Patent Publication Number: US-2009229248-A1

Title: Exhaust device comprising a cylinder head with a thermally decoupled integrated exhaust manifold

Description:
The invention relates to an exhaust device for a multicylinder internal combustion engine of a motor vehicle comprising a cylinder head of said engine in which are arranged exhaust ducts of which a first end of each is connected to a combustion chamber of said engine and of which the second end opens into a cavity or plenum of an exhaust manifold incorporated into the cylinder head of which an outlet duct is connected externally to a device for depolluting the exhaust gases of an exhaust line of the vehicle. 
     Many examples of devices of this type are known. 
     Conventionally, exhaust devices comprise exhaust ducts formed in the cylinder head of the engine which coincide with ducts of the manifold arranged facing the exhaust ducts. The manifold comprises at least one outlet duct which is connected to an exhaust line of the motor vehicle. 
     In order to reduce the content of pollutant products contained in the combustion gases, it is known practice to implant into the exhaust line depollution devices, notably catalyzers, which make it possible to convert the pollutant products present in the combustion gases, notably the unburnt hydrocarbons HC and the nitrogen oxides NOx. 
     A problem arises however when the engine is started up from cold. 
     Specifically, the effectiveness of the depollution devices is greatly dependent on their operating temperature. Having these devices at a distance from the combustion chambers of the engine slows down their rise in temperature during the first minutes of operation of the engine and reduces their effectiveness even though it is during these first minutes of operation of the engine that the largest quantity of pollutants is emitted. 
     To remedy this disadvantage, it has been proposed to produce exhaust manifolds that are partially or totally incorporated into the engine cylinder head, which makes it possible to considerably reduce the distance between the combustion chambers and the depollution device, which is then placed directly next to the cylinder head at the manifold outlet. 
     Document FR-2.738.289 describes notably a manifold of this type, wherein the exhaust ducts of the cylinder head open into a cavity or plenum half formed in the manifold and half in the cylinder head. 
     Document DE-A-3.815.408 describes, for its part, a manifold formed totally in the cylinder head and cast in one piece with the latter. 
     These known manifolds of the prior art however pose many problems. 
     Specifically, when a manifold of this type is only partly formed in the cylinder head, the distance between the combustion chambers and the depollution device is not reduced to the minimum and therefore the effectiveness of the depollution device is not total. 
     Furthermore, when the manifold is incorporated into the cylinder head, it is subjected to high temperature stresses and is therefore, like the material of the cylinder head, subjected to a considerable expansion as it rises in temperature. Therefore, it is practically impossible to couple such a manifold to the rest of the exhaust line if it is not provided with cooling cavities connected to the cavities for the circulation of liquid for cooling the cylinder head. 
     This design then reduces on the one hand from the efficiency of the engine since it is synonymous with reducing the thermal efficiency of the engine due to the need to cool the manifold, and, on the other hand, considerably reduces the benefits of having the manifold and the combustion chambers close together, the cooling of the manifold then no longer allowing its rapid rise in temperature to improve the effectiveness of the depollution device. 
     To remedy this disadvantage, the invention proposes a device of the type described above comprising a manifold incorporated into the cylinder head of the engine and furnished with means for thermally decoupling it from said cylinder head. 
     For this purpose, the invention proposes a manifold of the type described above, characterized in that the cavity or plenum of the manifold is formed in an element fitted to the inside of the cylinder head and in that said element is thermally decoupled from the cylinder head by thermal decoupling means. 
     According to other features of the invention:
         the second ends of the exhaust ducts open into a receiving bore formed in the cylinder head which receives a tubular insert which is fitted to the inside of the cylinder head in order to form the fitted element, which comprises orifices for communication with the second ends of the exhaust ducts, and which comprises the outlet duct opening to the outside of the cylinder head,   the tubular insert is made of stainless steel,   the tubular insert is mounted without clearance in the receiving bore of the cylinder head,   the thermal decoupling means are arranged in the cylinder head around the tubular insert and at least between two consecutive exhaust ducts,   the thermal decoupling means comprise at least one cavity which is formed in the material of the cylinder head around the receiving bore and which opens into said bore,   the device comprises a cavity which is arranged around the receiving bore between each pair of consecutive exhaust ducts, and a cavity which is arranged around the receiving bore between an end exhaust duct and an end outer face of the cylinder head,   each cavity is produced when the cylinder head is molded.       

