Abstract:
In measurement methods and a measuring equipment for flow of exhaust gas re-circulation, in order to prevent performances of exhaust, fuel efficiency, and power output from deteriorating due to reasons such as large loss in pressure and time, a control delay during an excessive operation, and reduction in an exhaust gas re-circulation gas flow rate upon measuring an exhaust gas re-circulation gas flow rate, an exhaust gas re-circulation gas flow rate is measured by a plurality of measurement methods on the basis of an intake air flow rate and pressure before and after a heat exchanger installed at an exhaust gas re-circulation passage and the measurement methods are carried out by performing a mutual comparison of the measurement flow rates. Accordingly, it is possible to measure the exhaust gas re-circulation gas flow rate with high precise in a short response time without increasing loss in pressure, and thus to improve performances of exhaust, fuel efficiency, and power output.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an exhaust gas re-circulator for a diesel engine, and more particularly, to a technique for appropriately measuring an exhaust gas re-circulation gas flow rate. 
       DESCRIPTION OF RELATED ART 
       [0002]    In order to reduce a discharge amount of nitrogen oxides of exhaust gas generated from an internal combustion engine, it is effective to restrict a combustion temperature by means of the exhaust gas re-circulation. Particularly, in a diesel engine, an exhaust gas re-circulation gas flow rate can be increased more than that of a gasoline engine. However, when the exhaust gas re-circulation gas flow rate is increased too much, a problem arises in that fuel efficiency deteriorates and soot increases. For this reason, it is necessary to appropriately maintain the exhaust gas re-circulation gas flow rate in accordance with a driving state. In addition, it is necessary to provide a measurement method for measuring the exhaust gas re-circulation gas flow rate with high precision in a short response time in order to cope with a variation in driving state. For this reason, for instance, JP-A-1-178760 discloses a technique for calculating the exhaust gas re-circulation gas flow rate on the basis of the measurement value of a gas density detector. Additionally, JP-A-2007-101426 discloses a technique for calculating the exhaust gas re-circulation gas flow rate by use of a hot wire type air flow sensor. 
         [0003]    In the technique disclosed in JP-A-1-178760, it is not appropriate to solve the above-described problems in that it takes time to measure gas density. Meanwhile, in the technique disclosed in JP-A-2007-101426, the thermal flow meter has a short response time, but it is difficult to measure a counter flow, and additionally, a heating resistor needs to be installed in a passage, which causes pressure loss. As a result, a problem arises in that the exhaust gas re-circulation gas flow rate decreases upon installing sensors for measuring the exhaust gas re-circulation gas flow rate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In order to attain the above-described object, according to an aspect of the invention, there is provided a measuring equipment for flow of exhaust gas re-circulation including: a control valve which is provided in an exhaust gas re-circulation passage of an internal combustion engine so as to control a flow rate in the exhaust gas re-circulation passage; a heat exchanger which cools exhaust gas re-circulation gas; pressure sensors which measure pressures of the exhaust gas re-circulation gas at two or more positions of the exhaust gas re-circulation passage before and after the heat exchanger; a temperature sensor which measures temperature of the exhaust gas re-circulation gas; and an intake air flow sensor which is provided in an intake air passage so as to measure a flow rate of intake air. The measuring equipment further includes: a first exhaust gas re-circulation gas flow measuring unit which calculates a first exhaust gas re-circulation gas flow rate on the basis of a phase-difference time of a pressure waveform at two or more positions measured by the pressure sensors, a distance of the exhaust gas re-circulation passage between two or more different pressure measurement positions and a sectional area of the exhaust gas re-circulation passage of the heat exchanger. According to the invention, it is possible to carry out the flow rate measurement in a short response time without additionally providing a pressure loss source. 
         [0005]    According to another aspect of the invention, the measuring equipment further includes: a second exhaust gas re-circulation gas flow measuring unit which calculates a second exhaust gas re-circulation gas flow rate on the basis of a difference between pressure values measured at two or more different measurement positions by the pressure sensors and the sectional area of the exhaust gas re-circulation passage of the heat exchanger. According to the invention, it is possible to simultaneously measure the exhaust gas re-circulation gas flow rate by means of a plurality of measurement methods and to determine whether the correction is necessary by comparing the measurement values. 
         [0006]    According to another aspect of the invention, the measuring equipment further includes: a third exhaust gas re-circulation gas flow measuring unit which calculates a third exhaust gas re-circulation gas flow rate on the basis of a difference between the intake air flow rate measured by the intake air flow sensor and a predetermined value for each operation state of the internal combustion engine. According to the invention, it is possible to simultaneously measure the exhaust gas re-circulation gas flow rate by means of a plurality of measurement methods and to determine whether the correction is necessary by comparing the measurement values. When the correction is necessary, it is possible to estimate a damage state of the heat exchanger by back calculating a sectional area value of the exhaust gas re-circulation passage of the heat exchanger on the basis of the flow rate. 
         [0007]    According to another aspect of the invention, the measuring equipment further includes: a calculation unit which calculates a quotient of a pressure difference between pressure values measured at two or more measurement positions by the pressure sensors and an amplitude of the pressure difference, and a difference between the quotient and a predetermined value. According to the invention, it is possible to estimate the measurement method having the highest precision by comparing the above calculation result with a previously researched relationship between measurement errors in the respective measurement methods and the difference. 
