The present invention relates to a control system of an internal combustion engine.
In the past, it has been known to arrange a catalyst and air-fuel ratio sensor in an exhaust passage of an internal combustion engine. By controlling an air-fuel ratio of an air-fuel mixture based on an output of the air-fuel ratio sensor, the exhaust gas is effectively purified at the catalyst and in turn the exhaust emissions are improved.
However, aging, individual variations, etc., sometimes cause deviation in the output of the air-fuel ratio sensor. For this reason, in a control system of an internal combustion engine described in PTL 1, the output of a downstream side air-fuel ratio sensor arranged at a downstream side of the catalyst is corrected. Specifically, an output air-fuel ratio of the downstream side air-fuel ratio sensor 41 is corrected based on a difference between the output air-fuel ratio of the downstream side air-fuel ratio sensor detected at a timing where the air-fuel ratio of exhaust gas flowing into the downstream side air-fuel ratio sensor becomes the stoichiometric air-fuel ratio due to rich control after fuel cut control and the stoichiometric air-fuel ratio.
[PTL 1] Japanese Unexamined Patent Publication No. 2016-031041
In the above downstream side air-fuel ratio sensor, at the initial setting, the applied voltage is set so that the output current when the air-fuel ratio of the exhaust gas flowing into the downstream side air-fuel ratio sensor is the stoichiometric air-fuel ratio becomes zero. When the output current is zero, current does not flow to the air-fuel ratio sensor, therefore variation in output current due to the fluctuation of the temperature or pressure of the exhaust gas, circuit error, etc., is reduced.
On the other hand, if deviation in the output of the downstream side air-fuel ratio sensor occurs, the output current when the air-fuel ratio of the exhaust gas flowing into the downstream side air-fuel ratio sensor is the stoichiometric air-fuel ratio becomes a value other than zero. For this reason, even if the air-fuel ratio of the exhaust gas flowing into the downstream side air-fuel ratio sensor is the stoichiometric air-fuel ratio, the variation in the output current at this time becomes larger.
In the above method of correction, the deviation in output of the downstream side air-fuel ratio sensor is corrected by processing, therefore the characteristics of the downstream side air-fuel ratio sensor remain off from the initial settings. For this reason, variation in the output current of the downstream side air-fuel ratio sensor is liable to cause the precision of detection of the air-fuel ratio to fall.
Therefore, in consideration of the above problem, an object of the present invention is to provide a control system of an internal combustion engine able to keep the precision of detection of the air-fuel ratio by the air-fuel ratio sensor arranged in an exhaust passage of the internal combustion engine from falling.
The summary of the present disclosure is as follows.
(1) A control system of an internal combustion engine comprising: an air-fuel ratio sensor arranged in an exhaust passage of the internal combustion engine and detecting an air-fuel ratio of exhaust gas, a current detecting device detecting an output current of the air-fuel ratio sensor, a voltage applying device applying voltage to the air-fuel ratio sensor, and a voltage control part configured to control voltage applied to the air-fuel ratio sensor through the voltage applying device, wherein the voltage control part is configured to set the applied voltage to a reference voltage determined so that the output current becomes zero when an air-fuel ratio of inflowing exhaust gas flowing into the air-fuel ratio sensor is a stoichiometric air-fuel ratio, and correct the reference voltage so that the output current detected by the current detecting device becomes zero when it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio.
(2) The control system of an internal combustion engine described in above (1), wherein a catalyst able to store oxygen is arranged in the exhaust passage, and the air-fuel ratio sensor is arranged at a downstream side of the catalyst.
(3) The control system of an internal combustion engine described in above (2), further comprising an air-fuel ratio control part configured to control an air-fuel ratio of an air-fuel mixture supplied to combustion chambers of the internal combustion engine, wherein the air-fuel ratio control part is configured to control the air-fuel ratio of the air-fuel mixture so that an oxygen storage amount of the catalyst changes between zero and a maximum oxygen storage amount, and the voltage control part is configured to correct the reference voltage so that the output current detected by the current detecting device becomes zero when an amount of change per predetermined time of the output current is equal to or less than a predetermined value.
(4) The control system of an internal combustion engine described in above (3), wherein the air-fuel ratio control part is configured to perform fuel cut control stopping supply of fuel to the combustion chambers and after the fuel cut control perform rich control making the air-fuel ratio of the air-fuel mixture an air-fuel ratio richer than a stoichiometric air-fuel ratio so that the oxygen storage amount of the catalyst becomes zero, and the voltage control part is configured to correct the reference voltage so that the output current detected by the current detecting device becomes zero when the rich control is performed and the amount of change per predetermined time of the output current is equal to or less than the predetermined value.
(5) The control system of an internal combustion engine described in above (3), wherein the air-fuel ratio control part is configured to perform active control switching the air-fuel ratio of the air-fuel mixture between an air-fuel ratio richer than the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio so that the oxygen storage amount of the catalyst changes between zero and the maximum oxygen storage amount, and the voltage control part is configured to correct the reference voltage so that the output current detected by the current detecting device becomes zero when the active control is performed and the amount of change per predetermined time of the output current is equal to or less than the predetermined value.
(6) The control system of an internal combustion engine described in any one of above (1) to (5), wherein the voltage control part is configured to switch the applied voltage between the reference voltage and a switching voltage different from the reference voltage and, when correcting the reference voltage, correct the switching voltage so that a difference between an oxygen concentration on an exhaust side electrode of the air-fuel ratio sensor corresponding to the reference voltage when the output current is zero and the oxygen concentration on the exhaust side electrode of the air-fuel ratio sensor corresponding to the switching voltage when the output current is zero becomes constant.
According to the present invention, there is provided a control system of an internal combustion engine able to keep the precision of detection of the air-fuel ratio by the air-fuel ratio sensor arranged in an exhaust passage of the internal combustion engine from falling.
