Abstract:
A method for coating at least one exhaust port of a cylinder arranged inside a cylinder head of a combustion engine is provided, wherein the exhaust port connects the cylinder to an exhaust system. One or more surface portions of the cylinder head defining the at least one exhaust port are at least partially coated by spraying material from both the cylinder side and the exhaust system side. An apparatus for performing the method is also provided.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The invention relates to a method for coating an exhaust port and an apparatus for performing the method. 
         [0002]    U.S. Pat. No. 5,987,882 discloses an engine which is coated on various portions with a layer such as a thermally insulating coating. Particularly, the inner surfaces of the exhaust manifold and the pipes prior to the turbocharger and optionally other areas of a cylinder head are coated, thus providing an increased temperature of the exhaust gases which can increase the efficiency of a turbocharger. Various deposition techniques are suggested to apply the coating to the inner surfaces, such as impregnation with a solution of soluble precursor followed by thermal or chemical decomposition, thermal spraying processes such as flame spraying or plasma spraying, or by application of a slurry followed by a thermal treatment to dry. However, an after treatment after a wet coating with a soluble precursor and/or a slurry is time consuming and the handling of the components is laborious. Further, some of the surfaces to be coated exhibit a complex geometry. 
         [0003]    It is desirable to provide a method for coating of complex inner surfaces of an exhaust port which provides a reliable deposition of material. It is also desirable to provide an apparatus for performing the method. 
         [0004]    A method is proposed for coating at least one exhaust port of a cylinder arranged inside a cylinder head of a combustion engine, wherein the exhaust port connects the cylinder to an exhaust system. One or more surface portions of the cylinder head defining the at least one exhaust port are at least partially coated by spraying material from both the cylinder side and the exhaust system side. 
         [0005]    Between inlet and outlet the exhaust port has a curved shape. By coating exhaust port from both sides, it is possible to coat the complicated shape of the exhaust ports with a high coating quality. Compared to other coating techniques such as wet coating and the like, where the cylinder head may have to undergo an after treatment, spray coating can be applied easily and reproducible. A geometrical modification of the engine can be avoided, particularly in the combustion chamber. As the coating is applied to the finished parts, a change in the casting process of the engine parts can be avoided. 
         [0006]    A high coverage of the exhaust outlet ports can be achieved by the heat insulating coating which yields a high thermal insulation. Preferably, the coating material can be a thermal barrier coating which reduces or eliminates a heat transfer from the hot exhaust gases to the cylinder head and/or the engine. The material can be sprayed in one global step with thicknesses up to several hundreds of micrometers. The coating can preferably be a thermal barrier coating applied by plasma spraying. Optionally, a basecoat can be deposited before a topcoat is applied. The topcoat preferably is a ceramic heat insulating material, by way of example yttria-stabilized zirconia (Y2O3-ZrO2), as well as magnesia stabilized zirconia (MgO—ZrO2)-, calcia stabilized zirconia (CaO—ZrO2)-, ceria stabilized zirconia (CeO2-ZrO2)-stabilized zirconia (ZrO2-ZrO2), as well as zircon (ZrSiO4), zirconates (such as CaZrO3), titanates (such as CaTiO3) and the like. 
         [0007]    Thus, the exhaust gases are at a high temperature when entering a turbocharger. More energy is available for the turbocharger which can provide more energy for driving a compressor for compressing air for the combustion process in the engine. 
         [0008]    According to a favourable embodiment of the invention, the at least one exhaust port can coated at least partially by coating separately a first portion and a second portion of the exhaust port. The coating of the exhaust port walls can be performed in a controlled way for each portion of the exhaust port. By coating the inlet and outlet region separately, it is possible to coat the complicated shape of the exhaust ports with a high coating quality. 
         [0009]    According to a further favourable embodiment of the invention, the first portion of the exhaust port can be coated by material supplied by a first spray gun. According to a favourable refinement, the first spray gun coating the first portion can be positioned outside the exhaust port. Preferably, material coating the first portion of the exhaust port can be deposited along a direction corresponding to a longitudinal extension of the first spray gun. Favourably, the spray gun can be rotated about an axis arranged crosswise to the spraying direction. 
         [0010]    According to a further favourable embodiment of the invention, the second portion of the exhaust port can be coated by material supplied by a second spray gun. According to a favourable improvement, the material for coating the second portion of the exhaust port can be supplied from inside of the exhaust port. Preferably, the material coating the second portion of the exhaust port can be deposited under an angle to a direction corresponding to a longitudinal extension of the second spray gun. Favourably, the second spray gun can be rotated about an axis arranged parallel to its longitudinal extension. The first and the second spray guns can be operative simultaneously or sequentially. A simultaneous operation shortens the process time for coating the one or more exhaust ports. A sequential operation allows for a less complex apparatus for performing the coating of the one or more exhaust ports. 
