Patent Publication Number: US-11035327-B2

Title: Engine device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/088,796 filed Sep. 26, 2018, which is a national stage application pursuant to 35 U.S.C. § 371 of International Application No. PCT/JP2017/010037 filed Mar. 13, 2017, which claims priority under 35 U.S.C. § 119 to Japanese Application No. 2016-066823 filed Mar. 29, 2016, and Japanese Application No. 2016-066824 filed Mar. 29, 2016, the disclosures of which are hereby incorporated by reference in their entireties. 
    
    
     DESCRIPTION 
     Technical Field 
     The present invention relates to an engine device configured to partially circulate exhaust gas to an intake side as EGR gas. 
     Background 
     Traditionally, as a countermeasure against exhaust gas of diesel engines and the like, there has been a technology that adopts an EGR device (exhaust-gas recirculation device), which circulates a portion of exhaust gas to an intake side, to keep the combustion temperature low, thereby reducing an amount of NOx (nitrogen oxide) in the exhaust gas. 
     Examples of such a type of EGR device are disclosed in Patent Literature 1 to Patent Literature 4 (hereinafter referred to as PTL 1 to PTL 4, respectively) and the like. In each of the EGR devices as described in PTL 1 to PTL 3, a recirculation flow pipe passage branched off from an exhaust manifold of a diesel engine is connected to an intake manifold. By supplying a portion of the exhaust gas (EGR gas) to the intake manifold through the recirculation flow pipe passage, the EGR gas is mixed with fresh air from the intake side, the mixed gas is introduced into cylinders (cylinders in an air intake step) of the diesel engine. 
     An installation space for a diesel engine varies depending on a work vehicle (such as a construction machine or an agricultural machine) to which the diesel engine is installed. Recently, due to demand for weight reduction and compactification, the installation space is often restricted (confined). It therefore is necessary that component parts of the diesel engine are arranged in a compact layout. In addition to such a problem of the restricted installation space, a structure with a high rigidity is required of a cylinder head because component parts such as an EGR device and a turbocharger are coupled to and supported by the cylinder head. 
     Further, when an EGR cooler (EGR heat exchanger) and an oil cooler (oil heat exchanger) are integrally structured as in PTL 1, it is necessary to seal oil and cooling water which are liquids and EGR gas which is a gas. Due to differences in the characteristics of these fluids, a sealing structure will become complicated. When the cylinder head is connected, thermal deformation in the cylinder head may influence the connection status. Therefore, integrating the EGR cooler with the oil cooler to form a large device limits the connecting portion to the cylinder head, consequently causing an increase in the size of the engine device. On the other hand, a structure in which cooling water flows to an EGR valve device as in PTL 2 will not only lead to a complicated structure of the EGR valve device itself but also necessitate connection of a cooling water pipe and an EGR gas pipe. 
     If the EGR cooler is connected through a pipe, the volume of the EGR gas increases due to an increase in the temperature of the EGR gas caused by generated heat of the diesel engine. Due to this, a sufficient amount of the EGR gas is cannot be maintained, and reduction of the NOx in the exhaust gas becomes difficult. On the other hand, if the EGR gas is excessively cooled by having the EGR pipe exposed to cooling air from a cooling fan and the like, the combustion in the cylinder is affected. For the reasons above, appropriate arrangement and structure of parts in the diesel engine and an appropriate cooling structure need to be considered for the purpose of supplying the EGR gas at an appropriate temperature. To add this, if there is unevenness in the mixture distribution of the EGR gas and fresh air, the amounts of EGR gas in the fresh air supplied to a plurality of cylinders will be uneven. This affects actions of reducing the NOx and combustion in each of the cylinders, thus deteriorating the operation efficiency of the diesel engine. 
     A technical problem of the present invention is to provide an engine device that is improved based on studies on the existing circumstances as mentioned above. 
     CITATION LIST 
     PTL 1: Japanese Patent No. 3852255 
     PTL 2: Japanese Patent No. 4071370 
     PTL 3: Japanese Patent No. 4484800 
     PTL 4: Japanese Patent Application Laid-Open No. 2000-008969 
     SUMMARY OF INVENTION 
     An aspect of the present invention is an engine device including: an exhaust manifold disposed on one of left and right sides of a cylinder head and an intake manifold disposed on the other one of left and right sides of the cylinder head; an EGR device configured to circulate, as EGR gas, a portion of exhaust gas exhausted from the exhaust manifold to the intake manifold; and an EGR cooler configured to cool the EGR gas and supply the EGR gas to the EGR device. In the engine device: the EGR cooler includes a heat exchanger in which coolant passages and EGR gas fluid passages are alternately stacked and a pair of left and right flange portions provided respectively at right and left end portions of one side surface of the heat exchanger; an inlet of a coolant is disposed in one of the left and right flange portions and an outlet of the coolant is disposed in the other of the left and right flange portions; an inlet of EGR gas is disposed in one of the left and right flange portions and an outlet of the EGR gas is disposed in the other of the left and right flange portions; and the left and right flange portions are connected to one of front and rear sides of the cylinder head. 
     The above engine device may be such that: a space is formed between the heat exchanger in the EGR cooler and the cylinder head. 
     The above engine device may be such that: the inlet of the coolant and the outlet of the EGR gas are provided to one of the left and right flange portions, and the outlet of the coolant and the inlet of the EGR gas are provided to the other one of the left and right flange portions; the inlet of the coolant and the outlet of the EGR gas are disposed one above the other in the flange portion, and the outlet of the coolant and the inlet of the EGR gas are disposed one above the other in the flange portion; and the inlet of the coolant and the inlet of the EGR gas are disposed at the same height, and the outlet of the coolant and the outlet of the EGR gas are disposed at the same height. 
     The above engine device may be such that the cylinder head includes: an upstream EGR passage communicating a side surface where the exhaust manifold is disposed to a side surface where the EGR cooler is disposed; a downstream EGR passage communicating a side surface where the intake manifold is disposed to a side surface where the EGR cooler is disposed; an upstream coolant passage communicating the inlet of the coolant; and a downstream coolant passage communicating the outlet of the coolant, the downstream coolant passage being provided nearby the upstream EGR passage, the upstream coolant passage being provided nearby the downstream EGR passage. 
     The above engine device may be such that a plate-shape gasket is interposed between the cylinder head and the flange portions in such a manner as to extend across the left and right flange portions, a ring-shape seal member is embedded in each of the outlet and the inlet of the coolant in the cylinder head respectively communicating with the inlet and the outlet of the coolant in the flange portions, and the seal member is surrounded by the flange portions. 
     The above engine device may be such that the EGR device includes a main body case configured to mix the fresh air with the EGR gas and supply the mixed gas to the intake manifold, the main body case being configured so that a fresh air flow direction and an EGR gas flow direction therein cross each other perpendicularly or with an obtuse angle, and that a direction in which a mixed gas of the EGR gas and the fresh air is taken into the intake manifold intersects each of the fresh air flow direction and the EGR gas flow direction. 
     The above engine device may be such that a fresh air inlet to which fresh air is supplied is opened in one of the front and rear sides of the main body case, whereas an EGR gas inlet to which the EGR gas is supplied is opened in the other of the front and rear sides of the main body case; an intake outlet communicating with the intake manifold is opened on one of the left and right sides of the main body case, the intake outlet and the EGR gas inlet are disposed at the same height, and the fresh air inlet and the EGR gas inlet are disposed at different heights. 
     The above engine device may be such that the main body case includes a first case with the fresh air inlet and a second case with the intake outlet and the EGR gas inlet coupled with each other. 
     The above engine device may be such that the first case is provided therein with a first EGR gas fluid passage constituting a part of the EGR gas passage where the EGR gas flows and a mixing chamber in which fresh air and the EGR gas are mixed; and the second case is provided with a second EGR gas fluid passage through which the first EGR gas fluid passage is in communication with the EGR gas inlet and a mixed gas fluid passage through which mixed gas obtained by mixing the fresh air with the EGR gas is supplied from the mixing chamber to the intake manifold. 
     The above engine device may be such that the first EGR gas fluid passage is coupled with an offset to a side surface of the mixing chamber opposite to a side surface thereof having the intake outlet relative to a central axis of the mixing chamber, and the first EGR gas fluid passage and the second EGR gas fluid passage are in communication with each other so that the EGR gas fluid passage is formed in a spiral manner. 
