This application is a national phase application of International Application No. PCT/JP2015/003788, filed Jul. 28, 2015, and claims the priority of Japanese Application No. 2014-153229, filed Jul. 28, 2014, the content of both of which is incorporated herein by reference.
The present invention relates to a control system of an internal combustion engine.
A control system of an internal combustion engine which is provided with an air-fuel ratio sensor or oxygen sensor in an exhaust passage of the internal combustion engine and controls an amount of fuel, which is fed to the internal combustion engine, based on an output of the air-fuel ratio sensor or oxygen sensor is well known. In particular, as such a control system, one which is provided with air-fuel ratio sensors at an upstream side and a downstream side, in a direction of exhaust flow, from an exhaust purification catalyst which is provided in the engine exhaust passage, has been proposed (for example, PTL 1).
In particular, in the control system described in PTL 1, a fuel feed device which feeds fuel to the inside of the exhaust passage is provided at the downstream side from the engine body and the upstream side from the exhaust purification catalyst. Further, when heating the exhaust purification catalyst, the amount of fuel which should be fed from the fuel feed device is calculated, based on the output of the air-fuel ratio (below, also referred to as the “output air-fuel ratio”) detected by the upstream side air-fuel ratio sensor, so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the stoichiometric air-fuel ratio. In addition, when the output air-fuel ratio of the downstream side air-fuel ratio sensor has not become the stoichiometric air-fuel ratio, the amount of fuel fed from the fuel feed device is corrected so that the output air-fuel ratio becomes the stoichiometric air-fuel ratio.
PTL 1: Japanese Patent Publication No. H8-312408 A
In this regard, according to the inventors of the present application, a control system performing control which is different from the control system described in the above-mentioned PTL 1, has been proposed. In this control system, when the output air-fuel ratio of the downstream side air-fuel ratio sensor has become a rich judged air-fuel ratio (air-fuel ratio slightly richer than the stoichiometric air-fuel ratio) or less, the target air-fuel ratio is set to an air-fuel ratio which is leaner than the stoichiometric air-fuel ratio (below, referred to as the “lean air-fuel ratio”). On the other hand, when the output air-fuel ratio of the downstream side air-fuel ratio sensor has become a lean judged air-fuel ratio (air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio) or more, the target air-fuel ratio is set to an air-fuel ratio which is richer than the stoichiometric air-fuel ratio (below, referred to as the “rich air-fuel ratio”). That is, in this control system, the target air-fuel ratio is alternately switched between the rich air-fuel ratio and the lean air-fuel ratio.
When performing such control, if the oxygen storage amount of the exhaust purification catalyst becomes a suitable amount between zero and a maximum storable oxygen amount, there is little outflow of oxygen, NOx, or unburned gas (HC or CO) from the exhaust purification catalyst. However, for example, when the flow amount of the exhaust gas flowing into the exhaust purification catalyst is large or when the ability of the exhaust purification catalyst to purify unburned gas, etc., falls, sometimes despite the oxygen storage amount of the exhaust purification catalyst being a suitable amount, oxygen, NOx, and unburned gas will flows out.
Therefore, in view of the above problem, an object of the present invention is to provide a control system of an internal combustion engine which can suppress the outflow of NOx or unburned gas from an exhaust purification catalyst.
To solve the above problem, the following inventions are provided.
(1) A control system of internal combustion engine, the engine comprising: an exhaust purification catalyst which is arranged in an exhaust passage of the internal combustion engine and which can store oxygen; a downstream side air-fuel ratio sensor which is arranged at a downstream side, in the direction of exhaust flow, from the exhaust purification catalyst and which detects the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst; and a flow velocity detecting device which detects or estimates a flow velocity of exhaust gas flowing through the exhaust purification catalyst, wherein the control system: controls the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, by feedback control, to become a target air-fuel ratio; sets the target air-fuel ratio to a lean air-fuel ratio which is leaner than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than a rich judged air-fuel ratio, which is richer than the stoichiometric air-fuel ratio; sets the target air-fuel ratio to a rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than a lean judged air-fuel ratio, which is leaner than the stoichiometric air-fuel ratio; and, when a change in the flow velocity of exhaust gas flowing through the exhaust purification catalyst, which is detected or estimated by the flow velocity detecting device, occurs so that the flow velocity becomes faster, sets the lean degree to lower than before, during at least part of the time period during which the target air-fuel ratio is set to the lean air-fuel ratio, and/or sets the rich degree to lower than before, during at least part of the time period during which the target air-fuel ratio is set to the rich air-fuel ratio.
(2) A control system of internal combustion engine, the engine comprising: an exhaust purification catalyst which is arranged in an exhaust passage of the internal combustion engine and which can store oxygen; a downstream side air-fuel ratio sensor which is arranged at a downstream side, in the direction of exhaust flow, from the exhaust purification catalyst and which detects the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst; and a purification ability detecting device which detects or estimates the value of a purification ability parameter which indicates a purification ability of the exhaust purification catalyst, wherein the control system: controls the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, by feedback control, to become a target air-fuel ratio; sets the target air-fuel ratio to a lean air-fuel ratio which is leaner than the stoichiometric air-fuel ratio when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than a rich judged air-fuel ratio, which is richer than the stoichiometric air-fuel ratio; sets the target air-fuel ratio to a rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than a lean judged air-fuel ratio, which is leaner than the stoichiometric air-fuel ratio; and, when a change in the value of the purification ability parameter, which is detected or estimated by the purification ability detecting device, occurs so that the purification ability falls, sets the lean degree to lower than before, during at least part of the time period during which the target air-fuel ratio is set to the lean air-fuel ratio, and/or sets the rich degree to lower than before, during at least part of the time period during which the target air-fuel ratio is set to the rich air-fuel ratio.
(3) The control system of an internal combustion engine according to the above (2), wherein the purification ability parameter is the temperature of the exhaust purification catalyst or the degree of deterioration of the exhaust purification catalyst.
