Method for controlling exhaust gas recirculation system for engine

A method for controlling an exhaust gas recirculation (EGR) system which is provided with an intake throttle valve and an EGR valve driven by a motor may include detecting an engine speed and an amount of intake air for each cylinder of an engine while the engine is operating, determining an amount of air flow supplied to the engine based on the engine speed and the amount of intake air for each cylinder, determining an equivalent cross-section of the EGR valve based on the amount of air flow, determining an opening angle of the EGR valve based on the engine speed, the amount of intake air for each cylinder, the amount of air flow, and the equivalent cross-section of the EGR valve, and controlling the EGR valve according to the opening angle of the EGR valve.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2014-0170350 filed Dec. 2, 2014, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method for controlling an exhaust gas recirculation (EGR) system for an engine, and more particularly, to a method for controlling an EGR system that may accurately monitor and calculate complex flow of EGR gas by forming and applying a formula to form a 3-dimensional map based on an equivalent orthogonal cross-section, an amount of intake air for each cylinder, and an engine speed in an EGR system which controls an intake throttle valve and an EGR valve using one motor.

Description of Related Art

Exhaust gas of an engine contains a large amount of toxic matters, such as CO, HC, and NOx (nitrogen oxides). Particularly, when a combustion temperature of the engine increases, the amount of generation of NOx increases, so that it is necessary to lower a combustion temperature of the engine in order to reduce the amount of NOx contained in the exhaust gas.

Among the reasons for the increase of the combustion temperature of the engine, a major reason is that high temperature heat is momentarily generated according to an increase of a spread speed of flames ignited by a spark plug in a state where an air-fuel ratio of an air-fuel mixed gas inside a combustion chamber is in a rich state.

A method of lowering a combustion temperature of the engine in order to reduce the amount of NOx contained in the exhaust gas includes an exhaust gas recirculation (EGR) method of lowering a combustion temperature of an engine by decreasing a density of mixed gas without changing an inherent air-fuel ratio of the mixed gas by mixing a part of exhaust gas with fresh air and making the mixed gas flow in a combustion chamber.

The exhaust gas recirculation (EGR) method is used for improving fuel efficiency of a gasoline engine, as well as reducing the amount of NOx contained in the exhaust gas. By using the exhaust gas recirculation (EGR) method, it is possible to simultaneously decrease the amount of NOx and advance ignition timing while avoiding a knocking generation region. Accordingly, it is possible to improve output of the engine and fuel efficiency.

In order to accurately control the recirculation of the exhaust gas, the amount of EGR gas recirculated to the intake manifold needs to be accurately controlled.

Among methods of controlling the recirculation of the exhaust gas, a method determines a present driving region, after pre-setting a displacement amount of a low pressure EGR valve for each driving region of an engine and forming a map table based thereon, extracts a control value from the map table, and then controls the low pressure EGR valve based on the extracted control value.

In the method, since it is required to make an EGR gas extracted from an exhaust system (extraction point) flow in an inlet (inflow point) of a compressor, a pressure difference between the two points may be insufficient for the EGR.

As shown inFIG. 1, although the insufficient pressure difference may be complemented by throttling a front side the compressor (not shown), since the amount of EGR gas non-linearly increases due to the throttling, it is difficult to accurately control the amount of EGR gas.

InFIG. 1, reference numbers10,20, and30refer to an intake throttle valve for throttling, an EGR valve for controlling the amount of the EGR gas, and a motor for controlling opening angles of the valves10and20, respectively.

According to the conventional structure in which one motor such as the motor30controls the intake throttle valve10and the EGR valve20, the amount of EGR gas passed through the EGR valve20may be one-dimensionally represented through the following equation.

However, according to the equation, since the pressure difference (PCOMP/PEGRV) between the opposite sides of the EGR valve20is small, and parameters of the equation are associated with the two valves10and20and are associated with intake/exhaust flow in which pressure pulsation exists, control based on an equation θ=f(ARED) related to an opening angle (θ) of a generally-used valve and an equivalent cross-section (ARED) is difficult, as shown inFIG. 2.

The reason why the control based on the equation (θ=f(ARED)) is difficult is that an average pressure of the pressures (PCOMP, PEGRV) according to the amount of the air flow ({dot over (m)}AIR) is changeable, and a pulsation phase of the pressures (PCOMP, PEGRV) according to an engine speed (N) is changeable.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing a method for controlling an EGR system that may accurately monitor and calculate complex flow of EGR gas by forming and applying a formula to form a 3-dimensional map based on an equivalent orthogonal cross-section, an amount of intake air for each cylinder, and an engine speed in an EGR system which controls an intake throttle valve and an EGR valve using one motor.

According to various aspects of the present invention, a method for controlling an exhaust gas recirculation (EGR) system which is provided with an intake throttle valve and an EGR valve driven by a motor may include detecting, by an engine controller, an engine speed and an amount of intake air for each cylinder of an engine while the engine is operating, determining, by the engine controller, an amount of air flow supplied to the engine based on the engine speed and the amount of intake air for each cylinder, determining, by the engine controller, an equivalent cross-section of the EGR valve based on the amount of air flow, determining, by the engine controller, an opening angle of the EGR valve based on the engine speed, the amount of intake air for each cylinder, the amount of air flow, and the equivalent cross-section of the EGR valve, and controlling, by the engine controller, the EGR valve according to the opening angle of the EGR valve.

The amount of air flow may be determined through the following equation:

m.AIR=n2⁢2⁢⁢π⁢⁢N60⁢MAIR,
wherein {dot over (m)}AIRis the amount of air flow, n is a number of cylinders of the engine, N is the engine speed, and MAIRis the amount of intake air for each cylinder.

