Abstract:
A multiple path air mass flow sensor assembly includes an inlet duct which is divided into at least two separate ducts. A first duct has an air mass flow sensor disposed in conventional fashion therein. A second duct includes a damper or valve which may be selectively, partially or fully opened. At low air flow rates, the damper is closed forcing all air through the first duct. As engine speed increases and the air mass flow rate increases, the damper opens, either fully at a predetermined flow rate or proportionally over a predetermined flow rate range, to allow a larger volume of air through the inlet duct. The signal from the air mass flow sensor is adjusted by the engine controller to compensate for the (bypass) air flowing through the second duct.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to air mass flow sensors and more particularly to a multiple path air mass flow sensor having improved measurement accuracy. 
     2. Description of the Prior Art 
     Modern internal combustion engines typically intended for use in passenger vehicles and trucks include extensive use of operational sensors and microprocessors to control engine operation, improve performance and fuel economy, and reduce emissions. In fact, the great improvements in these often conflicting operational parameters of the last several decades would simply not have been possible without the use of these systems. 
     One of the most commonly used sensors with an internal combustion engine and electronic controller is an air mass flow sensor. Typically, an air mass flow sensor includes a resistance element which is located in an air intake duct, often immediately after the air filter and before the duct is subdivided into the branches of the intake manifold or other structure. So disposed, the air mass flow sensor is exposed to the instantaneous flow of air to the engine. Data from the sensor is utilized by the engine controller to compute the instantaneous air mass flow which is utilized to, for example, adjust the air fuel mixture and optimize engine performance in accordance with predetermined parameters. 
     Ideally the air mass flow sensor will be located in an intake duct to ensure that the air impinging upon it is representative of the total air flow so that the output of the sensor accurately reflects the total instantaneous air mass flowing through the duct. This can be a challenge because the often convoluted duct and varying air velocities may create complex flow patterns which render accurate flow measurement difficult. This problem is especially acute at low flow rates which represent the most critical engine operating condition from the standpoints of emissions and fuel economy. Although reducing the size of the intake duct would seem to be a solution to this problem, since the size of the intake duct is mandated by the air flow requirements of the engine at full load, reducing its size at the sensor location to improve measurement accuracy reduces the volume of air provided to the engine, increases flow resistance and will thus interfere with achieving maximum engine power output. A larger intake duct, however, reduces the flow velocity and, as noted, renders air mass flow measurement more difficult, particularly under low flow conditions—precisely those conditions that are most critical to optimum engine operation. 
     The foregoing discussion suggests that improvements in the design of mass air flow sensor assemblies is desirable. 
     SUMMARY 
     A multiple path air mass flow sensor assembly for an internal combustion engine includes an intake duct which is divided into at least two separate ducts. A first duct has an air mass flow sensor disposed in conventional fashion therein. The second duct includes a damper or valve which may be selectively, partially or fully opened. At low air flow rates, the damper is closed forcing all air through the first duct. As engine speed increases and the air flow rate increases, the damper opens, either fully at a predetermined flow rate or proportionally over a predetermined range, to allow a larger volume of air through the intake duct. Processing of the signal from the air mass flow sensor by the engine controller or other processor is undertaken to compensate for the (bypass) air flowing through the second duct. 
     An air mass flow sensor assembly having a passive, spring biased damper is also disclosed. Here, the damper is opened by increased air flow and a sensor provides a signal indicating that the damper is open to the engine controller or other processor which recalculates the air mass flow based upon the open damper and the resulting bypass air flow. 
     The air mass flow sensor assembly according to the present invention exhibits improved measurement accuracy at low flow rates because the sensor is located in a duct smaller than those currently used with a comparable engine while providing reduced flow resistance and improved high volume air flow. Both parallel and non-parallel ducts defining parallel and non-parallel flow pathways as well as multiple, staged or sequenced bypass ducts and ducts having different areas or flow rates are also within the scope of this invention. 
     Thus it is an object of the present invention to provide an air mass flow sensor assembly having improved low flow rate measurement accuracy. 
     It is a further object of the present invention to provide an air mass flow sensor assembly having improved high flow rate capability. 
     It is a still further object of the present invention to provide an air mass flow sensor assembly having improved low flow rate measurement accuracy and high flow rate capacity. 
     It is a still further object of the present invention to provide an air mass flow sensor assembly having a damper disposed in a duct parallel to a duct containing the mass air flow sensor. 
     It is a still further object of the present invention to provide an air mass flow sensor assembly having a passive, spring biased damper and position sensor disposed in a duct parallel to a duct having a mass air flow sensor. 
     Further objects and advantages of the present invention will become apparent by reference to the following description and appended drawing wherein like reference numbers refer to the same component, element or feature. 
