Abstract:
A high-speed absorption spectrographic system employs a slit-less spectroscope to obtain high-resolution, high-speed spectrographic data of combustion gases in an internal combustion engine allowing precise measurement of gas parameters including temperature and species concentration.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   This invention was made with United States government support awarded by the following agencies: NSF 0238633. 
   The United States has certain rights in this invention. 

   CROSS-REFERENCE TO RELATED APPLICATIONS 
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   BACKGROUND OF THE INVENTION 
   The present invention relates to instruments for the study of combustion gases and in particular to an improved sensor system for providing high-speed optical measurements of combustion gas temperature, water mole fraction and the like. 
   Knowledge about combustion, including combustion temperature and combustion gas composition, can be important in the study and control of internal combustion engines. For this purpose of measuring combustion gas temperature, it is generally known to use an optical pyrometer observing light emitted from the combustion gases and/or materials contacting combustion gases. For example, U.S. Pat. No. 6,370,486 describes a sensor that measures infrared energy emitted at several preselected wavelengths from hot gas to calculate gas temperature. 
   A more sophisticated system is described in U.S. Pat. No. 6,640,199, which analyzes the emission spectrum of the combustion gases to deduce temperature and relative concentration of some chemical species making up the combustion gas. 
   SUMMARY OF THE INVENTION 
   The present invention provides a measurement of combustion gas temperature and species concentration using absorption spectroscopy techniques. In contrast to the measurement of emission spectra, such absorption spectroscopy requires the introduction of a known light signal into the combustion space and the extraction of sufficient energy at multiple light frequencies to perform the spectroscopic measurement. The present invention meets these requirements while using a light guide that may be as small as a fiber optic, by employing a sensing systems that eliminates the standard optical slit required of, for example, grating spectrometers. The elimination of the optical slit or similar aperture reducing structure improves the use of light energy and allows high-resolution spectrographs to be created at an extremely high rate. 
   Specifically, the present invention provides a high-speed spectrographic sensor for internal combustion engines having a plug receivable into a combustion chamber of an operating internal combustion engine and a light source providing light at multiple frequencies between 2000 and 3000 nm. A light guide, for example one or more optical fibers held by the plug, receives the light source to communicate the light into the combustion chamber for interaction with combustion gases. The light guide also communicates the light out of the combustion chamber for sensing. A sensor system distinguishes the strength of the light after interaction with the combustion gases at no less than twenty multiple frequencies and at a rate of no less than 1000 times per second. 
   Thus it is an aspect of at least one embodiment of the invention to provide real-time multi-spectral absorption measurements of combustion gases. 
   The sensor system may be a spatial heterodyne spectroscope receiving the light from the light guide after the light has passed through the combustion chamber. 
   Thus it is another aspect of at least one embodiment of the invention to provide for a spectrographic decomposition that avoids the energy loss incident to a standard slit or similar-type spectrometer. The spatial heterodyne spectroscope may operate with an input aperture that is as much as two orders of magnitude larger than a slit spectroscope. 
   The sensor system may further include a computer deducing and outputting temperature within the combustion chamber from the strengths of the multiple frequencies. 
   It is thus another aspect of at least one embodiment of the invention to provide for automatic temperature measurements of combustion gases. 
   The computer may deduce and output water concentration within the combustion chamber from the strengths of the multiple frequencies. 
   It is thus another aspect of at least one embodiment of the invention to provide for automatic measurements of water mole fractions. 
   The computer may deduce a physical parameter of combustion gases by matching the strengths of the multiple frequencies to corresponding multiple frequencies of signatures representing known different physical parameters within the combustion chamber. 
   It is thus another aspect of at least one embodiment of the invention to allow complex analysis of absorption spectra on an automatic basis. 
   The light source may provide frequencies substantially within a range of 2400-2600 nm. 
   It is thus another aspect of at least one embodiment of the invention to provide for absorption measurements in a novel frequency band for combustion gases. 
   The sensor system may distinguish the strength of no less than 100 multiple frequencies. 
   It is thus another aspect of at least one embodiment of the invention to provide for the measurement of high-resolution absorption spectra of combustion gases. 
   The sensor system may distinguish the strength of the multiple frequencies at no less than 10,000 times a second. 
   It is thus another aspect of at least one embodiment of the invention to provide for measurements that accurately capture the real-time dynamic process of combustion. 
   In an alternative embodiment, the sensor system may include a Fourier spectroscope positioned between the light source and the combustion chamber on the light guide. The Fourier spectroscope may measure and time-modulate the multiple frequencies passing into the combustion chamber. A demodulating intensity detector may be positioned on the light guide after the combustion chamber providing a time signal measuring a combination of the multiple frequencies and demodulating the time signal to distinguish the strength of the multiple frequencies. 
