Exhaust gas purification system for internal combustion engine and exhaust gas purification method for internal combustion engine

Abstract

To control methane, which is emitted from a combustion chamber due to rich purge for releasing and reducing NOx occluded in an NSR catalyst, to an appropriate amount.SOLUTION: A methane emission allowable value ΣAMe increases as a travel distance of a vehicle increases. When an NOx occlusion amount ΣNOx of an NSR catalyst becomes equal to or more than a purge request threshold value Dc, rich purge is executed (flag Fr=1). When the rich purge starts, a methane emission amount ΣMe is calculated. When the methane emission amount ΣMe exceeds the methane emission allowable value ΣAMe, a flag Fm is set to 1 to prohibit the rich purge.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present disclosure relates to an exhaust gas purification system for an internal combustion engine and a method for purifying exhaust gas of an internal combustion engine, and more particularly to an exhaust gas purification system and an exhaust gas purification method for an internal combustion engine equipped with a NOx storage reduction catalyst.

Background Art

[0002] In order to purify NOx (nitrogen oxides) contained in exhaust gas, an internal combustion engine equipped with a NOx storage reduction catalyst is known. The NOx storage reduction catalyst (hereinafter also referred to as an NSR catalyst) stores NOx when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is lean, and releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas becomes rich.

[0003] In order to reduce the NOx stored in the NSR catalyst, when making the air-fuel ratio of the exhaust gas rich, it is known to perform so-called post-injection, in which fuel is injected into the combustion chamber separately from the main injection during the expansion stroke of the internal combustion engine. For example, in Japanese Patent Application Laid-Open No. 2004-132262 (Patent Document 1), it is said that by performing post-injection, the fuel injected into the combustion chamber is reformed, the CO concentration (carbon monoxide concentration) in the exhaust gas increases, and the NOx released from the NSR catalyst can be efficiently reduced.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In order to release and reduce the NOx stored in the NSR catalyst, when post-injection is performed to make the air-fuel ratio of the exhaust gas rich, methane (CH4) is generated during fuel reforming, and methane is discharged from the combustion chamber. Methane has a weak reducing power for NOx and is difficult to be purified by the catalyst. Therefore, when the air-fuel ratio of the exhaust gas is made rich by post-injection to release and reduce the NOx stored in the NSR catalyst, methane is discharged into the atmosphere.

[0006] Although methane is not harmful, it is a greenhouse gas, so it is desirable to control its emission amount.

[0007] The object of the present disclosure is to control the amount of methane discharged when the air-fuel ratio of the exhaust gas is made rich by fuel injection to an appropriate amount in order to release and reduce the NOx stored in the NSR catalyst.

Means for Solving the Problem

[0008] The exhaust gas purification system of the internal combustion engine of the present disclosure is an exhaust gas purification system of an internal combustion engine including a NOx storage reduction catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas is rich, a fuel injection valve that injects fuel into the combustion chamber of the internal combustion engine, and a control device. When the NOx stored in the NOx storage reduction catalyst becomes equal to or more than the purge request threshold value, the control device makes the air-fuel ratio of the exhaust gas rich by the fuel injected from the fuel injection valve into the combustion chamber, and executes a rich purge to release and reduce the NOx stored in the NOx storage reduction catalyst. When executing the rich purge, the control device calculates the methane emission amount, which is the amount of methane discharged from the combustion chamber, and when the methane emission amount exceeds the methane emission allowable value, the rich purge is prohibited.

[0009] According to this configuration, when the amount of NOx stored in the NOx storage reduction catalyst becomes equal to or greater than the purge request threshold value, the control device for the exhaust gas purification system of the internal combustion engine enriches the air-fuel ratio of the exhaust gas with the fuel injected from the fuel injection valve into the combustion chamber to perform a rich purge, and releases and reduces the NOx stored in the NOx storage reduction catalyst. When executing the rich purge, the control device calculates the methane emission amount, which is the amount of methane discharged from the combustion chamber, and prohibits the rich purge when the methane emission amount exceeds the methane emission allowable value. When the methane emission amount exceeds the methane emission allowable value, the rich purge is prohibited, so the methane discharged into the atmosphere during the execution of the rich purge can be controlled to an appropriate amount.

[0010] Preferably, the internal combustion engine is mounted on a vehicle, and the control device may calculate the methane emission allowable value based on the driving distance of the vehicle. Also, when the methane emission amount exceeds the methane emission allowable value, the methane emission amount and the methane emission allowable value may be set to initial values.

[0011] According to this configuration, since the methane emission allowable value is calculated based on the driving distance of the vehicle, it becomes possible to appropriately manage the amount of methane discharged into the atmosphere in the same way as the regulation of the amount of carbon dioxide (CO2) discharged in the exhaust gas. Also, when the methane emission amount exceeds the methane emission allowable value, since the methane emission amount and the methane emission allowable value are set to initial values (which may be, for example, "0"), an overflow of the register that holds the calculated methane emission amount and the methane emission allowable value can be suppressed.

[0012] Preferably, a selective reduction type NOx catalyst is provided in the exhaust passage downstream of the NOx storage reduction catalyst, and when the rich purge is prohibited because the methane emission amount exceeds the methane emission allowable value during the execution of the rich purge, the control device may increase the temperature of the NOx storage reduction catalyst and perform a temperature increase purge to release the NOx stored in the NOx storage reduction catalyst.

[0013] According to this configuration, a selective reduction type NOx catalyst is provided in the exhaust passage downstream of the NOx storage reduction catalyst. When the rich purge is prohibited, the control device executes a temperature increase purge to raise the temperature of the NOx storage reduction catalyst and release the NOx stored in the NOx storage reduction catalyst. The NOx released from the NOx storage reduction catalyst by the temperature increase purge can be reduced by the selective reduction type NOx catalyst, and the emission of NOx into the atmosphere can be suppressed.

[0014] Preferably, when the NOx stored in the NOx storage reduction catalyst becomes equal to or less than the purge end threshold value, the control device may terminate the execution of the rich purge and the temperature increase purge.

[0015] According to this configuration, since the rich purge or the temperature increase purge is executed until the NOx stored in the NOx catalyst becomes equal to or less than the purge end threshold value, NOx can be sufficiently released from the NOx storage reduction catalyst, and the recovery of the NOx storage capacity of the NOx storage reduction catalyst is favorably performed.

[0016] The exhaust gas purification method of the internal combustion engine of the present disclosure is an exhaust gas purification method of an internal combustion engine including a NOx storage reduction catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas is rich, and a fuel injection valve that injects fuel into the combustion chamber of the internal combustion engine. The exhaust gas purification method of the internal combustion engine includes a step of making the air-fuel ratio of the exhaust gas rich with the fuel injected from the fuel injection valve into the combustion chamber and executing a rich purge to release and reduce NOx when the NOx stored in the NOx storage reduction catalyst becomes equal to or greater than the purge request threshold value, and a step of calculating the methane emission amount, which is the amount of methane discharged from the combustion chamber during the execution of the rich purge, and prohibiting the rich purge when the methane emission amount exceeds the methane emission allowable value.

[0017] According to this exhaust gas purification method, when the NOx stored in the NOx storage reduction catalyst reaches or exceeds the purge request threshold value, in order to release and reduce the NOx stored in the NOx storage reduction catalyst, the air-fuel ratio of the exhaust gas is made rich by the fuel injected from the fuel injection valve into the combustion chamber, and a rich purge is executed. When executing the rich purge, the methane emission amount, which is the amount of methane discharged from the combustion chamber, is calculated, and when the methane emission amount exceeds the methane emission allowable value, the rich purge is prohibited. Therefore, when the methane emission amount exceeds the methane emission allowable value, the rich purge is prohibited, so that the amount of methane discharged into the atmosphere can be controlled to an appropriate amount.

[0018] Preferably, the internal combustion engine is mounted on a vehicle, and the exhaust gas purification method may further include a step of calculating the methane emission allowable value based on the driving distance of the vehicle.

[0019] According to this exhaust gas purification method, since the methane emission allowable value is calculated based on the driving distance of the vehicle, it becomes possible to appropriately manage the amount of methane discharged into the atmosphere in the same way as the regulation of the amount of carbon dioxide (CO2) discharged in the methane exhaust gas.

[0020] Preferably, the internal combustion engine further includes a selective reduction type NOx catalyst in the exhaust passage downstream of the NOx storage reduction catalyst, and the exhaust gas purification method may further include a step of executing a temperature increase purge for raising the temperature of the NOx storage reduction catalyst to release NOx when the rich purge is prohibited in the step of prohibiting the rich purge.

[0021] According to this exhaust gas purification method, when the rich purge is prohibited, a temperature increase purge is executed to raise the temperature of the NOx storage reduction catalyst and release the NOx stored in the NOx storage reduction catalyst. The NOx released from the NOx storage reduction catalyst by the temperature increase purge can be reduced by the selective reduction type NOx catalyst, and the release of NOx into the atmosphere can be suppressed.

Effects of the Invention

[0022] According to the present disclosure, in order to release and reduce NOx stored in the NSR catalyst, methane discharged when the air-fuel ratio of the exhaust gas is made rich by fuel injection into the combustion chamber can be controlled to an appropriate amount.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following description, the same or corresponding parts in the drawings may be denoted by the same reference numerals and their description may not be repeated.

[0025] FIG. 1 is a diagram showing the overall configuration of a vehicle according to the present embodiment. The vehicle V includes an engine 1, a torque converter 2, and an automatic transmission 3.

[0026] In the present embodiment, the engine 1 is a compression ignition internal combustion engine (diesel engine) equipped with an exhaust gas purification system, and the output shaft of the engine 1 is connected to the input shaft of the torque converter 2. The torque converter 2 is a torque converter with a lock-up clutch, and includes a pump impeller, a turbine runner, a stator, and a lock-up clutch (not shown). The output shaft of the torque converter 2 is connected to the input shaft of the automatic transmission 3. The automatic transmission 3 is a planetary gear type multi-stage automatic transmission, and achieves each gear stage by controlling the combination of engagement and release of a plurality of friction engagement elements. The output shaft of the automatic transmission 3 is connected to the differential gear 4 via a propeller shaft. The differential gear 4 is connected to the rear wheels 5, which are drive wheels, via a drive shaft. The vehicle V is a rear-wheel drive vehicle, but may be a front-wheel drive vehicle or an all-wheel drive vehicle.

[0027] In FIG. 1, reference numeral 111 denotes a vehicle speed sensor that detects the vehicle speed SPD of the vehicle V. The engine 1 is controlled by an engine ECU ((Electronic Control Unit) 100, and the torque converter 2 and the automatic transmission 3 are controlled by a transmission ECU 200.

[0028] FIG. 2 is a schematic configuration diagram of the engine 1. The engine 1 is a compression ignition internal combustion engine (diesel engine) equipped with an exhaust gas purification system. Fuel is injected from a fuel injection valve (injector) 14 into the combustion chamber formed in the cylinder 12 of the engine body 10, and it is an internal combustion engine that performs compression self-ignition. In the present embodiment, the engine 1 has four cylinders. An air cleaner 22, an intercooler 24, and a throttle valve (diesel throttle valve) 26 are provided in the intake passage 20 of the engine 1. The fresh air (air) from which foreign substances have been removed by the air cleaner 22 is supercharged (compressed) by the compressor 32 of the turbocharger 30, cooled by the intercooler 24, supplied to the intake manifold 28, and supplied from the intake port to each combustion chamber.

[0029] Fuel is stored in the fuel tank 40. The fuel in the fuel tank 40 is supplied to the high-pressure fuel pump 42 by the feed pump 41, and the high-pressure fuel discharged from the high-pressure fuel pump 42 is pumped into the common rail 44 through the fuel passage 43. The high-pressure fuel stored in the common rail 44 is injected into the combustion chamber (inside the cylinder) from the injector 14.

[0030] The exhaust gas discharged from the combustion chamber is collected in the exhaust manifold 50 and released to the outside air through the exhaust passage 52. Also, a part of the exhaust gas is recirculated to the intake manifold 28 through the EGR (Exhaust Gas Recirculation) passage 60. The EGR passage 60 is provided with an EGR cooler 62 and an EGR valve 64.

[0031] In the exhaust passage 52, from the upstream side, there are provided a turbine 34 of the turbocharger 30, a NOx storage reduction catalyst (NSR catalyst) 70, a DPF (Diesel Particulate Filter) integrated selective reduction catalyst (hereinafter also referred to as SCR (Selective Catalytic Reduction) DPF) 72, and a selective reduction catalyst (hereinafter also referred to as SCR catalyst) 74.

[0032] The NSR catalyst 70 is a catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean (oxygen excess) and releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas becomes rich (reducing atmosphere). For example, it may be a carrier made of cordierite supporting barium (Ba) as a NOx storage component and a platinum group metal as a catalyst component. Note that the NSR catalyst 70 may also be a NOx storage reduction type three-way catalyst obtained by adding an alkaline substance as a NOx storage material to a three-way catalyst.

[0033] The SCRDPF72 is a catalyst that integrates a filter function of collecting particulate matter (PM) in exhaust gas and purifying it by appropriately burning and removing the collected PM, and a function of reducing and purifying NOx in exhaust gas. For example, it may be a honeycomb filter made of cordierite or silicon carbide, with copper (Cu) ion-exchanged zeolite supported as a catalyst. The SCR catalyst 74 is a catalyst for reducing and purifying NOx in exhaust gas. For example, it may be a carrier made of cordierite, with copper (Cu) ion-exchanged zeolite supported as a catalyst.

[0034] The SCRDPF72 and the SCR catalyst 74 exhibit a high NOx purification rate by using ammonia (NH3) as a reducing agent. The ammonia used as a reducing agent is generated by hydrolyzing the urea water supplied to the exhaust passage 52 upstream of the SCRDPF72 and upstream of the SCR catalyst 74. A urea addition valve (urea water injection injector) 82 is provided in the exhaust passage 52 upstream of the SCRDPF72, and a urea addition valve 83 is provided in the exhaust passage 52 upstream of the SCR catalyst 74. When reducing and purifying the NOx flowing into the SCRDPF72 and the SCR catalyst 74, the urea water pumped from the urea water tank 80 by a pump (not shown) is injected into the exhaust passage 52 from the urea addition valve 82 and the urea addition valve 83.

[0035] A fuel addition valve 90 is provided in the exhaust passage 52 upstream of the NSR catalyst 70 (in this embodiment, the exhaust passage 52 upstream of the turbine 34). Fuel in the fuel tank 40 is supplied to the fuel addition valve 90 through the fuel passage 45 by the feed pump 41. When the fuel addition valve 90 opens, fuel is added (injected) into the exhaust passage 52.

[0036] When the amount of PM deposited on the SCRDPF72 reaches or exceeds a predetermined value, fuel is added (injected) from the fuel addition valve 90 into the exhaust passage 52 to raise the temperature of the SCRDPF72 and burn and remove the deposited PM. When fuel is added from the fuel addition valve 90, the catalyst component of the NSR catalyst 70 functions as an oxidation catalyst, the fuel generates heat (burns), the temperature of the exhaust gas flowing into the SCRDPF72 rises, and the temperature of the SCRDPF72 increases.

[0037] The engine ECU 100 includes a CPU (Central Processing Unit) 101, a memory 102 composed of a ROM (Read Only Memory) and a RAM (Random Access Memory), an input / output port (not shown) for inputting and outputting various signals, etc. It executes predetermined arithmetic processing based on the information stored in the memory 102 and the information from various sensors, and controls the injector 14, the throttle valve 26, the high-pressure fuel pump 42, the urea addition valves 82, 83, the fuel addition valve 90, etc. The engine ECU 100 corresponds to the "control device" of the present disclosure.

[0038] Various sensors input to the engine ECU 100 include, for example, a vehicle speed sensor 111, an engine rotation speed sensor 112, an accelerator pedal sensor 113, an air flow meter 114, an NSR catalyst temperature sensor 115, an SCRDPF temperature sensor 116, etc. The engine rotation speed sensor 112 detects the rotation speed NE of the engine 1. The accelerator pedal sensor 113 detects the accelerator pedal operation amount (hereinafter also referred to as "accelerator opening") AP by the user. The air flow meter 114 detects the intake air amount (inhaled air amount) Ga of the engine 1. The NSR catalyst temperature sensor 115 detects the temperature Tc of the NSR catalyst 70. The SCRDPF temperature sensor 116 detects the temperature Tf of the SCRDPF72.

[0039] Engine 1 is a diesel engine and is operated with an air excess ratio of 1.5 to 2 even at full load, and the air-fuel ratio of the exhaust gas is lean. Therefore, in the present embodiment, an NSR catalyst 70 is provided in the exhaust passage 52, and when the air-fuel ratio of the exhaust gas is lean, the NSR catalyst 70 stores NOx and suppresses the emission of NOx into the atmosphere. Since there is a limit to the NOx storage amount of the NSR catalyst 70, when the amount of NOx stored in the NSR catalyst 70 exceeds the allowable value (storage capacity) of the NOx storage amount, the NOx in the exhaust gas flows out downstream of the NSR catalyst 70 without being stored in the NSR catalyst 70. For this reason, before the stored amount of NOx exceeds the allowable value, the air-fuel ratio of the exhaust gas is made rich (reducing atmosphere) to release and reduce the NOx stored in the NSR catalyst 70, and the NOx storage capacity of the NSR catalyst 70 is restored. Note that making the air-fuel ratio of the exhaust gas rich to release and reduce the NOx stored in the NSR catalyst 70 is also referred to as rich purge.

[0040] Also, in the NSR catalyst 70, when the temperature Tc of the NSR catalyst 70 increases, the NOx storage capacity decreases, and the allowable value of the NOx storage amount decreases. For this reason, (without making the air-fuel ratio of the exhaust gas rich,) the temperature Tc of the NSR catalyst 70 can be increased to decrease the allowable value of the NOx storage amount and release the NOx stored in the NSR catalyst 70. Hereinafter, increasing the temperature Tc of the NSR catalyst 70 to release the NOx stored in the NSR catalyst 70 is also referred to as temperature increase purge.

[0041] The SCRDPF 72 reduces and purifies the NOx discharged from the NSR catalyst 70 by temperature rise purge. An NOx sensor 117 is provided in the exhaust passage 52 on the inlet side (upstream side) of the SCRDPF 72. The NOx sensor 117 detects the NOx concentration of the exhaust gas flowing into the SCRDPF 72. Based on the NOx concentration of the exhaust gas flowing into the SCRDPF 72 and the exhaust gas amount (intake air amount Ga), the engine ECU 100 calculates the addition amount of the urea water to be added from the urea addition valve 82, and adds urea from the urea addition valve 82 to reduce and purify the NOx that has flowed into the SCRDPF 72. In this way, (regardless of the NOx discharged from the NSR catalyst 70 by temperature rise purge) the NOx flowing from the NSR catalyst 70 into the SCRDPF 72 is reduced and purified in the SCRDPF 72 using the urea water added from the urea addition valve 82.

[0042] An NOx sensor 118 is provided in the exhaust passage 52 on the inlet side (upstream side) of the SCR catalyst 74. Similarly, the NOx flowing from the SCRDPF 72 into the SCR catalyst 74 is reduced and purified in the SCR catalyst 74 using the urea water added from the urea addition valve 83.

[0043] In order to release and reduce the NOx stored in the NSR catalyst 70, a rich purge is performed. When performing the rich purge, in order to make the air-fuel ratio of the exhaust gas rich (reducing atmosphere), in this embodiment, the throttle valve 26 is controlled to reduce the intake air amount Ga, and post injection is performed after the main injection of the fuel injected from the injector 14. By performing post injection, the air-fuel ratio of the exhaust gas is made rich without causing an increase in output torque.

[0044] The engine ECU 100 calculates the fuel injection quantity Qf and the fuel injection timing based on the accelerator opening degree AP and the engine rotational speed NE. The fuel injection quantity Qf is preset as a map using the accelerator opening degree AP and the engine rotational speed NE as parameters. In the present embodiment, "pilot injection", "pre-injection", and "main injection" are performed, and the fuel injection timing for each injection is preset as a map using the accelerator opening degree AP and the engine rotational speed NE as parameters. Based on the accelerator opening degree AP and the engine rotational speed NE, the fuel injection quantity Qf and the fuel injection timing for each injection are calculated from each map. The calculated fuel injection quantity Qf is distributed to "pilot injection", "pre-injection", and "main injection" based on a preset ratio, and is injected from the injector 14 into the combustion chamber.

[0045] When performing rich purge, the throttle valve 26 is controlled to reduce the intake air quantity Ga, and the post-injection quantity Qp is calculated based on the intake air quantity Ga and the fuel injection quantity Qf. Post-injection is a fuel injection performed during the expansion stroke of the engine 1 after the main injection in order to make the air-fuel ratio of the exhaust gas rich (reducing atmosphere). For example, when the minimum intake air quantity G0 required to burn the total fuel injection quantity (Qf + Qp) obtained by adding the fuel injection quantity Qf and the post-injection quantity Qp is used, the post-injection quantity Qp required to make the air excess ratio λ represented by λ = Ga / G0 less than a predetermined value smaller than 1 is calculated. By performing post-injection of the post-injection quantity Qp in addition to the fuel injection quantity Qf, the air-fuel ratio of the exhaust gas becomes rich (reducing atmosphere), and the NOx stored in the NSR catalyst 70 is released and reduced. An A / F sensor 119 for detecting the air-fuel ratio of the exhaust gas is provided in the exhaust passage 52 on the upstream side (inlet side) of the NSR catalyst 70, and during the execution of rich purge, the post-injection quantity Qp is feedback-controlled so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio. The A / F sensor 119 may be a sensor capable of simultaneously detecting the air-fuel ratio and the NOx concentration.

[0046] In engine 1 that uses light oil as fuel, methane discharged from the combustion chamber during normal operation is extremely small. However, when performing rich purge and conducting post-injection to make the air-fuel ratio of the exhaust gas rich, more methane is discharged compared to normal operation. Although the reason for this is not clear, it is presumed that methane is generated when the post-injected fuel is reformed in the rich atmosphere combustion chamber. Methane is a greenhouse gas although it is not harmful, so it is desirable to control its emission amount.

[0047] In the present embodiment, when the amount of methane discharged from the combustion chamber exceeds the allowable value during the execution of rich purge, the rich purge is prohibited (stopped) to control the methane discharged by rich purge to an appropriate amount.

[0048] Figure 3 is a flowchart showing the NOx emission control process executed by engine ECU 100. This flowchart is repeatedly processed at regular intervals during the operation of engine 1 (not shown, from when the ignition switch is turned on until it is turned off). In step (hereinafter, step is abbreviated as "S") 10, it is determined whether flag Fp is 1. Flag Fp is a flag set to 1 when it is necessary to release the NOx stored in NSR catalyst 70 and recover the NOx storage capacity of NSR catalyst 70, and it is set by the procedure shown in Figure 4.

[0049] Figure 4 is a flowchart showing an example of the NOx storage amount calculation process executed by engine ECU 100. This flowchart is repeatedly processed at regular intervals during the operation of engine 1. In S30, it is determined whether flag Fp is 1. When the amount of NOx stored in NSR catalyst 70 does not exceed the allowable value of the NOx storage amount, there is no need to release NOx from NSR catalyst 70, so flag Fp is set to 0, and a negative determination is made in S30 and the process proceeds to S31.

[0050] In S31, based on the accelerator opening degree AP and the engine rotational speed NE, the NOx emission amount Ng of the engine 1 is calculated. For example, the amount of NOx emitted from the engine 1 per unit time has been obtained in advance through experiments or the like, using the accelerator opening degree AP and the engine rotational speed NE as parameters, mapped, and stored in the memory. The NOx emission amount Ng is calculated by multiplying the amount of NOx emitted per unit time read from this map by the time interval Δt between the previous process and the current process. Note that the NOx emission amount Ng may be calculated using the fuel injection amount Qf and the engine rotational speed NE as parameters.

[0051] In the subsequent S32, the NOx emission amount Ng is added to the previous NOx storage amount ΣNOx to calculate the (current) NOx storage amount ΣNOx, and the process proceeds to S33.

[0052] In S33, it is determined whether the NOx storage amount ΣNOx is greater than or equal to the purge request threshold value Dc. The purge request threshold value Dc is a value at which, if the NOx storage amount ΣNOx exceeds this value, there is a risk that NOx will not be stored in the NSR catalyst 70 and will be discharged downstream of the NSR catalyst 70, and it is set through experiments or the like. If the NOx storage amount ΣNOx is greater than or equal to the purge request threshold value Dc, an affirmative determination is made and the process proceeds to S34, where the flag Fp is set to 1 and the current routine ends. If the NOx storage amount ΣNOx is less than the purge request threshold value Dc, a negative determination is made and the current routine ends.

[0053] In S30, if the flag Fp is 1 and an affirmative determination is made, the process proceeds to S35. In S35, it is determined whether the flag Fr is 1. The flag Fr is a flag that is set to 1 during the execution of rich purge. When the flag Fr is 1, an affirmative determination is made in S35 and the process proceeds to S36.

[0054] In S36, the NOx emission amount Nr1 is calculated based on the post-injection amount Qp and the engine rotational speed NE. The NOx emission amount Nr1 is the amount of NOx released from and reduced by the NSR catalyst 70 when the air-fuel ratio of the exhaust gas becomes rich (reducing atmosphere). For example, the amount of NOx released and reduced from the NSR catalyst 70 per unit time has been obtained in advance through experiments or the like using the post-injection amount Qp and the engine rotational speed NE as parameters, mapped, and stored in the memory. The NOx emission amount Nr1 is calculated by multiplying the amount of NOx released and reduced per unit time read from this map by the time interval Δt between the previous process and the current process. Note that the amount of NOx released and reduced from the NSR catalyst 70 per unit time may be a constant value regardless of the engine rotational speed NE or the like.

[0055] In the subsequent S37, the NOx emission amount Nr1 is subtracted from the previous NOx storage amount ΣNOx to calculate the (current) NOx storage amount ΣNOx, and the process proceeds to S41.

[0056] When the rich purge is not being executed and the flag Fr is 0, in S35, a negative determination is made and the process proceeds to S38. In S38, it is determined whether the flag Ft is 1. The flag Ft is a flag set to 1 during the execution of the warm-up purge. When the flag Ft is 1, an affirmative determination is made in S38 and the process proceeds to S39. When the flag Ft is 0, a negative determination is made in S38 and the current routine is terminated.

[0057] In S39, the NOx emission amount Nr2 is calculated based on the temperature Tc of the NSR catalyst 70. The NOx emission amount Nr2 is the amount of NOx released from the NSR catalyst 72 when the NSR catalyst 72 is heated up. For example, using the temperature Tc as a parameter, the amount of NOx released from the NSR catalyst 70 per unit time has been obtained in advance through experiments or the like, mapped, and stored in the memory. The NOx emission amount Nr2 is calculated by multiplying the amount of NOx released per unit time read from this map by the time interval Δt between the previous process and the current process.

[0058] In the subsequent S40, the NOx emission amount Nr2 is subtracted from the previous NOx storage amount ΣNOx to calculate the (current) NOx storage amount ΣNOx, and the process proceeds to S41.

