Abstract:
An optical sensor for sensing combustion products avoids fouling and damage to optical components by using a “windowless” design in which an air curtain through an orifice provides a constantly refreshing transparent shield protecting the optical components from corrosive combustion gases and resisting the accumulation of particulates that might otherwise foul a static window.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    -- 
       CROSS REFERENCE TO RELATED APPLICATION 
       [0002]    -- 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to combustion gas sensors and in particular to an optical sensor adapted for continuous engine exhaust monitoring. 
         [0004]    Accurate, real-time information about the combustion gases and particulates produced by engines is increasingly important in the efficient control of combustion engines such as automotive gasoline engines but also including other mobile and stationary engines such as gas turbine engines. This information can be obtained by spectrographic analysis of combustion products using optical sensors that measure the absorption of light in different frequency bands as the light passes through combustion gases. 
         [0005]    While such spectrographic sensors are routinely used in experimental environments, practical use for engine control requires a highly reliable sensor that can operate with minimal maintenance over a long period of time. One significant obstacle to long-term use of such sensors is damage to sensitive optical elements caused by the combustion gases and particulates that can accumulate on the protective optical window of the optical components reducing light transmission and/or creating erroneous readings. One approach to minimizing such fouling and damage is the use of an air curtain that flushes air across an optical window, for example, as described in U.S. Pat. No. 5,592,296 incorporated by reference. In an alternative design taught by U.S. Pat. No. 4,583,859, also incorporated by reference, clean air is flushed along the optical path of the sensor to provide an “air shield”. 
         [0006]    A disadvantage to each of these techniques is need for an ample supply of fresh air normally obtained by filtration using filters that must be frequently be cleaned and changed. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an optical assembly that resists damage and fouling of the optical sensors through the use of air flow through an extremely small opening into the combustion conduit (less than a millimeter in diameter and typically on the order of 80 microns). This small opening is possible through the use of a light emitter with high spatial coherence (e.g. a laser or super luminescent source) and advantageously resists fouling by permitting higher air flow velocities for a given pump power and filter consumption. In addition, the extremely small hole size can block insect entry, moisture, and dust accumulation. 
         [0008]    Specifically then, at least one embodiment of the invention provides an engine sensor having a conduit communicating with a combustion engine to receive combustion gases channeled by walls of the conduit, the walls including at least one orifice having an area of less than one square millimeter therethrough. A high spatial coherence light emitter with a predetermined spectral content is positioned to direct light through the at least one orifice through combustion gases within the conduit and a light receiver is positioned to detect the light after passage through the combustion gases and the at least one orifice to produce an electrical signal indicating spectral absorption of the light by the combustion gases in at least two frequencies A source of air provides a pressure greater than a pressure within the conduit and communicates with the at least one orifice to provide a substantially continuous shielding airflow through the at least one orifice during operation of the combustion engine to prevent passage of combustion gases into the orifices and a buildup of particles from the combustion gas on the at least one orifice. 
         [0009]    It is thus a feature of at least one embodiment of the invention to provide accurate optical sensing for long-term combustion engine monitoring with reduced power usage and reduced degradation of the sensor. 
         [0010]    The high spatial coherence light emitter may be selected from the group consisting of a laser light emitter and a super luminescent light emitter. 
         [0011]    It is thus a feature of at least one embodiment of the invention to permit the use of small orifice sizes through the use of light emitters that can be readily collimated into small area beams. 
         [0012]    The light emitter may include a lens receiving light from the light emitter and providing a focal point of the light located in the at least one orifice or a light guide leading to the at least one orifice. 
         [0013]    It is thus a feature of at least one embodiment of the invention to maximize light transmission through a small orifice by approximately setting the focal length of the lens to the orifice center. 
         [0014]    The total area of the orifice maybe selected so that pressurized air introduced into the conduit through the orifice has a mass flow rate of less than 100 parts per million of the total mass flow rate of the gas passing through the conduit. 
         [0015]    It is thus a feature of at least one embodiment of the invention to provide a system that does not adversely affect measurement of the combustion gases. 
         [0016]    Each orifice may have a diameter of less than one square millimeter. 
         [0017]    It is thus a feature of at least one embodiment of the invention to provide sufficient air velocity for shielding while reducing air pumping costs. 
         [0018]    The light emitter and light receiver may have transparent optical elements through which the light passes and the transparent optical elements may be displaced away from the orifice to be protected from contact with the combustion gases by a shielding effect of the pressurized air. 
         [0019]    It is thus a feature of at least one embodiment of the invention to not only prevent blockage of the orifice but to shield the optical elements from corrosive gases. 