   It is thus another aspect of at least one embodiment of the invention to provide for a system that easily compensates for variation in the spectra of the exciting light signal. 
   The Fourier spectroscope may employ a photoelastic modulator to vary its effective optical length. 
   It is thus another aspect of at least one embodiment of the invention to provide a novel high speed Fourier spectroscope that can provide sufficiently fast measurements for combustion gas analysis. 
   The plug may be a spark plug providing a spark for the internal combustion engine. 
   It is thus another aspect of at least one embodiment of the invention to provide for measurement in the vicinity of the spark in operating the internal combustion engine. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a modified spark plug holding a light guide for receiving light to interact with combustion gases and transmitting the light back to a spectroscope for high-speed analysis; 
       FIG. 2  is a block diagram of a spatial heterodyne spectrometer suitable for use as the spectroscope of  FIG. 1 ; 
       FIG. 3  is a diagram of the process steps of converting an image from the spatial heterodyne spectrometer into a spectrum and in performing signature matching; 
       FIG. 4  is a block diagram of the alternative embodiment of the invention using the spark plug of  FIG. 1  but employing a Fourier spectrometer upstream from the spark plug; and 
       FIG. 5  is a figure similar to that of  FIG. 3  showing those steps of signal analysis in the embodiment of  FIG. 4  that differ from the embodiment of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a high-speed spectrographic sensor  10  of the present invention provides for a modified spark plug  12  that may be fit to a combustion chamber  18 . In the manner of conventional spark plugs, the spark plug  12  may provide a conductive threaded flange  14  fitting within a corresponding threaded bore in the wall  16  of the combustion chamber  18 , providing a seal therewith. 
   The spark plug  12  provides a center electrode  20  coaxially within a ceramic insulator  22  and passing from outside of the chamber  18  where it is accessible at a high-voltage terminal  24  to inside the chamber  18  where it extends out of the insulator  22  as an electrode tip  26 . A ground electrode  28  extends from the flange  14  into the combustion chamber  18  to a point opposite the electrode tip  26  across a spark gap  27  in a manner known in the art. 
   The insulator  22  or threaded flange  14  also holds a light guide  30  passing through the insulator  22  or threaded flange  14  from outside the combustion chamber  18  to a point within the combustion chamber  18  near the spark gap  27 . The light guide  30  may be, in a preferred embodiment, two adjacent optical fibers  32  and  34 , one for carrying light into the combustion chamber  18  and one for carrying light out of the combustion chamber  18  for sensing. 
   The fiber  32  carrying the light into the combustion chamber  18  may receive light from a broad spectrum light source, such as an incandescent bulb in the form of a quartz tungsten-halogen lamp, or a wideband LED or broadband laser, providing substantial energy in the range of 2000 nm to 3000 nm and preferably in a range of 2400 nm to 2600 nm and having a known spectrum. 
   A mirror  36  is positioned across a gap  37  from the point where the light guide  30  terminates in the combustion chamber  18 . The mirror  36  is positioned so that light passing through optical fiber  32  exits the light guide  30  and passes across the gap  37  to strike the mirror  36 , to be reflected back across the gap  37  and be received by fiber  34 . The optical path through the gap  37  may be as great as 10 mm to allow the light to interact with combustion gases in the region of the electrode tip  26 . 
   Light received from optical fiber  34 , after interacting with the combustion gases, passes through a filter  40 , for example, a band limiting filter of the desired frequency range (e.g. 2400-2600 nm). The filtered light is then received by a spectrometer  42  which in the preferred embodiment is a spatial heterodyne spectrometer. 
   The spectrometer  42  provides a digitized output  44  received by a computer  46 . The computer executes a program to display a high-resolution absorption spectrum  48  (based on known or measured spectrum of light source  38 ) extracted every 100 μs and no less than every 1000 μs and consisting of hundreds of resolved frequency points and no less then twenty resolved frequency points. The computer  46 , operating according to the stored program, may also identify a quantitative parameter value  49 , being for example a temperature of the combustion gases or a species mole fraction such as water concentration or other similar measurement, as will be described. 
   Referring now to  FIG. 2 , the spatial heterodyne spectrometer  42  provides an open aperture and high-speed response made possible by its efficient use of minor energy obtained through fiber  34 . Spectrometers of this type are described in U.S. Pat. No. 5,059,027, issuing Oct. 22, 1991, assigned to the assignee of the present invention, and hereby incorporated by reference. Such a spectrometer receives a light signal  50  from the fiber  34  and collimates this light using an optical assembly  52  to provide for a beam  53  having generally an aligned wavefront  54 . 
   A dispersive optical system  56  tips the wavefronts  55  of each of multiple frequency component in the light signal  50  (only two shown) to an angle α dependent on the wavelength of that frequency component. The wavefront-modified beam  58  is then received by an imaging optical assembly  60  to project an image on a solid-state image detector  62  such as an extended InGaAs line scan camera commercially available from Xenics Leuven, Belgium. The signal from the solid-state image detector  62  may be digitized and sampled per block  63  to produce an image  64  at approximately 1000 times per second or as much as 10,000 times per second. 
   Referring now also to  FIG. 3 , the image  64  from the solid-state image detector  62  will contain a series of bands of different intensities  66  caused by interference in the image produced by the constructive and destructive interference of the wavefronts  55  as tipped by dispersive optical system  56 . The information of this image  64  may be collapsed to a single dimension (x) to produce a spatially dependent signal  68  with improved signal-to-noise ratio that better utilizes all of light energy from the fiber  34  both improving the speed and the resolving power of the spectrum. 
   This signal  68 , when operated on by the Fourier transform  70 , as may be implemented in the computer  46  of  FIG. 1 , produces a high-resolution spectrum  48  providing resolvable points for more than 100 different frequencies. The high-resolution spectrum  48  may be compared to spectrum  74  of a library  76  of different signature spectra  74  by a correlator  78 , where each signature spectra  74  is associated with a known physical parameter that is to be extracted. For example, the multiple spectra  74  may each represent measurements of combustion gases at a different temperature. Alternatively the multiple spectra  74  may each represent a measurement of a different water concentration or another species concentration. 
   The correlator  78  finds the best correlation between high-resolution spectrum  48  and each of spectra  74  to output a measured temperature or other quantitative parameter value  49  as shown in  FIG. 1 , according to the parameter associated with the most highly correlated spectra  74 . 
   Referring now to  FIG. 4 , in an alternative embodiment the light source  38  provides light to a filter  40  operating in a manner described above with respect to filter  40  in  FIG. 1 . The filtered light is then provided to a Fourier spectrometer  71 . The Fourier spectrometer  71  operates in a manner similar to conventional Fourier spectrometers by separating the light beam into two paths one of which is changed in effective length to create interference between the light of the two paths. The interference effectively modulates by frequency each of the wavelengths of light from the light source  38  with that wavelength having highest frequency being modulated at the highest rate. A Fourier transform of this modulation reveals the spectrum of the light. Ideally the changing cavity length is a simple linear function, for example, following a triangle or sawtooth wave  75 . 
   The output of the Fourier spectrometer  71  is thus a modulated light beam which is sent to the fiber  32  and which may be sampled locally at a local sensor  73  to allow local characterization of the spectrum of the light before modification by combustion gases as will be described. The modulated light from the Fourier spectrometer  71  passes through the fiber  32  to the spark plug  12 , as described above with respect to  FIGS. 1 and 2 , and is modified by combustion gases and received by fiber  34  ultimately to be provided to a sensor  72 . Sensor  72  is not frequency discriminating and thus may employ an open aperture to efficiently measure multi-spectral light intensity. A Fourier transform of the modulated intensity at sensor  72  yields a spectrum which when compared to the spectrum calculated from the sensor  73  provides an absorption spectrum. 
   Referring still to  FIG. 4 , in order to provide the necessary speed and resolution for measuring combustion gases, the Fourier spectrometer  71  differs from those spectrometers of the prior art by eliminating a mechanically movable mirror or optical element that could not provide sufficiently responsive modulation. Instead the Fourier spectrometer  17  employs a non-mechanical cavity length control, for example, a photoelastic modulator  77  to provide for a sweeping of the cavity length at least 1000 times per second and as much as 10,000 times per second. 
   Referring to  FIG. 5 , the sensor  73  used in conjunction with the high speed Fourier spectrometer  71  thus produces a time signal  80  that provides a high resolution spectrum of more than 100 points at a sampling rate as described above. 
   The system of the present invention may be employed while the internal combustion engine is operating to measure gas temperatures in the vicinity of the electrode at an extremely high rate and accuracy. For example, it is beleived that a temporal resolution of 100 μs (˜1 deg. crank angle) with a better than 4 cm −1  spectral resolution to provide a temperature resolution of ˜5 degrees C. or less than 0.1% to 1000K and 1% to 3000K. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.