Abstract:
An exhaust pressure sensing system and method for an internal combustion engine. The system includes a pressure sensor positioned to sense exhaust pressure of an internal combustion engine. An attenuator reduces the relatively large pressure fluctuations that would otherwise impinge on the pressure sensor to provide an averaging of the exhaust backpressure to which the pressure sensor is exposed.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a system and method for sensing and controlling characteristics of certain internal combustion engines, and particularly to a system and method for improving the sensing of exhaust backpressure for an engine. 
     BACKGROUND OF THE INVENTION 
     Internal combustion engines are used in a wide variety of applications, including the providing the power for a variety of vehicles. Generally, such engines include one or more cylinders that each contain a piston designed for movement in a reciprocating manner. Each piston is connected to a crankshaft by a connecting rod that delivers force from the piston to the crankshaft in a manner that rotates the crankshaft. Power to drive the piston is provided by igniting an air-fuel mixture supplied to the cylinder on a side of the piston opposite the connecting rod. The air-fuel mixture is ignited, e.g. by an ignition device, such as a spark plug having electrodes across which a spark is provided. 
     Air and fuel may be supplied to each cylinder by a variety of mechanisms, such as carburetor or fuel injection systems. Adjusting or changing the air-fuel mixture according to operating conditions permits greater optimization of desired engine operation characteristics. For example, application of greater throttle for increased engine speed requires a greater quantity of fuel. On the other hand, maintaining the engine operation at a lower RPM, requires a lesser quantity of fuel supplied to each cylinder. Generally, greater control over combustion conditions, e.g. air-fuel mixture, provides an engine designer with a greater ability to bring about a desired engine performance under a greater range of operating conditions. 
     Modern engines often utilize electronic fuel injection systems that inject specific amounts of fuel based on a stored fuel map. The fuel map effectively acts as a guide to fuel injection quantities based on a variety of sensed parameters, such as engine speed, throttle position, exhaust pressure and engine temperature. For example, detection of pressure in the exhaust system under given operating conditions can be used to adjust one or more combustion parameters of the engine. 
     Pressure sensors can be used to detect pressure in an exhaust system and to output a signal representative of the pressure. However, the exhaust pressure is subject to large fluctuations that create a sometimes radically fluctuating output signal. Often, it is desirable to obtain a more useable signal that is representative of the average exhaust backpressure of the engine. 
     SUMMARY OF THE INVENTION 
     The present invention features a system and method for measuring the exhaust pressure of an internal combustion engine. The technique utilizes a pressure sensor placed into fluid communication with an exhaust flow path of the internal combustion engine. Additionally, an attenuator is utilized to automatically provide time averaging of the otherwise relatively large exhaust pressure fluctuations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
     FIG. 1 is a perspective view of a watercraft powered by an exemplary internal combustion engine utilizing an exhaust pressure sensing system, according to an embodiment of the present technique; 
     FIG. 2 is a schematic representation of the outboard motor that is illustrated in FIG.  1  and includes an internal combustion engine benefiting from an exhaust pressure sensing system, according to an embodiment of the present technique; 
     FIG. 3 is a schematic cross-sectional view of a single cylinder in an exemplary two-stroke engine, such as the engine illustrated in FIG. 2; 
     FIG. 4 is a schematic representation of an exhaust pressure sensing system, according to an embodiment of the present technique; 
     FIG. 5 is a graphical representation of an actual exhaust pressure and an attenuated exhaust pressure versus time; 
     FIG. 6 is a cross-sectional view of an exemplary exhaust pressure sensing system; and 
     FIG. 7 is an alternative embodiment of the exhaust pressure sensing system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the sake of clarity in explanation, the present technique is described in conjunction with engines that operate on a two-stroke cycle and utilize fuel injection. The present system and method are particularly amenable for use in two-stroke engines that inject fuel, such as gasoline, directly into each cylinder of the engine. The exemplary embodiment described herein should not be construed as limiting, however, and has potential uses in other types of two-stroke and four-stroke engine applications that may benefit from a control system that uniquely utilizes the sensing of combustion end products, e.g. exhaust gasses, to adjust the air-fuel mixture introduced into one or more of the engine cylinders. 
     Referring generally to FIG. 1, an exemplary application of the present system is illustrated. In this application, a watercraft  10 , such as a boat, is powered by an engine  12  disposed in an outboard motor  14 . Watercraft  10  can also be a personal watercraft or boat having an internally mounted engine. In the illustrated embodiment, outboard motor  14  is mounted to a transom  16  of watercraft  10 . Engine  12  is a two-stroke engine that utilizes direct fuel injection, as explained more fully below. 