         [0020]    A particulate filter may be positioned to filter particulates from the source of pressurized air that could block the orifice before the pressurized air is received by the at least one orifice. 
         [0021]    It is thus a feature of at least one embodiment of the invention to permit the use of available ambient air even if contaminated with exhaust or other particulates. 
         [0022]    A heater may be positioned for heating the air that provided to the at least one orifice. 
         [0023]    It is thus a feature of at least one embodiment of the invention to prevent moisture buildup or frost from interfering with the operation of the sensor when a small orifice size is used. 
         [0024]    The engine sensor may further include an electronic computer receiving the electrical signal to provide a control signal to the combustion engine controlling the combustion engine to change the combustion gases. 
         [0025]    It is thus a feature of at least one embodiment of the invention to provide sophisticated engine control possible with optical sensing. 
         [0026]    The source of pressurized air maybe controlled by an electronic circuit to vary the air pressure. 
         [0027]    It is thus a feature of at least one embodiment of the invention to provide both continuous protection and burst particulate removal. 
         [0028]    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 
         [0029]      FIG. 1  is a simplified block diagram of a combustion engine receiving air and fuel to provide mechanical power through an output shaft and showing a sensor assembly of the present invention measuring exhaust gases from the engine and receiving compressed air from the engine; 
           [0030]      FIG. 2  is a phantom view of the sensor assembly of  FIG. 1  providing an enclosed pressurized housing fitting about an exhaust conduit; 
           [0031]      FIG. 3  is a fragmentary cross-section along line  3 - 3  of  FIG. 2  showing the optical path employed by the sensor through the exhaust conduit in one embodiment and the flow of air through orifice is aligned with that optical path, the latter providing an air curtain protecting the optics and preserving a clear optical path through the exhaust conduit; 
           [0032]      FIG. 4  is a graph of a combination steady-state and air pulse flow that can be used in the present invention; 
           [0033]      FIGS. 5 and 6  are simplified block diagrams of different implementations of the light sensor and light emitter used in the optical sensing assembly for transmission measurements; 
           [0034]      FIGS. 7 and 8  are figures similar to those of  FIGS. 5 and 6  showing different implementations of the light sensor and light emitter used in the optical sensing assembly for reflection measurements; 
           [0035]      FIG. 9  is a cross-section similar to that of  FIG. 3  showing orientation of the optical path in reflection measurement elevating the orifice to further resist fouling; 
           [0036]      FIG. 10  is an alternative embodiment showing an electric blower used for generating the compressed air used in the air curtain of the present invention; 
           [0037]      FIG. 11  is an implementation of the present invention using an internally reflective hollow light pipe; 
           [0038]      FIG. 12  is a detail of the orifice as seen within the conduit of  FIG. 3 or 9  showing a moisture-diverting feature; 
           [0039]      FIG. 13  is a fragmentary cross-section similar to  FIG. 3  showing use of a lens system for focusing light through the small orifice; and 
           [0040]      FIG. 14  is a figure similar to that of  FIG. 13  showing the use of a light guide to displace the lens from the orifice with similar focusing effect. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    Referring now to  FIG. 1 , a combustion engine  10 , including but not limited to a gas turbine, diesel or gasoline engine, may receive fuel  12  over fuel line  14  and air through an air intake  16  to provide mechanical energy through a driveshaft  18  using energy released from the combustion of the air and fuel. The driveshaft  18  will typically drive a load  24 , for example, an electrical generator, pump, vehicle or the like. 
         [0042]    Combustion products from the combustion engine  10  are conducted through an exhaust pipe  20  that passes through a sensor system  22  of the present invention. These combustion products are then received by pollution control measures (not shown) and discharged into the atmosphere. Alternatively, the pollution control measures could be located before the sensor system, for example, when the sensor system is monitoring such control measures. 
         [0043]    A small portion of the power from the combustion engine  10  may be used to provide a source of pressurized air over air line  26 , for example, using a compressor  27  driven directly or indirectly by the driveshaft  18 . The invention also contemplates that a separate external air source may be used as will be discussed below. 
         [0044]    Pressurized air passing through air line  26  may be received by a heater  29  and then by particulate filter  28  and is then provided to the sensor system  22  for use as will be described below. 
         [0045]    Data from the sensor system  22  may be provided to an engine controller  30 , the latter, for example, including a processor  32  and a memory  34  holding a stored program for engine control. Based on the data from the sensor system  22  which may indicate concentrations of different gaseous species, particulates, and the like, the engine controller  30  may control the engine  10  for improved performance or may monitor the engine  10 , for example, with respect to maintenance. This control may be implemented by a variety of known techniques including controlling the fuel  12  passing through the fuel line  14  by valve  36  and/or controlling air passing through the air intake  16  by means of valve  38  or by other engine control inputs, for example, camshaft timing, piston displacement adjustments, or in the case of an ignition engine, ignition timing and the like. 