     With the above aspect of the present invention, since each of the pair of left and right flange portions has a coolant opening and an EGR gas opening, it is possible that the flange portions are made from a common member, and moreover material costs of the flange portions can be suppressed. In addition, a coupling portion where the flange portions are coupled to the heat exchanger can be minimized, so that the amount of heat transfer from the cylinder head to the heat exchanger can be reduced, which increases the effect of cooling the EGR gas by the heat exchanger. 
     In the above aspect of the present invention, a space is formed between the heat exchanger and the cylinder head. As a result, the EGR cooler is in a state where a wide area of the front and rear surfaces of the heat exchanger is exposed to outside air. Heat dissipation occurs in the heat exchanger, too. Thus, the effect of cooling the EGR gas by the EGR cooler is increased. This configuration can reduce the volume of the heat exchanger as compared to a configuration in which the entire surface of the heat exchanger is attached. Thus, the engine device can be downsized. 
     In the above aspect of the present invention, the coolant outlet and the EGR gas inlet are disposed one above the other in one of the flange portions, while the EGR gas outlet and the coolant outlet are disposed one above the other in the other of the flange portions. Thus, the flange portions having identical shapes with their postures mutually upside-down are attached to the heat exchanger. This can reduce the number of types of component parts included in the EGR cooler, thus improving an assemblability of the EGR cooler and reducing costs of the component parts. 
     In the EGR cooler of the above aspect of the present invention, the coolant outlet and the coolant inlet are disposed at diagonal positions, and the EGR gas inlet and the EGR gas outlet are disposed at diagonal positions. Since EGR gases having different quantities of heat and coolants having different quantities of heat are respectively supplied or discharged at diagonal positions, thermal deformations of coupling portions where the EGR cooler is coupled to the cylinder head can be mutually relieved, so that deflection or slackness of the coupling portions can be suppressed. Accordingly, leakage of an EGR gas or a coolant in the EGR cooler and in the cylinder head can be prevented, and moreover a decrease in the coupling strength can be prevented. 
     In the above aspect of the present invention, the EGR gas is sealed by the gasket and the coolant is sealed by an O-ring, thereby improving the sealability. This way, even though the EGR cooler where a liquid and a gas enter and exit is coupled to the cylinder head, a sealability for each of the liquid and the gas can be obtained, so that leakage of each of the EGR gas and the coolant can be prevented. 
     In the main body case of the EGR device in the above aspect of the present invention, the EGR gas flow direction is at an angle of 90° or more relative to the fresh air flow direction, and the fresh air flow and the EGR gas flow intersect each other, so that a distribution of mixture of the EGR gas with the fresh air can be made uniform, and an uneven flow of the EGR gas in the intake manifold can be suppressed. As a result, a concentration of the EGR gas in the mixed gas supplied from the intake manifold to each of the plurality of intake fluid passages can be made uniform. Thus, a variation in combustion action among cylinders of the engine device can be suppressed, and the EGR device can be compactly configured. 
     In the above aspect of the present invention, the fresh air taken into the fresh air inlet flows in the front-rear direction and then in the up-down direction while curving in an L-shape, whereas the EGR gas taken into the EGR gas inlet flows obliquely upward and mixed in the mixing chamber. Therefore, the EGR gas flows in toward a flow of the fresh air, which facilitates mixing of the EGR gas with the fresh air. The mixed gas of the fresh air and the EGR gas flows in the up-down direction and then in the left-right direction while curving in an L-shape, to flow into the intake manifold through the intake outlet. A direction in which the mixed gas is emitted intersects not only the directions in which the fresh air and the EGR gas are taken in but also the directions in which the fresh air and the EGR gas flow within the main body case. Consequently, a distribution of mixture of the EGR gas with the fresh air can be made uniformed. 
     With the above aspect of the present invention, since the main body case is divisible into the first case and the second case, a mixed fluid passage where the EGR gas flow and the fresh air flow intersect each other at an angle of 90° or more can be easily formed in the main body case. It therefore is possible that the main body case is formed as a casting with a high rigidity, and moreover, weight reduction of the collector can be obtained by forming the collector as an aluminum-based casting product. Furthermore, each of the EGR gas fluid passage, the mixing chamber, and the mixed gas fluid passage can be compactly configured within the main body case, and thus the main body case can be downsized. 
     In the above aspect of the present invention, the EGR gas inlet is disposed in the second case while the fresh air inlet and the mixing chamber are disposed in the first case. In the mixing chamber, therefore, the fresh air flowing from the fresh air inlet and the EGR gas flowing from the second case intersect each other, so that the fresh air and the EGR gas can be efficiently mixed. In addition, the intake outlet is disposed in the second case, and the fresh air having entered the first case tends to flow toward the second case. As a result, mixing of the EGR gas flowing toward the first case with the fresh air is made uniform. 
     In the above aspect of the present invention, a portion of the mixing chamber that is in communication with the EGR gas fluid passage is on the side opposite to the intake outlet. The EGR gas flowing into the mixing chamber, therefore, reaches the intake outlet while being guided by a fresh air flow, which allows the EGR gas to be uniformly mixed with the fresh air. The EGR gas flowing from the EGR gas fluid passage into the mixing chamber flows in a direction against the direction from the mixing chamber toward the mixed gas fluid passage. This causes the fresh air and the EGR gas to collide with each other while flowing within the mixing chamber. Accordingly, the EGR gas is smoothly mixed with the fresh air. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a front view of an engine. 
         FIG. 2  illustrates a rear view of the engine. 
         FIG. 3  illustrates a left side view of the engine. 
         FIG. 4  illustrates a right side view of the engine. 
         FIG. 5  illustrates a top plan view of the engine. 
         FIG. 6  illustrates a bottom plan view of the engine. 
         FIG. 7  illustrates a perspective view of the engine as viewed from diagonally front. 
         FIG. 8  illustrates a perspective view of the engine as viewed from diagonally rear. 
         FIG. 9  illustrates an enlarged perspective view of a cylinder head as viewed from an intake manifold side. 
         FIG. 10  illustrates an exploded perspective view of the cylinder head as viewed from an exhaust manifold side. 
         FIG. 11  illustrates an exploded perspective view of the cylinder head as viewed from the intake manifold side. 
         FIG. 12  illustrates a top plan view of the cylinder head. 
         FIG. 13  illustrates a front view of the cylinder head. 
         FIG. 14  illustrates a perspective cross-sectional view of the cylinder head and an EGR device. 
         FIG. 15  illustrates a perspective cross-sectional view of the cylinder head and the exhaust manifold. 
         FIG. 16  illustrates a perspective cross-sectional view of a coupling portion of the cylinder head coupled to an EGR cooler. 
         FIG. 17  illustrates a perspective cross-sectional view of the EGR device. 
         FIG. 18  illustrates a top plan view of the EGR device. 
         FIG. 19  illustrates an exploded perspective view of the EGR device. 
         FIG. 20  illustrates an exploded view of a collector in the EGR device. 
         FIG. 21  illustrates an exploded view of a collector in the EGR device. 
         FIG. 22  illustrates an exploded view of the coupling portion of the cylinder head coupled to the EGR cooler. 
         FIG. 23  illustrates a rear view of the EGR cooler. 
         FIG. 24  illustrates a cross-sectional view of the coupling portion of the cylinder head coupled to the EGR cooler. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, an embodiment of the present invention will be described with reference to the drawings. First, referring to  FIG. 1  to  FIG. 8 , an overall structure of a diesel engine (engine device)  1  will be described. In the descriptions below, opposite side portions parallel to a crankshaft  5  (side portions on opposite sides relative to the crankshaft  5 ) will be defined as left and right, a side where a flywheel housing  7  is disposed will be defined as front, and a side where a cooling fan  9  is disposed will be defined as rear. For convenience, these are used as a benchmark for a positional relationship of left, right, front, rear, up, and down in the diesel engine  1 . 
     As shown in  FIG. 1  to  FIG. 8 , an intake manifold  3  and an exhaust manifold  4  are disposed in one side portion and the other side portion of the diesel engine  1  parallel to the crankshaft  5 . In the embodiment, the intake manifold  3  provided on a right surface of a cylinder head  2  is formed integrally with the cylinder head  2 . The exhaust manifold  4  is provided on a left surface of the cylinder head  2 . The cylinder head  2  is mounted on a cylinder block  6  in which the crankshaft  5  and a piston (not shown) are disposed. 
