Method for detecting cylinder air amount introduced into cylinder of internal combustion engine with exhaust gas recirculation system and

Claims

What is claimed is:

1. In an electronic engine control system for an internal combustion engine, in which a cylinder air mass flow rate to be actually introduced into an engine cylinder is calculated using measured value of a parameter associated with engine operating condition, and a fuel injection amount is controlled corresponding to the calculated cylinder air mass flow rate, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving a mass flow rate of a recirculated exhaust gas flowing into an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a partial pressure of the exhaust gas within said induction passage on the basis of said mass flow rate of the recirculated exhaust gas; and

deriving said cylinder air mass flow rate on the basis of the estimated value of the partial pressure of the recirculated exhaust gas.

2. In an electronic engine control system for an internal combustion engine, in which a cylinder air mass flow rate to be actually introduced into an engine cylinder is calculated using measured value of a parameter associated with engine operating condition, and a fuel injection amount is controlled corresponding to the calculated cylinder air mass flow rate, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving a mass flow rate of a recirculated exhaust gas flowing into an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating an amount of the recirculated exhaust gas mass flow rate introduced into the engine cylinder on the basis of said mass flow rate of the recirculated exhaust gas; and

deriving said cylinder air mass flow rate on the basis of the estimated value of the recirculated exhaust gas mass flow rate introduced into the engine cylinder.

3. In an electronic engine control system for an internal combustion engine, in which a cylinder air mass flow rate to be actually introduced into an engine cylinder is calculated using measured value of a parameter associated with engine operating condition, and a fuel injection amount is controlled corresponding to the calculated cylinder air mass flow rate, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving a mass flow rate of a recirculated exhaust gas flowing into an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a partial pressure ratio of an intake air and said recirculated exhaust gas in said induction passage on the basis of an intake air mass flow rate and the recirculated exhaust gas mass flow rate flowing into said induction passage; and

deriving the cylinder air mass flow rate on the basis of predetermined parameters including said partial pressure ratio.

4. In an electronic engine control system for an internal combustion engine employing an exhaust gas recirculation system, in which a fuel injection amount is controlled corresponding to measured value of a parameter associated with engine operating condition, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving mass flow rates of an intake air and a recirculated exhaust gas introduced into an induction passage;

estimating a partial pressure of the intake air in the induction passage on the basis of the intake air mass flow rate flowing into the induction passage of the engine;

estimating a partial pressure of the recirculated exhaust gas in the induction passage on the basis of said mass flow rate of recirculated exhaust gas flowing into the induction passage of the engine; and

deriving the cylinder air mass flow rate on the basis of the estimated value of the partial pressure of the intake air and the estimated value of the partial pressure of the recirculated exhaust gas.

5. In an electronic engine control system for an internal combustion engine employing an exhaust gas recirculation system, in which a fuel injection amount is controlled corresponding to measured value of a parameter associated with engine operating condition, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving mass flow rates of an intake air and a recirculated exhaust gas introduced into an induction passage;

estimating a partial pressure of the intake air in the induction passage on the basis of the intake air mass flow rate flowing into the induction passage of the engine;

estimating a partial pressure of the recirculated exhaust gas in the induction passage on the basis of said recirculated exhaust gas mass flow rate;

estimating a total pressure of a gas in the induction passage and a partial pressure ratio of the intake air and the exhaust gas on the basis of the estimated value of the partial pressure of the intake air and the estimated value of the partial pressure of the exhaust gas; and

deriving the cylinder air mass flow rate on the basis of said total pressure and said partial pressure ratio.

6. In an electronic engine control system for an internal combustion engine employing an exhaust gas recirculation system, in which a fuel injection amount is controlled corresponding to measured value of a parameter associated with engine operating condition, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving mass flow rate of a recirculated exhaust gas introduced into an induction passage;

estimating a partial pressure of the recirculated exhaust gas in the induction passage on the basis of said recirculated exhaust gas mass flow rate;

estimating a partial pressure of an intake air on the basis of a measured value of the internal pressure in said induction passage and said partial pressure of the recirculated exhaust gas; and

deriving the cylinder air mass flow rate on the basis of the estimated value of the partial pressure of the intake air and the estimated value of the partial pressure of the recirculated exhaust gas.

