Abstract:
This invention relates to an exhaust recirculation system for a multi-cylinder internal combustion engine ( 1 ), wherein intake air aspirated to a collector ( 4 ) from an intake passage ( 6 ) of circular cross-section is provided to cylinders (# 1-#4 ) via branch pipes ( 5 A- 5 D) connected the collector ( 4 ). The exhaust gas recirculation system is provided with an exhaust gas recirculation pipe ( 7 ) which branches off from an exhaust passage ( 3 ) and connected to the intake passage ( 6 ), and an exhaust gas inlet ( 8, 9 ) which leads a recirculated exhaust gas of the exhaust recirculation pipe ( 7 ) into the intake passage ( 6 ) along a tangent line of the circular cross-section of the intake passage ( 6 ).

Description:
FIELD OF THE INVENTION 
     This invention relates to an exhaust recirculation system in an internal combustion engine such as a diesel engine of an automobile. 
     BACKGROUND OF THE INVENTION 
     In regard to an exhaust recirculation system (EGR system) wherein a part of the exhaust is recirculated into the intake air to perform combustion control of an internal combustion engine, various devices have been proposed to promote mixing of intake air and recirculated exhaust gas. 
     Jikkai Hei 3-114563 published by the Japanese Patent Office in 1991 discloses a system wherein an insulator is gripped between a throttle case and surge tank in the intake passage of the engine, a pair of exhaust inlets are provided at an 180-degree interval in the insulator towards the center of the intake passage, and exhaust gas is mixed with intake air through these exhaust gas inlets. 
     Jikkai Hei 3-114564 published in 1991 by the Japanese Patent Office discloses a device wherein an annular passage for leading exhaust gas surrounds the intake passage, and exhaust gas is led into the intake air via plural holes connecting the annular passage and intake passage. 
     Tokkai Hei 5-106519 published by the Japanese Patent Office in 1993 discloses a system wherein a minute throughhole connecting an exhaust port and intake port of an engine. 
     Further, this prior art discloses that the air-fuel mixture of fuel injected into the intake port and intake air is heated by the exhaust gas led to the intake port, and a swirl promoted by the throughhole arranged on a tangential line of the cross-section of the inlet port accelerates evaporation of the fuel adhering to the wall of the inlet port. 
     SUMMARY OF THE INVENTION 
     When exhaust gas is introduced from a right-angled direction as in Jikkai Hei 3-114563 and Jikkai Hei 3-114564 towards the center of the intake passage of a multi-cylinder engine, the exhaust gas forms an uneven flow in the passage due to the flow of intake air in the intake passage, and this causes unevenness in the exhaust concentration in each part of the cross-section of the passage. As a result, the exhaust gas concentration in the intake air will vary greatly depending on the cylinder. In general, in diesel engines which do not have a throttle, the flow of intake air is close to laminar flow, so this phenomenon is very pronounced, and as a result, a difference easily arises in the exhaust recirculation rate depending on the cylinder. 
     On the other hand, when the exhaust gas is supplied to the intake air through a minute throughhole formed in the cylinder head as in Tokkai Hei 5-106519, the exhaust recirculation rate is limited, and as the exhaust gas is mixed with the intake air just before the combustion chamber, the exhaust gas and intake air are aspirated into the combustion chamber without being properly mixed, and a variation in the running conditions of the engine has a direct effect on the exhaust recirculation rate (EGR rate). Such characteristics may makes the EGR rate of the engine unstable. 
     It is therefore an object of this invention to make the EGR rate of each cylinder of a multi-cylinder engine uniform, while preventing the variation of the running conditions of the engine from affecting the EGR rate. 