     The crankshaft  5  has its front and rear distal ends protruding from front and rear surfaces of the cylinder block  6 . The flywheel housing  7  is fixed to one side portion of the diesel engine  1  (in the embodiment, a front surface side of the cylinder block  6 ) intersecting the crankshaft  5 . A flywheel  8  is disposed in the flywheel housing  7 . The flywheel  8 , which is pivotally supported on the front end side of the crankshaft  5 , is configured to rotate integrally with the crankshaft  5 . The flywheel  8  is configured such that power of the diesel engine  1  is extracted to an actuating part of a work machine (for example, a hydraulic shovel, a forklift, or the like) through the flywheel  8 . The cooling fan  9  is disposed in the other side portion of the diesel engine  1  (in the embodiment, a rear surface side of the cylinder block  6 ) intersecting the crankshaft  5 . A rotational force is transmitted from the rear end side of the crankshaft  5  to the cooling fan  9  through a V-belt  10 . 
     An oil pan  11  is disposed on a lower surface of the cylinder block  6 . A lubricant is stored in the oil pan  11 . The lubricant in the oil pan  11  is suctioned by an oil pump (not shown) disposed on the right surface side of the cylinder block  6 , the oil pump being arranged in a coupling portion where the cylinder block  6  is coupled to the flywheel housing  7 . The lubricant is then supplied to lubrication parts of the diesel engine  1  through an oil cooler  13  and an oil filter  14  that are disposed on the right surface of the cylinder block  6 . The lubricant supplied to the lubrication parts is then returned to the oil pan  11 . The oil pump (not shown) is configured to be driven by rotation of the crankshaft  5 . 
     In the coupling portion where the cylinder block  6  is coupled to the flywheel housing  7 , a fuel feed pump  15  for feeding a fuel is attached. The fuel feed pump  15  is disposed below an EGR device  24 . A common rail  16  is fixed to a side surface of the cylinder block  6  at a location below the intake manifold  3  of the cylinder head  2 . The common rail  16  is disposed above the fuel feed pump  15 . Injectors (not shown) for four cylinders are provided on an upper surface of the cylinder head  2  which is covered with a head cover  18 . Each of the injectors has a fuel injection valve of electromagnetic-controlled type. 
     Each of the injectors is connected to a fuel tank (not shown) through the fuel feed pump  15  and the common rail  16  having a cylindrical shape. The fuel tank is mounted in a work vehicle. A fuel in the fuel tank is pressure-fed from the fuel feed pump  15  to the common rail  16 , so that a high-pressure fuel is stored in the common rail  16 . By controlling the opening/closing of the fuel injection valves of the injectors, the high-pressure fuel in the common rail  16  is injected from the injectors to the respective cylinders of the diesel engine  1 . 
     A blow-by gas recirculation device  19  is provided on an upper surface of the head cover  18  covering intake and exhaust valves (not shown), etc. disposed on the upper surface of the cylinder head  2 . The blow-by gas recirculation device  19  takes in a blow-by gas that has leaked out of a combustion chamber of the diesel engine  1  or the like toward the upper surface of the cylinder head  2 . A blow-by gas outlet of the blow-by gas recirculation device  19  is in communication with an intake part of a two-stage turbocharger  30  through a recirculation hose  68 . A blow-by gas, from which a lubricant component is removed in the blow-by gas recirculation device  19 , is then recirculated to the intake manifold  3  via the two-stage turbocharger  30 . 
     An engine starting starter  20  is attached to the flywheel housing  7 . The engine starting starter  20  is disposed below the exhaust manifold  4 . A position where the engine starting starter  20  is attached to the flywheel housing  7  is below a coupling portion where the cylinder block  6  is coupled to the flywheel housing  7 . 
     A coolant pump  21  for smoothing a coolant is provided in a portion of the rear surface of the cylinder block  6 , the portion being a little left-hand. The coolant pump  21  is disposed below the cooling fan  9 . Rotation of the crankshaft  5  causes the coolant pump  21  as well as the cooling fan  9  to be driven through the cooling fan driving V-belt  10 . Driving the coolant pump  21  causes a coolant in a radiator (not shown) mounted in the work vehicle to be supplied to the coolant pump  21 . The coolant is then supplied to the cylinder head  2  and the cylinder block  6 , to cool the diesel engine  1 . 
     The coolant pump  21  is disposed below the exhaust manifold  4 , and a coolant inlet pipe  22  is provided on the left surface of the cylinder block  6  and is fixed at a height equal to the height of the coolant pump  21 . The coolant inlet pipe  22  is in communication with a coolant outlet of the radiator. A coolant outlet pipe  23  that is in communication with a coolant inlet of the radiator is fixed to an upper rear portion of the cylinder head  2 . The cylinder head  2  has a coolant drainage  35  that protrudes rearward from the intake manifold  3 . The coolant outlet pipe  23  is provided on an upper surface of the coolant drainage  35 . 
     The inlet side of the intake manifold  3  is coupled to an air cleaner (not shown) via a collector (EGR main body case)  25  of an EGR device  24  (exhaust-gas recirculation device) which will be described later. Fresh air (outside air) suctioned by the air cleaner is subjected to dust removal and purification in the air cleaner, then fed to the intake manifold  3  through the collector  25 , and then supplied to the respective cylinders of the diesel engine  1 . In the embodiment, the collector  25  of the EGR device  24  is coupled to the right side of the intake manifold  3  which is formed integrally with the cylinder head  2  to form the right surface of the cylinder head  2 . That is, an outlet opening of the collector  25  of the EGR device  24  is coupled to an inlet opening of the intake manifold  3  provided on the right surface of the cylinder head  2 . In this embodiment, the collector  25  of the EGR device  24  is coupled to the air cleaner via an intercooler (not shown) and the two-stage turbocharger  30 , as will be described later. 
     The EGR device  24  includes: the collector  25  serving as a relay pipe passage that mixes a recirculation exhaust gas of the diesel engine  1  (an EGR gas from the exhaust manifold  4 ) with fresh air (outside air from the air cleaner), and supplies a mixed gas to the intake manifold  3 ; an intake throttle member  26  that communicates the collector  25  with the air cleaner; a recirculation exhaust gas tube  28  that constitutes a part of a recirculation flow pipe passage connected to the exhaust manifold  4  via an EGR cooler  27 ; and an EGR valve member  29  that communicates the collector  25  with the recirculation exhaust gas tube  28 . 
     The EGR device  24  is disposed on the right lateral side of the intake manifold  3  in the cylinder head  2 . The EGR device  24  is fixed to the right surface of the cylinder head  2 , and is in communication with the intake manifold  3  in the cylinder head  2 . In the EGR device  24 , the collector  25  is coupled to the intake manifold  3  on the right surface of the cylinder head  2 , and an EGR gas inlet of the recirculation exhaust gas tube  28  is coupled and fixed to a front portion of the intake manifold  3  on the right surface of the cylinder head  2 . The EGR valve member  29  and the intake throttle member  26  are coupled to the front and rear of the collector  25 , respectively. An EGR gas outlet of the recirculation exhaust gas tube  28  is coupled to the rear end of the EGR valve member  29 . 
     The EGR cooler  27  is fixed to the front surface of the cylinder head  2 . The coolant and the EGR gas flowing in the cylinder head  2  flows into and out of the EGR cooler  27 . In the EGR cooler  27 , the EGR gas is cooled. EGR cooler coupling bases  33 ,  34  for coupling the EGR cooler  27  to the front surface of the cylinder head  2  protrude from left and right portions of the front surface of the cylinder head  2 . The EGR cooler  27  is coupled to the coupling bases  33 ,  34 . That is, the EGR cooler  27  is disposed on the front side of the cylinder head  2  and at a position above the flywheel housing  7  such that a rear end surface of the EGR cooler  27  and the front surface of the cylinder head  2  are spaced from each other. 
     The two-stage turbocharger  30  is disposed on a lateral side (in the embodiment, the left lateral side) of the exhaust manifold  4 . The two-stage turbocharger  30  includes a high-pressure turbocharger  51  and a low-pressure turbocharger  52 . The high-pressure turbocharger  51  includes a high-pressure turbine  53  in which a turbine wheel (not shown) is provided and a high-pressure compressor  54  in which a blower wheel (not shown) is provided. The low-pressure turbocharger  52  includes a low-pressure turbine  55  in which a turbine wheel (not shown) is provided and a low-pressure compressor  56  in which a blower wheel (not shown) is provided. 
