Abstract:
A system and method comprises receiving a mass air flow signal having a frequency that varies based on mass air flow in an intake manifold of an engine, determining first period data from the mass air flow signal, deriving first mass data for the mass air flow signal based on the first period data, cumulating the first period data and the first mass data for N cylinder events, wherein N is an integer greater than 1, and calculating a mass air flow between the N cylinder events from the cumulated first period data and the cumulated first mass data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/958,065, filed on Jul. 2, 2007, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to vehicle control systems, and more particularly to methods and systems for determining mass air flow in vehicles. 
     BACKGROUND OF THE INVENTION 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Sensors gather information from components of an engine system. The information is received by a control module that controls the engine system based on the received information. For example, a mass air flow (MAF) sensor may measure mass air flow. The MAF sensor may have a square wave output. The frequency of the MAF sensor output may vary relative to the mass air flow to the MAF sensor. The relationship between the frequency of the MAF sensor output signal and mass air flow may be known such that the mass air flow at a particular frequency may be found using a mass air flow vs. frequency look-up table. 
     The control module uses the measured mass air flow to control fuel injection. It may be useful to know the mass air flow that enters a cylinder between particular cylinder events. A cylinder event may be a cylinder air intake event, and may also be referred to as a low resolution (LORES) event. Some systems determine mass air flow using the average frequency between the engine events. This average frequency is used as an index for the mass air flow vs. frequency look-up table. However, averaging techniques may not account for non-linearity in the relationship between mass air flow and frequency, and thus may result in an inaccurate average mass air flow. 
     Another way to determine mass air flow between cylinder events involves converting the frequency axis of the mass air flow versus frequency look-up table to a period axis. This conversion may be based on the relationship between frequency (cycles per second) and period (seconds per cycle). Mass air flow may also be converted to mass based on the relationship between mass air flow (mass per second), mass, and period. A timing module may receive the MAF output signal and measure the period of each cycle of the signal. The mass vs. period look-up table may be used by the timing module to determine a mass based on the period. The mass and period may then be accumulated between cylinder events. 
     The MAF sensor signal may not be synchronized with the cylinder events, such that an error may be associated with an uncounted partial MAF signal cycle between cylinder events. The magnitude of the error may be based on the period of the partial signal compared to the overall time between cylinder events. Vehicle operating conditions may occur where the output of the MAF sensor is at a low frequency (i.e., low mass air flow) and the cylinder events occur frequently (i.e., high RPM). A relatively small number of MAF sensor output signal cycles such as 5 may occur per cylinder event, such that a partial signal may create a potentially large error. 
     Referring to  FIG. 1 , a timing diagram illustrating a possible error due to cylinder event and MAF sensor output timing is demonstrated and generally identified at  10 . In  FIG. 1  there are five complete MAF sensor output pulses per cylinder event when the falling edges of the MAF sensor output line up with the cylinder (LORES) events. However, assuming that mass air flow calculations may be based on falling edges of the MAF sensor  34  output, five pulses occur between the first and second cylinder events and four pulses occur between the second and third cylinder events. This may cause different mass air flow readings for the same overall mass air flow. 
     Referring now to  FIG. 2 , a graph  12  illustrates sampling error in an exemplary  8  cylinder engine with varying engine RPM values and varying MAF sensor frequency values. Accuracy of 95 percent or greater may be considered acceptable. The accuracy is based on the percentage of time that a calculation may not yield an error in engine operation. The  FIG. 2  shows accuracy falling below the acceptable range at high engine RPM levels and/or low MAF sensor output frequency levels. For example, at a MAF sensor frequency of 1000 and an engine RPM of approximately 3300, the accuracy is approximately 75%. 
     Referring now to  FIG. 3 , a graph  14  illustrates sampling error in an exemplary 4 cylinder engine with varying engine RPM values and varying MAF sensor frequency values. Accuracy of 95 percent or greater may be considered acceptable. The graph  14  shows that the accuracy falls below the acceptable range at high engine RPM levels and/or low MAF sensor output frequency levels. For example, at a MAF sensor frequency of 1000 and an engine RPM of approximately 6500, the accuracy is approximately 77%. 
     SUMMARY OF THE INVENTION 
     A method comprises receiving a mass air flow signal having a frequency that varies based on mass air flow in an intake manifold of an engine, determining first period data from the mass air flow signal, deriving first mass data for the mass air flow signal based on the first period data, cumulating the first period data and the first mass data for N cylinder events, wherein N is an integer greater than 1, and calculating a mass air flow between the N cylinder events from the cumulated first period data and the cumulated first mass data. 
     A control system comprises a timing module that receives a mass air flow signal having a frequency that varies based on a mass air flow in an intake manifold to an engine, that determines first period data from the mass air flow signal, that derives first mass data based on the first period data, and that cumulates the first mass data and the first period data, and a mass air flow module that calculates a mass air flow for N cylinder events from the cumulated first mass data and the cumulated first period data, wherein N is an integer greater than 1. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a graph illustrating a sampling error in mass air flow measurement; 
         FIG. 2  is a graph illustrating the potential sampling error in a 8 cylinder engine for varying engine RPM values; 
         FIG. 3  is a graph illustrating the potential sampling error in a 4 cylinder engine for varying engine RPM values; 
         FIG. 4  is a functional block diagram of an engine system; 
         FIG. 5  is a functional block diagram of a control module of the engine system; 
         FIG. 6  is a graph illustrating an exemplary output of a MAF sensor as a function of frequency; 
         FIG. 7  is a graph of mass air flow as a function of frequency for an exemplary mass air flow sensor; 
         FIG. 8  is a graph of mass air flow as a function of period for an exemplary mass air flow sensor; 
         FIG. 9  is a graph of mass as a function of period for an exemplary mass air flow sensor; and 
         FIG. 10  is a flowchart illustrating the operation of a control system for determining mass air flow. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify the same elements. As used herein, the term module and/or device refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. 
     Referring now to  FIG. 4 , an engine system  15  utilizing the mass air flow system of the present application is shown. Engine system  15  may include engine  16  and control module  17 . Engine  16  may include a plurality of cylinders  18  each with one or more intake valves and/or exhaust valves (not shown). During operation, defined cylinder (or LORES) events may be used for mass air flow calculations based on an engine position sensor ring (EPSR)  32  capable of determining the position of engine  16  components based on a position of a crankshaft (not shown). 
     Engine system  15  may further include fuel injection system  20  to provide fuel to cylinders of engine  16 . Engine  16  may receive air that is combusted with fuel from fuel system  20  to drive pistons (not shown) of engine  16 . Electronic throttle control (ETC) module  26  may adjust a throttle blade  27  in an intake manifold  28  based upon a position of an accelerator pedal  30  and a throttle control algorithm that is executed by ETC module  26 . A position of accelerator pedal  30  may be sensed by accelerator pedal sensor  40 , which may generate a pedal position signal that is output to ETC module  26  through communication with control module  17 . A position of brake pedal  44  may be sensed by brake pedal sensor  48 , which may generate a brake pedal position signal that is output to ETC module  26  through communication with control module  17 . 
     It may be desired to determine a mass air flow or mass of air delivered to a cylinder  18  between cylinder events. In this manner, air supplied to cylinders  18  of engine  16  may be known or controlled, and these values may be used to provide proper fuel injection to achieve a desired air/fuel mixture for combustion. A mass air flow (MAF) sensor  34  may sense air passing to engine  16  through intake manifold  28 . MAF sensor  34  may generate a voltage based on mass air flow which may be input to a voltage-controlled oscillator of MAF sensor  34 . MAF sensor  34  may then output signal with a frequency that increases as the mass air flow input (represented by voltage) increases. 
     The relationship of frequency to mass air flow for MAF sensor  34  may be known and converted into a mass vs. period look-up table. The mass vs. period look-up table may be stored in memory of timing module  36 . Timing module  36  may be a separate module or may be a component of control module  17 . Timing module  36  may communicate mass and period values to control module  17  or components thereof. Control module  17  may use these values to determine a mass air flow between cylinder events. The mass air flow between cylinder events may be used to control engine  16  functions such as fuel injection from fuel injection system  20  to cylinders  18 . 
     Cylinder events may be based on an output of EPSR  32 . EPSR  32  may include a sensor capable of sensing a position of a crankshaft (not shown) of engine  16  such as by sensing a position of teeth on the crankshaft. From the crankshaft position it may be possible to determine the position of pistons within respective cylinders  18  of engine  16 . For example, a typical LORES event associated with a cylinder event may be based on a piston position in a range such as 68°-78° before top dead center (bTDC) as measured by EPSR  32 . The output of EPSR  32  may also be used to determine the elapsed time between cylinder events. 
     Control module  17  may also consider other inputs in controlling engine  16  functions such as fuel injection. Emissions sensors  50  and system sensors  52  may be received by control module  17 . System sensors  52  may be sensors such as a temperature sensor or a barometric pressure sensor, and other conventional sensor and/or controller signals. An output of engine  16  may be coupled by torque converter  58  and transmission  60  to front and/or rear wheels. 
     Referring now to  FIG. 5 , control module  17  and timing module  36  are shown in more detail. For purposes of  FIG. 5 , timing module  36  may be depicted as a component of control module  17 . Control module  17  may provide the functionality of determining mass air flow and may include timing module  36 , mass air flow (MAF) module  80 , and other control modules  84 . Timing module  36  may include data accumulation module  82 , mass conversion module  86 , and processing module  88 . 
     Timing module  36  may provide the first level of mass air flow calculations, freeing up processing time within other processors of control module  17 . Processing module  88  of timing module  36  may receive a signal from MAF sensor  34  and measure the period of the signal for each cycle of the signal. Mass conversion module  86  may convert period data from processing module  88  to mass data. For example, mass conversion module  86  may be a look-up table and may include mass vs. period data for the MAF sensor  34 . Processing module  88  may be in communication with mass conversion module  86  to receive a mass value for the measured period. Processing module  88  may then communicate with data accumulation module  82  to accumulate the latest mass and period values with running accumulations of total measured mass and period. Processing module  88  may communicate the accumulated values from data accumulation module  82  to MAF module  80  based on a request from MAF module  80 . 
     MAF module  80  may communicate with processing module  88  to receive accumulated mass and period data at desired times based on cylinder events as indicated by EPSR  32 . MAF module  80  may determine when a cylinder event occurs based on an output of EPSR  32 . At each cylinder event, MAF module  80  may query processing module  88  to receive the accumulated mass and period data for that cylinder event. MAF module  80  may then determine an overall mass or mass air flow between the engine events based on the accumulated mass and period data and an elapsed time between the cylinder events. 
     MAF module  80  may communicate mass and mass air flow values to other control modules  84 . Other control modules  84  may include control modules that utilize mass air flow information to determine combustion parameters such as fuel injection. For example, another control module  84  may modify the amount of fuel injected into cylinders  18  of engine  16  based on mass air flow to maintain a desired air/fuel mixture for combustion. 
     Referring now to  FIG. 6 , a graph illustrating an exemplary frequency output of MAF sensor  34  for an exemplary mass air flow pattern is shown and is generally identified at  90 . The MAF sensor output is depicted as the bottom signal and may be a square wave. As can be seen in  FIG. 6 , each cylinder event corresponds to a LORES value. As the mass air flow passing the MAF sensor increases, the frequency of the MAF sensor output signal also increases. The period associated with a complete cycle of MAF sensor  34  decreases as mass air flow increases. 
     Referring now to  FIG. 7 , graph  92  depicts a relationship between frequency and mass air flow for an exemplary MAF sensor  34 . Frequency may be on the x-axis and may be in units of kilohertz (kHz). Mass air flow may be on the y-axis and may be in units of grams per second (g/s). As can be seen from  FIG. 7 , the frequency output of MAF sensor  34  may increase with mass air flow in a nonlinear fashion. This is the manner in which most MAF sensor  34  manufacturers provide information regarding the MAF sensor  34 . 
     Referring now to  FIG. 8 , graph  94  depicts a relationship between period and mass air flow for an exemplary MAF sensor  34 . Graph  94  may be determined from graph  92  based on the relationship between frequency (cycles per second) and period (seconds per cycle). Period may be on the x-axis and may be in units of milliseconds (ms). Mass air flow may be on the y-axis and may be in units of g/s. The shorter the period of the cycle being considered, the higher the mass air flow for that cycle. Graph  94  may be useful for determining mass air flow because a signal received from MAF sensor  34  may have a period that is easily measured by determining the time between consecutive rising or falling edges of the signal. 
     Referring now to  FIG. 9 , graph  96  depicts a relationship between mass and period for an exemplary MAF sensor  34 . Graph  96  may be determined from graph  94  by multiplying a mass air flow value for a period in grams per second by the value of that period in milliseconds to determine a mass for the particular period. Period may be on the x-axis and may be in units of milliseconds. Mass may be on the y-axis and may be in units of milligrams (mg). Period data may be measured by processing module  88  of timing module  36  based on rising or falling edges of a signal from MAF sensor  34 . The shorter the period measured by processing module  88  of timing module  36 , the greater the mass for that cycle. The information of graph  96  may be used to create the look-up table of mass conversion module  86 . Mass vs. period information may be useful in determining an overall mass air flow between cylinder events because the units (mass and time) can be accumulated between cylinder events to determine an overall mass air flow (mass per unit time). 
     Referring now to  FIG. 10 , a flowchart illustrating steps for calculation of mass and mass air flow between cylinder events is depicted in control logic  100 . Control logic  100  may begin at step  102 . At step  102 , MAF module  80  may monitor an output of EPSR  32  for a new cylinder event. When a new cylinder event has occurred, MAF module  80  may store the time of the cylinder event and control logic  100  may continue to block  103 . If not, control logic  100  may continue looping about block  102  until a first cylinder event is encountered. At block  103 , MAF module processing module  80  may query processing module  88  of timing module  36  to get baseline accumulated mass and time data. Alternatively, MAF module  80  could communicate with processing module  88  to zero out any accumulated mass and time data. Control logic  100  may then continue to block  104  to wait for a falling edge from MAF sensor  34 . 
     At block  104 , processing module  88  may wait for a falling edge from MAF sensor  34 . Alternatively, processing module  88  could wait for a rising edge from MAF sensor  34 . Assuming a falling edge is used, control logic  100  could continue looping about block  104  until a falling edge is received. Once a falling edge is received processing module  88  may begin counting the time until the next falling edge and continue to block  105 . At block  105 , processing module  88  may wait for the next falling edge from MAF sensor  34 . When the next falling edge arrives at block  105 , control logic  100  may continue to block  106 . Until the falling edge arrives, control logic  100  may continue looping about step  105 . 
     At step  106 , processing module  88  may determine the elapsed time between the previous falling edge and the latest falling edge of the output of MAF sensor  34  (i.e., one cycle) to determine a period for that cycle. Control logic  100  may then proceed to step  108 . At step  108 , processing module  88  may determine the mass of air (mg) corresponding to the period of the most recent cycle using mass conversion module  86  of timing module  36 . Mass conversion module  86  may contain a table used to convert the period data to mass data as in graph  96 . Control logic  100  may proceed to step  110 . 
     At step  110 , processing module  88  may store the latest mass and period values in data accumulation module  82  of timing module  36 . Data accumulation module  82  may include a running accumulation of total air mass and time. This running accumulation may be performed by adding the latest mass and time values to previously accumulated values. Control logic  100  may continue to block  112 . At block  112  MAF module  80  may monitor EPSR  32  to determine whether another cylinder event has occurred. Steps  105  through  112  may continue to wait for falling edges, calculate a period between falling edges, determine a mass for the period, and accumulate mass and period data until another cylinder event occurs. Once another cylinder event occurs, MAF module  80  may store the time of the cylinder event and control logic  100  may continue to step  114 . 
     At step  114 , MAF module  80  may query processing module  88  of timing module  36  to get the latest accumulated mass and time values from data accumulation module  82 . Control logic  100  may then proceed to step  116 . At step  116 , MAF module  80  may access the previous accumulated mass and time values associated with the previous cylinder event. MAF module  80  may then subtract the latest accumulated mass and time from the previous accumulated mass and time to determine the mass and time between the two latest cylinder events. Control logic  100  may then proceed to step  118 . 
     At step  118 , MAF module  80  may divide the mass between the two cylinder events by the time between the two cylinder events to determine an average mass air flow between the two cylinder events. If a total mass value is desired, MAF module  80  may multiply this average mass air flow by the total elapsed time between the two cylinder events as determined from the EPSR  32  signals. MAF module  80  may communicate these values to other control modules  84  for use in vehicle operations such as fuel injection. Control may then return to step  105  to resume measuring mass and period until the next cylinder event occurs. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.