     Although engine  12  may be a single cylinder engine, it often includes a plurality of cylinders  18 , e.g. six cylinders, as illustrated schematically in FIG.  2 . In the application described above, engine  12  is mounted to an outboard motor frame  20  that supports engine  12  and encloses a drive shaft  22 . Generally, drive shaft  22  is vertical and connects to an output shaft  24  to which a propeller  26  is mounted. Engine  12  rotates drive shaft  22  which, in turn, rotates output shaft  24 . Output shaft  24  is connected to propeller  26  by, for example, splines that rotate the propeller to drive watercraft  10  along the surface of the water. A shroud or housing  28  encloses engine  12 . 
     Referring generally to FIG. 3, a single cylinder of an exemplary two-stroke engine  12  is illustrated. In this embodiment, engine  12  includes a cylinder  30  having an internal cylinder bore  32  through which a piston  34  reciprocates. Piston  34  typically includes one or more rings  36  that promote a better seal between piston  34  and cylinder bore  32  as piston  34  reciprocates within cylinder  30 . 
     Piston  34  is coupled to a connecting rod  38  by a pin  40 , sometimes referred to as a wrist pin. Opposite pin  40 , connecting rod  38  is connected to a crankshaft  42  at a location  43  offset from a crankshaft central axis  44 . Crankshaft  42  rotates about axis  44  in a crankshaft chamber  46  defined by a housing  48 . 
     At an end of cylinder  30  opposite crankshaft housing  48 , a cylinder head  50  is mounted to cylinder  30  to define a combustion chamber  52 . Cylinder head  50  may be used to mount a fuel injector  54  and a spark plug  56 , which are received in a pair of openings  58  and  60 , respectively. Openings  58  and  60  may be formed through the wall that forms either cylinder head  50  or cylinder  30 . In the illustrated embodiment, openings  58  and  60  are formed through the wall of cylinder head  50  for communication with combustion chamber  52  within a recessed internal region  62  of cylinder head  50 . 
     By way of example, fuel injector  54  may be centrally located at the top of cylinder head  50 , as illustrated in FIG.  3 . Spark plug  56  preferably is disposed at an angle such that its electrodes  64 , and consequently the spark, are positioned in an actual fuel spray pattern  66 . Fuel spray pattern  66  is the “cone” or other pattern of fuel spray injected by fuel injector  54 . 
     In operation, piston  34  travels towards cylinder head  50  to compress a charge of air within combustion chamber  52 . Simultaneously, fuel injector  54  injects fuel to create an air-fuel mixture that is ignited by an appropriately timed spark across electrodes  64 . As piston  34  travels towards cylinder head  50 , air is drawn through an inlet port  68  into crankshaft chamber  46  and cylinder  30  on a side of piston  34  opposite combustion chamber  52 . A valve  70 , such as a reed valve, allows the air to pass into engine  12  but prevents escape back through inlet port  68 . 
     Upon ignition of the air-fuel charge in combustion chamber  52 , piston  34  is driven away from cylinder head  50  past an exhaust port  72  through which the exhaust gasses are discharged. As piston  34  moves past exhaust port  72 , it ultimately exposes a transfer port  74 . Air from crankshaft chamber  46  is forced through port  74  and into cylinder  30  on the combustion chamber side of piston  34 . Effectively, the downward travel of piston  34  compresses the air in crankshaft chamber  46  and forces a fresh charge of air into cylinder  30  through transfer port  74  for the next ignition. 
     This reciprocal motion of piston  34  drives connecting rod  38  and crankshaft  32  to provide power to, for example, drive shaft  22  of outboard motor  14 . To provide the desired power to crankshaft  42 , it is necessary that ignition of the air-fuel mixture be carefully timed. If the ignition occurs too early, the resultant explosion works against the progress of piston  34  towards cylinder head  50 . On the other hand, if ignition is too late, less power is transferred to piston  34 . 
     Additionally, it is beneficial to optimize the air-fuel mixture introduced into a given cylinder to promote a desired result, e.g. power, efficiency of operation, reduced soot, etc. Existing fuel injection systems rely on various sensed parameters, such as throttle position, to adjust the amount of fuel injected. However, as illustrated in FIG. 3, a sensor assembly  76  may be coupled to a sensor port  78  of an exhaust system  80  at a desired location  82  downstream from the exhaust port  72 . The sensor assembly  76  may be coupled to a control assembly  84 , which is coupled to the two-stroke engine  12 . For example, the control assembly  84  may be configured to provide control signals to the fuel injector  54  and the spark plug  56  through control paths  86  and  88 , respectively. The control assembly  84  may utilize the sensed parameter from the sensor assembly  76 , and adjust one or parameters (e.g., ignition timing or fuel injection angle/timing) affecting the performance of the engine  12 . 