7. In an electronic engine control system for an internal combustion engine employing an exhaust gas recirculation system, in which a fuel injection amount is controlled corresponding to measured value of a parameter associated with engine operating condition, a method for calculating the cylinder air mass flow rate comprising the steps of:

deriving mass flow rate of a recirculated exhaust gas introduced into an induction passage;

estimating a partial pressure of the recirculated exhaust gas in the induction passage on the basis of said recirculated exhaust gas mass flow rate;

estimating a partial pressure of an intake air the basis of a measured value of the internal pressure in said induction passage and said partial pressure of the recirculated exhaust gas;

estimating a pressure ratio of the intake air and the recirculated exhaust gas on the basis of the estimated partial pressure of the intake air and the recirculated exhaust gas; and

deriving the cylinder air mass flow rate on the basis of the pressure in said induction passage and the partial pressure ratio.

8. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters of the engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

deriving mass flow rates of an intake air and a recirculated exhaust gas introduced into the induction passage;

estimating a total pressure in the induction passage on the basis of the intake air mass flow rate and the recirculated exhaust gas mass flow rate flowing in said induction passage; and

deriving a parameter representative of a fuel transfer characteristics in said induction passage on the basis of predetermined parameters including said total pressure.

9. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters of the engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

deriving mass flow rates of an intake air and a recirculated exhaust gas introduced into the induction passage of the engine having an exhaust gas recirculation system;

estimating a total mass flow rate of a gas flowing into the cylinder on the basis of the intake air mass flow rate and the recirculated exhaust gas mass flow rate flowing into said induction passage; and

deriving a parameter representative of a fuel transfer characteristics in said induction passage on the basis of predetermined parameters including said total mass flow rate.

10. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters of the engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

deriving a mass flow rate of a recirculated exhaust gas flowing into the induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a partial pressure of the recirculated exhaust gas in said induction passage on the basis of the mass flow rate of the recirculated exhaust gas flowing into the induction passage; and

deriving a parameter representative of the fuel transfer characteristics in said induction passage on the basis of predetermined parameters including said estimated partial pressure.

11. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters of the engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

deriving a mass flow rate of a recirculated exhaust gas flowing into the induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating an mass flow rate of the recirculated exhaust gas introduced into the engine cylinder on the basis of the mass flow rate of the recirculated exhaust gas flowing into the induction passage; and

deriving a parameter representative of the fuel transfer characteristics in said induction passage on the basis of predetermined parameters including said estimated recirculated exhaust gas mass flow rate introduced into the engine cylinder.

12. In an electronic engine control system for an internal combustion engine employing an exhaust gas recirculation system, in which a fuel injection amount is controlled corresponding to measured value of a parameter associated with engine operating condition, a method for calculating a cylinder air mass flow rate comprising the steps of:

estimating a partial pressure of the intake air in the induction passage on the basis of a measured intake air mass flow rate;

estimating a partial pressure of the recirculated exhaust gas flowing into the induction passage on the basis of a detected value of the internal pressure of said induction passage and said estimated partial pressure of the intake air; and

calculating said cylinder air mass flow rate on the basis of the estimated value of the partial pressure of the intake air and the estimated value of the partial pressure of the recirculated exhaust gas.

13. In an electronic engine control system for an internal combustion engine employing an exhaust gas recirculation system, in which a fuel injection amount is controlled corresponding to measured value of a parameter associated with engine operating condition, a method for calculating a cylinder air mass flow rate comprising the steps of:

detecting intake air mass flow rate and pressure in an induction passage of the engine;

estimating a partial pressure of the intake air in the induction passage on the basis of an intake air mass flow rate;

estimating a partial pressure of the recirculated exhaust gas flowing in the induction passage on the basis of a detected value of the internal pressure of said induction passage and said estimated partial pressure of the intake air; and

deriving a partial pressure ratio on the basis of the estimated value of the partial pressure of the intake air and the estimated value of the partial pressure of the recirculated exhaust gas

calculating said cylinder air mass flow rate on the basis of the measured internal pressure in said induction passage and said partial pressure ratio.