Below, referring to the figures, embodiments of the present invention will be explained in detail. Note that, in the following explanation, similar components are assigned the same reference numerals.
First, referring to
<Explanation of Internal Combustion Engine Overall>
Referring to
As shown in
The intake port 7 in each cylinder is connected through a corresponding intake runner 13 to a surge tank 14. The surge tank 14 is connected through an intake pipe 15 to an air cleaner 16. The intake port 7, intake runner 13, surge tank 14, intake pipe 15, etc., form an intake passage which leads air to the combustion chamber 5. 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 turned by the throttle valve drive actuator 17 to thereby change the opening area of the intake passage.
On the other hand, the exhaust port 9 in 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 header at which these runners are collected. The header of the exhaust manifold 19 is connected to an upstream side casing 21 which has an upstream side catalyst 20 built into it. The upstream side casing 21 is connected to a downstream side casing 24 which has a downstream side catalyst 23 built into it via an exhaust pipe 22. The exhaust port 9, exhaust manifold 19, upstream side casing 21, exhaust pipe 22, downstream side casing 24, etc., form an exhaust passage which discharges exhaust gas produced due to combustion of the air-fuel mixture in the combustion chamber 5.
Various control routines of the internal combustion engine arc performed by an electronic control unit (ECU) 31 based on outputs of sensors provided in the internal combustion engine, etc. The 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 air flow meter 39 detecting the flow rate of air which flows through the intake pipe 15 is arranged. The output of the air flow meter 39 is input through a corresponding AD converter 38 to the input port 36.
Further, at the header of the exhaust manifold 19, i.e., a upstream side of the upstream side catalyst 20, an upstream side air-fuel ratio sensor 40 detecting the air-fuel ratio of the exhaust gas which flows through the inside of the exhaust manifold 19 (that is, the exhaust gas which flows into the upstream side catalyst 20) is arranged. The output of the upstream air-fuel ratio sensor 40 is input through the corresponding AD converter 38 to the input port 36.
Further, inside the exhaust pipe 22, that is, at the downstream side of the upstream side catalyst 20, a downstream side air-fuel ratio sensor 41 for detecting an air-fuel ratio of the exhaust gas flowing through the inside of the exhaust pipe 22 (that is, exhaust gas flowing out from the upstream side catalyst 20) is arranged. The output of the downstream side air-fuel ratio sensor 41 is input through a corresponding AD converter 38 to the input port 36.
Further, an accelerator pedal 42 is connected to a load sensor 43 generating an output voltage proportional to the amount of depression of the accelerator pedal 42. The output voltage of the load sensor 43 is input through a corresponding AD converter 38 to the input port 36. The CPU 35 calculates an engine load based on the output of the load sensor 43.
A crank angle sensor 44 generates an output pulse every time the crankshaft rotates, for example, by 15 degrees. This output pulse is input to the input port 36. The CPU 35 calculates an engine speed based on the output of the 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 the throttle valve drive actuator 17.
Note that, the above-mentioned internal combustion engine is a nonsupercharged internal combustion engine fueled by gasoline, but the configuration of the internal combustion engine is not limited to the above configuration. Therefore, the cylinder array, mode of injection of fuel, configuration of the intake and exhaust systems, configuration of the valve operating mechanism, presence of any supercharger, and other specific parts of the configuration of the internal combustion engine may differ from the configuration shown in
<Explanation of Catalysts>
The upstream side catalyst 20 and the downstream side catalyst 23 arranged in the exhaust passage have similar configurations. The catalysts 20 and 23 are catalysts having oxygen storage abilities, for example, three-way catalysts. Specifically, the catalysts 20 and 23 are comprised of carriers made of ceramic on which a precious metal having a catalytic action (for example, platinum (Pt)) and a co-catalyst having an oxygen storage ability (for example, ceria (CeO2)) are carried.
Further, the catalysts 20 and 23 store or release oxygen in accordance with the air-fuel ratio of the exhaust gas by the co-catalyst. Specifically, the catalysts 20 and 23 store excess oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio. On the other hand, the catalysts 20 and 23 release the amount of additional oxygen required for making the unburned gas oxidize when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio. As a result, even if the air-fuel ratio of the exhaust gas is somewhat off from the stoichiometric air-fuel ratio, the air-fuel ratio on the surface of the catalysts 20 and 23 is maintained near the stoichiometric air-fuel ratio and the unburned gas and NOX are effectively removed at the catalysts 20 and 23.
Note that, so long as the catalysts 20 and 23 have catalytic actions and oxygen storage abilities, they may be catalysts other than three-way catalysts.
<Constitutions of Air-Fuel Ratio Sensors>
The upstream side air-fuel ratio sensor 40 and the downstream side air-fuel ratio sensor 41 arranged in the exhaust passage have similar constitutions.
As shown in
The diffusion regulating layer 54 is provided on one side surface of the solid electrolyte layer 51, while the protective layer 55 is provided on the side surface at the opposite side to the side surface of the solid electrolyte layer 51 side of the diffusion regulating layer 54. In the present embodiment, a measured gas chamber 57 is formed between the solid electrolyte layer 51 and the diffusion regulating layer 54. A part of the exhaust gas flowing through the exhaust passage is introduced through the diffusion regulating layer 54 to the measured gas chamber 57. Further, the exhaust side electrode 52 is arranged inside the measured gas chamber 57. Therefore, the exhaust side electrode 52 is exposed to the exhaust gas through the diffusion regulating layer 54. Note that, the measured gas chamber 57 does not necessarily have to be provided. Each of the air-fuel ratio sensors 40 and 41 may be configured so that the diffusion regulating layer 54 directly contacts the surface of the exhaust side electrode 52.