         [0011]    According to a further favourable embodiment of the invention, the material coating the first portion can be deposited with a deposition rate higher than the material coating the second portion. The first portion is subject to a higher thermal load during engine operation so that a thick coating improves a thermal insulation of the exhaust port. Thus it is advantageous according to a further favourable embodiment of the invention that the first portion on the cylinder head fire face side can be coated with a deposition rate higher than coating the second portion on an exhaust manifold side of the exhaust port. 
         [0012]    Favourably, the exhaust port can be coated by thermal spraying, preferably by plasma spraying. Thermal or plasma spraying results in a coating on the first and second surface portions with a reliable bonding strength and homogeneity. 
         [0013]    According to a further favourable embodiment of the invention, the exhaust port can be treated with a cleaning step prior to coating. A cleaning step can improve the bonding of the coating deposited on the first and second surface portions. Alternatively or additionally, the bond strength of the coating can be further improved by coating the first and second portions with a bond coat prior to coating with a topcoat. 
         [0014]    According to further aspect of the invention, an apparatus is proposed for performing coating of an exhaust port. A first spray gun and a second spray gun are provided for deposition of a material at a first and a second portion of an exhaust port of a cylinder head. 
         [0015]    According to a favourable embodiment of the invention, a nozzle of the first spray gun can be arranged to deposit material along a direction corresponding to a longitudinal extension of the first spray gun. The spray gun has a simple design spraying in a forward direction. 
         [0016]    According to a further favourable embodiment of the invention, a nozzle of the second spray gun can be arranged to deposit material under an angle to a direction corresponding to a longitudinal direction of the second spray gun. This allows depositing material from inside the exhaust port in a sidewise direction. 
         [0017]    According to a further favourable embodiment of the invention, the first and/or the second spray guns can be arranged rotatably with respect to the exhaust port. Alternatively, the exhaust port can be arranged rotatably with respect to first and/or the second spray guns. A homogeneous coating thickness can be achieved when rotating the first and/or second spray gun during spray coating. 
         [0018]    According to further aspect of the invention, a cylinder head is proposed comprising at least one exhaust port coated with a thermally heat insulating material according to a method where spray coating is performed at least partially of one or more surface portions of the cylinder head defining the at least one exhaust port from both the cylinder side and the exhaust system side. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention may best be understood from the following detailed description of the embodiments, but not restricted to the embodiments, wherein is shown schematically: 
           [0020]      FIG. 1  an arrangement comprising an engine with a cylinder head, a turbocharger and a catalyst system; 
           [0021]      FIG. 2   a ,  2   b  a view on a fire face side of the cylinder head ( FIG. 2   a ) and a view on an exhaust manifold side of the cylinder head ( FIG. 2   b ); and 
           [0022]      FIG. 3   a - 3   c  a longitudinal cut through a exhaust port with a first spray gun depositing material on a first portion of the exhaust port ( FIG. 3   a ), with a second spray gun depositing material on a second portion of the exhaust port ( FIG. 3   b ) according to the invention, and the surface portions to be coated in combination ( FIG. 3   c ). 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention. 
         [0024]      FIG. 1  depicts schematically an arrangement comprising an engine  10  with a cylinder head  12 , a turbocharger  50  connected with its turbine side to an exhaust manifold  18  of the engine  10  and an exhaust after treatment system  60  for reducing emissions contained in the exhaust gases. The general setup of such an arrangement is known in the art. 
         [0025]    In the cylinder head  12  of the engine  10  a multitude of cylinders  14  is provided in each of which a piston  16  is movable up and down by action of the combustion process in the engine  10  in the usual manner. Exhaust gases generated during combustion are discharged through exhaust ports  20  assigned to each cylinder  14  to the exhaust manifold  18 . An exhaust port  20  is a channel defined by the walls of the cylinder head  12 . 
         [0026]      FIG. 2   a  and  FIG. 2   b  illustrate a view on a fire face side  32  of a cylinder head  12  ( FIG. 2   a ) and a view on an exhaust manifold side  36  of a cylinder head  12  ( FIG. 2   b ) comprising by way of example six cylinders  14 , each equipped with an exhaust port  20 . 
         [0027]    The exhaust ports  20  on the fire face side  32  exhibit two openings  20   b,    20   c,  whereas on the exhaust manifold side  36  the exhaust ports  20  exhibit one opening  20   a.  Each cylinder  14  ( FIG. 1 ) also exhibits two inlet openings (not referred to with a reference number) for feeding air into the cylinder  14  ( FIG. 1 ). 
