Abstract:
An exhaust gas control method of an engine may include calculating a target mass flux of EGR gas by using air mass entering into a cylinder and target air mass supplied into the engine, calculating a target effective flow area (EFA d ) of the EGR valve by using the target mass flux of the EGR gas and front/rear condition of the EGR valve, and calculating a valve opening rate of the EGR valve by using a predetermined curve fitting formula and the target effective flow area (EFA d ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0123612 filed Dec. 6, 2010, the entire contents of which application is incorporated herein for all purposes by this reference. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to an exhaust gas control method that recirculates the exhaust gas of an exhaust line to an intake line and precisely controls the flux of the EGR gas. 
     2. Description of Related Art 
     Generally, an exhaust gas recirculation system is provided in most diesel engines that are equipped in a vehicle so as to take action with regard to exhaust gas regulations. 
     The exhaust gas recirculation system returns some of the exhaust gas exhausted from an engine to an intake device of a cylinder to lower combustion temperature of the engine and to decrease NOx thereof. 
     An EGR valve and an EGR cooler are disposed such that the EGR gas is cooled to a predetermined temperature to be supplied to the intake manifold. 
     The rotation speed of the engine and fuel injection amount are applied and the EGR valve is controlled by a feed forward control method so as to control the flux of the EGR gas, and a MAF detects the flux of the EGR gas. 
     Meanwhile, the flux of the EGR gas that is determined by the rotation speed and the fuel injection amount of the engine can be varied by several factors such that a real flowing of the EGR gas cannot be reflected and a feedback control can be interfered in a particular condition. 
     The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     SUMMARY OF INVENTION 
     Various aspects of the present invention provide for an exhaust gas control method of an engine having advantages of precisely controlling flux of exhaust gas that is recirculated from an exhaust line to an intake line, reducing a fuel consumption, and improving quality of exhaust gas. 
     One aspect of the present invention is directed to an exhaust gas control method of an engine may include calculating a target mass flux of EGR gas by using air mass entering into a cylinder and air mass supplied into the engine, calculating an effective flow area (EFA) of the EGR valve by using a target mass flux of the EGR gas and front/rear condition of the EGR valve, and calculating a valve opening rate of the EGR valve by using a predetermined curve fitting formula and the effective flow area (EFA). 
     The target mass flux of the EGR gas may be calculated by the following formula:
 
 {dot over (m)}   egr   d   ={dot over (m)}   a   −MAF   d  
 
     Here, {dot over (m)} egr   d  is a target mass flux of the EGR gas, MAF d  is a mass flux of the intake air detected, {dot over (m)} a  is a mass flux of the air entered into the cylinder. 
     The target effective flow area (EFA d ) is calculated by the following formula: 
     
       
         
           
             
               
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     Here, R exh  is a gas constant of the exhaust gas, T exh  is a temperature of the exhaust gas, PR is a front/rear pressure ratio of the EGR valve, k is a specific heat ratio, and P CR  is a critical pressure ratio of the orifice flow (e.g., pressure). 
     The curve fitting formula is represented by the following formula:
 
EGR valve lift= P   1   +P   2   ·EFA   d   +P   3 ·( EFA   d ) 2   +P   4 ·( EFA   d ) 3  
 
     Here, the EGR valve lift denotes a target valve opening of the EGR valve, P 1 , P 2 , P 3 , and P 4  are constants, and the EFA d  is a target effective flow area. 
     The {dot over (m)} a  is 
                   η   V     ·     ρ   int     ·     V   d     ·   N     120     ,         
η V  is a volumetric efficiency of a cylinder, ρ int  is a density of intake air, and V d  is a stroke volume of a cylinder, and N is an engine speed.
 
     The ρ int  is 
                 P   int       R   ·     T   int         ,         
R is a gas constant, and T int  is a temperature of the intake air.
 
     An exhaust gas control method of an engine according to various embodiments of the present invention calculates a target mass flux of EGR gas flowing an EGR line and applies an opening rate of the EGR valve thereby to control a flux of the EGR gas precisely and quickly. 
     The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary exhaust gas control method of an engine according to various embodiments of the present invention. 
         FIG. 2  is a flowchart showing an exemplary exhaust gas control method of an engine according to various embodiments of the present invention. 
         FIG. 3  shows control factors in an exemplary exhaust gas control method of an engine according to various embodiments of the present invention. 
         FIG. 4  is a graph showing effects by an exemplary exhaust gas control method of an engine according to various embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
       FIG. 1  is a schematic diagram of an exhaust gas control method of an engine according to various embodiments of the present invention. 
     Referring to  FIG. 1 , an exhaust gas control method of an engine may include an engine  100 , an exhaust line  120 , a catalyst  110 , an intake line  160 , an EGR line  140 , an EGR valve  130 , an EGR cooler  150 , and a control portion  170 . 
     Further, it may include a pressure difference sensor that detects a pressure difference between a front and a rear of the EGR valve  130 , a EGR flux detecting portion that detects a real flux of the EGR gas passing the EGR line  140 , and an intake flux detecting portion that detects a real flux of the intake air passing the intake line  160 . 
     The exhaust gas that is generated by the engine  100  is released outside through the exhaust line  120 , harmful material is reduced by the catalyst  110 , and a part thereof is recirculated to the intake line  160  through the EGR line  140 . 
     The control portion  170  controls opening of the EGR valve  130  so as to precisely control the EGR gas flowing the EGR line  140 . 
     Referring to  FIG. 2  and  FIG. 3 , a method for controlling the opening of the EGR valve will be detailed. 
       FIG. 2  is a flowchart showing an exhaust gas control method of an engine according to various embodiments of the present invention, and  FIG. 3  shows control factors in an exhaust gas control method of an engine according to various embodiments of the present invention. 
     Referring to (1) of  FIG. 2 , an intake manifold model is shown, the control portion  170  detects an intake pressure (P int ), an intake air temperature (T int ), an engine speed (RPM) as driving conditions and then perform the intake manifold model. Here, a Target M AF denotes a target flux of the intake air. 
     Referring to (1) of  FIG. 3 , a target EGR mass flux means a target mass flux of the EGR gas flowing the EGR line  140 , and {dot over (m)} egr   d  is calculated by a target mass flux of the intake air (MAF d ) and a mass flux of the air inhaled by a cylinder ({dot over (m)} a ). 
     More particularly, a target mass flux of the EGR gas is calculated by the following formula.
 
 {dot over (m)}   egr   d   {dot over (m)}   a   −MAF   d    Formula 1
 
     Here, {dot over (m)} a  is 
                   η   V     ·     ρ   int     ·     V   d     ·   N     120     ,         
η V  is a volumetric efficiency of a cylinder, ρ int  is a density of the intake air, V d  is cylinder stroke volume, N is an engine speed, the ρ int  is
 
                 P   int       R   ·     T   int         ,         
R is gas constant, and T int  is a temperature of the intake air.
 
     Referring to (2) of FIG  2 , an inverse valve flow model g −1  signifies a flux model of the EGR, and the control portion  170  detects an intake pressure (P int ), an intake air temperature (T int ), and an engine speed (RPM) as driving conditions and perform the flux model of the EGR. 
     Referring to (2) of  FIG. 3 , the EGR mass model is calculated by a target mass flux of the EGR gas {dot over (m)} egr   d . 
     More particularly, the EGR flux model is used to get a target effective flow area (EFA d ) of the EGR valve  130  and is calculated by the following formula: 
     
       
         
           
             
               
                 
                   
                     
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     Here, R exh  is a gas constant of the exhaust gas, T exh  is a temperature of the exhaust gas, PR is a pressure ratio between a front and a rear of the EGR valve  130 , k is a specific heat ratio, P CR  is a critical pressure ratio of the orifice flow, and a temperature of the exhaust gas and the front/rear pressure difference of the EGR valve  130  are detected by a pressure difference sensor and a temperature sensor. A detailed description for the pressure difference sensor and the temperature sensor will be omitted in various embodiments of the present invention. 
     Referring to (3) of  FIG. 2 , an inverse poppet valve model is used to get a target opening of the EGR valve  130  and is calculated by the target EFA (effective flow area) that is calculated in the (2) of  FIG. 3 . 
     More particularly, a target opening rate of the EGR valve  130  is calculated by the following formula:
 
EGR valve lift= P   1   +P   2   ·EFA   d   +P   3·(   EFA   d ) 2   +P   4 ·( EFA   d ) 3    Formula 3
 
     Here, the Formula 3 is a curve fitting formula that is made to be fitted to a graph that is drawn by experimental data, an EGR valve lift denotes a target valve opening of the EGR valve  130 , P 1 , P 2 , P 3 , and P 4  are constants, and the EFA d  denotes a target effective flow area. Further, the curve fitting formula can be varied depending on a design specification of the valve. 
     A target valve opening of the EGR valve  130  that is calculated by the formula 3 is set to a target value of the feed forward control so as to control the opening of the EGR valve  130  in various embodiments of the present invention. In a  FIG. 2 , a FF valve lift as a feed forward control signifies controlling the opening rate of the valve. 
       FIG. 4  is a graph showing effects by an exhaust gas control method of an engine according to various embodiments of the present invention. 
     Referring to  FIG. 4 , a horizontal axis denotes time, a vertical axis shows a mass flux of the intake air (MAF) flowing the line  160 . 
     As shown there, in a case that a control of the present invention is applied, a settling time is improved from about 1.43 to 0.45 second. 
     In other words, in a case that the control according to the present invention is applied, the EGR valve  130  quickly reacts such that the target mass flux of the EGR gas is quickly realized. 
     The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.