Abstract:
A spark plug system for measuring the thermal profile across the surface of a piston in an operating internal combustion engine in characterizing the combustion process within a combustion chamber of the engine is described that includes a specially constructed spark plug containing a coherent fiber optic bundle and lens system optically connected to a remote sensing thermometer and high-speed triggerable imaging infrared video camera.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to systems and methods for the diagnostic testing of internal combustion engines and more particularly to an instrumented spark plug useful for optically viewing the interior of a combustion chamber of an operating internal combustion engine. 
     The invention provides an instrumented spark plug system for measuring the thermal profile across a piston surface within an internal combustion engine during normal operation, and includes a coherent optical fiber bundle contained within a specially constructed spark plug, and lens system optically connected to a high speed triggerable imaging infrared (IR) video camera. The system images the piston surface onto the face of the optical fiber bundle in near real time and transmits the images to the video camera, thereby obtaining spot temperature measurements at each engine cycle. 
     It is therefore a principal object of the invention to provide an instrumented spark plug. 
     It is another object of the invention to provide a system for the diagnostic testing of an internal combustion engine. 
     It is a further object of the invention to provide a system for obtaining temperature profile information on a piston within an operating internal combustion engine. 
     It is a further object of the invention to provide an instrumented spark plug for diagnostic testing of an internal combustion engine. 
     These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds. 
     SUMMARY OF THE INVENTION 
     In accordance with the foregoing principles and objects of the invention, a spark plug system for measuring the thermal profile across the surface of a piston in an operating internal combustion engine in characterizing the combustion process within a combustion chamber of the engine is described that includes a specially constructed spark plug containing a coherent fiber optic bundle and lens system optically connected to a high-speed triggerable imaging infrared video camera. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein: 
     FIG l shows schematically two cylinders of an internal combustion engine and placement of the instrumented spark plug and associated equipment of the invention; 
     FIG. 2 is a view in axial section of a spark plug of the FIG. 1 system; 
     FIG. 3 is a schematic of representative imaging optics for imaging a piston surface onto the coherent fiber optic array of the system of FIGS. 1 and 2; and 
     FIG. 4 is an enlarged view of the imaging optics of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     In the drawings, FIG. 1 shows schematically two adjacent or complementary cylinders  12 , 13  of an internal combustion engine  10  (e.g. cylinders  1  and  4  of a conventional eight-cylinder gasoline engine), cylinders  12 , 13  being equipped with instrumented spark plugs  11 , 11 ′ of the invention. Cylinders  12 , 13  typically include respective combustion chambers  14 , 15 , pistons  16 , 17 , piston rods  18 , 19 , fuel intake ports  20 , 21 , exhaust ports  22 , 23 , spark plug wells  24 , 25  and cam  26  operatively connected to rods  18 , 19 . In the representative system of the invention of FIG. 1, two separate embodiments are presented for temperature observation of surfaces  16 ′, 17 ′ of pistons  16 , 17 . 
     In FIG. 1, incoherent fiber optic bundle  31  operatively connects plug  11  on cylinder  12  to the sensor head of spot temperature sensor  27  (commercially available remote sensing thermometer) for measuring the temperature of a spot on surface  16 ′ of piston  16 . Coherent fiber optic bundle  34  connects plug  11 ′ on cylinder  13  to relay optical system  35  and IR imaging radiometric temperature sensor  29  (e.g. platinum silicide IR video camera) for imaging surface  17 ′ of piston  17  and producing a temperature profile across surface  17 ′. Any suitably temperature resistant fiber optic bundle may be used as selected by one skilled in the applicable art guided by these teachings, an arsenic-selenium-tellurium imaging fiber bundle operating at wavelengths of about 3-5 microns (μm) being preferred and included in a system built and operated in demonstration of the invention. Fiber bundle  31  in the demonstration system was about one meter long with individual fibers having a 125 μm diameter core and 10 μm thick cladding, giving fiber-to-fiber spacing of about 135 μm. 
