Method for obtaining exhaust gas flow quantity, exhaust gas flow measurement apparatus, and exhaust gas recirculation control system

The present application claims priority from Japanese application serial no. 2004-285649, filed on Sep. 30, 2004, the contents of which are hereby incorporated by references into this application.

The present invention relates to an EGR, that is, an exhaust gas recirculation control system for properly re-circulating exhaust gases of an internal combustion engine to an air intake system, a method for obtaining flow quantity of the exhaust gases, and an exhaust gas flow measurement apparatus.

In recent years, an exhaust gas recirculation (EGR) for realizing reduction in NOx by re-circulating part of the exhaust to an air intake system is being widely used. A recirculation quantity (EGR amount) has to be controlled with high accuracy so as to also achieve stability of combustion. In particular, in the case of applying the EGR to a diesel engine, when the EGR becomes excessive, a large quantity of smoke is produced. Consequently, high-accuracy EGR control according to the operating conditions of the engine is necessary.

One of conventional EGR controls is disclosed in Japanese publication No. 2003-516496. The conventional technique is comprised of an intake air mass flowmeter for measuring an intake quantity of air taken into an intake passage, and a recirculation exhaust gas mass flowmeter for measuring an EGR gas quantity provided in an exhaust recirculation passage. The EGR control is performed on the basis of an EGR rate (=EGR gas quantity/(intake air flow quantity+EGR gas flow quantity)) from the intake air flow quantity and the EGR gas flow quantity measured by the respective flowmeters.

In such a conventional technique, high accuracy EGR control according to the operating conditions of the engine cannot be realized sufficiently. Because, in an internal combustion engine, although a quantity of oxygen contained in exhaust gases largely changes according to engine conditions, the conventional EGR system doesn't make special consideration in this respect. That is, in low or intermediate load, a large amount of oxygen is contained in the exhaust gases due to lean-burn operation mode with rich air in mixture, on the other hand, in high load, the oxygen in the exhaust gases decreases due to stoichiometric mixture ratio or rich-burn operation mode.

In the conventional technique, only information of an EGR gas quantity is obtained from the recirculation exhaust gas flowmeter but information of the oxygen in the exhaust gases cannot be obtained. Consequently, it is difficult to perform high-accuracy EGR control.

On the other hand, Japanese publication No. H9(1997)-126060 describes a conventional exhaust recirculation control method of performing EGR control in order to reduce smoke in accordance with changes in the absolute value of oxygen in the total intake air containing EGR gases and in order to reduce NOx in accordance with changes in the concentration of oxygen in the total intake air.

In the conventional technique, at the time of performing the exhaust gas recirculation control by adjusting the opening of the EGR valve in accordance with operating conditions of the engine, oxygen concentration in the total intake air including the EGR gases and the ratio between the quantity of oxygen in the total intake air and the fuel injection quantity (namely oxygen/fuel ratio) is obtained. The oxygen/fuel ratio and a smoke allowable limit value are compared with each other. When the oxygen/fuel ratio is low, the EGR valve is controlled so that the oxygen/fuel ratio becomes equal to the smoke allowable limit value. When the oxygen/fuel ratio is high, the EGR valve is controlled so that the oxygen concentration becomes equal to target oxygen concentration. By the control, an oxygen quantity necessary to maintain the target smoke level can be assured. While always suppressing the smoke exhaust quantity to be within the allowable value, NOx can be reduced maximally.

In this conventional EGR control system, in order to measure the oxygen concentration in the total intake air including the EGR gases and the oxygen quantity, the following techniques is adopted. On the downstream side (engine side) of the position of connection for the exhaust recirculation passage in the intake air passage of an internal combustion engine, a flow measurement means is provided for measuring the flow quantity of a total intake air including EGR gases. The flow measurement means is comprised of a pressure sensor for sensing intake pressure and an intake temperature sensor for measuring intake temperature. An O 2 sensor for measuring oxygen concentration in the total intake air is also provided on such a downstream side. The O 2 sensor senses the oxygen concentration by using an electromotive force of a solid electrolyte or the like.

The conventional technique constructed as described above has a problem such that actual O 2 concentration in the total intake air containing the EGR gas and O 2 concentration measured by the O 2 sensor are different from each other due to a chemical reaction delay in the O 2 sensor. Therefore, the control accuracy, particularly, control accuracy at the time of transition largely deteriorates. In case where the flowmeter for measuring the flow quantity of the total intake air is comprised of the intake pressure sensor and the intake temperature sensor, it is difficult to accurately measure mass flow of the total intake air. Because, although the ratio of gas components in the total intake air containing the EGR gas change according to engine operating conditions, the aforementioned flowsensor cannot sense such changes of the gas components.

An object of the present invention is to realize a method of obtaining a flow quantity of exhaust gases with high accuracy measurement and an exhaust gas flow quantity measurement apparatus, and to realize an exhaust gas recirculation control system having high response and capable of performing high-accuracy EGR control.

The object is achieved by the inventions described in the claims.

For example, the object is achieved the following method. A flow quantity of exhaust gases flowing from an exhaust side to an intake side of an internal combustion engine through a passage for exhaust gas recirculation, is measured by using a flowmeter provided in the passage. The flowmeter has a heating resistor controlled so as to have a predetermined temperature difference with respect to a temperature of the exhaust gases.

An oxygen concentration of the exhaust gases is predict or sensed, and the flow quantity of the exhaust gases flowing through the passage is obtained on the basis of a signal from the flowmeter and the oxygen concentration.

According to the invention, a method of obtaining a flow quantity of an exhaust gases with high accuracy, an exhaust gas flow quantity measurement apparatus, and an exhaust recirculation control system realizing excellent response and capable of performing EGR control with high accuracy can be realized.

FIG. 1 is a block diagram of an exhaust recirculation control system as an embodiment of the present invention.

FIG. 2 is a diagram illustrating functions in an exhaust recirculation control method.

FIG. 3 is a diagram illustrating prediction exhaust gas oxygen concentration.

FIG. 4 is a configuration diagram of a recirculation exhaust gas mass flowmeter 9 .

FIG. 5 shows results of calculation of an exhaust gas component.

FIG. 6 shows results of calculation of an output of the flowmeter 9 in the case of a different exhaust gas component.

FIG. 7 is an output-mass flow correction map.

FIG. 8 is an output-oxygen concentration-mass flow correction two-dimensional map.

FIG. 9 is a block diagram of an exhaust recirculation control system as a second embodiment.

FIG. 10 is a configuration diagram of a recirculation exhaust gas mass flowmeter 9 as the second embodiment.

FIG. 11 is a diagram illustrating functions in an exhaust recirculation control method as the second embodiment.

FIG. 12 is an output-oxygen concentration-exhaust gas temperature-mass flow correction three-dimensional map.

FIG. 13 is a block diagram of an exhaust recirculation control system as a third embodiment.

FIG. 14 is a diagram illustrating functions in an exhaust recirculation control method as the third embodiment.

Embodiments of the present invention will be described hereinafter with reference to the drawings.

FIG. 1 shows an exhaust recirculation control system as a first embodiment of the present invention, and the configuration will be described below. The exhaust recirculation control system mainly has an internal combustion engine 1 , an intake passage 4 in which an air cleaner 12 and a throttle valve 15 are disposed for feeding an intake air 2 to the internal combustion engine 1 , an exhaust passage 5 for discharging exhaust gases 3 via an exhaust gas catalyst 13 for purifying exhaust gases 3 , an EGR valve 7 provided at an exhaust gas recirculation passage 6 for circulating (re-circulating) the exhaust gases, an EGR cooler 10 for cooling the exhaust gases and for increasing cylinder intake efficiency, an intake air mass flowmeter 8 provided at the intake passage 4 for measuring an intake air mass flow quantity, a recirculation exhaust gas mass flowmeter 9 provided at the exhaust recirculation passage 6 for measuring a recirculation exhaust gas mass flow quantity, an accelerator position sensor 16 and an engine speed sensor 17 indicative of driving conditions, and a control unit 11 for operating the EGR valve 7 , the throttle valve 15 and a fuel injector 14 in accordance with information from the sensors and the engine operating states.

The exhaust gas recirculation passage 6 is a passage directly connecting between the exhaust passage 5 and the intake passage 4 so as to bypass the internal combustion engine 1 . Respective actuators of the EGR valve 7 and the throttle valve 15 are electrically connected to the control unit 11 . A part of exhaust gases is fed into the intake passage 4 by controlling the EGR valve 7 and the throttle valve 15 to flow into intake airflow, and re-circulates into the engine 1 .

The intake air mass flowmeter 8 and the recirculation exhaust gas mass flowmeter 9 are comprised of thermal mass flowmeters having at least heaters (namely heating resistor) as gas mass flow measurement elements, and directly sense mass flows with heat radiation characteristics (heat transfer efficiency) of heaters to the intake airflow and the recirculation exhaust gas-flow. The temperature of each heater is set to be higher than the temperature of each fluid to be measured as flow quantity by a predetermined temperature, or set to be a predetermined fixed temperature. These mass flow quantities are sensed by power supplied to the heater, the temperature distribution of the heater, and the like.

The intake air mass flowmeter 8 and the recirculation exhaust gas mass flowmeter 9 are electrically connected to the control unit 11 . As the other sensors for sensing operating conditions, the engine speed sensor 17 for sensing a rotational speed Ne of the engine 1 and the accelerator position sensor 16 for sensing an accelerator position are provided and electrically connected to the control unit 11 .

Next, the exhaust recirculation control method executed with the control unit 11 of the embodiment will be described with reference to FIG. 2 .

The control unit 11 is composed of an input/output unit, a storing unit, and a computing unit (CPU). The input/output unit is electrically connected to the various sensors and actuators. The storing unit stores the following information as maps: respective instruction quantities Qr of fuel injections corresponding to operating conditions such as the engine rotational speed Ne; an acceleration position VL, and the like; target oxygen concentrations in total intake air quantities containing the recirculation exhaust gases; target oxygen quantities/fuel injection quantities ratios, a smoke allowable limit (smoke limit), and the like.

The computing unit (CPU) executes the following computations. As a first step: a basic injection quantity is computed from data of the sensed engine speed Ne and accelerator position with reference of a map of an engine speed Ne and an accelerator position; and an injection quantity for obtaining a driving force necessary for accessories such as an air conditioner or the like is computed. A second step: an instruction fuel injection quantity Qr is computed by the sum of the basic injection quantity and the injection quantity for the accessories. Further, a computation for predicting a exhaust gas component concentration (oxygen concentration Do) inan engine cylinder is executed from the following information: an intake air mass Qa from the intake air mass flowmeter; a recirculation exhaust gas mass flow quantity Qeo before correction from the recirculation exhaust gas mass flowmeter 9 ; the engine speed Ne from the engine speed sensor 17 ; and the instruction fuel injection quantity Qr.

Next, based on the predicted exhaust gas component concentration (oxygen concentration Do), the recirculation exhaust gas mass flow quantity Qeo is corrected, thereby obtaining a corrected recirculation exhaust mass flow quantity Qe 1 . Further, an oxygen mass Qo(=Oa*0.21+Qe 1 *Do) in the total intake air containing the recirculation exhaust gases in the cylinder, and the other inactive gas mass Qetc (=Qa+Qe 1 −Qo) are computed from the corrected recirculation exhaust gas mass flow quantity Qe 1 , the predicted exhaust gas component concentration (oxygen concentration Do), and the intake air mass Qa. After that, oxygen concentration (Qo/(Qo+Qetc)) in the total intake air containing the recirculation exhaust gases in the cylinder, and the ratio between the quantity of oxygen in the total intake air and the fuel injection quantity (oxygen quantity/fuel injection quantity ratio) (Qo/Qr) is computed with high accuracy. The computed oxygen concentration (Qo/(Qo+Qetc)) in total intake air and quantity/fuel injection quantity ratio (Qo/Qr) are compared with a target oxygen concentration (Qo/(Qo+Qetc)) in the total intake air and a target oxygen quantity/fuel injection quantity ratio (Qo/Qr). The target oxygen concentration and the target oxygen quantity/fuel injection quantity ratio are pre-set according to engine operating conditions (engine speed Ne and the accelerator position) in the storing unit. Based on the result of such a comparison, the opening of the EGR valve 7 , the throttle valve 15 , or a fuel injection quantity Qr of the fuel injector 14 is controlled.

The computation for predicting exhaust gas component concentration for each of combustions in the cylinder, will be described as follows with reference to FIG. 3 .

As a first step, the concentration of oxygen in an exhaust gases is estimated. In FIG. 3 , the intake air mass flow quantity Qa (including oxygen mass flow Oa) and the recirculation exhaust gas mass flow Qe quantity (including oxygen mass flow Oe) are took into the cylinder 1 of the internal combustion engine. And fuel of the instruction fuel injection quantity (mass) Qr is injected from the fuel injector 14 into the cylinder 1 . When injecting such a fuel, mass Qt of burned exhaust gases discharged from the cylinder per revolution (including the oxygen mass flow Ot) is expressed by the following equation.

Qt=Qa+Qe+Qr   (1)

The oxygen mass flow quantity Ot in the burned exhaust gases is expressed by the following equation when oxygen mass consumed by burning of the instruction fuel injection quantity Qr in the cylinder is Oc.

Ot=Oa+Oe−Oc   (2)

Since the component ratio of oxygen and that of nitrogen doesn't change, the oxygen mass flow quantity Oa in the intake air is expressed as follows.

Oa= 0.21 ×Qa   (3)

The mass Oc of oxygen consumed for combustion in the cylinder with the instruction fuel injection quantity Qr is expressed by the following equation by using the stoichiometric air fuel ratio of combustion (14.6).

Oc= 0.21 ×Qr× 14.6=3.1 ×Qr   (4)

When Equations (3) and (4) are substituted for Equation (2), the following is obtained.

Ot= 0.21 ×Qa+Oe− 3.1 ×Qr   (5)

Since the oxygen concentration Do of the burned exhaust gas mass Qt (including the oxygen mass flow quantity Ot) and that of the recirculation exhaust gas mass flow quantity Qe (including the oxygen mass flow quantity Oe) are the same, the following is satisfied.

Do=Ot/Qt=Oe/Qe   (6)

When Equations (1) and (5) are substituted for Equation (6), the oxygen mass flow quantity Oe in the recirculation exhaust gases is obtained as follows.

Oe =(0.21 ×Qa− 3.1 ×Qr )/( Qa+Qr )  (7)

Therefore, the oxygen concentration Do of the recirculation exhaust gases is expressed as follows.

Do=Oe/Qe= (0.21 ×Qa− 3.1 ×Qr )/( Qa+Qr )/ Qe   (8)

Since Qa is obtained from the intake air mass follow meter 8 , Qe is obtained from the recirculation exhaust gas mass flowmeter 9 , and Qr is obtained as an instruction fuel injection quantity, respectively, the oxygen mass flow quantity Oe of the recirculation exhaust gases and the oxygen concentration (Do) of the recirculation exhaust gases can be computed from Equations (7) and (8).

Next, a method of performing correction computation (Qe 1 after correction) on the recirculation exhaust gas mass flow quantity Qeo from the recirculation exhaust gas mass flowmeter 9 on the basis of the computed predicted exhaust gas component concentration (oxygen concentration Do) will be described.

First, the operation principle of the recirculation exhaust gas mass flowmeter 9 will be described with reference to FIG. 4. 6 in FIG. 4 denotes the exhaust recirculation passage in which a heater 18 and an exhaust gas temperature sensor 19 are disposed. 20 denotes a power source, 21 denotes a transistor for passing heat current through the heater 18 , 22 a and 22 b denote resistors, 24 indicates a differential amplifier, and 11 indicates the control unit.

A bridge circuit is comprised of the heater (heating resistor) 18 , the exhaust gas temperature sensor 19 , and the resistors 22 a and 22 b . Voltages of terminals 23 a and 23 b of the bridge are inputted to the differential amplifier 24 . The differential amplifier 24 performs feedback control of setting the values of the resistors 22 a and 22 b so that temperature (Th) of the heater 18 becomes higher than the temperature (Te) of the exhaust gas temperature sensor 19 corresponding to the exhaust gas temperature by a predetermined temperature (ΔTh=Th−Te).

For measurement of the recirculation exhaust gas mass flow quantity Qe, a terminal voltage across the resistor 22 b corresponding to the heating current to be passed through the heater 18 is inputted to the control unit 11 . Specifically, since the feedback control is performed so that when the mass flow quantity Qe increases, the heater 18 is cooled by the heat transfer effect of the exhaust gases and the heat of the heater 18 is increased to keep the predetermined temperature (ΔTh=Th−Te). Accordingly, the heat current (corresponding to the terminal voltage of the resistor 22 ) increases and can be sensed as the mass flow quantity Qe.

The relation between discharged power Ph of the heater 18 and the recirculation exhaust gas mass flow quantity Qe is expressed by the following King's equation.

Ph =( a+b √/( Qe ))Δ Th   (9)

The first term (a) of Equation (9) is a contributory amout of the thermal conduction effect, and the second term (b) is a contributory amount of the thermal transfer effect of the exhaust gases. When attention is paid particularly to the second term, Equation (9) becomes the following approximate expression.

Ph≈ρ 0.5 *μ −0.13 *k 0.63 *Cp 0.37 *√( Ue )*Δ Th   (10)

where ρ denotes density of the exhaust gas, μ denotes viscosity coefficient, k denotes thermal conductivity, Cp denotes specific heat at constant pressure, and Ue indicates flow velocity.

From Equation (10), an output of the recirculation exhaust gas mass flowmeter 9 is a function of not only the heater temperature ATh but also physical property values (ρ, μ, k, and Cp) of the exhaust gases. In the case where the intake air mass flowmeter 8 has the same configuration as the gas mass flowmeter 9 , although the air which is a gas having a predetermined ratio of oxygen and nitrogen is ignorable, in the case of a recirculation exhaust gases, since the exhaust gas components largely change according to the operating conditions, the influence of the gas physical property values (ρ, μ, k, and Cp) cannot be ignored. That is, it is understood that the output Qeo from the recirculation exhaust gas mass flowmeter 9 has to be corrected on the basis of concentrations of the exhaust gas components (gas physical property values (ρ, μ, k, and Cp)).

When it is assumed that a diesel engine is used as the internal combustion engine, a general reaction in the case where the fuel is n-decane (C 10 H 22 ) is as follows.

C 10 H 22+a (15.5O 2 +58.3N 2 )→10CO 2 +11H 2 O+15.5(a−1)O 2 +58.3 aN 2   (11)

where “a” denotes an excess air ratio (≧1). When a=1 in Equation (11), complete combustion is performed, and the oxygen O 2 in the exhaust gas is zero. As “a” increases, the exhaust gas becomes leaner and the oxygen O 2 concentration increases. When the concentration (mole %) of oxygen O 2 in the exhaust gases is known from Equation (11), gas concentration (mole %) of carbon dioxide CO 2 , nitrogen N 2 , and moisture H 2 O as the other exhaust gas components are uniquely determined. FIG. 5 shows the result of calculation of Equation (11). In relation to the oxygen O 2 concentration Do (mole %) in the exhaust gases, the gas concentrations (mole %) of the carbon dioxide CO 2 , nitrogen N 2 , and moisture H 2 O as the other exhaust gas components are uniquely determined by the relational expression, and the concentration ratio of each of the exhaust gas components is computed.

After the concentration ratio of each of the exhaust gas components is determined, the gas physical property values (ρ, μ, k, and Cp) of the total exhaust gas are obtained by the following equations. When the concentration ratio (mole %) of each of the exhaust gas components is set as Xi, the density ρ of the total exhaust gases is expressed as follows.

ρ=Σ(ρ) i*Xi/ 100  (12)

The specific heat Cp at constant pressure is expressed as follows.

Cp=Σ ( Cp ) i*Mi*Xi/M/ 100  (13)

where Mi: molecular weight of each component

M: average molecular weight of total exhaust gases=ΣMi*Xi.

The viscosity coefficient μ is expressed as follows.

μ=Σμ i /(1+(1 /Xi )*Σ( XjΦij ))( i≠j )  (14)

where Φij is expressed as follows.

Φ ij =(1+(μ i/μj ) 1/2 ( Mj/Mi ) 1/4 )2/(4/√2)/(1 +Mi/Mj ) 1/2   (15)

The thermal conductivity “k” is expressed as follows.

K=Σki /(1+(1.065 /ηi )Σ( XjΦij ))( i≠j )  (16)

Since the molecular weights Mi of the exhaust gas components (for example, the molecular weights Mi of oxygen, carbon dioxide, moisture, and nitrogen are 32, 44, 18, and 28, respectively) and the physical property values of the respective single gases of the exhaust gas components are known, when the concentration (mole %) Do of oxygen O 2 in the exhaust gases is known, the concentration ratio (mole %) Xi of each of the exhaust gas components is obtained from the relational expression shown in FIG. 5 . Furthermore, when Equations (12) to (16) are used in addition, the density ρ of the total exhaust gases, the specific heat Cp at constant pressure, viscosity coefficient μ, and thermal conductivity k are obtained. Finally, the correction amount of the output Qeo from the recirculation exhaust gas mass flowmeter 9 can be calculated on the basis of the exhaust gas component concentrations (gas physical property values (ρ, μ, k, Cp)) by the Equation (10).

FIG. 6 shows a calculation result of the exhaust gas mass flow quantity Qe and the output (heat discharge amount Ph) of the recirculation exhaust gas mass flowmeter 9 , in the case where air temperature Ta and exhaust gas temperature Te are 20° C. and the heater temperature ΔTh=350° C., with respect to the air (oxygen concentration Do=21%) and the exhaust gas (oxygen concentration Do=10%). As shown in the diagram, a difference between the air (oxygen concentration Do=21%) and of the exhaust gas (oxygen concentration Do=10%) occurs in respective outputs (heat discharge amounts Ph).

Therefore, for example, in the case of adjusting the initial calibration of the recirculation exhaust gas mass flowmeter 9 with the air, when an output of the exhaust gases whose oxygen concentration Do is unknown is Qeo, the oxygen concentration (Do) of the recirculation exhaust gases is predicted by using Equation (8) using the Qeo value. Based on the predicted oxygen concentration (Do), the difference between the outputs (heat discharge amounts) shown in FIG. 6 is corrected to obtain a more-accurate recirculation exhaust gas mass flow Qe 1 (=Oeo±correction amount (function of Do)).

A method which does not use the relational expression as described above can be also employed. In the storage of the control unit 11 , the relation between the output V of the recirculation exhaust gas mass flowmeter 9 preliminarily subjected to the initial calibration with air and the mass flow quantity Qeo is stored as a map shown in FIG. 7 . Further, as shown in FIG. 8 , a corrected mass flow quantity Qij is similarly stored in the storage as a two-dimensional map of the output V of the recirculation exhaust gas mass flowmeter 9 and the oxygen concentration (Do) of the predicted recirculation exhaust gas.

When the output of the recirculation exhaust gas mass flowmeter 9 is V 2 , the recirculation exhaust gas mass flow quantity Qeo=Q 2 before correction is obtained from the map of FIG. 7 . Further, when the oxygen concentration predicted by using Qeo=Q 2 becomes Do 2 , the correction amount Qij=Q 22 is obtained from the output V 2 and the mass flow quantity Q 2 of the two-dimensional map of FIG. 8 , and the corrected circulation exhaust gas mass flow quantity Qe 1 can be calculated as follows.

Qe 1= Q 2+ Q 22  (17)

By pre-storing the maps shown in FIGS. 7 and 8 in the storage in the control unit 11 , the more-accurate recirculation exhaust gas mass flow quantity Qe 1 can be obtained with calculation time smaller than that in the case of using the relational expressions.

In the embodiment of the invention, the recirculation exhaust gas mass flow signal Qeo is corrected on the basis of the predicted exhaust gas component concentration Do. The oxygen concentration Do in the total intake air containing the recirculation exhaust gases in the cylinder and the ratio between the oxygen quantity Qo in the total intake air and the fuel injection quantity Qr (oxygen quantity/fuel injection quantity ratio, Qo/Qr) are computed with high accuracy by using the corrected recirculation exhaust gas mass flow quantity Qe 1 . Therefore, the exhaust recirculation control system having high response and capable of performing high-accuracy EGR control reliably achieving both of a lower quantity of smoke and reduction in NOx can be realized.

Since the method of directly measuring the mass flow quantity by the intake air mass flowmeter 8 and the recirculation exhaust gas mass flowmeter 9 is employed, as compared with a conventional volume flow measuring method using a pressure sensor, the following advantages is effected. That is, higher-accuracy EGR control can be performed without being influenced by situations peculiar to apparatuses such that intake resistance increases due to clogging of an air cleaner or the like and an intake air quantity decreases.

In the foregoing embodiment, the output V of the recirculation exhaust gas mass flowmeter 9 preliminarily subjected to the initial calibration with air is used as the recirculation exhaust gas mass flow signal QeO. In the case of repeating computation synchronously with rotation of the cylinder of the internal combustion engine, re-calculation can be also performed by using the corrected recirculation exhaust gas mass flow Qe 1 as the result of the immediately preceding calculation.

It is also possible to correct response delay time by using a transfer function model in order to correct physical time delay from the intake air mass flowmeter 8 and the recirculation exhaust gas mass flowmeter 9 into the cylinder.

Next, an exhaust recirculation control system as a second embodiment of the present invention will now be described with reference to FIG. 9 . In the diagram, the same reference numerals are given to components having the same configurations as those of the first embodiment of FIG. 1 . The second embodiment is different from the first embodiment with respect to the following points: that is a configuration of the recirculation exhaust gas mass flowmeter 9 disposed in the exhaust recirculation gas passage 6 ; and an exhaust gas temperature sensor 25 is additionally disposed near the configuration of the recirculation exhaust gas mass flowmeter 9 .

FIG. 10 shows a circuit configuration of the recirculation exhaust gas mass flowmeter 9 and the exhaust gas temperature sensor 25 of the second embodiment.

In the exhaust recirculation passage 6 in the diagram, the heater 18 and the exhaust gas temperature sensor 25 are disposed. The configuration in the exhaust recirculation passage 6 is the same as that of the first embodiment but the circuit configuration is different from that of the first embodiment. In the first embodiment of FIG. 4 , the heater 18 and the exhaust gas temperature sensor 19 construct a bridge circuit. In the second embodiment, the exhaust gas temperature sensor 19 is replaced with a resistor 22 c . Consequently, in the first embodiment, the feedback control is performed so that the temperature (Th) of the heater 18 becomes higher than the temperature (Te) of the exhaust gas temperature sensor 19 corresponding to the exhaust gas temperature by predetermined temperature (ΔTh=Th−Te). In the second embodiment, the temperature is controlled to be a predetermined heater temperature (Th) irrespective of the exhaust gas temperature (Te).

By controlling the temperature to the predetermined heater temperature (Th), even in the case where the exhaust gas temperature (Te) largely changes, the heater temperature (Th) becomes constant. Consequently, the heater is not heated to an excessively high temperature, and occurrence of a trouble in the heater can be prevented. The exhaust gas also contains a large amount of suspended matters such as carbons. When the suspended matters such as carbon are adhered to the heater 18 , the heat discharge effect (heat transmission) of the heater changes, and a sensing error occurs in the recirculation exhaust gas mass flowmeter. However, by controlling the heater temperature (Th) to a predetermined value of 400° C. or higher at which a baking effect is expected, adhesion of suspended matters such as carbon can be prevented, and a very reliable configuration can be realized.

On the other hand, when the control is performed to the predetermined heater temperature (Th), an output of the recirculation exhaust gas mass flowmeter 9 strongly depends on the exhaust gas temperature (Te) as shown in the Equations (9) and (10). Consequently, as shown in FIG. 10 , the exhaust gas temperature sensor 25 is disposed in the exhaust recirculation passage 6 , an output of the exhaust gas temperature (Te) is received from a terminal 23 c to the control unit 11 , and an output of the heat current flowing in the heater 18 (corresponding to the voltage at the end of the resistor 22 b ) is corrected.

FIG. 11 shows the exhaust recirculation control method executed by the control unit 11 of the embodiment. The point different from the first embodiment is that the exhaust gas temperature sensor 25 is added. Consequently, in the control unit 11 , computation for predicting the exhaust gas component concentration (oxygen concentration Do) every combustion in the cylinder is performed. After that, based on the predicted exhaust gas component concentration (oxygen concentration Do) and the exhaust gas temperature (Te) of the exhaust gas temperature sensor 25 , the recirculation exhaust gas mass flow quantity Qeo is corrected to obtain the corrected recirculation exhaust gas mass flow quantity Qe 1 .

In the first embodiment, as shown in FIG. 8 , correction is made by using the two-dimensional map of the predicted exhaust gas component concentration (oxygen concentration Do) and the output (V) of the recirculation exhaust gas mass flowmeter 9 . In the second embodiment, as shown in FIG. 12 , correction is made by using a three-dimensional map of the predicted exhaust gas component concentration (oxygen concentration Do), the exhaust gas temperature (Te), and the output (V) of the recirculation exhaust gas mass flowmeter 9 .

For example, when the output of the recirculation exhaust gas mass flowmeter 9 is Vi, the recirculation exhaust gas mass flow quantity QeO before correction is obtained as QeO=Qi from the map of FIG. 7 . When the value of the predicted exhaust gas component concentration (oxygen concentration Do) is equal to Dok (Do=Dok), and the exhaust gas temperature (Te) is equal to Tej (Te=Tej), the correction amount is obtained as Qkji from the three-dimensional map of FIG. 12 on the basis of the output Vi, the oxygen concentration Dok, and the exhaust gas temperature Tej. And the corrected recirculation exhaust gas mass flow quantity Qe 1 can be calculated as follows.

Qe 1= Qi+Qkji   (18)

From the recirculation exhaust gas mass flow quantity Qe 1 corrected by the Equation (18), the predicted exhaust gas component concentration (oxygen concentration Do), and the intake air mass quantity Qa, the oxygen mass quantity Qo in the total intake air containing the recirculation exhaust gas in the cylinder and the other inert gas mass Qetc are computed. After that, the oxygen concentration (Qo/(Qo+Qetc)) in the total intake air containing the recirculation exhaust gas in the cylinder, and the ratio between the oxygen quantity in the total intake air and the fuel injection quantity (oxygen quantity/fuel injection quantity ratio) (Qo/Qr) are computed with high accuracy. The computed oxygen concentration (Qo/(Qo+Qetc)) in total intake air and quantity/fuel injection quantity ratio (Qo/Qr) are compared with a target oxygen concentration (Qo/(Qo+Qetc)) in the total intake air and a target oxygen quantity/fuel injection quantity ratio (Qo/Qr). The target oxygen concentration and the target oxygen quantity/fuel injection quantity ratio are pre-set according to engine operating conditions (engine speed Ne and the accelerator position) in the storing unit. Based on the result of such a comparison, the opening of the EGR valve 7 , the throttle valve 15 , or a fuel injection quantity Qr of the fuel injector 14 is controlled.

According to the embodiment, the reliability of the recirculation exhaust gas mass flowmeter 9 is improved, and the recirculation exhaust gas mass flow quantity Qe 1 corrected with higher accuracy can be obtained.

Next, an exhaust recirculation control system as a third embodiment of the present invention will be described with reference to FIG. 13 . In the diagram, the same reference numerals are given to components having the same configurations as those of the second embodiment of FIG. 9 . The third embodiment is different from the second embodiment with respect to the point that an oxygen concentration sensor 26 is additionally disposed in the exhaust recirculation passage 6 . FIG. 14 shows an exhaust recirculation controlling method executed by the control unit 11 as the third embodiment.

In the third embodiment, learning of an error in the predicted exhaust gas component concentration (oxygen concentration Do) is performed by using a sensing value of the oxygen concentration sensed by the oxygen concentration sensor 26 , which is disposed in the exhaust circulation passage 6 . By performing integral learning in a steady state, accuracy in the steady state improves. It is determined that the operating state of the internal combustion engine is the steady state by monitoring whether or not a change with time in the engine speed Ne, the accelerator position, or the fuel injection quantity Qr is a predetermined value within certain time.

In the control unit 11 , a correction factor is stored as an error learn map every operating range determined by the engine speed Ne, the accelerator position, or the fuel injection quantity Qr. After error learning in the steady state, the correction factor is corrected and re-stored. After the error learning, the predicted exhaust gas component concentration (oxygen concentration Do 2 ) is calculated every combustion in the cylinder.

According to the method, as compared with the case where the exhaust oxygen concentration is directly sensed by the oxygen concentration sensor attached to the exhaust passage 5 (the conventional technique using no prediction exhaust gas), there is no time delay which occurs during the period of time until the exhaust gases reach the oxygen concentration sensor, or there is no chemical reaction delay in the oxygen concentration sensor itself. Consequently, the exhaust oxygen concentration Do 2 can be predicted with high accuracy. Therefore, by performing the EGR control or injection quantity control in accordance with the predicted exhaust oxygen concentration Do 2 , excellent response is obtained and control accuracy, particularly, control accuracy at the time of transition largely improves. Although the oxygen concentration sensor 26 is disposed in the exhaust recirculation passage 6 in the embodiment, similar effects of the invention can be produced even if such a disposition is adopted.

The recirculation exhaust gas mass flowmeter 9 as embodiments of the present invention, is adopted with a so-called direct heating type mass flowmeter in which the temperature of the heater 18 is heated so as to be higher than the exhaust gas temperature (Te) by predetermined temperature (ΔTh=Th−Te) or so as to become predetermined heater temperature (Th). Obviously, similar effects of the invention can be produced also in the case of using a temperature difference type mass flowmeter, in which temperature sensors such as thermal sensitive resistors are respectively upstream and downstream of the heater and the mass flow is sensed from the difference of temperatures of the upstream and downstream temperature sensors.

With respect to the computation for predicting the exhaust gas component concentration of each combustion in the cylinder, the oxygen concentration in the exhaust gas is estimated in the embodiments. Furthermore, in place of the oxygen concentration as seen in Equation (11) and FIG. 5 , it is possible to use information of the concentration of any of carbon dioxide, moisture, and nitrogen, because the concentration ratio of the oxygen, carbon dioxide, moisture, and nitrogen as the exhaust gas components can be known. Consequently, similar effects of the aforementioned embodiments can be also produced by using prediction value of the gas concentration or sensed value of gasflow sensor other than that of the oxygen concentration or the oxygen concentration sensor.

According to the embodiments, the high-accuracy EGR control which also deals with a rapid change in the operating conditions can be performed. And excellent effects such that the maximum reduction in NOx can be achieved while suppressing the smoke ejection quantity to be within the allowable range are obtained.