     An exhaust gas inlet  57  of the high-pressure turbine  53  is coupled to the exhaust manifold  4 . An exhaust gas inlet  60  of the low-pressure turbine  55  is coupled to an exhaust gas outlet  58  of the high-pressure turbine  53  via a high-pressure exhaust gas tube  59 . An exhaust gas introduction side end portion of an exhaust gas discharge pipe (not shown) is coupled to an exhaust gas outlet  61  of the low-pressure turbine  55 . A fresh air supply side (fresh air outlet side) of the air cleaner (not shown) is connected to a fresh air inlet port (fresh air inlet)  63  of the low-pressure compressor  56  via an air supply pipe  62 . A fresh air inlet port  66  of the high-pressure compressor  54  is coupled to a fresh air supply port (fresh air outlet)  64  of the low-pressure compressor  56  via a low-pressure fresh air passage pipe  65 . A fresh air introduction side of the intercooler (not shown) is connected to a fresh air supply port  67  of the high-pressure compressor  54  via a high-pressure fresh air passage pipe (not shown). 
     The high-pressure turbocharger  51  is coupled to the exhaust gas outlet  58  of the exhaust manifold  4 , and is fixed to the left lateral side of the exhaust manifold  4 . On the other hand, the low-pressure turbocharger  52  is coupled to the high-pressure turbocharger  51  via the high-pressure exhaust gas tube  59  and the low-pressure fresh air passage pipe  65 , and is fixed above the exhaust manifold  4 . Thus, the exhaust manifold  4  and the high-pressure turbocharger  51  with a small diameter are disposed side-by-side with respect to the left-right direction below the low-pressure turbocharger  52  with a large diameter. As a result, the two-stage turbocharger  30  is arranged so as to surround the left surface and the upper surface of the exhaust manifold  4 . That is, the exhaust manifold  4  and the two-stage turbocharger  30  are arranged so as to form a rectangular shape in a rear view (or front view), and are compactly fixed to the left surface of the cylinder head  2 . 
     Next, referring to  FIG. 9  to  FIG. 16 , a configuration of the cylinder head  2  will be described. As shown in  FIG. 9  to  FIG. 16 , the cylinder head  2  is provided with a plurality of intake fluid passages  36  for taking fresh air into a plurality of intake ports (not shown) and a plurality of exhaust fluid passages  37  for emitting an exhaust gas from a plurality of exhaust ports. The intake manifold  3  which aggregates the plurality of intake fluid passages  36  is formed integrally with a right side portion of the cylinder head  2 . Since the cylinder head  2  is integrated with the intake manifold  3 , a gas sealability between the intake manifold  3  and the intake fluid passages  36  can be enhanced, and in addition, the rigidity of the cylinder head  2  can be increased. 
     The cylinder head  2  is configured such that the exhaust manifold  4  is coupled to the left surface of the cylinder head  2  which is opposite to the right surface where the intake manifold  3  is provided, and the EGR cooler  27  is coupled to the front surface (a surface on the flywheel housing  7  side) of the cylinder head  2  which is adjacent to the left and right surfaces. Coupling bases (EGR cooler coupling bases)  33 ,  34  to which the EGR cooler  27  is coupled are provided so as to protrude from the front surface of the cylinder head  2 . The coupling bases  33 ,  34  are provided therein with EGR gas fluid passages (EGR gas relay fluid passages)  31 ,  32  and coolant passages (coolant relay fluid passages)  38 ,  39 . 
     Since the EGR gas relay fluid passages  31 ,  32  and the coolant passages  38 ,  39  are provided in the coupling bases  33 ,  34  to which the EGR cooler  27  is coupled, it is not necessary that coolant piping and EGR gas piping are disposed between the EGR cooler  27  and the cylinder head  2 . This can give a sealability to a coupling portion coupled to the EGR cooler  27  without any influence of, for example, extension and contraction of piping caused by the EGR gas or the coolant. This can also enhance a resistance (structural stability) against external fluctuation factors such as heat and vibration, and moreover can make the configuration compact. 
     The cylinder head  2  includes an upstream EGR gas relay fluid passage  31  through which a front portion of the left surface is in communication with the front surface. An EGR gas outlet  41  disposed at the front end of the exhaust manifold  4  is in communication with the upstream EGR gas relay fluid passage  31 . The cylinder head  2  also includes a downstream EGR gas relay fluid passage  32  through which a front portion of the right surface (on the front side of the intake manifold  3 ) is in communication with the front surface. The EGR gas inlet of the recirculation exhaust gas tube  28  is in communication with the downstream EGR gas relay fluid passage  32 . The cylinder head  2  has the EGR cooler coupling bases  33 ,  34  which are formed by left and right edges of the front surface of the cylinder head  2  (a front-left corner portion and a front-right corner portion of the cylinder head  2 ) being protruded frontward. The upstream EGR gas relay fluid passage  31  is provided inside the coupling base  33 , and the downstream EGR gas relay fluid passage  32  is provided inside the coupling base  34 . 
     The EGR device  24  is coupled to the intake manifold  3  which is provided on the right surface of the cylinder head  2  so as to protrude therefrom. The intake manifold  3  is disposed in a portion of the right surface of the cylinder head  2 , the portion being relatively close to the rear side (the cooling fan  9  side). The intake manifold  3  is formed by a lower portion of the right surface of the cylinder head  2  being protruded rightward. The intake manifold  3  has an intake inlet  40  at its middle portion with respect to the front-rear direction. An intake outlet  83  of the collector  25  of the EGR device  24  is coupled to the intake inlet  40  of the intake manifold  3  which protrudes from the right surface of the cylinder head  2 , and the EGR device  24  is fixed to the right lateral side of the cylinder head  2 . 
     On the front side (the flywheel housing  7  side) of the right surface of the cylinder head  2 , the coupling base  34  coupled to the EGR cooler  27  protrudes frontward, and an EGR gas outlet of the downstream EGR gas relay fluid passage  32  is opened in a right surface of the coupling base  34 . One end of the recirculation exhaust gas tube  28  of the EGR device  24  is coupled to the right surface of the coupling base  34 , and thereby the collector  25  of the EGR device  24  is in communication with the downstream EGR gas relay fluid passage  32  provided inside the cylinder head  2  via the recirculation exhaust gas tube  28  and the EGR valve member  29 . 
     On the rear side (the cooling fan  9  side) of the right surface of the cylinder head  2 , the coolant drainage (thermostat case)  35  whose upper surface is opened to communicate with a coolant outlet pipe (thermostat cover)  23  protrudes rearward, and a thermostat (not shown) is installed therein. The coolant drainage  35  is offset at the rear of the right surface of the cylinder head  2 , and therefore it is possible that the V-belt  10  wound on a fan pulley  9   a  to which the cooling fan  9  is fixed extends through a space below the coolant drainage  35 . Thus, the length of the diesel engine  1  in the front-rear direction can be shortened. The coolant drainage  35  also protrudes from the right surface of the cylinder head  2 . On the right surface of the cylinder head  2 , the intake manifold  3  and the coolant drainage  35  are arranged one behind the other with respect to the front-rear direction. 
     On the front side (the flywheel housing  7  side) of the left surface of the cylinder head  2 , the coupling base  33  coupled to the EGR cooler  27  protrudes frontward, and an EGR gas inlet of the upstream EGR gas relay fluid passage  31  is opened in a left surface of the coupling base  33 . That is, in the left surface of the cylinder head  2 , the EGR gas inlet of the upstream EGR gas relay fluid passage  31  and exhaust gas outlets of the plurality of exhaust fluid passages  37  are disposed in the front-rear direction, and are opened. The exhaust manifold  4  has, in its right surface which is coupled to the left surface of the cylinder head  2 , the EGR gas outlet  41  which is in communication with the upstream EGR gas relay fluid passage  31  and exhaust gas inlets  42  which are in communication with the plurality of exhaust fluid passages  37  are arranged in the front-rear direction, and are opened. Since the EGR inlet and the exhaust gas outlets are disposed side-by-side in the same surface of the cylinder head  2 , it is easy for a coupling portion where the cylinder head  2  is coupled to the exhaust manifold  4  to obtain an airtightness (gas sealability) by sandwiching a single gasket  45  therebetween. 