         [0028]    Referring now to the illustrations in  FIGS. 2   a ,  2   b  in combination with  FIGS. 3   a ,  3   b , the fire face side  32  and the exhaust manifold side  36  are oriented perpendicular to each other, the exhaust ports  20  have two portions  22   b,    22   c  and  22   a  which are bent between the perpendicularly oriented fire face side  32  and the exhaust manifold side  36 . The two portions  22   b,    22   c  at the fire face side  32  are merged into the portion  22   a  at the exhaust manifold side  36 , which can be more clearly seen in  FIGS. 3   a    3   b  and  3   c.    
         [0029]    A longitudinal cut through an exhaust port  20  is depicted in  FIG. 3   a  and  FIG. 3   b  with a first spray gun  100  depositing material on a first surface portion  22   b,    22   c  of the exhaust port  20  ( FIG. 3   a ) and with a second spray gun  110  depositing material on a second surface portion  22   a  of the exhaust port  20  ( FIG. 3   b ).  FIG. 3   c  illustrates the first surface portions  22   b,    22   c  and the second surface portion  22   a  of the exhaust port  20  to be coated in combination. According to the example embodiment of  FIG. 3   c , the first and second portions  22   b,    22   c  and  22   a  can be spray coated simultaneously. 
         [0030]    A nozzle  106  of the spray gun  100  coating the first portion  22   b,    22   c  is positioned outside the exhaust port  20  under an angle to the walls of the exhaust port  20  to deposit material inside the first portions  22   b,    22   c  of the exhaust ports  20  ( FIG. 3   a ). The material from the first spray gun  100  is deposited along a direction  102  corresponding to a longitudinal extension of the first pray gun  100 . The first spray gun  100  can be rotated about an axis  102   b  in the first of the first portions  20   b  and about an axis  120   c  in the second of the first portions  20   c.  The axes  120   b,    120   c  are virtually parallel to the walls close to the openings  20   b,    20   c  of the two first portions  22   b,    22   c.    
         [0031]    The slash-dotted lines in the two first portions  22   b,    22   c  indicate the surface areas where the material from the spray gun  100  can be deposited. Preferably, the spray gun  100  is operated by a robot unit (not shown) for precise control of the deposition of the thermal insulating coating. 
         [0032]    The two first portions  22   b,    22   c  can be coated with one first spray gun  100  sequentially or with two first spray guns  100  simultaneously. 
         [0033]      FIG. 3   b  illustrates how the coating in the second portion  22   a  of the exhaust port  20  is performed. The second portion  22   a  of the exhaust port  20  is coated by material supplied by a second spray gun  110 . The material sprayed by the second spray gun  110  is supplied from a nozzle  116  arranged inside of the exhaust port  20 , wherein the material coating the second portion  22   a  is deposited in a direction  114  arranged under an angle to a direction  112  corresponding to a longitudinal extension of the second spray gun  110 . 
         [0034]    The second spray gun  110  is positioned virtually parallel to the walls close to the opening  20   a  of the second portion  22   a.  By rotating the second spray gun  110  about an axis  120   a  the second portion  22   a  of the exhaust ports  20  can be coated. The axis  120   a  is arranged parallel to the direction  112 . Preferably, the second spray gun  110  is operated by a robot unit (not shown) for precise control of the deposition of the thermal insulating coating. 
         [0035]    Favourably, the coating of each portion  22   a  and  22   b,    22   c  can be performed in a compact process. Preferably, a surface treatment step is performed prior to the coating step. By way of example, the surfaces to be coated can be treated with grit blasting or the like. In a subsequent optional step, a first coating can be applied for improving the bond strength of the thermal insulation coating by depositing a bond coat layer, e.g. a metal based layer via the spray guns  100  and  110 . The thickness of the optional bond coat layer can be in the range of a few micrometers to a few tens of micrometers. 
         [0036]    After the bond coat deposition or after the surface treatment step, if no bond coat layer is applied, the topcoat layer is deposited in the above mentioned way. 
         [0037]    Preferably, the topcoat layer can deposited in the two first portions  22   b,    22   c  with a high deposition rate and in the second portion  22   a  with a lower deposition rate as the sizes of the spray guns  100 ,  110  differ: since the spray gun  110  used for coating portion  22   a  is much smaller to fit in the port  20   a,  it may have less available power to melt the coating particles, as well as a lower powder feed. For instance, in a test power for the portion  22   a  can reach approximately 6 kW, compared with 40 kW for the portions  22   b  and  22   c.    
         [0038]    Advantageously, the topcoat layer can be deposited with thicknesses up to several hundreds of micrometers which result in a favourable thermal insulation of the hot exhaust gases. 
         [0039]    By providing a thermal insulating barrier between the hot exhaust gases and the cylinder head  12  it is possible to increase the exhaust gas temperature at the exit of the cylinder head  13  by reducing the heat losses to the cylinder head  12  and its coolant. Thus, the power available in the turbocharger  50  ( FIG. 1 ) can be increased. As a consequence, the fuel consumption of the engine  10  can be decreased.