     In order to achieve the above object, this invention provides a exhaust recirculation system for a multi-cylinder internal combustion engine, wherein intake air aspirated to a collector from an intake passage of circular cross-section is provided to cylinders via branch pipes connected to the collector, and part of exhaust gas of the engine is recirculated to the intake passage from an exhaust passage. The exhaust recirculation system comprises an exhaust recirculation pipe which branches off from the exhaust passage and is connected to the intake passage, and an exhaust gas inlet which leads a recirculated exhaust gas of the exhaust recirculation pipe into the intake passage along a tangent line of the circular cross-section of the intake passage. 
     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an exhaust recirculation system according to this invention. 
     FIG. 2 is a cross-sectional view of an intake passage according to this invention showing a construction of an exhaust gas inlet. 
     FIG. 3 is a schematic perspective view of a collector and the intake passage according to this invention describing the flow of intake air and exhaust gas. 
     FIG. 4 is a cross-sectional view of the intake passage taken at the exhaust gas inlet for the purpose of describing a flow of exhaust gas led into the intake passage. 
     FIG. 5 is a diagram showing an EGR rate in each cylinder of an engine with the exhaust recirculation system according to this invention. 
     FIG. 6 is a schematic cross-sectional view of an intake passage with a pair of exhaust gas inlets according to a second embodiment of this invention. 
     FIG. 7 is a cross-sectional view of an intake passage with exhaust gas inlets according to a third embodiment of this invention. 
     FIG. 8 is similar to FIG. 7, but showing a fourth embodiment of this invention. 
     FIGS. 9A and 9B are cross-sectional views of exhaust gas inlets taken along lines IXA—IXA and IXB—IXB of FIG.  8 . 
     FIG. 10 is similar to FIG. 7, but showing a fifth embodiment of this invention. 
     FIGS. 11A and 11B are cross-sectional views of exhaust gas inlets taken along lines XIA—XIA and XIB—XIB of FIG.  11 . 
     FIG. 12 is similar to FIG. 7, but showing a sixth embodiment of this invention. 
     FIG. 13 is similar to FIG. 7, but showing a seventh embodiment of this invention. 
     FIG. 14 is a cross-sectional view of an exhaust gas inlet and a rib taken along a line XIV—XIV of FIG.  13 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, an intake manifold  2  is installed on one side of a four-cylinder diesel engine  1 , and an exhaust manifold  3  is installed on the other side. The intake manifold  2  comprises a collector  4  extending in the direction of the row of cylinders of the engine, and branch pipes  5 A- 5 D which connect the collector  4  with cylinders # 1  to # 4  of the engine. The collector  4  has a longitudinal axis X as shown in the figure and the branch pipes  5 A- 5 D are connected to the collector  4  from a direction perpendicular to the longitudinal axis X. 
     An intake passage  6  is connected to one end of the collector  4  from a direction corresponding to the longitudinal axis X. Fresh air is introduced into the intake passage  6  through an air cleaner, not shown in the figure. 
     An exhaust recirculation pipe  7  is connected to the intake passage  6  at right-angles to the intake passage  6  via an exhaust gas inlet  8 . The exhaust recirculation pipe  7  branches from an exhaust passage  14  connected to the exhaust manifold  3 , and recirculates part of the exhaust gas of the engine  1  to the intake passage  6 . An exhaust recirculation control valve or the like, not shown in FIG. 1, is installed in the exhaust recirculation pipe  7 . 
     Next, referring to FIG. 2, the exhaust gas inlet  8  comprises an exhaust gas conduit  8 B and an opening  8 A located on a tangent line of a circular cross-section of the intake passage  6 . Due to the construction of this exhaust gas inlet  8 , recirculated exhaust gas flowing from the exhaust recirculation pipe  7  into the intake passage  6  sets up a flow along the cylindrical wall of the intake passage  6  as shown in FIG.  4 . 
     The intake passage  6  comprises a pipe of circular cross-section of molded aluminum alloy. The exhaust gas inlet  8  is precast as a boss together with the pipe, and the exhaust recirculation pipe  7  comprising a metal pipe is connected to this boss. 
