Exhaust gas recirculation control system

An engine exhaust gas cleaning apparatus is provided to suppress EGR ratio fluctuations when the excess air ratio is adjusted and thereby stabilize the engine operating performance. The engine exhaust gas cleaning apparatus comprises an exhaust gas recirculation control valve configured to recirculate a portion of the exhaust gas from the exhaust system to the air intake system and feedback control the quantity of the recirculated exhaust; an intake air throttle valve arranged in the air intake system at a position upstream of the position where an exhaust gas recirculation passage connects to the air intake system; an excess air ratio control section configured to change the intake air quantity by using the intake air throttle valve in response to a request to adjust the excess air ratio; and a feedback control restricting section configured to temporarily prohibit feedback control of the exhaust gas recirculation quantity delivered by the exhaust gas recirculation control valve, or to temporarily lower the gain of the feedback control, when the excess air ratio is being adjusted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to exhaust gas recirculation control apparatuses for internal combustion engines. More specifically, the present invention relates to an exhaust gas recirculation apparatus that regenerates an exhaust gas cleaning device by changing the excess air ratio.

2. Background Information

In internal combustion engines such as diesel engines, an exhaust gas recirculating system (EGR system) is widely used wherein a part of the exhaust gas is recirculated to lower the combustion temperature in order to reduce discharge of nitrogen oxide (NOx). A NOx trapping catalytic converter traps NOx in the exhaust gas when an air-fuel ratio in the exhaust gas is in a lean range and purifies (releases) the trapped NOx when the air-fuel ratio is in a rich range. The NOx deposited in the NOx trapping catalytic converter is typically purified when the amount of adsorbed and held NOx reaches a prescribed upper limit value.

One example of an internal combustion engine having a NOx trapping catalytic converter provided in an exhaust system for cleaning NOx discharged from the engine is disclosed in U.S. Pat. No. 5,732,554 (also see, Japanese Laid-Open Patent Publication No. 08-218920). The NOx trapping catalytic converter described in that document is configured to adsorb and hold NOx contained in the exhaust gas when the excess air ratio is lean, and to desorb and deoxidize the adsorbed NOx when the excess air ratio is rich. These types of NOx trapping catalytic converters are generally applied to internal combustion engines that are normally operated at a lean excess air ratio. When the amount of adsorbed and held NOx reaches a prescribed upper limit value, the NOx trapping catalytic converter can no longer adsorb and hold more NOx. Consequently, the amount of held NOx is estimated and the NOx trapping catalytic converter is regenerated.

When the NOx trapping catalytic converter is regenerated, an intake air throttle valve or the like is used to reduce the amount of intake air into the engine to reduce the excess air ratio to a richer target value. Simultaneously, the fuel injection amount is increased such that the excess air ratio converges on the richer target value.

SUMMARY OF THE INVENTION

It has been discovered that in cases where exhaust gas recirculation (EGR) is being conducted in conjunction with using a NOx trap catalyst in order to reduce NOx emissions, if the opening degree of the intake air throttle valve is reduced, the negative pressure of the air intake system will increase and the EGR quantity will increase even if the opening degree of the EGR valve does not change. Therefore, if the opening degree of the intake air throttle valve is suddenly reduced in order to adjust the excess air ratio during regeneration of the NOx trap catalyst, the EGR quantity will increase accordingly and the engine operating performance will degrade, including an increase in smoke.

Therefore, during regeneration of the NOx trap catalyst, it is necessary to reduce the opening degree of the EGR valve simultaneously with the reduction of the opening degree of the intake air throttle valve in order to maintain the target EGR ratio during rich operation and prevent a sudden increase in the EGR quantity. However, if the opening degree of the EGR valve is feedback controlled, the feedback control will undergo hunting if the opening degree of the EGR valve is corrected in correspondence with the opening degree of the intake air throttle valve because the actual change in the intake air quantity is delayed with respect to the change in the opening degree of the intake air throttle valve.