(4) The control system of an internal combustion engine according to any one of the above (1) to (3), wherein the control system: sets the target air-fuel ratio to a lean set air-fuel ratio, which is leaner than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio; sets the target air-fuel ratio to a lean air-fuel ratio with a smaller lean degree than the lean set air-fuel ratio from a lean degree change timing after the target air-fuel ratio is set to the lean set air-fuel ratio and before the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio, until the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio; and lowers a lean degree of the lean set air-fuel ratio when the change occurs.
(5) The control system of an internal combustion engine according to the above (4), wherein when the change occurs, the control system lowers the lean degree of the air-fuel ratio from the lean degree change timing to when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio.
(6) The control system of an internal combustion engine according to any one of the above (1) to (3), wherein the control system: sets the target air-fuel ratio to a lean set air-fuel ratio, which is leaner than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio; sets the target air-fuel ratio to a lean air-fuel ratio with a smaller lean degree than the lean set air-fuel ratio from a lean degree change timing after the target air-fuel ratio is set to the lean set air-fuel ratio and before the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio until the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio; and, when the change occurs, lowers the lean degree of the air-fuel ratio from the lean degree change timing to when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio or more.
(7) The control system of an internal combustion engine according to any one of the above (4) to (6), wherein even when lowering the lean degree, the target air-fuel ratio is set to equal to or greater than the lean judged air-fuel ratio.
(8) The control system of an internal combustion engine according to any one of the above (1) to (7), wherein the control system: sets the target air-fuel ratio to a rich set air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratio; sets the target air-fuel ratio to a rich air-fuel ratio with a smaller rich degree than the rich set air-fuel ratio from a rich degree change timing after the target air-fuel ratio is set to the rich set air-fuel ratio and before the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio, until the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio; and lowers a rich degree of the rich set air-fuel ratio when the change occurs.
(9) The control system of an internal combustion engine according to the above (8), wherein when the change occurs, the control system lowers the rich degree of the air-fuel ratio from the rich degree change timing to when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio.
(10) The control system of an internal combustion engine according to any one of the above (1) to (7), wherein the control system: sets the target air-fuel ratio to a rich set air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or greater than the lean judged air-fuel ratios; sets the target air-fuel ratio to a rich air-fuel ratio with a smaller rich degree than the rich set air-fuel ratio from a rich degree change timing after the target air-fuel ratio is set to the rich set air-fuel ratio and before the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio until the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio or less; and, when the change occurs, lowers the rich degree of the air-fuel ratio from the rich degree change timing to when the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes equal to or less than the rich judged air-fuel ratio or less.
(11) The control system of an internal combustion engine according to any one of the above (8) to (10), wherein even when lowering the rich degree, the target air-fuel ratio is set to equal to or less than the rich judged air-fuel ratio.
According to the present invention, a control system of an internal combustion engine which can suppress the outflow of NOx or unburned gas from an exhaust purification catalyst is provided.
Below, referring to the drawings, embodiments of the present invention will be explained in detail. Note that, in the following explanation, similar components are assigned the same reference numerals.
As shown in
The intake port 7 of each cylinder is connected to a surge tank 14 through a corresponding intake runner 13, while the surge tank 14 is connected to an air cleaner 16 through an intake pipe 15. The intake port 7, intake runner 13, surge tank 14, and intake pipe 15 form an intake passage. Further, inside the intake pipe 15, a throttle valve 18 which is driven by a throttle valve drive actuator 17 is arranged. The throttle valve 18 can be operated by the throttle valve drive actuator 17 to thereby change the aperture area of the intake passage.
On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19. The exhaust manifold 19 has a plurality of runners which are connected to the exhaust ports 9 and a collected part at which these runners are collected. The collected part of the exhaust manifold 19 is connected to an upstream side casing 21 which houses an upstream side exhaust purification catalyst 20. The upstream side casing 21 is connected through an exhaust pipe 22 to a downstream side casing 23 which houses a downstream side exhaust purification catalyst 24. The exhaust port 9, exhaust manifold 19, upstream side casing 21, exhaust pipe 22, and downstream side casing 23 form an exhaust passage.
The electronic control unit (ECU) 31 is comprised of a digital computer which is provided with components which are connected together through a bidirectional bus 32 such as a RAM (random access memory) 33, ROM (read only memory) 34, CPU (microprocessor) 35, input port 36, and output port 37. In the intake pipe 15, an airflow meter 39 is arranged for detecting the flow rate of air flowing through the intake pipe 15. The output of this airflow meter 39 is input through a corresponding AD converter 38 to the input port 36. Further, at the collected part of the exhaust manifold 19, an upstream side air-fuel ratio sensor 40 is arranged which detects the air-fuel ratio of the exhaust gas flowing through the inside of the exhaust manifold 19 (that is, the exhaust gas flowing into the upstream side exhaust purification catalyst 20). In addition, in the exhaust pipe 22, a downstream side air-fuel ratio sensor 41 is arranged which detects the air-fuel ratio of the exhaust gas flowing through the inside of the exhaust pipe 22 (that is, the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 and flowing into the downstream side exhaust purification catalyst 24). The outputs of these air-fuel ratio sensors 40 and 41 are also input through the corresponding AD converters 38 to the input port 36. Furthermore, at the upstream side exhaust purification catalyst 20, an upstream side temperature sensor 46 which detects the temperature of the upstream side exhaust purification catalyst 20 is arranged, while at the downstream side exhaust purification catalyst 24, a downstream side temperature sensor 47 which detects the temperature of the downstream side exhaust purification catalyst 24 is arranged. The outputs of these temperature sensors 46 and 47 are also input through the corresponding AD converters 38 to the input port 36.
Further, an accelerator pedal 42 is connected to a load sensor 43 generating an output voltage which is proportional to the amount of depression of the accelerator pedal 42. The output voltage of the load sensor 43 is input to the input port 36 through a corresponding AD converter 38. The crank angle sensor 44 generates an output pulse every time, for example, a crankshaft rotates by 15 degrees. This output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of this crank angle sensor 44. On the other hand, the output port 37 is connected through corresponding drive circuits 45 to the spark plugs 10, fuel injectors 11, and throttle valve drive actuator 17. Note that the ECU 31 functions as a control device for controlling the internal combustion engine.