     In FIG. 2, the exhaust gas inlet  8  and the branch pipes  5 A- 5 D are located on the same side of the intake passage  6 , but the location of the exhaust gas inlet  8  relative to the intake passage  6  is not limited to this and the angle between the exhaust gas inlet  8  and the branch pipes  5 A- 5 D may be freely set. 
     Next, referring to FIG. 3, fresh air aspirated through the intake passage  6  sets up a laminar flow as shown by arrows with broken lines. 
     The recirculated exhaust gas flowing into the intake passage  6  from the exhaust gas inlet  8  forms a swirl along the cylindrical wall surface of the intake passage  6 , as shown by arrows with solid lines in FIGS. 3 and 4. Due to the effect of the flow of fresh air, this swirl sets up a spiral flow towards the collector  4 , and due to the dispersion of the recirculated exhaust gas toward the inside, it mixes with the fresh air flowing inside the spiral. On the other hand, as the direction of the spiral flow is effectively perpendicular to the flow of fresh air, turbulence is set up at the boundary between the fresh air and the recirculated exhaust gas. The energy of this turbulence promotes mixing of the fresh air and the recirculated exhaust gas. 
     Thus, the exhaust concentration of the intake air flowing into the collector  4  is made more uniform by the spiral flow due to the exhaust gas inlet  8  which promotes mixing of the fresh air and recirculated exhaust gas, and deviation of the EGR rate in the cylinders # 1 -# 4  become small as shown in FIG.  5 . Moreover, due to the mixing of recirculated exhaust gas and fresh air upstream of the collector  4 , the exhaust recirculation rate is not easily influenced by the running conditions of the engine  1 . 
     FIG. 6 shows a second embodiment of this invention. 
     In this embodiment, two exhaust gas inlets  8  and  9  are provided on the outer circumference of the intake passage  6  at a 180-degree interval. 
     The exhaust gas inlets  8  and  9  are arranged so as to set up a swirl in the same direction. The exhaust gas inlet  8  comprises an exhaust gas conduit  8 B and opening  8 A. the exhaust gas inlet  9  comprises an exhaust gas conduit  9 B and opening  9 A. The exhaust gas conduits  8 B and  9 B are respectively connected to the exhaust recirculation pipe  7 . The exhaust gas inlets  8  and  9  are formed on the same cross-section of the intake passage  6  as shown in FIG. 6, but they may be slightly offset from each other in the direction of the fresh air flow in the intake passage  6 . According to this embodiment, exhaust gas flows into the intake passage  6  from two positions respectively in tangential directions, so the swirl of recirculated exhaust gas is enhanced. 
     FIG. 7 shows a third embodiment of this invention. 
     This embodiment provides two exhaust gas inlets  8  and  9  in the intake passage  6  as in the second embodiment. 
     The exhaust gas conduits  8 B and  9 B are precast as a part of the exhaust gas inlets  8  and  9  together with the intake passage  6 . 
     The exhaust gas conduit  8 B leading to the opening  8 A has a short straight conduit, but the exhaust gas conduit  9 B leading to the opening  9 A forms a long circular arc along the outer circumference of the intake passage  6 . The exhaust gas conduits  8 B,  9 B are connected to the exhaust recirculation pipe  7  via a common flange  10  which is also precast as a part of the exhaust gas inlets  8  and  9 . 
     The cross-sectional area of the exhaust gas conduit  9 B gradually decreases as it approaches the opening  9 A as shown by S 1 -S 5  in the figure. The cross-sectional area of the exhaust gas conduit  8 B also gradually decreases as it approaches the opening  8 A as shown by S 6  and S 7  in the figure. 
     This arrangement of the exhaust gas conduits  8 B,  9 B has the following effect. The long, arc-shaped exhaust gas conduit  9 B has a supercharging effect due to the inertial force of the exhaust gas flowing inside it. In addition, as the cross-sectional areas of the exhaust gas conduits  8 B and  9 B respectively decrease toward the openings  8 A and  9 A, the speed of the exhaust gas flowing into the intake passage  6  increases, and the exhaust gas is therefore mixed sufficiently with fresh air even at a low exhaust recirculation rate. 
