EGR controller for internal combustion engine

By means of an air model, an estimated quantity of an exhaust gas flowing into a cylinder and a target quantity of the exhaust gas are computed. A deviation between the estimated quantity and the target quantity is multiplied by a feedback gain to obtain a feedback correction quantity. A reference opening degree of an EGR valve is defined according to an engine driving condition and the feedback correction quantity is added to the reference opening degree to obtain a command opening degree of the EGR valve. According to the engine driving condition and the deviation between the estimated quantity and the target quantity, a feedback gain is established so that control accuracy and control stability of a feedback control can be ensured.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2010-244207 filed on Oct. 29, 2010, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an EGR controller for an internal combustion engine, which controls an opening degree of an EGR valve adjusting a quantity of recirculated exhaust gas flowing into a cylinder.

BACKGROUND OF THE INVENTION

An internal combustion engine equipped with an EGR system is well known. In the EGR system, a part of exhaust gas emitted from the engine is recirculated into an intake passage through an EGR passage. The quantity of the recirculated exhaust gas is controlled by an EGR valve. U.S. Pat. No. 7,269,497-B2 shows an EGR system in which the quantity of exhaust gas passing through the EGR valve is estimated by means of an EGR valve model and this estimated quantity is filtered to estimate a quantity of the exhaust gas flowing into a cylinder. An opening degree of the EGR valve is feedback controlled in such a manner that the quantity of the exhaust gas flowing into the cylinder agrees with a target value.

JP-2008-101626A shows, in detail, a model for estimating the quantity of the exhaust gas flowing into a cylinder. The quantity of the exhaust gas passing through the EGR valve is computed by means of an EGR valve model which simulates a behavior of the exhaust gas passing through the EGR valve. When the engine is stably running, it is deemed that the quantity of the exhaust gas passing through the EGR valve is equal to the quantity of the exhaust gas flowing into a cylinder. When the engine is in a transitional running condition, the quantity of the exhaust gas flowing into the cylinder is computed based on the quantity of the exhaust gas passing through the EGR valve by means of a EGR diffusion model (time delay+first order lag) which simulates a behavior of the exhaust gas flowing in an intake pipe toward an intake port of the engine.

It should be noted that the opening degree of the EGR valve is feedback controlled in such a manner that the quantity of the exhaust gas passing through the EGR valve agrees with a target value.

In order to improve fuel economy of the engine equipped with an EGR system, it is necessary to control the quantity of the recirculated exhaust gas flowing into a cylinder (or an EGR ratio) according to an engine driving condition so that gas including exhaust gas is efficiently combusted in a cylinder. Thus, it is necessary to feedback control the opening degree of the EGR valve in such a manner that the quantity of the exhaust gas flowing into a cylinder (or an EGR ratio) agrees with the target value with high accuracy.

However, in the EGR system shown in the above two patent documents, it became apparent that a control accuracy of the quantity of the exhaust gas flowing into a cylinder does not satisfy the required accuracy to improve the fuel economy. This reason can be considered as follows:

(1) In the conventional system, it is difficult to obtain high control accuracy and high control stability in the feedback control of an EGR valve.

(2) An accuracy of the model which estimates the quantity of the exhaust gas flowing into a cylinder is not high.

SUMMARY OF THE INVENTION

The present invention is made in view of the above matters, and it is an object of the present invention to provide an EGR controller having high accuracy for controlling an opening degree of an EGR valve, whereby a quantity of recirculated exhaust gas flowing into a cylinder can be accurately controlled.

According to the present invention, an EGR controller includes: an EGR valve controlling a quantity of an exhaust gas recirculated from an exhaust passage into an intake passage through an EGR passage; a measuring means for measuring a quantity of an intake air flowing through the intake passage; an estimating means for estimating a quantity of the exhaust gas flowing into a cylinder of the engine; and an EGR control means for controlling an opening degree of the EGR valve.