14. In an electronic engine control system for an internal combustion engine, in which a cylinder air mass flow rate to be actually introduced into an engine cylinder is calculated using measured value of a parameter associated with engine operating condition, and a fuel injection amount is controlled corresponding to the calculated cylinder air mass flow rate, a method for calculating a cylinder air mass flow rate comprising the steps of:

detecting intake air mass flow rate and pressure in an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a partial pressure of a recirculated exhaust gas in the induction passage on the basis of an intake air mass flow rate and a detected value of the internal pressure in the induction passage; and

deriving said cylinder air mass flow rate on the basis of predetermined parameters including said estimated partial pressure.

15. In an electronic engine control system for an internal combustion engine, in which a cylinder air mass flow rate to be actually introduced into an engine cylinder is calculated using measured value of a parameter associated with engine operating condition, and a fuel injection amount is controlled corresponding to the calculated cylinder air mass flow rate, a method for calculating a cylinder air mass flow rate comprising the steps of:

detecting intake air mass flow rate and pressure in an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a mass flow rate of a recirculated exhaust gas into the cylinder on the basis of an intake air flow mass rate and a detected value of the internal pressure in the induction passage; and

deriving said cylinder air mass flow rate on the basis of predetermined parameters including said estimated predicted recirculated exhaust gas mass flow rate.

16. In an electronic engine control system for an internal combustion engine, in which a cylinder air mass flow rate to be actually introduced into an engine cylinder is calculated using measured value of a parameter associated with engine operating condition, and a fuel injection amount is controlled corresponding to the calculated cylinder air mass flow rate, a method for calculating a cylinder air mass flow rate comprising the steps of:

detecting intake air mass flow rate and pressure in an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a partial pressure ratio of an intake air and a recirculated exhaust gas in the induction passage on the basis of an intake air mass flow rate and a detected value of the internal pressure in the induction passage; and

deriving said cylinder air mass flow rate on the basis of predetermined parameters including said estimated partial pressure ratio.

17. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters of the engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

detecting an internal pressure in an induction passage;

deriving a mass flow rate of a recirculated exhaust gas into the induction passage of the engine, said engine having exhaust gas recirculation system;

deriving a total mass flow rate of a gas flowing into the engine cylinder on the basis of a detected internal pressure in the induction passage and the recirculated exhaust gas mass flow rate introduced into the induction passage; and

deriving a parameter representative of said fuel transfer characteristics on the basis of predetermined parameters including the calculated total mass flow rate flowing into the engine cylinder.

18. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters associated with engine operating condition, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

detecting intake air mass flow rate and pressure in an induction passage of the engine, said engine having an exhaust gas recirculation system;

deriving a total mass flow rate of a gas flowing into the engine cylinder on the basis of a detected internal pressure in the induction passage and an intake air mass flow rate; and

deriving a parameter representative of said fuel transfer characteristics on the basis of predetermined parameters including the calculated total mass flow rate flowing into the engine cylinder.

19. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters including an internal pressure in an induction passage of the engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

detecting intake air mass flow rate and pressure in an induction passage of the engine, said engine having an exhaust gas recirculation system;

estimating a partial pressure of a recirculated exhaust gas in the induction passage on the basis of a detected internal pressure in the induction passage and an intake air mass flow rate; and

deriving a parameter representative of said fuel transfer characteristics on the basis of predetermined parameters including the estimated partial pressure.

20. In an electronic engine control system for an internal combustion engine, in which a parameter representative of a fuel transfer characteristics is derived using measured values of parameters associated with engine running condition engine, and a fuel injection amount is controlled depending upon the parameter representative of the fuel transfer characteristics, a method for controlling a fuel injection amount comprising the steps of:

detecting intake air mass flow and pressure in an induction passage of the engine, said engine having an exhaust gas recirculation system;

deriving a mass flow rate of a recirculated exhaust gas introduced into the engine cylinder on the basis of a detected internal pressure in the induction passage and an intake air mass flow rate; and

deriving a parameter representative of said fuel transfer characteristics on the basis of predetermined parameters including the calculated recirculated exhaust gas flow rate.

BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling an internal combustion engine associated with an exhaust gas recirculation system (hereafter referred to as "EGR" as an exhaust emission control system. More specifically, the invention relates to a method for deriving an accurate value of an intake air mass flow rate (cylinder air mass flow rate) actually introduced into an engine cylinder during active state of the EGR system, which is necessary for engine control, and a method for precisely determining a fuel injection amount with taking delay of transportion of fuel through an intake manifold.

For the purpose of exhaust emission control and fuel economy, modern automotive internal combustion engines employ electronic control systems which maintain air/fuel mixture ratio at a target value throughout the engine operation. In order to control air/fuel ratio with high precision, a cylinder air mass flow rate to be actually introduced into an engine cylinder is precisely detected and necessary fuel flow amount is determined on the basis of the intake air mass flow rate value and the target air/fuel ratio value.

The co-pending U.S. patent application Ser. No. 07/640,598, filed by the same applicant to the present invention on Jan. 10, 1991, pending, and commonly assigned to the assignee of the present invention, discloses a method for determining the intake air mass flow rate to be introduced into the engine cylinder by measuring an intake air mass flow rate flowing into an intake manifold by means of an air flow meter and deriving an internal pressure in the intake manifold on the basis of the measured intake air mass flow rate and further deriving the actually introduced intake air mass flow rate on the basis of the intake air pressure in the intake manifold and an engine speed. In this method, taking the intake air pressure and the engine speed as parameters, the cylinder air mass flow rate corresponding to both parameters is preliminarily measured through experiments. The experimentally measured cylinder air mass flow rate values are mapped in a form of a table. Therefore, the cylinder air mass flow rate is determined by looking up the table in terms of the intake air pressure and the engine speed.

Furthermore, in Japanese Unexamined Patent Publication (Kokai) No. 58-8238, for Japanese Patent Application filed by Toyota Motor Company, Limited on Jul. 6, 1981, discloses a method for determining a fuel injection amount with taking an intake manifold wetting fuel ratio which adhere on the inner periphery of the intake manifold and delivery ratio of the wetting fuel into the engine cylinder, with taking the intake manifold wetting fuel ratio and the wetting fuel delivery ratio as parameters representative of the engine operating condition.

Here, in the recent years, for the purpose of environment protection, emission restriction value for nitrogen oxides (NO.sub.x) is set by a strict emission control regulation. In order to satisfy such restriction value, most of the internal combustion engines employs EGR systems. As is well known, the ERG system recirculates part of the exhaust gas exhausted from an exhaust port of the engine into the air intake manifold trough an EGR valve to introduce into the engine cylinder together with the fresh intake air. By effecting recirculation of the exhaust gas through the EGR system, maximum temperature of combustion in a combustion chamber is lowered to reduce generation amount of NO.sub.x. On the other hand, the charge efficiency of the intake air into the engine cylinder is variable between active state and inactive state of EGR even when the intake air pressure, the engine speed and the intake air temperature are maintained constant.

Accordingly, when the cylinder air mass flow rate value derived at inactive state of the EGR is applied for derivation of the fuel injection amount in the active state of EGR, the actual air/fuel ratio cannot be controlled to the target air/fuel ratio and can become over rich or over lean.

On the other hand, since mobility of the gas (air and exhaust gas) in the intake manifold is variable depending upon active and inactive states of EGR, a transfer characteristics of the injected fuel into the engine cylinder is variable even at the constant engine operating condition. Accordingly, when the fuel transfer characteristics determined at the inactive state of the EGR is applied for derivation of the fuel injection amount at the active state of EGR, a problem can be encountered to cause an error in air/fuel ratio control.

Despite of the fact as set forth above, since the applicants' co-pending application and the Japanese publication are not directed to derivation of the cylinder air mass flow rate or the fuel injection amount at the active state of EGR, the foregoing problems can be encountered.

SUMMARY OF THE INVENTION

Therefore, it is a first object of the present invention to provide a method for deriving a cylinder air mass flow rate of an internal combustion engine, which can derive the cylinder air mass flow rate with high precision even when an exhaust gas recirculation is effected.

A second object of the invention is to provide a method for controlling fuel injection for the internal combustion engine, in which can compensate fuel transfer delay at high precision even when the exhaust gas recirculation is active.