     FIG. 4 is a diagram illustrating an exemplary sensor assembly  76  of the present technique. In this exemplary embodiment, the sensor port  78  may be configured as a pressure sensing port  90  for exhaust gases discharging through the exhaust system  80 . The sensor assembly  76  may have an attenuation chamber  92  and diaphragm assembly  94  disposed between the pressure sensing port  90  and a pressure sensor  96 , as illustrated. Accordingly, pressure variations in the exhaust gases sampled through the pressure sensing port  90  may be significantly attenuated or eliminated (e.g., to produce a mean or average pressure), prior to pressure reading by the pressure sensor  96 . 
     FIG. 5 is an exemplary plot of pressure versus time comparing an actual exhaust pressure  98  with a reading from the sensor assembly  76  of the present technique. As illustrated, the actual exhaust pressure  98  fluctuates significantly about an average pressure  100 . In a conventional pressure sensor assembly, the actual exhaust pressure  98  is measured at sampling points, such as point  102 , at time intervals At (e.g., every 20 milliseconds). Accordingly, the pressure fluctuations (e.g., fluctuation  104 ) read by the conventional pressure sensor assembly require signal filtering and time averaging subsequent to the pressure reading, resulting in a relatively slow response time for the control assembly  84 . 
     The present technique substantially reduces these fluctuations, providing an attenuated pressure  106  relatively close to the average pressure  100 , and reads the attenuated pressure  106  to provide a pressure reading to the control assembly  84 . The control assembly  84  is then able to use the pressure reading to calculate real-time corrections for various parameters affecting combustion in the engine  12 , and to provide adjusted control signals to components such as the fuel injector  54  and the spark plug  56 . For example, the real-time corrections may relate to a fuel injection rate and timing for the fuel injector  54 , and ignition timing for the spark plug  56 . 
     FIG. 6 is a cross-sectional view of an exemplary embodiment of the sensor assembly  76 . As illustrated, the sensor assembly  76  has a series of hoses or conduits for coupling the various components. The sensor assembly  76  may be coupled to the sensor port  78  by a coupling member  108 , which may be fixedly disposed within the sensor port  78 . A conduit  110  then couples the coupling member  108  to the attenuation chamber  92 . The conduit  110  may be secured to the coupling member  108  and attenuation chamber  92  via flanged ends  112  and  114 , respectively. External clamps also may be provided to secure the conduit  110 . Similarly, the attenuation chamber  92  is coupled to the diaphragm assembly  94  by a conduit  116 , which may be secured by pressure fitting the conduit  116  about flanged ends  118  and  120  of the attenuation chamber  92  and diaphragm assembly  94 , respectively. External clamps also may be provided to secure the conduit  116  to the flanged ends  118  and  120 . 
     A conduit  122  is also provided for coupling the diaphragm assembly  94  to the pressure sensor  96 , which may be an integral part of the control assembly  84 . The conduit  122  is pressurably secured about flanged ends  124  and  126  of the diaphragm assembly  94  and the pressure sensor  96 , respectively, and may have external clamps for additional securement of the conduit  122  about the flanged ends  124  and  126 . 
     As illustrated, the attenuation chamber  92  is a rectangular chamber, although a variety of geometries and configurations are contemplated for the present technique. For example, the attenuation chamber  92  may have a conduit extending inwardly from either of the flanged ends  114  and  118 . The attenuation chamber  92  also may have a plurality of internal dampening members for dispersing and attenuating pressure fluctuations of incoming exhaust gases. 
     The diaphragm assembly  94  may have sections  128  and  130  adjacent to the conduits  116  and  122 , respectively. As illustrated, a diaphragm member  132  is disposed between the sections  128  and  130  to form opposite cavities  134  and  136 , respectively. The sections  128  and  130  may then be fixedly attached (e.g., welded), or removably coupled by securement members  138  (e.g., nut and bolt). Within the diaphragm assembly  94 , the diaphragm member  132  advantageously attenuates pressure pulses from the exhaust gases, and may isolate the exhaust gases from the pressure sensor  96 . The diaphragm member  132  can comprise a variety of metallic or other suitable materials, depending on the particular application, configuration of the sensor assembly  76 , and properties of the exhaust gases (e.g., temperature, pressure, etc.). The diaphragm member  132  also may have a variety of geometries, and may have ribs or other structural features, such as rib  140  (e.g., a circular rib). 
