JP4311169B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine provided with an NOx storage reduction catalyst and a reducing agent supply means in an exhaust passage.

  A technology for purifying exhaust gas by arranging a NOx occlusion reduction catalyst (hereinafter referred to as “NOx catalyst”) in the exhaust passage of a diesel engine is known, but such NOx catalyst is capable of oxidizing sulfur in exhaust gas. Since the object (SOx) is gradually held, so-called SOx poisoning occurs in which NOx in the exhaust gas cannot be held when the amount of SOx held increases.

  Therefore, after raising the bed temperature of the NOx catalyst to 600 ° C. or higher, the fuel is added through the reducing agent addition valve arranged upstream of the NOx catalyst in the exhaust system, and the SOx held in the NOx catalyst is reduced and removed to reduce the SOx. A so-called SOx poisoning elimination process for eliminating the poisoning is executed. However, the fuel added to the exhaust system exhibits the function of decomposing SOx held in the NOx catalyst under high temperature conditions, and has the property of further increasing the temperature of the NOx catalyst, so that the NOx catalyst is overheated. There is a risk that.

  On the other hand, as a measure for preventing or suppressing SOx poisoning while preventing overheating of the NOx catalyst, Patent Document 1 discloses that a fuel is supplied through a reducing agent addition valve provided in an exhaust passage upstream of the NOx catalyst. It is described that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is intermittently rich and lean by repeating addition and stop at appropriate timing.

Further, in Patent Document 2, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is made rich, in addition to the addition of fuel by the reducing agent addition valve, the air-fuel ratio in the cylinder is made rich compared to that during normal operation. It is described to switch to.
JP 2003-166415 A JP 2002-38943 A JP 2002-38939 A

  However, the fuel added by the reducing agent addition valve adheres to the exhaust passage, or even if a part of the fuel is vaporized by the heat of the exhaust gas, the fuel that has not been sufficiently vaporized has a large particle diameter and a heavy mass. Since it is difficult to reach the catalyst along with the gas flow, not all of it evaporates immediately after the addition, and not all of it reaches the NOx catalyst immediately.

  For this reason, if the air-fuel ratio in the cylinder is switched to richer than in normal operation at the same time as fuel addition is started by the reducing agent addition valve, the air-fuel ratio in the cylinder is lowered and the exhaust gas discharged from the cylinder is reduced. Even though the air-fuel ratio is lowered, the fuel added by the reducing agent addition valve does not sufficiently reach the NOx catalyst, so the air-fuel ratio of the exhaust gas flowing into the NOx catalyst does not become the target rich air-fuel ratio. There is a fear. As a result, the fuel used while lowering the air-fuel ratio in the cylinder is wasted, and the fuel efficiency is deteriorated.

On the other hand, when the SOx poisoning elimination process is finished, if the air-fuel ratio in the cylinder is switched to the air-fuel ratio during normal operation at the same time as stopping the fuel addition by the reducing agent addition valve, it is discharged from the cylinder. Despite the fact that the air-fuel ratio of the exhaust gas has increased, the fuel that has been added so far and adhered to the exhaust passage and the fuel that has delayed evaporation reach the NOx catalyst with a delay, so the added fuel is wasted. The fuel consumption will worsen.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can reduce and remove SOx held in a NOx catalyst with high fuel efficiency.

The control and purification device for an internal combustion engine according to the present invention employs the following means in order to solve the above-described problems. That is, a NOx storage reduction catalyst provided in the exhaust passage, a reducing agent supply means provided in an exhaust passage upstream of the NOx storage reduction catalyst, an air fuel ratio control means for controlling the air fuel ratio in the cylinder, The air-fuel ratio control means lowers the air-fuel ratio in the cylinder as compared with the air-fuel ratio in normal operation , lowers the air-fuel ratio of the exhaust gas discharged from the cylinder, and the reducing agent supply means The exhaust gas purifying apparatus for an internal combustion engine that releases and reduces the sulfur oxide held in the NOx occlusion reduction type catalyst by reducing the air-fuel ratio of the exhaust gas flowing into the NOx occlusion reduction type catalyst to a theoretical air fuel ratio or less by supplying A reducing agent amount calculating means for calculating a reducing agent amount that has actually reached the NOx occlusion reduction type catalyst among the reducing agents supplied from the reducing agent supply means; hand Is characterized by reducing the air-fuel ratio in the cylinder based on the calculated amount of reducing agent in the catalyst reaches the reducing agent amount calculating means.

  In order to release and reduce sulfur oxide (SOx) held in the NOx catalyst, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst needs to be less than the stoichiometric air-fuel ratio. Assuming that the timing for lowering the air-fuel ratio of the exhaust gas discharged from the cylinder by lowering the air-fuel ratio and the timing for supplying the reducing agent by the reducing agent supply means are the same, the reducing agent supplied to the exhaust passage is exhausted. Even if the fuel adhering to the passage or part of it is vaporized due to the heat of the exhaust gas, the fuel that has not been sufficiently vaporized has a large particle size and heavy mass, so it is difficult to reach the catalyst along with the flow of exhaust gas. Despite the fact that the exhaust gas resulting from combustion at the air-fuel ratio in the cylinder has reached the NOx catalyst, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is kept below the stoichiometric air-fuel ratio and maintained in the NOx catalyst. Amount of reducing agent needed to release reducing SOx may not reach the NOx catalyst. As a result, the fuel used while the air-fuel ratio in the cylinder is lowered is wasted and fuel consumption is deteriorated.