    
    
     
       DRAWINGS 
         FIG. 1  is a diagrammatic view of a first embodiment of an air mass flow sensor assembly according to the present invention associated with an air intake system of an internal combustion engine; 
         FIG. 2  is a diagrammatic view of the first embodiment of an air mass flow sensor assembly according to the present invention with the bypass damper open; 
         FIG. 3  is a diagrammatic view of another embodiment of an air mass flow sensor assembly according to the present invention; and 
         FIG. 4  is a graph presenting two relationships between air flow through an intake duct of an internal combustion engine and damper opening of an air mass flow sensor assembly according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , an inlet or intake duct assembly for an internal combustion engine is illustrated and designated by the reference number  10 . The intake duct assembly  10  includes an air filter assembly  12  which draws in ambient air and includes an air filtration element  14 . The air filter assembly  12  communicates with an air inlet or intake duct  16  which provides combustion air to an internal combustion engine  18 . In a first embodiment of an air mass flow sensor assembly  20  according to the present invention, the air inlet or intake duct  16  is separated into a first, sensing duct  22  and a second, bypass duct  24  by a divider or partition  26 . While illustrated as parallel, it should be understood that the first, sensing duct  22  and the second, bypass duct  24  may be separate, non-parallel and sinuous or convoluted, for example, to fit within available under-hood space or match and connect to air inlet and engine components. 
     Disposed within the first, sensing duct  22  is a conventional air mass flow sensor  30 . The air mass flow sensor  30  is configured and arranged to provide an accurate indication of the instantaneous air mass flow through the first, sensing duct  22 . Typically, the air mass flow sensor  30  may include a resistive element such as a carbon or wirewound resistor. The air mass flow sensor  30  also includes an output conductor or cable  32  which carries the data signal from the air mass flow sensor  30  to a microprocessor such as an engine controller or engine control module  36 . The intake duct  16  extends to and includes an exemplary intake manifold  40 . Although not illustrated, the intake duct assembly  10  may also include a turbo-charger or supercharger, if desired. 
     Disposed within the second, bypass duct  24  is a calibrated orifice  42  having a known or defined area. The calibrated orifice  42  improves the accuracy of air mass flow computations undertaken by the engine control module  36  as will be more fully described below. The second, bypass duct  24  also includes a moveable damper, flap, partition or valve  44 . The damper or valve  44  is sized to completely close off the second, bypass duct  24  when it is in its closed position, perpendicular or substantially perpendicular to the walls of the bypass duct  24  and the direction of air flow, as illustrated in  FIG. 1 . Thus, if the second, bypass duct  24  is square or rectangular in cross-section, for example, the damper or valve  44  is correspondingly shaped. About the periphery of the opening in the second, bypass duct  24  and adjacent the damper or valve  44  may be disposed a sealing lip, bead or flange  46  which assists in maintaining a tight seal between the damper or valve  44  and the walls of the second, bypass duct  22  when the damper or valve  44  is in its closed position, as illustrated in  FIG. 1 . 
     As illustrated in  FIGS. 1 and 2 , the damper or valve  44  is moved from its closed position, illustrated in  FIG. 1 , through intermediate positions, to a fully open position, illustrated in  FIG. 2 , by an electric, electronic or pneumatic actuator  48 . The actuator  48  receives signals from the engine control module  36  through a conductor or cable  52  which command it to a fully closed, a fully open or an intermediate position. 
       FIG. 3  illustrates another embodiment  20 ′ of the present invention wherein the majority of components are the same but the damper actuator  48  has been replaced by an assembly  54  having a two position switch or rotary transducer  56  which provides data regarding the position of the damper or valve  44  to the engine control module  36  through a conductor or cable  58 . As indicated by the arrow, the assembly  54  includes a spring biasing feature which urges the damper or valve  44  toward the closed position, against the sealing bead or flange  46 . As the volume of air in the intake duct  16  increases, pressure against the damper or valve  44  increases, overcoming the resistance of the spring and the damper or valve  44  moves toward an open position, as illustrated in  FIG. 3 . As the damper or valve  44  rotates and opens, the switch or transducer  56  changes state or provides a continuous signal indicating the position of the damper or valve  44  to the engine control module  36 . 
       FIG. 4  is a graph which presents two exemplary operating relationships between the total instantaneous air mass flow in the air intake duct  16  and the percent of opening of the damper or valve  44  relating to the embodiment of the invention illustrated in  FIGS. 1 and 2 . The lower line, designated “A,” illustrates that operational mode in which the actuator  48  is commanded by the engine control module  36  to move the damper or valve  44  from its fully closed position to its fully opened position (without any intermediate position(s)) when air mass flow in the intake duct  16  reaches or exceeds a certain predetermined rate. Stated somewhat differently, when total instantaneous air mass flow is below the line “A,” the damper or valve  44  will be closed; when it is above the line “A,” the damper or valve  44  will be open. This threshold value may be determined by, for example, either the speed or acceleration of the internal combustion engine  18  or data from the air mass flow sensor  30 . 