     Alternatively, the sensor assembly  76  may have combined and/or modified components. FIG. 7 is a cross-sectional view of an alternative embodiment of the sensor assembly  76 , wherein the attenuation chamber  92  and diaphragm assembly  94  are combined/replaced by a pressure attenuation assembly  142 . In this exemplary embodiment, the pressure attenuation assembly  142  has sections  144  and  146  disposed about a diaphragm structure  148  to form opposite cavities  150  and  152 , respectively. The sections  144  and  146  also have coupling members  154  and  156  with flanged ends  158  and  160  for pressure fitting within the conduits  110  and  116 , respectively. Accordingly, exhaust gases may enter through the coupling member  154 , enter the cavity  150 , and pressuringly interact with the diaphragm structure  148 . The diaphragm structure  148  then advantageously attenuates pressure fluctuations from the exhaust gases, and typically isolates the exhaust gases from the pressure sensor  96 . The cavity  150  also may be configured for attenuating or softening pressure fluctuations or pulses. For example, the cavity  150  may be relatively larger than the cavity  146  (e.g., similar to the attenuation chamber  92 ), and may have the coupling member  154  extending partially into the cavity  150 . The larger volume of the cavity  150  allows the exhaust gases space to attenuate pressure fluctuations, while additional elements may be added to further attenuate the pressure fluctuations. 
     The pressure attenuation assembly  142  can have a variety of elements, modifications, and geometric configurations to accommodate a particular application and/or properties of the exhaust gases (e.g., temperature, pressure, etc.). In one example, the pressure attenuation assembly  142  comprises a variety of metallic or other suitable materials, and has internal elements for breaking up and/or directing incoming exhaust gases so as to attenuate pressure fluctuations. The pressure attenuation assembly  142  itself may be multi-sectional, as illustrated, or a single component. For example, the sections  144  and  146  can be fixedly attached (e.g., welded), or removably coupled by securement members  162  (e.g., nut and bolt). The diaphragm structure  148  also may comprise multiple sections and/or diaphragms, and may have one or more structural features such as rib  164 . For example, the pressure attenuation assembly  142  can be formed with a plurality of chambers separated by diaphragms. The sensor assembly  76  also may have one or more filters (e.g., to filter out liquid or particulate), valves, gauges, insulation layers, circuits, or other electrical or mechanical devices. For example, sensor assembly  76  can include an attenuation circuit to attenuate (e.g., by time averaging) any residual pressure fluctuations apparent in the reading provided by the sensor  96 . 
     The pressure reading from the pressure sensor  96  may be utilized by the control assembly  84 , or for servicing the engine  12 . The control assembly  84  utilizes the pressure reading to quickly respond to performance variations in the engine  12  and to provide corrections (e.g., corrected control signals) for improved performance. The pressure readings may essentially be a real-time average pressure of the exhaust gases that moves over time as operational characteristics of the engine change. As noted above, the control signals may relate to ignition and fuel injection timing, or other controllable parameters. In one example, control assembly  84  uses the pressure readings to ensure that the air-fuel mixture is at a desired fuel-air mixture, such as a stoichiometric mixture. The exemplary control assembly  84  comprises a processor, circuits, memory, and various control parameters stored on the memory. Various other sensors and gauges (e.g., oxygen sensors, temperature sensors, etc.) also may be coupled to the control assembly  84 . 
     Accordingly, the present technique relates to combustion control and engine performance, and more particularly to determining combustion conditions and control parameters for the control assembly  84 . For example, the control assembly  84  may be coupled to a number of sensors for sensing properties of the combustion chamber and/or for sensing downstream properties (e.g., exhaust pressure). The properties measured by the sensors are then be used by the control assembly  84  to adjust various controllable engine parameters (e.g., fuel injection and ignition) to ensure desired operational characteristics of the engine (i.e., fuel-air mixture, emission reduction, etc.). 
     Control assembly  84  stores previously mapped values for fuel injection (e.g., fuel amount and timing), for ignition (e.g., spark timing), and for other controllable parameters for various operational characteristics of the engine (e.g., percent throttle, speed or engine rpm, temperature, pressure, etc.). The previously mapped values are then used by the control assembly  84  as reference or default values for controlling the engine, subject to adjustment according to the sensed properties. For example, as the pressure sensor  96  measures the exhaust pressure, the control assembly  84  determines whether the engine is operating at the desired operational characteristics, makes adjustments from the mapped values to achieve the desired operational characteristics, compares the actual values with the mapped values, and then determines a correction factor for the previously mapped values to improve performance of the engine. 
     The present technique is particularly amenable for use in fuel-injected, two-stroke engines, such as the direct injection engine described above. By way of example, at a particular operating condition, the fuel injection rate actually applied to the single cylinder is compared to a previously stored fuel map value for a desired mixture (e.g., stoichiometric). If the fuel injection rate deviates from the previously mapped value, then a correction factor is determined to account for the deviation (e.g., a ratio between the actual and mapped fuel injection rates or amounts). The control assembly  84  then utilizes the correction factor to adjust the mapped value to provide the fuel injection rate corresponding to the desired mixture for the particular operating conditions. As the control assembly  84  continuously evaluates the sensed properties, and adjusts the engine, the sensor assembly  76  of the present technique is particularly advantageous in ensuring rapid adjustment of the controllable properties. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.