More specifically, if the opening degree of the EGR valve is greatly reduced before the actual intake air quantity matches the change in the opening degree of the intake air throttle valve, the EGR quantity will decrease too much. Then, the system will detect the excessive decrease and attempt to increase the opening degree of the EGR valve so as to increase the EGR quantity. In short, this kind of correcting operation will be performed repeatedly (i.e., hunting will occur).

Thus, when the excess air ratio is adjusted, the exhaust gas recirculation ratio fluctuates greatly and causes the engine output to fluctuate. As a result, vibrations, noise, and other phenomena occur which have an adverse effect on the operating performance.

One object of the present invention is to provide an engine exhaust gas recirculation control system that avoids these problems. In other words, the engine exhaust gas recirculation control system of the present invention was basically contrived to avoid to the greatest extent possible the adverse effects on operating performance that can result from hunting of the EGR control when the excess air ratio is reduced.

In view of the forgoing, an engine exhaust gas recirculation control system is provided for an engine that basically comprises an exhaust gas recirculation control device, an intake air regulating device and a control unit. The exhaust gas recirculation control device is configured to recirculate a portion of exhaust gas from an exhaust system of an engine to an air intake system of the engine. The intake air regulating device is arranged in the air intake system at a position upstream of a position where the exhaust gas recirculation control device connects to the air intake system. The control unit is configured to control the exhaust gas recirculation control device and the intake air regulating device. The control unit includes a feedback control section, an excess air ratio control section and a feedback control restricting section. The feedback control section is configured to feedback control an exhaust gas recirculation quantity of the exhaust gas to be recirculated to the air intake system. The excess air ratio control section is configured to adjust an excess air ratio in response to a request to change the excess air ratio by using the intake air regulating device to change an intake air quantity. The feedback control restricting section is configured to temporarily restrict feedback control of the exhaust gas recirculation quantity delivered by the exhaust gas recirculation control device when the excess air ratio is being adjusted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIG. 1, an exhaust gas recirculation control apparatus or system is illustrated for an internal combustion engine such as a supercharged diesel engine1in accordance with a first embodiment of the present invention. The exhaust gas recirculation control apparatus in accordance with the present invention can be applied to other internal combustion engines used in automobiles and the like.

As shown in toFIG. 1, the engine1includes a common rail fuel injection system including a common rail2, a plurality of fuel injection valves3, and a high-pressure fuel pump (not shown) so as to be supplied with pressurized fuel. The fuel pump (not shown) pumps fuel to the common rail2, where the pressurized fuel accumulates, and high-pressure fuel is delivered to the inside of the combustion chambers when the fuel injection valves3are opened. Thus, the fuel injection valves3inject fuel directly into respective combustion chambers (not shown) of each cylinder.

The fuel injection valves3are configured and arranged to execute a pilot injection before the main injection or executing a post-injection following the main injection. By changing the accumulation pressure of the common rail2, the fuel injection pressure can be controlled in a variable manner.

A turbocharger (supercharger)4having a compressor4ais arranged in an air intake passage5of the air intake system. The compressor4aserves to pressurize the intake air. The compressor4ais rotated by a turbine4bthat is driven by exhaust gas flowing through an exhaust passage6. The supercharger4is positioned downstream of an air flow meter7in the air intake passage5of the engine1. Preferably, the supercharger4is a variable-capacity type supercharger having a variable nozzle provided on the turbine4b. By using a variable-capacity type supercharger4, the variable nozzle can be constricted when the engine1is operating in a low speed region to increase the turbine efficiency. The variable nozzle of the supercharger4can be opened when the engine1is operating in a high speed region to increase the turbine capacity. Thus, this arrangement enables a high supercharging effect to be obtained over a wide range of operating conditions.

An intake air throttle valve8is installed inside the air intake passage5at a location downstream of the compressor4a. The intake air throttle valve8serves as an intake air regulating device to make it possible to control the quantity of intake air drawn into the engine1. The intake air throttle valve8is, for example, an electronically controlled throttle valve whose opening degree can be varied freely using a stepper motor.

The exhaust passage6is provided with an exhaust gas recirculation (EGR) passage9that branches from a position between the engine1and the turbine4b. The EGR passage9connects to the air intake passage5downstream of the intake air throttle valve8.

The exhaust system is provided with an exhaust gas recirculation (EGR) control valve10that is installed in the EGR passage9. The EGR valve10serves to control the exhaust gas recirculation quantity in accordance with the engine operating conditions. The EGR valve10is electronically controlled using a stepper motor such that the opening degree of the EGR valve10regulates the flow rate of the exhaust gas recirculated to the air intake system, i.e., the EGR quantity drawn into the engine1. The EGR valve10is feedback (closed-loop) controlled to regulate the EGR quantity in such a manner as to achieve an EGR ratio set in accordance with the operating conditions.

The flow rate of the air drawn into the engine main body1is determined according to the engine rotational speed at that particular time, which depends on the fuel injection quantity, and is equal to the total of the flow rate of fresh air from upstream of the intake air throttle valve8(hereinafter also called simply “intake air quantity”) and the flow rate of recirculated exhaust gas (hereinafter called “EGR quantity”) introduced downstream of the intake air throttle valve8. Assuming the engine operating state does not change, the total flow rate does not change and, therefore, the fresh air intake flow rate decreases when the EGR quantity increases and increases when the EGR quantity decreases. The fresh air intake flow rate also changes depending on the opening degree of the intake air throttle valve8, the fresh air intake flow rate being maximum when the intake air throttle valve8is fully open and decreasing as the opening degree becomes smaller.

Thus, once the target EGR ratio is determined, the target intake air quantity at that particular time, i.e., the target fresh air intake flow rate, is determined depending on the fuel injection quantity and the opening degree of the intake air throttle valve. As a result, the EGR quantity is relatively large when the actual intake air quantity is smaller than the target intake air quantity and, conversely, the EGR quantity is small when the actual intake air quantity is larger than the target intake air quantity.

Therefore, the EGR ratio can be feedback controlled by comparing the target intake air quantity to the actual intake air quantity that is measured and outputted by the air flow meter7.

The exhaust system is also provided with an oxidation catalytic converter11having an HC adsorbing function, a NOx trapping catalytic converter12having a NOx trapping function, and an exhaust gas fine particle capturing filter (DPF=diesel particulate filter)13arranged in sequence in the exhaust passage6at a position downstream of the turbine4bof the turbocharger4.

The oxidation catalytic converter11has the characteristic of adsorbing exhaust HCs when the temperature is low and releasing the HCs when the temperature is high and it functions to oxidize HCs and CO when in an active state. The NOx trapping catalytic converter12adsorbs or traps NOx contained in the exhaust gas when the excess air ratio λ is greater than 1, i.e., when the air fuel mixture is lean, and releases the NOx when the excess air ratio λ is rich. The NOx trapping catalytic converter12also functions to deoxidize the NOx when in an active state. The particulate filter13captures fine particles (PM=particulate matter) contained in the exhaust gas and the captured PM is combusted by raising the exhaust gas temperature using regeneration control.

A control unit20is provided to control the exhaust gas recirculation control system of the present invention. In particular, the control unit20determines and sets the intake air quantity Qa, the fuel injection quantity Qf and the injection timing IT based on detection signals from various sensors (described below) that serve to detect the operating state of the engine1and executes the controls based on these signals as explained below. Thus, the control unit20also controls the drive of the fuel injection valves3, controls the opening degree of the intake throttle valve8and the EGR valve10in response to detection signals from various sensors (described below).

The control unit20is a microcomputer comprising of a central processing unit (CPU) and other peripheral devices. The control unit20can also include other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. The control unit20preferably includes an engine control program that controls various components as discussed below. The control unit20receives input signals from various sensors (described below) that serve to detect the operating state of the engine1and executes the aforementioned controls based on these signals. It will be apparent to those skilled in the art from this disclosure that the precise structure and algorithms for the control unit20can be any combination of hardware and software that will carry out the functions of the present invention. In other words, “means plus function” clauses as utilized in the specification and claims should include any structure or hardware and/or algorithm or software that can be utilized to carry out the function of the “means plus function” clause.

The intake air quantity Qa is detected by the air flow meter7, which outputs a signal to the control unit20that is indicative of the intake air quantity Qa. The control unit20is also operatively coupled to a rotational speed sensor14, an accelerator position sensor15, an engine coolant temperature sensor16, a rail pressure sensor17, a plurality of exhaust system temperature sensors21,22and23, and an exhaust gas sensor or oxygen sensor24. The rotational speed sensor14is configured and arranged to detect the engine rotational speed Ne of the engine1, and output a signal to the control unit20that is indicative of the engine rotational speed Ne of the engine1. The accelerator position sensor15is configured and arranged to detect the accelerator position APO, and output a signal to the control unit20that is indicative of the accelerator position APO.

The coolant temperature sensor16is configured and arranged to detect the temperature of the engine coolant Tw, and output a signal to the control unit20that is indicative of the temperature of the engine coolant Tw. The rail pressure sensor17is configured and arranged to detect the fuel pressure (fuel injection pressure) inside the common rail2, and output a signal to the control unit20that is indicative of the fuel pressure (fuel injection pressure) inside the common rail2. The temperature sensors21,22and23are configured and arranged to detect the exhaust gas temperature in the general vicinity of the outlets of the oxidation catalytic converter11, the NOx trapping catalytic converter12, and the particulate filter13, respectively. The temperature sensors21,22and23are configured and arranged to output signals to the control unit20that are indicative of the exhaust gas temperature in the general vicinity of the outlets of the oxidation catalytic converter11, the NOx trapping catalytic converter12, and the particulate filter13, respectively. The exhaust gas sensor24is configured and arranged in the exhaust passage6at a position upstream of the turbine4bto detect the air fuel ratio or the oxygen concentration of the exhaust gas. The exhaust gas sensor24is configured and arranged to output a signal to the control unit20that is indicative of the air fuel ratio or the oxygen concentration of the exhaust gas.

Accordingly, the control unit20controls the regeneration of the NOx trapping catalytic converter12and the particulate filter13. In other words, the control unit20controls the fuel injection quantity Qf delivered by the fuel injection valves3, the injection timing IT of the fuel injection valves3in accordance with various engine operating conditions (e.g., accelerator position). The control unit20further configured to execute control to adjust the opening degree of the intake air throttle valve8so as to obtain the excess air ratio (lean or rich) requested based on the operating conditions. The control unit20further configured to execute feedback control to adjust the opening degree of the EGR valve10so as to achieve the target EGR ratio in each operating circumstance. The control unit20further configured to execute regeneration control to desorb and deoxidize the NOx held by the NOx trapping catalytic converter12when it is determined that the total NOx absorbed to the NOx trapping catalytic converter12has reached a prescribed value. The control unit20further controls the regeneration of the particulate filter13by executing regeneration control to raise the exhaust gas temperature and thereby combust/remove the particulate matter when the amount of particulate matter captured in the particulate filter13has reached a prescribed amount.

A basic characteristic feature of in this embodiment of the present invention is that the feedback control of the EGR ratio is restricted by the control unit20when the excess air ratio is adjusted in order to prevent hunting of the EGR ratio and avoid degradation of the engine operating performance. In other words, in the present invention, the feedback control of the exhaust gas recirculation is restricted when the excess air ratio is adjusted by using the intake throttle valve8, e.g., when the excess air ratio is reduced. Consequently, even if a response delay occurs between the operation of the intake throttle valve8and the actual change in the intake air quantity, the EGR quantity will not undergo temporary hunting and the degradation of the operating performance can be avoided. Meanwhile, when the intake air quantity stabilizes, the feedback control is resumed so that the targeted EGR ratio can be maintained with good precision.