     FIGS. 8,  9 A and  9 B show a fourth embodiment of this invention. 
     This embodiment provides exhaust gas conduits  8 B and  9 B which are also disposed around the intake passage  6  as in the case of the above-mentioned third embodiment, but have different cross sections. The openings  8 A,  9 A of the exhaust gas inlets  8 ,  9  are formed in an elliptical shape with identical dimensions as shown in FIGS. 8A and 9A. The long axis of the ellipse is set parallel to the direction of fresh air flow of the intake passage. The cross-sections of the exhaust gas conduits  8 B and  9 B accordingly have an elliptical shape, and respectively decrease as they approach the openings  8 A and  9 A as in the third embodiment. 
     According to this embodiment, the thickness of the swirl set up by the recirculated exhaust gas flowing into the intake passage  6  is thinner. Therefore, the flow of recirculated exhaust gas flowing into the intake passage  6  is not swept away easily by the flow of fresh air of the intake passage  6 , and a swirl is more firmly established along the cylindrical walls of the intake passage  6 . 
     FIGS. 10,  11 A and  11 B show a fifth embodiment of this invention. 
     This embodiment differs from the above-mentioned fourth embodiment in that the opening  9 A of the exhaust gas inlet  9  is set larger than the opening  8 A of the exhaust gas inlet  8 . Accordingly, the cross sectional area of the exhaust gas conduit  9 B is set to be larger than that of the exhaust gas conduit  8 B. 
     The exhaust gas conduit  9 B is longer than the exhaust gas conduit  8 B, so by setting the opening  9 A to be larger than opening  8 A in this manner, the flowrate ratio of the exhaust gas conduits  8 B,  9 B become almost equal, which creates a desirable environment for setting up a swirl. 
     FIG. 12 shows a sixth embodiment of this invention. 
     According to this embodiment, throughholes  11  are formed in the cylindrical wall of the intake passage  6  to connect the exhaust gas conduits  8 B,  9 B and the intake passage  6 . 
     A part of the recirculated exhaust gas of the exhaust gas conduits  8 B,  9 B flows into the intake passage  6  through these throughholes  11 , so turbulence in the intake passage  6  is promoted, and mixing of exhaust gas with fresh air is promoted. Moreover, since the exhaust gas flows in from various points on the cylindrical wall of the intake passage  6 , the concentration of the exhaust gas in the intake passage  6  is made uniform. 
     FIGS. 13 and 14 show a seventh embodiment of this invention. In this embodiment, ribs  12 ,  13  are provided immediately upstream of the openings  8 A,  9 A in the intake passage  6 . The ribs  12  and  13  respectively project from the inner wall of the intake passage  6  into the intake passage  6  like a weir which is perpendicular to the flow of fresh air of the intake passage  6 . Crests  12 A and  13 A of the weir are on a straight line of the same height as the openings  8 A,  9 A. In other words, the ribs  12 ,  13  have a crescent shape, and the maximum value of the height from the inner circumference of the intake passage  6  to the crests  12 A and  13 A is equal to the length of the short axis of the elliptically-shaped cross-sections of the openings  8 A,  9 A. 
     The ribs  12 ,  13  thus arranged prevent the flow of intake air shown by arrows with broken lines in FIG. 14 from colliding with the recirculated exhaust gas flowing into the intake passage  6  from the exhaust gas inlets  8  and  9 . As a result, the recirculated exhaust gas which flows into the intake passage  6  is not easily swept out by fresh air, and the swirl flow along the cylindrical walls of the intake passage  6  is set up more easily. 
     The contents of Tokugan Hei 10-336420, with a filing date of Nov. 27, 1999 in Japan, are hereby incorporated by reference. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. 
     The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: