DE19918909A1 - Air fuel ratio control system for engine with oxidation- and nitrogen oxides storage catalysts - Google Patents

Abstract

Engine lean burn operation alternates with NOx catalyst rich regeneration. A sensor (27) upstream of the NOx catalyst (14) in the exhaust pipe (12) measures exhaust oxygen concentration. A controller (30) controls rich combustion and degree of enrichment of exhaust gases for reaction in the NOx catalyst, by monitoring the signal from the oxygen concentration sensor (27) Preferred features: The catalyst (13) upstream of the oxygen concentration sensor (27) has oxygen storage capacity. There is a further catalyst (not shown) upstream of the sensor and NOx catalyst, with small oxygen storage capacity. An NOx computer (31) determines NOx adsorbed in the catalyst (14). A computer (31) determines a reference rich quantity on the basis of the calculated quantity of NOx adsorbed in the NOx catalyst. The control unit (30, 31) determines combustion of the rich mixture corresponding to the calculated reference amount. The quantity of rich components accumulated is determined from the oxygen sensor output. Rich combustion is continued until the value reaches the reference amount for rich components. The controller calculates an interval from the oxygen sensor, required to vary air fuel ratio to a rich value exceeding stoichiometric, during rich combustion. Rich combustion is continued until the computed reference quantity of rich components has been reached. In a similar operation, the time and quantity criteria can be combined.

Description

The present invention relates to an air force Substance ratio control system for an internal combustion engine around in an area where an air-fuel ratio is lean to allow a lean mixture combustion. In particular, the invention relates to an air force material ratio control system for an internal combustion engine, which is a NOx occlusion and reduction catalyst Reduction of nitrogen oxides (NOx), which at the time of Lean mixture combustion are given off.

In an air-fuel ratio control system for one Internal combustion engines have become one in recent years Lean mixture combustion control (control for a lean A / F (air / fuel ratio) run to a Air-fuel mixture on the lean air force material ratio side in relation to the stoichiometric Run ratio to increase fuel consumption improve. In the case of execution of the Lean-burn combustion contains exhaust gas from that Internal combustion engine is released, a large amount of NOx. This makes a lean NOx catalyst to reduce the NOx emissions to the atmosphere required. That is why proposed a NOx occlusion and in an engine exhaust system Reduction catalyst (NOx catalyst) to provide NOx emitted from the engine, or to clean one To provide the NOx sensor in the engine exhaust system to the Monitor NOx concentration in the exhaust gas.

In a system for occlusion or absorption of during the Lean-burn combustion NOx generated by the NOx catalytic converter reaches its NOx cleanability Limit when the absorption of NOx in the NOx catalyst is saturated. Consequently, it is proposed in the engine  at times a combustion of a rich mixture to carry out the emission of NOx by restoration suppress the cleanability of the NOx catalyst.

According to the above conventional system, it becomes necessary to Time to burn the fat mixture to determine the To compensate for gas transport delay in the exhaust pipe. If the time of the fat burn this way is prolonged, fuel enrichment tends to to last a long time, worsening fuel economy becomes. The engine torque is increased during the combustion of the increase fat mixture. If this lasts a long time, it takes Fluctuation in engine speed too, resulting in an undesirable Driving property of the vehicle. When the time of the fat If the combustion of the mixture is too short, the reduction takes place insufficiently absorbed NOx in the NOx catalyst, which in the emission of unpurified NOx into the atmosphere results.

For example, in JP-A-7-189660 there is a NOx-Ab sorbent (NOx catalyst) in an engine exhaust system provided and an air-fuel ratio sensor (A / F sensor) is upstream of the NOx absorbent provided so that the NOx absorption amount of the NOx-Ab sorbent based on the output signal of the A / F sensor is estimated.

The present invention therefore has the task of Air-fuel ratio control system for an internal combustion engine to create the delivery of the amount of fat Can control fuel gas, which is necessary to that in the NOx catalyst absorbed NOx to an optimal value to reduce.

In order to solve the problem, an inventive one Air-fuel ratio control system for an internal combustion engine the combustion of a lean mixture in one area of a  lean air-fuel ratio, so that NOx, the Ejected exhaust gas mixture combustion time is absorbed by a lean NOx catalyst. The Lean mixture combustion is temporarily due to a combustion of a rich mixture to switch the absorbed NOx reduce and expel. The burning period of the fat The mixture is determined by the degree of enrichment of the exhaust gas is supplied to the NOx catalyst in response to a Output signal of the oxygen concentration sensor is provided upstream of the NOx catalyst, controlled.

Preferably an additional catalyst is one Has oxygen storage capacity upstream of that Oxygen concentration sensor provided. Alternatively, it is an additional catalyst that is only a minor Has oxygen storage capacity at an upstream Provided the NOx catalyst and the Oxygen concentration sensor is on the upstream Place of the additional catalyst provided.

Other tasks, features and advantages of the present Invention will be detailed based on the following Description with reference to the accompanying drawings obviously. The following is shown in the drawings:

Fig. 1 is a schematic block diagram of an air-fuel ratio control system for an internal combustion engine according to a first embodiment of the invention.

Fig. 2 is a flow diagram of a portion of an air-fuel ratio control, which is executed in the first embodiment.

Fig. 3 is the other part of an air-fuel ratio control, which is carried out in the first embodiment.

FIGS. 4 (a) and (b) are diagrams for determining a NOx base amount and a correction value.

Fig. 5 is a diagram for determining a reference value based on the NOx absorption ability.

Fig. 6 is a diagram for determining a reference rich area corresponding to an accumulated NOx amount.

FIGS. 7 (a) and (b) are timing diagrams of the operation of the first embodiment and the prior art.

Fig. 8 is a schematic diagram of a control system according to a second embodiment of the present invention.

Fig. 9 is a flow diagram of a portion of an air-fuel ratio control, which is executed in the second embodiment.

Fig. 10 is the other part of the air-fuel ratio control which is carried out in the second embodiment.

Fig. 11 is a diagram for determining a reference fat area DRAFN in correspondence to an accumulated amount of NOx.

Fig. 12 is a schematic diagram of a control system according to a third embodiment of the present invention.

Embodiments of the present invention are in the following with reference to the drawings  described in which the same or similar throughout Reference signs the same or similar components describe. In an air-fuel ratio control system that following embodiments will Lean-burn combustion control executed, for fixing a target air-fuel ratio (A / F) of a mixture, that is delivered to an internal combustion engine, to a lean one Air-fuel ratio side in relation to the stoichiometric Air-fuel ratio, and to grant one Lean burn based on this Target air-fuel ratio to be executed. As The basic structure of the system is one Three-way catalyst and a NOx absorption and -Reduction catalyst (NOx catalyst) in an exhaust line of the Internal combustion engine provided, and an air force Material ratio sensor (A / F sensor) of the limit current type is between the three-way catalyst and the NOx catalyst arranged. One mainly from a microcomputer composite electronic control unit (ECU) occupies Detection result of the A / F sensor to a Air-fuel ratio control with feedback (F / B) on the Execute the basis of the acquisition result.

(First embodiment)

Referring to FIG. 1, the internal combustion engine is a spark ignition four-cylinder four-stroke engine 1. Intake air flows from the upstream side through an air filter 2 , an intake pipe 3 , a throttle valve 4 , an intermediate store 5 and an intake manifold 6 . The intake air is mixed with fuel that is injected from fuel injection valves 7 of the respective cylinders in the intake manifold 6 . The mixture is supplied to the respective cylinders in a predetermined air-fuel ratio.

A high voltage supplied from an ignition circuit (IG) 9 is distributed through a distributor 10 to a spark plug 8 provided for each cylinder in the engine 1 , and the spark plug 8 ignites the mixture of each cylinder at a predetermined timing. Exhaust gas discharged from each cylinder after the mixture is burned flows through an exhaust manifold 11 , an exhaust pipe 12 , a three-way catalyst 13 that purifies the exhaust gas from three gas components (HC, CO and NOx), and one NOx catalyst 14 , which purifies the exhaust gas of NOx before it is discharged into the atmosphere. The three-way catalytic converter 13 has a purifying capacity smaller than that of the NOx catalytic converter 14 and serves as a starting catalytic converter that is quickly activated after an engine start at a low temperature to purify the exhaust gas. The NOx catalyst 14 absorbs NOx during combustion of a lean air-fuel mixture and reduces the absorbed NOx with rich gas components such as CO and HC during combustion of a mixture with a rich air-fuel ratio.

The intake line 3 is provided with an intake temperature sensor 21 and an intake pressure sensor 22 . The intake temperature sensor 21 detects a temperature of the intake air (intake air temperature Tam) and the intake pressure sensor 22 detects a negative pressure in the intake line (intake air pressure PM) downstream of the throttle valve 4 . The throttle valve 4 is provided with a throttle valve sensor 23 in order to detect the opening angle of the throttle valve 4 (throttle valve opening angle TH). The throttle valve sensor 23 generates an analog signal according to the throttle valve opening angle TH. The throttle valve sensor 23 has an idle switch and generates a detection signal indicating that the throttle valve 4 is substantially closed.

A cylinder block of the engine 1 is provided with a coolant temperature sensor 24 . The coolant temperature sensor 24 measures the temperature of the cooling water (cooling water temperature Thw) circulating in the engine 1 . The distributor 10 is provided with a speed sensor 25 in order to measure the speed of the engine 1 (engine speed Ne). The speed sensor 25 generates 24 pulse signals at the same intervals every two rotations of the engine 1 , ie every 720 ° crank angle.

Further, the limit current type A / F s sensor 27 provided between the catalysts 13 and 14 generates linear signals of a linear air-fuel ratio in a wide range in proportion to the oxygen concentration of the exhaust gas discharged from the engine 1 (or for the CO concentration of unburned gases). The A / F s sensor 27 has a heater 47 to activate the sensing element (solid electrolyte and diffusion resistance layer). The A / F s sensor 27 may be of a cup-shaped type that has a cup-shaped solid electrolyte, or it may be of a layered type that has a stack of a plate member and a heater.

The ECU 30 is constructed as a logical operation unit, the main components of which are a CPU (central processing unit) 31 , a ROM (read-only memory) 32 , a RAM (random access memory) 33 and a backup RAM (back-up RAM) ) 34 , and which are connected to an input connection (IP) 35 for receiving the detection signals from the sensors, and to an output connection (OP) 36 for delivering control signals to the actuators and the like via a bus 37 . The ECU 30 receives detection signals (intake air temperature Tam, intake air pressure PM, throttle opening angle TH, cooling water temperature Thw, engine speed Ne, air-fuel ratio signal and the like) from the various sensors via the input port 35 . The ECU 30 generates control signals such as a fuel injection amount TAU and an ignition timing (timing) Ig based on the received values, and outputs the control signals to the fuel injection valve 7 , the ignition circuit (Ig) 9, and the like via the output terminal 36 .

The CPU 31 keeps the A / F sensor 27 in an active state by performing power control on the heater excitation amount of each A / F sensor 27 . In the exemplary embodiment, the necessary amount of electrical energy is supplied to the heater 47 of the A / F sensor 27 , as a result of which the element temperature of the sensor 27 is kept within an activation temperature range.

The air-fuel ratio control in the first Embodiment will now be described in detail.

A fuel injection control routine performed by the CPU 31 shown in FIGS. 2 and 3 is executed every fuel injection of each cylinder (every 180 ° crank angle in the embodiment). In this control routine, the fuel injection amount is normally feedback controlled when the air-fuel ratio is lean with respect to the stoichiometric ratio based on the detection result of the A / F sensor 27 upstream of the NOx catalyst 14 . During this lean combustion control, the air-fuel ratio is temporarily changed to a rich air-fuel ratio.

If the routine of FIGS. 2 and 3 begins, the CPU 31 checks in step 101 whether or not a rich A / F control flag XREX that represents the execution of the rich mixture control or the combustion thereof is 0. XREX = 0 means that no enriched mixture combustion takes place, ie there is a lean mixture combustion, while XREX = 1 means that a rich mixture combustion takes place. It should be noted that the flag XREX is reset to 0 by initialization at the time the electric power is turned on by an ignition switch (not shown).

If XREX = 0 (lean burn), the CPU estimates the amount of NOx NOMOL (mol) that is in the exhaust gas at step 102 . This estimate of NOMOL can be performed with reference to a data table that defines the NOx base amount in relation to the engine speed Ne and the intake pressure PM, as shown in Fig. 4 (a). At this moment, the A / F correction value is further determined by referring to the data table which defines the A / F correction in relation to the A / F as shown in Fig. 4 (b). The NOx base amount and the A / F correction value are multiplied to determine the NOx amount (NOMOL), that is, NOMOL = NOx base amount × A / F correction value).

In Fig. 4 (a), the basic NOx amount increases as the engine speed Ne or the intake pressure increases. In Fig. 4 (b), the A / F correction value for the stoichiometric air-fuel ratio is set to 1 (λ = 1 corresponds to an A / F = 14.8). It is set to a larger value when A / F is on the lean side (λ <1). Although not shown, it converges to a predetermined value when A / F is very much on the lean side (A / F <16) because the combustion temperature decreases and no further correction is necessary.

The CPU 31 then calculates an amount NOMOLAD from an accumulated NOx amount in step 103 . That is, the value NOMOL continuously calculated in step 102 is added to the previously calculated NOMOLAD in order to determine the current accumulated value (NOMOLAD = NOMOLAD + NOMOL).

The CPU 31 further checks in step 104 whether the NOMOLAD value of collected NOx is larger than a predetermined reference value NOMOLSD. This reference value NOMOLSD can be a fixed value or a variable value that changes with the NOx absorption capacity of the NOx catalytic converter according to FIG. 5. The NOx absorption ability corresponds to the deterioration of the NOx catalyst 14 . That is, when the NOx catalyst 14 deteriorates, the NOx absorbency decreases.

If the check result in step 104 is NO (NOMOLAD ≦ NOMOLSD), the CPU 31 sets a TARGET A / F (AFTG) to 1.5 in step 105 and calculates a fuel injection amount (injection period) TAU based on the AFTG in step 106 . Value in the well-known way. More specifically, the basic fuel injection amount is calculated and corrected by an A / F correction value calculated based on a difference between the target ratio AFTG and an actual air-fuel ratio AF (detection value of the A / F sensor 27 ) and other various correction values. The fuel injection valves 7 are driven by a pulse signal that corresponds to the calculated period TAU. In this way, the lean A / F control for the lean mixture combustion continues as long as the test result in step 104 is NO.

If the test result in step 104 is due to the increase of the value NOMOLAD of accumulated NOx YES (NOMOLAD <NOMOLSD), the CPU 31 F-controller sets the FLAG XREX for the rich A / in step 107 to 1. In the subsequent step 108 calculates the CPU 31 has a reference rich range DRAFND which corresponds to the NOMOLAD value of the accumulated NOx. The reference rich range DRAFND means the enriched A / F control amount required to reduce and exhaust all of the NOx absorbed into the NOx catalyst 14 . This DRAFND can be determined based on a data table that defines a relationship between the value of the collected NOx NOMOLAD and the suction pressure PM, as shown in FIG. 6. According to Fig. 6 of the reference fat area DRAFND is increased when the value of the accumulated NOx NOMOLAD increases and if the intake pressure PM decreases. Alternatively, the reference grease range DRAFND can be determined independently of the intake pressure PM or depending on other parameters such as the engine speed Ne and the intake air quantity.

The CPU 31 then sets the target ratio AFTG to a rich value 0.75 in step 109 and calculates the fuel injection amount TAU based on the target ratio AFTG in step 106 . In this way, the A / F control is changed from the lean A / F control to the rich A / F control when the test result in step 104 changes from NO to YES.

After the A / F control is changed to the rich A / F control, the CPU 31 determines NO in step 101 and executes step 110 ( FIG. 3). The CPU 31 subtracts the actual AF from the A / F reference value AFSD (for example, the stoichiometric ratio) and determines the difference as a fat deviation DRAF (DRAF = AFSD - AF). The CPU 31 then checks in step 110 whether the fat deviation DRAF is positive (DRAF <0), that is, whether the actual AF is on the richer side than the A / F reference value AFSD.

If the check result in step 110 is NO (DRAF ≦ 0), the CPU 31 executes step 109 ( Fig. 2) to continue the above rich A / F control. If the check result in step 111 is YES (DRAF <0), the CPU 31 executes step 112 to calculate an actual fat range DRAFAD. More specifically, the value DRAF currently calculated in step 110 is added to the previously calculated value DRAFAD in order to determine the current value DRAFAD (DRAFAD = DRAFAD + DRAF).

The CPU 31 then checks in step 113 whether the actual fat range DRAFAD is greater than the reference fat range DRAFND, which was calculated in step 108 . If the check result in step 113 is NO (DFAFAD ≦ DRAFND), the CPU 31 executes step 109 ( Fig. 2) to continue the rich A / F control with steps 109 and 106 . If the check result in step 113 is YES (DRAFAD <DRAFND), the CPU 31 executes step 114 to reset the rich A / F control flag XREX (XREX = 0) and clears the value of the accumulated NOx (NOMOLAD = 0) and the actual fat range (DRAFAD = 0). Thereafter, the CPU 31 executes step 105 . In this way, the rich A / F control is ended and the lean A / F control is resumed when the test result changes to YES in step 113 .

The first embodiment operates as shown in Figs. 7 (a) and (b), wherein t1-t4 and t11-t13 mean periods of the rich A / F control, which are temporarily during the normal lean A / F control be performed. During that period, the A / F ratio shifts upstream and downstream from the three-way catalyst 13 to the rich side. In fact, the A / F ratio changes downstream of the three-way catalyst 13 with a transportation delay from the upstream side of the three-way catalyst 13 . In Fig. 7 such changes are shown briefly occur at the same time.

According to Fig. 7 (a) the lean A / F control before the time t1 is performed so that the amount of NOx NOMOL and its accumulated value NOMOLAD is calculated (steps 102 and 103 in Fig. 2). When the value of the accumulated NOx NOMOLAD reaches the reference value NOMOLSD at time t1, the target A / F ratio and the resulting actual A / F ratio are changed from a lean to a rich ratio. At this time t1, the reference fat range DRAFND is calculated based on the value of the accumulated NOx NOMOLAD (step 108 in FIG. 2).

At time t2, the A / F ratios upstream and downstream of the three-way catalyst 13 reach the stoichiometric ratio (λ = 1). The A / F ratio upstream of the three-way catalyst 13 changes immediately to the richer side from the stoichiometric ratio. However, since the three-way catalyst 13 stores oxygen released during the lean A / F control, the stored oxygen reacts with the rich gas components (HC, CO, etc.) in the exhaust gas in response to the rich A / F control was generated. In this way, the A / F ratio remains downstream of the three-way catalyst 13 for a moment at the stoichiometric ratio. After the stored oxygen reacts with the rich gas components, the A / F ratio changes downstream of the three-way catalyst to the rich side. After the time t3, the rich gas components enter the NOx catalyst 14 and the NOx absorbed in the NOx catalyst 14 is reduced and discharged into the atmosphere.

After changing to the rich A / F control, the actual fat range DRAFAD is calculated (step 112 in FIG. 3) when the A / F sensor 27 detects the rich A / F ratio, the richer than the stoichiometric one, at time t3 Ratio is. Then, the target A / F ratio is changed to the lean A / F ratio at time t4 when the actual fat range DRAFAD reaches the reference fat range DRAFND (YES in step 113 in FIG. 3).

Thereafter, the A / F ratio downstream of the three-way catalyst 13 is kept at the stoichiometric ratio for a period (time t5-t6) during which the lean gas components in the exhaust gas supplied from the engine 1 with the rich gas components, which are stored in the three-way catalyst 13 , react. After the time t6, the actual A / F ratio returns from the downstream side of the three-way catalyst 13 to the lean A / F ratio.

It should be noted that in Fig. 7 (a) the actual fat range DRAFAD calculated for the period t3-t4 is approximately half the fat range for a period (t3-t5) in which the A / F- Ratio downstream of the three-way catalyst 13 is actually the rich A / F ratio. In this case, the reference fat range DRAFND is set to correspond to the actual fat range DRAFAD (approximately half of the range for the period t3-t4). Alternatively, it is advantageous to compare the double value of the actual fat area DRAFAD of the period t3-t4 with the reference fat area DRAFND if the reference fat area DRAFND is determined so that it corresponds to the fat area of the period t3-t5.

According to Fig. 7 (b), in contrast to the operation shown in Fig. 7 (a), the target A / F ratio from the lean A / F ratio at the time t11 to the rich A / F Ratio changes and the actual A / F ratios upstream and downstream of the three-way catalyst 13 begin to change to the rich side. At time t12, the A / F ratio downstream of the three-way catalyst 13 is kept at the stoichiometric ratio for a moment due to the oxygen stored in the three-way catalyst 13 . However, the rich A / F ratio control can be ended at time t13 before the A / F ratio downstream of the three-way catalyst 13 changes to the rich side. This happens when the period of the rich A / F control (t11-t13) is fixed. In this way, only a small amount of NOx absorbed in the NOx catalyst 14 can be reduced and exhausted, resulting in an increase in the emission of unpurified NOx.

In this way, it is understood that the unpurified NOx increases due to the shortness of the actual rich A / F ratio time in the prior art shown in FIG. 7 (b), while the unpurified NOx in this embodiment shown in FIG. 7 (a) decreases.

The first embodiment provides the following Advantages.

The three-way catalyst 13 and the NOx catalyst 14 are provided upstream and downstream in the exhaust pipe 12 , and the A / F sensor 27 is located between the catalysts 13 and 14 . The A / F sensor 27 monitors the degree of richness of the exhaust gas supplied to the NOx catalyst 14 to control the rich A / F ratio control period. In this way, it becomes possible to supply an appropriate amount of rich gas components necessary to reduce the NOx absorbed and to keep the amount of NOx absorbed below a certain value.

More specifically, oxygen is stored in the three-way catalyst 13 that is upstream of the NOx catalyst 14 . It is possible to check from time to time by monitoring the degree of richness of the exhaust gas by the output signal of the A / F sensor 27 whether the rich gas components and the oxygen in the three-way catalyst 13 have reacted or have finished the reaction . As a result, it becomes possible to perform the rich A / F ratio control in accordance with the reaction amount with the stored oxygen. That is, an appropriate amount of rich gas components necessary to reduce NOx absorbed in the NOx catalyst 14 can be supplied. As a result, deterioration in fuel economy, torque fluctuation, increase in HC and CO output can be avoided, all of which otherwise occur with excessive supply of rich gas components. Furthermore, the emission of NOx, which occurs in the case of insufficient rich gas components, can be prevented.

As is well known, rich gas components and lean gas components in the A / F sensor 27 react at its electrode, thereby generating its signal corresponding to the excessive amount of gas components that are not responding. In this case, the exhaust gas component supplied to the NOx catalyst 14 located on a downstream side of the A / F sensor 27 is not the exhaust gas itself that is output from the engine 1 , but the gas that is in the A / F sensor 27 has responded. Therefore, it is advantageous to detect the amount of the excessive gas component after the reaction in the A / F sensor 27 and to control the rich ratio control depending on the detected amount of the excessive gas component, because then the reduction of the absorbed NOx in the NOx Catalyst 14 can be achieved effectively.

Further, during the lean A / F ratio control, the amount of NOx absorbed in the NOx catalyst 14 (value of accumulated NOx NOMOLAD) is calculated, and the reference amount of the rich gas component (reference rich range DRAFND) for the rich A / F ratio control is calculated calculated on the basis of the calculated value NOMOLAD. The rich A / F ratio control is continued for a period that corresponds to the reference fat range DRAFND. More specifically, the accumulated value of the rich amount (actual rich area DRAFAD immediately upstream of the NOx catalyst) is determined based on the output signal of the A / F sensor 27 during the rich A / F ratio control. The rich A / F ratio control is continued until the determined value DRAFAD reaches the reference fat range DRAFND. In this way, an appropriate amount of rich components can be supplied to the NOx catalyst 14 .

Furthermore, the rich control is carried out to obtain the actually required reference fat range DRAFND by monitoring the output signal of the A / F sensor 27 . Therefore, it is still possible to supply the appropriate amount of rich components to the NOx catalyst 14 when the oxygen storage capacity of the three-way catalyst 13 changes due to temperature changes and a change in the degree of deterioration of the three-way catalyst changes.

(Second embodiment)

In the second embodiment shown in FIG. 8, an O ₂ sensor 28 , which generates an electromotive force-voltage signal VOX2 depending on whether the A / F ratio is richer or leaner than the stoichiometric ratio, is between the three-way catalyst 13 and the NOx catalyst 14 are provided.

The CPU 31 of the ECU 30 is programmed to execute the A / F ratio control routine shown in FIGS. 9 and 10. In steps 101-103 , the CPU 31 calculates the value of the accumulated NOx NOMOLAD when the rich A / F ratio control flag XREX = 0. Since an O ₂ sensor 28 is used in this embodiment, no A / F feedback correction is performed when the NOx amount NOMOL is calculated in step 102 . That is, the NOx base amount is set as the value NOMOL, as shown in Fig. 4 (a).

If the check result in step 104 is NO (NOMOLAD ≦ NOMOLSD), the CPU 31 sets the A / F correction value FAF to 0.5 in step 201 so that the lean A / F ratio control is carried out based on this FAF value becomes.

If the check result in step 104 is YES (NOMOLAD <NOMOLSD), the CPU 31 sets the flag XREX to 1 in step 107. Then, in step 202 , the CPU 31 calculates a reference rich time DRAFTM corresponding to the accumulated amount NOMOLAD. This reference rich time DRAFTM corresponds to the rich A / F control amount required to reduce and exhaust all of the NOx absorbed in the NOx catalyst 14 . The reference rich time DRAFTM can be determined from the value of the NOx NOMOLAD collected and the intake pressure PM, as shown in FIG. 11. More specifically, the reference rich time DRAFTM increases as the value of the NOx NOMOLAD collected increases and the intake pressure PM decreases. The intake pressure PM used as a parameter for determining the reference rich time DRAFTM may be omitted, or other parameters such as the engine speed Ne or the intake air amount may be used instead.

The CPU 31 sets the A / F feedback correction value FAF to 1.25 in step 203 to perform the rich A / F control based on the FAF value, thereby moving the A / F control from the lean A / F Control for bold A / F control is changed.

After the change to the rich A / F control, the CPU 31 checks in step 204 in FIG. 10 whether the output signal (VOX2) of the O ₂ sensor 28 indicates the rich A / F ratio. If the check result is YES, the CPU 31 calculates the actual rich time CRAFAD in step 205 by increasing the previously calculated rich time CRAFAD by one.

The CPU 31 checks whether the actual rich time CRAFAD reaches the reference rich time DRAFTM calculated in step 202 . If the check result in step 206 is NO (CRAFAD ≦ DRAFTM), the CPU 31 executes step 203 in Fig. 9 to continue the rich A / F ratio control.

If the check result in step 206 is YES (CRAFAD <DRAFTM), the CPU 31 executes step 207 to reset the rich control flag XREX, the value of the accumulated NOx NOMOLAD, and the actual rich time CRAFAD to zero. The CPU 31 then executes step 201 in FIG. 9. In this manner, in response to YES in step 206, the rich A / F ratio control is ended and the lean A / F control is resumed.

According to the second embodiment, as in the first embodiment, the most appropriate amount of rich components can be supplied to reduce and exhaust NOx absorbed in the NOx catalyst 14 . Furthermore, the rich A / F ratio control is carried out only for the period actually required, ie for the reference rich time DRAFTM, by monitoring the output signal of the O ₂ sensor 28 . Accordingly, the appropriate amount of rich components can be supplied to the NOx catalyst 14 even if the oxygen storage capacity of the three-way catalyst 13 changes with changes in the temperature of the three-way catalyst 13 and the degree of deterioration thereof.

(Third embodiment)

In the third embodiment shown in FIG. 12, a three-way catalyst 15 having a small oxygen storage capacity is provided upstream of the NOx catalyst 14 . This three-way catalyst 15 is constructed by carrying a noble metal (platinum Pt) and has no oxygen storage capacity on a carrier. More specifically, a stainless steel or ceramic such as cordierite is coated with a platinum Pt catalyst layer supported on a porous aluminum oxide AL ₂ O ₃ . An A / F sensor 29 is provided upstream of the three-way catalytic converter 15 .

In this case, A / F ratio changes upstream and downstream of the three-way catalyst 15 are substantially the same because the storage of oxygen in the three-way catalyst 15 is limited. That is, the exhaust components actually delivered to the NOx catalyst 14 follow changes in the target A / F ratio. For example, when the lean mixture combustion is changed to the rich mixture combustion, rich gas components flow through the three-way catalyst 15 without being stored and enter the NOx catalyst 14 .

From time to time, the A / F sensor 29 monitors the exhaust gas components that get into the NOx catalytic converter 14 .

Therefore, in this embodiment, the A / F ratio control is carried out by the A / F ratio control routine shown in Figs. 2 and 3. That is, the amount of NOx in the NOx catalyst 14 is absorbed during the lean A / F ratio control, and the rich A / F ratio control is carried out by the reference amount of rich components based on the amount of NOx absorbed was determined to achieve. In this way, similarly to the first and second embodiments, the appropriate amount of rich components required to reduce and exhaust the NOx absorbed in the NOx catalyst 14 can be supplied.

Furthermore, the distance that the exhaust gas travels from the engine 1 to the sensor 29 is shortened because the A / F sensor 29 is arranged upstream of the three-way catalyst 15 , so that the sensor response time is also shortened and the A / F Ratio detection accuracy at the time of a transient operating state of the engine is improved. Furthermore, it is not necessary to provide a separate sensor immediately upstream of the NOx catalyst, which leads to a simplified construction.

(Variations)

In the first embodiment, the A / F ratio can be during the lean A / F control in an open control loop and during the fat A / F control without one Feedback control (regulation) can be controlled.

In the second exemplary embodiment, the time of the rich A / F ratio control can be determined using the A / F sensor of the limit current type instead of the O ₂ sensor 28 . In this case, the time to change to the rich ratio that is richer than the stoichiometric ratio and a high peak during this period are calculated. In this way, the actual rich control amount is determined based on the calculated rich time and the rich peak value, and the rich A / F ratio control continues until the actual rich control amount reaches the reference rich amount (the amount of richness that the accumulated amount of NOx corresponds). In this case, the appropriate amount of rich components can be supplied to the NOx catalyst 14 required to reduce and exhaust the NOx absorbed.

In the third embodiment, the A / F sensor provided upstream of the three-way catalyst 15 can be replaced by the O ₂ sensor. In this case, as in the second exemplary embodiment, the rich A / F control is carried out by monitoring the output signal of the O ₂ sensor for the reference rich time which is actually required for the rich A / F ratio control.

In the third embodiment, the three-way catalyst 15 , which has a small oxygen storage capacity, can be realized as follows. The support is not coated with an auxiliary catalyst which has a large oxygen storage capacity, or its amount is limited to a small amount. Such a catalyst, which has a large oxygen storage capacity, can consist of cerium dioxide CeO ₂ , barium B, lantan La and the like. Alternatively, the amount of noble metal (Rh, Pd) that has an oxygen storage capacity is reduced. If rhodium rh is used, its amount should be less than 0.2 grams / liter. If Paladium Pd is used, its amount should be less than 2.5 grams / liter.

In each of the exemplary embodiments, a NOx concentration sensor immediately upstream of the NOx catalyst may be provided, and the amount of NOx absorbed can be determined based on this sensor output signal become. In this case, it is preferable to have a sensor after  the hybrid type is built, d. H. a sensor that both the oxygen concentration (A / F) as well as the NOx concentration detected.

In each of the embodiments, the three-way catalyst 13 can be replaced by an oxidation catalyst. In the first and second embodiments, the three-way catalysts 13 can be omitted while only the NOx catalyst 14 is used.

A three-way catalyst 13 and a NOx absorption (occlusion) and reduction catalyst 14 are arranged in series in an engine exhaust pipe 12 . An A / F sensor 27 is disposed between the catalysts 13 , 14 to detect an oxygen concentration (degree of enrichment) in the exhaust gas that flows immediately upstream of the NOx catalyst 14 . A lean mixture combustion is normally performed so that the NOx catalyst 14 absorbs NOx from the exhaust gas discharged at the time of the lean mixture combustion. The lean mixture combustion is temporarily changed to a rich mixture combustion in order to reduce and exhaust the NOx absorbed by the NOx catalyst 13 . During the lean control, the accumulated amount of NOx absorbed in the NOx catalyst 14 is calculated to calculate a reference amount of rich components required for rich control. The rich control continues until the output of the A / F sensor 27 indicates that the rich control has continued to obtain the reference amount of rich components.

Claims (7)

1. Air-fuel ratio control system for an internal combustion engine ( 1 ), comprising the following components:
a NOx catalyst ( 14 ) provided in an engine exhaust system ( 12 ) to absorb NOx contained in the engine exhaust gas discharged during a lean mixture combustion and the absorbed NOx by the exhaust gas discharged during a rich mixture combustion , to reduce,
an oxygen concentration sensor ( 27 , 28 , 29 ) which is provided upstream of the NOx catalytic converter ( 14 ) for detecting the oxygen concentration in the exhaust gas,
a control device ( 30 , 31 ) for controlling the rich mixture combustion by monitoring the degree of enrichment of the exhaust gas supplied to the NOx catalyst ( 14 ) in response to an output signal of the oxygen concentration sensor ( 27 , 28 , 29 ). 2. System according to claim 1, characterized in that it further comprises an upstream catalyst ( 13 ) which is provided upstream of the oxygen concentration sensor ( 27 , 28 ) and has an oxygen storage capacity. 3. System according to claim 2, characterized in that it further comprises an upstream catalyst ( 15 ) which is provided upstream of the NOx catalyst ( 14 ) and has a low oxygen storage capacity, the oxygen concentration sensor ( 29 ) upstream of the upstream Catalyst ( 15 ) is provided. 4. System according to one of claims 1 to 3, characterized in that it further comprises the following components:
a NOx amount calculator ( 31 ) for calculating an amount of NOx absorbed in the NOx catalyst ( 14 );
a reference fat amount calculation device ( 31 ) for calculating a reference amount of rich components based on the calculated amount of NOx absorbed in the NOx catalyst ( 14 ),
wherein the control device ( 30 , 31 ) continues the combustion of the rich mixture according to the calculated reference amount of rich components. 5. System according to claim 4, characterized in that the control device ( 30 , 31 ) during the rich mixture combustion on the basis of the output signal of the oxygen concentration sensor ( 27 , 28 , 29 ) calculates an amount of accumulated fat components and continues the rich mixture combustion, until the amount of fat components accumulated reaches the reference amount of fat components. 6. System according to claim 4, characterized in that the control device ( 30 , 31 ) from the output signal of the oxygen concentration sensor ( 28 ) calculates a time required to change the air-fuel ratio to a rich air-fuel ratio that during the rich mixture combustion is richer than a stoichiometric ratio, and which continues the rich mixture combustion for the calculated time until the calculated reference amount of rich components is reached. 7. System according to claim 4, characterized in that the control device ( 30 , 31 ) from the output signal of the oxygen concentration sensor ( 28 ) calculates a time required to change the air-fuel ratio to a rich air-fuel ratio that during the rich mixture combustion is richer than a stoichiometric ratio, as well as its rich peak value, and which continues the rich mixture combustion until a control value, which is determined by the calculated time and the rich peak value, reaches the calculated reference amount of rich components.