The above-mentioned first object can be achieved by a system, in an electronic engine control system calculating a cylinder air mass flow rate on the basis of various detected value of the engine operating condition and controlling the fuel supply amount to an engine cylinder based thereon, wherein, when an exhaust gas is recirculated, the cylinder air mass flow rate is derived by estimating a mass flow rate of a recirculated exhaust gas flowing into the cylinder, a partial pressure of the recirculated exhaust gas, or a partial pressure ratio of the intake air and the recirculated exhaust gas in the intake manifold on the basis of an intake air flow rate flowing into intake manifold, and an internal pressure in the intake manifold or detected mass flow rate of a recirculated exhaust gas flowing into the intake manifold, and performing calculation for deriving the cylinder air mass flow rate on the basis of one of the estimated values.

The above-mentioned second object is achieved by a system deriving a parameter representative of a fuel transfer characteristics in an intake manifold on the basis of various detected values of engine operating condition, and controlling fuel supply amount for the engine cylinder based thereon, wherein, when the exhaust gas is recirculated, the fuel transfer character is derived by detecting mass flow rate of the recirculated exhaust gas, estimating total mass flow rate flowing into the engine cylinder, flow rate of the recirculated exhaust gas flowing into the engine cylinder, a partial pressure of the recirculated exhaust gas in the intake manifold or a total pressure in the intake manifold on the basis of at least the detected value of the recirculated exhaust gas flow rate, and by performing calculation to derive the parameter of the fuel transfer characteristics on the basis on at least one of the estimated value.

With the method according to the present invention, since the cylinder air mass flow rate is calculated with taking into account the exhaust gas partial pressure, the recirculated exhaust gas flow rate flowing into the engine cylinder or a partial pressure ratio between the intake air and the recirculated exhaust gas in the intake manifold, as dominant factor for charge efficiency of the intake air into the engine cylinder, the intake air mass flow rate can be calculated with high precision. Also, since the parameter representative of the fuel transfer characteristics is derived with taking into account the total mass flow rate of gas flowing into the engine cylinder, the mass flow rate of the recirculated gas into the engine cylinder, the partial pressure of the exhaust gas in the intake manifold or the total pressure in the intake manifold, which are the dominant factor for the fuel transfer characteristics to deliver the fuel into the engine cylinder, compensation for the fuel transfer delay can be done with high accuracy.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram showing overall construction of an electronic engine control system of a type employing an air flow sensor, to which a control method according to the present invention is applied;

FIG. 2 is a block diagram showing method of the present invention for deriving a cylinder air mass flow rate;

FIG. 3 is a block diagram showing another method of the present invention for deriving the cylinder air mass flow rate;

FIG. 4 is a flowchart showing a method of FIG. 2 for deriving the cylinder air mass flow rate;

FIG. 5 is a flowchart showing a method of FIG. 3 for deriving the cylinder air mass flow rate;

FIG. 6 is a diagrammatic illustration showing general construction of a fuel injection system, to which a control method according to the present invention is applied;

FIG. 7 is a block diagram of a fuel injection control system at one cylinder;

FIG. 8 is a flowchart of a program for calculating a fuel injection pulse width;

FIG. 9 is a block diagram showing the overall construction of the electronic engine control system of a type employing a intake air pressure sensor, to which the control method according to the present invention is applied;

FIG. 10 is a block diagram showing method of the present invention for deriving a cylinder air mass flow rate in the control system of FIG. 9;

FIG. 11 is a block diagram showing another method of the present invention for deriving the cylinder air mass flow rate in the control system of FIG. 9;

FIG. 12 is a flowchart showing a method of FIG. 10 for deriving the cylinder air mass flow rate;

FIG. 13 is a flowchart showing a method of FIG. 11 for deriving the cylinder air mass flow rate;

FIG. 14 block diagram showing process for calculation of response of an air/fuel ratio;

FIG. 15 is a chart showing response characteristics of an air/fuel ratio sensor

FIG. 16 is a chart showing an example of calculation of response of air/fuel ratio;

FIG. 17 is a block diagram showing method of the present invention for deriving a cylinder air mass flow rate in another electronic engine control system of a type employing the air flow sensor and the intake air pressure sensor;

FIG. 18 is a block diagram showing another method of the present invention for deriving cylinder air mass flow rate in a further electronic engine control system of a type employing the air flow sensor and the intake air pressure sensor;

FIG. 19 is a schematic diagram showing a practical example of an EGR system; and

FIG. 20 is a chart showing an example of calculation of response of air/fuel ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be discussed herebelow with reference to the accompanying drawings.

