of the exhaust pipe portion constant. 
         [0024]    The exhaust pipe portion may be provided with: a bent portion, connected to an upstream side of the catalyst, and formed by bending the exhaust pipe portion; and a projecting portion, one end of which is opened to a wall surface of the bent portion, and which projects to an opposite side of the catalyst in a direction of a central axis of the catalyst. The additive injection valve may be provided at the other end of the projecting portion. 
         [0025]    An additive mixing portion may be formed at the exhaust pipe portion located at upstream side of the catalyst, through which the injection flow of the additives injected by the additive injection valve passes, and have a shape being identical with a section of an injection region of the injection flow. 
         [0026]    The additive mixing portion may be formed in such a manner as to keep an area of a flow path from an upstream side of the exhaust pipe portion constant. 
         [0027]    An outlet of the additive mixing portion may be expanded into a bell mouth shape. 
         [0028]    The exhaust pipe portion may be provided with: a bent portion, connected to an upstream side of the catalyst, and formed by bending the exhaust pipe portion; and a projecting portion, one end of which is opened to a wall surface of the bent portion, and which projects to an opposite side of the catalyst in a direction of a central axis of the catalyst. The additive injection valve may be provided at the other end of the projecting portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a partially sectional side view showing the construction of an emission control system according to a first embodiment of the invention. 
           [0030]      FIG. 2  is a sectional view of an exhaust gas induction portion taken along the line A-A in  FIG. 1 . 
           [0031]      FIG. 3  is a schematic perspective view showing the exhaust gas induction portion. 
           [0032]      FIG. 4  is a sectional side view showing the construction of an emission control system according to a second embodiment of the invention. 
           [0033]      FIG. 5  is a sectional view taken along the line B-B in  FIG. 4 . 
           [0034]      FIG. 6  is a sectional side view showing in an enlarged fashion of the vicinity of an inlet of a catalyst of an emission control system according to a third embodiment of the invention. 
           [0035]      FIG. 7  is a sectional view taken along the line C-C in  FIG. 6 . 
           [0036]      FIG. 8  is a side view showing an emission control system according to a fourth embodiment of the invention. 
           [0037]      FIG. 9  is a sectional view taken along the line D-D in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0038]    Hereinafter, the invention will be described based on a first embodiment shown in  FIGS. 1 to 3 . 
         [0039]      FIG. 1  shows an exhaust system of an internal combustion engine, for example, a diesel engine. In  FIG. 1 , reference numeral  1  denotes an engine main body of the diesel engine,  1   a  an exhaust manifold (only part of which is shown) of the engine main body  1  of the diesel engine, and  2  a supercharger or, in this embodiment, a turbocharger connected to an outlet of the exhaust manifold  1   a.    
         [0040]    An emission control system  3  is provided at an exhaust outlet of the turbocharger  2 . A system in which a NOx removing system  3   a  for adsorbing NOx (oxides of nitrogen) contained in exhaust gases and periodically reduction removing the adsorbed NOx and a PM (particulate matter) capturing system  3   b  for capturing PM are combined together is used for the emission control system  3 . 
         [0041]    For example, used for the NOx removing system  3   a  is a configuration in which a catalytic converter  6 , a catalytic converter  9  and a fuel addition valve (an additive injection valve)  23  are combined together. The catalytic converter  6  is coupled to the turbocharger  2  in such a manner as to extend downwards from the exhaust outlet of the turbocharger  2  and incorporates therein a pre-stage oxidation catalyst  5  (corresponding to the catalyst of the subject patent application). The catalytic converter  9  is coupled horizontally to the rear of the catalytic converter  6  and incorporates therein a NOx trap catalyst  8 . The fuel addition valve  23  supplies a catalyst reaction promoting fuel which constitutes additives to the oxidation catalyst  5 , which will be described later. In addition, a configuration is adopted for the capturing system  3   b  in which a catalytic converter  12  which incorporates therein a particulate filter  11  is coupled to the catalytic converter  9 . An exhaust pipe portion  15  for inducing exhaust gases discharged from the diesel engine (the engine main body  1 ) to the outside thereof is made up of the catalytic converters  6 ,  9 ,  12  and connecting portions  13  which connect the catalytic converters. 