    
    
     
       Other features and advantages of the invention will appear on reading the following detailed description, for the comprehension of which reference will be made to the appended drawing in which the single FIGURE is a schematic view of a cylinder head for an exhaust device according to the invention. 
     
    
    
     The single FIGURE shows a cylinder head  10  for an exhaust device made according to the invention. 
     In a known manner, exhaust ducts  12  are arranged in the cylinder head  10 . A first end  14  of each duct  12  is connected to a combustion chamber  16  of the engine and the second end  18  of each duct  12  opens into a cavity  20  or plenum of an exhaust manifold  22  incorporated into the cylinder head  10 . An outlet duct  24  of the manifold  22  is connected externally to a device (not shown) for depolluting the exhaust gases of an exhaust line (not shown) of the vehicle. The cylinder head  10  comprises, between each combustion chamber  16 , cavities  30  for the circulation of a liquid for cooling the engine. 
     According to the invention, the cavity  20  or plenum of the manifold  22  is formed in an element  26  which is fitted to the inside of the cylinder head  10  and said element  26  is thermally decoupled from the cylinder head  10  by thermal decoupling means  28 . 
     More particularly, the second ends  18  of the exhaust ducts  12  open into a receiving bore  32  which is formed in the cylinder head  10 . This bore  32  receives a tubular insert  26  forming the fitted element  26  which is fitted to the inside of the cylinder head, which comprises orifices  34  for communication with the second ends  18  of the exhaust ducts  12 , and which comprises the outlet duct  24  opening to the outside of the cylinder head  10 . 
     In the embodiment, the tubular insert  26  is made of stainless steel, because this material has thermal expansion characteristics suitable for ensuring a satisfactory seal when it is connected to the depollution device, irrespective of its temperature. It should be noted that this characteristic does not limit the invention, and that any other material having similar characteristics could be used for the correct application of the invention. 
     In the preferred embodiment of the invention, the tubular insert  26  is mounted without clearance in the receiving bore  32  of the cylinder head  10 . The tubular insert  26  is for example sleeve-fitted into the bore  32  with an adjustment of the tight type. 
     Advantageously, the thermal decoupling means  28  are arranged in the cylinder head around the tubular insert  26  and at least between two consecutive exhaust ducts  12 . 
     More particularly, the thermal decoupling means  28  comprise at least one cavity  28  which is formed in the material of the cylinder head  10  around the receiving bore  32 , and which opens into said bore  32 . This cavity is filled with air and therefore allows a thermal decoupling of the insert  26  relative to the cylinder head, preventing the heat of the cylinder head  10  from communicating with the insert  26  by conduction. 
     Preferably, as illustrated in the single FIGURE, the device comprises a cavity  28  which is arranged around the receiving bore  32  between each pair of consecutive exhaust ducts  12 , and a cavity  28  which is arranged around the receiving bore  32  between an end exhaust duct  12  and an end external face  36  of the cylinder head  10 . 
     Any production method may be used for the correct production of the cavities  28 . However, in the preferred embodiment of the invention, each cavity  28  is made when the cylinder head  10  is molded. 
     The invention therefore makes it possible to benefit from a device comprising a manifold  26  fitted into the cylinder head  10  as close as possible to the combustion chambers  12 , which makes it possible to optimize the performance of a depollution device connected to its outlet duct  24  and which does not have the disadvantages of the incorporated manifolds known in the prior art because of its thermal decoupling with respect to the cylinder head  10 .