     Conventional personal computer  37  may provide means for controlled temperature data acquisition and image capture for displaying the imagery on monitor  38 . Measurements may be made at any selected position of the piston  16 , 17  strokes using triggering off cam  26 . 
     Referring now to FIG. 2, shown therein is a view in axial section of an instrumented spark plug  40 , similar in structure to plugs  11 , 11 ′ of FIG. 1, representative of the invention. For optimum operation of the invention, plug  40  should be structured for operating as a normal spark plug but be capable also of relaying the maximum practical optical energy characterizing the piston surface. Plug  40  therefore includes a central coherent optical fiber bundle  41  having at first end  42  thereof one or more imaging lenses  43 , 44  and an optical window  45  of sapphire, diamond, zinc selenide or other suitable temperature resistant optical material transparent to the wavelength range of interest (3-5 μm). Lenses  43 , 44 , fiber bundle  41  and window  45  are all axially aligned central of the structure of plug  40  in order to minimize effects of engine vibration and reflection losses. Fiber bundle  41  is surrounded by electrically conducting tube  47  serving as a charge carrier (cathode) to spark gap  48 . Fiber bundle  41  and conducting tube  47  are surrounded by tubular ceramic insulator  49  composed of any suitable material customarily used in conventional spark plug construction. Typical dielectric strengths of extruded type ceramics useful for insulator  49  range from about 400 to 800 volts/mil and the spark plug generates up to about 30,000 volts under full load. Insulator  49  is therefore about 80 mils thick for adequate insulation. Insulator  49  separates tube (cathode)  47  of plug  40  from the main body (anode)  51  which terminates in tip  52  and defines spark gap  48  between tip  52  and tube  47 . Spark gap  48  is disposed to one side of optical axis O along which fiber bundle  41 , lenses  43 , 44  and window  45  are disposed, which allows line-of-sight measurement of the surface of the piston (not shown in FIG. 2) within its combustion chamber. Water jacket  55  may be included in the structure of plug  40  as suggested in FIG.  2  and operatively connected to a source (not shown) of coolant water to avoid overheating of plug  40  constituent parts from engine heat during operational use of the invention. Thermally insulating outer shells  57 , 58  of material such as DELRIN®, any suitable ceramic, or the like may be included to insulate cathode  46  and conducting tube  47  from engine heat. Means such as threads  59  may be included on the outer surface of plug  40  so that plug  40  may be received by engine  1 O as suggested in FIG.  1 . 
     The size of a standard spark plug requires that the instrumented plug of the invention include miniature lenses  43 , 44  for transmitting an image of the piston surface into fiber bundle ( 41 , FIG.  2 ). Lens requirements for the instrumented plug may be determined using a PC based lens design program such as ZEMAX (mfgd by Focus Software, Inc.). FIGS. 3 and 4 are schematic illustrations of representative two-lens imaging optics suitable for imaging a surface  60  of a piston onto the optical fiber bundle  41  of the invention. Optical window  45  (nominally 1 inch thick sapphire) protects imaging lenses  43 , 44  from the heat of combustion chamber  61 . Lenses  43 , 44  are nominally about 6 mm in diameter in order to accommodate size restrictions imposed on plug  40 , and may comprise any suitable materials as would occur to the skilled artisan guided by these teachings, such as germanium or zinc selenide as illustrated in FIG.  4 . Lenses  43 , 44  are cemented together and fiber bundle  41  is cemented to the back side of lens  44  as suggested in FIG. 4 using conventional IR index matching cement  63 . The first (input) surface of lens  43  serves as the stop and may preferably have an antireflection coating . Lenses  43 , 44  correct some field curvature, coma, astigmatism and spherical aberrations and image the scene onto the input surface of fiber bundle  41 . 
     The invention therefore provides an instrumentation spark plug for viewing the combustion process within a combustion chamber of an internal combustion engine. It is understood that modifications to the invention may be made to the invention as might occur to one with skill in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder which achieve the objects of the invention have therefore not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims.