Abstract:
An exhaust manifold which includes a flange to be fixed on an engine cylinder head. A wide exhaust gas collecting cavity is connected to the flange to receive exhaust gases discharged by exhaust pipes of the cylinder head. The wide collecting cavity has a recessed wall element subdividing the cavity into two mutually communicating sub-cavities. An end portion is connected to the collecting cavity outlet and has a discharge outlet for the exhaust gases.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to exhaust manifolds for internal combustion engines, and more particularly to those installed in internal combustion engines in association with a means for cleaning the exhaust gases, in particular a start catalyst, which for example is mounted on the manifold. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide such a manifold whose heat loss is as small as possible in order to shorten the time for starting the cleaning means. 
     Another object of the invention is to ensure that the gas velocity field over the inlet face of the cleaning means is as uniform as possible. 
     Yet another object of the invention is to permit easy assembly of the manifold in the factory. 
     The invention therefore provides an exhaust manifold for internal combustion engines. According to one general characteristic of the invention, this manifold comprises a flange for fixation on the engine cylinder head, a wide exhaust-gas collecting cavity joined to the flange to receive the exhaust gases delivered by the cylinder head exhaust pipes, this wide collecting cavity having a reentrant wall element subdividing the said cavity into two mutually communicating sub-cavities. The manifold also comprises an end portion joined to the collecting cavity outlet and having an outlet orifice for the exhaust gases. 
     The combination of the “plenum” form of the manifold (wide exhaust-gas collecting cavity) subdivided into two “lungs” ensures that the heat loss will be minimal and contributes to achieving a uniform velocity field and uniform distribution of the exhaust gases at the inlet of a gas cleaning means. 
     According to one embodiment of the invention, the reentrant wall element provides, on the outside surface of the collecting cavity, a central indentation extending substantially perpendicular to the flange fixation plane, the two sub-cavities being symmetric with respect to the central plane of this central indentation. 
     This embodiment thus not only makes it possible easily to construct the wide cavity formed by these two lungs but also permits, by virtue of the central indentation, easy introduction of a wrench for establishing a point for fixation of the manifold on the cylinder head. 
     The wide collecting cavity has an appropriate internal volume. More particularly, it has been found that it is advantageous for the volume of the collecting cavity to be greater than about 0.8 times the cubic capacity of the engine in order to achieve a noteworthy reduction of heat loss as well as good uniformity of the velocity field at the inlet of the cleaning means. In addition, and especially for reasons of overall size, it is preferable that the volume of the collecting cavity be smaller than about 1.5 times the cubic capacity of the engine. 
     According to a preferred embodiment of the invention, the volume of the collecting cavity is chosen substantially equal to the cubic capacity of the engine, thus permitting an optimal compromise between the criteria of heat loss and uniformity of the velocity field and distribution, and overall manifold size. 
     It is also particularly advantageous for the end portion to have substantially the form of a bowl, thus permitting the uniformity of the velocity field to be further improved. 
     In addition, the end portion is preferably provided with an elliptical throttle plate, the major axis of the ellipse being parallel to the plane of fixation of the fixation flange. This also contributes to obtaining better uniformity of the velocity field. 
     The fixation flange is advantageously provided with four inlet ports to receive the exhaust gases and with five points of fixation on the cylinder head, those points being disposed in crisscross arrangement around the four inlet ports. Furthermore, the line segment connecting the respective centers of two consecutive fixation zones passes substantially through the center of the port situated between these two fixation zones. 
     Such an embodiment makes it possible to minimize the number of bolts for fixation of the manifold on the cylinder head and thus to minimize the assembly stresses associated with the introduction of wrenches. In addition, the criss-cross disposition of the fixation bolts relative to the centers of two consecutive fixation zones aligned with the center of the inlet port situated between these two fixation zones contributes to ensuring excellent leaktightness of fixation of the manifold on the cylinder head. 
     According to another embodiment of the invention, the manifold also comprises an intermediate connecting portion which connects the fixation flange to the collecting cavity and is provided with exhaust conduits joined respectively to the inlet ports of the fixation flange. Advantageously, the exhaust conduits disposed at the ends of the intermediate connecting portion are convergent. Consequently, all the exhaust conduits make it possible to force the exhaust-gas streams toward the manifold center, thus additionally contributing to achievement of a uniform velocity field over the inlet face of the cleaning means. In addition, this intermediate connecting portion contributes to increasing the rigidity of the manifold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and characteristics of the invention will become evident upon examination of the detailed description of one embodiment, which in no way is limitative, and of the attached drawings, wherein: 
     FIG. 1 schematically shows a manifold according to the invention, 
     FIG. 2 is a section through line II—II of FIG. 1, 
     FIG. 3 is a section through line III—III of FIG. 2, 
     FIG. 4 shows the section of the throttle plate of the manifold through line IV—IV of FIG. 2, 
     FIG. 5 shows a section through line V—V of FIG. 2, 
     FIG. 6 is a view in the direction of arrow F of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In these FIGS., reference  1  denotes in general an exhaust manifold according to the invention. This manifold is provided with a fixation flange  2  having five fixation holes  20  to  24 , by means of which the manifold can be fixed by bolts onto the engine cylinder head. 
     The manifold is also provided with a wide exhaust-gas collecting cavity  3 , which is connected to the fixation flange by an intermediate connecting portion  5 . 
     This wide collecting cavity  3  is prolonged by an end connecting portion  4  provided with a portion  41  having substantially the form of a bowl followed by a fixation flange  42  provided with an exhaust-gas outlet orifice  49 , a start catalyst CT being fixed, by welding in the present case, on the said fixation flange. 
     The outlet orifice  40  of bowl  41  is a throttle plate having elliptical section (FIG.  4 ), the major axis of the ellipse being parallel to the plane of fixation of the manifold on the cylinder head. 
     As illustrated more particularly in FIG. 3, the wide exhaust-gas collection cavity  3 , which is cast in one piece, supports a reentrant wall element  32 , which forms on the outside surface of cavity  3  a central indentation  34 , thus creating two symmetric lateral portions  30  and  31 . In this way collection cavity  3  is divided into two sub-cavities  300  and  310 , which are in mutual communication at the level of constricted section  320  of cavity  3 , situated in the plane of symmetry thereof. 
     Central fixation zone  20  of fixation flange  2  is situated in the extension of central indentation  34 , thus permitting introduction of central wrench VS. In addition, lateral portions  30 ,  31  are provided with reentrant wall elements  39 ,  38  respectively, which form on the outside surface of these lateral portions two auxiliary indentations  37 ,  36  respectively. 
     It will be noted here that fixation zones  22  and  23  are situated respectively in the extension of auxiliary indentations  37  and  36 , thus permitting easy introduction of automatic wrenches. 
     As illustrated more particularly in FIG. 5, it is evident that fixation flange  2  is provided with four ports O 1  to O 4  corresponding to the four exhaust pipes made in the cylinder head and joined to the four cylinders of the engine. Fixation holes  20  to  24  are situated in criss-cross pattern around these ports O 1  to O 4 . The center of each inlet port of the fixation flange is substantially aligned with the centers of the two fixation orifices disposed on both sides of this inlet port. Such an arrangement permits the number of manifold fixation bolts to be minimized while at the same time ensuring excellent leaktightness of fixation on the cylinder head. 
     Intermediate connecting portion  5  is provided with four exhaust conduits  51  to  54  prolonging the exhaust pipes of the cylinder head. As illustrated more particularly in FIG. 6, axes A 51  and A 54  of the two exhaust conduits situated at the end of intermediate connecting portion  5  converge. Thus the entire group of exhaust conduits  51  to  54 , and in particular the two end conduits, ensure that the exhaust-gas streams are forced toward the manifold center. 
     Finally, at the top of collecting cavity  3 , and in the extension of wall element  32  and consequently of central indentation  34 , there is disposed a port  35  for housing any desired transducer or sensor, especially a sensor for measuring the oxygen concentration, commonly referred to as “lambda sensor” by those skilled in the art, which sensor is used traditionally in air/fuel ratio control circuits. 
     The location of this port  35  permits lambda sensor SL to be positioned centrally, which is particularly advantageous. 
     The invention makes it possible to obtain an exhaust manifold with mounted-on start catalyst, which manifold is particularly efficient as regards heat loss and also as regards uniformity of the velocity field of the gases delivered to the start catalyst. In fact, except for exhaust conduits  51  to  54 , which prolong the exhaust pipes for a distance of about 30 mm and incidentally contribute to ensuring good manifold rigidity because of this fact, the pulsations arriving from the four cylinders are collected in a large volume and circulate toward outlet orifice  40  of the bowl as illustrated by arrows CG in FIG.  3 . In addition, bowl  41  permits even better homogenization of the gases, especially by causing vortexing action MC (FIG. 3) of the gases, this bowl followed by elliptical throttle plate  40  contributing to the creation of a uniform velocity field at the inlet of the start catalyst. 
     It will be understood that the invention as described hereinabove is not limited to an exhaust manifold equipped with a mounted-on start catalyst, but that it is also suitable for any gas cleaning means, such as a three-way catalyst, a particulate filter, an NO x  trap, an SO x  trap, etc., whether it is mounted on the manifold or effectively connected thereto by, for example, a tube portion.