  Also, the timing at which the air-fuel ratio control means lowers the air-fuel ratio in the cylinder to lower the air-fuel ratio of the exhaust gas discharged from the cylinder and the reducing agent supply means finishes supplying the reducing agent. Assuming that the timing is the same, the amount of reducing agent necessary for releasing and reducing the SOx retained in the NOx catalyst has reached the NOx catalyst, but the exhaust gas exhausted from the cylinder is empty. Since the fuel ratio is not lowered, the exhaust gas flowing into the NOx catalyst may not be less than the stoichiometric air fuel ratio. As a result, the supplied reducing agent is wasted and fuel consumption is deteriorated.

  According to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the catalyst reaching reducing agent amount calculating means calculates the reducing agent amount reaching the NOx catalyst among the reducing agents supplied from the reducing agent supplying means, and the calculation is performed. Since the air-fuel ratio control means lowers the air-fuel ratio in the cylinder based on the reduced amount of reducing agent, the exhaust gas resulting from combustion at the reduced air-fuel ratio in the cylinder has sufficiently reached the NOx catalyst. The air-fuel ratio of the exhaust gas flowing into the NOx catalyst without reaching a sufficient amount of the reducing agent does not fall below the stoichiometric air-fuel ratio, or even though a sufficient amount of the reducing agent has reached the NOx catalyst. Therefore, it is possible to prevent the exhaust gas that has flown into the NOx catalyst from reaching the exhaust gas due to combustion at the air-fuel ratio in the cylinder and not to be less than the stoichiometric air-fuel ratio. As a result, it is possible to suppress the wasteful reduction of the air-fuel ratio in the cylinder and the wasteful supply of the reducing agent, and to release and reduce SOx held in the NOx catalyst with high fuel efficiency.

When the amount of reducing agent calculated by the catalyst reaching reducing agent amount calculating means is equal to or greater than the amount of reducing agent necessary for releasing and reducing the sulfur oxide held in the NOx occlusion reduction type catalyst, air-fuel ratio control means, Rukoto to maintain the air-fuel ratio in the cylinder while being lower than the air-fuel ratio during normal operation is suitable. Thereby, it is possible to suppress the wasteful reduction of the air-fuel ratio in the cylinder and the wasteful supply of the reducing agent, and it is possible to release and reduce the SOx held in the NOx catalyst with high fuel efficiency.

  However, the air-fuel ratio control means operates so as to lower the air-fuel ratio in the cylinder, and the exhaust gas due to combustion at the actually lowered air-fuel ratio flows through the exhaust passage and reaches the NOx catalyst accordingly. Takes time. Therefore, the reducing agent that has reached the NOx catalyst during this time may be wasted.

Therefore, the air-fuel ratio control means until the exhaust gas resulting from combustion at the air-fuel ratio in the cylinder lowered by the air-fuel ratio control means compared to the air-fuel ratio during normal operation reaches the NOx storage reduction catalyst. It is preferable to reduce the air-fuel ratio in the cylinder on the basis of the exhaust gas arrival time which is the above-mentioned time. For example, the amount of reducing agent calculated by the catalyst reducing agent amount calculating means reaches the exhaust gas more than the amount of reducing agent necessary for releasing and reducing sulfur oxide held in the NOx storage reduction type catalyst. If it is more partial small amount of reducing agent amount to reach in time, the air-fuel ratio control means, in Rukoto to maintain the air-fuel ratio in the cylinder while being lower than the air-fuel ratio during normal operation Further, it is possible to more accurately suppress the wasteful reduction of the air-fuel ratio in the cylinder and the wasteful supply of the reducing agent, and it is possible to release and reduce SOx held in the NOx catalyst with high fuel efficiency.

  The air-fuel ratio control means preferably controls the air-fuel ratio in the cylinder by limiting at least the amount of air taken into the cylinder. As a method of limiting the amount of air sucked into the cylinder, the opening degree of the intake throttle valve provided in the intake passage is adjusted, and the opening degree of the exhaust throttle valve provided in the exhaust passage downstream of the NOx catalyst is adjusted. Examples of the adjustment include adjusting the opening degree of both the intake throttle valve and the exhaust throttle valve.

  As described above, the exhaust gas purification apparatus for an internal combustion engine according to the present invention can reduce and remove SOx held in the NOx catalyst with good fuel efficiency.

  Exemplary embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent.

  FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine provided with an exhaust purification device according to the present invention. An internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cylinder diesel engine having four cylinders 2.

  The internal combustion engine 1 includes a fuel injection valve 3 that injects fuel directly into the combustion chamber of each cylinder 2. Each fuel injection valve 3 is connected to a pressure accumulating chamber (common rail) 4, and the common rail 4 communicates with a fuel pump 6 through a fuel supply pipe 5.

  An intake passage 7 is connected to the internal combustion engine 1, and the intake passage 7 is connected to an air cleaner box 8. An air flow meter 9 that outputs an electrical signal corresponding to the mass of the intake air flowing through the intake passage 7 is attached to the intake passage 7 downstream of the air cleaner box 8.

A compressor housing 10 a of a supercharger (turbocharger) 10 is provided in the middle of the intake passage 7. An intercooler 11 is attached to the intake passage 7 downstream of the compressor housing 10a. Further, an intake throttle valve 12 for adjusting the flow rate of intake air flowing through the intake passage 7 is provided in the intake passage 7 downstream of the intercooler 11. An intake throttle actuator 13 that opens and closes the intake throttle valve 12 is attached to the intake throttle valve 12.