Note that, the internal combustion engine according to the present embodiment is a non-supercharged internal combustion engine which is fueled by gasoline, but the internal combustion engine according to the present invention is not limited to the above configuration. For example, the internal combustion engine according to the present invention may have cylinder array, state of injection of fuel, configuration of intake and exhaust systems, configuration of valve mechanism, presence of supercharger, and/or supercharged state, etc. which are different from the above internal combustion engine.
The upstream side exhaust purification catalyst 20 and downstream side exhaust purification catalyst 24 in each case have similar configurations. The exhaust purification catalysts 20 and 24 are three-way catalysts having oxygen storage abilities. Specifically, the exhaust purification catalysts 20 and 24 are formed such that on substrate consisting of ceramic, a precious metal having a catalytic action (for example, platinum (Pt)) and a substance having an oxygen storage ability (for example, ceria (CeO2)) are carried. The exhaust purification catalysts 20 and 24 exhibit a catalytic action of simultaneously removing unburned gas (HC, CO, etc.) and nitrogen oxides (NOx) and, in addition, an oxygen storage ability, when reaching a predetermined activation temperature.
According to the oxygen storage ability of the exhaust purification catalysts 20 and 24, the exhaust purification catalysts 20 and 24 store the oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is leaner than the stoichiometric air-fuel ratio (lean air-fuel ratio). On the other hand, the exhaust purification catalysts 20 and 24 release the oxygen stored in the exhaust purification catalysts 20 and 24 when the air-fuel ratio of the inflowing exhaust gas is richer than the stoichiometric air-fuel ratio (rich air-fuel ratio).
The exhaust purification catalysts 20 and 24 have a catalytic action and oxygen storage ability and thereby have the action of purifying NOx and unburned gas according to the stored amount of oxygen. That is, in the case where the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is a lean air-fuel ratio, as shown in
On the other hand, in the case where the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is the rich air-fuel ratio, as shown in
In the above way, according to the exhaust purification catalysts 20 and 24 used in the present embodiment, the purification characteristics of NOx and unburned gas in the exhaust gas change depending on the air-fuel ratio and stored amount of oxygen of the exhaust gas flowing into the exhaust purification catalysts 20 and 24. Note that, as long as having a catalytic action and oxygen storage ability, the exhaust purification catalysts 20 and 24 may be any catalyst.
Next, referring to
As will be understood from
Note that, in the above example, as the air-fuel ratio sensors 40 and 41, limit current type air-fuel ratio sensors are used. However, as the air-fuel ratio sensors 40 and 41, it is also possible to use air-fuel ratio sensor not a limit current type or any other air-fuel ratio sensor, as long as the output current varies linearly with respect to the exhaust air-fuel ratio. Further, the air-fuel ratio sensors 40 and 41 may have structures different from each other.
Next, air-fuel ratio control in the control system of an internal combustion engine of the present invention will be explained in brief. In the present embodiment, feedback control is performed to control the fuel injection amount from the fuel injector 11, based on the output air-fuel ratio of the upstream side air-fuel ratio sensor 40, so that the output air-fuel ratio of the upstream side air-fuel ratio sensor 40 becomes the target air-fuel ratio. Note that, “output air-fuel ratio” means an air-fuel ratio corresponding to the output value of the air-fuel ratio sensor.
Further, in air-fuel ratio control of the present embodiment, the target air-fuel ratio setting control is performed to set the target air-fuel ratio based on the output air-fuel ratio of the downstream side air-fuel ratio sensor 41, etc. In the target air-fuel ratio setting control, when the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes a rich judged air-fuel ratio which is just slightly richer than the stoichiometric air-fuel ratio (for example, 14.55) or less, it is judged that the air-fuel ratio of the exhaust gas detected by the downstream side air-fuel ratio sensor 41 has become the rich air-fuel ratio. At this time, the target air-fuel ratio is set to a lean set air-fuel ratio. Note that, the “lean set air-fuel ratio” is a predetermined air-fuel ratio which is leaner than the stoichiometric air-fuel ratio by a certain degree, for example, 14.65 to 20, preferably 14.65 to 18, more preferably 14.65 to 16 or so.
After that, if, in the state where the target air-fuel ratio is set to the lean set air-fuel ratio, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes an air-fuel ratio which is leaner than a rich judged air-fuel ratio (air-fuel ratio which is closer to stoichiometric air-fuel ratio than rich judged air-fuel ratio), it is judged that the air-fuel ratio of the exhaust gas detected by the downstream side air-fuel ratio sensor 41 has become substantially the stoichiometric air-fuel ratio. At this time, the target air-fuel ratio is set to a slight lean set air-fuel ratio. Note that, the slight lean set air-fuel ratio is a lean air-fuel ratio with a smaller lean degree than the lean set air-fuel ratio (smaller difference from stoichiometric air-fuel ratio), for example, 14.62 to 15.7, preferably 14.63 to 15.2, more preferably 14.65 to 14.9 or so.
On the other hand, when the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes a lean judged air-fuel ratio which is slightly leaner than the stoichiometric air-fuel ratio (for example, 14.65) or more, it is judged that the air-fuel ratio of the exhaust gas detected by the downstream side air-fuel ratio sensor 41 has become the lean air-fuel ratio. At this time, the target air-fuel ratio is set to a rich set air-fuel ratio. Note that, the “rich set air-fuel ratio” is a predetermined air-fuel ratio which is richer by a certain extent from the stoichiometric air-fuel ratio, for example, 10 to 14.55, preferably 12 to 14.52, more preferably 13 to 14.5 or so.