         [0008]    According to the invention, it is possible to measure the exhaust gas re-circulation gas flow rate with high precision in a short response time, and to accurately measure the exhaust gas re-circulation gas flow rate even when the internal combustion engine is transitionally operated. Accordingly, it is possible to high-precisely set the internal combustion engine&#39;s output performances such as fuel efficiency, nitrogen oxide, soot and noise in such a manner that a comparison result between the exhaust gas re-circulation gas flow rate measurement value and the exhaust gas re-circulation gas flow rate target value is reflected in an opening degree of the exhaust gas re-circulation gas flow control valve. 
         [0009]    Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWINGS 
         [0010]      FIG. 1  is an explanatory diagram illustrating a position setting and a connection configuration of units necessary for a measurement related to the invention. 
           [0011]      FIG. 2  is an explanatory diagram illustrating an example in which another sensor is attached to perform the measurement related to the invention (Embodiment 1). 
           [0012]      FIG. 3  is an explanatory diagram illustrating a method for performing the measurement related to the invention (Embodiment 1). 
           [0013]      FIG. 4  is an explanatory diagram illustrating the method for performing the measurement related to the invention (Embodiment 1). 
           [0014]      FIG. 5  is an explanatory diagram illustrating the method for performing the measurement related to the invention (Embodiment 2). 
           [0015]      FIG. 6  is an explanatory diagram illustrating the method for performing the measurement related to the invention (Embodiment 2). 
           [0016]      FIG. 7  is a flowchart illustrating a method for performing the measurement related to the invention (Embodiment 3). 
           [0017]      FIG. 8  is a flowchart illustrating another method for performing the measurement related to the invention (Embodiment 4). 
           [0018]      FIG. 9  is a flowchart illustrating another method for performing the measurement related to the invention (Embodiment 5). 
           [0019]      FIG. 10  is a flowchart illustrating another method for performing the measurement related to the invention (Embodiment 6). 
           [0020]      FIG. 11  is an explanatory diagram illustrating the method for performing the measurement related to the invention (Embodiment 7). 
           [0021]      FIG. 12  is an explanatory diagram illustrating the method for performing the measurement related to the invention (Embodiment 8). 
           [0022]      FIG. 13  is a diagram illustrating a relationship between a correlation coefficient and a pulsation amplitude ratio. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Hereinafter, Embodiments according to the invention will be described with reference to the accompanying drawings. 
         [0024]      FIG. 1  is a configuration diagram illustrating a control device for an engine according to Embodiment 1 of the invention. Reference Numeral  19  shown in  FIG. 1  denotes an engine. An air cleaner  17 , an air flow sensor  2 , a compressor  6 ( b ) of a supercharger, an intercooler  16 , a throttle  13  for adjusting an intake air flow rate, an intake pipe  20 , and a fuel injection valve (hereinafter, an injector)  5  are arranged on the upstream side of the engine  19 . In this embodiment, an intake air flow controlling unit includes the compressor  6 ( b ), the intercooler  16  and the throttle  13 , and an intake air flow detecting unit includes the air flow sensor  2 . The injector  5  is configured to directly inject fuel into a combustion chamber  18 . It is desirable that the throttle  13  is an electronic control throttle which drives a throttle valve by an electric actuator. In this embodiment, an intake pressure sensor  14  is disposed in the intake pipe  20  so as to appropriately control an intake amount by detecting a pressure within the intake pipe  20 . An exhaust pressure/exhaust temperature sensor  3  is disposed in an exhaust pipe  23 . An exhaust gas re-circulation passage  9  for re-circulating the exhaust gas to the intake pipe  20 , an exhaust gas re-circulation gas heat exchanger  10 , and an exhaust gas re-circulation gas flow control valve  11  are installed in the exhaust pipe  23 . In addition, the invention is characterized in that an exhaust gas re-circulation gas pressure/temperature sensor  12  is disposed in the exhaust gas re-circulation passage  9  so as to detect a pressure and a temperature of the exhaust gas re-circulation gas. In this embodiment, the exhaust gas re-circulation gas pressure/temperature sensor  12 , the exhaust pressure/exhaust temperature sensor  3 , and the air flow sensor  2  are used to measure the exhaust gas re-circulation gas flow rate. The injector  5  injects a predetermined amount of fuel in accordance with target engine torque calculated by an opening degree signal α output from an accelerator opening degree sensor  1 , and appropriately corrects the fuel amount in accordance with outputs such as an opening degree signal θtp output from the throttle  13 , an opening degree signal θegr output from the exhaust gas re-circulation gas flow control valve  11 , and a supercharge pressure Ptin output from the compressor  6 ( b ). Reference Numeral  8  denotes an engine control unit (hereinafter, ECU) which determines a combustion mode or a control amount of the engine  19  in accordance with a user request such as an accelerator opening degree α or a brake state, a vehicle state such as a vehicle speed, and an engine driving condition such as a temperature of an engine cooling water or a temperature of exhaust gas. 
       EMBODIMENT 1 
       [0025]      FIGS. 2 to 4  illustrate an exemplary measurement method among the measurement methods according to the invention. 