The heater part 56 is provided on the other side surface of the solid electrolyte layer 51. A reference gas chamber 58 is formed between the solid electrolyte layer 51 and the heater part 56. Reference gas is introduced inside the reference gas chamber 58. In the present embodiment, the reference gas chamber 58 is open to the atmosphere, so the atmosphere is introduced into the reference gas chamber 58 as the reference gas. The atmosphere side electrode 53 is arranged inside the reference gas chamber 58. Therefore, the atmosphere side electrode 53 is exposed to the reference gas (atmosphere).
The heater part 56 is provided with a plurality of heaters 59. Due to the heaters 59, it is possible to control the temperature of each of the air-fuel ratio sensors 40 and 41, in particular the temperature of the solid electrolyte layer 51. The heater part 56 has a heat generating capacity sufficient for heating the solid electrolyte layer 51 until activation.
The solid electrolyte layer 51 is a thin sheet member having oxide ion conductivity. The solid electrolyte layer 51 is, for example, a sintered body comprised of ZrO2 (zirconia), HfO2, ThO2, Bi2O3, etc., to which CaO, MgO, Y2O3, Yb2O3, etc., is added as a stabilizer. Further, the diffusion regulating layer 54 is formed by a porous sintered body of alumina, magnesia, silica, spinel, mullite, or other heat resistant inorganic substance. Furthermore, the exhaust side electrode 52 and the atmosphere side electrode 53 are formed by platinum or another precious metal having a high catalytic activity.
Further, an electrical circuit 70 is connected to the exhaust side electrode 52 and the atmosphere side electrode 53. The electrical circuit 70 includes a voltage application device 60 and a current detection device 61. The voltage application device 60 applies voltage to each of the air-fuel ratio sensors 40 and 41 so that the potential of the atmosphere side electrode 53 becomes higher than the potential of the exhaust side electrode 52. Therefore, the exhaust side electrode 52 functions as a negative electrode, while the atmosphere side electrode 53 functions as a positive electrode. The output port 37 of the ECU 31 is connected through a corresponding drive circuit 45 to the voltage application device 60. Therefore, the ECU 31 can control the voltage applied to each of the air-fuel ratio sensors 40 and 41 through the voltage application device 60.
Further, the current detection device 61 detects the current flowing between the exhaust side electrode 52 and the atmosphere side electrode 53, that is, the output current of each of the air-fuel ratio sensors 40 and 41. The output of the current detection device 61 is input through the corresponding AD converter 38 to the input port 36 of the ECU 31. Therefore, the ECU 31 can acquire the output current of each of the air-fuel ratio sensors 40 and 41 detected by the current detection device 61.
<Operation of Air-Fuel Ratio Sensors>
Next, referring to
As explained above, each solid electrolyte layer 51 has oxide ion conductivity. For this reason, if a difference arises in the concentrations of oxygen between the both side surfaces of the activated solid electrolyte layer 51, an electromotive force E is generated trying to make the oxide ions move from the high concentration surface side to the low concentration surface side. This characteristic is called the “oxygen cell characteristic”.
On the other hand, if a potential difference is given between the both side surfaces of the solid electrolyte layer 51, oxide ions move so that a ratio of oxygen concentration corresponding to the potential difference occurs between the both side surfaces of the solid electrolyte layer. This characteristic is called the “oxygen pumping characteristic”.
When the air-fuel ratio of the exhaust gas flowing into each of the air-fuel ratio sensors 40 and 41 is leaner than the stoichiometric air-fuel ratio, since the oxygen concentration in the exhaust gas is high, the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 is not so large. For this reason, if setting the voltage Vr applied to each of the air-fuel ratio sensors 40 and 41 to a suitable value, the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 becomes smaller than the ratio of oxygen concentration corresponding to the sensor applied voltage Vr. For this reason, to enable the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 to become close to the ratio of oxygen concentration corresponding to the sensor applied voltage Vr, as shown in
On the other hand, when the air-fuel ratio of the exhaust gas flowing into each of the air-fuel ratio sensors 40 and 41 is richer than the stoichiometric air-fuel ratio, the oxygen on the exhaust side electrode 52 reacts with the unburned gases in the exhaust gas and is removed. For this reason, the oxygen concentration at the exhaust side electrode 52 becomes extremely low and the ratio of oxygen concentration between the both side surfaces of the solid electrolyte layer 51 becomes larger. For this reason, if setting the sensor applied voltage Vr to a suitable value, the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 becomes larger than the ratio of the oxygen concentration corresponding to the sensor applied voltage Vr. For this reason, as shown in
Further, when the air-fuel ratio of the exhaust gas flowing into each of the air-fuel ratio sensors 40 and 41 is the stoichiometric air-fuel ratio, the amounts of oxygen and unburned gases in the exhaust gas become the chemical equivalent ratios. For this reason, due to the catalytic action of the exhaust side electrode 52, the two completely burn and the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 is maintained at the oxygen concentration ratio corresponding to the applied voltage Vr. For this reason, as shown in
Therefore, the values of the output currents of the air-fuel ratio sensors 40 and 41 fluctuate according to the air-fuel ratio of the exhaust gas flowing into the air-fuel ratio sensors 40 and 41. For this reason, the ECU 31 can estimate the air-fuel ratio of the exhaust gas based on the currents detected by the current detection device 61. Note that, “the air-fuel ratio of the exhaust gas” means the ratio of the mass of the air to the mass of the fuel supplied until the exhaust gas is generated (mass of air/mass of fuel) and is estimated from the oxygen concentration and reducing gas concentration in the exhaust gas.
<Specific Example of Electrical Circuits>
As will be understood from
If the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio and the ratio of the oxygen concentration between the both side surfaces of a solid electrolyte layer 51 does not change, the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 becomes a ratio of oxygen concentration corresponding to the sensor applied voltage Vr. In this case, the electromotive force E and the potential difference Vs match the sensor applied voltage Vr, so the current Ir does not flow.