     A second line, “B,” in  FIG. 4  presents an alternative operational mode in which increased air mass flow, above a certain predetermined threshold, causes a corresponding or proportional opening of the damper or valve  44  as the air mass flow continues to increase. It will be appreciated that the proportional relationship between the air mass flow and the percent of opening of the damper or valve  44  a presented by line “B” while beginning at 0 percent, effectively is no longer proportional after the damper or valve  44  is significantly open. This is the result of a well understood feature of control dampers and valves whereby little or no proportional control of the controlled fluid is achieved after the valve or damper is about two-thirds open. It should, however, be appreciated that the 75 percent value is exemplary only and that this value may be higher or lower depending upon particular applications and operating conditions. 
     When the damper or valve  44  is partially or fully open, it is necessary that the engine control module  36  correct or recalibrate its computation of the instantaneous air mass flow through the air intake duct  16  (from its sensing and computations undertaken when the damper or valve  44  is fully closed) since all of the air passing through the intake duct  16  is no longer passing through the first, sensing duct  22 . This correction or recalibration can be undertaken electronically in the engine control module  36  or, if the sensor assembly  10  is a stand alone device, within a self-contained microprocessor (not illustrated). In the case of operation according to line “A,” the correction simply requires multiplying the sensed air mass flow by the inverse of the ratio of air passing through the first, sensing duct  22  and the intake duct  16  when the damper or valve  44  is fully open. In the case of operation according to line “B,” the relationship between the percent of opening of the valve or damper  44  and the actual flow through the second, bypass duct  24  must first be known or established and this relationship utilized to make the necessary correction in accordance with the inverse ratio of sensed air mass flow and total air mass flow described immediately above. 
     With regard to the embodiment  20 ′ of the invention illustrated in  FIG. 3 , similar operational considerations, particularly correcting or recalibrating the air mass flow value sensed by the air mass flow sensor  30  based upon the flow rate through the second, bypass duct  24 , to reflect the actual total air mass flow must be undertaken, Here, however, the two position switch or transducer  56  provides data to the engine control module  36  regarding the present position of the damper or valve  44  resulting from the force of the moving air on the damper or valve  44 . The engine control module  36  (or other processor) then computes the actual, instantaneous air mass flow passing through the inlet duct  16  in accordance with known relationships between the position of the damper or valve  44  and the air mass flow sensed by the air mass flow sensor  30  in the first, sensing duct  22 . 
     While the foregoing description has centered upon an air mass flow sensor assembly  20  having two paths or ducts: a first main or sensing path or duct and a second auxiliary or bypass path or duct, it should be appreciated that the invention and its operating principles encompasses a multiple path air mass flow sensor having two, three, four or more bypass ducts in operational parallel with a single sensing duct. In any such multiple path or duct sensor assembly, the sensing duct is always open and one, two or more bypass ducts are opened in response to higher engine speeds and loads. As the bypass ducts are sequentially and cumulatively opened, the engine control module  36  or other controller corrects or recalibrates the output of the air mass flow sensor  30  to compensate for the reduced air mass flow through the sensing duct and increased flow through the bypass ducts. For example, if the assembly includes three equal flow rate ducts, i.e., one sensing duct and two bypass ducts, and all three ducts are open, the output of the air mass flow sensor would be tripled to compensate for the fact that it is sensing only one-third the total air mass flow passing through the inlet duct  16 . 
     Additionally, it should be understood that while the foregoing description has centered upon an air mass flow sensor assembly  20  having two ducts of equal flow rates, an air mass flow sensor assembly having two or more ducts of known but unequal flow rates or areas is also within the scope of the present invention. For example, a main sensing duct  22  may have a flow rate or area of one unit whereas a bypass duct  24  and its calibrated orifice  42  may have a flow rate or area of two units or three units. When such a bypass duct  24  is opened, the engine control module  36  or other controller is programmed to and applies the appropriate ratio correction or recalibration to the output of the air mass flow sensor  30 . If the main to bypass flow ratio is 1:2, so that only one-third of the total air mass flow is passing through the sensing duct  22  when the bypass duct is fully open, the engine control module  36  triples the sensed value of the air mass flow sensor  30 ; if the main to bypass flow ratio is 1:3, the engine control module  36  multiplies the sensed value by four when the bypass duct is fully open. 
     The foregoing description of the present invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be, and are, within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the present invention.