The equivalent cross-section (ARED) of the EGR valve may be determined through the following equation:

ARED=m.EGR2⁢γ(γ-1)⁢RTEGRV⁢PEGRV⁢ΨEGR,
wherein AREDis an axis of the equivalent cross-section of the EGR valve,

ΨEGR={(PCOMPPEGRV)2γ-(PCOMPPEGRV)γ+1γ⁢,PCOMPPEGRV>0.520.2588,PCOMPPEGRV<0.52,
and γ is a specific heat ratio.

The opening angle of the EGR valve may be calculated through the following equation: θ=f1(ARED,{dot over (m)}AIR(N, MAIR),N)=f2(ARED,MAIR,N), wherein AREDis an axis of the equivalent cross-section of the EGR valve, {dot over (m)}AIRis the amount of air flow, N is the engine speed, and MAIRis the amount of intake air for each cylinder.

The opening angle of the EGR valve may be formed as a 3-dimensional map with respect to the equation for calculating the opening angle of the EGR valve.

The 3-dimensional map may include an axis of the engine speed, an axis of the amount of intake air for each cylinder of the engine, and the axis of the equivalent cross-section of the EGR valve.

According to various embodiments of the present invention, a method for controlling an EGR system may be provided to accurately monitor and calculate complex flow of EGR gas by forming and applying a formula to form a 3-dimensional map based on an equivalent orthogonal cross-section, an amount of intake air for each cylinder, and an engine speed in an EGR system which controls an intake throttle valve and an EGR valve using one motor, thereby improving control stability and reliability of the engine.

DETAILED DESCRIPTION

FIG. 3is a block diagram of a system for implementing a method for controlling an EGR system according to various embodiments of the present invention.

Referring toFIG. 3, the system for implementing the method for controlling the EGR system according to various embodiments of the present invention may include an EGR controller100configured to control general operation of the EGR system; an EGR sensor50configured to detect whether EGR is performed, and an air flow sensor60configured to detect an amount of air. In the system, an EGR valve20and an intake throttle valve10are controlled by one motor30.

The motor30, the EGR valve20, the intake throttle valve10, the EGR sensor50, and the air flow sensor60may be similar to those shown inFIG. 1or may be those typically applied in the conventional art.

The EGR controller100may be one or more microprocessors and/or hardware including a microprocessor that can be operated by a predetermined program, wherein the predetermined program may include a series of commands for executing the method for controlling the EGR system to be described later according to various embodiments of the present invention.

The EGR controller100may be included in an engine electronic control unit (ECU)11configured to control an engine1as shown inFIG. 3, or may include the ECU11.

Hereinafter, a method for controlling an EGR system according to various embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 4is a flowchart of a method for controlling an EGR system according to various embodiments of the present invention.

The operation of the engine1may be detected or determined through signals outputted from the ECU11, as is well-known to a person skilled in the art.

When the operation of the engine1is detected or determined at step S100, the EGR controller100detects an engine speed (N) of the engine1and an amount (MAIR) of intake air for each cylinder of the engine1(S200).

The engine speed (N) may be detected or determined through the ECU11, and the amount (MAIR) of intake air for each cylinder may be detected or determined by the air flow sensor60and/or through the ECU11.

When the engine speed (N) and the amount (MAIR) of intake air for each cylinder is detected, the EGR controller100calculates an amount ({dot over (m)}AIR) of air flow supplied to the engine1based on the engine speed (N) and the amount (MAIR) of intake air for each cylinder (S300). For example, the amount ({dot over (m)}AIR) of air flow may be calculated through the following equation.

m.AIR=n2⁢2⁢⁢π⁢⁢N60⁢MAIR
(where n is the number of cylinders of the engine)

When the amount ({dot over (m)}AIR) of air flow is calculated, the EGR controller100calculates an equivalent cross-section (ARED) of the EGR valve20based on the amount ({dot over (m)}AIR) of air flow (S400). For example, the equivalent cross-section (ARED) may be calculated through the following equation.

(where γ is the specific heat ratio, R is a gas constant of the EGR gas, PEGRVis a front pressure of the EGR valve, PCOMPis a rear pressure of the EGR valve, and TEGRVis a temperature of the EGR gas)

When the equivalent cross-section (ARED) is calculated, the EGR controller100calculates an opening angle (θ) of the EGR valve20through the following equation based on the engine speed (N), the amount (MAIR) of intake air for each cylinder, the amount ({dot over (m)}AIR) of air flow, and the equivalent cross-section (ARED) of the EGR valve20(S500), and then controls the EGR valve20according to the calculated opening angle (θ) of the EGR valve20(S600).
θ=f1(ARED,{dot over (m)}AIR(N,MAIR),N)=f2(ARED,MAIR,N).

As shown inFIG. 5, the opening angle (θ) of the EGR valve20may be formed as a 3-dimensional orthogonal map with respect to the equation for calculating the opening angle (θ) of the EGR valve20, wherein the 3-dimensional orthogonal map may have an axis of the engine speed (N), an axis of the amount (MAIR) of intake air for each cylinder of the engine1, and an axis of the equivalent cross-section (ARED) of the EGR valve20.

The 3-dimensional orthogonal map may be formed with respect to every predetermined engine speed to accurately set values of the opening angle of the EGR valve20.

Accordingly, according to various embodiments of the present invention, it is possible to accurately monitor and calculate complex flow of EGR gas by forming and applying a formula to form a 3-dimensional map based on an equivalent orthogonal cross-section, an amount of intake air for each cylinder, and an engine speed in an EGR system which controls an intake throttle valve and an EGR valve using one motor, thereby improving control stability and reliability for the engine.