         [0026]    The drawings show the arrangement positions of the exhaust gas re-circulation gas heat exchanger  10  for cooling the exhaust gas re-circulation gas, exhaust pressure/exhaust temperature sensors  3 ,  3 ′,  3 ″, and the exhaust gas re-circulation gas pressure/temperature sensors  12 ,  12 ′,  12 ″ which are arranged at two or more positions in the passage before and after the exhaust gas re-circulation gas heat exchanger  10  so as to measure the pressure and the temperature of the exhaust gas re-circulation gas. 
         [0027]    Since the measurement values of the temperature and the pressure are used to calculate a sonic speed or a gas density, the measurement needs to be carried out in a space having the same condition. Thus, static pressure measurement portions  3 ( a ),  3 ′(a),  3 ″(a),  12 ( a ),  12 ′(a),  12 ″(a), positive-flow-direction dynamic pressure measurement portions  3 ′( b ),  3 ″( b ),  12 ′( b ),  12 ″( b ), and negative-flow-direction dynamic pressure measurement portions  3 ″(c),  12 ″(c) among pressure measurement portions and temperature measurement portions  3 ( t ),  3 ′( t ),  3 ″( t ),  12 ( t ),  12 ′( t ),  12 ″( t ) need to be respectively approximated to each other. 
         [0028]    The measurement positions of the temperature measurement portions  3 ( t ),  3 ′( t ),  3 ″( t ),  12 ( t ),  12 ′( t ),  12 ″( t ) are set to the center of a passage section so as to measure an average temperature of gas in space and the temperature measurement portions  3 ′( t ),  3 ″( t ),  12 ( t ),  12 ′( t ),  12 ″( t ) are installed at two positions before and after the heat exchanger  10 . An average value in space of the exhaust gas re-circulation gas as a measurement object within the entire passage is obtained in such a manner that a sum of the temperatures measured at two positions is obtained and a quotient is obtained by dividing the sum by two, thereby reducing an error of a measured temperature due to a heat loss by the heat exchanger  10 . The pressure measurement is carried out for the purpose of a gas density calculation, a gas flow direction determination, or a pressure loss detection and a pressure propagation detection of the heat exchanger  10 . 
         [0029]    For this reason, in the minimum configuration shown in  FIG. 2 , the pressure measurement is carried out by the static pressure measurement portions  3 ( a ),  12 ( a ) among the exhaust pressure/exhaust temperature sensor  3  and the exhaust gas re-circulation gas pressure/temperature sensor  12 , and the above-described purpose is attained by measuring only static pressure values before and after the heat exchanger  10 . 
         [0030]      FIG. 3  shows a case that only the pressure propagation detection is carried out by the positive-flow-direction dynamic pressure measurement portions  3 ′( b ),  12 ′( b ) among the exhaust pressure/exhaust temperature sensor  3 ′ and the exhaust gas re-circulation gas pressure/temperature sensor  12 ′ installed before and after the heat exchanger  10 . Since pressure measurement holes of the dynamic pressure measurement portions face the gas flow direction, speed energy is changed into pressure energy in the pressure measurement holes, and thus it is possible to obtain more accurate amplitude signals. Accordingly, it is possible to improve precision of a first exhaust gas re-circulation gas flow rate which is measured by the use of a phase-difference time of a pressure waveform, described in detail later. In addition, in a case that a check valve is attached instead of the exhaust gas re-circulation gas flow control valve  11  or a case that the exhaust gas re-circulation gas flow control valve  11  is closed when it is determined as a negative flow condition on the basis of a difference between static pressure values measured before and after the heat exchanger  10  by the exhaust pressure/exhaust temperature sensor  31  and the exhaust gas re-circulation gas pressure/temperature sensor  12 ′, the negative-flow-direction flow rate measurement is not necessary, and thus, the dynamic pressure measurement is carried out only in an one-way direction. In this case, the intake pressure and the exhaust pressure are simultaneously measured by the exhaust gas/exhaust temperature sensor  3  and the intake pressure sensor  14 , and a control for re-opening the exhaust gas re-circulation gas flow control valve  11  is carried out at the time point when the positive-flow-direction condition is determined. 
         [0031]      FIG. 4  shows a case that the flow rate can be measured in the negative flow direction, where only the pressure propagation detection is carried out by the positive-flow-direction dynamic pressure measurement portions  3 ″( b ),  12 ″( b ) and the negative-flow-direction dynamic pressure measurement portions  3 ″( c ),  12 ″( c ) among the exhaust pressure/exhaust temperature sensor  3 ″ and the exhaust gas re-circulation gas pressure/temperature sensor  12 ″ which are installed before and after the heat exchanger  10 . When it is determined as the positive-flow-condition on the basis of a difference between static pressure values measured before and after the heat exchanger  10  by the exhaust pressure/exhaust temperature sensor  3 ″ and the exhaust gas re-circulation gas pressure/temperature sensor  12 ″, the pressure propagation detection is carried out by the measurement pressure value obtained by the positive-flow-direction dynamic pressure measurement portions  3 ″( b ),  12 ″( b ). In addition, when it is determined as the negative-flow-condition on the basis of a difference between static pressure values measured before and after the heat exchanger  10  by the exhaust pressure/exhaust temperature sensor  3 ″ and the exhaust gas re-circulation gas pressure/temperature sensor  12 ″, the pressure propagation detection is carried out by the measurement pressure value obtained by the positive-flow-direction dynamic pressure measurement portions  3 ″( c ),  12 ″( c ), thereby performing the high-precise flow rate measurement of the exhaust gas re-circulation gas even in the negative flow condition. 
       EMBODIMENT 2 
       [0032]      FIGS. 5 and 6  show an exemplary flow rate calculation of the exhaust gas re-circulation gas in the measurement methods according to the invention. In  FIG. 5 , a phase time difference is set to a time difference when the pressure waveforms measured at different positions are compared with each other. Here, a speed at which pressure propagates in gas having a gas flow corresponds to a value obtained by adding a sonic speed to a gas flow speed. Accordingly, it is possible to obtain the gas flow rate by using the following expression: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    wherein 
         [0033]    Q EGR [kg/h]: exhaust gas re-circulation gas mass flow rate 
         [0034]    L[m]: distance in an exhaust gas re-circulation passage between two or more different pressure measurement positions 
         [0035]    dt[sec]: phase-difference time 
         [0036]    k[−]: specific heat ratio 
         [0037]    R[J/kg·K]: gas constant 
         [0038]    T 1 [K]: temperature of exhaust gas re-circulation gas on the upstream side of the exhaust gas re-circulation gas heat exchanger 
         [0039]    T 2 [K]: temperature of exhaust gas re-circulation gas on the downstream side of the exhaust gas re-circulation gas heat exchanger 
         [0040]    A[m 2 ]: sectional area of the exhaust gas re-circulation passage 
         [0041]    p 1 [Pa]: pressure of exhaust gas re-circulation gas on the upstream side of the exhaust gas re-circulation gas heat exchanger 
         [0042]    p 2 [Pa]: pressure of exhaust gas re-circulation gas on the downstream side of the exhaust gas re-circulation gas heat exchanger. 
         [0043]      FIG. 6  shows a relationship between the phase-difference time and the flow rate upon applying the formula 1. With the above-described method, it is possible to measure the exhaust gas re-circulation gas flow rate without using the differential pressure. Accordingly, it is possible to measure the exhaust gas re-circulation gas flow rate even when the differential pressure cannot be measured due to the low gas flow speed. Here, although parameters except for the gas pressure, the gas temperature and the phase-difference time are constant and are uniquely determined, actually an error occurs due to a drift, a turbulence, a gas component, humidity, and a heat exchange amount of the heat exchanger  10 . Accordingly, it is desirable that the correction is carried out by simultaneously using another measurement method. 
       EMBODIMENT 3 
       [0044]      FIG. 7  shows another exemplary flow rate calculation of the exhaust gas re-circulation gas in the measurement methods according to the invention. Since the flow speed is proportional to a value obtained by a square root of the pressure difference between two positions, it is possible to obtain the gas flow rate by using the following expression. 
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         [0045]    Here, Δp[Pa]: pressure difference of exhaust gas re-circulation gases before and after exhaust gas re-circulation gas heat exchanger (=p 1 −p 2 ). 
         [0046]      FIG. 7  shows a relationship between the differential pressure and the flow rate upon applying the formula 2. With the above-described method, it is possible to measure the exhaust gas re-circulation gas flow rate without using the pulsation component of the pressure waveform. Accordingly, it is possible to measure the exhaust gas re-circulation gas flow rate even when the pulsation component of the pressure waveform cannot be detected because a pressure loss is large between the pulsation component generation source and the pressure measurement position. Here, although parameters except for the gas pressure, the gas temperature, and the differential pressure are constant and are uniquely determined, actually an error occurs due to a drift, a turbulence, a gas component, humidity, and a heat exchange amount of the heat exchanger  10 . Accordingly, it is desirable that the correction is carried out by simultaneously using another measurement method. 
       EMBODIMENT 4 
       [0047]      FIG. 8  is a flowchart illustrating an exemplary flow rate calculation procedure of the exhaust gas re-circulation gas in the measurement method according to the invention. 
         [0048]    The measurement method according to the invention is carried out by periodically repeating the measurement and the calculation. 
         [0049]    First, the pressure value and the temperature value are obtained by the exhaust pressure/exhaust temperature sensor  3  and the exhaust gas re-circulation gas pressure/temperature sensor  12  which are installed before and after the heat exchanger  10  (Blocks  1001  and  1011 ). 
         [0050]    Subsequently, a sonic speed is calculated by comparing with a database stored in advance the pressure value measured at a certain time in Block  1001  and the temperature value measured at a certain time in Block  1011  (Block  1012 ). 
         [0051]    Subsequently, a pulsation frequency is detected by recognizing an interval (N shown in  FIG. 5 ) between peaks of the pressure waveform obtained by a history of the pressure value for a predetermined time obtained in Block  1001  (Block  1002 ). 
         [0052]    Subsequently, a band-pass frequency is set to a value obtained by adding the pulsation frequency detected in Block  1002  to a predetermined constant in terms of an experiment, and unnecessary noise components or undulation components are removed by performing a band-pass filter process of the frequency to the pressure waveform obtained by the history of the pressure value for the predetermined time (Block  1003 ). 
         [0053]    Subsequently, a history of the measurement pressure value necessary for a phase comparison of the pressure waveform is extracted. An extraction period is calculated from a quotient of a predetermined cycle and the pulsation frequency detected in Block  1002 , and a history of the measurement pressure value centering around a certain time is extracted (Block  1004 ). 
         [0054]    Subsequently, a pressure attenuation correction is carried out. The pressure waveform on the downstream side of the heat exchanger  10  obtained by the history of the pressure value extracted in Block  1003  is attenuated by the pressure loss of the heat exchanger  10 . For this reason, for instance, when comparing the phase of the peak of the pressure waveform on the upstream side of the heat exchanger  10  with that of the peak of the pressure waveform on the downstream side of the heat exchanger  10 , it is supposed that precision of the phase comparison deteriorates if the pressure values at both peaks are different from each other. Accordingly, a ratio between the amplitude before the attenuation (a shown in  FIG. 5 ) and the amplitude after the attenuation (b shown in  FIG. 5 ) is set to an attenuation ratio (a/b), and the pressure attenuation correction is carried out by multiplying the pressure value after the attenuation by the attenuation ratio so that the maximum value and the minimum value of both pressure values for a history are identical with each other. As a result, it is possible to reduce deterioration of precision of the phase comparison due to the attenuation (Block  1005 ). 
         [0055]    Subsequently, a phase-difference time (dt 1  shown in  FIG. 5 ) is detected. A time difference between a certain time (t 1  shown in  FIG. 5 ) and a time at which the pressure value on the upstream side of the heat exchanger  10  is identical with the pressure value subjected to the attenuation correction on the downstream side of the heat exchanger  10  at the above certain time is recognized. When there are a plurality of the pressure values having the same value, the phase-difference time is set to a time difference of which an absolute value is the smallest (Block  1006 ). 
         [0056]    Subsequently, a passage length value setting is carried out. This corresponds to the gas passage length (L shown in  FIG. 5 ) between the pressure sensor attachment positions (Block  1013 ). 
         [0057]    Subsequently, a pressure propagation speed calculation is carried out. A quotient is obtained from a quotient of the phase-difference time obtained in Block  1006  and the passage length set in Block  1013  (Block  1007 ). 
         [0058]    Subsequently, a flow speed calculation is carried out. A difference is obtained between the pressure propagation speed obtained in Block  1007  and the sonic speed obtained in Block  1012  (Block  1008 ). 
         [0059]    Subsequently, a passage sectional area setting is carried out. This corresponds to a gas passage sectional area in the heat exchanger  10  (Block  1014 ). 
         [0060]    Subsequently, a volume flow rate calculation is carried out. A value is obtained by multiplying the flow speed obtained in Block  1008  by the passage sectional area obtained in Block  1014  (Block  1009 ). 
         [0061]    Subsequently, a mass flow rate calculation is carried out. Density of the exhaust gas re-circulation gas is obtained by a gas constant calculated from the pressure value at a certain time obtained in Block  1001 , the gas temperature at a certain time obtained in Block  1011 , the component of standard exhaust gas measured in advance by the experiment, and the humidity of the standard exhaust gas measured in advance by the experiment, and is multiplied by the volume flow rate obtained in Block  1009  (Block  1010 ). 
         [0062]    In this way, it is possible to calculate the first exhaust gas re-circulation gas flow rate. 
       EMBODIMENT 5 
       [0063]      FIG. 9  is a flowchart illustrating an exemplary flow rate calculation procedure of the exhaust gas re-circulation gas in the measurement method according to the invention. 
         [0064]    The measurement method according to the invention is carried out by periodically repeating the measurement and the calculation. 
         [0065]    First, the pressure value and the temperature value are obtained by the exhaust pressure/exhaust temperature sensor  3  and the exhaust gas re-circulation gas pressure/temperature sensor  12  which are installed before and after the heat exchanger  10  (Blocks  1101  and  1108 ). 
         [0066]    Subsequently, a pulsation frequency is detected by recognizing an interval (N shown in  FIG. 5 ) between peaks of the pressure waveform obtained by a history of the pressure value for a predetermined time obtained in Block  1101  (Block  1102 ). 
         [0067]    Subsequently, a cut-off frequency of the filter is set to a value obtained by multiplying the pulsation frequency detected in Block  1102  by a predetermined constant in terms of an experiment, and unnecessary noise components are removed by performing a low-pass filter process to the pressure waveform obtained by a history of the pressure value for the predetermined time (Block  1103 ). 
         [0068]    Subsequently, a differential pressure is set to a pressure difference at a certain time among history points of the pressure waveform obtained in Block  1103  (Block  1104 ). 
         [0069]    Subsequently, a flow speed is obtained from the pressure value measured at a certain time in Block  1101  and the temperature value measured at a certain time in Block  1108  and the differential pressure at a certain time obtained in Block  1104  (Block  1105 ). 
         [0070]    Subsequently, a passage sectional area setting is carried out. This corresponds to a gas passage sectional area in the heat exchanger  10  (Block  1109 ). 
         [0071]    Subsequently, a volume flow rate calculation is carried out. A value is obtained by multiplying the flow speed obtained in Block  1105  by the passage sectional area obtained in Block  1109  (Block  1106 ). 
         [0072]    Subsequently, a mass flow rate calculation is carried out. Density of the exhaust gas re-circulation gas is obtained by a gas constant calculated from the pressure value at a certain time obtained in Block  1101 , the gas temperature at a certain time obtained in Block  1108 , the component of standard exhaust gas measured in advance by the experiment, and the humidity of the standard exhaust gas measured in advance by the experiment, and is multiplied by the volume flow rate obtained in Block  1106  (Block  1107 ). 
         [0073]    In this way, it is possible to calculate the second exhaust gas re-circulation gas flow rate. 
       EMBODIMENT 6 
       [0074]      FIG. 10  is a flowchart illustrating an exemplary flow rate calculation procedure of the exhaust gas re-circulation gas in the measurement method according to the invention. 
         [0075]    The measurement method according to the invention is carried out by periodically repeating the measurement and the calculation. 
         [0076]    First, the pressure value and the temperature value are obtained by the exhaust pressure/exhaust temperature sensor  3  and the exhaust gas re-circulation gas pressure/temperature sensor  12  which are installed before and after the heat exchanger  10  (Blocks  1201  and  1212 ). 
         [0077]    Subsequently, a sonic speed is calculated by comparing the pressure value measured at a certain time in Block  1201  and the temperature value measured at a certain time in Block  1212  with a database stored in advance (Block  1213 ). 
         [0078]    Subsequently, a pulsation frequency is detected by recognizing an interval (N shown in  FIG. 5 ) between peaks of the pressure waveform obtained by a history of the pressure value for the predetermined time obtained in Block  1201  (Block  1202 ). 
         [0079]    Subsequently, a cut-off frequency of the filter is set to a value obtained by multiplying the pulsation frequency detected in Block  1202  to a predetermined constant in terms of an experiment, and unnecessary noise components are removed by performing a low-pass filter process of the frequency to the pressure waveform obtained by a history of the pressure value for the predetermined time (Block  1203 ). 
         [0080]    Subsequently, a history of the measurement pressure value necessary for a phase comparison of the pressure waveform is extracted. An extraction period is calculated on the basis of the pulsation frequency detected in Block  1202  and a predetermined cycle, and a history of the measurement pressure value centering around a certain time is extracted (Block  1204 ). 
         [0081]    Subsequently, a pressure attenuation correction is carried out. The pressure waveform on the downstream side of the heat exchanger  10  obtained by the history of the pressure value extracted in Block  1204  is attenuated by the pressure loss of the heat exchanger  10 . For this reason, for instance, when comparing the phase of the peak of the pressure waveform on the upstream side of the heat exchanger  10  with that of the peak of the pressure waveform on the downstream side of the heat exchanger  10 , it is supposed that precision of the phase comparison deteriorates if the pressure values at both peaks are different from each other. Accordingly, a ratio between the amplitude before the attenuation (a shown in  FIG. 5 ) and the amplitude after the attenuation (b shown in  FIG. 5 ) is set to an attenuation ratio (a/b), and the pressure attenuation correction is carried out by multiplying the pressure value after the attenuation by the attenuation ratio so that the maximum value and the minimum value in the history of both pressure values are identical with each other. As a result, it is possible to reduce deterioration of precision of the phase comparison due to the attenuation (Block  1205 ). 
         [0082]    Subsequently, a phase-difference time (dt 1  shown in  FIG. 5 ) is detected. A time difference between a certain time (t 1  shown in  FIG. 5 ) and a time at which the pressure value on the upstream side of the heat exchanger  10  is identical with the pressure value subjected to the attenuation correction on the downstream side of the heat exchanger  10  at the above certain time is recognized. When there are a plurality of the time differences, the phase-difference time is set to a time difference of which an absolute value is the smallest (Block  1206 ). 
         [0083]    Subsequently, a phase correction of the pressure waveform is carried out. In order to correct the phase difference (dt shown in  FIG. 5 ) caused when the pressure propagates in the heat exchanger  10 , the phase-difference time obtained in Block  1206  is added to an upstream pressure measurement time (Block  1207 ). 
         [0084]    Subsequently, a differential pressure is set to a pressure difference at a certain time among history points of the pressure waveform obtained in Block  1207  (Block  1208 ). 
         [0085]    Subsequently, a flow speed is obtained from the pressure value measured at a certain time in Block  1201 , the temperature value measured at a certain time in Block  1212  and the differential pressure at a certain time obtained in Block  1208  (Block  1209 ). 
         [0086]    Subsequently, a passage sectional area setting is carried out. This corresponds to a gas passage sectional area in the heat exchanger  10  (Block  1214 ). 
         [0087]    Subsequently, a volume flow rate calculation is carried out. A value is obtained by multiplying the flow speed obtained in Block  1209  by the passage sectional area obtained in Block  1214  (Block  1210 ). 
         [0088]    Subsequently, a mass flow rate calculation is carried out. Density of the exhaust gas re-circulation gas is obtained by a gas constant calculated from the pressure value at a certain time obtained in Block  1201 , the gas temperature at a certain time obtained in Block  1212 , the component of standard exhaust gas measured in advance by the experiment, and a gas constant calculated from the humidity of the standard exhaust gas measured in advance by the experiment, and is multiplied by the volume flow rate obtained in Block  1210  (Block  1211 ). 
         [0089]    In this way, it is possible to calculate the third exhaust gas re-circulation gas flow rate. 
       EMBODIMENT 7 
       [0090]      FIG. 11  is a flowchart illustrating another exemplary flow rate calculation procedure of the exhaust gas re-circulation gas in the measurement method according to the invention. 
         [0091]    The measurement method according to the invention is carried out by periodically repeating the measurement and the calculation and by simultaneously calculating the first exhaust gas re-circulation gas flow rate, the second exhaust gas re-circulation gas flow rate, and the third exhaust gas re-circulation gas flow rate by the use of the measurement values. 
         [0092]    Here, the third exhaust gas re-circulation gas flow rate is obtained as follows. First, an intake air flow rate  24  experimentally measured is stored in advance for each engine rpm as well as the intake pressure value measured by the intake pressure sensor  14  while the exhaust gas re-circulation gas flow control valve  11  shown in  FIG. 1  is closed. Subsequently, a difference is obtained between the stored intake air flow rate  24  and an intake air flow rate  24 ′ measured when the exhaust gas re-circulation gas flow control valve  11  is opened, so as to obtain the third exhaust gas re-circulation gas flow rate by using the above difference. 
         [0093]    Validity of the third exhaust gas re-circulation gas flow rate is determined by a comparison of correlation data of the engine rpm, the intake air flow rate, and the opening degree of the exhaust gas re-circulation gas flow control valve  11 , which are obtained in advance by checking operations. When a large error occurs, it is recognized that the driving state of the engine  19  or the measurement equipment is abnormal. 
         [0094]    First, the upstream pressure value of the heat exchanger  10  is compared with the downstream pressure value thereof (Block  1301 ). 
         [0095]    Subsequently, when the comparison result obtained in Block  1301  satisfies the positive flow condition, the exhaust gas re-circulation gas flow control valve  11  is opened (Block  1302 ). 
         [0096]    Subsequently, when the comparison result obtained in Block  1301  satisfies the negative flow conciliation, the exhaust gas re-circulation gas flow control valve  11  is closed (Block  1303 ). 
         [0097]    Subsequently, when the exhaust gas re-circulation gas flow control valve  11  is opened in accordance with the determination in Block  1302 , the first exhaust gas re-circulation gas flow rate is compared with the third exhaust gas re-circulation gas flow rate (Block  1304 ). 
         [0098]    Subsequently, when a difference between the first exhaust gas re-circulation gas flow rate and the third exhaust gas re-circulation gas flow rate is not less than a predetermined value (for instance, ±5% relative to the third exhaust gas re-circulation gas flow rate) in accordance with the determination in Block  1304 , the exhaust gas re-circulation passage sectional area value used to calculate the first exhaust gas re-circulation gas flow rate is corrected so that both flow rates are identical with each other (Block  1305 ). 
         [0099]    Subsequently, when the exhaust gas re-circulation gas flow control valve  11  is opened in accordance with the determination in Block  1302 , the second exhaust gas re-circulation gas flow rate is compared with the third exhaust gas re-circulation gas flow rate (Block  1306 ). 
         [0100]    Subsequently, when a difference between the second exhaust gas re-circulation gas flow rate and the third exhaust gas re-circulation gas flow rate is not less than a predetermined value (for instance, ±5% relative to the third exhaust gas re-circulation gas flow rate) in accordance with the determination in Block  1306 , the exhaust gas re-circulation passage sectional area value used to calculate the second exhaust gas re-circulation gas flow rate is corrected so that both flow rates are identical with each other (Block  1307 ). 
         [0101]    Subsequently, when the exhaust gas re-circulation gas flow control valve  11  is closed in accordance with the determination in Block  1303 , the second exhaust gas re-circulation gas flow rate is compared with a predetermined value (Block  1308 ). 
         [0102]    Subsequently, when it is determined that the second exhaust gas re-circulation gas flow rate is larger than the predetermined value in accordance with the determination in Block  1308 , it is recognized that the exhaust gas re-circulation gas flow control valve  11  has a functional trouble (Block  1309 ). 
         [0103]    Subsequently, when it is determined that the second exhaust gas re-circulation gas flow rate is the predetermined value or less in accordance with the determination in Block  1308 , the first exhaust gas re-circulation gas flow rate is compared with the predetermined value (Block  1310 ). 
         [0104]    Subsequently, when it is determined that the first exhaust gas re-circulation gas flow rate is larger than the predetermined value in accordance with the determination in Block  1310 , the exhaust gas re-circulation passage distance value between two or more different pressure measurement positions used to calculate the first exhaust gas re-circulation gas flow rate is corrected so that the first exhaust gas re-circulation gas flow rate becomes 0 (Block  1311 ). 
         [0105]    Subsequently, a quotient is obtained from a difference between static pressures on the upstream side and the downstream side of the heat exchanger  10  at a certain time and the amplitude of the static pressure difference, and a difference is obtained between the quotient and a predetermined value (Block  1312 ). 