         [0046]    Referring now to  FIGS. 2 and 3 , the sensor system  22  may provide for a tubular conduit  40  receiving combustion gases  42  from the exhaust pipe  20 . Diametrically opposed orifices  44   a  and  44   b  may be cut through the walls of the conduit  40  to define a generally straight-line optical path  46  therebetween. 
         [0047]    Positioned in opposition on either side of the conduit  40  and aligned with the optical path  46  is a light emitter  48  and a light receiver  50  that may respectively transmit and receive multispectral light to make spectrographic measurements of the gas in the conduit  40 . The light emitter  48  is such as to provide a high spatial coherence in the manner of a laser or super luminescent light emitter so as to provide focused power through an extremely small orifice thereby compensating for the small orifice area. 
         [0048]    One or both of the light emitter  48  and light receiver  50  may have optical elements  52  such as lenses, windows, collimators, or the like through which light is transmitted or received and may provide an interface to the external environment. These optical elements  52  will be displaced away from the orifices  44   a  and  44   b  to allow airflow  54  past the optical elements  52  through the orifices  44   a  and  44   b  into the internal volume of the conduit  40 . 
         [0049]    To provide this airflow  54 , the conduit  40  may be surrounded by a secondary chamber  56  receiving air line  26  after filtration by filter  28  to provide an enclosed and pressurized source of particulate-free air at a higher pressure than the peak pressure found in the conduit  40  to distribute this air to the orifices  44   a  and  44   b  supporting a substantially continuous airflow  54  through the orifices  44   a  and  44   b . The airflow  54  provides the functions of shielding the optical elements  52  from corrosive pitting by the combustion gases in the manner of an air curtain and further prevents the accumulation of particulates  57  within or over orifices  44   a  and  44   b  that might block the optical path  46 . Because the airflow  54  is substantially transparent, it creates an effective constantly regenerating window resistant to fouling. 
         [0050]    The inventor has determined that the beneficial properties of the airflow  54  in maintaining unobstructed optical path  46  can be obtained without affecting the validity of the measurement of the combustion gases  42  by using small diameter orifices  44   a  and  44   b  and a small diameter light beam, each which may be, in one example, 80 microns in diameter. More generally the total area of each of the orifices  44   a  and  44   b  may be less than one square millimeter or preferably 10,000 square microns. In all cases, the mass airflow  54  may be generally less than 1000 parts per million and preferably less than 10 parts per million of the total mass of the combustion gases  42  while still providing suitable protective function, 
         [0051]    Referring now to  FIGS. 1 and 2 , in one embodiment the pressure  60  of the air in the outer chamber  56  will be maintained above the pressure  62  of the combustion gases in the conduit  40  on a substantially continuous basis at any time when the engine  10  is operating. Ideally, this pressure  62  rises slightly ahead of the pressure rise in the conduit  40  and may continue at an elevated pressure for a short period of time (for example, through the use of an accumulator) after operation of engine  10  ceases. In one embodiment the pressure is periodically substantially increased in pressure bursts  64  of less than the second, for example, peak pressure, and the pressure bursts  64  have twice a pressure difference between the peak pressure of the pressure bursts  64  and the conduit pressure  62  when compared to the difference between the steady-state pressure  60  and the conduit pressure  62 . This burst operation effects a flexible trade-off between energy use, combustion gas dilution, and cleaning effectiveness. During the pressure bursts  64  spectrographic measurement may be temporarily ceased. 
         [0052]    Referring now to  FIG. 5 , in one embodiment the light emitter  48  may provide for multiple narrowband light transmitters  68  that may be sequentially activated to individually transmit light to a broadband light receiver  50 . The sequential activation of the narrowband light transmitters  68  allows the light receiver  50  to record the separate absorption signals for each of the frequencies of the narrowband light transmitters  68 . These absorption measurements may compare the amplitude of the received signal to a baseline signal, for example, obtained when there is no combustion gas within the conduit  40 , to deduce an absorption value. 
         [0053]    Referring to  FIG. 6 , alternatively; the light emitter  48  may be a broadband or multispectral light transmitter having a bandwidth spanning the entire intended spectrum of the spectrographic measurement (typically including infrared frequencies) and the light receiver  50  may provide for multiple narrowband light sensors  70 , for example, using appropriate light filters or the like to provide frequency-selective sensitivity. Here each of the narrowband light sensors  70  may provide a separate attenuation signal to the engine controller. 