In relation to the present invention, the control unit20carries out the functions of a feedback control section, an excess air ratio control section and a feedback control restricting section. The feedback control section is configured to feedback-control an exhaust gas recirculation quantity of the exhaust gas to be recirculated to the air intake system. The excess air ratio control section is configured to adjust an excess air ratio in response to a request to change the excess air ratio by using the intake air regulating device to change an intake air quantity. The feedback control restricting section is configured to temporarily restrict feedback control of the exhaust gas recirculation quantity delivered by the exhaust gas recirculation control device when the excess air ratio is being adjusted.

These control routines ofFIGS. 2 and 3are periodically executed in a cyclic manner at a prescribed fixed time interval when the engine1is operating in accordance with certain predetermined engine operating conditions. First, the exhaust gas cleaning control executed by the control unit20will now be described with reference to FIG.2.

In step S1, the control unit20reads in various signals from each of the sensors shown inFIG. 1that represent engine operating conditions including, but not limited to, the engine rotational speed Ne, the accelerator position APO, the fuel injection quantity, and the engine coolant temperature. In other words, the engine operating state, e.g., load condition and rotational speed condition, of the engine1is determined by the control unit20receiving signals from each of the sensors shown in FIG.1.

In step S2, the control unit20calculates the amount of NOx trapped and accumulated (adsorbed) in the NOx trapping catalytic converter12. There are various known methods of calculating the NOx accumulation amount using theses signals from the sensors of FIG.1. For example, the NOx accumulation amount can be estimated based on the distance the vehicle has traveled and/or a value obtained by integrating the engine rotational speed Ne. When the value of an integral is used, the integral value is reset at the point in time when regeneration control of NOx trapping catalytic converter12is completed.

In step S3, the control unit20calculates the amount of particulate matter (PM) captured and accumulated in the DPF13. One method of calculating the PM accumulation amount is to utilize the fact that the exhaust gas pressure in the vicinity of the inlet of the DPF13increases when the amount of PM accumulated in the DPF increases, and then estimate the PM accumulation amount by comparing the detected exhaust gas pressure in the vicinity of the inlet with a reference exhaust gas pressure for the current engine operating conditions (e.g., engine rotational speed and fuel injection quantity). The PM accumulation amount can also be estimated based on a combination of an integral of the engine rotational speed since the last regeneration of the DPF, the traveling distance since the last regeneration of the DPF, and the exhaust gas pressure.

In step S4, the control unit20determines if the reg1flag is on, indicating that the apparatus is in the DPF regeneration mode. If the value of the reg1flag is 1, then the control unit20proceeds to step S5and executes regeneration control of the DPF13. If the reg1flag is not set to 1, then the control unit20proceeds to step S6.

During regeneration control of the DPF13, the exhaust temperature is raised by using such techniques as executing a post-injection of fuel in which the PM captured in the DPF13is combusted. When regeneration control of the DPF13is finished, the value of the reg1flag is set to 0.

In step S6, the control unit20determines if the sp flag is on, indicating that the NOx trapping catalytic converter12is in a regeneration mode, i.e., rich spike mode (shifting of the excess air ratio λ to a rich value) for the purpose of desorbing and cleaning the NOx adsorbed by the NOx trapping catalytic converter12. If the value of the sp flag is 1, then the control unit20proceeds to step S7where it executes rich spike control (NOx trapping catalytic converter regeneration control). If the sp flag is not set to 1, then the control unit20proceeds to step S8.

The NOx trap catalyst regeneration control involves lowering the excess air ratio to a rich target value. The opening degree of the intake air throttle valve8is reduced, causing the excess air ratio of the exhaust gas to become rich, and the NOx accumulated (adsorbed and held) in the NOx trapping catalytic converter12during lean operation is desorbed and deoxidized. When the excess air ratio is adjusted, the fuel injection quantity is not changed and thus remains the same both before and after the adjustment. In short, the fuel injection quantity is computed based on the accelerator position, engine rotational speed, etc., and the excess air ratio is adjusted by adjusting the opening degree of the intake air throttle valve8so that the engine torque does not fluctuate during the period before and after the adjustment. When the rich spike control of the NOx trapping catalytic converter12is finished, the sp flag is set to 0.

In step S8, the control unit20determines if the PM accumulation amount of the DPF13calculated in step S3has reached the prescribed amount PM1, indicating that it is time to regenerate the DPF13. If the PM accumulation amount is larger than PM1, then the control unit20determines that it is time to regenerate the DPF13and turns on the reg1flag (i.e., sets the value of the reg1flag to 1) in step S9to indicate that regeneration is in progress. If the PM accumulation amount is smaller than PM1, then the control unit20proceeds to step S10.

In step S10, the control unit20determines if the amount of the NOx accumulated in the NOx trapping catalytic converter12has reached the prescribed amount NOx1, indicating that it is time to regenerate the NOx trapping catalytic converter12. If the NOx accumulation amount is larger than the NOx1, then the control unit20determines that it is time to regenerate the NOx trapping catalytic converter12and turns on the sp flag (i.e., sets the value of the sp flag to 1) in step S11to indicate that regeneration of the NOx trap flag is in progress.

The feedback control of the EGR valve10executed by the control unit20will now be described with reference to FIG.3.

In step S21, the control unit20reads in signals representing the operating state of the engine1, but not limited to such factors as the engine rotational speed, and the fuel injection quantity. In step22, the control unit20refers to a map to set the target EGR ratio in accordance with the operating state of the engine1. In other words, the target EGR ratio is set at least in accordance with whether an excess air ratio is required to be lean or rich.

In step S23, the control unit20calculates and sets the opening degree of the EGR valve10in order to achieve the target EGR ratio in accordance with the adjustment of the intake air throttle valve8.

In step S24, the control unit20calculates or estimates the actual intake air quantity based on the output of the air flow meter7.

In step S25, the control unit20calculates the target air intake quantity based on the target EGR ratio, the fuel injection quantity delivered to the engine main body1, and the intake air throttle valve opening degree at that particular time. Then the control unit20estimates the actual EGR quantity based on the target intake air quantity and the actual intake air quantity and calculates the actual EGR ratio based on the actual air intake quantity and the actual EGR quantity.

In step S26, the control unit20compares the target EGR ratio to the actual EGR ratio. If the actual EGR ratio is smaller than the target EGR ratio, then the control unit20proceeds to step S27and increases the opening degree of the EGR valve10by a fixed amount so as to increase the EGR ratio. Conversely, the if the actual EGR ratio is larger than the target EGR ratio, then the control unit20proceeds to step S28and decreases the opening degree of the EGR valve10by a fixed prescribed amount.

In this way, the control unit20feedback controls the opening degree of the EGR valve10so as to make the actual EGR ratio match a target EGR ratio set according to the operating state of the engine1.

The control of the EGR valve10executed when the excess air ratio is adjusted from a lean value to a rich value will now be described using the flowchart of FIG.4.

The control sequence shown inFIG. 4is executed when, during the processing ofFIG. 2, the NOx accumulation amount reaches the prescribed value NOx1, the sp flag is set to 1, and the excess air ratio is adjusted from lean to rich.

First, in step S31, the control unit20determines if the excess air ratio will be adjusted from lean to rich, e.g., a request to adjust the excess air ratio from lean to rich. If so, the control unit20proceeds to step S32and restricts the feedback control of the EGR ratio. In this embodiment, the restriction is a temporary prohibition of the feedback control of the EGR ratio.

In step S33, the control unit20sets the target EGR ratio based on the rich excess air ratio that is desired or needed for executing regeneration control of the NOx trapping catalytic converter12. In step S34, the control unit20refers to the aforementioned map and sets the opening degree of the EGR valve10so as to achieve the new target EGR ratio having a rich value.

In step S35, the control unit20determines if the time t elapsed since the opening degree of the EGR valve10was set has reached a predetermined fixed waiting time tset. The waiting time tset is set, for example, to a value corresponding to the amount of time (delay time) required for the intake air quantity to change after the opening degree of the intake air throttle valve8is reduced for the purpose of switching the excess air ratio from a lean value to a rich value.

If the waiting time tset has elapsed, the control unit20proceeds to step S36, where it begins feedback control of the opening degree of the EGR valve, and ends the processing sequence.

The feedback control of the EGR valve10will now be described centering on the operations performed when adjusting the excess air ratio from a lean state to a rich state.

During lean operation, the opening degree of the EGR valve10is feedback controlled so as to achieve the targeted EGR ratio.

After the target EGR ratio is determined, then the target intake air quantity and the target opening degree of the EGR valve10are determined in accordance with the opening degree of the intake air throttle valve8at that particular point in time. So long as the operating state does not change and the total flow rate of the intake air and recirculated exhaust gas drawn into the engine1remain the same, then the intake air quantity or flow rate decreases in a relative manner when the EGR quantity increases and increases in a relative manner when the EGR quantity decreases.

Thus, the actual EGR ratio can be calculated based on the output of the air flow meter7, which measures the intake air quantity. Thus, the opening degree of the EGR valve10can be feedback controlled such that the actual EGR ratio matches the target EGR ratio.

Although EGR reduces the amount of NOx contained in the exhaust gas, it can cause an increase in smoke if the EGR ratio becomes larger than the target EGR ratio. Therefore, by feedback controlling the opening degree of the EGR valve10as described, the EGR ratio can be controlled accurately to the target EGR ratio value and both the reduction of NOx and the prevention of an increase in smoke can be accomplished.

When the accumulated amount of NOx adsorbed and held by the NOx trapping catalytic converter12during lean operation reaches a prescribed upper limit value, the NOx trapping catalytic converter12can no longer adsorb NOx. When this occurs, the control unit12executes a catalyst regeneration operation in which the excess air ratio is adjusted from lean to rich.

In order to adjust the excess air ratio to a rich value, the opening degree of the intake air throttle valve8is reduced to a prescribed target opening degree. Since the negative pressure of the air intake system increases as a result, the amount of the exhaust gas recirculated from the EGR passage9(which is connected downstream of the intake air throttle valve8) will increase greatly if the opening degree of the EGR valve10is left unchanged. Therefore, the opening degree of the EGR valve10is also reduced to a target EGR ratio suited to rich operation.

However, the following problem will occur if the feedback control of the EGR ratio is executed continuously through the adjustment process.

Although the opening degrees of the intake air throttle valve8and the EGR valve10are adjusted simultaneously, there is a delay time between when the opening degree of the intake air throttle valve8changes and when the intake air quantity changes in response. In short, the intake air quantity does not change immediately.

If feedback control of the EGR valve10is continued, the EGR quantity will be insufficient when the opening degree of the EGR valve10is adjusted to the rich target value because the intake air quantity will be large in comparison with the target EGR ratio. Then, the feedback control will correct the opening degree of the EGR valve10to a much larger value based on the output of the air flow meter7. As a result, the EGR quantity will increase while the intake air quantity decreases with time and EGR ratio will momentarily become much larger than the requested target value. After that, the feedback control will again correct the opening degree of the EGR valve10to a smaller opening. In short, the feedback control will undergo hunting and the EGR ratio will exhibit large fluctuations, causing the engine to exhibit unstable operating performance and such effects as noise and vibrations.

When the excess air ratio is reduced to the rich state, the EGR ratio is also set to a smaller value than is used during lean operation in order to avoid increasing the amount of smoke in the exhaust gas. If, as just described, the EGR ratio increases, even momentarily, after the excess air ratio is reduced, the effect on the exhaust performance will be too significant to be ignored.

In view of the problem just described, this embodiment temporarily prohibits feedback control of the EGR valve10when the excess air ratio is adjusted from lean to rich. Thus, this embodiment instead uses open-loop control (feed forward control) to adjust the opening degree of the EGR valve10to the target value.

Thus, even if a response delay occurs between reduction of the opening degree of the intake air throttle valve8and the change in the intake air quantity, the opening degree of the EGR valve10(which is adjusted simultaneously) will not repeatedly increase and decease due to hunting. Instead, during this response delay period, the actual EGR ratio does not change immediately to the target EGR ratio but converges on the target EGR ratio with some degree of delay.

As a result, engine output fluctuations and worsened noise and vibration resulting from temporary excessive EGR can be avoided.

When the intake air quantity response delay associated with changing the excess air ratio is over, normal feedback control is resumed. Thereafter, the opening degree of the EGR valve10is accurately feedback controlled so as to maintain the target EGR ratio. In short, stable, precise EGR control is executed without increasing the smoke or other emissions in the exhaust gas.

As previously explained, with this embodiment, the feedback control of the exhaust gas recirculation is restricted when the excess air ratio is adjusted by using the intake air throttle valve8, e.g., when the excess air ratio is reduced. Consequently, even if a response delay occurs between the operation of the intake air throttle valve8and the actual change in the intake air quantity, the EGR valve will not respond excessively and the EGR quantity will not undergo temporary hunting, thus enabling degradation of the operating performance to be avoided. Meanwhile, when the intake air quantity stabilizes, the feedback control is resumed so that the targeted EGR ratio can be maintained with good precision and NOx reduction and smoke prevention can be accomplished.

By setting the time period during which feedback control of the EGR is temporarily restricted to be equivalent to the time period of the response delay of the actual intake air quantity with respect to the change in the opening degree of the intake air throttle valve8, the amount of time during which feedback control is restricted can be held to the minimum necessary and degradation of the exhaust performance can be curbed to the greatest degree possible.

The feedback control of the EGR in this embodiment is executed while estimating the actual EGR ratio based on the fuel injection quantity and the opening degree of the intake air throttle valve (both of which are calculated by the control unit20in accordance with the operating state), as well as the intake air quantity detected by the air flow meter7. As a result, the feedback control can be executed with good precision without installing any special sensors or the like.

Although in this embodiment the feedback control of the EGR valve10is temporarily prohibited when the excess air ratio is adjusted from rich to lean, it is also possible to temporarily lower the control gain of the feedback control so as to delay the response of the feedback control and prevent hunting. When the control gain of the feedback control is lowered, corrections imposed by the feedback control do not take effect immediately and the opening degree of the EGR valve10is prevented from repeatedly increasing and decreasing, thus increasing the stability of the control during adjustment of the excess air ratio.

Additionally, during the period when the excess air ratio is being adjusted, in addition to prohibiting feedback control of the EGR valve10, it is possible to calculate the EGR ratio during the adjustment period by interpolating based on the lean and rich target EGR ratios and the actual excess air ratio, use the interpolated EGR ratio as the target EGR ratio during the response delay period of the intake air quantity, and then execute open-loop control of the opening degree of the EGR valve10based on this target value.

In such a case, during the period from when the opening degree of the intake air throttle valve8is adjusted from lean to rich until the actual intake air quantity catches up with the change, the opening degree of the EGR valve10is controlled successively to target opening degrees calculated by interpolation and the EGR quantity is accurately controlled in accordance with the actual excess air ratio. As a result, the stability of the engine combustion performance can be improved during the period when the excess air ratio is transient.

Although in the previously described embodiment the feedback control of the EGR is temporarily restricted when the excess air ratio is adjusted from lean to rich, it is also possible to execute a similar control (restriction) during adjustments from rich to lean.

It is also possible to feedback control the EGR quantity by calculating the excess air ratio of the exhaust gas based on the detection value of the exhaust gas sensor24and estimating the EGR quantity based on the calculated excess air ratio of the exhaust gas, the intake air quantity, and the opening degree of the intake air throttle valve.

Adjusting the excess air ratio from lean to rich is not limited to regeneration control of the NOx trapping catalytic converter12. It is also acceptable to adjust the excess air ratio in response to requests related to such factors as the activity state of the catalyst.

The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.

This application claims priority to Japanese Patent Application No. 2003-283288. The entire disclosure of Japanese Patent Application No. 2003-283288 is hereby incorporated herein by reference.