  The intake air that has flowed into the compressor housing 10a and is compressed in the compressor housing 10a to a high temperature is cooled by the intercooler 11, and then the flow rate is adjusted by the intake throttle valve 12 as necessary. The fuel is distributed to the combustion chambers of the respective cylinders 2 through the passages 7 and burned by using the fuel injected from the fuel injection valves 3 of the respective cylinders 2 as an ignition source.

  Further, an exhaust passage 14 is connected to the internal combustion engine 1, and this exhaust passage 14 is connected downstream with a muffler (not shown). Further, a turbine housing 10b of the supercharger 10 is disposed in the middle of the exhaust passage 14, and an NOx storage reduction catalyst (hereinafter, unless otherwise specified) is provided in a portion of the exhaust passage 14 downstream from the turbine housing 10b. "NOx catalyst") 15 is provided. The exhaust gas discharged from the turbine housing 10b flows into the NOx catalyst 15 through the exhaust passage 14, and the substance in the exhaust gas is purified. In addition, an air-fuel ratio sensor 16 that outputs an electric signal corresponding to the air-fuel ratio of the exhaust gas flowing in the exhaust passage 14 and the temperature of the exhaust gas flowing in the exhaust passage 14 are disposed in the exhaust passage 14 upstream of the NOx catalyst 15. And an exhaust gas temperature sensor 17 for outputting an electrical signal corresponding to the above.

  Further, there is provided a reducing agent supply means for supplying (adding) fuel (light oil) as a reducing agent into the exhaust gas flowing through the exhaust passage 14 upstream from the NOx catalyst 15. As shown in FIG. 1, this reducing agent supply means is attached to the cylinder head of the internal combustion engine 1 so that its nozzle hole faces the exhaust passage 14, and opens when fuel of a predetermined valve opening pressure or higher is applied. There are provided a reducing agent addition valve 18 that injects fuel by valve and a reducing agent supply passage 19 that guides the fuel discharged from the fuel pump 6 to the reducing agent addition valve 18.

  In the reducing agent supply means configured as described above, the reducing agent added from the reducing agent addition valve 18 into the exhaust passage 14 flows into the turbine housing 10 b together with the exhaust gas flowing from the upstream of the exhaust passage 14. The exhaust gas and the reducing agent flowing into the turbine housing 10b are agitated by the rotation of the turbine wheel to form a homogeneously mixed exhaust gas.

  The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 20 for controlling the internal combustion engine 1. The ECU 20 is an arithmetic logic operation circuit including a CPU, a ROM, a RAM, a backup RAM, and the like.

  The ECU 20 includes various sensors such as a crank position sensor (not shown) and a water temperature sensor (not shown) attached to the internal combustion engine 1 in addition to the air flow meter 9, the air-fuel ratio sensor 16, and the exhaust temperature sensor 17 described above. And the output signals of the various sensors described above are input to the ECU 20.

  On the other hand, the fuel injection valve 3, the intake throttle actuator 13, the reducing agent addition valve 18 and the like are connected to the ECU 20 via electric wiring, and the ECU 20 is connected to the fuel injection valve 3, the intake throttle actuator 13 and the reducing agent addition valve. 18 etc. can be controlled.

Next, the NOx storage reduction catalyst 15 according to the present embodiment will be described.
When the air-fuel ratio of the exhaust gas flowing into the catalyst is a lean air-fuel ratio, the NOx catalyst 15 retains (occludes) NOx in the exhaust gas so as not to be released into the atmosphere, and the exhaust gas flowing into the catalyst When the air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio (below the stoichiometric air-fuel ratio), the retained NOx is released and reduced.

  Further, since the NOx catalyst 15 holds SOx in the exhaust gas by the same mechanism as NOx, when the amount of SOx held increases, the NOx holding capacity of the NOx catalyst 15 decreases accordingly, so-called SOx poisoning occurs. appear.

  When SOx poisoning occurs in the NOx catalyst 15 in this manner, the NOx retention capacity is saturated, and NOx in the exhaust gas is released into the atmosphere without being purified by the NOx catalyst 15. Therefore, in the present embodiment, the SOx poisoning elimination control for releasing and reducing SOx held in the NOx catalyst 15 is executed.

  In this SOx poisoning elimination control, the ECU 20 first executes a catalyst temperature raising process for raising the bed temperature of the NOx catalyst 15 to about 600 ° C., and then sets the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 to be less than the stoichiometric air-fuel ratio. And so on.

  Specifically, in the catalyst temperature raising process of the present embodiment, as a means for increasing the temperature of the NOx catalyst 15 at an early stage, in addition to normal main fuel injection near the top dead center of the compression stroke of the internal combustion engine 1, exhaust gas It is effective to perform sub-injection such as post-injection, which injects fuel into the cylinder during the stroke or expansion stroke, or bi-rubber injection, which injects fuel into the cylinder near the top dead center of the intake or exhaust stroke. It is. In post-injection, fuel injected during the exhaust stroke or expansion stroke flows into the NOx catalyst as unburned fuel, and the temperature of the catalyst rises due to heat of reaction with the catalyst. On the other hand, in the rubber injection, the fuel injected near the top dead center of the intake stroke or the exhaust stroke evaporates in the subsequent stroke and easily ignites and stabilizes the combustion. The amount of energy consumed for the piston motion is reduced, and the exhaust gas whose temperature has increased accordingly reaches the NOx catalyst 15, whereby the temperature of the catalyst rises. Further, the unburned portion of the injected fuel is supplied to the NOx catalyst 15, which causes an oxidation reaction on the catalyst, thereby increasing the temperature of the catalyst.

  Further, by adding fuel as a reducing agent from the reducing agent addition valve 18 into the exhaust gas instead of the above-mentioned auxiliary injection or together with the auxiliary injection, these unburned fuel components are oxidized in the NOx catalyst 15 and oxidized. You may make it raise the bed temperature of a NOx catalyst with the heat | fever generate | occur | produced in the case.

  When the bed temperature of the NOx catalyst 15 rises to about 600 ° C. by the catalyst temperature raising process as described above, the ECU 20 adds the reducing agent so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 is less than the stoichiometric air-fuel ratio. Reducing agent addition control for adding fuel as a reducing agent from the valve 18 is executed.

  However, the reducing agent added to the exhaust gas has a function of further reducing the temperature of the NOx catalyst 15 while having a function of reducing SOx held in the NOx catalyst 15 under a high temperature condition. For this reason, if the reducing agent is continuously added from the state where the temperature of the NOx catalyst 15 is about 600 ° C., the temperature of the NOx catalyst 15 continues to rise, and the thermal deterioration due to the temperature of the NOx catalyst 15 becoming excessively high. Will be caused.

  Therefore, when executing the reducing agent addition control of the present embodiment, the reducing agent is intermittently added through the reducing agent addition valve 18 so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 is equal to or lower than the stoichiometric air-fuel ratio. By stopping the addition of the reducing agent at a predetermined timing and repeating this, the NOx catalyst 15 is prevented from overheating while releasing and reducing the SOx held in the NOx catalyst 15.

  Specifically, when there is a request that the SOx retained and accumulated in the NOx catalyst 15 should be released and the condition that the bed temperature of the NOx catalyst 15 is maintained at 600 ° C. or higher is satisfied, The ECU 20 intermittently outputs a command signal for opening the reducing agent addition valve 18 (hereinafter referred to as “valve opening command signal”) for a predetermined period (hereinafter referred to as “reducing agent addition period”). Thus, fuel as a reducing agent is intermittently added from the reducing agent addition valve 18 so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 becomes equal to or lower than the stoichiometric air-fuel ratio (see FIG. 2). Thereafter, the ECU 20 pauses the output of the valve opening command signal for a predetermined period (hereinafter referred to as “addition suspension period”) so as to suppress overheating of the NOx catalyst 15 so as to obtain a predetermined lean air-fuel ratio ( (See FIG. 2). Thereafter, after a suspension period of addition, the addition of the reducing agent is resumed so that it again becomes the stoichiometric air-fuel ratio or less (see FIG. 2). Then, after the reducing agent is added again during the reducing agent addition period, the reducing agent addition is stopped during the addition suspension period so that the predetermined lean air-fuel ratio is obtained again (see FIG. 2). Thus, when the reducing agent addition control is started, basically, the NOx catalyst 15 is intermittently supplied until the SOx retained in the NOx catalyst 15 is released and the function of the NOx catalyst 15 sufficiently recovers. The addition and pause of the reducing agent are repeated so that the air-fuel ratio of the inflowing exhaust gas is less than the stoichiometric air-fuel ratio. In this way, as reducing agent addition control, intermittently adding a reducing agent and making the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 intermittently lower than the stoichiometric air-fuel ratio is referred to as intermittent rich control. FIG. 2 illustrates the case where the valve opening command signal is intermittently output a plurality of times within the reducing agent addition period. However, the present invention is not particularly limited to this case, and the reducing agent addition valve 18 is not particularly limited. May be opened once and the reducing agent may be added during the reducing agent addition period.

  In the present embodiment, in order to make the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 equal to or lower than the stoichiometric air-fuel ratio, the reductant is added in the reductant addition valve 18 in synchronism with the empty air in the cylinder. The fuel ratio (hereinafter referred to as “in-cylinder air / fuel ratio”) is made to be slightly lean (for example, A / F = 18 to 23), which is an air / fuel ratio richer than the air / fuel ratio (A / F = 25 to 30) during normal operation. To do.

  As a technique for making the in-cylinder air-fuel ratio weakly lean, it is possible to exemplify reducing the amount of air sucked into the cylinder by reducing the opening of the intake throttle valve 12. Further, when the exhaust passage 14 downstream of the NOx catalyst 15 is provided with an exhaust throttle valve that adjusts the flow rate of exhaust gas discharged to the atmosphere, the air sucked into the cylinder with the opening of the exhaust throttle valve being reduced. The in-cylinder air-fuel ratio may be made slightly lean. Furthermore, the opening degree of the intake throttle valve 12 may be reduced and the opening degree of the exhaust throttle valve may be reduced. In the present embodiment, the in-cylinder air-fuel ratio is reduced to a weak lean according to the in-cylinder air-fuel ratio lowering control described below.

[First embodiment of in-cylinder air-fuel ratio lowering control]
The fuel as the reducing agent added by the reducing agent addition valve 18 is added to the exhaust passage 14 in a liquid state, so that it is attached to the exhaust passage 14 as it is or partly vaporized by the heat of the exhaust gas. However, the fuel that is not sufficiently vaporized has a large particle size and a large mass, so that it may be difficult to reach the catalyst along with the flow of the exhaust gas. Therefore, if the in-cylinder air-fuel ratio is made lean simultaneously with the addition of the reducing agent by the reducing agent addition valve 18, the reducing agent added by the reducing agent addition valve 18 arrives at the NOx catalyst 15 with a delay. In some cases, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 does not become a target air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio at which SOx can be released / reduced, even though the air-fuel ratio is weakly lean.