         [0042]    In these catalytic converters, a cylindrical housing  17  of the catalytic converter  6  which accommodates therein the oxidation catalyst  5  is bent at an upper portion in such a way as is shown in  FIG. 1 , so that an inlet portion  17   a  is disposed horizontally for connection to the turbocharger  2  which is situated thereabove. In additions an outlet portion  17   b  which communicates with the catalytic converter  9  lying therebelow is disposed in such a manner as to be oriented downwards. A bent portion  15   a  of the exhaust pipe portion  15  is formed in such a manner as to be bent in a position lying directly after an exhaust side of the diesel engine by the housing  17 . A catalyst installation space is secured in a portion lying directly below the bent portion  15   a . The oxidation catalyst  5  is installed in a position lying in the vicinity of the exhaust side of the diesel engine. 
         [0043]    The fuel addition valve  23  is made to inject a fuel required for catalyst reaction to the oxidation catalyst  5  and is provided in a position lying directly above the oxidation catalyst  5 , for example, on the bent portion  15   a  or on an outer circumferential wall portion lying downstream of the bent portion  15   a  to fulfill the aforesaid function. The fuel addition valve  23  has a fuel injecting portion for injecting the fuel at a distal end portion thereof. The fuel addition valve  23  is installed at an end of a cylindrical portion  24  which branches off at an outer circumferential portion of the exhaust pipe portion  15  which lies downstream of the bent portion  15   a  in such a manner as to extend outwards by the use of a mounting flange  24   a  and a base seat  25 . The cylindrical portion  24  is a projecting portion projecting to an opposite side of the oxidation catalyst  5  in a direction of a central axis of the oxidation catalyst  5 . One end of the projecting portion is opened to the wall portion of the bent portion  15   a  and the fuel addition valve  23  is provided at the other end of the projecting portion. By this configuration, the fuel injecting portion at the distal end portion of the fuel addition valve  23  is made to face a fuel injection path  24   b  which is formed by an interior space of the cylindrical portion  24 . The fuel injection path  24   b  extends in such a manner as to be inclined to an opposite side to a direction in which the bent portion  15   a  is bent, and an outlet end thereof is oriented towards a circumferential edge of an inlet end face of the oxidation catalyst  5  rather than towards a center thereof (that is, towards the inlet portion  17   a ). By this configuration, the fuel promoting the reaction of the oxidation catalyst  5  is made to be injected from the position lying far apart from a flow of exhaust gases to the oxidation catalyst  5  from a direction which intersects the flow of exhaust gases which passes through the bent portion  15   a . Specifically, the fuel is made to be injected downwards, that is, towards a mixing chamber  19  defined in front of the inlet end face of the oxidation catalyst  5  from a direction which intersects a flowing direction a of the flow of exhaust gases at an acute angle θ 1  as is shown in  FIG. 1 . By this configuration, a downstream portion of a flow of injected fuel α where a spray penetration becomes weak is made to collide with exhaust gases in a position directly upstream of the oxidation catalyst  5 . In addition, reference numeral  25   a  denotes a coolant channel formed in an interior of the base seat  25 . 
         [0044]    On the other hand, on an exhaust pipe part which lies upstream of the oxidation catalyst  5 , an exhaust gas induction portion  28  is provided in a position where the exhaust pipe part intersects the flow of injected fuel α injected from the fuel addition valve  23 , that is, at an exhaust pipe part S which constitutes an area extending, for example, from the bent portion  15   a  to the mixing chamber  19 . As is shown in a sectional view shown in  FIG. 2  and a perspective view shown in  FIG. 3 , the exhaust gas induction portion  28  is formed in such a manner that a sectional shape of a flow path of the exhaust pipe part S which intersects the flow of injected fuel α equates to a side view of an injection region of the flow of injected fuel α as viewed from a transverse direction in which the flow of injected fuel α intersects exhaust gases. Specifically, the exhaust gas induction portion  28  is formed in such a manner that the sectional shape of the flow path of the exhaust pipe part S is formed into a fan-like shape which is substantially the same shape as a shape of a downstream side of the flow of injected fuel α as projected from the transverse direction. By being formed in the way described above to thereby have the same shape as that of the flow of injected fuel α which expands towards the leading end thereof, the exhaust gas induction portion  28  is made to have a flow path whose sectional shape is such that a portion corresponding to an upstream side of the flow of injected fuel α becomes thin, whereas an opposite portion corresponding to the downstream portion of the flow of injected fuel α becomes thick. In  FIG. 2 , reference numeral  28   a  denotes one of the portions where the flow path section becomes thin, and  28   b  denotes the other portion where the flow path section becomes thick. A flow of exhaust gases is made to be distributed uniformly over the flow of injected additives in the exhaust gas induction portion by the exhaust gas induction portion  28  being configured as has been described above, whereby an opportunity is given where the flow of injected additives is allowed to contact exhaust gases sufficiently. 
         [0045]    A flow path area of the portion of the exhaust pipe portion  15  which extends from the inlet portion  17   a  which constitutes an upstream end of the exhaust pipe portion  15  to the exhaust gas induction portion  28  is held constant to a predetermined flow path area. Of course, the sectional shape of the flow path is constructed in such a manner as to change gradually so as to follow the shape of the flow of injected additives α in the exhaust gas induction portion  28 , whereby the generation of an unnecessary passage resistance is prevented. 
         [0046]    In addition, a portion of the exhaust pipe portion  15  which lies downstream of the exhaust gas induction portion  28  is formed in such a manner as to be expanded in a radial direction. The fuel and exhaust gases which have collided with each other completely are supplied to the inlet end face of the oxidation catalyst  5  while being expanded in the radial direction by an expanded portion  29  which results from the radial expansion of the portion of the exhaust pipe portion  15 . It is effective to form the expanded portion  29  into a flared shape (a bell mouth shape) as well as a conical shape in supplying the mixture of fuel and exhaust gases to the oxidation catalyst  5  under uniform distribution. 
         [0047]    In addition, the fuel injected from the fuel addition valve  23  is used to generate a reducing agent as a result of reaction with the oxidation catalyst  5 , so as to reduce to remove NOx and SOx which are adsorbed on to the NOx trap catalyst  8  by the use of the reducing agent so generated or to obtain heat as a result of reaction with the oxidation catalyst  5 , so as to burn to remove PM captured by the particulate filter  11 . Because of this, the fuel addition valve  23  is controlled by a control unit for controlling the diesel engine, for example, an ECU (not shown) to inject the fuel when a catalytic reaction is required during operation of the diesel engine for reduction removal of NOx and SOx and burning removal of PM. 
         [0048]    Next, the function of the emission control system  3  configured as has been described above will be described. 
         [0049]    Exhaust gases discharged from the diesel engine during operation thereof are, as is shown in  FIG. 1 , expelled to the outside air through the exhaust manifold  1   a , the turbocharger  2 , the bent portion  15   a , the exhaust gas induction portion  28 , the oxidation catalyst  5 , the NOx trap catalyst  8  and the particulate filter  11 . 
         [0050]    NOx and SOx which are contained in exhaust gases are adsorbed on to the NOx trap catalyst  8 , and PM is similarly captured by the particulate filter  11 . 
         [0051]    Assuming that the fuel addition valve  23  is activated as time has arrived to remove the adsorbed NOx and SOx and captured PM. 
         [0052]    Then, the fuel for removing NOx, SOx and PM is injected from the fuel injecting portion of the fuel addition valve  23  from the acute angle direction through the fuel injection path  24   b  to the flow of exhaust gases which is flowing in the exhaust pipe part S. By this action, the flow of injected fuel α and the flow of exhaust gases are made to collide with each other. 
         [0053]    As this occurs, since the exhaust gas induction portion  28 , which has the flow path whose sectional shape is substantially the same as that of the injection region of the flow of injected fuel α as viewed from the transverse direction, is formed in the exhaust pipe part S which intersects the flow of injected fuel α at the portion directly upstream of the downstream portion of the flow of injected fuel α where the spray penetration becomes weak, the exhaust gases which have passed through the exhaust gas induction portion  28  pass over the flow of injected fuel α while being distributed uniformly thereover. 
         [0054]    By this action, there is provided an opportunity where the exhaust gases in the flow of exhaust gases and the fuel in the flow of injected fuel α are allowed to contact each other sufficiently, whereby the exhaust gases and the fuel contact each other uniformly for sufficient mixture of the exhaust gases with the furl. 
         [0055]    As this occurs, since the flow velocities of the fuel and the exhaust gases are such that fuel in the upstream part of the flow of injected fuel α which is near from the fuel addition valve  23  and in which the spray penetration is strong collides with exhaust gases of a high flow velocity whose flow velocity has been increased at the portion  28   a  where the section of the flow path is narrow, while fuel in the downstream part of the flow of injected fuel α which is far away from the fuel addition valve  23  and in which the spray penetration is weak collides with exhaust gases of a low flow velocity whose flow velocity is reduced at the portion  28   b  where the section of the flow path is wide, a further uniform mixture of additives and exhaust gases can be obtained. The fuel and exhaust gases which are mixed uniformly then flow downwards towards the oxidation catalyst  5  while expanding in the radial direction due to the expanded portion  29 . 
         [0056]    Here, since the flow of exhaust gases and the flow of injected fuel α intersect each other at the acute angle to thereby suppress the deflection of the flow of injected fuel α that would otherwise be produced by the flow of exhaust gases, the fuel and exhaust gases which have been mixed with each other in the way described above are supplied to a predetermined position of the oxidation catalyst  5 , for example, to a substantially central portion of the inlet end face thereof. 
         [0057]    Consequently, even though the necessary distance or fuel spray travel is ensured between the fuel addition valve  23  and the oxidation catalyst  5 , the fuel which is uniformly mixed with the exhaust gases can be supplied to the catalyst due to the formation of the exhaust gas induction portion  28 . 
         [0058]    Therefore, the function of the oxidation catalyst  5  can be made to be exhibited sufficiently by the use of the exhaust gas induction portion  28 . Of course, when the expanded portion  29  is used in parallel, the fuel can be supplied to the oxidation catalyst  5  while being distributed more uniformly. On top of this, since the exhaust gas induction portion  28  is formed in such a manner that the flow path area is held constant to the predetermined flow path area from the upstream of the exhaust pipe portion  15 , there is caused no increase in flow path resistance in the exhaust pipe portion  15 , whereby a reduction in engine output can be suppressed. 
         [0059]    In particular, in the event that the exhaust gas induction portion  28  is adopted in the fuel injection construction in which the flow of exhaust gases is made to intersect the flow of injected fuel α at the acute angle, the exhaust gases and the fuel can be mixed together sufficiently while enjoying the benefit of the method in which the fuel is injected to the predetermined position while suppressing the deflection of the flow of injected fuel α. 
         [0060]      FIGS. 4 and 5  show a second embodiment of the invention. In  FIGS. 4 and 5 , like reference numerals are imparted to like portions to those of the first embodiment, and the description thereof will be omitted. 
         [0061]    In the second embodiment, the invention is applied to an emission control system in which a flow of exhaust gases does not intersect a flow of injected fuel α at an acute angle as done in the first embodiment but intersects the flow of injected fuel α at substantially right angles. 
         [0062]    Specifically, in the emission control system of this embodiment, as with the first embodiment, an injecting direction of a fuel addition valve  23  is determined to lie in a position which is offset from an oxidation catalyst  5 . A path is formed in an exhaust pipe part lying upstream of the oxidation catalyst  5  in such a manner as to intersect a flow of injected fuel α injected from the fuel addition valve  23  in a substantially right angle direction so as to be directed towards the oxidation catalyst  5 . By this configuration, on the contrary to the first embodiment, a construction is realized in which the flow of injected fuel α is pushed by the flow of exhaust gases, so as to deflect the flow of injected fuel α from the initial offset position to a desired position, whereby the fuel is injected to a predetermined position. In addition, as with the first embodiment, an exhaust gas induction portion  28  whose flow path section at the exhaust pipe part, which is something like one shown in a sectional view of  FIG. 5 , is made to have substantially the same shape as a side view of an injection region of the flow of injected fuel α is provided directly upstream of an exhaust pipe part S of the exhaust pipe part lying upstream of the oxidation catalyst  5  which intersects the flow of injected fuel α injected from the fuel addition valve  23 . 
         [0063]    By this configuration, even with the emission control system in which the flow of injected fuel α is deflected by the flow of exhaust gases, which is opposite to or differs from the configuration of the first embodiment, so that the fuel is injected to the predetermined position, by enjoying the benefit of the method adopted therein, the exhaust gases can be mixed with the fuel sufficiently by the use of the exhaust gas induction portion  28 . 
         [0064]    In particular, in the second embodiment, for example, a curved recessed portion  30  is formed as a release portion on a wall surface of a downstream side exhaust pipe part T which intersects the flow of injected fuel α in such a manner that nothing in the exhaust pipe portion  15  is affected by the flow of injected fuel α when it is deflected. By this recessed portion  30 , the contact of the flow of injected fuel α which is deflected with the wall portion of the exhaust pipe part T can be avoided, whereby a good mixture of the exhaust gases with the fuel can be promised. Moreover, since a wall portion of the recessed portion  30  causes exhaust gases which have come into contact with the recessed portion  30  to bounce back towards the flow of injected fuel α as is indicated by arrows b so as to cause the exhaust gases to collide with the fuel in the flow of injected fuel α, it can be expected that the mixture of the fuel with the exhaust gases is promoted further. 
         [0065]    In addition,  FIGS. 6 and 7  show a third embodiment of the invention. In  FIGS. 6 and 7 , like reference numerals are imparted to like portions to those of the first embodiment, and the description thereof will be omitted. 
         [0066]    As is shown in an enlarged fashion in  FIG. 6 , a fuel mixing portion (an additive mixing portion)  38  is provided in a position of an exhaust pipe part lying upstream of an oxidation catalyst  5  where a flow of injected fuel α injected from a fuel addition valve  23  passes, specifically, an exhaust pipe part U on an outlet side of a bent portion  15   a  which constitutes a portion where injected fuel and exhaust gases are made to collide with each other. This fuel mixing portion  38  is configured in such a manner that only a sectional shape of a flow path at the exhaust pipe part U is, as is shown in  FIG. 7 , has substantially the same shape as a sectional shape of an injection region of the flow of injected fuel α which passes through the exhaust pipe part U, that is, that the sectional shape of the flow path has the same shape of the sectional shape of the injection region. Fuel injected is made to be given by the fuel mixing portion  38  an opportunity where the fuel is uniformly distributed in an interior of the exhaust pipe part so as to be brought into sufficient contact with exhaust gases directed to the oxidation catalyst  5 . In addition, a flow path area of a portion, including the fuel mixing portion  38 , of an exhaust pipe portion  15  extending up to an inlet portion  17   a  which constitutes an upper end of the exhaust pipe portion  15  is held constant to a predetermined flow path area from upstream, so that an unnecessary passage resistance is not generated. 
         [0067]    As is shown in  FIG. 7 , a wall surface of an end of a housing  17  which continuously follows an outlet of the fuel mixing portion  38  is formed into a bell mouth shape which expands in a radial direction. Fuel injected from the fuel mixing portion  38  is made to be supplied to an inlet end face of the oxidation catalyst  5  while being allowed to expand in the radial direction by a bell mouth portion  39  formed at the outlet of the fuel mixing portion  38 . 
         [0068]    An exhaust pipe part lying in a position where it collides with a flow of injected fuel α and exhaust gases is formed into the fuel mixing portion  38  which has the flow path section which is substantially identical in shape to the section of the injection region of the flow of injected fuel α joins the exhaust gases thereat. Because of this, when the flow of injected fuel α passes through the fuel mixing portion  38 , the flow of injected fuel α passes therethrough while being distributed uniformly thereover. 
         [0069]    By this configuration, since an opportunity is given where exhaust gases in the flow of exhaust gases and fuel in the flow of injected fuel α are allowed to contact with each other sufficiently, the exhaust gases and the fuel are allowed to contact uniformly, and a sufficient mixture of the exhaust gases with the fuel occurs. 
         [0070]    The fuel and exhaust gases which have emerged from the fuel mixing portion  38  are atomized while being allowed to expand in the radial direction by the bell mouth portion  39 , so as to be supplied to the inlet end face of the oxidation catalyst  5  while the fuel is being distributed uniformly. 
         [0071]    Consequently, by the formation of the fuel mixing portion  38 , even though a required distance or fuel spray travel for mixture with exhaust gases is not ensured between the fuel addition valve  23  and the oxidation catalyst  5 , uniformly atomized fuel can be supplied to the catalyst. Needless to say, of course, in the event that this configuration is adopted in parallel with the construction in which fuel is injected from a position which lies far away from the flow of exhaust gases, a good mixture of exhaust gases with fuel can be achieved. 
         [0072]    Consequently, the function of the oxidation catalyst  5  can be exhibited sufficiently. In particular, since exhaust gases from a turbocharger  2  are introduced into the exhaust pipe portion  15  while being swirled, a better mixture of exhaust gases with fuel can be expected due to the swirling flow of exhaust gases produced then. 
         [0073]    Moreover, since the flow path area of the exhaust pipe part including the fuel mixing portion  38  is configured in such as to be held constant to the predetermined flow path area from the upstream portion of the exhaust pipe portion  15 , the flow path resistance in the exhaust pipe portion  15  is increased in no case, whereby a reduction in engine output can be suppressed. 
         [0074]    On top of this, since the exhaust gases and fuel which have flowed out from the fuel mixing portion  38  are directed uniformly to the oxidation catalyst  5  (the catalyst) while being allowed to expand in the radial direction of the housing  17  due to the formation of the bell mouth portion  39  at the outlet of the fuel mixing portion  38 , the fuel can be supplied to the oxidation catalyst  5  while being distributed uniformly. 
         [0075]      FIGS. 8 and 9  show a fourth embodiment of the invention. In  FIGS. 8 and 9 , like reference numerals are imparted to like portions to those of the first embodiment, and the description thereof will be omitted here. 
         [0076]    In this embodiment, the invention is applied to an emission control system  3  in which a fuel addition valve  23  and an oxidation  5  are installed in an exhaust pipe portion  15  without forming a bent portion  15   a.    
         [0077]    Specifically, in the emission control system  3  of this embodiment, a straight-line exhaust pipe portion  15  is used, and an oxidation catalyst  5  is provided in the exhaust pipe portion  15 . In addition, a fuel addition valve  23  is provided directly upstream of the oxidation catalyst  5 . Additionally, a sectional shape of a flow path of a straight-line exhaust pipe part U of the exhaust pipe portion  15  where a flow of injected fuel α passes is made to equate to a sectional shape of an injection region of the flow of injected fuel α which passes through the exhaust pipe portion  15 t whereby a fuel mixing portion  31  is provided.  FIG. 9  shows the sections of the fuel mixing portion  38  and the flow of injected fuel α. 
         [0078]    Even with the emission control system  3  configured as has been described above, in the event that the fuel mixing portion  38  is formed, as with the first embodiment, even though a required distance of fuel spray travel for mixture is not ensured between the fuel addition valve  23  and the oxidation catalyst  5 , uniformly distributed atomized fuel can be supplied to the oxidation catalyst  5  (the catalyst). 
         [0079]    Note that the invention is not limited to the embodiments that have been described heretofore, and hence, the invention may be modified variously without departing from the spirit and scope thereof. For example, in the embodiments, while the exhaust gases are described as being made to intersect the flow of injected fuel or additives at the acute angle or at right angles, the invention is not limited thereto. In addition, in the embodiments that have been described heretofore, while the invention is described as being applied to the emission control system in which the oxidation catalyst is used as a catalyst lying directly downstream of the bent portion and the NOx trap catalyst and the particulate filter are provided downstream of the oxidation catalyst, the invention is not limited thereto. Thus, the invention may be applied to emission control systems which adopt other exhaust gas purifying approaches. For example, the invention may be applied to an emission control system in which a NOx trap catalyst is used as a catalyst to be installed directly downstream of a bent portion, a particulate filter is provided downstream of the NOx trap catalyst, and an addition valve is provided upstream of the NOx trap catalyst, an emission control system in which a NOx trap catalyst is used as a catalyst to be installed directly downstream of a bent portion, a NOx trap catalyst, an oxidation catalyst and a particulate filter are provided downstream of the NOx trap catalyst, and an addition valve is provided upstream of the NOx trap catalyst, or an emission control system in which an addition valve is provided upstream of a selective reduction type catalyst and a particulate filter. 
         [0080]    Furthermore, in the embodiments, while the fuel is described as being used as the additives, any substances may be adopted, provided that they can be supplied to catalysts. For example, as a reducing agent, substances may be used which include gas oil, gasoline, ethanol, dimethyl ether, natural gas, propane gas, urea, ammonia, hydrogen, carbon monoxide and the like. In addition, any substances other than reducing agents may be used, and these substances include, for example, air, nitrogen or carbon dioxide used to cool the catalyst, or air or ceria which promotes the burning removal of soot captured on to the particulate filter. 
         [0081]    In addition, in the embodiments that have been described above, while the injecting configuration of the fuel addition valve  23  is described as being formed into a conical shape, a fuel addition valve having an injecting configuration which expands in a flat fan-shaped fashion or a fuel addition valve in which additives is injected from a plurality of injection holes may be adopted. In the case of a plurality of injection holes being provided, an outline of a plurality of flows of injected fuel constitutes an injection region. 
         [0082]    According to an aspect of the invention, since the flow of exhaust gases is distributed uniformly over the flow of injected additives by the exhaust gas induction portion, the opportunity can be given where exhaust gases in the flow of exhaust gases and additives in the flow of injected additives are allowed to contact with each other sufficiently. 
         [0083]    Consequently, the exhaust gases can be mixed with the additives sufficiently, and even though a required distance or fuel or additive spray travel for mixture is not ensured between the additive injection valve and the catalyst, the additives which is mixed with the exhaust gases uniformly can be supplied to the catalyst. As a result, the function of the catalyst can be exhibited sufficiently. 
         [0084]    According to an aspect of the invention, the flow velocities of the additives and the exhaust gases after they have collided with each other are such that additives in the upstream part of the flow of injected additives in which the spray penetration is strong collides with exhaust gases of a high flow velocity whose flow velocity has been increased at the portion where the section of the flow path is narrow, while additives in the downstream part of the flow of injected additives in which the spray penetration is weak collides with exhaust gases of a low flow velocity whose flow velocity is reduced at the portion where the section of the flow path is wide, whereby a uniform mixture of additives and exhaust gases can be promoted, and the additives and exhaust gases which are mixed uniformly can be supplied to the oxidation catalyst  5 . 
         [0085]    According to an aspect of the invention, furthermore, by enjoying the benefit of the method in which the additives are injected to the predetermined position by suppressing the deflection of the flow of injected additives, the exhaust gases and the additives can be mixed with each other sufficiently. 
         [0086]    According to an aspect of the invention, furthermore, by enjoying the benefit of the method in which the additives are injected to the predetermined position by deflecting the flow of injected additives by the flow of exhaust gases, which is opposite to or different from the method described above, the exhaust gases and the additives can be mixed with each other sufficiently. 
         [0087]    According to an aspect of the invention, even though the flow of injected additives is deflected by the flow of exhaust gases, the additives in the flow of injected additives which is so deflected are prevented from being brought into contact with the wall surface of the exhaust pipe part by the release portion, a good mixture of the additives with the exhaust gases can be promised. 
         [0088]    According to an aspect of the invention, since the flow path area of the exhaust pipe portion is held constant from the upstream portion to the exhaust gas induction portion, the flow path resistance is increased in no case, whereby a reduction in engine output occurs in no case. 
         [0089]    According to an aspect of the invention, the opportunity can be given by the additive mixing portion where the exhaust gases in the flow of exhaust gases and the additives in the flow of injected additives are allowed to contact with each other sufficiently upstream of the catalyst, whereby the exhaust gases and the additives can be mixed together to a sufficient level. 
         [0090]    Consequently, even though a required distance or additive spray travel for mixture is not ensured between the additive injection valve and the catalyst, the uniformly distributed atomized additives can be supplied to the catalyst, whereby the function of the catalyst can be exhibited sufficiently. 
         [0091]    According to an aspect of the invention, since the flow path area of the exhaust pipe portion is held constant from the upstream portion to the additive mixing portion, the flow path resistance is increased in no case, whereby a reduction in engine output occurs in no case. 
         [0092]    According to an aspect of the invention, furthermore, in addition to the advantages that have been raised above, since the exhaust gases and the additives which have flowed out from the additive mixing portion are directed uniformly towards the catalyst while being allowed to expand in the radial direction by the bell mouth-shaped outlet of the additive mixing portion, the additives can be supplied to the catalyst while being distributed uniformly.