The estimating means includes, as shown inFIG. 2, a first means for computing a total quantity of a gas flowing into a cylinder by means of an intake valve model which simulates a behavior of gas flowing through the intake passage toward a cylinder;

a second means for computing a quantity of the exhaust gas passing through the EGR valve by means of an EGR valve model which simulates a behavior of the recirculated exhaust gas passing through the EGR valve;

a third means for computing a temporal quantity of the exhaust gas flowing into the cylinder based on the quantity of the exhaust gas passing through the EGR valve by means of an EGR diffusion model which simulates a behavior of the exhaust gas flowing through the intake passage toward an intake port of the internal combustion engine;

a fourth means for obtaining the quantity of the intake air flowing into the cylinder by subtracting the temporal quantity of the exhaust gas flowing into the cylinder from the total quantity of the gas flowing into the cylinder;

a fifth means for correcting the quantity of the intake air flowing into the cylinder by means of the intake air quantity measured by the measuring means;

a sixth means for computing an intake air pressure based on the quantity of the intake air corrected by the fifth means, and

a seventh means for computing a quantity of the recirculated exhaust gas flowing into the cylinder based on at least the intake air pressure.

The EGR control means executes a feedback control of an opening degree of the EGR valve in such a manner that a deviation between a target quantity established according to an engine driving condition and the quantity of the exhaust gas flowing into the cylinder which the estimating means estimates, or a deviation between a target ratio established according to the engine driving condition and an EGR ratio which is computed based on the quantity of the exhaust gas flowing into the cylinder becomes smaller.

The estimating means computes a total quantity of gas flowing into a cylinder by means of an intake valve model. A temporal quantity of the exhaust gas flowing into the cylinder is computed by means of an EGR valve model and an EGR diffusion model. After that, the quantity of the intake air flowing into the cylinder is obtained by subtracting the temporal quantity of the exhaust gas flowing into the cylinder from the total quantity of the gas flowing into the cylinder.

Further, the computed quantity of the intake air is corrected by means of the intake air quantity measured by the measuring means. An intake air pressure is computed based on the corrected quantity of the intake air. Then, a quantity of the recirculated exhaust gas flowing into the cylinder is computed based on at least the intake air pressure.

Thus, the quantity of the recirculated exhaust gas can be computed with high accuracy based on the exhaust gas quantity which the measuring means measures. A combustibility of air fuel mixture is improved and emission can be reduced.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention, which is applied to an internal combustion engine equipped with a turbocharger, will be described hereinafter.

Referring toFIG. 1, an engine control system will be schematically explained. An air cleaner13is arranged upstream of an intake pipe12(intake passage) of an internal combustion engine11. An airflow meter14detecting an intake air flow rate is provided downstream of the air cleaner13. An exhaust pipe15(exhaust passage) of the engine11is provided with a three-way catalyst16which reduces CO, HC, NOx, and the like contained in exhaust gas.

The engine11is provided with the turbocharger17. The turbocharger17includes an exhaust gas turbine18arranged upstream of the catalyst16in the exhaust pipe15and a compressor19arranged downstream of the airflow meter14in the intake pipe12. This turbocharger17has well known configuration which supercharges the intake air into the combustion chamber.

An intercooler22cooling the intake air is provided in the intake pipe21downstream of the compressor19. A throttle valve23driven by a DC-motor20and a throttle position sensor24detecting a throttle position are provided downstream of the intercooler22.

A surge tank25is connected to an intake pipe21adownstream of the throttle valve23. An intake manifold26which introduces air into each cylinder of the engine11is provided downstream of the surge tank25, and a fuel injector (not shown) which injects fuel is provided for each cylinder. An intake air pressure sensor28detecting intake air pressure Pm is provided in the surge tank25. A spark plug (not shown) is mounted on a cylinder head of the engine11corresponding to each cylinder to ignite air-fuel mixture in each cylinder.

An exhaust manifold27(exhaust passage) is connected to each exhaust port of the cylinder. A confluent portion of the exhaust manifold27is connected to the exhaust pipe15aupstream of the exhaust gas turbine18. Between the exhaust passage15a,27upstream of the exhaust gas turbine18and the intake passage21a,25,26downstream of the throttle valve23, an EGR pipe30of the EGR system29is provided in order to recirculate a part of exhaust gas into the intake passage downstream of the throttle valve23. An EGR cooler31cooling the recirculated exhaust gas and an EGR valve32controlling the quantity of the recirculated exhaust gas flowing into the intake passage are provided in the EGR pipe30. An opening degree of the EGR valve32is adjusted by a motor (not shown).

Also, the engine11is provided with a variable valve timing controller (not shown) which adjusts valve timings of an intake valve and an exhaust valve. A crank angle sensor (not shown) is provided on a cylinder block to output crank angle pulses when a crank shaft rotates a predetermined angle. Based on this crank angle pulses, a crank angle and an engine speed Ne are detected.

The outputs from the above sensors are inputted into an electronic control unit37, which is referred to an ECU37hereinafter. The ECU37detects engine driving condition based on the outputs of the sensors. When the engine driving condition is a specified condition to perform an EGR control, the ECU37opens the EGR valve32, whereby a part of exhaust gas is recirculated into the intake passage downstream of the throttle valve23. At this moment, the ECU37executes an EGR quantity computing program, which will be described later referring toFIG. 4, so that a function of each unit “A” to “G” of an air model “H” shown inFIG. 2is realized. And, the quantity of the exhaust gas flowing into a cylinder is commuted. The quantity of the recirculated exhaust gas flowing into the cylinder is denoted by “QEGR”, hereinafter. The function of each unit “A” to “G” of the air model 2H” will be described hereinafter.

A first unit “A” computes “QTOTAL” (=QINTAKE+QEGR) by means of an intake valve model which simulates a behavior of the intake air flowing into a cylinder with the recirculated exhaust gas. It should be noted that “QTOTAL” represents a total quantity of gas flowing into the cylinder and “QINTAKE” represents a quantity of intake air flowing into the cylinder.

The intake valve model is approximated by a following formula (1) of which input parameters are an estimated intake air pressure Pm, coefficients A0, A1, and A2.
QTOTAL=A2×Pm2+A1×Pm+A0(1)

The coefficients A0, A1, A2are computed based on an intake air characteristic map which is previously defined by simulations or experiments. The intake air characteristic map is comprised of a plurality of maps which is defined at every opening degree of the EGR valve32. The coefficients A0, A1, A2are defined for every opening degree of the EGR valve32. The estimated intake air pressure Pm is computed by means of an intake pipe model. The details of the intake pipe model are described in JP-2008-101626A.

A second unit “B” computes the quantity of the exhaust gas passing through the EGR valve32by means of an EGR valve model which simulates a behavior of the recirculated exhaust gas passing through the EGR valve32. The quantity of the exhaust gas passing through the EGR valve32is denoted by “QVALVE”, hereinafter.

The EGR valve model is approximated by a following formula (2) of which input parameters are an estimated intake air pressure Pm, coefficients B0, B1, and B2.
QVALVE=B2×Pm2B1×Pm+B0(2)

The coefficients B0, B1, B2are computed based on an EGR characteristic map which is previously defined by simulations or experiments. The EGR characteristic map is comprised of a plurality of maps which is defined at every opening degree of the EGR valve32. The coefficients B0, B1, B2are defined for every opening degree of the EGR valve32. The estimated intake air pressure Pm is computed by means of the above intake pipe model.

A third unit “C” computes a temporal “QEGR” based on “QVALVE” by means of an EGR diffusion model which simulates a behavior of the exhaust gas flowing in an intake pipe toward an intake port of the engine. The EGR diffusion model is approximated by “time delay+first order lag”.

A fourth unit “D” subtracts the above temporal “QEGR” from “QTOTAL” computed by the first unit “A” in order to obtain “QINTAKE”.
QINTAKE=QTOTAL−TemporalQEGR(3)
[Fifth Unit “E”]

A fifth unit “E” corrects the computed “QINTAKE” by means of intake air quantity measured by the airflow meter14. Specifically, the fifth unit “E” has a switching means which switches the correction quantity “QC” of “QINTAKE” between for stable driving condition and for transitional driving condition. When a variation ΔPm in the estimated intake air pressure Pm per unit time is not greater than a specified value, it is determined that the engine11is in the stable driving condition. When the variation ΔPm is greater than the specified value, it is determined that the engine11is in the transitional driving condition.

When the engine is in the transitional driving condition, the correction quantity “QC” of “QINTAKE” is set to a differential value between the computed “QINTAKE” and the measured intake air quantity measured by the airflow meter14. The measured intake air quantity is denoted by “QMETER”, hereinafter.
QC=QINTAKE−QMETER(4)
CorrectedQINTAKE=QINTAKE+QC=QINTAKE(QINTAKE−QMETER)  (5)

When the engine is in the stable condition, “QC” is set to zero. Therefore, when the engine is in the stable condition, “QINTAKE” computed by the fourth unit “D” is used without any correction.

A sixth unit “F” computes an estimated intake air pressure Pm based on “QINTAKE” corrected by the fifth unit “E”, by means of an inverse model of the intake valve model which is used in the first unit “A”.

A seventh unit “G” computes “QEGR” based on the estimated intake air pressure Pm, an engine speed Ne, an intake valve timing, an exhaust valve timing, and the opening degree of the EGR valve32, by means of the EGR valve model.

The ECU37executes an EGR quantity computing program of which processing is shownFIG. 4, whereby the above functions of the first to the seventh unit “A” to “G” of the air model “H” are realized. Furthermore, the ECU37feedback controls an opening degree of the EGR valve32by means of QEGR-Feedback-control system shown inFIG. 3.

In the QEGR-Feedback-control system of the present embodiment, “QEGR” is computed by means of the air model “H” and a target “QEGR” is computed according to an engine driving condition, such as engine speed and intake air quantity. Then, a deviation between “QEGR” and the target “QEGR” is multiplied by a feedback gain to obtain a feedback correction quantity “QFBC”. A reference opening degree of the EGR valve32is established according to the engine driving condition. “QFBC” is added to the reference opening degree of the EGR valve32to obtain a command opening degree of the EGR valve32. This command opening degree is converted into voltage which is applied to the motor driving the EGR valve32, whereby the opening degree of the EGR valve32is brought into the command opening degree. The opening degree of the EGR valve32detected by an EGR valve position sensor (not shown) is fed back to the air model “H”, whereby the opening degree of the EGR valve32is controlled in such a manner that the deviation between “QEGR” and the target “QEGR” becomes smaller.

Alternatively, an EGR ratio is computed based on “QEGR” which the air model “H” estimates, and the opening degree of the EGR valve32is controlled in such a manner that the deviation between “QEGR” and the target “QEGR” becomes smaller.

It should be noted that “QTOTAL” represents a total quantity of gas flowing into the cylinder and “QINTAKE” represents a quantity of intake air flowing into the cylinder.

The feedback gain may be a predetermined constant value. According to the present embodiment, the feedback gain is established according to the engine driving condition and the deviation between “QEGR” and the target “QEGR” in order to improve the accuracy and the stability of the feedback control. Specifically, a map defining a relationship between the deviation, the engine driving condition and the feedback gain is previously formed by simulations or experiments. Based on this map, the feedback gain is computed.

The above described feedback control is executed by the ECU37according to processings shown inFIGS. 4 and 5,

An EGR quantity computing program of which processing is shown inFIG. 4is a sub-routine executed in step202ofFIG. 5. This program corresponds to an exhaust gas quantity estimating means of the present invention.