FIG. 1 shows the overall construction of a control system as implemented by a digital control unit. A control unit 10 includes CPU 1, ROM 2, RAM 3, a timer 4, I/O LSI circuit 5 and bus 6 which electrically connecting these elements. Detecting information from a throttle angle sensor 11, an intake air mass flow rate sensor 12, an engine coolant temperature sensor 13, a crank angle sensor 14 and an oxygen (O.sub.2) sensor 15 are written in the RAM 3 through the I/O LSI circuit 5. On the other hand, a fuel injection valve drive signals for fuel injection valves 16 (only for one cylinder is shown for simplification of illustration) are output through the I/O LSI circuit 5. Here, a mechanical or electronically controlled EGR system as shown in FIG. 19, is employed. A mass flow rate Q.sub.EGR of a recirculated exhaust gas is detected through a predetermined arithmetic operation. An EGR valve 17 is disposed within an exhaust gas recirculation pipe 20 which connects an air intake manifold 18 and an exhaust manifold 19. In case of an electronically controlled EGR valve, an EGR signal is provided from the control unit 10 to the EGR valve 17 for controlling valve open ratio for recirculating the desired amount of the exhaust gas.

Here, discussion will be given for the EGR system shown in FIG. 19. The EGR system shown in FIG. 19 is the electronically controlled EGR valve of the internal combustion engine, to which the method according to the present invention is applied. The exhaust gas recirculation amount through the EGR valve 17 is controlled by controlling pressure difference between upstream and downstream ports of a throttle valve 40 in a throttle chamber through duty control for two electromagnetic valves 41 and 42. The electromagnetic valves 41 and 42 are controlled by an EGR control signal from the control unit 10. The EGR control signal is derived by the control unit 10 on the basis of a throttle valve open angle, the engine speed and an engine coolant temperature measured by the engine coolant temperature sensor 43 and output of the pressure sensor 44 and so forth. It should be noted that the EGR system, to which the method of the invention, is not specified to the specific construction of the EGR system shown in FIG. 19 and can be any other constructions of systems.

Next, for example, the mass flow rate Q.sub.EGR of the recirculated exhaust gas can be derived indirectly on the basis of a target EGR ratio r.sub.EGR and a measured intake air mass flow rate Q.sub.a through the following equation: ##EQU1##

On the other hand, the exhaust gas mass flow amount Q.sub.EGR can also be derived through the following equation. In this case, the pressure P.sub.2 upstream of an orifice is detected by an absolute pressure sensor 44. Based on the pressure detecting value P.sub.2, the exhaust gas mass flow amount Q.sub.EGR can be expressed by: ##EQU2## where C: flow coefficient

S: cross sectional area of the orifice

P.sub.1 : exhaust gas pressure (about 1 bar)

Since the electronic control unit cannot calculate the root operation, the control unit looks up a table wherein the data of root operation are stored, so as to get the value of ##EQU3## The value of Q.sub.EGR can be obtained by the methods disclosed in Japanese Unexamined Patent Publications Nos. JP-A-63-208657 and JP-A-63-239352.

Next, with reference to FIGS. 2 and 3, discussion will be made for methods for deriving cylinder air mass flow rate during active state of EGR control. FIG. 2 show a discrete block diagram showing process of deriving the cylinder air mass flow rate in the active state of the EGR control in the system of FIG. 1. The shown process includes five processes of a process for compensating delay of response of the intake air flow amount sensor 12, a process for estimating partial pressure of air in the air intake manifold 18, a process for estimating partial pressure of the exhaust gas in the air intake manifold 18, a process for calculating a cylinder air mass flow rate Q.sub.map, and a process for calculating an exhaust gas mass flow rate Q.sub.EGRP to be actually introduced into the engine cylinder (cylinder recirculated exhaust gas mass flow rate). Discussion for respective process will be given herebelow.

At first, in a respo