         [0054]    Referring now to  FIG. 7 , the dual orifices  44   a  and  44   b  shown in  FIG. 3  may be replaced with a single orifice  44  and the light may be transmitted through the single orifice  44  into the conduit  40  to scatter from an inner wall of the conduit  40  to return out of the same orifice  44 . In one embodiment similar to that of  FIG. 6 , the light emitter  48  may be a single broadband light transmitter and the light receiver  50  may have multiple narrowband light sensors  70 . Alternatively as shown in  FIG. 8 , light emitter  48  as discussed with respect to  FIG. 5 , may include multiple narrowband light transmitters  68  sequentially operated and the light receiver  50  may be a broadband light receiver whose signal is demodulated by the engine controller  30  to obtain separate spectral measurements. The invention anticipates that at least two and more typically three different spectral bands will be measured. 
         [0055]    Referring to  FIG. 9 , using the backscattering system of  FIGS. 7 and 8 , the optical path  46  may be positioned, for example, at an angle with respect to a vertical direction  74  (with the axis of the conduit  40  being generally horizontal) so as to elevate the orifice  44  reducing the tendency of particulates  57  from collecting thereon under the influence of gravity. In this case, the backscattering surface  76  of the interior of the conduit  40  is also removed from the lowest point in the conduit  40  so as to reduce backscattering changes that may be caused by the accumulation of material along the bottom wall of the conduit  40 . 
         [0056]    Referring now to  FIG. 10 , the source of pressurized air on air line  26  may alternatively be provided by a dedicated electrical blower or fan  78  receiving air from filter  28  and conducting it to the outer chamber  56  of the sensor system  22 . This fan  78  may be controlled by the engine controller  30  in the manner described above to provide continuous operation with pulse pressure changes. The electrical power for the fan  78  may be obtained by batteries and/or a generator attached to the combustion engine  10 . 
         [0057]    The heater  29  may be positioned upstream or downstream from the fan and may incorporate one or both of an electrical resistance heater  79  communicating with the air line  26  and a heat exchanger between the air line  26  and the exhaust pipe  20  for preheating the air in the air line  26 . This preheated air helps to remove moisture from the environment of the sensors and/or icing in cold weather. As such, the resistance heater  79  may be activated before the exhaust pipe  20  is fully heated to provide advanced defrosting of the sensor system. 
         [0058]    Referring now to  FIG. 11 , in one embodiment, the orifices  44  (including any of the embodiments described above including orifices  44   a  and  44   b ) may be formed by a center channel  80  of a hollow light pipe  82 , for example, the light pipe  82  being a metal tube with internally backscattering inner surfaces. This light pipe  82  may conduct light  84  along its inner channel via internal reflections from the light emitter  48  or to the light receiver  50  which may be thereby giving greater separation of these elements from the combustion gases  42  from inside the conduit  40  by the length of the light pipe  82 . In addition the central channel of the light pipe  82  may conduct the airflow  54 . Again a windowless optical interface is generated that is constantly flushed through the use of particulate free fresh air. 
         [0059]    Referring now to  FIG. 12 , in one embodiment, the orifice  44  may provide for a shield  88  projecting from the inner surface of the conduit wall perpendicularly therefrom to partially surround the orifice  44  thereby diverting any condensation flow  90  caused by the force of gravity and such as may wash particulate matter into the orifice  44  around the orifice  44 . 
         [0060]    Referring now to  FIG. 13 , the light emitter  48  may be associated with a lens  100  (or mirror) receiving light  102  from the light emitter  48  to focus it at a focal point  104  centered within the orifice  44  to maximize light transmission there through. 
         [0061]    Alternatively, as shown in  FIG. 14 , the lens  100  may have its focal point  104  adjusted to lie at the entrance aperture of a light guide  106  such as an optical fiber or the light pipe discussed with respect to  FIG. 11 . This allows the light emitter  48  to be further separated from the orifice  44  while still providing a high amount of light transmission through a small area. 
         [0062]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0063]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0064]    References to “a controller”, “a circuit” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more circuits or microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. A single microprocessor or similar computer executing different programs may provide different circuits by virtue of the programs control of current flow in hardware. Accordingly, it will be understood that the invention contemplates that the described circuits may be implemented flexibly through discrete circuitry, microcoding (firmware) and associated processing circuitry, gate arrays, and general-purpose processors executing programs including special application programs and/or operating system functions. 
         [0065]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to 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. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.