     The exhaust manifold  4  is provided therein with an exhaust aggregate part  43  which is in communication with the EGR gas outlet  41  and the exhaust gas inlets  42 . The exhaust aggregate part  43  is disposed such that its longitudinal direction is parallel to the front-rear direction. An exhaust gas outlet  44  which is in communication with the exhaust aggregate part  43  is opened in a rear portion of the left surface of the exhaust manifold  4 . The exhaust manifold  4  is configured such that, after an exhaust gas coming from the exhaust fluid passages  37  of the cylinder head  2  flows into the exhaust aggregate part  43  via the exhaust gas inlets  42 , part of the exhaust gas serves as an EGR gas and flows into the upstream EGR gas relay fluid passage  31  of the cylinder head  2  via the EGR gas outlet  41  while the rest of the exhaust gas flows into the two-stage turbocharger  30  via the exhaust gas outlet  44 . 
     On the front surface of the cylinder head  2 , the left and right pair of EGR cooler coupling bases  33 ,  34  are disposed on the exhaust manifold  4  side and on the intake manifold  3  side, respectively. The EGR cooler coupling base  33  has the upstream EGR gas relay fluid passage  31  through which the EGR gas fluid passage of the exhaust manifold  4  communicates with the EGR gas fluid passage of the EGR cooler  27 . The EGR cooler coupling base  34  has the downstream EGR gas relay fluid passage  32  through which the EGR gas fluid passage of the EGR device  24  communicates with the EGR gas fluid passage of the EGR cooler  27 . The EGR cooler coupling base  33  also has the downstream coolant passage  38  to which a coolant is discharged from the EGR cooler  27 . The EGR cooler coupling base  34  has the upstream coolant passage  39  that supplies a coolant to the EGR device  24  and to the EGR cooler  27 . 
     Since the EGR cooler coupling bases  33 ,  34  are configured in a protruding manner, there is no need for EGR gas piping that communicates the exhaust manifold  4 , the EGR cooler  27 , and the EGR device  24 . Thus, the number of coupling portions of the EGR gas fluid passage is small. Accordingly, in the diesel engine  1  that aims to reduce NOx by the EGR gas, EGR gas leakage can be reduced, and moreover deformation can be suppressed which may otherwise be caused by a change in a stress due to extension and contraction of piping. Since the EGR gas relay fluid passages  31 ,  32  and the coolant passages  38 ,  39  are provided in the EGR cooler coupling bases  33 ,  34 , the shapes of the fluid passages  31 ,  32 ,  38 ,  39  formed in the cylinder head  2  are simplified, so that the cylinder head  2  can be easily formed by casting without using a complicated core. 
     The EGR cooler coupling base  33  on the intake manifold  3  side and the EGR cooler coupling base  34  on the exhaust manifold  4  side are distant from each other. This can suppress a mutual influence between thermal deformations of the coupling bases  33 ,  34 . Accordingly, gas leakage and damage of coupling portions where the EGR cooler coupling bases  33 ,  34  are coupled to the EGR cooler  27  can be prevented, and in addition, a balance of the rigidity of the cylinder head  2  can be maintained. Moreover, the volume of the front surface of the cylinder head  2  can be reduced, which leads to weight reduction of the cylinder head  2 . Furthermore, it is possible that the EGR cooler  27  is disposed at a distance from the front surface of the cylinder head  2 , to provide a space on the front and rear sides of the EGR cooler  27 . This enables cool air to flow around the EGR cooler  27 , thus increasing the cooling efficiency of the EGR cooler  27 . 
     In the EGR cooler coupling base  33 , the downstream coolant passage  38  is disposed above the upstream EGR gas relay fluid passage  31 . In the EGR cooler coupling base  34 , the downstream EGR gas relay fluid passage  32  is disposed above the upstream coolant passage  39 . A coolant inlet of the downstream coolant passage  38  and an EGR gas inlet of the downstream EGR gas relay fluid passage  32  are disposed at the same height. A coolant outlet of the upstream coolant passage  39  and an EGR gas outlet of the downstream EGR gas relay fluid passage  32  are disposed at the same height. 
     Since the EGR gas relay fluid passages  31 ,  32  and the coolant passages  38 ,  39  are provided in the EGR cooler coupling bases  33 ,  34  protruding at a distance from each other, a mutual influence between thermal deformations of the EGR cooler coupling bases  33 ,  34  is relieved. In the EGR cooler coupling bases  33 ,  34 , the EGR gas flowing in the EGR gas relay fluid passages  31 ,  32  is cooled by the coolant flowing in the coolant passages  38 ,  39 , so that thermal deformations of the EGR cooler coupling bases  33 ,  34  are suppressed. In addition, the up-down positional relationship of the EGR gas relay fluid passages  31 ,  32  and the coolant passages  38 ,  39  in one of the EGR cooler coupling bases  33 ,  34  is reverse to that in the other of the EGR cooler coupling bases  33 ,  34 . As a result, heat distributions in the respective EGR cooler coupling bases  33 ,  34  are in opposite directions with respect to the up-down direction, which can reduce an influence of thermal deformation in the height direction in the cylinder head  2 . 
     An outer peripheral wall of the cylinder head  2  stands upward at a peripheral edge of the upper surface of the cylinder head  2 , to provide a spacer  46  which is coupled to a peripheral edge of a lower surface of the head cover  18 . The spacer  46  has, in a right surface thereof, a plurality of openings  47 . Fuel pipes  48  which couple injectors (not shown) provided in the cylinder head  2  to the common rail  16  pass through the openings  47 . Since the spacer  46  integrated with the cylinder head  2  is disposed above the cylinder head  2 , the rigidity of the cylinder head  2  is increased, which can reduce distortion of the cylinder head  2  itself and also can allow component parts coupled to the cylinder head  2  to be supported with a high rigidity. 
     A configuration of the EGR device  24  will now be described with reference to  FIG. 9  to  FIG. 15 , and  FIG. 17  to  FIG. 21 . As shown in  FIG. 9  to  FIG. 15 , and  FIG. 17  to  FIG. 21 , the EGR device  24  includes the collector (main body case)  25  that mixes fresh air with an EGR gas, and supplies a mixture to the intake manifold  3 . The intake manifold  3  and the intake throttle member  26  for taking fresh air in are connected in communication with each other via the collector  25 . The EGR valve member  29  which leads to an outlet side of the recirculation exhaust gas tube  28  is connected in communication with the collector  25 . 
     In the collector  25 , a fresh air flow direction and an EGR gas flow direction cross each other perpendicularly or with an obtuse angle, and a direction in which a mixed gas of the EGR gas and the fresh air is taken into the intake manifold  3  intersects each of the fresh air flow direction and the EGR gas flow direction. A fresh air inlet  81  to which the fresh air is supplied is opened in one of front and rear surfaces of the collector  25 , whereas an EGR gas inlet  82  to which the EGR gas is supplied is opened in the other of the front and rear surfaces of the collector  25 . The intake outlet  83  which is coupled to the intake manifold  3  is opened in a left surface of the collector  25 . The EGR gas inlet  82  and the intake outlet  83  are disposed at the same height, and the fresh air inlet  81  and the EGR gas inlet  82  are disposed at different heights. 
     In the collector  25 , fresh air taken from the intake throttle member  26  into the fresh air inlet  81  flows in the front-rear direction and then in the up-down direction while curving in an L-shape, whereas an EGR gas taken from the EGR valve member  29  into the EGR gas inlet  82  flows obliquely upward. As a result, the EGR gas flows in toward a flow of the fresh air, which facilitates mixing of the EGR gas with the fresh air. The mixed gas of the fresh air and the EGR gas flows in the up-down direction and then in the left-right direction while curving in an L-shape, to flow into the intake manifold  3  through the intake outlet  83 . A direction in which the mixed gas is emitted intersects not only the directions in which the fresh air and the EGR gas are taken in but also the directions in which the fresh air and the EGR gas flow within the collector  25 . Consequently, a distribution of mixture of the EGR gas with the fresh air can be made uniformed. 
     In the collector  25 , as described above, the EGR gas flow direction is at an angle of 90° or more relative to the fresh air flow direction, and the fresh air flow and the EGR gas flow intersect each other, so that a distribution of mixture of the EGR gas with the fresh air can be made uniform, and an uneven flow of the EGR gas in the intake manifold  3  can be suppressed. As a result, a concentration of the intake EGR gas supplied to each of the plurality of intake fluid passages  36  of the cylinder head  2  can be made uniform. Thus, a variation in combustion action among cylinders of the diesel engine  1  can be suppressed. Consequently, generation of black smoke is suppressed, and the amount of NOx can be reduced while a good combustion state of the diesel engine  1  is maintained. That is, purifying (cleaning) the exhaust gas by a recirculation flow of the EGR gas can be achieved without causing a misfire in a specific cylinder. 