  Specifically, FIG. 3 shows the added reducing agent adhesion rate, which is the ratio of the added reducing agent that adheres to the exhaust passage 14 in a liquid state. As shown in FIG. 3, the additive reducing agent adhesion rate is correlated with the exhaust gas flow rate (flow velocity) and the exhaust gas temperature, and as the exhaust gas flow rate decreases, the ratio of the reducing agent adhering to the exhaust passage increases. As the exhaust gas temperature decreases, the ratio of the reducing agent that adheres to the exhaust passage 14 increases.

  On the other hand, the reducing agent adhering to the exhaust passage 14 in the liquid state as described above gradually evaporates over time. Specifically, FIG. 4 shows the attached reducing agent evaporation rate, which is the ratio of the reducing agent that evaporates within a predetermined time (32 ms) of the attached reducing agent. As shown in FIG. 4, the attached reducing agent evaporation rate has a correlation with the exhaust gas flow rate (flow velocity) and the exhaust gas temperature, and as the exhaust gas flow rate increases, the ratio of the reducing agent that evaporates increases. The ratio of the reducing agent that evaporates increases as the value increases.

Thus, since a part of the reducing agent added by the reducing agent addition valve 18 adheres to the exhaust passage 14 in a liquid state, the reducing agent that reaches the NOx catalyst 15 at the beginning of the reducing agent addition period is: This is the amount obtained by subtracting the amount of reducing agent adhering to the exhaust passage 14 from the amount of reducing agent added. Since a part of the reducing agent attached to the exhaust passage 14 evaporates with time and reaches the NOx catalyst 15, after the start of evaporation, the reducing agent attached to the exhaust passage 14 from the amount of the added reducing agent. An amount obtained by adding an amount obtained by subtracting the amount to which a part of the reducing agent that has adhered so far has evaporated is the amount of reducing agent that reaches the NOx catalyst 15. Therefore, considering the time taken by the exhaust gas from the position where the added reducing agent is added to the NOx catalyst 15, as shown in FIG. 2B, the NOx catalyst 15 is delayed by ΔT1 from the start of the reducing agent addition period. The amount of reducing agent necessary to bring the air-fuel ratio of the exhaust gas flowing into the target air-fuel ratio below the stoichiometric air-fuel ratio at which SOx can be released and reduced reaches the NOx catalyst 15. The amount of necessary reducing agent is calculated based on the following calculation formula (1).
Necessary reducing agent amount = (the amount of air sucked into the cylinder when the in-cylinder air-fuel ratio is weakly lean) / (target air-fuel ratio of exhaust gas flowing into the NOx catalyst 15) −the in-cylinder air-fuel ratio is weakly lean Amount of fuel injected into the cylinder in some cases (1)

  Therefore, in the present embodiment, when the amount of reducing agent necessary to bring the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 to the target air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio reaches the NOx catalyst 15, Reduce the in-cylinder air-fuel ratio to make it lean. That is, as shown in FIG. 2 (c), the in-cylinder air-fuel ratio is switched to weak lean after a delay of ΔT1 from the start of the reducing agent addition period. Thereby, it is possible to prevent the fuel used while reducing the in-cylinder air-fuel ratio from being wasted. As a result, the SOx retained in the NOx catalyst 15 can be reduced and removed with good fuel efficiency.

  On the other hand, as described above, the reducing agent adhering to the exhaust passage 14 in a liquid state gradually evaporates over time, and is transported from the position where the added reducing agent is added by the exhaust gas to the NOx catalyst 15. Since it takes a corresponding time, the reducing agent reaches the NOx catalyst 15 for a while even after the execution of the reducing agent addition is completed. Therefore, as shown in FIG. 2 (b), during the period ΔT2 from the end of the reducing agent addition period, the amount of reducing agent necessary to bring the air-fuel ratio of the exhaust gas to the target air-fuel ratio at which SOx can be released and reduced is increased. The NOx catalyst 15 is reached.

  Therefore, in the present embodiment, the in-cylinder air-fuel ratio remains weakly lean for ΔT2 even after the reducing agent addition by the reducing agent addition valve 18 is finished (even after the reducing agent addition period is finished). . Thereby, it is possible to prevent the added reducing agent from being wasted and fuel consumption from being deteriorated.

  Hereinafter, the control routine of in-cylinder air-fuel ratio lowering control according to the present embodiment will be described with reference to the flowchart of FIG.

  This control routine is a routine stored in the ROM of the ECU 20 in advance, and is triggered by the passage of a fixed time during execution of the reducing agent addition control (intermittent rich control) or the input of a pulse signal from the crank position sensor. This routine is executed by the ECU 20 as the interrupt process.

  In this control routine, the ECU 20 first determines whether or not both the fuel injection valve 3 and the air flow meter 9 are normal in step (hereinafter may be simply referred to as “S”) 101. This is determined because it is difficult to control the in-cylinder air-fuel ratio when the fuel injection valve 3 and the air flow meter 9 are abnormal. The command value for the amount of fuel to be injected by the fuel injection valve 3 is determined. The determination can be made based on the detection value of the air flow meter 9 or the detection value of the air-fuel ratio sensor 16. If an affirmative determination is made, the process proceeds to S102, and if a negative determination is made, execution of this routine is terminated.

  In S102, the amount of catalyst reaching reducing agent is calculated from the flow rate of exhaust gas flowing into the NOx catalyst 15, the temperature of the exhaust gas, and the like. This is because of the relationship between the exhaust gas flow rate, the exhaust gas temperature, and the additive reducing agent deposition rate as shown in FIG. 3, the relationship between the exhaust gas flow rate, the exhaust gas temperature, and the deposition reducing agent evaporation rate as shown in FIG. Based on the map, the relationship between the exhaust gas flow rate, the temperature of the exhaust gas, and the amount of the catalyst reducing agent is mapped in advance by experiments and stored in the ROM, and is calculated based on the map, the exhaust gas flow rate, and the exhaust gas temperature. Is. The exhaust gas flow rate is estimated based on the detection value of the air flow meter 9. Further, the exhaust gas temperature may be estimated based on the detected value of the exhaust temperature sensor 17, or a sensor that outputs an electrical signal corresponding to the exhaust gas temperature is provided in the exhaust passage 14 upstream of the NOx catalyst 15. May be detected. Alternatively, it may be estimated from a map of engine load and engine speed.

  Thereafter, the process proceeds to S103, and in this step, the amount of catalyst reaching reducing agent calculated in S102 is an amount necessary to bring the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 15 to a target air-fuel ratio equal to or lower than the theoretical air-fuel ratio. It is determined whether or not the amount is greater than a certain necessary reducing agent amount. The amount of necessary reducing agent is calculated by the above-described calculation formula (1). If a positive determination is made, the process proceeds to S104, and if a negative determination is made, the process proceeds to S105.

  In S104, the in-cylinder air-fuel ratio is set to the target weak lean state because the amount of reducing agent sufficient to achieve the target air-fuel ratio at which SOx of the NOx catalyst 15 can be released and reduced has reached the NOx catalyst 15. . As described above, this is to output a command signal to the intake throttle actuator 13 so that the opening degree of the intake throttle valve 12 is relatively smaller than the opening degree during normal operation.

  On the other hand, in S105, the in-cylinder air-fuel ratio is not made weakly lean because the amount of reducing agent necessary to achieve the target air-fuel ratio at which SOx of the NOx catalyst 15 can be released and reduced has not reached the NOx catalyst 15. Like that. In other words, as long as the amount of reducing agent necessary to reach the target air-fuel ratio at which the SOx of the NOx catalyst 15 can be released and reduced even when the reducing agent addition period starts, the in-cylinder emptying is not achieved. The fuel ratio is kept during normal operation. On the other hand, after the reducing agent addition period ends, the amount of reducing agent necessary to achieve the target air-fuel ratio at which the SOx of the NOx catalyst 15 can be released and reduced does not reach the NOx catalyst 15, and is switched to weak lean. Release the status. That is, a command signal is output to the intake throttle actuator 13 to adjust the opening of the intake throttle valve 12 so as to be relatively larger than the opening of the intake throttle valve 12 in S104.

  By doing so, even if the reducing agent addition valve 18 starts to add the reducing agent, the amount of reducing agent necessary for achieving the target air-fuel ratio at which the SOx of the NOx catalyst 15 can be released and reduced is still in the NOx catalyst 15. Since the in-cylinder air-fuel ratio is not weakly lean while it has not reached (between ΔT1 from the start of the reducing agent addition period), the fuel used while the in-cylinder air-fuel ratio is lowered is wasted. Can be prevented.

On the other hand, even when the addition of the reducing agent by the reducing agent addition valve 18 is completed, the amount of reducing agent necessary to achieve the target air-fuel ratio at which SOx of the NOx catalyst 15 can be reduced and released has reached the NOx catalyst 15. Since the in-cylinder air-fuel ratio remains weak during the interval (between the end of the reducing agent addition period and ΔT2), it is possible to suppress the added reducing agent from being wasted and deteriorating fuel consumption.

  In the above-described in-cylinder air-fuel ratio lowering control, the amount of the catalyst reducing agent that is the amount of reducing agent that reaches the NOx catalyst 15 is calculated, and the amount of the catalyst reaching reducing agent is equal to or greater than the above-described necessary reducing agent amount. However, the present invention is not particularly limited to this, and the time when the required reducing agent amount starts to reach the NOx catalyst 15 is calculated, and the time has elapsed. Sometimes the in-cylinder air-fuel ratio is switched from the air-fuel ratio during normal operation to weak lean, and the time during which the required reducing agent amount does not reach the NOx catalyst 15 is calculated. You may make it switch to the air fuel ratio at the time of normal driving | operation.

[Second embodiment of in-cylinder air-fuel ratio lowering control]
In general, until exhaust gas resulting from combustion at a desired in-cylinder air-fuel ratio reaches the NOx catalyst 15, a command signal for controlling the fuel injection valve 3 and each actuator is output from the ECU 20 and the desired cylinder is output. After the internal air-fuel ratio is set, combustion and exhaust are actually performed and it is necessary to pass through the exhaust system such as the exhaust passage 14 and the supercharger 10, and accordingly, a corresponding time is required. Therefore, the in-cylinder air-fuel ratio is made slightly lean, and an exhaust gas arrival time, which is a corresponding time, is required until the exhaust gas resulting from combustion at this air-fuel ratio reaches the NOx catalyst 15.

  Therefore, as in the first embodiment, the air / fuel ratio of the exhaust gas flowing into the NOx catalyst 15 is set to the target air / fuel ratio in consideration of the amount of adhesion and evaporation of the reducing agent added by the reducing agent addition valve 18. When the in-cylinder air-fuel ratio is switched from the air-fuel ratio in normal operation to weak lean simultaneously with the time when the amount of reducing agent necessary to do so reaches the NOx catalyst 15, a delay corresponding to the exhaust gas arrival time described above occurs. Even if the reducing agent added by the reducing agent addition valve 18 arrives within the exhaust gas arrival time, it may be wasted. FIG. 6 shows the correlation between the exhaust gas arrival time and the exhaust gas flow rate. As shown in FIG. 6, the exhaust gas arrival time becomes shorter as the exhaust gas flow rate increases. It becomes longer as it decreases.

  Therefore, in this embodiment, the amount of reducing agent necessary to achieve the target air-fuel ratio that can release and reduce the SOx of the NOx catalyst 15 reaches the NOx catalyst 15, and at the same time, the weak lean in-cylinder air-fuel ratio. In consideration of the exhaust gas arrival time, the in-cylinder air-fuel ratio is switched from the air-fuel ratio during normal operation to weak lean so that the exhaust gas due to combustion at NOx reaches the NOx catalyst 15. That is, the in-cylinder air-fuel ratio is switched to weak lean earlier by the exhaust gas arrival time than ΔT1 elapses from the start of the reducing agent addition period in FIG. Thereby, it is possible to prevent the reducing agent added within the exhaust gas arrival time from being wasted and deteriorating fuel consumption.

  On the other hand, when the addition of the reducing agent is terminated, the exhaust gas flowing into the NOx catalyst 15 is taken into consideration, as in the first embodiment, in consideration of the amount of adhesion and evaporation of the reducing agent added by the reducing agent addition valve 18. The in-cylinder air-fuel ratio is switched from weak lean to normal air-fuel ratio at the same time when the amount of reducing agent required to make the gas air-fuel ratio the target air-fuel ratio does not reach the NOx catalyst 15 (releasing weak lean) In this case, a delay corresponding to the exhaust gas arrival time described above occurs, so that the fuel used while the in-cylinder air-fuel ratio is reduced within the exhaust gas arrival time may be wasted.

Therefore, in this embodiment, the amount of reducing agent necessary for achieving the target air-fuel ratio at which SOx of the NOx catalyst 15 can be released / reduced does not reach the NOx catalyst 15, and at the same time, the weak lean in-cylinder air In consideration of the exhaust gas arrival time, the in-cylinder air-fuel ratio is switched from weak lean to the air-fuel ratio during normal operation so that the exhaust gas due to combustion at the fuel ratio does not reach the NOx catalyst 15 (releasing the weak lean). ) That is, the in-cylinder air-fuel ratio is switched to a weak lean earlier by the exhaust gas arrival time than ΔT2 elapses from the end of the reducing agent addition period in FIG. Thereby, it is possible to prevent the fuel used while reducing the in-cylinder air-fuel ratio from being wasted.

  Hereinafter, the control routine of the cylinder air-fuel ratio lowering control according to the present embodiment will be described with reference to the flowchart of FIG.

  This control routine is a routine stored in the ROM of the ECU 20 in advance, and the ECU 20 serves as an interrupt process triggered by elapse of a fixed time during execution of the reducing agent addition control or input of a pulse signal from the crank position sensor. Is a routine to be executed.

  Since the processing of S201 and S202 in this control routine is the same as the processing of S101 and S102 in FIG. 5, detailed description thereof will be omitted.

  In S203, the amount of reducing agent that reaches the NOx catalyst 15 during the exhaust gas arrival time is calculated. This is because of the relationship between the exhaust gas flow rate, the exhaust gas temperature, and the additive reducing agent deposition rate as shown in FIG. 3, the relationship between the exhaust gas flow rate, the exhaust gas temperature, and the deposition reducing agent evaporation rate, as shown in FIG. Based on the relationship between the exhaust gas flow rate and the exhaust gas arrival time as shown in FIG. 6, the relationship between the exhaust gas flow rate, the temperature of the exhaust gas, and the amount of catalyst reaching reducing agent within the exhaust gas arrival time is previously mapped by experiments or the like And is stored in the ROM and calculated based on the map, the exhaust gas flow rate, and the exhaust gas temperature.

  Thereafter, the process proceeds to S204, and the catalyst reaching reducing agent amount is calculated by excluding the catalyst reaching reducing agent amount within the exhaust gas arrival time. This subtracts the reducing agent amount calculated in S203 from the reducing agent amount calculated in S202.

  Thereafter, the process proceeds to S205, in which it is determined whether or not the amount of catalyst reaching reducing agent excluding the amount of catalyst reaching reducing agent within the exhaust gas arrival time calculated in S204 is equal to or greater than the necessary amount of reducing agent. This is basically the same as the process of S103 in FIG. 5 and will not be described in detail. However, the required reducing agent amount in this embodiment is the required reduction calculated by the above formula (1). This is calculated by subtracting the catalyst reaching reducing agent amount within the exhaust gas arrival time calculated in S203 from the agent amount.

S206, a detailed description thereof is omitted since processing in S20 7 is the same as the processing in S104, S105 in each Figure 5.

  In this way, even if the reducing agent addition valve 18 starts to add the reducing agent, the amount of reducing agent necessary to achieve the target air-fuel ratio that can release and reduce the SOx of the NOx catalyst 15 is supplied to the NOx catalyst 15. At the same time, the exhaust gas resulting from the combustion at the weak lean in-cylinder air / fuel ratio reaches the NOx catalyst 15, so that it is more certain that the fuel consumption will deteriorate due to the weak lean air / fuel ratio in the cylinder. Can be suppressed.

  On the other hand, even when the addition of the reducing agent by the reducing agent addition valve 18 is completed, the amount of reducing agent necessary to achieve the target air-fuel ratio at which SOx of the NOx catalyst 15 can be released and reduced does not reach the NOx catalyst 15. At the same time, the exhaust gas generated by the combustion at the weak lean in-cylinder air / fuel ratio does not reach the NOx catalyst 15, so that it is possible to more reliably suppress the added reducing agent from being wasted and deteriorating the fuel consumption.

In the above-described in-cylinder air-fuel ratio lowering control, the catalyst reaching reducing agent amount is calculated by subtracting the reducing agent amount reaching within the exhaust gas arrival time, and the catalyst reaching reducing agent amount is equal to or greater than the necessary reducing agent amount. In this case, the in-cylinder air-fuel ratio is reduced to a weak lean state. However, the present invention is not particularly limited to this, and the reducing agent necessary for achieving the target air-fuel ratio that can release and reduce the SOx of the NOx catalyst 15 is not limited thereto. The time when the amount starts to reach is calculated, the exhaust gas arrival time is subtracted from the time, and when the subtracted time has elapsed, the in-cylinder air-fuel ratio is switched from the air-fuel ratio during normal operation to weak lean, and the NOx catalyst 15 Calculate the time when the amount of reducing agent required to reach the target air-fuel ratio at which SOx can be released / reduced does not reach, subtract the exhaust gas arrival time from that time, and when the subtracted time has elapsed, Reduce the fuel ratio It may be switched to the air-fuel ratio during normal operation from the emissions.

1 is a diagram showing a schematic configuration of an internal combustion engine and an intake / exhaust system thereof according to an embodiment of the present invention. It is the figure which showed the transition of a reducing agent addition pulse, a catalyst arrival reducing agent, a cylinder air fuel ratio, and an exhaust air fuel ratio in time series. It is a figure which shows the relationship between an exhaust-gas flow volume, exhaust-gas temperature, and an addition reducing agent adhesion rate. It is a figure which shows the relationship between exhaust-gas flow volume, exhaust-gas temperature, and an adhesion reducing agent evaporation rate. It is a flowchart figure which shows the control routine of the 1st Example of in-cylinder air fuel ratio fall control. It is a figure which shows the relationship between exhaust gas flow volume and exhaust gas arrival time. It is a flowchart figure which shows the control routine of the 2nd Example of in-cylinder air fuel ratio fall control.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Cylinder 3 Fuel injection valve 4 Common rail 5 Fuel supply pipe 6 Fuel pump 7 Intake passage 8 Air cleaner 9 Air flow meter 10 Supercharger 11 Intercooler 12 Intake throttle valve 13 Intake throttle actuator 14 Exhaust passage 15 Exhaust purification device ( NOx catalyst)
16 Air-fuel ratio sensor 17 Exhaust temperature sensor 18 Reducing agent addition valve 19 Reducing agent supply path 20 ECU

Claims (5)

A NOx occlusion reduction catalyst provided in the exhaust passage;
Reducing agent supply means provided in the exhaust passage upstream of the NOx storage reduction catalyst;
Air-fuel ratio control means for controlling the air-fuel ratio in the cylinder;
Have
The air-fuel ratio control means lowers the air-fuel ratio in the cylinder as compared with the air-fuel ratio in normal operation , lowers the air-fuel ratio of the exhaust gas discharged from the cylinder, and the reducing agent supply means supplies the reducing agent. In the exhaust gas purification apparatus for an internal combustion engine, the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is made lower than the stoichiometric air-fuel ratio to release and reduce the sulfur oxide held in the NOx storage-reduction catalyst.
Among the reducing agents supplied from the reducing agent supply means, there is a catalyst reaching reducing agent amount calculating means for calculating the amount of reducing agent that has actually reached the NOx storage reduction type catalyst,
The exhaust gas purification apparatus for an internal combustion engine, wherein the air-fuel ratio control means lowers the air-fuel ratio in the cylinder based on the reducing agent amount calculated by the catalyst reaching reducing agent amount calculating means. When the reducing agent amount calculated by the catalyst reaching reducing agent amount calculating means is equal to or greater than the reducing agent amount necessary for releasing and reducing the sulfur oxide held in the NOx storage reduction catalyst, the air-fuel ratio control means, exhaust gas control apparatus according to claim 1, characterized that you keep the air-fuel ratio in the cylinder in a state of being lower than the air-fuel ratio at the time of normal operation. The air-fuel ratio control means is a time until the exhaust gas resulting from combustion at the air-fuel ratio in the cylinder , which has been lowered by the air-fuel ratio control means compared to the air-fuel ratio during normal operation , reaches the NOx storage reduction catalyst. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio in the cylinder is lowered based on the exhaust gas arrival time. The reducing agent amount calculated by the catalyst reaching reducing agent amount calculating means is within the exhaust gas arrival time than the reducing agent amount required for releasing and reducing the sulfur oxide held in the NOx occlusion reduction type catalyst. If it is more partial small amount of reducing agent amount that reaches the said air-fuel ratio control means includes a feature that you keep the air-fuel ratio in the cylinder in a state of being lower than the air-fuel ratio during normal operation The exhaust emission control device for an internal combustion engine according to claim 3. The exhaust purification of an internal combustion engine according to any one of claims 1 to 4, wherein the air-fuel ratio control means controls the air-fuel ratio in the cylinder by limiting at least the amount of air taken into the cylinder. apparatus.

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