Diesel engine emission control system

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FIELD OF THE INVENTION

The present invention relates to a system for controlling emissions from a diesel engine by both recirculating engine exhaust gases and modulating the fuel injection amount.

BACKGROUND OF THE INVENTION

Recirculation of a controlled amount of diesel engine exhaust gas to the engine air intake is generally known to provide a beneficial reduction in diesel engine emissions of oxides of nitrogen NOx. Limits are conventionally imposed on the amount of recirculated exhaust gas EGR to avoid excessive engine intake air charge dilution that may degrade engine performance and increase particulate emission levels. An adequate amount of excess air must be maintained for a smoke-free combustion by imposing limits on the amount of fuel injection.

Both open-loop and closed-loop EGR control operate to deliver EGR to the engine air intake in an amount reflecting a compromise between the competing goals of minimizing NOx and particulate emissions in an engine that deliver a satisfying level of performance. The open-loop approaches deliver EGR according to an open-loop EGR schedule determined through a modeled or calibrated relationship between desired EGR and certain EGR parameters. Such open-loop approaches are sensitive to variations in that modeled or calibrated relationship, such as may result from sensor or actuator degradation over time.

Closed-loop approaches attempt to compensate for system disturbances by including some measure of the actual performance of the EGR control in the determination of a desired EGR amount. Since recirculated exhaust gases displace intake air that would otherwise be drawn into the cylinders of the diesel engine, a sensed engine mass airflow (MAF) is decreased with increasing levels of the exhaust gas recirculation. Thus, a closed-loop EGR control is known, which is responsive to the MAF. MAF is currently a sensed parameter on many conventional engine control systems. MAF is commonly generated through a sensor in the intake air path to the engine at a point after an air filter has filtered the intake air has been filtered by an air filter. Accordingly the MAF sensor is exposed to a minimal level of contaminants. Further, the MAF sensor is commonly spaced a considerable distance away from high temperature components, reducing potential sensor wear due to temperature.

Generally, the greater the amount of exhaust gases recirculated the lower the emission levels of oxides of nitrogen NOx. However, the air-to-fuel ratio of the mixture in the cylinders is decreased with increasing levels of exhaust gas recirculation. Therefore, in order to prevent undesirable smoke emissions, the amount of exhaust gases recirculated must be limited to levels that do not result in excessively rich air-to-fuel ratios that produce smoke emissions.

For preventing undesirable smoke emissions, the amount of fuel to be drawn into the cylinder during engine cycle must be lower than the upper maximum fuel that is limited in accordance with the actual performance of EGR control.

An EGR valve of the EGR control system is subject to the harsh environment of the EGR path, so that operation loss resulting from valve contamination and valve exposure to temperature may occur.

Accordingly, it would be desirable to take into account occurrence of operation loss of an EGR control valve that is subject to the harsh environment of the EGR path in determining the maximum fuel and desired EGR.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an internal combustion engine, comprising:

a cylinder including a combustion space;

an intake manifold from which air is drawn into said combustion space during operation of the engine;

an exhaust manifold into which exhaust gas resulting from combustion event in said combustion space is discharged;

an EGR conduit providing a path through which a portion of the exhaust gas passes into said intake manifold;

an EGR valve forming a part of an EGR passage in said EGR conduit, said EGR valve having different valve openings; and

a control arrangement monitoring performance of EGR control and limiting the maximum fuel, in amount, to be drawn into said cylinder in response to the result from monitoring performance of EGR control.

The present invention provides improvements in EGR control system of an internal combustion engine by limiting the maximum fuel, in amount, to be drawn into the engine cylinder and EGR rate in response to result of monitoring performance of EGR control.

In the event when actual performance of EGR control (hereinafter abbreviated as "actual EGR performance") remains outside of a predetermined window around normal performance of EGR control (hereinafter abbreviated as "normal EGR performance"), the maximum fuel, in amount, to be drawn into the engine cylinder is reduced. At the same time, the EGR rate is suppressed by fully opening a throttle valve in the intake air path. This avoids temperature increase of the engine exhaust gases and reduces inflow of the EGR gases to a minimum level.

The reduction in the maximum fuel is determined as a function of a deviation between actual EGR performance and normal EGR performance.