     The collector  25  includes an upper case (first case)  84  with the fresh air inlet  81  and a lower case (second case)  85  with the EGR gas inlet  82  and the intake outlet  83  being coupled to each other. Since the collector  25  is divisible in the up-down direction into the upper case  84  and the lower case  85 , a mixed fluid passage where the EGR gas flow and the fresh air flow intersect each other at an angle of 90° or more can be easily formed in the collector  25 . It therefore is possible that the collector  25  is formed as a casting with a high rigidity, and moreover, weight reduction of the collector  25  can be obtained by forming the collector  25  as an aluminum-based casting product. 
     The upper case  84  is provided therein with a downstream EGR gas fluid passage (first EGR gas fluid passage)  86   a  which is a part of the EGR gas fluid passage  86  where the EGR gas flows and a mixing chamber  87  in which the fresh air and the EGR gas are mixed. The lower case  85  is provided therein with an upstream EGR gas fluid passage (second EGR gas fluid passage)  86   b  through which the downstream EGR gas fluid passage  86   a  is in communication with the EGR gas inlet  82  and a mixed gas fluid passage  88  through which a mixed gas obtained by mixing the fresh air with the EGR gas is supplied from the mixing chamber  87  to the intake manifold  3 . 
     The EGR gas inlet  82  is disposed in the lower case  85  while the fresh air inlet  81  and the mixing chamber  87  are disposed in the upper case  84 . In the mixing chamber  87 , therefore, the fresh air flowing from the fresh air inlet  81  and the EGR gas flowing from the lower case  85  intersect each other, so that the fresh air and the EGR gas can be efficiently mixed. In addition, the intake outlet  83  is disposed in the lower case  85 , and the fresh air having entered the upper case  84  tends to flow toward the lower case  85 . As a result, mixing of the EGR gas flowing toward the upper case  84  with the fresh air is made uniform. Furthermore, each of the EGR gas fluid passage  86 , the mixing chamber  87 , and the mixed gas fluid passage  88  can be compactly configured within the collector  25 , and thus the collector  25  can be downsized. 
     In a plan view, the downstream EGR gas fluid passage  86   a  is coupled with an offset to a side surface (right side surface) of the mixing chamber  87  opposite to a side surface (left side surface) thereof having the intake outlet  83  relative to a central axis of the mixing chamber  87 , and the downstream EGR gas fluid passage  86   a  and the upstream EGR gas fluid passage  86   b  are in communication with each other so that the EGR gas fluid passage  86  is formed in a spiral manner. The EGR gas fluid passage  86  composed of the downstream EGR gas fluid passage  86   a  and the upstream EGR gas fluid passage  86   b  has a bent shape curved toward the side (right side) opposite to the intake outlet  83  in a plan view. A bottom of the upstream EGR gas fluid passage  86   b  is constituted by a slope (a slope inclined upward toward the rear) extending from the EGR gas inlet  82  toward the upper case  84 . 
     A portion of the mixing chamber  87  that is in communication with the EGR gas fluid passage  86  is on the side opposite to the intake outlet  83 . The EGR gas flowing into the mixing chamber  87 , therefore, reaches the intake outlet  83  while being guided by a fresh air flow, which allows the EGR gas to be uniformly mixed with the fresh air. The EGR gas flowing from the EGR gas fluid passage  86  into the mixing chamber  87  flows in a direction against the direction from the mixing chamber  87  toward the mixed gas fluid passage  88 . This causes the fresh air and the EGR gas to collide with each other while flowing within the mixing chamber  87 . Accordingly, the EGR gas is smoothly mixed with the fresh air. 
     Since the EGR gas flows along the EGR gas fluid passage  86  having a spiral shape, the EGR gas creates a swirling flow having a clockwise vortex when flowing into the mixing chamber  87 . Such a turbulent EGR gas flows in a direction against the fresh air gas flow. Thus, simultaneously with flowing into the mixing chamber  87 , the EGR gas is smoothly mixed with the fresh air flowing within the mixing chamber  87 . In the collector  25 , therefore, the fresh air and the EGR gas can be efficiently mixed (the EGR gas can be smoothly dispersed in the mixed gas) by agitation before they are fed to the intake manifold  3 , so that a variation (unevenness) in the gas mixing state within the collector  25  can be suppressed more reliably. As a result, a mixed gas having less unevenness can be distributed to the respective cylinders of the diesel engine  1 , and a variation in the EGR gas amount among the cylinders can be suppressed. Accordingly, it is possible to suppress generation of black smoke, and to reduce the amount of NOx while maintaining a good combustion state of the diesel engine  1 . In addition, the EGR gas fluid passage  86  having a spiral shape gives sufficient swirling properties to the EGR gas flowing into the mixing chamber  87 . Thus, the collector  25  can be shaped with a shortened length in the front-rear direction. 
     A lower surface flange  84   a  of the upper case  84  and an upper surface flange  85   a  of the lower case  85  are fastened with bolts, to form the collector  25  having openings (the fresh air inlet  81 , the EGR gas inlet  82 , and the intake outlet  83 ) in three directions (toward the front, rear, and left). The upper case  84  has a rear surface flange  84   b  in which the fresh air inlet  81  is opened, and a fresh air outlet of the intake throttle member  26  is fastened to the rear surface flange  84   b  with bolts. The intake throttle member  26  adjusts the degree of opening of an intake valve (butterfly valve)  26   a  provided therein, to thereby adjust the amount of fresh air supply to the collector  25 . 
     The lower case  85  has a front surface flange  85   b  in which the EGR gas inlet  82  is opened, and an EGR gas outlet of the EGR valve member  29  is fastened with bolts to the front surface flange  85   b  with interposition of a relay flange  89  having a rectangular pipe shape. The EGR valve member  29  adjusts the degree of opening of an EGR valve (not shown) provided therein, to thereby adjust the amount of EGR gas supply to the collector  25 . A reed valve  90  inserted in the EGR gas inlet  82  is fixed inside the front surface flange  85   b  of the lower case  85 . The relay flange (spacer)  89  which is fastened to the front surface flange  85   b  with bolts covers the front side of the reed valve  90 . As a result, the collector  25  is provided therein with the reed valve  90  disposed in a portion of the EGR gas fluid passage  86 , the portion being on the EGR gas inlet  82  side. 
     The relay flange  89  has, in its rear surface coupled to the collector  25 , an EGR gas outlet  89   a  which is in communication with the EGR gas inlet  82 . The relay flange  89  has a front surface from which valve coupling bases  89   b ,  89   c  to be coupled to the EGR valve member  29  protrude. Openings of the valve coupling bases  89   b ,  89   c  are in communication with the EGR gas outlet of the EGR valve member  29 . In the relay flange  89 , the EGR gas is merged at EGR gas inlets of the upper and lower valve coupling bases  89   b ,  89   c , and then is caused to flow from the EGR gas inlet  82  into the EGR gas fluid passage  86  provided inside the collector  25  via the reed valve  90 . 
     The EGR valve member  29  is configured such that: a valve body  29   e  has an EGR gas fluid passage  29   f  in which an EGR valve (not shown) is disposed; an actuator  29   d  for adjusting the degree of opening of the EGR valve is disposed above the valve body  29   e ; the EGR valve member  29  has its longitudinal direction in parallel to the up-down direction; and the EGR valve member  29  is coupled to the front side of the collector  25  with interposition of the relay flange  89 . The EGR valve member  29  has, in a rear surface of the valve body  29   e  which is arranged lower, outlet side flanges  29   a ,  29   b  to be coupled respectively to the valve coupling bases  89   b ,  89   c  of the relay flange  89 . The outlet side flanges  29   a ,  29   b  are arranged one above the other. The EGR valve member  29  also has, in its front surface, an inlet side flange  29   c  having an EGR gas inlet that is in communication with the EGR gas outlet of the recirculation exhaust gas tube  28 . 
     The EGR valve member  29  is configured such that: after an EGR gas cooled by the EGR cooler  27  flows into the EGR gas inlet of the inlet side flange  29   c  through the downstream EGR gas relay fluid passage  32  of the EGR cooler coupling base  34  and the recirculation exhaust gas tube  28 , the EGR gas is distributed to upper and lower parts via the EGR gas fluid passage  29   f  of the valve body  29   e . The EGR gas flow distributed to upper and lower parts through the EGR gas fluid passage  29   f  is then subjected to a flow rate adjustment by the EGR valve, and then enters the relay flange  89  through the EGR gas outlets of the upper and lower outlet side flanges  29   a ,  29   b.    
