JP2008133744A - Exhaust system for internal combustion engine - Google Patents

Abstract

An exhaust system for an internal combustion engine capable of suppressing a decrease in the purification rate of NOx while suppressing fluctuations in the output torque of the internal combustion engine due to a rich spike.
An exhaust system of an internal combustion engine according to the present invention is applied to an internal combustion engine 1 provided with an exhaust gas recirculation device 9 that extracts a part of exhaust gas from an exhaust passage 4 as EGR gas and introduces the exhaust gas into an intake passage 3. A filter 17 that reduces the oxidized nitrogen in the presence of fuel, and a fuel addition valve 18 that is disposed upstream of the EGR gas take-out position by the exhaust gas recirculation device 9 and can inject fuel toward the filter 17. , And temporarily changing the exhaust air-fuel ratio to the rich side to inject fuel from the fuel adding means so that the exhaust air-fuel ratio becomes the target air-fuel ratio, and when the EGR rate is higher than when it is lower Also set the target air-fuel ratio to the lean side.
[Selection] Figure 1

Description

  The present invention relates to an exhaust system applied to an internal combustion engine provided with an exhaust gas recirculation device that introduces a part of exhaust gas into an intake passage as EGR gas.

  2. Description of the Related Art Exhaust gas purification apparatuses having NOx catalysts that store nitrogen oxide (NOx) in exhaust gas and reduce and purify the stored NOx in the presence of fuel as a reducing agent are widely known. Also, an internal combustion engine provided with an exhaust gas recirculation device that takes out a part of exhaust gas as EGR gas from an exhaust passage and introduces it into the intake passage in order to lower the combustion temperature and reduce the amount of NOx emission is widely known. For example, a diesel equipped with a turbocharger provided with an exhaust gas purification device having a NOx catalyst downstream of the turbine of the turbocharger, and provided with an exhaust gas recirculation device that takes out the exhaust gas from the downstream of the exhaust gas purification device and introduces the exhaust gas upstream of the turbocharger compressor An engine is known (Patent Document 1). In addition, there is Patent Document 2 as a prior art document related to the present invention.

JP-A-2005-76456 JP-A-6-50134

  Since the exhaust gas of such a diesel engine has a high oxygen concentration and the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the NOx stored by the NOx catalyst cannot be reduced sufficiently. Therefore, the NOx occluded by the NOx catalyst is reduced by supplying fuel into the exhaust passage and executing a rich spike that temporarily sets the air-fuel ratio of the exhaust to the rich side. Since the EGR gas is taken out from the downstream side of the exhaust gas purification device in the device of Patent Document 1, if rich spike is executed during the introduction of the EGR gas into the intake passage, the exhaust gas set to the rich side becomes the intake passage as EGR gas. As a result, the output torque of the internal combustion engine fluctuates. In order to suppress such fluctuations in output torque, the amount of fuel supplied during the rich spike must be reduced to refrain from setting the air-fuel ratio to the rich side. As a result, the air-fuel ratio cannot be lowered sufficiently during a rich spike, and the NOx purification rate of the exhaust system may be reduced.

  Accordingly, an object of the present invention is to provide an exhaust system for an internal combustion engine that can suppress a decrease in the NOx purification rate while suppressing fluctuations in the output torque of the internal combustion engine due to a rich spike.

  An exhaust system for an internal combustion engine according to the present invention is an exhaust system for an internal combustion engine applied to an internal combustion engine provided with an exhaust gas recirculation device that takes out part of exhaust gas from an exhaust passage as EGR gas and introduces the exhaust gas into an intake passage. An exhaust purification means for reducing the stored nitrogen oxides at a rich air-fuel ratio, and an exhaust purification means arranged in the exhaust passage upstream of the EGR gas take-out position by the exhaust gas recirculation device. Fuel addition means capable of injecting fuel toward the fuel, and fuel addition for injecting fuel from the fuel addition means so as to temporarily change the air-fuel ratio of the exhaust to the rich side so that the air-fuel ratio of the exhaust becomes the target air-fuel ratio The target air-fuel ratio is set on the lean side when the EGR ratio as a ratio of the control means and the EGR gas to the intake gas of the internal combustion engine is high than when the EGR ratio is low That a target air-fuel ratio setting means, by providing, for solving the above problems (claim 1).

  According to the exhaust system of the present invention, the fuel as the reducing agent of the exhaust purification means is injected by the fuel addition means toward the exhaust purification means, so that the fuel is in a droplet state before being uniformly diffused into the exhaust. Is led to the exhaust gas purification means. Since the fuel in the droplet state remains in the exhaust purification unit and volatilizes, an atmosphere in which the air-fuel ratio is locally rich can be formed in the exhaust purification unit. As a result, NOx reduction of the exhaust purification unit can be performed. As described above, when the local rich atmosphere can be formed, the entire air-fuel ratio can be set to the lean side as compared with the case where the fuel is uniformly diffused into the exhaust gas. For this reason, even when part of the exhaust gas is introduced into the intake passage as EGR gas, the fluctuation of the air-fuel ratio can be reduced, and the fluctuation of the output torque can be suppressed. Moreover, in the exhaust system of the present invention, when the EGR rate is high, the target air-fuel ratio is set to be leaner than when the EGR rate is low. Therefore, while suppressing fluctuations in output torque due to an increase in the EGR rate, The necessary NOx purification rate can be ensured by the formation.

  In one aspect of the exhaust system of the present invention, the target air-fuel ratio setting means may set the target air-fuel ratio to the lean side when the intake gas temperature is low at the same EGR rate, rather than when the intake gas temperature is high. Item 2). As the intake gas temperature (intake air temperature) is higher, the output torque is less likely to fluctuate due to a decrease in the oxygen concentration of the intake gas. In other words, the lower the intake air temperature, the easier it is for the output torque to fluctuate with a decrease in the oxygen concentration of the intake gas. According to this aspect, when the intake air temperature is low at the same EGR rate, the target air-fuel ratio is set to be leaner than when the intake air temperature is high. Therefore, even if the EGR rate is the same, the target air-fuel ratio corresponding to the intake air temperature is Is set. As a result, it is possible to suppress fluctuations in the output torque caused by changes in the intake air temperature.

  In one aspect of the exhaust system of the present invention, the internal combustion engine has a turbine disposed in the exhaust passage and a compressor disposed in the intake passage and driven by the turbine, and uses exhaust energy. In addition, a turbocharger for supercharging may be provided, the exhaust purification unit may be disposed on the downstream side of the turbine, and the fuel addition unit may be disposed between the turbine and the exhaust purification unit ( Claim 3). As described above, in the aspect in which the internal combustion engine is provided with the turbocharger, even when the supercharging pressure by the turbocharger is low at the same EGR rate, the target air-fuel ratio is set to be leaner than when it is high. Good (Claim 4). The lower the supercharging pressure, the more likely the output torque fluctuates with a decrease in the oxygen concentration of the intake gas. According to this aspect, when the supercharging pressure is low at the same EGR rate, the target air-fuel ratio is set to be leaner than when the supercharging pressure is high. Therefore, even if the EGR rate is the same, the target air / fuel ratio corresponding to the supercharging pressure is set. The fuel ratio is set. As a result, it is possible to suppress fluctuations in the output torque caused by changes in the supercharging pressure.

  In one aspect of the exhaust system of the present invention, the fuel addition control means is configured so that the fuel atomization by the fuel addition means increases as the target air-fuel ratio is set to the rich side. The injection state may be controlled (claim 5). Since the air-fuel ratio of the entire exhaust gas becomes leaner as the target air-fuel ratio is set to the lean side, in order to reduce NOx by the exhaust purification means at such an air-fuel ratio, the formation of a local rich atmosphere is promoted. Therefore, it is necessary to suppress a decrease in the NOx purification rate. According to this aspect, the atomization of the fuel is increased as the target air-fuel ratio is set to the rich side. In other words, since the atomization decreases as the target air-fuel ratio is set to the lean side, the fuel is easily guided to the exhaust gas purification means in a droplet state, and the formation of a local rich atmosphere is promoted. Thereby, since the atomization of the fuel is adjusted in accordance with the change in the target air-fuel ratio, it is possible to suppress a decrease in the NOx purification rate due to the change in the target air-fuel ratio to the lean side.

  In one aspect of the exhaust system of the present invention, the fuel addition control means may be configured such that, in the injection period in which the fuel addition means injects fuel so as to temporarily change the air-fuel ratio of the exhaust to the rich side, The fuel injection state by the fuel addition means may be controlled such that the atomization of the fuel in the fuel is lower than in the later stage of the injection period. Since atomization in the initial stage of the injection period by the fuel addition means is lower than that in the latter period, the fuel is easily supplied to the exhaust purification means in a droplet state. The fuel in the droplet state is volatilized while staying in the exhaust purification unit, whereby an atmosphere in which the air-fuel ratio is locally rich is formed in the exhaust purification unit. Thereafter, since atomization increases in the later stage of the injection period, the exhaust gas purification unit is guided to the exhaust gas whose air-fuel ratio has decreased as a whole. Thereby, reduction of NOx desorbed from the exhaust gas purification means can be promoted by a local rich atmosphere. Therefore, NOx desorbed from the exhaust purification unit due to the formation of a local rich atmosphere can be suppressed from being discharged without being reduced, and the NOx purification rate is improved.

  In one aspect of the exhaust system of the present invention, the internal combustion engine includes, as the exhaust gas recirculation device, an EGR gas extraction position located downstream of the turbine of the turbocharger and an EGR gas introduction position defined as the turbocharger. A first exhaust gas recirculation device located upstream of the compressor, an EGR gas take-out position is located upstream of the turbine of the turbocharger, and an EGR gas introduction position is located downstream of the compressor of the turbocharger. A second exhaust gas recirculation device is provided, and is disposed downstream of the EGR gas take-out position by the second exhaust gas recirculation device and upstream of the fuel addition means, and is disposed in the exhaust passage. And further comprising an upstream side fuel addition means capable of injecting fuel, wherein the fuel addition control means comprises the first exhaust return. In a state where the introduction of the EGR gas by the device and the introduction of the EGR gas by the second exhaust gas recirculation device are respectively performed, the air-fuel ratio of the exhaust gas is temporarily changed to the rich side so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio. And the ratio of the fuel injected by the fuel adding means to the total fuel to be injected by each of the upstream fuel adding means and the fuel adding means becomes higher as the target air-fuel ratio is set to the lean side. The fuel is injected from each of the upstream fuel addition means and the fuel addition means, and the target air-fuel ratio setting means is configured such that the EGR gas introduced by the first exhaust gas recirculation device with respect to the intake gas of the internal combustion engine The target air-fuel ratio may be set on the lean side when the EGR rate as a ratio is high than when it is low (Claim 7). Since the upstream fuel addition means is disposed downstream of the EGR gas take-out position of the second exhaust gas recirculation device, the injected fuel is not introduced into the intake passage through the second exhaust gas recirculation device. Therefore, regarding the setting of the target air-fuel ratio, it is not necessary to consider the EGR rate related to the second exhaust gas recirculation device regardless of whether the EGR gas is introduced by the second exhaust gas recirculation device. Therefore, the target air-fuel ratio in this aspect is set to the lean side when the EGR rate related to the first exhaust gas recirculation device is high, rather than when it is low. In a state where the introduction of the EGR gas by the first exhaust gas recirculation device and the introduction of the EGR gas by the second exhaust gas recirculation device are respectively performed, the upstream fuel addition means and the fuel addition means respectively correspond to the total fuel to be injected. The ratio of the fuel injected by the fuel addition means increases as the target air-fuel ratio is set to the lean side. Thereby, formation of a local rich atmosphere can be promoted while suppressing fluctuations in output torque due to an increase in the EGR rate by the first exhaust gas recirculation device, so that a necessary NOx purification rate can be ensured.

  In the present invention, the rich side and the lean side mean the change direction of the air-fuel ratio, and do not mean an absolute air-fuel ratio that is leaner or richer than the stoichiometric air-fuel ratio. Therefore, the target air-fuel ratio according to the present invention is not always richer than the stoichiometric air-fuel ratio.

  As described above, according to the present invention, the fuel as the reducing agent of the exhaust gas purification unit is injected by the fuel addition unit toward the exhaust gas purification unit, so that the atmosphere in which the air-fuel ratio is locally rich is purified. Can be formed in the means. When the local rich atmosphere can be formed, the entire air-fuel ratio can be set to the lean side rather than the case where the fuel is diffused uniformly in the exhaust, so that a part of the exhaust is introduced into the intake passage as EGR gas However, since fluctuations in the air-fuel ratio are small, fluctuations in output torque can be suppressed. In addition, when the EGR rate is high, the target air-fuel ratio is set to be leaner than when the EGR rate is low. Therefore, while suppressing fluctuations in the output torque due to the increase in the EGR rate, the necessary NOx is reduced by forming a local rich atmosphere. A purification rate can be secured.

(First form)
FIG. 1 shows a main part of an internal combustion engine to which an exhaust system of the present invention is applied. The internal combustion engine 1 is mounted on a vehicle (not shown) as a driving power source, and is configured as an in-line four-cylinder diesel engine in which four cylinders 2 are arranged in a row. An intake passage 3 and an exhaust passage 4 are connected to each cylinder 2. The intake passage 3 is provided with a throttle valve 5, a compressor 6a of a turbocharger 6, and an intercooler 7 for cooling the intake air. In the exhaust passage 4, a turbine 6 b of the turbocharger 6 and an exhaust purification device 8 are provided. The internal combustion engine 1 is also provided with an exhaust gas recirculation device 9 that extracts a part of the exhaust gas from the exhaust passage 4 as EGR gas and introduces it into the intake passage 3. The exhaust gas recirculation device 9 adjusts the amount of EGR gas introduced into the intake passage 3, an EGR passage 10 that connects the exhaust passage 4 and the intake passage 3, an EGR cooler 11 that cools the EGR gas that passes through the EGR passage 10. EGR valve 12 is provided. The internal combustion engine 1 includes a fuel injection valve 13 provided in each cylinder 2 and a common rail 14 that stores high-pressure fuel to be supplied to each fuel injection valve 13. The common rail 14 includes a fuel pump (not shown). The fuel is supplied in a high pressure state.

  The exhaust purification device 8 includes a casing 15 that forms part of the exhaust passage 4, and an oxidation catalyst 16 and a filter 17 that are accommodated in the casing 15. The filter 17 has a base material (not shown) capable of collecting particulates in the exhaust gas, and the base material is coated with an NOx catalyst material of occlusion reduction type. As a result, the filter 17 has a function of collecting particulates, and also functions as an exhaust purification unit according to the present invention that reduces the stored NOx in the presence of fuel, in other words, reduces the rich air-fuel ratio. Instead of the filter 17, a NOx catalyst that does not have a particulate collection function may be provided as an exhaust purification means. In order to reduce the NOx occluded by the filter 17, the internal combustion engine 1 is provided with a fuel addition valve 18 that can inject fuel toward the filter 17. A fuel pump 19 is connected to the fuel addition valve 18, and fuel of a predetermined pressure is supplied by the fuel pump 19. Although the detailed internal structure of the fuel addition valve 18 is omitted, the fuel addition valve 18 is a known electromagnetically driven type that opens and closes the nozzle hole formed in the nozzle body by moving the valve body using an electromagnet. It is configured as an injector. The fuel addition valve 18 is configured to be able to arbitrarily set the lift amount of the valve body by adjusting the energization time (duty ratio) to the electromagnet. The fuel pump 19 has an adjustment mechanism (not shown) that can independently adjust the fuel injection pressure by the fuel addition valve 18. If it is not necessary to independently adjust the injection pressure, the fuel pump 19 may also be used as a fuel pump (not shown) that supplies fuel to the common rail 14. The fuel addition valve 18 injects a predetermined amount of fuel, so that the air-fuel ratio of the exhaust gas is temporarily set to the rich side. Such an operation is a well-known operation called a rich spike. The air-fuel ratio set by the rich spike is set to a value smaller than the air-fuel ratio when the rich spike is not executed, that is, a value on the rich side with respect to the air-fuel ratio. Therefore, the target air-fuel ratio at the time of rich spike is not always set to a value richer than the theoretical air-fuel ratio.

  The rich spike is controlled by an engine control unit (ECU) 20 that is a computer that controls the operating state of the internal combustion engine 1. The ECU 20 includes a rotation speed sensor 21 that can detect the rotation speed of the internal combustion engine 1, an air flow meter 22 that outputs a signal corresponding to the intake air flow rate, an air / fuel ratio sensor 23 that outputs a signal corresponding to the air / fuel ratio of the exhaust, and a cylinder 2. Output signals from various sensors such as an intake air temperature sensor 24 that outputs a signal corresponding to the temperature of the introduced intake gas and a supercharging pressure sensor 25 that outputs a signal corresponding to the supercharging pressure are input, and the ECU 20 Each part of the internal combustion engine 1 is controlled with reference to the output signal. The control executed by the ECU 20 is diverse. Here, the control executed by the ECU 20 will be described in relation to the present invention.

  The ECU 20 determines whether or not to introduce EGR gas (exhaust gas recirculation) into the intake passage 3 by the exhaust gas recirculation device 9 and prohibition thereof according to the operating state of the internal combustion engine 1 and executes exhaust gas recirculation by the exhaust gas recirculation device 9. At this time, the opening amounts of the throttle valve 5 and the EGR valve 12 provided in the intake passage 3 are operated to control the introduction amount of the EGR gas so that a predetermined EGR rate is obtained. The EGR rate is defined as the ratio of the EGR gas to the intake gas (the mixed gas of air and EGR gas) of the internal combustion engine 1. In the internal combustion engine 1, since the fuel supply position by the fuel addition valve 18 is provided upstream of the position where the exhaust gas recirculation device 9 extracts the exhaust gas, when the rich spike is executed during the exhaust gas recirculation by the exhaust gas recirculation device 9. The exhaust gas whose air-fuel ratio is set to the rich side is introduced into the intake passage 3 through the exhaust gas recirculation device 9, and the exhaust gas is mixed with fresh air in the intake passage 3 and introduced into the cylinder 2. The present embodiment is characterized in that the ECU 20 sets the target air-fuel ratio at the time of rich spike according to the EGR rate in order to suppress the fluctuation of the output torque of the internal combustion engine 1 due to the introduction of the exhaust gas.

  FIG. 2 is a flowchart illustrating an example of a control routine for rich spike control executed by the ECU 20. The program for this routine is held in storage means such as a ROM of the ECU 20, and is repeatedly executed at a predetermined cycle. First, the ECU 20 estimates the NOx occlusion amount S by the filter 17 in step S1. Since the occlusion amount S can be estimated by a well-known method, a detailed description is omitted. Next, in step S2, it is determined whether or not the occlusion amount S is larger than a predetermined threshold value Sth1. The threshold value Sth1 determines whether or not the rich spike needs to be executed, and is set to a value lower than the limit value of the storage amount of NOx. If the occlusion amount S is larger than the threshold value Sth1 in step S2, the process proceeds to step S3. Otherwise, it is not necessary to execute a rich spike, so the subsequent processing is skipped and the current routine is terminated.

  In step S3, it is determined whether exhaust gas recirculation by the exhaust gas recirculation device 9 is being performed (EGR is being performed). This determination may be performed based on information from an opening sensor (not shown) that detects the opening degree of the EGR valve 12, or processing of an exhaust gas recirculation control routine (not shown) that is executed in parallel with the routine of FIG. You may perform based on a state. If the EGR is not being executed, there is no fear that the output torque will fluctuate due to the execution of the rich spike. Therefore, the process proceeds to step S6, where the target air-fuel ratio is set to a predetermined value. Run a rich spike so that That is, the ECU 20 controls the injection period of the fuel addition valve 18 to inject a predetermined amount of fuel into the exhaust passage 4 so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio. On the other hand, when the EGR is being executed, the process proceeds to step S4 to acquire the EGR rate, the intake air temperature, and the supercharging pressure. The EGR rate is acquired based on the processing state of the above-described exhaust gas recirculation control routine executed in parallel with the routine of FIG. The intake air temperature is acquired based on information from the intake air temperature sensor 24 in FIG. 1, and the supercharging pressure is acquired based on information from the supercharging pressure sensor 25 in FIG.

  Next, in step S5, a target air-fuel ratio is set in accordance with the EGR rate, intake air temperature, and boost pressure acquired in step S4. Here, a method for setting the target air-fuel ratio will be described. When the EGR rate is high, more exhaust gas whose air-fuel ratio is set to the rich side is introduced into the cylinder 2 than when the EGR rate is low. For this reason, if the air-fuel ratio is constant, the higher the EGR rate, the lower the oxygen concentration in the intake gas, the worse the combustion state such as ignition delay, and the lower the output torque of the internal combustion engine 1. Therefore, in this embodiment, in order to suppress such fluctuations in output torque due to fluctuations in the combustion state, the target air-fuel ratio is set to the lean side when the EGR rate is high compared to when it is low. In other words, when the EGR rate is low, the oxygen concentration in the intake gas is reduced, and the combustion state such as ignition delay is deteriorated, so that the output torque of the internal combustion engine 1 does not decrease and the upper limit EGR rate is obtained. it can.

  FIG. 3 shows a change in the ignition delay with respect to the oxygen concentration of the intake gas. As is apparent from FIG. 3, even when the oxygen concentration of the intake gas is the same, the ignition delay becomes smaller as the intake temperature is higher or as the supercharging pressure is higher. Therefore, in this embodiment, the target air-fuel ratio at the time of rich spike is set to a leaner side when the EGR rate is high than when it is low, and is set to a lean side when the intake air temperature is low at the same EGR rate. And when the supercharging pressure is low at the same EGR rate, it is set on the lean side than when it is high. For example, as shown in FIG. 4, a map for giving the target air-fuel ratio is stored in the ROM of the ECU 20 with reference to the EGR rate, the intake air temperature, and the supercharging pressure as variables. Can be realized. After the target air-fuel ratio is set in step S5, a rich spike is executed in step S7 so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio, and this routine is terminated.

  According to the above embodiment, since the fuel is injected toward the filter 17 by the fuel addition valve 18, the fuel is guided to the filter 17 in a droplet state before being uniformly diffused into the exhaust gas. Since the fuel in the droplet state remains in the filter 17 and volatilizes, an atmosphere in which the air-fuel ratio is locally rich can be formed in the filter 17 and NOx can be reduced. When a locally rich atmosphere can be formed, the entire air-fuel ratio can be set on the lean side than when fuel is uniformly diffused into the exhaust, so a part of the exhaust is introduced into the intake passage as EGR gas. Even in this case, the fluctuation of the output torque can be suppressed because the fluctuation of the air-fuel ratio is small. Further, when the EGR rate is high according to the routine of FIG. 2, the target air-fuel ratio is set to be leaner than when the EGR rate is low, so that a local rich atmosphere is formed while suppressing fluctuations in output torque due to an increase in the EGR rate. Thus, the required NOx purification rate can be ensured. Further, in the routine of FIG. 2, it is considered that when the target air-fuel ratio is set, the degree of fluctuation of the output torque due to the decrease in the oxygen concentration of the intake gas varies depending on the intake air temperature and the boost pressure. Even with the EGR rate, it is possible to set the target air-fuel ratio corresponding to the intake air temperature and the boost pressure.

(Second form)
Next, the 2nd form of the exhaust system of this invention is demonstrated. This embodiment is characterized in that the fuel injection state by the fuel addition valve 18 is controlled in accordance with the target air-fuel ratio, and is otherwise the same as the first embodiment. Therefore, the description of the common part with the first embodiment is omitted below. Since the air-fuel ratio of the entire exhaust gas becomes leaner as the target air-fuel ratio in the rich spike is set to the lean side, it is necessary to promote the formation of a local rich atmosphere and increase the NOx purification rate.

  FIG. 5 shows the change of the droplet diameter of the injected fuel and the change of the purification rate of NOx with respect to the change of the fuel injection pressure by the fuel addition valve 18. In this figure, the change in the NOx purification rate is indicated by broken lines when the air-fuel ratio is the stoichiometric air-fuel ratio (stoichiometric) and when it is leaner than stoichiometric. As is apparent from this figure, when the air-fuel ratio is stoichiometric, the NOx purification rate increases as the injection pressure increases and the fuel droplet diameter decreases, in other words, the fuel atomization increases. On the other hand, when the air-fuel ratio is lean, the NOx purification rate decreases as the injection pressure increases and the fuel droplet diameter decreases, in other words, the fuel atomization increases. That is, in order to suppress the reduction in the NOx purification rate, it can be said that it is desirable to promote the formation of a local rich atmosphere by reducing the atomization of fuel as the target air-fuel ratio is set to the lean side. On the other hand, the necessity of forming a local rich atmosphere decreases as the target air-fuel ratio is set to the rich side, so it can be said that it is desirable to increase fuel atomization in order to suppress a decrease in the NOx purification rate.

  Therefore, in this embodiment, the fuel injection state is controlled so that the fuel atomization by the fuel addition valve 18 increases as the target air-fuel ratio is set to the rich side when the rich spike is executed. The injection state control for changing the atomization of the fuel is performed by controlling the operation of the fuel pump 19 in FIG. 1 according to the target air-fuel ratio when the ECU 20 executes step S7 in FIG. This can be achieved by adjusting the pressure of the fuel supplied to the tank. This is because when the pressure of the fuel supplied to the fuel addition valve 18 increases, the fuel injection pressure by the fuel addition valve 18 increases and fuel atomization increases. Thereby, since the atomization of the fuel is adjusted in accordance with the change in the target air-fuel ratio, it is possible to suppress a decrease in the NOx purification rate due to the change in the target air-fuel ratio to the lean side.

(Third form)
Next, the 3rd form of the exhaust system of this invention is demonstrated. This embodiment is characterized in that the fuel injection state by the fuel addition valve 18 is changed within the injection period in the rich spike, and is otherwise the same as the first embodiment. Therefore, the description of the common part with the first embodiment is omitted below.

  In each of the embodiments described above, the fuel injection state during the injection period in the rich spike is substantially constant, and the atomization of the fuel is not actively changed during the injection period. In the present embodiment, in the injection period in the rich spike, the fuel injection state by the fuel addition valve 18 is controlled by the ECU 20 so that the atomization of fuel in the initial stage of the injection period is lower than in the later stage of the injection period.

  Specifically, as shown in FIG. 6, when executing the rich spike, the ECU 20 changes the duty ratio for the fuel addition valve 18 to be small at the beginning of the injection period and large at the later stage. FIG. 6 also shows a comparative example in which the duty ratio is constant during the injection period for comparison. At the beginning of the injection period with a small duty ratio, the lift amount of the valve body of the fuel addition valve 18 is smaller than in the latter period when the duty ratio is large, so that a pressure loss occurs between the valve body and the nozzle body, resulting in the fuel addition valve. The fuel injection pressure due to 18 decreases. Thereby, the droplet diameter of the fuel injected by the fuel addition valve 18 increases at the initial stage of the injection period and decreases at the latter stage of the injection period. That is, the atomization of fuel in the early stage of the injection period is lower than in the later stage. Instead of changing the duty ratio, the operation of the fuel pump 19 can be controlled to change the initial and late injection pressures in the injection period.

  According to this embodiment, in the initial stage of the injection period, the fuel injected from the fuel addition valve 18 is easily supplied to the filter 17 in a droplet state. As a result, the formation of a local rich atmosphere is promoted at the early stage of the injection period, and then the atomization of the fuel is increased at the later stage of the injection period. . Thereby, reduction of NOx desorbed from the filter 17 by a locally rich atmosphere can be promoted. Therefore, NOx desorbed from the filter 17 due to the formation of a local rich atmosphere can be suppressed from being discharged without being reduced, and the NOx purification rate is improved.

(4th form)
Next, the 4th form of the exhaust system of this invention is demonstrated. FIG. 7 shows a main part of an internal combustion engine to which the exhaust system according to this embodiment is applied. Hereinafter, the same configurations as those of the first embodiment are denoted by the same reference numerals in FIG. In addition to the exhaust gas recirculation device 9, the internal combustion engine 40 includes an exhaust gas recirculation device 41 that extracts EGR gas from the exhaust passage 4 upstream of the turbine 6b of the turbocharger 6 and introduces it into the intake air passage 3 downstream of the compressor 6a. In this embodiment, the exhaust gas recirculation device 9 corresponds to the first exhaust gas recirculation device according to the present invention, and the exhaust gas recirculation device 41 corresponds to the second exhaust gas recirculation device according to the present invention. The exhaust gas recirculation device 41 includes an EGR passage 42 that connects the exhaust passage 4 upstream of the turbine 6b and the intake passage 3 downstream of the compressor 6a, and an EGR valve 43 that adjusts the amount of EGR gas introduced into the intake passage 3. . The internal combustion engine 40 includes a throttle valve 44 that is used to adjust the EGR rate related to the exhaust gas recirculation device 41. The internal combustion engine 40 acquires the operating state based on information from various sensors such as the rotational speed sensor 21 and the air flow meter 22, and appropriately uses the two exhaust gas recirculation devices 9 and 41 according to the operating state. There are three patterns of usage, the first pattern is that the exhaust gas recirculation device 41 alone introduces the EGR gas into the intake passage 3, and the second pattern is the exhaust gas recirculation device 9 only and the EGR gas is introduced into the intake passage 3. In the third pattern, the EGR gas is introduced into the intake passage 3 by both the exhaust gas recirculation device 9 and the exhaust gas recirculation device 11. For example, the internal combustion engine 40 has a first pattern when the load is low, a second pattern when the load is high, and a third pattern when the load is medium. Perform exhaust gas recirculation respectively.

  As shown in FIG. 7, in the exhaust system of the present embodiment, as an upstream fuel addition means disposed downstream of the EGR gas take-out position by the exhaust gas recirculation device 41 and upstream of the fuel addition valve 18. A fuel addition valve 45 is further provided. The fuel addition valve 45 has the same configuration as the fuel addition valve 18 on the downstream side, and fuel at a predetermined pressure is supplied to the fuel addition valve 45 by the fuel pump 19.

  Next, rich spike control executed by the ECU 20 will be described. FIG. 8 is a flowchart showing an example of a control routine for rich spike control according to the present embodiment. The program for this routine is held in storage means such as a ROM of the ECU 20, and is repeatedly executed at a predetermined cycle. First, the ECU 20 estimates the NOx storage amount S by the filter 17 in step S11. This process is the same as step S1 in FIG. Next, in step S12, it is determined whether or not the occlusion amount S is larger than a predetermined threshold value Sth1. This process is the same as step S2 in FIG. If the occlusion amount S is larger than the threshold value Sth1 in step S12, the process proceeds to step S13. Otherwise, the rich spike need not be executed, and the subsequent routine is terminated by skipping subsequent processes.

  In step S13, it is determined whether exhaust gas recirculation is being executed in any of the three patterns described above (whether EGR is being executed). If EGR is not being executed, there is no concern that the output torque will fluctuate due to execution of the rich spike, so the routine proceeds to step S23 and normal control is executed. In the normal control, fuel is injected into the exhaust passage 4 from either or both of the fuel addition valve 18 and the fuel addition valve 45 so that a predetermined target air-fuel ratio is obtained. On the other hand, if EGR is being executed, the process proceeds to step S14 to determine whether exhaust gas recirculation is being executed in the first pattern. If exhaust gas recirculation is being executed in the first pattern, step S20 is performed. If not, the process proceeds to step S15. Since the exhaust gas recirculation device 41 is located upstream of the fuel addition valves 18 and 45, the exhaust gas recirculation device 41 has the fuel injected into the exhaust passage 4 when exhaust gas recirculation is being executed in the first pattern. The torque is not changed by being introduced into the intake passage 3 by the recirculation device 41. Accordingly, in step S20, the target air-fuel ratio is set to stoichiometric regardless of the EGR rate, and in step S21, a rich spike is executed using the upstream fuel addition valve 45 (upstream addition). Thereafter, the current routine is terminated.

  On the other hand, when it is not the first pattern, that is, in the case of the second pattern or the third pattern, the exhaust gas recirculation is performed by the exhaust gas recirculation device 9 in which the EGR gas extraction position is located downstream of the fuel addition valves 18 and 45. Therefore, the target air-fuel ratio is set according to the EGR rate related to the exhaust gas recirculation device 9 in step S15 and step S16 based on the same concept as in the first embodiment described above. Step S15 is the same processing as step S4 in FIG. 2, and step S16 is the same processing as step S5 in FIG.

  Next, in step S17, it is determined whether exhaust gas recirculation is being executed in the third pattern. If it is not executed in the third pattern, it is executed in the second pattern. Therefore, the process proceeds to step S22, and the rich spike is executed using the fuel addition valve 18 as in the first embodiment. Thereafter, the current routine is terminated. On the other hand, if exhaust gas recirculation is being executed in the third pattern, the flow proceeds to step S18 to set the blowing ratio. This blowing rate is defined as the ratio of the fuel injected by the fuel addition valve 18 to the total amount of fuel to be injected by each of the fuel addition valve 18 and the fuel addition valve 45. The air-blowing rate is associated with the target air-fuel ratio so that it increases as the target air-fuel ratio is set to the lean side. This setting can be realized by storing in advance in the ECU 20 a map that gives the target air-fuel ratio as a variable, and the ECU 20 refers to this map. Next, in step S19, rich spike is executed by injecting fuel from each of the two fuel addition valves 18 and 45 in accordance with the determined blowing rate. Thereafter, the current routine is terminated.

  According to the routine of FIG. 8, in the third pattern, the target air-fuel ratio is set according to the EGR rate related to the exhaust gas recirculation device 9, and the above-described air-blowing rate becomes higher as the target air-fuel ratio is set to the lean side. Is set to be As a result, the formation of a local rich atmosphere can be promoted while suppressing fluctuations in the output torque associated with an increase in the EGR rate by the exhaust gas recirculation device 9, so that the necessary NOx purification rate can be ensured.

  In each of the above embodiments, the ECU 20 functions as the fuel addition control means according to the present invention by executing the routine of FIG. 2 or FIG. Further, by executing step S5 of FIG. 2 or step S16 of FIG. 8, the ECU 20 functions as a target air-fuel ratio setting unit according to the present invention.

  However, the present invention is not limited to the above embodiments, and can be implemented in various forms within the scope of the gist of the present invention. In each of the above embodiments, the intake air temperature and the boost pressure are considered in addition to the EGR rate when setting the target air-fuel ratio, but it is not essential to consider the intake air temperature and the boost pressure. It does not have to be. Further, when setting the target air-fuel ratio, either the intake air temperature or the supercharging pressure may be considered in addition to the EGR rate. In each of the above embodiments, the present invention is applied to an internal combustion engine with a turbocharger, but the internal combustion engine to be applied may not have a turbocharger.