According to the embodiment, amount of air to be drawn into the engine cylinder is anticipated by performing arithmetic operation involving, as a variable, mass air flow measured by an airflow meter in the intake air path. For brevity, this amount of air is referred to as "cylinder air charge". Since the engine cylinder displacement volume is invariable, cylinder air charge varies with amount of recirculated exhaust gases to be drawn into the engine cylinder. For example, with the same engine speed, cylinder air charge increases as EGR rate decreases or decreases as EGR rate increases. Thus, cylinder air charge can be used as a measure of actual EGR performance, making it possible to compare and calculate deviation between actual EGR performance and normal EGR performance.

It is determined that EGR should be inhibited when the actual EGR performance remains outside of the predetermined window around the normal EGR performance. If the EGR is to be inhibited, the maximum fuel limit criteria is altered by changing the level of maximum fuel to a lower level that may be stored as a look-up table in a memory or given by correcting a normal level of maximum fuel. Simultaneously with this alteration of the maximum fuel, desired EGR rate in EGR control is set to a minimum or zero and the throttle valve in the intake air path is fully opened to minimize inflow of EGR gases to the intake air path.

The normal EGR performance is altitude compensated since the cylinder gas charge density varies with atmospheric pressure, resulting in a decrease in the air-to-fuel ratio with increasing altitude levels. This is accomplished by monitoring the barometric pressure and adjusting the normal EGR performance in accordance with the sensed barometric pressure to provide for altitude compensation.

The normal EGR performance is altitude compensated such that the lower the barometric pressure the smaller the correction term and the higher the barometric pressure the greater the correction term.

The actual EGR performance is derived from the desired EGR rate and engine RPM.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general diagram of the engine control hardware used in accordance with a preferred embodiment of the present invention;

FIG. 2 is a block diagram showing control of cylinder charge through fuel injection pump control, EGR control and throttle valve control when the actual EGR performance remains outside of the normal EGR performance;

FIG. 3 is a computer flowchart illustrating steps to determine mass airflow Qas0;

FIG. 4 is an Us-Qas0.sub.-- d conversion look-up table illustrating characteristics of an airflow meter;

FIG. 5 is a computer flowchart illustrating steps to determine cylinder air charge Qac, i.e., amount of air to be drawn into the engine cylinder;

FIG. 6 is a computer flowchart illustrating steps to determine maximum fuel Qful, i.e., an upper limit of amount of fuel to be injected into the engine cylinder;

FIG. 7 is a computer flowchart illustrating steps to determine maximum fuel Qfuln for normal operation mode where the actual EGR performance Qac falls in a predetermined window around the normal EGR performance Qacf;

FIG. 8 is a look-up table illustrating variations of a coefficient Klamb against engine RPM Ne;

FIG. 9 is a computer flowchart illustrating steps to determine a base fuel amount Qsol1, i.e., a base amount of fuel to be injected into the engine cylinder;

FIG. 10 is a look-up table illustrating changes in variations of a correction coefficient Mrdrv against engine RPM Ne with increasing pedal position CI of a gas pedal;

FIG. 11 is a computer flowchart illustrating steps to determine a final fuel amount Qsol, i.e., a final amount of fuel to be injected into the engine cylinder;

FIG. 12 is a look-up table illustrating changes in variations of a fuel injection pump applied voltage U.alpha.sol against engine RPM Ne with increasing final fuel amount Qsol;

FIG. 13 is a computer flowchart illustrating steps to determine desired opening area Aevf of the EGR valve;

FIG. 14 is a look-up table illustrating variations of weight Nik that decreases with increasing flow speed Cqe;

FIG. 15 is a conversion look-up table in the case where the EGR valve is actuated by an actuator including a stepping motor;

FIG. 16 is a computer flowchart illustrating steps to determine a desired EGR amount per unit amount of time Tqek;

FIG. 17 is a computer flowchart illustrating steps to determine a desired EGR rate Megr;

FIG. 18 is a look-up table illustrating changes in variations of base desired EGR rate Megrb against engine RPM Ne with increasing final fuel amount Qsol;

FIG. 19 is a look-up table illustrating variations of a correction coefficient Kegr.sub.-- tw against coolant temperature Tw;

FIG. 20 is a computer flowchart illustrating steps to monitor combustion events;

FIG. 21 is a computer flowchart illustrating steps to monitor EGR valve;

FIG. 22 is a computer flowchart illustrating steps to determine normal EGR performance Qacf;

FIG. 23 is a computer flowchart illustrating steps to determine a base value Qacfb that is used in determining the normal EGR performance Qacf;

FIG. 24 is a computer flowchart illustrating another manner of determining a base value Qacfb;

FIG. 25 is a computer flowchart illustrating other manner of determining a base value Qacfb;

FIG. 26 is a look-up table illustrating how the base value Qacfb varies against engine RPM Ne before altitude correction;

FIG. 27 is a look-up table illustrating how altitude correction coefficient Kqacfb varies against barometric pressure Pa;

FIG. 28 is a look-up table illustrating changes in variations base value Qacfb1 prior to altitude correction against engine RPM Ne with increasing desired EGR rate Megr;

FIG. 29 is a computer flowchart illustrating steps to determine maximum fuel Qfuldg for abnormal operation mode where the actual EGR performance remains outside of the predetermined window around the normal EGR performance;

FIG. 30 is a computer flowchart illustrating another manner of determining a maximum fuel Qfuldg for abnormal operation mode;

FIG. 31 is a look-up table illustrating variations of Qfuldg against Ne in comparison with variations of Qful against Ne; and

FIG. 32 is a look-up table illustrating a correction coefficient Kqful against deviation between actual and normal EGR performance Qac-Qacf.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a single cylinder of a diesel engine 10 is shown for illustrative purpose only, it being understood that the engine 10 may have any number of other cylinders as desired. The cylinder illustrated includes a combustion space 12, and a piston 14. Air is drawn into the combustion space 12 during an engine operation from an intake manifold 16 that is open to the atmosphere via an air intake conduit 18.

The conduit 18 includes a throttle valve 20. When the throttle valve 20 is fully opened, the air conduit 18 is not throttled so that the pressure in the intake manifold 16 is substantially atmospheric pressure. The conduit 18 also includes an airflow meter 22, such as of the hot wire type airflow meters, for providing a measure of the mass airflow into the engine 10. The airflow meter 22 provides an output signal Us indicative of the mass airflow (MAF) into the engine 10.

Fuel is metered into the engine cylinders through the operation of an electric governor controlled fuel pump 24 that delivers fuel pulses timed to engine rotation events to fuel injectors, such as an injector 26 that injects fuel to the combustion space 12. The pump 24 is controlled by an engine control unit 28 to meter appropriate amounts of fuel to the engine cylinders with each fuel injection event, such as determined from the timing of the engine rotation events. Generally, the vehicle operator dictates the appropriate amounts of fuel to be metered by positioning a gas pedal 30, the position of which is converted by a pedal position sensor 32. The pedal position sensor 32 may be a potentiometer providing an output signal CI indicative of pedal 30 displacement away from a rest position to the control unit 28. A RPM signal Ne the period of which is proportional to the rate of rotation of an engine output shaft (not shown) is provided to the engine control unit 28. The control unit 28 determines final fuel amount Qsol responsive to the input signals CI and Ne. The control unit 28 generates a fuel injection amount command U.alpha.sol responsive to the final fuel amount Qsol and the RPM signal Ne, and provides the command U.alpha.sol to an electric governor of the pump 24. The pump 24, which includes the electric governor, may take the form of a known fuel injection pump that is described on pages B-81 to B-84 of a new model introduction manual (R50-0") entitled "NISSAN TERRANO" published in September 1995 by Nissan Motor Co., Ltd.

The electric governor can move a control sleeve for adjustment of fuel amount to be injected. Electric current passing through the electric governor induces magnetic field, causing a rotor to rotate. A shaft of the rotor is operatively connected via its eccentrically mounted ball to the control sleeve such that rotation of the rotor causes displacement of the control sleeve. The strength of the magnetic field and a force of a return spring acting on the rotor determine an angle through which the rotor rotates away from its rest position. The setting is such that increasing the current passing through the electric governor causes the rotor to increase its angle of rotation, thereby to increase displacement of the control sleeve in a direction to increase supply of fuel. The current is varied by varying duty ratio of ON-OFF of a ground circuit of the electric governor.

Exhaust gas resulting from engine cylinder combustion events is discharged into an exhaust manifold 34 and thereafter is passed through an exhaust gas conduit 36. EGR conduit 38 is provided as a path through which a controlled portion of the exhaust gas is recirculated to the engine intake 30 manifold 16, to reduce levels of NOx discharged from the engine 10, and to provide control authority over the inlet air quantity through inlet air charge dilution. An EGR passage 40 in the EGR conduit 38 contains an EGR valve 42 actuated by vacuum pressure in a vacuum actuator 44 to control a degree of the valve opening. Authority over the EGR valve 42 is thus provided by the degree of vacuum applied to the vacuum actuator 44 from a vacuum line 46. A vacuum source 48, such as a conventional vacuum pump applies a substantially steady vacuum to a vacuum line 50 when power is applied to the pump, such as when the engine 10 is operating.