After that, if, in the state where the target air-fuel ratio is set to the rich set air-fuel ratio, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes an air-fuel ratio which is richer than the lean judged air-fuel ratio (air-fuel ratio which is closer to stoichiometric air-fuel ratio than lean judged air-fuel ratio), it is judged that the air-fuel ratio of the exhaust gas detected by the downstream side air-fuel ratio sensor 41 has become substantially the stoichiometric air-fuel ratio. At this time, the target air-fuel ratio is set to a slight rich set air-fuel ratio. Note that, the “slight rich set air-fuel ratio” is a rich air-fuel ratio with a smaller rich degree than the rich set air-fuel ratio (smaller difference from stoichiometric air-fuel ratio), for example, 13.5 to 14.58, preferably 14 to 14.57, more preferably 14.3 to 14.55 or so.
As a result, in the present embodiment, if the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio or less, first, the target air-fuel ratio is set to the lean set air-fuel ratio. After that, if the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes larger than the rich judged air-fuel ratio, the target air-fuel ratio is set to the slight lean set air-fuel ratio. On the other hand, if the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio or more, first, the target air-fuel ratio is set to the rich set air-fuel ratio. After that, if the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes smaller than the lean judged air-fuel ratio, the target air-fuel ratio is set to the slight rich set air-fuel ratio. After that, similar control is repeated.
Note that, the rich judged air-fuel ratio and lean judged air-fuel ratio are set to air-fuel ratios within 1% of the stoichiometric air-fuel ratio, preferably within 0.5%, more preferably within 0.35%. Therefore, the differences from the stoichiometric air-fuel ratio of the rich judged air-fuel ratio and the lean judged air-fuel ratio when the stoichiometric air-fuel ratio is 14.6 are 0.15 or less, preferably 0.073 or less, more preferably 0.051 or less. Further, the difference of the target air-fuel ratio (for example, slight rich set air-fuel ratio or lean set air-fuel ratio) from the stoichiometric air-fuel ratio is set to be larger than the above difference.
Referring to
In the illustrated example, in the state before the time t1, the target air-fuel ratio AFT is set to a slight rich set air-fuel ratio AFTsr. Along with this, the output air-fuel ratio of the upstream side air-fuel ratio sensor 40 becomes the rich air-fuel ratio. The unburned gas contained in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is purified by the upstream side exhaust purification catalyst 20. Along with this, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases. On the other hand, due to the purification at the upstream side exhaust purification catalyst 20, the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 does not contain unburned gas, and therefore the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes substantially the stoichiometric air-fuel ratio.
If the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, the oxygen storage amount OSA approaches zero at the time t1 (for example, in
In the present embodiment, if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio AFrich or less, the target air-fuel ratio AFT is switched to the lean set air-fuel ratio AFTl so as to make the oxygen storage amount OSA increase. Therefore, the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio.
Note that, in the present embodiment, the target air-fuel ratio AFT is switched not right after the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 changes from the stoichiometric air-fuel ratio to the rich air-fuel ratio, but after reaching the rich judged air-fuel ratio AFrich. This is because even if the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is sufficient, sometimes the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 shifts slightly from the stoichiometric air-fuel ratio. Conversely speaking, the rich judged air-fuel ratio is made an air-fuel ratio which the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 will never reach when the oxygen storage amount of the upstream side exhaust purification catalyst 20 is sufficient. Note that, the same can be said for the above-mentioned lean judged air-fuel ratio.
If, at the time t2, the target air-fuel ratio is switched to the lean air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes from the rich air-fuel ratio to the lean air-fuel ratio. Further, along with this, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes a lean air-fuel ratio (in actuality, a delay occurs from when switching the target air-fuel ratio to when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes, but in the illustrated example, for convenience, it is assumed that they change simultaneously). If, at the time t2, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes to the lean air-fuel ratio, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 increases.
If, in this way, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 increases, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 changes toward the stoichiometric air-fuel ratio. In the example shown in
Therefore, in the present embodiment, when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 changes to a value larger than the rich judged air-fuel ratio AFrich, the target air-fuel ratio AFT is switched to a slight lean set air-fuel ratio AFTsl. Therefore, at the time t3, the lean degree of the target air-fuel ratio is decreased. Below, the time t3 is called the “lean degree change timing”.
At the lean degree change timing of the time t3, if the target air-fuel ratio AFT is switched to the slight lean set air-fuel ratio AFTsl, the lean degree of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 also becomes smaller. Along with this, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes smaller and the speed of increase of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 falls.
After the time t3, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually increases, though the speed of increase is slow. If the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually increases, the oxygen storage amount OSA finally approaches the maximum storable oxygen amount Cmax (for example, Cuplim of
In the present embodiment, if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio AFlean or more, the target air-fuel ratio AFT is switched to the rich set air-fuel ratio AFTr so as to make the oxygen storage amount OSA decrease. Therefore, the target air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio.
If, at the time t5, the target air-fuel ratio is switched to the rich air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes from the lean air-fuel ratio to the rich air-fuel ratio. Further, along with this, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes the rich air-fuel ratio (in actuality, a delay occurs from when switching the target air-fuel ratio to when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes, but in the illustrated example, for convenience, it is assumed that they change simultaneously). If, at the time t5, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes to the rich air-fuel ratio, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 decreases.
If, in this way, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 decreases, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 changes toward the stoichiometric air-fuel ratio. In the example shown in
Therefore, in the present embodiment, when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 changes to a value smaller than the lean judged air-fuel ratio AFlean, the target air-fuel ratio AFT is switched from the rich set air-fuel ratio to a slight rich set air-fuel ratio AFTsr.
If, at the time t6, the target air-fuel ratio AFT is switched to the slight rich set air-fuel ratio AFTsr, the rich degree of the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 also becomes smaller. Along with this, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 increases and the speed of decrease of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 falls.
After the time t6, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, through the speed of decrease is slow. If the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, the oxygen storage amount OSA finally approaches zero at the time t7 in the same way as the time t1 and falls to the Cdwnlim of
According to the above-mentioned basic air-fuel ratio control, at the time right after the time t2 when the target air-fuel ratio is changed from the rich air-fuel ratio to the lean air-fuel ratio, and at the time right after the time t5 when the target air-fuel ratio is changed from the lean air-fuel ratio to the rich air-fuel ratio, the difference between the target air-fuel ratio and the stoichiometric air-fuel ratio is large (that is, the rich degree or lean degree is large). For this reason, it is possible to make the unburned gas which flowed out from the upstream side exhaust purification catalyst 20 at the time t2 and the NOx which flowed out from the upstream side exhaust purification catalyst 20 at the time t5 rapidly decrease. Therefore, it is possible to suppress the outflow of the unburned gas and NOx from the upstream side exhaust purification catalyst 20.
Further, according to the air-fuel ratio control of the present embodiment, at the time t2, the target air-fuel ratio is set to the lean set air-fuel ratio, and then after the outflow of unburned gas from the upstream side exhaust purification catalyst 20 is stopped and the oxygen storage amount OSA thereof recovers to a certain extent, the target air-fuel ratio is switched to the slight lean set air-fuel ratio at the time t3. By making the rich degree (difference from stoichiometric air-fuel ratio) of the target air-fuel ratio small in this way, even if NOx flows out from the upstream side exhaust purification catalyst 20, the amount of outflow per unit time can be decreased. In particular, according to the above air-fuel ratio control, although NOx flows out from the upstream side exhaust purification catalyst 20 at the time t5, it is possible to keep the amount of outflow at this time small.
In addition, according to the air-fuel ratio control of the present embodiment, at the time t5, the target air-fuel ratio is set to the rich set air-fuel ratio, and then after the outflow of NOx (oxygen) from the upstream side exhaust purification catalyst 20 stops and the oxygen storage amount OSA thereof decreases by a certain extent, the target air-fuel ratio is switched to the slight rich set air-fuel ratio at the time t6. By making the rich degree of the target air-fuel ratio (difference from stoichiometric air-fuel ratio) smaller in this way, even if unburned gas flows out from the upstream side exhaust purification catalyst 20, it is possible to decrease the amount of outflow per unit time. In particular, according to the above air-fuel ratio control, although unburned gas flows out from the upstream side exhaust purification catalyst 20 at the times t2 and t8, at this time as well, the amount of outflow thereof can be kept small.
Furthermore, in the present embodiment, as the sensor for detecting the air-fuel ratio of the exhaust gas at the downstream side, the air-fuel ratio sensor 41 is used. This air-fuel ratio sensor 41, unlike an oxygen sensor, does not have hysteresis. For this reason, according to the air-fuel ratio sensor 41, which has a high response with respect to the actual exhaust air-fuel ratio, it is possible to quickly detect the outflow of unburned gas and oxygen (and NOx) from the upstream side exhaust purification catalyst 20. Therefore, by this as well, according to the present embodiment, it is possible to suppress the outflow of unburned gas and NOx (and oxygen) from the upstream side exhaust purification catalyst 20.
Further, in an exhaust purification catalyst which can store oxygen, if maintaining the oxygen storage amount substantially constant, a drop in the oxygen storage capacity will be invited. Therefore, to maintain the oxygen storage capacity as much as possible, at the time of use of the exhaust purification catalyst, it is necessary to make the oxygen storage amount change up and down. According to the air-fuel ratio control according to the present embodiment, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 repeatedly changes up and down between near zero and near the maximum storable oxygen amount. For this reason, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 can be maintained high as much as possible.
Note that, in the above embodiment, when, at the time t3, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes a value larger than the rich judged air-fuel ratio AFrich, the target air-fuel ratio AFT is switched from the lean set air-fuel ratio AFTl to the slight lean set air-fuel ratio AFTsl. Further, in the above embodiment, when, at the time t6, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes a value smaller than the lean judged air-fuel ratio AFlean, the target air-fuel ratio AFT is switched from the rich set air-fuel ratio AFTr to the slight rich set air-fuel ratio AFTsr. However, the timings for switching the target air-fuel ratio AFT do not necessarily have to be determined based on the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 and may also be determined based on other parameters.
For example, the timings for switching the target air-fuel ratio AFT may also be determined based on the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20. For example, as shown in
In this case, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is estimated based on the cumulative oxygen excess/deficiency of exhaust gas flowing into the upstream side exhaust purification catalyst 20. The “oxygen excess/deficiency” means the oxygen which becomes in excess or the oxygen which becomes deficient (amount of excessive unburned gas, etc.) when trying to make the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 the stoichiometric air-fuel ratio. In particular, when the target air-fuel ratio becomes the lean set air-fuel ratio, the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes excessive. This excess oxygen is stored in the upstream side exhaust purification catalyst 20. Therefore, the cumulative value of the oxygen excess/deficiency (below, referred to as “cumulative oxygen excess/deficiency”) can be said to express the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20. As shown in
Note that, the oxygen excess/deficiency is calculated based on the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 and the estimated value of the amount of intake air into the combustion chamber 5 which is calculated based on the air flow meter 39, etc., or the amount of feed of fuel from the fuel injector 11, etc. Specifically, the oxygen excess/deficiency OED is, for example, calculated by the following formula (1):
OEF=0.23·Qi·(Afup−14.6) (1)
Here, 0.23 is the oxygen concentration in the air, Qi indicates the fuel injection amount, and AFup indicates the output air-fuel ratio of the upstream side air-fuel ratio sensor 40.
Alternatively, the timing (lean degree change timing) of switching the target air-fuel ratio AFT to the slight lean set air-fuel ratio AFTsl may be determined based on the elapsed time or the cumulative amount of intake air, etc., from when switching the target air-fuel ratio to the lean air-fuel ratio (time t2). Similarly, the timing of switching the target air-fuel ratio AFT to the slight rich set air-fuel ratio AFCsr (rich degree change timing) may be determined based on the elapsed time or the cumulative amount of intake air, etc., from when switching the target air-fuel ratio to the rich air-fuel ratio (time t5).
In this way, the rich degree change timing or lean degree change timing is determined based on various parameters. Whatever the case, the lean degree change timing is set to a timing after the target air-fuel ratio is set to the lean set air-fuel ratio and before the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio or more. Similarly, the rich degree change timing is set to a timing after the target air-fuel ratio is set to the rich set air-fuel ratio and before the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio or less.
Further, in the above embodiment, from the time t2 to the time t3, the target air-fuel ratio AFT is maintained constant at the lean set air-fuel ratio AFTl. However, during this time period, the target air-fuel ratio AFT need not necessarily be maintained constant and, for example, may also change so as to gradually fall (approach the stoichiometric air-fuel ratio). Similarly, in the above embodiment, from the time t3 to the time t5, the target air-fuel ratio correction amount AFT is maintained constant at the slight lean set air-fuel ratio AFTl. However, during this time period, the target air-fuel ratio AFT does not necessarily have to be maintained constant. For example, it may also change so as to gradually fall (approach the stoichiometric air-fuel ratio). Further, the same can be said for the times t5 to t6 and the times t6 to t8.
In this regard, the amount of flow of exhaust gas flowing through the upstream side exhaust purification catalyst 20 changes in accordance with the amount of intake air to the combustion chamber 5. Further, if the flow amount of exhaust gas flowing through the upstream side exhaust purification catalyst 20 increases, along with this, the flow rate of exhaust gas when flowing through the upstream side exhaust purification catalyst 20 becomes faster. In this way, if the flow rate of exhaust gas becomes faster, the time, during which the exhaust gas can contact the precious metal which is carried at the upstream side exhaust purification catalyst 20, becomes shorter. Therefore, the faster the flow rate of the exhaust gas, the less the amount of NOx or the amount of unburned gas which can be purified from the exhaust gas (these together being referred to as the “purifiable amount”) while a unit volume of exhaust gas is flowing through the upstream side exhaust purification catalyst 20.
This state is shown in
As a result, for example, when the amount of flow of exhaust gas flowing through the upstream side exhaust purification catalyst 20 is large and the air-fuel ratio is rich with a large rich degree, exhaust gas containing unpurified unburned gas flows out from the upstream side exhaust purification catalyst 20. Similarly, for example, when the flow amount of exhaust gas flowing through the upstream side exhaust purification catalyst 20 is large and the air-fuel ratio is lean with a large lean degree, exhaust gas containing unpurified NOx flows out from the upstream side exhaust purification catalyst 20. Therefore, from the viewpoint of purifying the NOx or unburned gas which is contained in the exhaust gas, it is necessary to make the rich degree or lean degree of the air-fuel ratio of the exhaust gas smaller, the larger the flow amount of exhaust gas flowing through the upstream side exhaust purification catalyst 20.
Therefore, in the present embodiment, the rich degree of the rich set air-fuel ratio AFTr and the lean degree of the lean set air-fuel ratio AFTl are changed in accordance with the amount of intake air to the combustion chamber 5, that is, the amount of flow of exhaust gas flowing through the upstream side exhaust purification catalyst 20. Specifically, as shown in
Further, in the present embodiment, as shown in
In the example shown in
Therefore, if, at the time t1, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio AFrich or less, the target air-fuel ratio AFT is switched to the first lean set air-fuel ratio AFTl1. Further, if, at the time t3, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio AFlean or more, the target air-fuel ratio AFT is switched to the first rich set air-fuel ratio AFTr1. This cycle is repeated up to the time t5.
In the example shown in
Similarly, if, at the time t8, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio AFlean or more, the target air-fuel ratio AFT is set to a rich air-fuel ratio with a smaller rich degree than the first rich set air-fuel ratio AFTr1. In addition, if, at the time t12, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio or more, the target air-fuel ratio AFT is set to a rich air-fuel ratio with a further smaller rich degree than the first rich set air-fuel ratio AFTr1.
In the example shown in
Further, in the present embodiment, even if the amount of intake air changes, neither of the slight lean set air-fuel ratio AFTsl and the slight rich set air-fuel ratio AFTsr are changed. Therefore, in the example shown in
In this regard, the lean set air-fuel ratio AFTl is larger in lean degree than the slight lean set air-fuel ratio AFTsl, and therefore when the amount of intake air increases, the NOx in the exhaust gas easily flows out without being purified at the upstream side exhaust purification catalyst 20. Further, the rich set air-fuel ratio AFTr is larger in rich degree than the slight rich set air-fuel ratio AFTsr, and therefore when the amount of intake air increases, the unburned gas in the exhaust gas easily flows out without being purified at the upstream side exhaust purification catalyst 20. According to the present embodiment, the larger the amount of intake air to the combustion chamber 5, the more the lean degree of the lean set air-fuel ratio AFTl and the rich degree of the rich set air-fuel ratio AFTr can be decreased. Therefore, it is possible to effectively suppress the outflow of NOx or unburned gas from the upstream side exhaust purification catalyst 20.
Note that, in the above embodiment, both the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr are changed in accordance with the amount of intake air. However, it is also possible to change only one of the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr in accordance with the amount of intake air and maintain the other constant as it is.
Further, in the above embodiment, as the parameter which expresses the flow rate of exhaust gas flowing through the upstream side exhaust purification catalyst 20, the amount of intake air to the combustion chamber 5 is used, and the lean set air-fuel ratio AFTl, etc., is changed based on the amount of intake air. However, the flow rate of the exhaust gas flowing through the upstream side exhaust purification catalyst 20 may be calculated based on other parameters as well. Therefore, for example, the flow rate of the exhaust gas may be calculated based on the engine load and engine speed, and in this case, the lean set air-fuel ratio AFTl, etc., is changed based on the engine load and engine speed.
As shown in
At step S12, it is judged if the lean set flag Fl is set to OFF. The lean set flag Fl is a flag which is set to ON when the target air-fuel ratio is set to the lean air-fuel ratio, and is set to OFF otherwise. When it is judged at step S12 that the lean set flag Fl is set to OFF, the routine proceeds to step S13. At step S13, it is judged if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is the rich judged air-fuel ratio AFrich or less.
When, at step S13, it is judged that the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is larger than the rich judged air-fuel ratio AFrich, the routine proceeds to step S14. At step S14, it is judged if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is smaller than the lean judged air-fuel ratio AFlean. When it is judged that the output air-fuel ratio AFdwn is the lean judged air-fuel ratio AFlean or more, the routine proceeds to step S15. At step S15, the target air-fuel ratio AFT is set to the rich set air-fuel ratio AFTr and the control routine is ended.
Then, if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 approaches the stoichiometric air-fuel ratio and becomes smaller than the lean judged air-fuel ratio AFlean, at the next control routine, the routine proceeds from step S14 to step S16. At step S16, the target air-fuel ratio AFT is set to the slight rich set air-fuel ratio AFTsr and the control routine is ended.
Then, if the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 becomes substantially zero and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio AFrich or less, at the next control routine, the routine proceeds from step S13 to step S17. At step S17, the target air-fuel ratio AFT is set to the lean set air-fuel ratio AFTl. Next, at step S18, the lean set flag Fl is set to ON and the control routine is ended.
If the lean set flag Fl is set to ON, at the next control routine, the routine proceeds from step S12 to step S19. At step S19, it is judged if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is the lean judged air-fuel ratio AFlean or more.
When it is judged at step S19 that the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is smaller than the lean judged air-fuel ratio AFlean, the routine proceeds to step S20. At step S20, it is judged if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is larger than the rich judged air-fuel ratio AFrich. If it is judged that the output air-fuel ratio AFdwn is the rich judged air-fuel ratio AFrich or less, the routine proceeds to step S21. At step S21, the target air-fuel ratio AFT is continued to be set to the lean set air-fuel ratio AFTl and the control routine is ended.
Then, if the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 approaches the stoichiometric air-fuel ratio and becomes larger than the rich judged air-fuel ratio AFrich, at the next control routine, the routine proceeds from step S20 to step S22. At step S22, the target air-fuel ratio AFT is set to the slight lean set air-fuel ratio AFCsl and the control routine is ended.
Then, if the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 becomes the substantially maximum storable oxygen amount and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio AFlean or more, at the next control routine, the routine proceeds from step S19 to step S23. At step S23, the target air-fuel ratio AFT is set to the rich set air-fuel ratio AFTr. Next, at step S24, the lean set flag Fl is reset to OFF and the control routine is ended.
First, at step S31, the amount of intake air to the combustion chamber 5 is calculated by the air flow meter 39. Next, at step S32, the rich set air-fuel ratio AFTr is calculated based on the amount of intake air Ga detected at step S31 by using the map shown in
Next, referring to
Specifically, as shown in
Similarly, in this modification, as shown in
Therefore, if, at the time t2, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 changes from the rich judged air-fuel ratio AFrich or less to an air-fuel ratio larger than the rich judged air-fuel ratio AFrich, the target air-fuel ratio AFT is switched to the first slight lean set air-fuel ratio AFTsl1. Further, if, at the time t4, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 changes from the lean judged air-fuel ratio AFlean or more to an air-fuel ratio which is smaller than the lean judged air-fuel ratio AFlean, the target air-fuel ratio AFT is switched to the first slight rich set air-fuel ratio AFTsr1. Then, this cycle is repeated until the time t7.
In the example shown in
In the example shown in
In this regard, the slight lean set air-fuel ratio AFTsl is smaller in lean degree than the lean set air-fuel ratio AFTl. Further, the slight rich set air-fuel ratio AFTsr is also smaller in rich degree than the rich set air-fuel ratio AFTr. However, even if the lean degree or the rich degree is small in this way, when the amount of intake air increases, there is a possibility of the NOx or the unburned gas flowing out.
Further, if referring to
In this regard, in the control system of the present modification, the larger the amount of intake air to the combustion chamber 5, the more the lean degree of the slight lean set air-fuel ratio AFTsl and the rich degree of the slight rich set air-fuel ratio AFTsr are lowered. Therefore, it is possible to effectively suppress the outflow of NOx or unburned gas from the upstream side exhaust purification catalyst 20 when the target air-fuel ratio AFT is set to the slight lean set air-fuel ratio AFTsl or the slight rich set air-fuel ratio AFTsr. In addition, it is possible to suppress the outflow of unburned gas around the times t1 to t3 of
Note that, in the above embodiment and its modification, when the amount of intake air increases, the lean degree of the lean set air-fuel ratio AFTl and the rich degree of the rich set air-fuel ratio AFTr are set smaller. However, as shown in
Further, in the example shown in
Further, in the above embodiment and its modification, the rich degree is decreased while the target air-fuel ratio AFT is set to the rich air-fuel ratio (for example, at the time t6 of
If expressing the above together, in the present embodiment, when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio AFrich or less, the target air-fuel ratio is set to the lean air-fuel ratio. In addition, when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio or more, the target air-fuel ratio is set to the rich air-fuel ratio. Further, if the flow rate of the exhaust gas flowing through the upstream side exhaust purification catalyst 20, which is detected or estimated by the flow rate detecting device (for example, the air flow meter 39), is changed to become faster, the lean degree is set lower than before, during at least part of the time period during which the target air-fuel ratio AFT is set to the lean air-fuel ratio, and/or the rich degree is set lower than before, during at least part of the time period during which the target air-fuel ratio AFT is set to the rich air-fuel ratio.
Next, referring to
The purification ability of the upstream side exhaust purification catalyst 20 changes according to its temperature. That is, the higher the temperature of the upstream side exhaust purification catalyst 20, the higher the activity of the precious metal which is carried on the upstream side exhaust purification catalyst 20. As a result, the NOx and unburned gas in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 become easier to be purified. Considered conversely, the lower the temperature of the upstream side exhaust purification catalyst 20, the more the purification rate of NOx and unburned gas in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 falls.
