Abstract:
A cylinder head assembly for an internal combustion engine including a cylinder head at least partially defining a flow path through the engine, including an intake port, an exhaust port, and a combustion chamber disposed between the intake port and the exhaust port. A valve is coupled to the cylinder head and movable relative to the cylinder head to selectively open the combustion chamber to one of the intake port and the exhaust port. A valve spring is positioned between the valve and the cylinder head and biases the valve to a closed position. A thermally insulating washer is positioned between the cylinder head and the valve spring.

Description:
BACKGROUND 
   Known internal combustion engines include valves that control the flow of intake air into a combustion chamber and the flow of exhaust gases out of the combustion chamber. A valve assembly, part of which is positioned within a rocker box of the engine, includes a spring configured to bias the valve to a closed position. A valve stem seal is provided on a stem of the valve to prevent oil within the rocker box from entering the combustion chamber and conversely, to prevent exhaust gases from entering the rocker box. Typically, valve springs and valve stem seals are subject to conduction heating from the heat of combustion absorbed into a cylinder head. High temperatures and repeated temperature cycling (between periods of operation and periods of non-operation) of the valve springs and valve stem seals can lead to decreased closing force on the valve, valve stem seal degradation, and increased oil consumption by the engine. 
   SUMMARY 
   In one embodiment, the invention provides a cylinder head assembly for an internal combustion engine. The cylinder head assembly includes a cylinder head at least partially defining a flow path through the engine. The flow path includes an intake port, an exhaust port, and a combustion chamber disposed between the intake port and the exhaust port. A valve is coupled to the cylinder head and movable relative to the cylinder head to selectively open the combustion chamber to one of the intake port and the exhaust port. A valve spring is positioned between the valve and the cylinder head and biases the valve to a closed position. A thermally insulating washer is positioned between the cylinder head and the valve spring. 
   Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of an internal combustion engine including a cylinder head assembly according to the present invention. 
       FIG. 2  is perspective view of a portion of the cylinder head assembly of  FIG. 1 . 
       FIG. 3  is an exploded view of the portion of the cylinder head assembly shown in  FIG. 2 . 
       FIG. 4  is a cross-sectional view of the cylinder head assembly, taken along line  4 - 4  of  FIG. 2 . 
       FIG. 5  is a perspective view of a valve assembly removed from a cylinder head of the cylinder head assembly. 
   

   Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates a motorcycle engine  20  (e.g., a V-twin type internal combustion engine). The motorcycle engine  20  includes cylinders  24  defining a pair of cylinder bores (not shown). Each of the cylinders  24  receives a piston, which reciprocates therein to compress fuel and air prior to combustion within a combustion chamber  28  ( FIG. 4 ). A cylinder head assembly  32  is positioned above each one of the cylinders. The cylinder head assemblies  32  include cylinder heads  36  ( FIG. 2 ) that combine with the pistons to define the combustion chambers  28 . A cylinder head cover  40  of each cylinder head assembly  32  is coupled to each cylinder head  36 . 
     FIG. 2  is a partial view of one of the cylinder heads  36  having the cylinder head cover  40  removed to illustrate a valve assembly  44 . As shown in  FIGS. 3-5 , the valve assembly  44  includes a valve such as an exhaust valve  48 .  FIG. 4  is a section view illustrating the valve assembly  44  assembled in the cylinder head  36  with the exhaust valve  48  in a closed position. The exhaust valve  48  is axially movable to selectively open the combustion chamber  28  to an exhaust port  52  in the cylinder head  36  to allow exhaust gases to escape the combustion chamber  28 . A valve spring  56 , such as a coil spring, of the valve assembly  44  biases the exhaust valve  48  to the closed position, shown in  FIG. 4 , in which the exhaust valve  48  seals against a valve seat  60  in the cylinder head  36 . Although not shown, the cylinder head  36  also defines an intake port in selective fluid communication with the combustion chamber  28  (via an intake valve similar to the exhaust valve  48 ) to provide intake air and/or fuel into the combustion chamber  28 . 
   During operation of the engine  20 , and after the power stroke of the piston is completed in one of the cylinders, the exhaust stroke commences to expel the exhaust gases out of the cylinder. During the exhaust stroke, the exhaust valve  48  is actuated (i.e., by a cam—not shown) to an open position. The exhaust valve  48  is moved from the closed position to the open position against the bias of the valve spring  56 . As described in further detail below, the valve assembly  44  is provided with additional components for fluidly and thermally isolating the valve spring  56  from the combustion chamber  28  and the exhaust port  52 . 
   In addition to the valve spring  56 , the valve assembly  44  includes a valve guide  64  that contacts an outer surface of a valve stem  68  of the exhaust valve  48 , as shown in  FIG. 4 . The valve guide  64  guides the exhaust valve  48  for axial sliding movement between the closed and open positions. A valve stem seal  72  is coupled to an end of the valve guide  64  adjacent the valve spring  56  and remote from the combustion chamber  28 . The valve stem seal  72  provides a sliding seal with the valve stem  68 . The valve stem seal  72  fluidly separates the exhaust port  52  from the area surrounding the valve spring  56 . For example, the valve stem seal  72  prevents lubricant in the area of the valve spring  56  from reaching the exhaust port  52  and combustion chamber  28 , and further prevents exhaust gases from reaching the area surrounding the valve spring  56 . 
   A lower collar  76  of the valve assembly  44  is coupled to the valve stem seal  72 . The lower collar  76  is formed to fit an irregular outer surface  72 A of the valve stem seal  72  ( FIG. 4 ). In this way, the valve stem seal  72  is axially positioned by the lower collar  76 . The lower collar  76  includes a lower flange  80  that extends radially outward between the valve spring  56  and a support surface  84  of the cylinder head  36  ( FIGS. 3 and 4 ). The bias force of the valve spring  56  presses the lower collar  76  towards the support surface  84  so that the lower collar  76  is fixed in one position. Thus, the lower collar  76  defines a substantially stationary position of the valve stem seal  72  during movement of the exhaust valve  48 . As the valve stem  68  moves, the valve stem seal  72  remains stationary, and a fluid seal is maintained therebetween. 
   The valve spring  56  is constrained between a first surface of the lower flange  80  on a lower end of the valve spring  56  (closest to the exhaust port  52 ) and an upper collar  88  at a second, upper end of the valve spring  56 . The upper collar  88  includes an upper flange  92  that extends radially outward to support the upper end of the valve spring  56 . As shown in  FIG. 4 , the upper collar  88  is coupled to an upper end  96  of the valve stem  68 . One or more retainers  98  are positioned to mutually engage the upper end  96  of the valve stem  68  and the upper collar  88 . In this way, the upper collar  88  moves with the valve stem  68 . When the cam actuates the exhaust valve  48  to the open position, the upper collar  88  moves towards the support surface  84  of the cylinder head  36 , compressing the valve spring  56 . When the cam ceases actuation of the exhaust valve  48 , the valve spring  56  returns the exhaust valve  48  to the closed position by acting upon the upper flange  92  of the upper collar  88 , which is fixed to the valve stem  68  via the retainers  98 . 
   A thermally insulating element, such as a washer  100 , is positioned between the lower collar  76  and the cylinder head  36 . Specifically, the thermally insulating washer  100  is positioned between the lower flange  80  of the lower collar  76  and the support surface  84  of the cylinder head  36 , the washer  100  having a generally planar surface facing each of the lower flange  80  and the support surface  84 . The lower flange  80  includes a second surface (opposite the first surface of the lower flange  80  that faces and supports the valve spring  56 ) facing the washer  100 . 
   The washer  100  is constructed of a material having relatively low thermal conductivity and a relatively high melting point. The washer  100  thermally insulates the valve spring  56  and the valve stem seal  72  from the high temperatures of the cylinder head  36  in the area of the exhaust port  52 . In some embodiments, the thermally insulating washer  100  is as little as 1.0 millimeter thick, although greater thicknesses provide increased insulating effect. 
   In some embodiments, both the cylinder head  36  and the lower collar  76  are constructed of metallic materials and have relatively high thermal conductivity. For example, the cylinder head  36  may be aluminum and the lower collar  76  may be steel. The thermally insulating washer  100  provides a barrier of high resistance for the conduction of heat from the cylinder head  36  to the lower collar  76  and has a thermal conductivity less than the material used for the cylinder head  36  and the valve spring  56 . By limiting heat conduction to the lower collar  76 , heat conduction to the valve spring  56  and to the valve stem seal  72  is limited. The presence of the washer  100  lowers the respective material temperatures of the valve spring  56  and the valve stem seal  72  during normal operation of the engine  20 . All of the engine components are subject to temperature cycles between periods of operation and periods of non-operation. By limiting the high end of the material temperatures, the magnitude of each temperature cycle and the effects thereof are reduced. Particularly, the valve spring  56  maintains a higher, more consistent closing force upon the exhaust valve  48  when it is thermally insulated by the washer  100 . Thermal degradation to the valve stem seal  72  and engine oil consumption are also reduced or prevented by the use of the thermally insulating washer  100  between the lower collar  76  and the cylinder head  36 . 
   In addition to the benefits above, the washer  100  provides a layer of frictional protection between the base of the valve spring  56  and the support surface  84  of the cylinder head  36 . Repeated compression and release of the valve spring  56  causes torsional instability, which can lead to erosion of the cylinder head  36  as the bottom end of the valve spring  56  twists. The erosion of the support surface  84  by the valve spring instability is vastly reduced or prevented by use of the washer  100  between the lower collar  76  and the support surface  84 . 
   Although illustrated in the figures as being a simple wafer or washer disposed below the lower flange  80  of the lower collar  76 , it is conceived that the washer  100  may be fixed or coupled with the lower collar  76  prior to assembly in the cylinder head  36  (e.g., by inter-engaging recesses and protrusions, adhesive, etc.). Alternatively, the washer  100  may be integrally formed with the lower collar  76 , for example by overmolding a thermally insulating material onto the lower flange  80  of the lower collar  76 . In order to reduce the number of parts in the valve assembly  44 , the lower collar  76  may be primarily constructed of a thermally insulating material rather than providing the separate washer  100 . The thickness of the lower flange  80  of the lower collar  76  may be sized accordingly to thermally insulate the valve spring  56  and the valve stem seal  72  from the heat present at the combustion chamber  28  and the exhaust port  52  during operation of the engine  20 . 
   In some embodiments, the thermally insulating washer  100  is constructed primarily of a polyimide material. For example, the washer  100  is constructed of a polyimide material sold under the registered trademark VESPEL of E.I. du Pont de Nemours and Company, available from DuPont Engineering Polymers, Newark, Del. In some embodiments, the washer  100  may be constructed of polyimide with a graphite filler or additive of between about 15 percent and about 40 percent by weight, which provides increased wear resistance and reduced friction compared to an unfilled polyimide base resin. However, in some embodiments, the washer  100  may be constructed of an unfilled polyimide base resin, having a lower thermal conductivity than a graphite-filled polyimide. In some embodiments, the washer  100  has a thermal conductivity less than 0.5 W/m*K. The above-described washer  100  has a thermal conductivity of about 0.3 W/m*K in some embodiments. 
   Various features and advantages of the invention are set forth in the following claims.