     The recirculation exhaust gas tube  28  includes a gas pipe portion  28   a  and a rib  28   b , the gas pipe portion  28   a  being bent to have an L-shape in a plan view, the rib  28   b  having a flat-plate shape protruding from an inner peripheral side of an outer wall of the gas pipe portion  28   a . The recirculation exhaust gas tube  28  has, at one end (rear end) of the gas pipe portion  28   a , an outlet side flange  28   c  to be coupled to the inlet side flange  29   c  of the EGR valve member  29 , and also has, at the other end (left end) of the gas pipe portion  28   a , an inlet side flange  28   d  to be coupled to the right surface of the EGR cooler coupling base  34 . The recirculation exhaust gas tube  28  further has, in an upper surface of a bent portion of the gas pipe portion  28   a , a sensor attachment base  28   e  to which an EGR gas temperature sensor is attached. 
     In the EGR device  24 , the collector  25  can be configured with a shortened length, and therefore the distance between the EGR valve member  29  and the intake throttle member  26  can be shortened, which enables the length of the EGR device  24  in the front-rear direction to be shortened. In the EGR valve member  29 , the actuator  29   d  is disposed on the upper side. It therefore is possible that topmost portions of the EGR valve member  29 , the collector  25 , and the intake throttle member  26  are at the same height. This can lower the height of the EGR device  24  in the up-down direction, and also can narrow the width of the EGR device  24  in the left-right direction. Since the EGR device  24  can be configured compactly, coupling the EGR device  24  to the right side of the cylinder head  2  integrated with the intake manifold  3  can be easily implemented merely by adjusting the recirculation exhaust gas tube  28 . In addition, such a configuration contributes to downsizing of the diesel engine  1 . 
     The recirculation exhaust gas tube  28  has the flat-plate rib  28   b  that is coupled so as to connect the opposite ends of the gas pipe portion  28   a . This gives a high rigidity to the recirculation exhaust gas tube  28 , and also increases a strength with which the front end side of the EGR device  24  is supported on the cylinder head  2 . In addition, the recirculation exhaust gas tube  28  has the flat-plate rib  28   b  that is disposed along an EGR gas fluid passage  28   f  provided inside the gas pipe portion  28   a . Due to the rib  28   b , the gas pipe portion  28   a  has a wide heat dissipation area, which increases the effect of cooling the EGR gas flowing in the EGR gas fluid passage  28   f . This contributes to cooling a mixed gas prepared in the EGR device  24 , and exerts an effect that reduction in the amount of NOx generated from the mixed gas can be easily kept in a proper state. 
     A configuration of the EGR cooler  27  will now be described with reference to  FIG. 9  to  FIG. 16 , and  FIG. 22  to  FIG. 24 . As shown in  FIG. 9  to  FIG. 16 , and  FIG. 22  to  FIG. 24 , the EGR cooler  27  includes a heat exchanger  91  and a pair of left and right flange portions  92 ,  93 . The heat exchanger  91  has a coolant passage and an EGR gas fluid passage alternately stacked. The pair of left and right flange portions  92 ,  93  are disposed in left and right end portions of one side surface of the heat exchanger  91 . The coolant outlet  94  is disposed in one of the left and right flange portions  92 ,  93 , while the coolant inlet  95  is disposed in the other of the left and right flange portions  92 ,  93 . The EGR gas inlet  96  is disposed in one of the left and right flange portions  92 ,  93 , while the EGR gas outlet  97  is disposed in the other of the left and right flange portions  92 ,  93 . The left and right flange portions  92 ,  93  are coupled to the front surface of the cylinder head  2 , so that the EGR cooler  27  is fixed to the cylinder head  2 . 
     Since each of the pair of left and right flange portions  92 ,  93  has a coolant opening and an EGR gas opening, it is possible that the flange portions  92 ,  93  are made from a common member, and moreover material costs of the flange portions  92 ,  93  can be suppressed. The flange portions  92 ,  93  are formed by a flat plate being bored to have through holes  94  to  97  corresponding to the coolant and the EGR gas, the flat plate being coupled to the cylinder head  2 . Thus, forming the flange portions  92 ,  93  in the EGR cooler  27  is easy. In addition, a coupling portion where the flange portions  92 ,  93  are coupled to the heat exchanger  91  can be minimized, so that the amount of heat transfer from the cylinder head  2  to the heat exchanger  91  can be reduced, which increases the effect of cooling the EGR gas by the heat exchanger  91 . 
     Since the EGR cooler  27  has the flange portions  92 ,  93  protruding from the rear surface of the heat exchanger  91 , a space is formed between the heat exchanger  91  and the cylinder head  2 . As a result, the EGR cooler  27  is in a state where a wide area of the front and rear surfaces of the heat exchanger  91  is exposed to outside air. Heat dissipation occurs in the heat exchanger  91 , too. Thus, the effect of cooling the EGR gas by the EGR cooler  27  is increased. This configuration can reduce the degree of stacking in the heat exchanger  91  as compared to a configuration in which the rear surface and the front surface of the heat exchanger  91  are attached. The length of the EGR cooler  27  in the front-direction can be shorted, and thus the diesel engine  1  can be downsized. 
     The left flange portion  92  has the coolant outlet  94  and the EGR gas inlet  96 , while the right flange portion  93  has the coolant inlet  95  and the EGR gas outlet  97 . In the left flange portion  92 , the coolant outlet  94  is disposed above the EGR gas inlet  96 , while in the right flange portion  93 , the EGR gas outlet  97  is disposed above the coolant inlet  95 . The coolant outlet  94  and the EGR gas outlet  97  are disposed at the same height, while the coolant inlet  95  and the EGR gas inlet  96  are disposed at the same height. 
     The left and right flange portions  92 ,  93  of the EGR cooler  27  are coupled respectively to the EGR cooler coupling bases  33 ,  34  protruding from the front surface of the cylinder head  2 . The upstream EGR gas relay fluid passage  31  and the downstream coolant relay fluid passage  38  of the left EGR cooler coupling base  33  are in communication with the EGR gas inlet  96  and the coolant outlet  94  of the left flange portion  92 , respectively. The downstream EGR gas relay fluid passage  32  and the upstream coolant relay fluid passage  39  of the right EGR cooler coupling base  34  are in communication with the EGR gas outlet  97  and the coolant inlet  95  of the right flange portion  93 , respectively. 
     The EGR gas relay fluid passages  31 ,  32  and the coolant passages  38 ,  39  are provided in the coupling bases  33 ,  34  to which the flange portions  92 ,  93  of the EGR cooler  27  are coupled, and are in communication with the EGR gas inlet and outlet  96 ,  97  and the coolant outlet and inlet  94 ,  95  of the flange portions  92 ,  93 . It is not necessary that coolant piping and EGR gas piping are disposed between the EGR cooler  27  and the cylinder head  2 . Accordingly, a sealability can be given to a coupling portion where the EGR cooler  27  and the cylinder head  2  are coupled to each other without any influence of, for example, extension and contraction of piping caused by the EGR gas or the coolant. In addition, the EGR cooler  27  is given an enhanced resistance against external fluctuation factors such as heat and vibration, and can be compactly installed in the cylinder head  2 . 
     The coolant outlet  94  is disposed above the EGR gas inlet  96  in the flange portion  92 , while the EGR gas outlet  97  is disposed above the coolant inlet  95  in the flange portion  93 . Thus, the flange portions  92 ,  93  having identical shapes with their postures mutually upside-down are attached to the heat exchanger  91 . This can reduce the number of types of component parts included in the EGR cooler  27 , thus improving an assemblability of the EGR cooler  27  and reducing costs of the component parts. 
     The flange portion  92  is provided with the coolant outlet  94  and the EGR gas inlet  96  through which a coolant or an EGR gas having a large quantity of heat passes, while the flange portion  93  is provided with the coolant inlet  95  and the EGR gas outlet  97  through which a coolant or an EGR gas having a small quantity of heat passes. Accordingly, distortion caused by thermal deformation of each of the flange portions  92 ,  93  can be suppressed. In addition, the flange portions  92 ,  93  are configured as separate members whose thermal deformation is less influential to each other, and therefore damage and breakdown of the EGR cooler  27  can be prevented. 