         [0106]    Subsequently, when the value obtained in Block  1312  is not less than 0 or when it is recognized that the exhaust gas re-circulation gas flow control valve  11  has the functional trouble in Block  1309 , the first exhaust gas re-circulation gas flow rate is set to a calculation result (Block  1313 ). 
         [0107]    Subsequently, when the value obtained in Block  1312  is smaller than 0, the second exhaust gas re-circulation gas flow rate is set to a calculation result (Block  1314 ). 
       EMBODIMENT 8 
       [0108]      FIG. 12  is a flowchart illustrating another exemplary flow rate calculation procedure of the exhaust gas re-circulation gas in the measurement method according to the invention. 
         [0109]    The measurement method according to the invention is carried out by storing the calculation result while periodically repeating the measurement and the calculation. 
         [0110]    First, the first exhaust gas re-circulation gas flow rate, the second exhaust gas re-circulation gas flow rate, and the third exhaust gas re-circulation gas flow rate are calculated, and then the calculation results are stored (Blocks  1401  to  1403 ). 
         [0111]    Subsequently, a quotient is obtained from a difference between static pressures measured at two or more positions at a certain time by the pressure measuring portions of the exhaust pressure/exhaust temperature sensor  3  and the exhaust gas re-circulation gas pressure/temperature sensor  12  and the amplitude of the static pressure difference, and then the quotient is stored as the pulsation amplitude ratio (Block  1404 ). 
         [0112]    Subsequently, calculation results for a predetermined number of times are extracted from the calculation results stored in Block  1404 , an average value of the extracted calculation results is calculated, and then the calculation result is stored as an average value of the pulsation amplitude ratio (Block  1407 ). 
         [0113]    Subsequently, calculation results for a predetermined number of times are extracted from the calculation results stored in Blocks  1401  and  1402 , a correlation efficient is calculated by a least square method using the values of the first exhaust gas re-circulation gas flow rate obtained in Block  1401  and the third exhaust gas re-circulation gas flow rate obtained in Block  1402 , and then the calculation result is stored as a correlation coefficient  1  (Block  1405 ). 
         [0114]    Subsequently, calculation results for a predetermined number of times are extracted from the calculation results stored in Blocks  1402  and  1404 , a correlation efficient is calculated by the least square method using the values of the second exhaust gas re-circulation gas flow rate obtained in Block  1403  and the third exhaust gas re-circulation gas flow rate obtained in Block  1402 , and then the calculation result is stored as a correlation coefficient  2  (Block  1406 ). 
         [0115]    Subsequently, calculation results for a predetermined number of times are extracted from the calculation results stored in Blocks  1405  to  1407 , and then an approximation line (a) (see  FIG. 13 ) is obtained by the least square method using the values of the average value of the pulsation amplitude ratio obtained in Block  1407  and the correlation coefficient  1  obtained in Block  1405 . At the same time, an approximation line (b) (see  FIG. 13 ) is obtained by the least square method using the values of the average value of the pulsation amplitude ratio obtained in Block  1407  and the correlation coefficient  2  obtained in Block  1406 . Subsequently, a pulsation amplitude ratio is obtained at a point where the approximation line (a) intersects the approximation line (b), and then the pulsation amplitude ratio is set to a selection determination value (Block  1408 ). 
         [0116]    Subsequently, it is determined whether the pulsation amplitude ratio is larger than the selection determination value by comparing the selection determination value obtained in Block  1408  with the pulsation amplitude ratio at a certain time obtained in Block  1404 . Subsequently, depending on the determined result, the output value is changed into the first exhaust gas re-circulation gas flow rate obtained in Block  1401  and the second exhaust gas re-circulation gas flow rate obtained in Block  1403  (Block  1409 ). 
       EMBODIMENT 9 
       [0117]      FIG. 13  is an exemplary method for selecting a high-precise measurement method for flow rate of exhaust gas re-circulation with respect to the pulsation amplitude ratio in Block  1408  of Embodiment 8. 
         [0118]    It is supposed that the measurement precision of the first exhaust gas re-circulation gas flow rate and the second exhaust gas re-circulation gas flow rate is concerned with the pulsation amplitude ratio obtained from the quotient of the difference between the static pressures on the upstream side and the downstream side of the heat exchanger  10  and the amplitude of the static pressure difference. For this reason, it is possible to select the high-precise measurement method by obtaining the relative correlation coefficients between the first and third exhaust gas re-circulation gas flow rates, and between the second and third exhaust gas re-circulation gas flow rates. In the example shown in  FIG. 13 , the selection determination value is set to a condition capable of measuring a pulsation amplitude ratio  100  in which both approximation lines intersect with each other, and thus the measurement method having high correlation coefficient is selected. This is obtained in advance in terms of an experiment, and is used as an initial value of the selection determination value. Since the optimal value of the selection determination value varies in accordance with a soiled and damaged state of the passage in the heat exchanger  10 , the selection determination value is examined and corrected while a correlation estimation is carried out whenever measuring the exhaust gas re-circulation gas flow rate after starting the engine  19 , thereby reducing deterioration in the measurement precision of the exhaust gas re-circulation gas flow rate. 
         [0119]    The present invention is not limited to the application of the internal combustion engine, but may be applied to a high-precise pulsation flow meter as an industrial product in other industrial fields. 
         [0120]    It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.