A vacuum modulator 52 is disposed in the vacuum line 46 between the vacuum actuator 44 and the vacuum source 48. The vacuum modulator 52 includes an electrically-controlled solenoid valve (not shown) that opens and closes at a duty cycle dictated by a control signal EGR(Aevf) supplied to the vacuum modulator 52 from the engine control unit 28. For example, EGR(Aevf) may be a fixed frequency, fixed amplitude, variable duty ratio or cycle electrical signal.

The throttle valve 20 within the intake conduit 18 is actuated by vacuum pressure in a vacuum actuator 54. Authority over the throttle valve 20 is thus provided by the degree of vacuum applied to the vacuum actuator 54 from a vacuum line 56.

A vacuum modulator 58 is disposed in the vacuum line 56 between the vacuum actuator 54 and the vacuum source 48. The vacuum modulator 58 includes an electrically controlled solenoid valve (not shown) which opens or closes as dictated by a control signal THc supplied to the vacuum modulator 58 from the engine control unit 28.

An intake manifold pressure sensor 60 is exposed to the pressure in the intake manifold 16 developed at a portion downstream of where the recirculated exhaust gas is admitted to the intake manifold 16. The pressure sensor 60 outputs an intake manifold pressure signal Pm indicative of that pressure to the engine control unit 28. An exhaust manifold pressure sensor 62 is exposed to the pressure in the exhaust manifold 34 and outputs an exhaust manifold pressure signal Pexh indicative of that pressure. Other input signals generally recognized in conventional engine control are provided to the engine control unit 28, such as engine coolant temperature Tw, output from a conventional temperature sensor 64 in the engine coolant path, and a barometric pressure Pa, output from a conventional barometric pressure sensor (not shown).

The engine control unit 28 may include a digital computer containing such generally-known components as a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM) and an input/output interface circuit (I/O). The computer periodically reads and processes inputs from engine sensors, such as the described CI, Ne, Us, Pm, Pexh, Tw, and Pa inputs, and, through execution of control routines stored in the ROM, generates and outputs a series of actuator commands, such as the described U.alpha.sol, EGR(Aevf), and THc commands.

A preferred implementation of the present invention can be understood with reference to the control diagram in FIG. 2. The driver signal from the gas pedal (not shown) is used as the input to a fuel command generator 32. The fuel command generator 32 may be as simple as a pedal position sensor. The fuel command generator 32 outputs a pedal position indicative signal CI. A RPM signal Ne from an engine speed sensor (not shown) and the pedal position indicative signal CI are input to a fuel injection pump control routine 70 which outputs a base fuel amount command Qsol1 indicative of an amount of fuel to be injected into the engine cylinder. The fuel injection pump control routine 70 may include a two-dimensional look-up table (FIG. 10) in a computer memory. As shown in FIG. 10, the look-up table contains various values of Mqdrv against various combinations of values of Ne and CI. Table look-up operation of this table using the input signals CI and Ne results in generating an output signal Mqdrv. This signal Mqdrv is corrected in response to a coolant temperature signal Tw that is provided also to the box 70. The signal Mqdrv as corrected is set as the base fuel amount command Qsol1. The base fuel amount command Qsol1 is limited in a box 72 as will be explained below. The box 72 outputs a final fuel amount command Qsol indicative of a final amount of fuel to be injected into the engine cylinder. This command Qsol controls the amount of fuel to be injected into the engine cylinder of the diesel engine 10 by varying voltage signal U.alpha.sol applied to the fuel injection pump.

The output signal Us of the airflow meter 22 (see FIG. 1) is converted into instantaneous airflow Qas0.sub.-- d by a conversion table (see FIG. 4) in computer memory. Weighted mean of Qas0.sub.-- d is calculated and the result is set as mass airflow Qas0. Using the mass airflow Qas0 and RPM Ne, an amount of air to be distributed to the engine cylinder Qac0 is determined by calculating the following equation.

where:

KC is a constant.

The calculation of this equation is repeated in timed relation with the engine RPM. The calculation results are stored in an L-tuple register (L is an integer greater than 1) one after another and data shifted out of the register are stored one after another in a 2-tuple register as Qac.sub.n and Qac.sub.n-1. Data Qac.sub.n-1 is older than data Qac.sub.n. Using Qac.sub.n-1 and Qac.sub.n, cylinder air charge Qac can be given by calculating the following equation.

where:

KV is a constant.

The