     In the EGR cooler  27 , the coolant outlet  94  and the coolant inlet  95  are disposed at diagonal positions, and the EGR gas inlet  96  and the EGR gas outlet  97  are disposed at diagonal positions in a rear view. Since EGR gases having different quantities of heat and coolants having different quantities of heat are respectively supplied or discharged at diagonal positions, thermal deformations of coupling portions where the EGR cooler  27  is coupled to the cylinder head  2  can be mutually relieved, so that deflection or slackness of the coupling portions can be suppressed. Accordingly, leakage of an EGR gas or a coolant in the EGR cooler  27  and in the cylinder head  2  can be prevented, and moreover a decrease in the coupling strength can be prevented. 
     A plate-shaped gasket  98  is sandwiched between the cylinder head  2  and the flange portions  92 ,  93  so as to extend across the left and right flange portions  92 ,  93 . A coolant inlet and a coolant outlet of the cylinder head  2 , which are respectively in communication with the coolant outlet  94  and the coolant inlet  95  of the flange portions  92 ,  93 , have O-rings  99  embedded therein, the O-rings  99  being ring-shape seal members. The O-rings  99  are covered with the flange portions  92 ,  93 . 
     Since the flange portions  92 ,  93  configured as separate members are coupled to the coupling bases  33 ,  34  of the cylinder head  2  with the gasket  98  interposed therebetween, a tension is exerted on the gasket  98  due to thermal deformation of the coupling portion coupled to the cylinder head  2 . This enhances a sealability (hermetic sealing performance) of the gasket  98  in a coupling portion of each of the EGR gas inlet  96  and the EGR gas outlet  97 . Thus, leakage of an EGR gas flowing from one to the other between the cylinder head  2  and the EGR cooler  27  can be prevented. The O-rings  99  are embedded in spaces defined by rear end surfaces of the flange portions  92 ,  93  and the coolant inlet and the coolant outlet of the coupling bases  33 ,  34  of the cylinder head  2 . When a coolant flows, therefore, the coolant is in contact with the O-rings  99  in communication portions where the coupling bases  33 ,  34  are in communication with the flange portions  92 ,  93 . Thus, a sealability (hermetic sealing performance) of the coupling portions of the coolant outlet and inlet can be obtained. Accordingly, even though the EGR cooler  27  where a liquid and a gas enter and exit is coupled to the cylinder head  2 , a sealability for each of the liquid and the gas can be obtained, so that leakage of each of the EGR gas and the coolant can be prevented. 
     An outer peripheral portion of each of the flange portions  92 ,  93  is bored to have through holes  100  for bolt fastening, at outer positions. Specifically, the left flange portion  92  has five through holes  100  disposed in its upper, lower, and left sides, and the right flange portion  93  has five through holes  100  disposed in its upper, lower, and right sides. Since the left flange portion  92  has the through holes  100  disposed above the coolant outlet  94 , below the EGR gas inlet  96 , and to the left of a portion between the coolant outlet  94  and the EGR gas inlet  96 , a sealability of the coolant outlet  94  and the EGR gas inlet  96  can be exerted when the left flange portion  92  is fastened to the coupling base  33  of the cylinder head  2  with bolts. Likewise, since the right flange portion  93  has the through holes  100  disposed below the coolant inlet  95 , above the EGR gas outlet  97 , and to the right of a portion between the coolant inlet  95  and the EGR gas outlet  97 , a sealability of the coolant inlet  95  and the EGR gas outlet  97  can be exerted when the right flange portion  93  is fastened to the coupling base  34  of the cylinder head  2  with bolts. 
     The gasket  98  is constituted by a lamination of two plates  98   a ,  98   b  each having through holes  101  to  103 . The EGR gas passes through the through holes (EGR gas through holes)  101 . The coolant passes through the through holes (coolant through holes)  102 . Fastening bolts are inserted into the through holes (bolt through holes)  103 . The gasket  98  has such a shape that an inner peripheral edge at the EGR gas through hole  101  is branched so as to be warped in the front-rear direction and is configured such that the open areas of the coolant through holes  102  are larger than the open areas of the coolant outlet and inlet  94 ,  95 . 
     In the gasket  98 , the front plate  98   a  has its inner peripheral edge at the EGR gas through hole  101  being warped frontward, while the rear plate  98   b  has its inner peripheral edge at the EGR gas through hole  101  being warped rearward. The front plate  98   a  and the rear plate  98   b  are bonded by welding, so that the inner peripheral edge at the EGR gas through hole  101  has a Y-shaped cross-section. Since the inner peripheral edge at the EGR gas through hole  101  is warped in the front-rear direction, front and rear surfaces of the inner peripheral edge at the EGR gas through hole  101  can be in tight contact with end surfaces of the coupling bases  33 ,  34  and the flange portions  92 ,  93 . Accordingly, a sufficient airtightness can be obtained. 
     The gasket  98  is configured such that the openings of the coolant through holes  102  is larger than those of the coolant outlet and inlet  94 ,  95 . Thus, the O-rings  99  are inserted in the coolant through holes  102 . Communication portions where the coolant outlet and inlet of the flange portions  92 ,  93  are in communication with the coolant relay fluid passages  38 , 39  of the coupling bases  33 ,  34  are hermetically sealed by the O-rings  99  fitted in the coolant through holes  102  of the gasket  98 . 
     The coupling bases  33 ,  34  of the cylinder head  2  have the coolant outlet and inlet opened with steps, and thereby the openings of the coolant outlet and inlet are given larger diameters than the fluid passage diameters of the coolant relay fluid passages  38 ,  39  formed inside the coupling bases  33 ,  34 . The O-rings  99  disposed to the coolant outlet and inlet of the coupling bases  33 ,  34  are fitted on the outer circumferential sides of the coolant relay fluid passages  38 ,  39 . The O-rings  99  are inserted in the gasket  98 , and also fitted in the step portions of the coolant outlet and inlet in the coupling bases  33 ,  34 . Thereby, the O-rings  99  are sandwiched between the coupling bases  33 ,  34  and the flange portions  92 ,  93 . When a coolant passes inside the O-rings  99  made of an elastic material, the O-rings  99  are deformed to expand outward and come into tight contact with the coupling bases  33 ,  34  and the flange portions  92 ,  93 , thus providing a sealability for the coolant. 
     The ring-shape O-ring has its inner circumferential portion bulging frontward and rearward. A coolant passing through the inner circumferential portion of the O-ring  99  pushes the inner circumferential portion, so that its front and rear edges are deformed to protrude frontward and rearward. This brings the inner circumferential portion of the O-ring  99  into tight contact with the coupling bases  33 ,  34  and the flange portions  92 ,  93 . Thus, a sealability for the coolant can be enhanced in the coupling portion where the cylinder head  2  is coupled to the EGR cooler  27 . 
     The ring-shape O-ring  99  whose inner circumferential portion is bulged frontward and rearward is shaped such that its inner circumferential surface has a recessed portion. The inner circumferential surface of the O-ring is warped frontward and rearward so as to have a Y-shaped cross-section. A coolant passing through the inner circumferential portion of the O-ring  99  pushes the inner circumferential portion, so that its front and rear edges are further protruded frontward and rearward, to increase the degree of tight contact of the inner circumferential portion of the O-ring  99  with the coupling bases  33 ,  34  and the flange portions  92 ,  93 . Accordingly, a sealability for the coolant can be enhanced in the coupling portion where the cylinder head  2  is coupled to the EGR cooler  27 . 
     The configurations of respective parts of the present invention are not limited to those of the illustrated embodiment, but can be variously changed without departing from the gist of the invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  engine 
               2  cylinder head 
               3  intake manifold 
               4  exhaust manifold 
               5  crankshaft 
               6  cylinder block 
               7  flywheel housing 
               8  flywheel 
               9  cooling fan 
               24  EGR device 
               25  collector (EGR main body case) 
               26  intake throttle member 
               27  EGR cooler 
               28  recirculation exhaust gas tube 
               29  EGR valve member 
               31  upstream EGR gas relay fluid passage 
               32  downstream EGR gas relay fluid passage 
               33  EGR cooler 
               34  EGR cooler 
               35  coolant drainage 
               36  intake fluid passage 
               37  exhaust fluid passage 
               38  downstream coolant relay fluid passage 
               39  upstream coolant relay fluid passage 
               40  intake inlet 
               91  heat exchanger 
               92  flange member 
               93  flange member 
               94  coolant outlet 
               95  coolant inlet 
               96  EGR gas inlet 
               97  EGR gas outlet 
               98  gasket 
               98   a  front plate 
               98   b  rear plate 
               99  O-ring 
               100  through hole (for bolt fastening) 
               101  EGR gas through hole 
               102  coolant through hole 
               103  bolt through hole