referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures , there is shown in fig1 a portion of a cylinder head assembly 10 for a direct injection internal combustion engine , not shown . the cylinder head assembly 10 includes a cylinder head 12 formed from a cast metal such as aluminum , iron , magnesium , etc . having a fuel injector assembly 14 mounted thereto . the cylinder head 12 defines an injector bore 16 and partially defines a combustion chamber 18 . the injector bore 16 is in communication with the combustion chamber 18 . the fuel injector assembly 14 includes an injector body 20 having an injector tip portion 22 extending therefrom . the injector bore 16 is configured to receive the injector tip portion 22 such that the injector tip portion 22 is in communication with the combustion chamber 18 . a fuel rail 24 is mounted with respect to the fuel injector assembly 14 and is operable to provide a source of pressurized fuel 26 to the fuel injector assembly 14 . the fuel injector assembly 14 is operable to communicate metered and timed amounts of pressurized fuel 26 from the fuel rail 24 directly into the combustion chamber 18 for subsequent combustion therein . as such , the fuel injector assembly 14 may be characterized as a direct injection fuel injector . an isolator member 28 is disposed between the cylinder head 12 and the fuel injector assembly 14 and is operable to provide a measure of compliance such that hard contact or grounding between the fuel injector assembly 14 and the cylinder head 12 is prevented . in so doing , the transmission of noise producing vibrations between the fuel injector assembly 14 and the cylinder head 12 is reduced . a secondary seal 29 is operable to seal the injector body 20 with respect to the cylinder head 12 . the injector bore includes first and second generally cylindrical portions 30 and 32 , respectively , having a generally tapering bore portion 34 therebetween . the generally tapering bore portion 34 is operable to guide or pilot the injector tip portion 22 into the second generally cylindrical portion 32 during insertion of the fuel injector assembly 14 . since the fuel injector assembly 14 is in direct communication with the combustion chamber 18 , the fuel injector assembly 14 is subject to high pressure loads and temperatures of the combustion process . therefore , the fuel injector assembly 14 must be sealed with respect to the cylinder head 12 . the injector tip portion 22 defines an annular groove 36 configured to receive a portion of a combustion seal 38 . the annular groove 36 includes a generally tapering portion 37 operable to radially expand the combustion seal 38 as the combustion seal 38 is biased thereon by combustion gases , indicated by arrows 39 , within the combustion chamber 18 . the combustion seal 38 is generally annular and sleeve - like in shape and is operable to sealingly engage the second generally cylindrical portion 32 of the injector bore 16 thereby preventing combustion gases 39 from traversing the injector bore 16 during operation of the internal combustion engine . the combustion seal 38 is preferably formed from glass or carbon filled polytetrafluoroethylene ; however , other materials may be employed possessing the requisite temperature and chemical resistance while remaining within the scope of that which is claimed . referring to fig2 and with continued reference to fig1 , there is shown a portion of the cylinder head assembly 10 enlarged to illustrate further aspects of the present embodiment . in operation , the combustion seal 38 is subject to pressure forces exerted thereon by the combustion gases 39 . as a result , the combustion seal 38 is biased upward , as viewed in fig2 , against the generally tapering portion 37 of the annular groove 36 . as the combustion seal 38 is biased against the generally tapering portion 37 , the generally tapering portion 37 expands the combustion seal 38 radially outward to sealingly engage the second generally cylindrical portion 32 of the injector bore 16 . as such , the combustion seal 38 may be characterized as an “ energized ” combustion seal . that is , the combustion seal 38 is energized by the pressure forces of the combustion gases 39 to effect sealing of the injector tip portion 22 with respect to the injector bore 16 . therefore , the sealing effectiveness of the combustion seal 38 is proportional to the pressure forces acting on the combustion seal by the combustion gases 39 . the generally tapering portion 37 of the annular groove 36 has a taper angle θ , shown in fig2 . in the preferred embodiment the taper angle θ is approximately 12 to approximately 20 degrees . this range of taper angles will enable ease of insertion of the injector tip portion 22 within the injector bore 16 during assembly , while maintaining adequate sealing effectiveness of the combustion seal over a range of friction coefficients for the injector bore 16 , injector tip portion 22 , and the combustion seal 38 . in operation , the fuel injector assembly 14 will exhibit small axial movements , as illustrated by arrow 44 , as a result of variations in pressure within the combustion chamber 18 and the pressurized fuel 26 within the fuel rail . the isolation member 28 may tend to increase the magnitude of this movement . since the combustion seal 38 is energized by pressure forces exerted by the combustion gases 39 , the combustion seal 38 is operable to maintain a seal between the injector tip portion 22 and the second generally cylindrical portion 32 of the injector bore 16 during axial movement of the fuel injector assembly 14 . referring to fig3 a and with continued reference to fig1 and 2 , there is shown the combustion seal 38 in an un - deformed state , i . e . prior to installation within the generally annular groove 36 . as shown , the combustion seal 38 has a bore 40 that extends the length of the combustion seal 38 . the bore 40 is shown in fig3 a as generally cylindrical ; however , the bore 40 may have a generally tapering shape to complement the taper angle θ of the generally tapering portion 37 . referring to fig3 b and with continued reference to fig1 and 2 , an alternate embodiment of the combustion seal 38 is indicated at 38 a . the combustion seal 38 a defines a bore 42 that extends the length of the combustion seal 38 a . the bore 42 includes first and second tapering portions 45 and 46 , respectively , extending from the midline or center of the combustion seal 38 a and tapering outward . the first and second tapering portions 45 and 46 have a taper configured to generally complement the taper angle θ of the generally tapering portion 37 . the first and second tapering portions 45 and 46 provide a measure of error - proofing for assembly purposes since the orientation of the combustion seal 38 a as the combustion seal 38 a is inserted into the generally annular groove 36 is irrelevant . referring now to fig4 , there is shown an alternate embodiment of the cylinder head assembly 10 of fig1 , generally indicated at 10 a . the cylinder head assembly 10 a includes a fuel injector assembly 14 a . the fuel injector assembly 14 a includes the injector body 20 having an injector tip portion 22 a extending therefrom . the annular groove 36 defined by injector tip portion 22 of fig1 is lacking in the injector tip portion 22 a of fig4 . an injector piloting sleeve 48 is mounted with respect to the cylinder head 12 and defines the generally tapering bore portion 34 and the generally cylindrical bore portion 32 of the injector bore 16 . the injector piloting sleeve 48 may be retained within the cylinder head 12 by interference fit , threaded engagement , or other fastening means . the injector piloting sleeve 48 defines an annular groove 50 configured to receive a portion of the combustion seal 38 . the annular groove 50 may be formed by a machining operation . the injector piloting sleeve 48 may be formed by a powdered metal process . the combustion seal 38 sealingly engages the injector tip portion 22 a of the fuel injector assembly 14 a . the annular groove 50 is preferably formed in the second generally cylindrical bore portion 32 . the annular groove 50 includes a generally tapering portion 52 operable to radially compress the combustion seal 38 as the combustion seal 38 is biased thereon by combustion gases 39 within the combustion chamber 18 . the combustion seal 38 is operable to sealingly engage the second injector tip portion 22 a of the fuel injector assembly 14 a thereby preventing combustion gases 39 from traversing the injector bore 16 during operation of the engine . referring to fig5 and with continued reference to fig4 , there is shown a portion of the cylinder head assembly 10 a enlarged to illustrate further aspects of the present embodiment . in operation , the combustion seal 38 is subject to pressure forces exerted thereon by the combustion gases 39 . as a result , the combustion seal 38 is biased upward , as viewed in fig5 , against the generally tapering portion 52 of the annular groove 50 . as the combustion seal 38 is biased against the generally tapering portion 52 , the generally tapering portion 52 biases or urges the combustion seal 38 radially inward to sealingly engage the injector tip portion 22 a of the fuel injector assembly 14 a . the combustion seal 38 is energized by the pressure forces of the combustion gases 39 to effect sealing of the injector tip portion 22 a with respect to the injector bore 16 . therefore the sealing effectiveness of the combustion seal 38 is proportional to the pressure forces acting on the combustion seal by the combustion gases 39 . the generally tapering portion 52 of the annular groove 50 has a taper angle φ , shown in fig5 . in the preferred embodiment the taper angle φ is approximately 10 to approximately 20 degrees . this range of taper angles will enable ease of insertion and removal of the injector tip portion 22 a within the injector bore 16 , while maintaining adequate sealing effectiveness of the combustion seal 38 over a range of friction coefficients for the injector bore 16 , injector tip portion 22 a , and the combustion seal 38 . as described hereinabove , the fuel injector assembly 14 a will exhibit small axial movements , as illustrated by arrow 44 , as a result of variations in pressure within the combustion chamber 18 and the pressurized fuel 26 within the fuel rail . the isolation member 28 may tend to increase the magnitude of this movement . since the combustion seal 38 is energized by pressure forces exerted by the combustion gases 39 , the combustion seal 38 is operable to maintain a seal between the injector tip portion 22 a and the injector bore 16 during the axial movement of the fuel injector assembly 14 a . additionally , the injector piloting sleeve 48 may be formed from a material having a similar coefficient of thermal expansion as that of the injector tip portion 22 a . in this case , close tolerances may be maintained between the second generally cylindrical bore portion 32 and the injector tip portion 22 a over a wide range of temperatures . furthermore , the injector piloting sleeve 48 may be formed without the annular groove 50 for use with the fuel injector assembly 14 of fig1 . while the best modes for carrying out the invention have been described in detail , those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims .