JP2019167848A - Internal combustion engine system - Google Patents

Abstract

An internal combustion engine system capable of suppressing an increase in NOx concentration of exhaust gas from an engine during a transition period when switching between a stoichiometric combustion mode and a lean combustion mode and suppressing an increase in an integrated NOx emission amount during the transition period. I will provide a. An internal combustion engine system includes an engine, a three-way catalyst and a NOx reduction catalyst, an exhaust gas recirculation device, an exhaust gas recirculation amount adjusting valve, a turbocharger, a turbine bypass flow path, It includes a wastegate valve 42, an intake throttle valve 26, and a combustion switching control device 11 that switches the combustion mode of the engine 12. During the transition period, the combustion switching control device 11 performs an air-fuel ratio switching step of switching from the lean combustion condition to the stoichiometric combustion condition so that the exhaust gas recirculation rate becomes a predetermined first target value, and an exhaust gas recirculation process while maintaining the stoichiometric combustion condition. And an exhaust gas recirculation rate switching step of reducing the circulation rate. [Selection diagram] Fig. 1

Description

  The present invention relates to an internal combustion engine system, and more particularly to an internal combustion engine system including a three-way catalyst and a NOx reduction catalyst as exhaust gas purification catalysts.

  Patent Document 1 describes a compression ignition engine including a three-way catalytic converter for reducing harmful exhaust gas. In this compression ignition engine, in the first mode with low engine load, the engine is operated at a high exhaust gas recirculation (EGR) rate in normal diesel combustion conditions to reduce NOx emissions. Also, in the second mode with medium to high engine load, the engine is operated in a stoichiometric state that can reduce NOx emissions using a three-way catalytic converter. Also, in a third mode with very high engine load and / or engine speed, the engine is operated at normal diesel combustion conditions and low EGR rates to obtain maximum torque.

  Patent Document 2 discloses NOx emission based on estimated values obtained by online estimation of NOx emission amount and torque fluctuation amount downstream of the exhaust purification device at the time of combustion region switching between a combustion region richer than stoichiometric and lean combustion region. In order to reduce the amount and torque fluctuation amount, there is described an engine control device including combustion control means for controlling the intake air amount sucked into the combustion chamber in a manner different from that in the normal state. This is supposed to prevent deterioration of exhaust emission and operability when switching the combustion region.

JP 2012-197794 A International Publication No. 2005/075803 Pamphlet

  In the technique of Patent Document 1, the NOx reduction catalyst is abolished, and the lean combustion mode and the stoichiometric combustion mode are switched according to the engine load. In the stoichiometric combustion mode, a three-way catalyst is used to reduce the NOx emission amount. Yes.

  By the way, in the prior art such as Patent Document 1, when switching between the lean combustion mode and the stoichiometric combustion mode, the NOx concentration in the engine exhaust gas temporarily in the combustion region leaner than the stoichiometric combustion region where the three-way catalyst functions. There is a problem that the accumulated NOx emission increases. However, Patent Document 1 does not mention control at the time of switching between the stoichiometric combustion mode and the lean combustion mode.

  Patent Document 2 discloses the estimation of NOx emission based on the air-fuel ratio of the air-fuel mixture, the engine speed, the torque, and the EGR amount at the time of switching between the combustion region richer than the stoichiometric region and the lean combustion region. Describes a method of reducing the NOx emission amount by switching the air-fuel ratio of the three-way catalyst inlet in a short time, which is performed by adjusting the intake air amount by adjusting the lift amount of the catalyst. However, Patent Document 2 does not specifically mention the control for reducing the NOx emission amount in the transition period for switching between the stoichiometric combustion region and the lean combustion region, and controls the external EGR device or the supercharger. There is no specific mention of the accompanying case.

  An object of the present invention is to suppress an increase in NOx concentration of exhaust gas from an engine during a transition period at the time of combustion switching between a stoichiometric combustion mode and a lean combustion mode, and to suppress an increase in accumulated NOx emission amount during the transition period. It is to provide an internal combustion engine system.

  A first internal combustion engine system according to the present invention includes an engine, a three-way catalyst and a NOx reduction catalyst that purify exhaust gas exhausted from the engine, a temperature acquisition unit that acquires the temperature of the NOx reduction catalyst, and an engine speed. A rotation speed acquisition means for acquiring the fuel, an injection control unit for controlling the fuel injection amount in the engine, an exhaust gas recirculation device for recirculating a part of the exhaust gas of the engine, and an exhaust gas recirculation amount for adjusting the amount of exhaust gas recirculated to the engine An adjusting device, a supercharging device that supercharges air sucked into the engine, a supercharging pressure adjusting device that adjusts the supercharging pressure by the supercharging device, an air amount adjusting device that adjusts the amount of air sucked in, Based on the NOx reduction catalyst temperature acquired by the temperature acquisition means, the engine rotation speed acquired by the rotation speed acquisition means, and the fuel injection amount acquired from the injection control unit, the engine And a combustion mode switching control unit for switching between the combustion mode and the lean combustion mode and the stoichiometric combustion mode. The combustion switching control unit controls the exhaust gas recirculation amount adjusting device, the supercharging pressure adjusting device, and the air amount adjusting device during the transition period for switching from the lean combustion mode to the stoichiometric combustion mode, so that the exhaust gas recirculation rate is a predetermined first value. After the air-fuel ratio switching step of switching the air-fuel ratio from lean combustion conditions to stoichiometric combustion conditions so as to reach the target value, and after the air-fuel ratio switching step, the exhaust gas recirculation rate is maintained from the first target value while maintaining the stoichiometric combustion conditions. And an exhaust gas recirculation rate switching step for reducing to a predetermined second target value.

  A second internal combustion engine system according to the present invention includes an engine, a three-way catalyst and a NOx reduction catalyst for purifying exhaust gas exhausted from the engine, a temperature acquisition means for acquiring the temperature of the NOx reduction catalyst, and an engine speed. A rotation speed acquisition means for acquiring the fuel, an injection control unit for controlling the fuel injection amount in the engine, an exhaust gas recirculation device for recirculating a part of the exhaust gas of the engine, and an exhaust gas recirculation amount for adjusting the amount of exhaust gas recirculated to the engine An adjusting device, a supercharging device that supercharges air sucked into the engine, a supercharging pressure adjusting device that adjusts the supercharging pressure by the supercharging device, an air amount adjusting device that adjusts the amount of air sucked in, Based on the NOx reduction catalyst temperature acquired by the temperature acquisition means, the engine rotation speed acquired by the rotation speed acquisition means, and the fuel injection amount acquired from the injection control unit, the engine And a combustion mode switching control unit for switching between the combustion mode and the lean combustion mode and the stoichiometric combustion mode. The combustion switching control unit controls the exhaust gas recirculation amount adjusting device, the supercharging pressure adjusting device, and the air amount adjusting device at the time of combustion switching to switch from the stoichiometric combustion mode to the lean combustion mode, while maintaining the stoichiometric combustion condition, After the exhaust gas recirculation rate switching step for increasing the recirculation rate to a predetermined first target value and the exhaust gas recirculation rate switching step, the exhaust gas recirculation rate is increased from the first target value to the predetermined second target value. And an air-fuel ratio switching step of switching the air-fuel ratio from the lean combustion condition to the stoichiometric combustion condition.

  According to the internal combustion engine system of the present invention, the step of switching between the lean combustion condition and the stoichiometric combustion condition so as to achieve a predetermined target EGR rate that suppresses the increase in the NOx concentration, and the stoichiometric combustion condition are maintained and the three-way catalyst is maintained. By performing the combustion switching control comprised of the step of adjusting the EGR rate while functioning NOx purification by NO, the increase in the NOx concentration of exhaust gas from the engine during the transition period at the time of combustion switching can be suppressed, As a result, an increase in the accumulated NOx emission amount during the transition period can be suppressed.

1 is a diagram schematically showing an overall configuration of an internal combustion engine system according to an embodiment. It is a graph which shows the relationship between the air fuel ratio of an engine, NOx emission amount, and supercharging pressure. It is a graph which shows the relationship between an engine intake oxygen concentration and NOx discharge | emission amount. It is a graph which shows the change of each parameter by fuel change control concerning an example of an embodiment. 2 is a flowchart showing an example of processing executed in the combustion switching control device of the internal combustion engine system shown in FIG. 1. It is a graph which shows the change of each parameter by fuel change control concerning other examples of an embodiment. 7 is a flowchart showing another example of processing executed in the combustion switching control device of the internal combustion engine system shown in FIG. 1.

  Embodiments according to the present invention will be described below in detail with reference to the accompanying drawings. In this description, specific shapes, materials, numerical values, directions, and the like are examples for facilitating the understanding of the present invention, and can be appropriately changed according to the application, purpose, specification, and the like. In addition, when a plurality of embodiments and modifications are included in the following, it is assumed from the beginning that these characteristic portions are used in appropriate combinations.

  In the following, the case where the engine is a compression ignition type diesel engine will be described. However, the present invention is not limited to this, and the present invention may be applied to an internal combustion engine system including a spark ignition type gasoline engine.

<First Embodiment>
FIG. 1 is a diagram schematically showing an overall configuration of an internal combustion engine system 10 according to a first embodiment of the present invention. The internal combustion engine system 10 includes an engine 12, a combustion switching control device (combustion switching control unit) 11, a fuel injection device 16, and an injection control device (injection control unit) 18. In the present embodiment, the engine 12 is a compression ignition type diesel engine and includes, for example, four cylinders 14. Each cylinder 14 is provided with a fuel injection device 16. Each fuel injection device 16 is controlled by the injection control device 18 that has received a signal from the combustion switching control device 11 and the number of fuels, the number of injections, the amount, and the timing.

  The internal combustion engine system 10 further includes a rotation speed sensor (rotation speed acquisition means) 20 attached to the engine 12. The rotation speed sensor 20 has a function of acquiring the rotation speed of the crankshaft connected to the piston in each cylinder of the engine 12 as the engine rotation speed Ne. The engine rotational speed Ne acquired by the rotational speed sensor 20 is transmitted to the combustion switching control device 11 for combustion mode switching in the engine 12 or the like.

  The internal combustion engine system 10 further includes an intake system 21, an exhaust system 30, an exhaust gas recirculation device 50, and a turbocharger (supercharger) 60.

  The intake system 21 is an air passage for supplying air to the engine 12. The direction of air intake in the intake system 21 is indicated by an arrow A. The intake system 21 includes a first intake passage 22 and a second intake passage 24. One end of the first intake passage 22 is opened to the atmosphere via a filter or the like (not shown), and the other end is connected to the compressor chamber 62 of the turbocharger 60. The second intake passage 24 has one end connected to the compressor chamber 62 and the other end connected to the intake port of the engine 12. An intake throttle valve 26 is provided in the second intake passage 24.

  The intake throttle valve 26 is an air amount adjusting device that adjusts the amount of air taken into the engine 12, and is preferably configured by, for example, an electromagnetic on-off valve. In the present embodiment, the intake throttle valve 26 is installed in the vicinity of the compressor chamber 62 of the turbocharger 60. The intake throttle valve 26 is adjusted in opening degree in response to a signal from the combustion switching control device 11.

  The exhaust system 30 is an exhaust gas passage for exhausting exhaust gas exhausted from the engine 12 to the outside. The exhaust system 30 includes a first exhaust passage 32, a second exhaust passage 34, and a turbine bypass passage 36. The first exhaust passage 32 has one end connected to the exhaust port of the engine 12 and the other end connected to the turbine chamber 64 of the turbocharger 60. The second exhaust passage 34 has one end connected to the turbine chamber 64 and the other end open to the atmosphere via a muffler (or a silencer) not shown.

  A three-way catalyst 38 and a NOx reduction catalyst 40 are provided in the second exhaust passage 34. While the exhaust gas passes through the three-way catalyst 38 and the NOx reduction catalyst 40, HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxide), etc. are removed and purified from the exhaust gas to the atmosphere. Discharged. In this embodiment, other catalysts (for example, HC trap catalyst and particulate filter (DPF)) of the three-way catalyst 38 and the NOx reduction catalyst 40 are not provided, but other catalysts may be further provided.

  The three-way catalyst 38 has a function of removing and purifying HC, CO, and NOx contained in the exhaust gas by oxidation / reduction action, and the purification efficiency is stoichiometric combustion conditions in which the air-fuel ratio is a stoichiometric ratio (stoichiometric combustion). Area) and can be kept relatively high even at high temperatures. On the other hand, the NOx reduction catalyst 40 mainly has a function of removing and purifying NOx in the exhaust gas by a reducing action, and the purification efficiency is determined based on the lean combustion condition (the lean combustion condition where the air-fuel ratio is leaner than the stoichiometric ratio). It is very high during operation in the lean combustion region), but tends to be slightly lower at higher temperatures.

  In the present embodiment, a selective reduction catalyst (SCR) is preferably used as the NOx reduction catalyst 40. However, the present invention is not limited to this, and the NOx reduction catalyst 40 may be constituted by an occlusion reduction type catalyst (NSR) or may be constituted by a combination of SCR and NSR. Note that any known or later-developed catalyst may be used for these three-way catalysts, SCR and NSR.

  A temperature sensor 41 is disposed in the NOx reduction catalyst 40. The temperature sensor 41 constitutes a temperature acquisition unit that acquires the temperature Tg of the NOx reduction catalyst 40. The temperature sensor 41 is preferably arranged so as to detect the internal temperature of the NOx reduction catalyst 40. The temperature Tg of the NOx reduction catalyst 40 acquired by the temperature sensor 41 is transmitted to the combustion switching control device 11. In this embodiment, an example in which the temperature sensor 41 detects the temperature Tg of the NOx reduction catalyst 40 will be described. However, the present invention is not limited to this, and the exhaust gas flowing through the first exhaust passage 32 or the second exhaust passage 34 is not limited thereto. The combustion switching control device 11 may predict the NOx reduction catalyst temperature Tg based on this temperature.

  In the present embodiment, the NOx reduction catalyst 40 is disposed on the downstream side of the three-way catalyst 38 with respect to the exhaust gas discharge direction (arrow E direction). In other words, the three-way catalyst 38 is arranged on the upstream side of the NOx reduction catalyst 40 with respect to the exhaust gas discharge direction E. The three-way catalyst 38 has a higher high temperature resistance than the NOx reduction catalyst 40 and maintains the purification characteristics of pollutants such as NOx even at a high temperature. Therefore, the three-way catalyst 38 is disposed on the upstream side exposed to higher temperature exhaust gas. Is preferred. However, the present invention is not limited to this, and the NOx reduction catalyst 40 may be disposed upstream of the three-way catalyst 38.

  The turbine bypass passage 36 is connected to the first exhaust passage 32 on the upstream side of the turbine chamber 64 of the turbocharger 60, and the other end is connected to the second exhaust passage 34 on the upstream side in the exhaust gas discharge direction E of the three-way catalyst 38. Has been. A wastegate valve 42 is provided in the turbine bypass passage 36. The wastegate valve 42 has a function of adjusting the supercharging pressure of intake air by the turbocharger 60. Further, the wastegate valve 42 has a function of preventing the supercharging pressure from exceeding a specified value and protecting the engine 12 and the turbocharger 60 from being damaged.

  The wastegate valve 42 is preferably configured by, for example, an electromagnetic on-off valve. The wastegate valve 42 is adjusted in opening degree in response to a signal from the combustion switching control device 11. When the opening degree of the waste gate valve 42 increases, the exhaust gas bypassed to the second exhaust passage 34 through the turbine bypass passage 36 without flowing into the turbine chamber 64 increases. As a result, the engine 12 and the turbocharger 60 are protected from damage. The turbine bypass passage 36 and the waste gate valve 42 in the present embodiment correspond to the “supercharging pressure adjusting device” in the present invention.

  An exhaust gas recirculation device 50 is provided between the second intake passage 24 and the first exhaust passage 32. The exhaust gas recirculation device 50 includes an exhaust gas recirculation passage 52 that connects the first exhaust passage 32 and a downstream side of the intake throttle valve 26 of the second intake air passage 24, and an exhaust gas recirculation amount installed in the middle of the exhaust gas recirculation passage 52. And an adjusting valve (exhaust gas recirculation amount adjusting device) 54. The exhaust gas recirculation amount adjusting valve 54 is adjusted in opening degree in response to a signal from the combustion switching control device 11. By adjusting the opening degree of the exhaust gas recirculation amount adjustment valve 54 in this way, the amount of exhaust gas recirculated or recirculated from the first exhaust passage 32 to the second intake passage 24 via the exhaust recirculation passage 52 is adjusted.

  The turbocharger 60 includes a compressor wheel 63 accommodated in the compressor chamber 62, a turbine 65 accommodated in the turbine chamber 64, and a shaft 66 connecting the compressor wheel 63 and the turbine 65. The exhaust gas is sprayed from the first exhaust passage 32 to the turbine 65 in the turbine chamber 64, whereby the turbine 65 rotates, and this rotational power is transmitted to the compressor wheel 63 via the shaft 66. As a result, the compressor wheel 63 is driven to rotate, and the air supplied to the engine 12 via the second intake passage 24 is pressurized (ie, supercharged).

  The combustion switching control device 11 is preferably configured by, for example, a microcomputer including a processing device, a storage unit, and an I / O interface. The processing device reads and executes a program, data, or the like stored in the storage unit. The storage unit is configured by a storage element such as a RAM or a ROM, and stores the program, and the engine rotational speed Ne acquired and transmitted by the rotational speed sensor 20, the NOx reduction catalyst temperature Tg acquired by the temperature sensor 41, Stores map and predetermined value.

  The combustion switching control device 11 transmits a command signal for controlling the number, amount, and injection timing of fuel injection into each cylinder 14 of the engine 12 to the injection control device 18. Further, the combustion switching control device 11 transmits an opening degree adjustment signal to each of the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54, and adjusts the opening degree of each valve. The combustion switching control device 11 may be configured as a chip integrated with the injection control device 18 or may be configured as a separate chip.

  Next, the combustion switching control executed by the combustion switching control device 11 in the internal combustion engine system 10 of the present embodiment will be described in detail. FIG. 2 is a graph showing an example of combustion switching control according to the present embodiment. In the graph of FIG. 2, the air-fuel ratio (A / F) is shown on the horizontal axis, and the NOx concentration (unit: g / kWh) in the exhaust gas immediately after being discharged from the engine 12 (engine output gas) is shown on the vertical axis. .

In the graph of FIG. 2, point A indicates the lean combustion mode, that is, the coordinates of the start point of the combustion switching control of the present embodiment, and point B relays from the first step to the second step in the combustion switching control of the present embodiment. The coordinates of a point (described later) are shown, and the point C shows the coordinates of the stoichiometric combustion mode, that is, the end point of the combustion switching control of the present embodiment. In the graph of FIG. 2, the fuel injection amount m _f from the fuel injection device 16 is constant.

The combustion switching between the lean combustion mode (A) and the stoichiometric combustion mode (C) is performed when the fuel injection amount m _f from the fuel injection device 16 is constant and the supercharging pressure when the intake valve is closed (IVC) ( This is done by adjusting the intake pressure (P) and the EGR rate (egr). In the lean combustion mode (A), exhaust gas is recirculated by the exhaust gas recirculation device 50 at a predetermined EGR rate (egr ₀ ) and is supercharged by the turbocharger 60 at a predetermined supercharging pressure (P ₀ ). It is assumed that the fuel ratio is adjusted to (A / F) ₀ . Similarly, in the stoichiometric combustion mode (C), the exhaust gas recirculation is recirculated by the exhaust gas recirculation device 50 at a predetermined EGR rate (egr ₂ ), and is supercharged by the turbocharger 60 at a predetermined supercharging pressure (P ₂ ). Thus, the air-fuel ratio is adjusted to the air-fuel ratio (A / F) _stoich which is the stoichiometric ratio (14.6). In this case, since the fuel injection amount mf is constant, both the EGR rate and the supercharging pressure in the stoichiometric combustion mode (C) are lower than those in the lean combustion mode (A) (egr ₀ > egr ₂ , P ₀ > P ₂ ). As a result, as shown in FIG. 2, in the stoichiometric combustion mode (C), the NOx concentration of the engine output gas increases as compared with the lean combustion mode (A). However, in the stoichiometric combustion mode (C) in which the air-fuel ratio is the stoichiometric ratio, the three-way catalyst 38 functions and purifies NOx in the engine exhaust gas, so that after the three-way catalyst 38 and the NOx reduction catalyst 40 have passed. The NOx concentration of the exhaust gas at the tail pipe is remarkably reduced as compared with the engine exhaust gas (see FIG. 4 (g)).

  Here, as shown by a broken line in FIG. 2, a case where only the lean combustion mode (A) is switched to the stoichiometric combustion mode (C) (A → C) will be considered. In the transition period in which the lean combustion mode (A) is switched to the stoichiometric combustion mode (C), the NOx concentration in the engine exhaust gas increases as the EGR rate decreases and as the stoichiometric combustion mode (C) approaches. However, since the air-fuel ratio during the transition period is higher than the stoichiometric ratio, the three-way catalyst 38 cannot function properly and NOx in the engine exhaust gas cannot be purified by reduction, and the exhaust gas remains as it is. It will be discharged from the tailpipe at a concentration. Therefore, in the above case, the accumulated NOx emission amount during the transition period from the lean combustion mode (A) to the stoichiometric combustion mode (C) increases.

Therefore, in this embodiment, the combustion switching control from the lean combustion mode (A) to the stoichiometric combustion mode (C) is performed, and the air-fuel ratio is changed from the lean combustion condition to the stoichiometric combustion so that the exhaust gas recirculation rate becomes a predetermined target value. An air-fuel ratio switching step (hereinafter referred to as “first step”) for switching to the conditions, and an exhaust gas recirculation rate switching step (hereinafter referred to as “second step”) for reducing the exhaust gas recirculation rate while maintaining the stoichiometric combustion conditions. The two steps are assumed to be performed. Specifically, as shown in FIG. 2, the relay point (B) having the predetermined EGR rate (egr ₁ ), which is the air-fuel ratio ((A / F) _stoichi ) of the stoichiometric combustion condition, is set, and the EGR rate is set. In addition, in the first step, the lean combustion mode (A) is switched to the relay point (B) by controlling the supercharging pressure, and in the second step, the stoichiometric point is maintained from the relay point (B) while maintaining the air-fuel ratio of the stoichiometric combustion condition. Switch to combustion mode (C).

EGR rate of the relay point (B) (egr _1), for example, is set based on the intake oxygen concentration _{x i} to be introduced into the engine 12. Figure 3 is a graph showing the relationship between the intake oxygen concentration x _i and the engine output NOx concentration in the gas engine 12. As shown in the graph of FIG. 3, the fuel injection amount m _f constant, that is, when it is equal torque, has a strong correlation with the NOx concentration in the engine output in the gas and the intake oxygen concentration x _i, NOx in the engine out gas concentrations are known to be substantially determined by the intake oxygen concentration _{x i} (JSAE papers, Vol.43 (2012 years), pp Nanba1,109～114). Therefore, for example, as the first target EGR rate (first target value) in the first step, the intake oxygen concentrations before and after the first step are equal based on the intake oxygen concentration x _i0 in the lean combustion mode (A). Set the EGR rate (egr ₁ ). Then, in the first step, as shown in FIG. 2, while the air-fuel ratio is reduced to the stoichiometric combustion condition toward the relay point (B) having the EGR rate (egr ₁ ) set from the lean combustion mode (A). In addition, the NOx concentration of the engine exhaust gas becomes constant. Therefore, an increase in the accumulated NOx emission amount during this period can be suppressed.

When the first step is completed (relay point (B)), the stoichiometric combustion condition is established, so that the three-way catalyst 38 can function properly and the NOx in the engine exhaust gas can be purified. Therefore, in the second step of reducing the EGR rate from egr ₁ to egr ₂ , which is the second target EGR rate (second target value), and switching from the relay point (B) to the stoichiometric combustion mode (C), engine output is reduced. Although the NOx concentration of the gas increases, the NOx concentration in the exhaust gas at the tail pipe is reduced by the purification function of the three-way catalyst 38. Thus, in the combustion switching control according to the present embodiment, the first step of switching from the lean combustion condition to the stoichiometric combustion condition toward the relay point (B) having the first target EGR rate set based on the intake oxygen concentration, By executing the second step of reducing the EGR rate to the second target EGR rate while maintaining the stoichiometric combustion condition, an increase in the accumulated NOx emission amount in the tail pipe can be suppressed.

  The effect | action of the combustion switching control which the combustion switching control apparatus 11 which concerns on this embodiment performs is demonstrated in detail, referring FIG. In each graph of FIG. 4, the horizontal axis indicates the passage of time, and the vertical axis indicates the change of each parameter by the combustion switching control. That is, the combustion switching control starts at time t0 and ends at time t2. 4 (a) shows the change in the air-fuel ratio by the combustion switching control, FIG. 4 (b) shows the change in the EGR rate, FIG. 4 (c) shows the change in the supercharging pressure, and FIG. 4 (d) shows the same. FIG. 4 (e) shows the change in the NOx concentration of the engine exhaust gas, FIG. 4 (f) shows the change in the NOx purification rate of the three-way catalyst, and FIG. 4 (g) shows the tail. The change of the NOx concentration of the exhaust gas in the pipe is shown respectively.

  The solid line in each graph of FIG. 4 shows the change due to the combustion switching control according to the present embodiment. The engine 12 is operated in the lean combustion mode (A) until time t0, and in the transition period from time t0 to t2. The combustion switching control composed of the first step (t0 to t1) and the second step (t1 to t2) is executed, and after the time t2, the engine 12 is operated in the stoichiometric combustion mode (C). Show.

In the lean combustion mode (A), the air-fuel ratio is (A / F) ₀ , the EGR rate is egr ₀ , the supercharging pressure is P ₀ , and the intake oxygen concentration is x _i0 . When the combustion switching control according to the present embodiment is started (time t0), the first step of reducing the air-fuel ratio to the stoichiometric combustion condition (A / F) _stoich while reducing the EGR rate and the supercharging pressure. to go into. As shown in FIGS. 4A to 4D, in the first step, the EGR rate and the supercharging pressure are reduced toward the first target EGR rate egr ₁ and the first target supercharging pressure P ₁ . The first target EGR rate egr ₁ and the first target boost pressure P ₁ are such that the intake oxygen concentration x _i1 at the end of the first step is maintained equal to the intake oxygen concentration x _{i0 in the} lean combustion mode (A). Are preset values. When the EGR rate and the supercharging pressure decrease and the air-fuel ratio, the EGR rate, and the supercharging pressure reach the respective target values (relay point B in FIG. 2), the first step is finished (time t1). During the first process from time t0 to t1, the intake oxygen concentration is kept constant as shown in FIGS. 4D and 4E, so the NOx concentration of the engine output gas does not increase. Therefore, as shown in FIG. 4G, in the first step, the lean combustion condition ((A / F) ₀ ) is maintained while maintaining the NOx emission amount in the tail pipe comparable to that in the lean combustion mode (A). To the stoichiometric combustion condition ((A / F) _stoic ).

Subsequently, the second step of reducing the EGR rate while maintaining the air-fuel ratio at the stoichiometric combustion condition is entered. In the second step, the EGR rate and the supercharging pressure are reduced to the second target EGR rate egr ₂ and the second target supercharging pressure P ₂ , respectively. The second target EGR rate egr ₂ is the EGR rate in the stoichiometric combustion mode (C) after the second step, and is a value set based on the temperature of the engine exhaust gas or the NOx reduction catalyst temperature Tg, etc. the second target boost pressure _{P 2} is determined by the second target EGR rate egr _2. In the second step, when the EGR rate and the supercharging pressure reach the respective target values by decreasing, the second step ends (time t2), and the engine 12 is operated in the stoichiometric combustion mode (C). As shown in FIG. 4 (f), since the air-fuel ratio is close to the stoichiometric ratio in the vicinity of the relay point (B) from the first step to the second step, the three-way catalyst 38 functions and the engine NOx in the output gas is purified. Therefore, in the second step, the NOx concentration of the engine exhaust gas increases as the intake oxygen concentration increases due to the decrease in the EGR rate and the supercharging pressure (FIG. 4E), but the purification function of the three-way catalyst 38 As a result, the amount of NOx discharged from the tailpipe is reduced (FIG. 4G).

On the other hand, the broken line in each graph of FIG. 4 does not perform the combustion switching control composed of two steps as in this embodiment, but simply switches from the lean combustion mode (A) to the stoichiometric combustion mode (C). The change of each parameter at the time of performing is shown (refer FIG. 2). In this example, the target values of the air-fuel ratio and the EGR rate at the start (t0) of the transition period for switching from the lean combustion mode (A) to the stoichiometric combustion mode (C) are the air-fuel ratio ((A / F) _stoic ) and EGR rate (egr ₂ ), and combustion switching control for reducing the EGR rate and supercharging pressure is executed. Then, as shown by the broken lines in FIGS. 4A to 4D, in the transition period (t0 → t2), the intake oxygen concentration increases due to the decrease in the EGR rate and the supercharging pressure, and accordingly, the engine output gas NOx concentration increases. However, in most of the transition period excluding the vicinity of time t2, the air-fuel ratio is not in the vicinity of the stoichiometric ratio (FIG. 4 (a)), and the three-way catalyst does not exhibit the NOx purification function (FIG. 4 (f)). ). Therefore, as shown in FIG. 4G, the NOx concentration of the exhaust gas in the tail pipe increases as the NOx concentration in the engine exhaust gas increases, and the accumulated NOx emission amount in the tail pipe increases.

  On the other hand, in the combustion switching control according to the present embodiment, when the combustion switching from the lean combustion mode (A) to the stoichiometric combustion mode (C) is performed, the above-described first step and second step are simply performed. Compared with the case where the air-fuel ratio and EGR rate in the stoichiometric combustion mode (C) are set as target values and the combustion mode is switched in one step, the integrated NOx corresponding to the region surrounded by the broken line in FIG. Emissions can be reduced.

The setting of each target value in the combustion switching control according to the above-described embodiment will be described. The combustion switching control device 11 first acquires the intake oxygen concentration x _i0 in the lean combustion mode, and the intake oxygen concentration x _{i1 that} is the target value for the first step is set based on the intake oxygen concentration x _i0 . The intake oxygen concentration x _{i, O2} can be obtained from the following equation, for example.

In formula (1), (A / F) ₀ is the air-fuel ratio in the lean combustion mode, (A / F) _stoich is the stoichiometric ratio (14.6), egr ₀ is the EGR rate in the lean combustion mode, η _comb represents the combustion efficiency. Here, the expression (1) is derived as follows. First, the oxygen mole fraction xe, o2 of the engine output gas is obtained from the following equation (2).

Where W _a is the average molecular weight of the air, We is the average molecular weight of the recycled exhaust gas, ma is the mass of air in the cylinder 14, me is the mass of the recycled exhaust gas in the cylinder 14, mf represents the mass of the injected fuel, respectively. When the above formula (2) is arranged with Wa = We, the following formula (3) is obtained.

Substituting ma / mf = A / F into the above equation yields the following equation (4).

The oxygen mole fraction xi, o2 of the intake air to the engine 12 can be obtained from the following equation (5).

  In the above equation (5), by setting Wa = We, substituting egr = me / (ma + me) and equation (4), and further setting the volume ratio of oxygen in the atmosphere to 21%, the above equation (1) Is obtained.

The air-fuel ratio A / F ₀ in the equation (1) is acquired by the combustion switching control device 11 using, for example, a known air-fuel ratio sensor or an estimated air-fuel ratio estimated by a known method. For example, the air-fuel ratio sensor is provided in a portion of the first exhaust passage 32 where the exhaust gas discharged from the exhaust port of each cylinder 14 of the engine 12 collects, and a signal corresponding to the detected oxygen concentration of the exhaust gas is switched to combustion. It transmits to the control apparatus 11. As the EGR rate (egr ₀ ) in the lean combustion mode, for example, a value that is set in the combustion switching control device 11 and stored in the storage unit in the lean combustion mode can be used. Alternatively, based on values obtained from various sensors such as a pressure sensor, an oxygen concentration sensor, and a flow meter provided in the first intake passage 22, the second intake passage 24, the first exhaust passage 32, and the exhaust recirculation passage 52, The EGR rate (egr ₀ ) may be estimated by the combustion switching control device 11. Further, the combustion efficiency η _comb is acquired by, for example, an estimation formula or map value from atmospheric conditions, engine rotation, and load.

Based on the intake oxygen concentration x _i0 in the lean combustion mode, the combustion switching control device 11 sets the intake oxygen concentration x _i1 that is the target value of the first step. As shown in FIGS. 4D and 4E, when the NOx concentration of the engine exhaust gas before and after the first step is maintained to be equal, the intake oxygen concentration x _{i1 is set} to the acquired intake oxygen concentration in the lean combustion mode. Set to the same value as x _i0 . Further, when it is desired to further reduce the NOx concentration of the engine exhaust gas in the first step, the intake oxygen concentration x _i1 is set to a value lower than the intake oxygen concentration x _i0 . For example, the combustion switching control device 11 stores the ratio (x _i1 / x _i0 ) of the intake oxygen concentration before and after the first step in the storage unit in advance, and executes the combustion switching control according to the present embodiment. _Is read from the storage unit and the inspiratory oxygen concentration x _i1 is set.

The combustion switching control device 11 calculates and sets the first target EGR rate (egr ₁ ) and the first target boost pressure (P ₁ ) of the first step based on the set intake oxygen concentration x _i1 . For example, when the intake oxygen concentration x _i1 is set equal to the intake oxygen concentration x _i0 , the first target EGR rate (egr ₁ ) and the first target boost pressure (P ₁ ) are expressed by the following equations (6) and (7). Desired.

In the formula (7), T represents the temperature in the cylinder 14 at the time of IVC, and m _f represents the fuel injection amount. T0 and T1 can be estimated, for example, by referring to a predetermined map stored in the storage unit based on a value acquired from a temperature sensor (not shown) provided in the first exhaust passage 32. Further, the value stored in the storage unit may be used as the fuel injection amount m _f0 in the lean combustion mode. The fuel injection amount m _f1 after the first step is stored in, for example, a storage unit so that the operating conditions of the engine 12 such as the target torque Tq and the target engine speed Ne input from a host controller (not shown) are realized. It is determined by the combustion switching control device 11 with reference to the predetermined map.

The combustion switching control device 11 further sets a second target EGR rate (egr ₂ ) in the second step. The second target EGR rate is preferably 0%. When the second target EGR rate is 0%, the generation of smoke in the engine exhaust gas is suppressed, and supercharging in the stoichiometric combustion mode is not required, so the turbocharger 60 and the like are downsized. Can do. On the other hand, for example, when the engine is operated under high-speed conditions without supercharging in the stoichiometric combustion mode, the durability of the exhaust system 30 may be reduced due to the temperature rise of the engine exhaust gas. In such a case, the second target EGR rate is set to a value larger than 0% based on the determination as to whether the temperature of the engine exhaust gas or the NOx reduction catalyst temperature Tg is equal to or higher than a predetermined value. As a result, the temperature of the engine exhaust gas is lowered and the durability of the exhaust system 30 is improved.

The combustion switching control device 11 calculates and sets the second target boost pressure (P ₂ ) in the second step based on the set second target EGR rate (egr ₂ ). The second target supercharging pressure (P ₂ ) is obtained by the following formula (8).

T2 in the equation (8) is estimated by referring to a predetermined map stored in the storage unit based on a value acquired from a temperature sensor (not shown) provided in the first exhaust passage 32, for example. Can do. The fuel injection amount mf2 after the second step is stored in, for example, a storage unit so that the operating conditions of the engine 12 such as the target torque Tq and the target engine speed Ne input from a host controller (not shown) are realized. It is determined by the combustion switching control device 11 with reference to the predetermined map. Further, when the second target EGR rate (egr ₂ ) is 0%, the second target supercharging pressure (P ₂ ) is obtained by the following formula (9).

  Next, specific control of the internal combustion engine system 10 of the present embodiment will be described with reference to FIG. FIG. 5 is a flowchart showing an example of processing executed in the combustion switching control device 11 of the internal combustion engine system 10 shown in FIG.

  The process shown in FIG. 5 is executed when the combustion switching control device 11 determines that the start condition of the combustion switching control for switching from the lean combustion mode to the stoichiometric combustion mode is satisfied. For example, it may be determined that the engine 12 is operating in the lean combustion mode based on the number and amount of fuel injections, the opening degree of the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54, and the like. it can. Further, for example, when the NOx reduction catalyst temperature Tg of the NOx reduction catalyst 40 acquired by the temperature sensor 41 is a high temperature equal to or higher than a predetermined value, the start condition for the combustion switching control for switching from the lean combustion mode to the stoichiometric combustion mode is satisfied. Can be determined. On the other hand, when it is not determined that the engine is operating in the lean combustion mode, or when the NOx reduction catalyst temperature Tg does not exceed the predetermined value, it can be determined that the start condition for the combustion switching control is not satisfied.

As shown in FIG. 5, the combustion switching control device 11 first acquires the intake oxygen concentration x _i0 in the current lean combustion mode in step S1. The intake oxygen concentration x _i0 can be calculated by the above equation (1) based on the acquired air-fuel ratio (A / F) ₀ and EGR rate egr ₀ in the lean combustion mode.

In step S2, the combustion switching control device 11 sets the intake oxygen concentration x _i1 that is the target value of the first step, based on the intake oxygen concentration x _i0 obtained in step S1. For example, by setting the intake oxygen concentration x _i1 to a value equal to or lower than the intake oxygen concentration x _i0 , the NOx concentration of the engine exhaust gas in the first step can be maintained or reduced.

In step S3, the combustion switching control device 11 calculates the first target EGR rate (egr ₁ ) and the first target boost pressure (P ₁ ) based on the intake oxygen concentration x _i1 obtained in step S2. Set. The first target EGR rate and the first target supercharging pressure can be calculated by the above formulas (6) and (7).

  In step S4, the combustion switching control device 11 starts the first step, and transmits an opening degree adjustment signal to the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54. Reduce supercharging pressure. In step S4, when adjusting the EGR rate and the supercharging pressure, for example, a pressure sensor or an oxygen concentration sensor provided in the first intake passage 22, the second intake passage 24, the first exhaust passage 32, the exhaust recirculation passage 52, or the like. The EGR rate or the supercharging pressure may be corrected based on values acquired from various sensors such as a flow meter.

  In step S5, the combustion switching control device 11 determines whether or not the first step is completed. Specifically, the combustion switching control device 11 estimates the EGR rate and the supercharging pressure, or calculates based on the values acquired from the various sensors described above. A comparison is made between the one target EGR rate and the first target supercharging pressure. As a result, if it is determined that the EGR rate is greater than the first target EGR rate or the supercharging pressure is greater than the first target supercharging pressure and the first step is not completed (NO), step The process of S4 is repeated. On the other hand, if it is determined that the EGR rate is equal to the first target EGR rate, the supercharging pressure is equal to the first target supercharging pressure, and the first step is completed (YES), the process proceeds to step S6. move on.

In step S6, the combustion switching control device 11 sets a second target EGR rate (egr ₂ ) and a second target boost pressure (P ₂ ). For example, when the second target EGR rate is set to 0%, the second target supercharging pressure can be calculated by the above equation (9). In step S6, when the engine exhaust gas temperature or the NOx reduction catalyst temperature Tg is equal to or higher than a predetermined value, the second target EGR rate may be set to a value larger than 0%. The supply pressure (P ₂ ) can be calculated by the above equation (8).

  In step S7, the combustion switching control device 11 starts the second step, and transmits an opening degree adjustment signal to the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54, whereby the EGR rate and Reduce supercharging pressure. In step S7, the EGR rate and the supercharging pressure may be corrected as in step S4.

  In step S8, the combustion switching control device 11 determines whether or not the second step has been completed. Specifically, the combustion switching control device 11 estimates the EGR rate and the supercharging pressure, or calculates based on the values acquired from the various sensors described above. Comparison between the 2 target EGR rate and the second target supercharging pressure is performed. As a result, if it is determined that the EGR rate is greater than the second target EGR rate or the boost pressure is greater than the second target boost pressure and the second step is not completed (NO), step The process of S7 is repeated. On the other hand, if it is determined that the EGR rate is equal to the second target EGR rate, the boost pressure is equal to the second target boost pressure, and the second step is completed (YES), the combustion switching control is performed. Then, the engine 12 is controlled so as to be operated in the stoichiometric combustion mode.

  As described above, in the combustion switching control in the internal combustion engine system 10 of the present embodiment, the EGR is performed by the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54 at the time of combustion switching from the lean combustion mode to the stoichiometric combustion mode. A first step of controlling the air-fuel ratio from the lean combustion condition to the stoichiometric combustion condition so that the EGR rate becomes the first target EGR rate by performing control for adjusting the rate and the supercharging pressure, and after the first step, By performing the second step of further reducing the EGR rate while maintaining the stoichiometric combustion conditions, it is possible to reduce the NOx concentration of the engine output gas during the transition period for switching the combustion mode, and as a result, the tail during the transition period The accumulated NOx emission amount in the pipe can be reduced.

Second Embodiment
Next, the combustion switching control of the internal combustion engine system 10 of the second embodiment will be described with reference to FIGS. In addition, since the internal combustion engine system 10 of 2nd Embodiment is the same as that of 1st Embodiment mentioned above, it abbreviate | omits about the description of the structure.

In the present embodiment, the combustion switching control from the stoichiometric combustion mode to the lean combustion mode is performed in the exhaust gas recirculation rate switching step (hereinafter referred to as “reducing the exhaust gas recirculation rate” to a predetermined first target value while maintaining the stoichiometric combustion conditions). (Described as “third step”) and an air-fuel ratio switching step (hereinafter referred to as “fourth step”) that increases the exhaust gas recirculation rate to a predetermined second target value and switches the air-fuel ratio from lean combustion conditions to stoichiometric combustion conditions. 2 steps). Specifically, a relay point having a predetermined EGR rate (egr ₃ ) (first target value) is set, and the air-fuel ratio under stoichiometric combustion conditions is maintained in the third step by controlling the EGR rate and the supercharging pressure. While switching from the stoichiometric combustion mode to the relay point, in the subsequent fourth step, the relay point is switched to the lean combustion mode having a predetermined EGR rate (egr ₄ ) (second target value) and supercharging pressure (P ₄ ). . That is, the combustion switching control from the stoichiometric combustion mode to the lean combustion mode according to the present embodiment reaches the stoichiometric combustion mode (C) through the relay point (B) from the lean combustion mode (A) of the first embodiment ( This corresponds to a case where the start point and end point of the combustion switching control configured in the first step and the second step are reversed.

  The operation of the combustion switching control according to the present embodiment will be described in detail with reference to FIG. The parameters indicated by the horizontal and vertical axes in each graph of FIG. 6 are the same as those in FIG. The solid line in each graph of FIG. 6 has shown the change by the combustion switching control which concerns on this embodiment, and in the transition period of the time t0-t2, the 3rd process (t0-t1) and the 4th process (t1-t2). ) Is executed, and the combustion mode is switched from the stoichiometric combustion mode before t0 to the lean combustion mode after t2.

When the combustion switching control according to the present embodiment is started (time t0), the third step of increasing the EGR rate while maintaining the air-fuel ratio at the stoichiometric combustion condition (A / F) _stoich is entered. In the third step, the EGR rate increases to the third target EGR rate (egr ₃ ), and the supercharging pressure increases to the third target supercharging pressure (P ₃ ). In the third step, when the EGR rate and the supercharging pressure increase to reach each target value, the third step ends (time t1). By this third step, as shown in FIG. 6D, the intake oxygen concentration decreases and reaches the intake oxygen concentration xi0 in the lean combustion mode. Further, during the third step from time t0 to t1, the air-fuel ratio becomes a value close to the stoichiometric ratio as shown in FIG. NOx is purified. Therefore, the NOx emission amount in the tail pipe remains reduced by the purification function of the three-way catalyst 38 (FIG. 6 (g)).

Subsequently, while maintaining the intake oxygen concentration, the EGR rate and the boost pressure are increased, and the air-fuel ratio is increased from the stoichiometric combustion condition ((A / F) _stoich ) to the lean combustion condition ((A / F) ₄ ). The fourth step is performed. As described above, at the start of the fourth step (t2), the intake oxygen concentration is reduced to the lean combustion mode (x _i4 ). As shown in FIG. 6D, in the fourth step, while maintaining the intake oxygen concentration, the EGR rate and the supercharging pressure are increased toward the fourth target EGR rate and the fourth target supercharging pressure, which are the final target values. To increase. As shown in FIGS. 6F and 6G, since the air-fuel ratio becomes larger than the stoichiometric ratio immediately after the start of the fourth step, the purification function of the three-way catalyst 38 does not work, and the tail pipe The NOx concentration of the exhaust gas increases at. However, since the intake oxygen concentration x _i0 has already decreased to the lean combustion mode at the end of the third step, the NOx concentration of the engine exhaust gas in the second step is kept low, so that the NOx concentration in the tail pipe is reduced. Increase can be suppressed. Then, when the EGR rate and the supercharging pressure reach the final target value, the fourth step ends (time t2), and the engine 12 is operated in the lean combustion mode.

On the other hand, the broken line in each graph of FIG. 6 does not perform the combustion switching control composed of two steps as in the present embodiment, but simply executes the control for switching from the stoichiometric combustion mode to the lean combustion mode. This shows a change in parameters (corresponding to control in the reverse direction of the broken line in FIG. 2). In this example, at the start of the transition period (t0), the target values of the air-fuel ratio and the EGR rate are set to the air-fuel ratio ((A / F) ₄ ) and the EGR rate (egr ₄ ) in the lean combustion mode, and the EGR rate And the combustion switching control which increases a supercharging pressure is performed. Then, as shown in FIGS. 6 (e) to 6 (g), the purification function of the three-way catalyst 38 does not work immediately after the start of the control, so that the engine exhaust gas having a high NOx concentration on the lean side from the stoichiometric combustion condition remains as it is. Discharged from the tailpipe. As the time elapses, the EGR rate and the supercharging pressure increase, and the NOx concentration of the engine exhaust gas and the exhaust gas in the tail pipe decreases. In this way, the amount of accumulated NOx discharged from the tail pipe increases as the exhaust gas having a high NOx concentration is discharged from the tail pipe immediately after the start of control.

  On the other hand, in the combustion switching control according to the present embodiment, when performing the combustion switching from the stoichiometric combustion mode to the lean combustion mode, the air-fuel ratio in the lean combustion mode is simply performed by performing the third step and the fourth step described above. And compared with the case where the EGR rate is set as a target value and the combustion mode is switched in one step, the integrated NOx emission amount corresponding to the region surrounded by the broken line in FIG. 6G can be reduced.

  Next, the combustion switching control of this embodiment will be described with reference to FIG. FIG. 7 is a flowchart showing another example of processing executed in the combustion switching control device 11 of the internal combustion engine system 10 shown in FIG.

  The process shown in FIG. 7 is executed when the combustion switching control device 11 determines that the start condition of the combustion switching control for switching from the stoichiometric combustion mode to the lean combustion mode is satisfied. For example, it may be determined that the engine 12 is operating in the stoichiometric combustion mode based on the number and amount of fuel injections, the opening degree of the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54, and the like. it can. Further, for example, when the NOx reduction catalyst temperature Tg of the NOx reduction catalyst 40 acquired by the temperature sensor 41 is a low temperature equal to or lower than a predetermined value, it can be determined that the combustion switching control start condition is satisfied. On the other hand, when it is not determined that the engine is operating in the stoichiometric combustion mode, or when the NOx reduction catalyst temperature Tg exceeds a predetermined value, it can be determined that the start condition of the combustion switching control is not satisfied.

As shown in FIG. 7, the combustion switching control device 11 first, in step S11, the fourth target air-fuel ratio ((A / F) ₄ ) and the fourth target EGR rate (egr ₄ ) in the lean combustion mode that is the final target value. ) And the fourth target boost pressure (P ₄ ). These target values are acquired with reference to, for example, a map stored in the storage unit.

In step S12, the combustion switching control device 11 acquires the fourth target intake oxygen concentration (x _i4 ) that is the intake oxygen concentration in the lean combustion mode. The fourth target intake oxygen concentration can be calculated with reference to the above equation (1) based on the fourth target air-fuel ratio and the fourth target EGR rate acquired in step S11.

In step S13, the combustion switching control device 11 sets a third target intake oxygen concentration (x _i3 ) as a target value in the third step based on the intake oxygen concentration obtained in step S12. For example, by setting the intake oxygen concentration (x _i3 ) to a value equal to or lower than the intake oxygen concentration (x _i4 ), the NOx concentration of the engine exhaust gas in the third step can be maintained or reduced.

In step S14, the combustion switching control device 11 calculates the third target EGR rate (egr ₃ ) and the third target boost pressure (P ₃ ) based on the intake oxygen concentration x _i1 obtained in step S13. Set. The third target EGR rate and the third target supercharging pressure can be calculated by the above formulas (6) and (7).

  In step S15, the combustion switching control device 11 starts the third step, and transmits an opening degree adjustment signal to the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54, whereby the EGR rate and Control to increase the boost pressure. In step S15, when adjusting the EGR rate and the supercharging pressure, for example, pressure sensors, oxygen concentration sensors provided in the first intake passage 22, the second intake passage 24, the first exhaust passage 32, the exhaust recirculation passage 52, and the like. The EGR rate or the supercharging pressure may be corrected based on values acquired from various sensors such as a flow meter.

  In step S16, the combustion switching control device 11 determines whether or not the third step has been completed. Specifically, the combustion switching control device 11 estimates the EGR rate and the supercharging pressure, or calculates based on the values acquired from the various sensors described above. A comparison is made between the 3 target EGR rate and the 3rd target boost pressure. As a result, if it is determined that the EGR rate is smaller than the third target EGR rate or the supercharging pressure is smaller than the third target supercharging pressure and the third step is not completed (NO), step The process of S15 is repeated. On the other hand, if it is determined that the EGR rate is equal to the third target EGR rate, the boost pressure is equal to the third target boost pressure, and the third step is completed (YES), the process proceeds to step S17. move on.

  In step S17, the combustion switching control device 11 starts the fourth step. In the fourth step, the EGR rate and the supercharging pressure are acquired in step S11 by transmitting an opening degree adjustment signal to the intake throttle valve 26, the wastegate valve 42, and the exhaust gas recirculation amount adjustment valve 54. Control to increase to the target air-fuel ratio and the fourth target EGR rate is performed. In step S17, the EGR rate and the supercharging pressure may be corrected similarly to step S14.

  In step S18, the combustion switching control device 11 determines whether or not the fourth step has been completed. Specifically, the combustion switching control device 11 estimates the EGR rate and the supercharging pressure, or calculates based on the values acquired from the various sensors described above. A comparison is made between the 4 target EGR rate and the 4th target boost pressure. As a result, if it is determined that the EGR rate is smaller than the fourth target EGR rate, or the supercharging pressure is smaller than the fourth target supercharging pressure and the second step is not completed (NO), step The process of S17 is repeated. On the other hand, if it is determined that the EGR rate is equal to the fourth target EGR rate, the boost pressure is equal to the fourth target boost pressure, and the fourth step is completed (YES), the combustion switching control is performed. Then, the engine 12 is controlled so as to be operated in the lean combustion mode.

  As described above, the present embodiment can also provide the operational effects corresponding to the first embodiment. That is, when the combustion is switched from the stoichiometric combustion mode to the lean combustion mode, the control for adjusting the EGR rate and the supercharging pressure is executed by the intake throttle valve 26, the waste gate valve 42 and the exhaust gas recirculation amount adjusting valve 54, and the stoichiometric combustion is performed. A third step of increasing the EGR rate while maintaining the conditions, and a fourth step of switching the air-fuel ratio from the stoichiometric combustion condition to the lean combustion condition so that the EGR rate becomes the fourth target EGR rate after the third step, By performing the above, it is possible to reduce the NOx concentration of the engine exhaust gas during the transition period for switching the combustion mode, and consequently reduce the accumulated NOx emission amount in the tail pipe during the transition period.

  The internal combustion engine system according to the present invention is not limited to the above-described embodiment, and various modifications and improvements can be made within the scope of matters described in the claims of the present application and the equivalent scope thereof. .

  For example, in the above description, an example in which the turbocharger 60 is used as the supercharging device has been described. However, the supercharging device is not limited to this, and a mechanical type that performs supercharging operation by engine power instead of the turbocharger 60 is described. A supercharger may be used, or an electric compressor that assists supercharging by the turbocharger 60 may be used. Further, a supercharging device selected from a turbocharger, a mechanical supercharger, and an electric compressor may be provided in a plurality of stages. Furthermore, in addition to the turbocharger, the mechanical supercharger, and the electric compressor, a pressure accumulation tank for storing compressed air may be provided, and the compressed air supplied from the pressure accumulation tank may be used for supercharging. Supercharging by these supercharging devices is performed in response to a command from the combustion switching control device 11.

  Further, the turbocharger 60 of the internal combustion engine system may have a turbine with a variable nozzle vane that varies the exhaust gas flow velocity. By making the exhaust gas flow velocity hitting the turbine variable with the variable nozzle vanes, the supercharging pressure by the turbocharger 60 can be adjusted. The opening adjustment of the variable nozzle vane is performed in response to a command from the combustion switching control device 11. In this configuration, in the turbocharger 60, it is possible to prevent the supercharging pressure from exceeding a specified value due to the variable nozzle vane, and therefore the turbine bypass passage 36 and the wastegate valve 42 can be omitted. In this configuration, the variable nozzle vane forms a part of the supercharging pressure adjusting device.

  Furthermore, in the first and second embodiments, when the NOx reduction catalyst is SCR, it is desirable to provide the exhaust system 30 with a device for adding a reducing agent (for example, urea) upstream thereof.

  DESCRIPTION OF SYMBOLS 10 Internal combustion engine system, 11 Combustion switching control apparatus (combustion switching control part), 12 Engine, 14 Cylinder, 16 Fuel injection apparatus, 18 Injection control apparatus (injection control part), 20 Speed sensor (rotation speed acquisition means), 21 Intake system, 22 First intake passage, 24 Second intake passage, 26 Intake throttle valve (air amount adjusting device), 30 Exhaust system, 32 First exhaust passage, 34 Second exhaust passage, 36 Turbine bypass passage (supercharging Pressure regulator), 38 Three-way catalyst, 40 NOx reduction catalyst, 41 Temperature sensor, 42 Wastegate valve (supercharging pressure regulator), 50 Exhaust gas recirculation device, 52 Exhaust gas recirculation passage, 54 Exhaust gas recirculation amount adjustment valve (Exhaust gas return) Flow control device), 60 turbocharger (supercharger), 62 compressor chamber, 63 compressor wheel, 64 turbine chamber, 65 tar Down, 66 shaft, A suction direction, E the exhaust gas discharge direction.

Claims (11)

Engine,
A three-way catalyst for purifying exhaust gas exhausted from the engine and a NOx reduction catalyst;
Temperature acquisition means for acquiring the temperature of the NOx reduction catalyst;
A rotational speed acquisition means for acquiring the rotational speed of the engine;
An injection control unit for controlling a fuel injection amount in the engine;
An exhaust gas recirculation device for recirculating part of the exhaust gas of the engine;
An exhaust gas recirculation amount adjusting device for adjusting the amount of exhaust gas recirculated to the engine;
A supercharging device for supercharging air sucked into the engine;
A supercharging pressure adjusting device for adjusting a supercharging pressure by the supercharging device;
An air amount adjusting device for adjusting the amount of air taken in, and
Based on the NOx reduction catalyst temperature acquired by the temperature acquisition means, the engine rotation speed acquired by the rotation speed acquisition means, and the fuel injection amount acquired from the injection control unit, the combustion mode of the engine is leaned. A combustion switching control unit that switches between the combustion mode and the stoichiometric combustion mode;
With
The combustion switching control unit controls the exhaust gas recirculation amount adjusting device, the supercharging pressure adjusting device, and the air amount adjusting device in a transition period for switching from the lean combustion mode to the stoichiometric combustion mode,
An air-fuel ratio switching step of switching the air-fuel ratio from the lean combustion condition to the stoichiometric combustion condition so that the exhaust gas recirculation rate becomes a predetermined first target value;
An exhaust gas recirculation rate switching step for reducing the exhaust gas recirculation rate from the first target value to a predetermined second target value while maintaining stoichiometric combustion conditions after the air-fuel ratio switching step;
Internal combustion engine system.   The first target value of the exhaust gas recirculation rate is set so that the intake oxygen concentration at the end of the air-fuel ratio switching step does not exceed the intake oxygen concentration at the start of the air-fuel ratio switch step. An internal combustion engine system according to claim 1.   The internal combustion engine system according to claim 1 or 2, wherein the second target value of the exhaust gas recirculation rate is 0%.   The combustion switching control unit sets the second target value of the exhaust gas recirculation rate to a value exceeding 0% when a catalyst temperature of the NOx reduction catalyst acquired by a temperature sensor is equal to or higher than a predetermined value. Item 4. The internal combustion engine system according to any one of Items 1 to 3. Engine,
A three-way catalyst for purifying exhaust gas exhausted from the engine and a NOx reduction catalyst;
Temperature acquisition means for acquiring the temperature of the NOx reduction catalyst;
A rotational speed acquisition means for acquiring the rotational speed of the engine;
An injection control unit for controlling a fuel injection amount in the engine;
An exhaust gas recirculation device for recirculating part of the exhaust gas of the engine;
An exhaust gas recirculation amount adjusting device for adjusting the amount of exhaust gas recirculated to the engine;
A supercharging device for supercharging air sucked into the engine;
A supercharging pressure adjusting device for adjusting a supercharging pressure by the supercharging device;
An air amount adjusting device for adjusting the amount of air taken in, and
Based on the NOx reduction catalyst temperature acquired by the temperature acquisition means, the engine rotation speed acquired by the rotation speed acquisition means, and the fuel injection amount acquired from the injection control unit, the combustion mode of the engine is leaned. A combustion switching control unit that switches between the combustion mode and the stoichiometric combustion mode;
With
The combustion switching control unit controls the exhaust gas recirculation amount adjusting device, the supercharging pressure adjusting device, and the air amount adjusting device at the time of combustion switching to switch from the stoichiometric combustion mode to the lean combustion mode,
An exhaust gas recirculation rate switching step for increasing the exhaust gas recirculation rate to a predetermined first target value while maintaining stoichiometric combustion conditions;
After the exhaust gas recirculation rate switching step, the exhaust gas recirculation rate is increased from the first target value to a predetermined second target value, and the air fuel ratio is switched from the lean combustion condition to the stoichiometric combustion condition; ,I do,
Internal combustion engine system.   The first target value of the exhaust gas recirculation rate is set so that the intake oxygen concentration at the start of the air-fuel ratio switching step does not exceed the intake oxygen concentration at the end of the air-fuel ratio switch step. An internal combustion engine system according to claim 1.   The internal combustion engine system according to any one of claims 1 to 6, wherein the supercharging device includes at least one of a turbocharger, a mechanical supercharger, and an electric compressor.   The internal combustion engine system according to any one of claims 1 to 7, wherein the exhaust gas recirculation amount adjusting device is an exhaust gas recirculation amount adjusting valve installed in an exhaust gas recirculation passage connected to an intake passage and an exhaust passage of the engine. .   The internal combustion engine system according to any one of claims 1 to 8, wherein the supercharging pressure adjusting device is a turbine bypass passage and a wastegate valve provided in an exhaust passage of the engine.   The internal combustion engine system according to any one of claims 1 to 9, wherein the air amount adjusting device is an intake throttle valve provided in an intake passage of the engine.   The internal combustion engine system according to any one of claims 1 to 10, wherein the NOx reduction catalyst is a selective reduction catalyst (SCR) and / or an occlusion reduction catalyst (NSR).