The figure which showed the principal part of the internal combustion engine to which the exhaust system of this invention was applied. The flowchart which shows an example of the control routine of rich spike control. Explanatory drawing which showed the change of the ignition delay with respect to the oxygen concentration of inhalation gas. The figure which showed an example of the map used for the setting of a target air fuel ratio. Explanatory drawing which showed the change of the droplet diameter of the injected fuel with respect to the change of the fuel injection pressure by a fuel addition valve, and the change of the purification rate of NOx, respectively. Explanatory drawing explaining the electricity supply state with respect to a fuel addition valve. The figure which showed the principal part of the internal combustion engine to which the exhaust system which concerns on a 4th form was applied. The flowchart which shows an example of the control routine of the rich spike control which concerns on a 4th form.

Explanation of symbols

1 Internal combustion engine 3 Intake passage 4 Exhaust passage 6 Turbocharger 6a Compressor 6b Turbine 9 Exhaust gas recirculation device (first exhaust gas recirculation device)
17 Filter (exhaust gas purification means)
18 Fuel addition valve (fuel addition means)
20 ECU (fuel addition control means, target air-fuel ratio setting means)
40 Internal combustion engine 41 Exhaust gas recirculation device (second exhaust gas recirculation device)

Claims (7)

In an exhaust system of an internal combustion engine applied to an internal combustion engine provided with an exhaust gas recirculation device that takes out a part of exhaust gas from an exhaust passage as EGR gas and introduces the exhaust gas into an intake passage.
An exhaust gas purifying means disposed in the exhaust passage for reducing the stored nitrogen oxides at a rich air-fuel ratio; and disposed in the exhaust passage upstream of an EGR gas take-out position by the exhaust gas recirculation device. Fuel addition means capable of injecting fuel toward the purification means, and fuel is injected from the fuel addition means so that the air-fuel ratio of the exhaust gas becomes a target air-fuel ratio by temporarily changing the air-fuel ratio of the exhaust gas to the rich side Fuel addition control means, and target air-fuel ratio setting means for setting the target air-fuel ratio to the lean side when the EGR ratio as a ratio of EGR gas to the intake gas of the internal combustion engine is high, rather than when it is low An exhaust system for an internal combustion engine, comprising:   2. The exhaust gas of the internal combustion engine according to claim 1, wherein the target air-fuel ratio setting means sets the target air-fuel ratio to the lean side when the intake gas temperature is low at the same EGR rate than when it is high. system. The internal combustion engine is provided with a turbocharger that has a turbine disposed in the exhaust passage and a compressor disposed in the intake passage and driven by the turbine, and supercharges using the energy of the exhaust. And
3. The internal combustion engine according to claim 1, wherein the exhaust purification unit is disposed on a downstream side of the turbine, and the fuel addition unit is disposed between the turbine and the exhaust purification unit. Exhaust system.   4. The internal combustion engine according to claim 3, wherein the target air-fuel ratio setting unit sets the target air-fuel ratio to a leaner side when the boost pressure by the turbocharger is low at the same EGR rate than when it is high. Engine exhaust system.   The fuel addition control means controls the fuel injection state by the fuel addition means so that the atomization of the fuel by the fuel addition means increases as the target air-fuel ratio is set to the rich side. The exhaust system of the internal combustion engine according to any one of claims 1 to 4.   In the injection period in which the fuel addition means injects fuel so that the air-fuel ratio of the exhaust gas temporarily changes to the rich side, the atomization of fuel at the initial stage of the injection period The exhaust system for an internal combustion engine according to any one of claims 1 to 5, wherein an injection state of the fuel by the fuel addition means is controlled so as to be lower than in a later stage. In the internal combustion engine, as the exhaust gas recirculation device, a first EGR gas take-out position is located downstream of the turbine of the turbocharger, and an EGR gas introduction position is located upstream of the compressor of the turbocharger. An exhaust gas recirculation apparatus, and a second exhaust gas recirculation apparatus in which an EGR gas extraction position is located upstream of the turbine of the turbocharger and an EGR gas introduction position is located downstream of the compressor of the turbocharger. Provided,
The apparatus further comprises an upstream fuel addition means disposed downstream of the EGR gas take-out position by the second exhaust gas recirculation device and upstream of the fuel addition means and capable of injecting fuel into the exhaust passage. ,
The fuel addition control means temporarily sets the air-fuel ratio of the exhaust to a rich side in a state where introduction of EGR gas by the first exhaust gas recirculation device and introduction of EGR gas by the second exhaust gas recirculation device are respectively performed. The ratio of the fuel injected by the fuel addition means to the total amount of fuel to be injected by each of the upstream fuel addition means and the fuel addition means is the target air fuel ratio. Injecting fuel from each of the upstream fuel addition means and the fuel addition means so that the fuel ratio becomes higher as the lean side is set,
The target air-fuel ratio setting means makes the target air-fuel ratio leaner when the EGR rate as a ratio of the EGR gas introduced by the first exhaust gas recirculation device to the intake gas of the internal combustion engine is high than when the EGR rate is low. 4. The exhaust system for an internal combustion engine according to claim 3, wherein the exhaust system is set on the side.

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