As a result, for example, when the temperature of the upstream side exhaust purification catalyst 20 is low and the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is rich with a large rich degree, exhaust gas which contains unpurified unburned gas flows out from the upstream side exhaust purification catalyst 20. Similarly, for example, when the temperature of the upstream side exhaust purification catalyst 20 is low and the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is lean with a large lean degree, exhaust gas which contains unpurified NOx flows out from the upstream side exhaust purification catalyst 20. Therefore, from the viewpoint of purifying the NOx or unburned gas contained in the exhaust gas, it is necessary to make the rich degree or lean degree of the air-fuel ratio of the exhaust gas smaller, as the temperature of the upstream side exhaust purification catalyst 20 becomes lower.
Therefore, in the present embodiment, the rich degree of the rich set air-fuel ratio AFTr and the lean degree of the lean set air-fuel ratio AFTl are changed in accordance with the temperature of the upstream side exhaust purification catalyst 20. Specifically, as shown in
In the example shown in
In the example shown in
Further, in the present embodiment, even if the temperature of the upstream side exhaust purification catalyst 20 changes, neither of the slight lean set air-fuel ratio AFTsl and slight rich set air-fuel ratio AFTsr is changed. Therefore, in the example shown in
In this way, in the present embodiment, if the temperature of the upstream side exhaust purification catalyst 20 becomes lower, that is, if the purification ability of the upstream side exhaust purification catalyst 20 falls, the lean degree of the lean set air-fuel ratio AFTl and the rich degree of the rich set air-fuel ratio AFTr are made to fall. Therefore, it is possible to effectively keep NOx or unburned gas from flowing out from the upstream side exhaust purification catalyst 20 along with a drop in the purification ability of the upstream side exhaust purification catalyst 20.
Note that, in the above embodiment, both of the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr are changed in accordance with the temperature of the upstream side exhaust purification catalyst 20. However, it is also possible to change only one of the lean set air-fuel ratio AFTl and the rich set air-fuel ratio AFTr in accordance with the temperature of the upstream side exhaust purification catalyst 20 and maintain the other constant as it is.
Further, in the above embodiment, the lean set air-fuel ratio AFTl, etc., are changed in accordance with the temperature of the upstream side exhaust purification catalyst 20, that is, the ability of the upstream side exhaust purification catalyst 20 to purify NOx and unburned gas. However, it is also possible to change the lean set air-fuel ratio AFTl, etc., in accordance with a parameter other than the temperature of the upstream side exhaust purification catalyst 20, as long as the parameter is a purification ability parameter which shows the purification ability of the upstream side exhaust purification catalyst 20.
As such a purification ability parameter, for example, degree of deterioration of the upstream side exhaust purification catalyst 20 may be mentioned. If the degree of deterioration of the upstream side exhaust purification catalyst 20 is high, the surface area of the precious metal which is carried at the upstream side exhaust purification catalyst 20 is decreased and the purification ability of the upstream side exhaust purification catalyst 20 falls. Therefore, if the degree of deterioration of the upstream side exhaust purification catalyst 20 becomes higher, the lean set air-fuel ratio AFTl, etc., are changed in the same way as when the temperature of the upstream side exhaust purification catalyst 20 falls.
In this regard, the degree of deterioration of the upstream side exhaust purification catalyst 20 can be detected by various methods. For example, if the degree of deterioration of the upstream side exhaust purification catalyst 20 becomes higher, the maximum storable oxygen amount Cmax of the upstream side exhaust purification catalyst 20 falls. Therefore, when performing control such as shown in
Next, referring to
Specifically, as shown in
Similarly, in the present modification, as shown in
In addition, in the example shown in
In this regard, even when the lean degree or the rich degree is small such as with the slight lean set air-fuel ratio AFTsl or the slight rich set air-fuel ratio AFTsr, when the temperature of the upstream side exhaust purification catalyst 20 is low, there is a possibility of NOx or unburned gas flowing out. To the contrary, in the control system of the present embodiment, the lower the temperature of the upstream side exhaust purification catalyst 20, the lower the lean degree of the slight lean set air-fuel ratio AFTsl and the rich degree of the slight rich set air-fuel ratio AFTsr are set. Therefore, it is possible to effectively suppress outflow of NOx or unburned gas from the upstream side exhaust purification catalyst 20 when the target air-fuel ratio AFT is set to the slight lean set air-fuel ratio AFTsl or the slight rich set air-fuel ratio AFTsr. In addition, the amount of outflow of unburned gas around the times t1 to t3 of
First, at step S41, the temperature sensor 46 of the upstream side exhaust purification catalyst 20 detects the temperature Tc of the upstream side exhaust purification catalyst 20. Next, at step S42, the rich set air-fuel ratio AFTr is calculated based on the temperature Tc detected at step S41, by using the map shown in
Next, at step S44, the slight rich set air-fuel ratio AFTsr is calculated based on the temperature Tc detected at step S41, by using the map shown in
Note that, in the above embodiment and its modification, when the temperature of the upstream side exhaust purification catalyst 20 falls, the lean degree of the lean set air-fuel ratio AFTl and the rich degree of the rich set air-fuel ratio AFTr are set smaller. However, as shown in
Further, in the example shown in
If expressing the above together, in the present embodiment, when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the rich judged air-fuel ratio AFrich or less, the target air-fuel ratio is set to the lean air-fuel ratio. In addition, when the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 becomes the lean judged air-fuel ratio or more, the target air-fuel ratio is set to the rich air-fuel ratio. Further, when the value of the parameter of the purification ability which is detected or estimated by the purification ability detection device (for example, the temperature sensor of the upstream side exhaust purification catalyst 20) is changed so that the purification ability id decreased, the lean degree is set lower than before, during at least part of the time period during which the target air-fuel ratio AFT is set to the lean air-fuel ratio and/or the rich degree is set lower than before, during at least part of the time period during which the target air-fuel ratio AFT is set to the rich air-fuel ratio.
Number | Date | Country | Kind |
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2014-153229 | Jul 2014 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2015/003788 | 7/28/2015 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2016/017154 | 2/4/2016 | WO | A |
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