On the other hand, if the air-fuel ratio of the exhaust gas is an air-fuel ratio different from the stoichiometric air-fuel ratio and the ratio of the oxygen concentration between the both side surfaces of a solid electrolyte layer 51 changes, the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 differs from the oxygen concentration ratio corresponding to the sensor applied voltage Vr. In this case, the electromotive force E becomes a value different from the sensor applied voltage Vr. As a result, due to negative feedback control, a potential difference Vs is given to the electrodes 52 and 53 so as to make the oxide ions move between the both side surfaces of the solid electrolyte layer 51 so that the electromotive force E matches the sensor applied voltage Vr. Further, current Ir flows along with movement of oxide ions. As a result, the electromotive force E converges at the sensor applied voltage Vr, while the potential difference Vs also converge at the sensor applied voltage Vr.
Further, the current detection device 61 detects the voltage E0 for detecting the current Ir. Here, E0 is expressed like in the following formula (1):
E0=Vr+V0+IrR (1)
Here, V0 is the offset voltage (for example, 3V) applied so that E0 does not become a negative value, while R is the value of the resistance shown in
In formula (1), the sensor applied voltage Vr, the offset voltage V0, and the resistance value R are constant, so the voltage E0 changes in accordance with the current Ir. Therefore, the current detection device 61 can calculate the current Ir based on the voltage E0.
Note that, the electrical circuits 70 may be different from the configuration shown in
<Output Characteristics of Air-Fuel Ratio Sensors>
As a result of the above-mentioned principle, each of the air-fuel ratio sensors 40 and 41 has a voltage-current (V-I) characteristic such as shown in
In the proportional region, the sensor applied voltage Vr is low, so the flow rate of oxide ions able to move through the solid electrolyte layer 51 is small. In this case, the speed of movement of oxide ions moving through the solid electrolyte layer 51 along with the application of voltage becomes slower than the speed of introduction of the exhaust gas introduced into the measured gas chamber 57 through the diffusion regulating layer 54. For this reason, the flow rate of the oxide ions able to move through the solid electrolyte layer 51 changes according to the sensor applied voltage Vr and the output current Ir increases along with the increase of the sensor applied voltage Vr. Note that, the output current Ir becomes a negative value when the sensor applied voltage Vr is 0 because an electromotive force corresponding to the ratio of the oxygen concentration between the both side surfaces of the solid electrolyte layer 51 is generated due to the oxygen cell characteristic.
As shown in
As shown in
For example, if the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio (14.6), the value of the sensor applied voltage Vr when the output current Ir becomes zero is 0.45V. If the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio (is rich), the value of the sensor applied voltage Vr when the output current Ir becomes zero is higher than 0.45V. On the other hand, if the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio (is lean), the value of the sensor applied voltage Vr when the output current Ir becomes zero is lower than 0.45V.
<Control System of Internal Combustion Engine>
Below, a control system of an internal combustion engine according to the first embodiment of the present invention will be explained.
In the present embodiment, the ECU 31 has the voltage control part 81 and air-fuel ratio control part 82. The voltage control part 81 and air-fuel ratio control part 82 are functional blocks realized by a program stored in the ROM 33 of the ECU 31 being run by the CPU 35 of the ECU 31.
The air-fuel ratio control part 82 controls the air-fuel ratio of the air-fuel mixture supplied to the combustion chambers 5 and in turn the air-fuel ratio of the exhaust gas flowing into the upstream side catalyst 20. Specifically, the air-fuel ratio control part 82 changes the amount of fuel supplied from the fuel injectors 11 to the combustion chambers 5 to control the air-fuel ratio of the air-fuel mixture.
The voltage control part 81 controls the voltage applied to the downstream side air-fuel ratio sensor 41 (below, simply referred to as the “applied voltage”) through the voltage applying device 60. As shown in
If current flows at the downstream side air-fuel ratio sensor 41, the output current of the downstream side air-fuel ratio sensor 41 changes due to fluctuations in the temperature or pressure of the exhaust gas, circuit error, etc. On the other hand, if the output current of the downstream side air-fuel ratio sensor 41 is zero, variation of the output current of the downstream side air-fuel ratio sensor 41 due to fluctuation of the temperature or pressure of the exhaust gas, circuit error, etc. is reduced.
In the present embodiment, the voltage control part 81 sets the applied voltage to the reference voltage determined so that the output current of the downstream side air-fuel ratio sensor 41 when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio becomes zero. By doing this, it is possible to precisely detect the air-fuel ratio of the inflowing exhaust gas being the stoichiometric air-fuel ratio and in turn possible to quickly detect changes in the characteristics of the exhaust gas flowing out from the upstream side catalyst 20. For this reason, it is possible to control the air-fuel ratio of the air-fuel mixture based on the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 to thereby keep the exhaust emissions from deteriorating.
However, aging, individual variations, etc., sometimes cause deviation in the output of the downstream side air-fuel ratio sensor 41. If deviation in the output of a downstream side air-fuel ratio sensor 41 occurs, even if the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio, the output current of the downstream side air-fuel ratio sensor 41 becomes a value other than zero. As a result, the precision of detection of the air-fuel ratio by the downstream side air-fuel ratio sensor 41, in particular the precision of detection of the stoichiometric air-fuel ratio, deteriorates.
For this reason, in order to keep the exhaust emissions from deteriorating due to deterioration of the precision of detection of the air-fuel ratio, it is necessary to correct deviation of the output of the downstream side air-fuel ratio sensor 41. For example, it may be considered to set the output current of the downstream side air-fuel ratio sensor 41 detected when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio to a correction amount and subtract the correction amount from the actually detected output current of the downstream side air-fuel ratio sensor 41.
However, with this method, the deviation in the output of the downstream side air-fuel ratio sensor 41 is corrected by processing, therefore the characteristics of the downstream side air-fuel ratio sensor 41 remain off from the initial settings. For this reason, variation in the output current of the downstream side air-fuel ratio sensor 41 is liable to cause the precision of detection of the air-fuel ratio to fall.