In step101, the ECU37computes the quantity of intake air passing through the throttle valve23, which is denoted by “QTHR”, based on the throttle opening detected by the throttle position sensor24by means of a throttle model which simulates the behavior of the intake air passing through the throttle valve23. The details of the throttle model are described in JP-2008-101626A.

In step102, the estimated intake air pressure Pm is computed by means of the intake pipe model. In step103, “QTOTAL” is computed by means of the intake valve model of the first unit “A”.

In step104, the temporal “QEGR” is computed by means of the EGR valve model of the second unit “B” and the EGR diffusion model of the third unit “C”. In step105, “QINTAKE” of before correction is obtained by subtracting the temporal “QEGR” computed in step104from “QTOTAL” computed in step103.
UncorrectedQINTAKE=QTOTAL−TemporalQEGR(7)

Then, the procedure proceeds to step106in which the ECU37determines whether the variation *Pm (absolute value) in the estimated intake air pressure Pm is less than or equal to the specified value. When the answer is YES in step106, the procedure proceeds to step107in which “QC” is set to zero and “QINTAKE” of before correction is defined as the corrected “QINTAKE”.
CorrectedQINTAKE=UncorrectedQINTAKE(8)

When the answer is NO in step106, the procedure proceeds to step108in which “QINTAKE” is corrected.

Then, the procedure proceeds to step109in which the estimated intake air pressure Pm is computed based on the corrected “QINTAKE” by means of the inverse model of the intake valve model. Then, the procedure proceeds to step110in which an estimated “QEGR” is computed based on the pressure Pm, the engine speed Ne, the intake valve timing, the exhaust valve timing, and the opening degree of the EGR valve32, by means of the EGR valve model.

[EGR Quantity Feedback Control Program]

An EGR quantity feedback control program of which procedure is shown inFIG. 5is executed at specified time intervals while the engine is running. This program corresponds to an EGR control means of the present invention. In step201, the target “QEGR” is computed according to an engine driving condition by use of a map. In step202, the EGR quantity computing program is executed to compute the estimated “QEGR”.

Then, the procedure proceeds to step204in which the reference opening degree of the EGR valve32is computed according to the engine driving condition in view of the map. In step205, the feedback gain is computed according to the engine driving condition and the deviation between the target “QEGR” and the estimated “QEGR” in view of the map. It should be noted that the feedback gain may be established according to only the deviation. Alternatively, the feedback gain may be a predetermined constant value.

In step206, the deviation is multiplied by the feedback gain to obtain “QFBC” relative to the reference opening degree of the EGR valve32.
QFBC=Deviation×Feedback Gain  (10)

Then, the procedure proceeds to step207in which the command opening degree of the EGR valve32is obtained by adding “QFBC” to the reference opening degree.
Command Opening Degree=Reference Opening Degree+QFBC(11)

Then, the procedure proceeds to step208in which the voltage corresponding to the command opening degree is applied to the motor driving the EGR valve32, whereby the opening degree of the EGR valve32agrees with the command opening degree.

According to the above embodiment, since the feedback gain is established according to at least the deviation between the estimated “QEGR” and the target “QEGR”, the feedback gain can be set larger as long as the stability of the feedback control is ensured. Therefore, both the accuracy and the stability of the feedback control can be improved, whereby the control accuracy of “QEGR” can be improved.

Further, since the feedback gain is established also according to the engine driving condition, both the accuracy and the stability of the feedback control can be ensured even if the engine driving condition is suddenly changed.

Furthermore, since the computed value of “QINTAKE” is corrected by means of “QMETER”, the computing accuracy of “QEGR” can be improved.

The model for computing “QEGR” is not limited to the air model “H” shown inFIG. 2. For example, the model described in shown in JP-2008-101626A can be used. Also in this case, the feedback gain is established according to at least the deviation.

The present invention can be applied to an engine having a supercharger and an engine having no supercharger. Especially, in a case that the present invention is applied to an engine equipped with a supercharger, it is restricted that a knocking occurs. A combustibility of air fuel mixture is improved and emission can be reduced.