As opposed to this, in the present embodiment, if deviation in the output of the downstream side air-fuel ratio sensor 41 occurs, the applied voltage is changed to render the output current of the downstream side air-fuel ratio sensor 41 corresponding to the stoichiometric air-fuel ratio zero. Specifically, the voltage control part 81 corrects the reference voltage so that the output current of the downstream side air-fuel ratio sensor 41 detected by the current detecting device 61 when it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio becomes zero. By doing this, the characteristics of the downstream side air-fuel ratio sensor 41 become the initial ideal state and the variation in the output current of the downstream side air-fuel ratio sensor 41 is reduced. As a result, it is possible to keep the precision of detection of the air-fuel ratio by the downstream side air-fuel ratio sensor 41 from falling.
As explained above, in order to correct the reference voltage, it is necessary to make the air-fuel ratio of the inflowing exhaust gas the stoichiometric air-fuel ratio. If the oxygen storage amount of the upstream side catalyst 20 changes between zero and the maximum oxygen storage amount, the air-fuel ratio of the inflowing exhaust gas at least temporarily becomes the stoichiometric air-fuel ratio by the exhaust purification characteristics of the upstream side catalyst 20. For this reason, the air-fuel ratio control part 82 controls the air-fuel ratio of the air-fuel mixture so that the oxygen storage amount of the upstream side catalyst 20 changes between zero and the maximum oxygen storage amount.
Further, when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio, the amount of change of the output current of the downstream side air-fuel ratio sensor 41 becomes smaller. For this reason, the voltage control part 81 corrects the reference voltage so that the output current of the downstream side air-fuel ratio sensor 41 detected by the current detecting device 61 becomes zero when the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 is equal to or less than a predetermined value. By doing this, it is possible to precisely correct the reference voltage applied to the downstream side air-fuel ratio sensor 41 based on the output current when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio.
In the present embodiment, the air-fuel ratio control part 82 performs fuel cut control to stop the supply of fuel to the combustion chambers 5 when predetermined conditions for execution are satisfied. In the fuel cut control, the air-fuel ratio control part 82 stops the injection of fuel from the fuel injectors 11 to stop the supply of fuel to the combustion chambers 5. The predetermined conditions for execution, for example, are satisfied when the amount of depression of the accelerator pedal 42 is zero or substantially zero (that is, the engine load is zero or substantially zero) and the engine speed is equal to or higher than a predetermined speed higher than the speed at the time of idling.
If fuel cut control is performed, air or a gas similar to air is discharged into the exhaust passage and flows into the upstream side catalyst 20. As a result, a large amount of oxygen flows into the upstream side catalyst 20 and the oxygen storage amount of the upstream side catalyst 20 reaches the maximum oxygen storage amount. Further, if the oxygen storage amount of the upstream side catalyst 20 reaches the maximum oxygen storage amount, a large amount of oxygen also flows into the downstream side catalyst 23 and the oxygen storage amount of the downstream side catalyst 23 also reaches the maximum oxygen storage amount.
For this reason, if fuel cut control is continued for equal to or greater than a predetermined time, the oxygen storage amounts of the upstream side catalyst 20 and downstream side catalyst 23 become the maximum oxygen storage amounts. When the oxygen storage amounts of the upstream side catalyst 20 and downstream side catalyst 23 are the maximum oxygen storage amounts, the upstream side catalyst 20 and downstream side catalyst 23 cannot store the excess oxygen in the exhaust gas. For this reason, if, after fuel cut control, exhaust gas leaner than the stoichiometric air-fuel ratio flows into the upstream side catalyst 20 and downstream side catalyst 23, the NOX in the exhaust gas is not removed at the upstream side catalyst 20 and downstream side catalyst 23 and the exhaust emissions are liable to deteriorate.
Therefore, in the present embodiment, after fuel cut control, the air-fuel ratio control part 82 performs rich control to make the air-fuel ratio of the air-fuel mixture richer than the stoichiometric air-fuel ratio so that the oxygen storage amount of the upstream side catalyst 20 becomes zero. By doing this, it is possible to decrease the oxygen storage amounts of the upstream side catalyst 20 and downstream side catalyst 23 and possible to keep the exhaust emissions from deteriorating after fuel cut control.
In rich control, the air-fuel ratio control part 82 sets the target air-fuel ratio of the air-fuel mixture to a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio and performs feedback control on the amount of fuel supplied to the combustion chambers 5 so that the air-fuel ratio detected by the upstream side air-fuel ratio sensor 40 matches the target air-fuel ratio. Note that, the air-fuel ratio control part 82 may control the amount of fuel supplied to the combustion chambers 5 so that the air-fuel ratio of the inflowing exhaust gas matches the target air-fuel ratio without using the upstream side air-fuel ratio sensor 40. In this case, the air-fuel ratio control part 82 supplies the combustion chambers 5 with an amount of fuel calculated from the amount of intake air detected by the air flow meter 39 and the target air-fuel ratio of the air-fuel mixture so that the ratio of the fuel and air supplied to the combustion chambers 5 matches the target air-fuel ratio of the air-fuel mixture.
Further, when it is judged that the total of the amounts of intake air from when starting rich control has reached a predetermined amount, the air-fuel ratio control part 82 ends the rich control. The predetermined amount is made larger than the amount required for the oxygen storage amount of the upstream side catalyst 20 to fall from the maximum oxygen storage amount to zero. Note that, the air-fuel ratio control part 82 may end the rich control when the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 has reached a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio.
Due to the rich control, the air-fuel ratio of the inflowing exhaust gas changes from an air-fuel ratio leaner than the stoichiometric air-fuel ratio toward the stoichiometric air-fuel ratio. While the oxygen storage amount of the upstream side catalyst 20 is in a suitable range, the air-fuel ratio of the inflowing exhaust gas is maintained at the stoichiometric air-fuel ratio and the output current of the downstream side air-fuel ratio sensor 41 becomes substantially constant. For this reason, the voltage control part 81 corrects the reference value so that the output current of the downstream side air-fuel ratio sensor 41 detected by the current detecting device 61 becomes zero when rich control is performed and the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 is equal to or less than a predetermined value.
<Explanation of Control Using Time Chart>
In the example of
In the example of
The reference voltage is applied to the downstream side air-fuel ratio sensor 41, therefore if there is no deviation in the output current of the downstream side air-fuel ratio sensor 41, the output current Ist of the downstream side air-fuel ratio sensor 41 detected at the time Tst when it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio becomes zero. On the other hand, if deviation in the output current of the downstream side air-fuel ratio sensor 41 occurs, the output current Ist of the downstream side air-fuel ratio sensor 41 becomes a value other than zero.
In the example of
<Processing for Correction of Voltage>
Below, referring to the flow chart of
First, at step S101, the voltage control part 81 judges whether the conditions for execution for correcting the reference voltage are satisfied. The conditions for execution are satisfied for example, when the temperature of the sensor element of the downstream side air-fuel ratio sensor 41 is equal to or greater than an activation temperature and a predetermined time has elapsed from when the reference voltage was corrected the previous time. The temperature of the sensor element of the downstream side air-fuel ratio sensor 41 is, for example, calculated based on the impedance of the sensor element. If at step S101 it is judged that the conditions for execution are not satisfied, the present control routine ends. On the other hand, if it is judged at step S101 that the conditions for execution are satisfied, the present control routine proceeds to step S102.
At step S102, the voltage control part 81 judges whether rich control is being performed after fuel cut control. If it is judged that rich control is not being performed after fuel cut control, the present control routine ends. On the other hand, if it is judged that rich control is being performed after fuel cut control, the present control routine proceeds to step S103.
At step S103, the voltage control part 81 judges whether the output current Idwn of the downstream side air-fuel ratio sensor 41 is equal to or less than a reference value Iref. The output current Idwn of the downstream side air-fuel ratio sensor 41 is detected by the current detecting device 61. The reference value Iref is determined in advance and, as shown in
At step S104, the voltage control part 81 judges whether the amount of change per predetermined time ΔIdwn of the output current Idwn is equal to or less than a predetermined value A. The predetermined value A is determined in advance and is, for example, set to the maximum value of the amount of change ΔIdwn detected when the air-fuel ratio of the inflowing exhaust gas is maintained at the stoichiometric air-fuel ratio. If at step S104 it is judged that the amount of change ΔIdwn is larger than the predetermined value A, the present control routine ends. On the other hand, if at step S104 it is judged that the amount of change ΔIdwn is equal to or less than the predetermined value A, the present control routine proceeds to step S105. In this case, it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio.
At step S105, the voltage control part 81 updates the sum output current ΣIdwn of the downstream side air-fuel ratio sensor 41. Specifically, the voltage control part 81 sets the value obtained by adding the newly detected output current Idwn to the current sum output current ΣIdwn as the new sum output current ΣIdwn.
Next, at step S106, the voltage control part 81 adds “1” to the number of times of detection N. The initial value of the number of times of detection N is zero.
Next, at step S107, the voltage control part 81 judges whether the number of times of detection N is equal to or greater than a reference number of times Nref. The reference number of times Nref is determined in advance. If at step S107 it is judged that the number of times of detection N is less than the reference number of times Nref, the present control routine ends. On the other hand, if at step S107 it is judged that the number of times of detection N is equal to or greater than the reference number of times Nref, the present control routine proceeds to step S108.
At step S108, the voltage control part 81 calculates the output current Ist of the downstream side air-fuel ratio sensor 41 detected when it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio. The voltage control part 81 calculates the output current Ist by averaging of the plurality of values of the output current Idwn added at step S105. Specifically, the voltage control part 81 divides the sum output current ΣIdwn of the downstream side air-fuel ratio sensor 41 by the reference number of times Nref to thereby calculate the output current Ist. Note that, the value obtained by excluding the maximum value and minimum value from the plurality of values of the output current Idwn may be used to calculate the output current Ist.
Next, at step S109, the voltage control part 81 corrects the reference voltage based on the output current Ist. Specifically, the voltage control part 81 corrects the reference voltage so that the output current Ist becomes zero. For example, the voltage control part 81 uses the map shown in
Note that, the lower limit value and upper limit value of the reference voltage are determined in advance so that the reference voltage does not become outside the limit current region. In the present embodiment, the upper limit value is set to 0.8V, while the lower limit value is set to 0.1V. That is, the reference voltage is set to 0.45V±0.35V in range. If due to the correction, the reference voltage reaches the upper limit value or lower limit value, the voltage control part 81 suspends the correction of the reference voltage. In this case, the output current Ist may be set to the amount of correction and the air-fuel ratio of the inflowing exhaust gas may be calculated based on the value of the actually detected output current of the downstream side air-fuel ratio sensor 41 minus the output current Ist. That is, the output current of the downstream side air-fuel ratio sensor 41 may be corrected by processing.
By correction of the reference voltage, the value of the reference voltage is updated and the applied voltage is changed to the value of the reference voltage after correction. The timing at which the applied voltage is changed is, for example, when the reference voltage is corrected or when, after the reference voltage is corrected, the internal combustion engine is restarted.
Next, at step S110, the voltage control part 81 resets the sum output current ΣIdwn and number of times of detection N to zero. After step S110, the present control routine ends.
Note that, step S105 and step S106 may be omitted and, at step S108, the voltage control part 81 may acquire the output current Idwn of the downstream side air-fuel ratio sensor 41 detected by the current detecting device 61 as the output current Ist. That is, the output current Ist need not be calculated as the average value of a plurality of values of the output current Idwn.
Further, in the present embodiment, the voltage control part 81 corrects the reference voltage so that the output current Ist becomes zero by a single correction operation. However, the voltage control part 81 may correct the reference voltage so that the output current Ist becomes zero by a plurality of correction operations. In this case, for example, the amount of correction of the reference voltage is calculated based on the value of the output current Ist divided by a predetermined value or value of the amount of correction of the reference voltage calculated based on the output current Ist divided by a predetermined value is set as the final amount of correction. By doing this, if error occurs in the output current Ist, it is possible to keep the precision of detection of the air-fuel ratio by the downstream side air-fuel ratio sensor 41 from deteriorating due to the correction of the reference voltage.
The configuration and control of a control system of an internal combustion engine according to a second embodiment are basically the same as the control system of an internal combustion engine according to the first embodiment except for the points explained below. For this reason, below, the second embodiment of the present invention will be explained centered on the parts different from the first embodiment.
In the second embodiment, in order to correct the reference voltage, the air-fuel ratio is controlled differently from the first embodiment so that the air-fuel ratio of the inflowing exhaust gas is rendered the stoichiometric air-fuel ratio. Specifically, the air-fuel ratio control part 82 performs active control switching the air-fuel ratio of the air-fuel mixture between an air-fuel ratio richer than the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio so that the oxygen storage amount of the upstream side catalyst 20 changes between zero and the maximum oxygen storage amount.
In active control, the air-fuel ratio control part 82 switches the target air-fuel ratio of the air-fuel mixture from the rich set air-fuel ratio to the lean set air-fuel ratio when the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 reaches the rich judged air-fuel ratio and switches the target air-fuel ratio of the air-fuel mixture from the lean set air-fuel ratio to the rich set air-fuel ratio when the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 reaches the lean judged air-fuel ratio.
The rich set air-fuel ratio is determined in advance and is set to an air-fuel ratio richer than the stoichiometric air-fuel ratio. The lean set air-fuel ratio is determined in advance and is set to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. The rich judged air-fuel ratio is determined in advance and is set to an air-fuel ratio richer than the stoichiometric air-fuel ratio and leaner than the rich set air-fuel ratio. For this reason, the oxygen storage amount of the upstream side catalyst 20 when the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 reaches the rich judged air-fuel ratio becomes zero. The lean judged air-fuel ratio is determined in advance and is set to an air-fuel ratio leaner than the stoichiometric air-fuel ratio and richer than the lean set air-fuel ratio. For this reason, the oxygen storage amount of the upstream side catalyst 20 when the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 reaches the lean judged air-fuel ratio becomes the maximum oxygen storage amount.
Further, the air-fuel ratio control part 82 performs feedback control of the amount of fuel supplied to the combustion chambers 5 so that the air-fuel ratio detected by the upstream side air-fuel ratio sensor 40 in active control matches the target air-fuel ratio of the air-fuel mixture. Note that, the air-fuel ratio control part 82 may control the amount of fuel supplied to the combustion chambers 5 so that the air-fuel ratio of the inflowing exhaust gas matches the target air-fuel ratio of the air-fuel mixture without using the upstream side air-fuel ratio sensor 40. In this case, the air-fuel ratio control part 82 supplies the combustion chambers 5 with the amount of fuel calculated from the amount of intake air detected by the air flow meter 39 and the target air-fuel ratio of the air-fuel mixture so that the ratio of the fuel and air supplied to the combustion chambers 5 matches the target air-fuel ratio of the air-fuel mixture.
By switching the target air-fuel ratio from the rich set air-fuel ratio to the lean set air-fuel ratio, the air-fuel ratio of the inflowing exhaust gas changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio toward the stoichiometric air-fuel ratio. On the other hand, by switching the target air-fuel ratio from the lean set air-fuel ratio to the rich set air-fuel ratio, the air-fuel ratio of the inflowing exhaust gas changes from an air-fuel ratio leaner than the stoichiometric air-fuel ratio toward the stoichiometric air-fuel ratio. While the oxygen storage amount of the upstream side catalyst 20 is in a suitable range, the air-fuel ratio of the inflowing exhaust gas is maintained at the stoichiometric air-fuel ratio and the output current of the downstream side air-fuel ratio sensor 41 becomes substantially constant. For this reason, the voltage control part 81 corrects the reference voltage so that the output current of the downstream side air-fuel ratio sensor 41 detected by the current detecting device 61 becomes zero when active control is performed and the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 is equal to or less than a predetermined value.
<Explanation of Control Using Time Chart>
At the time to, the target air-fuel ratio is set to the lean set air-fuel ratio AFL. The lean set air-fuel ratio AFL is, for example, set to 15.1. After the time to, at the time t1, the output current of the downstream side air-fuel ratio sensor 41 reaches the lean judged current Ilean. The lean judged current Ilean is the output current corresponding to the lean judged air-fuel ratio (for example, 14.65).
At the time t1, the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 reaches the lean judged air-fuel ratio, therefore the target air-fuel ratio is switched from the lean set air-fuel ratio AFL to the rich set air-fuel ratio AFR. The rich set air-fuel ratio AFR is, for example, set to 14.1.
Due to the switching of the target air-fuel ratio, the output current of the downstream side air-fuel ratio sensor 41 decreases toward zero. That is, the air-fuel ratio of the inflowing exhaust gas changes toward the stoichiometric air-fuel ratio. After that, at the time t2, the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 becomes equal to or less than a predetermined value. As a result, at the time t2 to the time t3, it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio.
In the example of
After the time t3, at the time t4, the output current of the downstream side air-fuel ratio sensor 41 reaches the rich judged current Irich. The rich judged current Irich is the output current corresponding to the rich judged air-fuel ratio (for example, 14.55).
At the time t4, the air-fuel ratio detected by the downstream side air-fuel ratio sensor 41 has reached the rich judged air-fuel ratio, therefore the target air-fuel ratio is switched from the rich set air-fuel ratio AFL to the lean set air-fuel ratio AFL.
By switching the target air-fuel ratio, the output current of the downstream side air-fuel ratio sensor 41 increases toward zero. That is, the air-fuel ratio of the inflowing exhaust gas changes toward the stoichiometric air-fuel ratio. After that, at the time t5, the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 becomes equal to or less than a predetermined value. As a result, at the time t5 to the time t6, it is judged that the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio.
In the example of
After the time t6, at the time t7, the output current of the downstream side air-fuel ratio sensor 41 again reaches the lean judged current Ilean and the target air-fuel ratio is switched from the lean set air-fuel ratio AFL to the rich set air-fuel ratio AFR.
<Processing for Correction of Voltage>
First, at step S201, in the same way as step S101 of
At step S202, the air-fuel ratio control part 82 performs active control. Next, at step S203, in the same way as step S104 of
Step S204 to step S209 are similar to step S105 to step S110 of
<Third Embodiment>
The configuration and control of a control system of an internal combustion engine according to a third embodiment are basically the same as the control system of an internal combustion engine according to the first embodiment except for the points explained below. For this reason, below, the third embodiment of the present invention will be explained centered on the parts different from the first embodiment.
In the third embodiment, the voltage control part 81 switches the applied voltage between a reference voltage and a switching voltage different from the reference voltage. In order to use the downstream side air-fuel ratio sensor 41 to precisely detect a predetermined air-fuel ratio, it is desirable to make the output current of the downstream side air-fuel ratio sensor 41 corresponding to the predetermined air-fuel ratio approach zero.
As will be understood from
If correcting the reference voltage to correct deviation of the output of the downstream side air-fuel ratio sensor 41, the switching voltage also has to be corrected. However, if the amount of correction of the reference voltage is also added to the switching voltage to correct the switching voltage, the correspondence relationship between the air-fuel ratio corresponding to the reference voltage when the output current is zero and the air-fuel ratio corresponding to the switching voltage when the output current is zero is liable to change due to the correction.
Further,
When correcting the reference voltage, the voltage control part 81 corrects the switching voltage so that the difference between the oxygen concentration corresponding to the reference voltage when the output current is zero and the oxygen concentration corresponding to the switching voltage when the output current is zero becomes constant. By doing this, it is possible to keep the correspondence relationship between the air-fuel ratio detected precisely at the reference voltage (stoichiometric air-fuel ratio) and the air-fuel ratio detected precisely at the switching voltage from changing due to the correction.
<Processing for Correction of Voltage>
Step S301 to step S309 are similar to step S101 to step S109 of
In the present control routine, after step S309, at step S310, the voltage control part 81 corrects the switching voltage so that the difference between the oxygen concentration corresponding to the reference voltage when the output current is zero and the oxygen concentration corresponding to the switching voltage when the output current is zero becomes constant.
Specifically, the voltage control part 81 uses a map or calculation formula to calculate an oxygen concentration corresponding to the corrected reference voltage when the output current is zero. Next, the voltage control part 81 adds the initial concentration difference to the oxygen concentration corresponding to the corrected reference voltage when the output current is zero to thereby calculate the target oxygen concentration. The initial concentration difference is the difference between the oxygen concentration corresponding to the initial value of the reference voltage when the output current is zero and the oxygen concentration corresponding to the initial value of the switching voltage when the output current is zero and is determined in advance by experiments, simulation, etc. Finally, the voltage control part 81 uses a map or calculation formula to calculate the applied voltage where the oxygen concentration becomes the target oxygen concentration when the output current is zero as the corrected switching voltage.
Next, at step S311, the voltage control part 81 resets the sum output current ΣIdwn and number of times of detection N to zero. After step S311, the present control routine ends. Note that, the present control routine can be modified in the same way as the control routine of
Above, preferred embodiments according to the present invention were explained, but the present invention is not limited to these embodiments. Various corrections and changes can be made within the language of the claims.
Regardless of the type of the air-fuel ratio control, when the output current of the downstream side air-fuel ratio sensor 41 is within a predetermined range and the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 is equal to or less than a predetermined value, there is a high possibility of the air-fuel ratio of the inflowing exhaust gas becoming the stoichiometric air-fuel ratio due to the purification of the exhaust gas by the upstream side catalyst 20. For this reason, the voltage control part 81 may correct the reference voltage so that the output current of the downstream side air-fuel ratio sensor 41 detected by the current detecting device 61 becomes zero when the output current of the downstream side air-fuel ratio sensor 41 is within a predetermined range and the amount of change per predetermined time of the output current of the downstream side air-fuel ratio sensor 41 is equal to or less than a predetermined value. In this case, it is not necessarily required that the air-fuel ratio control part 82 perform the predetermined air-fuel ratio control so as to correct the reference voltage.
Further, the downstream side air-fuel ratio sensor 41 may be arranged at the downstream side of the downstream side catalyst 23. Further, the control system of the internal combustion engine may be provided with an upstream side air-fuel ratio sensor 40 in addition to the downstream side air-fuel ratio sensor 41 or instead of the downstream side air-fuel ratio sensor 41. That is, in the same way as the downstream side air-fuel ratio sensor 41, the reference voltage and the switching voltage applied to the upstream side air-fuel ratio sensor 40 may be corrected. In this case, for example, the air-fuel ratio control part 82 sets the target air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio, and the voltage control part 81 corrects the reference voltage so that the output current of the upstream side air-fuel ratio sensor 40 detected when the amount of change per predetermined time of the output current of the upstream side air-fuel ratio sensor 40 is equal to or less than a predetermined value becomes zero.
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JP2019-009564 | Jan 2019 | JP | national |
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