[0059] In S41, it is determined whether the NOx storage amount ΣNOx is less than or equal to the purge end threshold Df. The purge end threshold Df is the value of the NOx storage amount ΣNOx when the NOx stored in the NSR catalyst 70 is released and the NOx storage capacity of the NSR catalyst 70 is sufficiently restored, and may be, for example, 0. When the NOx storage amount ΣNOx becomes less than or equal to the purge end threshold Df, an affirmative determination is made and the process proceeds to S42, where the flag Fp is set to 0 and the current routine ends. When the NOx storage amount ΣNOx is greater than the purge end threshold Df, a negative determination is made and the current routine ends.

[0060] In this way, when the NOx storage amount ΣNOx becomes greater than or equal to the purge request threshold Dc, the flag Fp is set to 1. Also, when the NOx storage amount ΣNOx becomes less than or equal to the purge end threshold Df by rich purge or temperature increase purge, the flag Fp is set to 0.

[0061] Referring to FIG. 3, when the flag Fp is set to 1 and it is necessary to release the NOx stored in the NSR catalyst 70 and restore the NOx storage capacity of the NSR catalyst 70 by rich purge or temperature increase purge, an affirmative determination is made in S10 and the process proceeds to S11. When the flag Fp is set to 0, a negative determination is made and the process proceeds to S20.

[0062] In S11, it is determined whether rich purge can be performed. For example, based on the engine rotational speed NE and the fuel injection amount Qf, it is determined whether the operating region of engine 1 is an operating region in which the air-fuel ratio of the exhaust gas can be made rich by controlling the throttle valve 26 to reduce the intake air amount Ga and performing post-injection. Then, when the operating region of engine 1 is an operating region in which the air-fuel ratio of the exhaust gas can be made rich and the temperature Tc of the NSR catalyst 70 is equal to or higher than the activation temperature, it is determined that rich purge can be performed. In S11, if it is determined that rich purge can be performed, the process proceeds to S11. If rich purge cannot be performed, a negative determination is made in S11 and the process proceeds to S17.

[0063] In S12, it is determined whether the flag Fm is 1. The flag Fm is a flag for prohibiting rich purge. When the amount of methane discharged from the combustion chamber exceeds the allowable value during the execution of rich purge, it is set to 1 and rich purge is prohibited. Details of the flag Fm will be described later. If the flag Fm is 0, a negative determination is made in S12 and the process proceeds to S13.

[0064]

[0065] In the subsequent S14, the methane emission amount ΣMe [mg], which is the amount of methane discharged from the combustion chamber, is calculated. For example, the amount of methane m [mg / s] discharged per unit time is calculated from a methane generation model using, as parameters, the intake air amount Gn per cycle in each cylinder of the engine 1, the temperature in the combustion chamber (in-cylinder temperature), the post-injection amount Qp, and the like. Then, the methane amount Me [mg] is calculated by multiplying the methane amount m discharged per unit time by the time interval Δt [s] between the previous process and the current process (Me = m × Δt). Then, the methane amount Me is added to the previous methane emission amount ΣMe to obtain the (current) methane emission amount ΣMe [mg] (ΣMe = ΣMe (previous value) + Me). Note that the methane amount m discharged per unit time may be obtained in advance through experiments or the like using the intake air amount Ga, the total fuel injection amount (Qf + Qp), and the engine speed NE as parameters, mapped, and the methane amount m discharged per unit time may be calculated using this map.

[0066] In S15, it is determined whether the flag Fp is 0. When the NOx storage capacity of the NSR catalyst 70 has not fully recovered, the flag Fp is 1 and a negative determination is made, and the routine returns to S11 to continue releasing the NOx stored in the NSR catalyst 70.

[0067] FIG. 5 is a flowchart showing the methane emission allowable value calculation process executed by the engine ECU 100. This flowchart is repeatedly processed at predetermined intervals during the operation of the engine 1 (not shown, from when the ignition switch is turned on until it is turned off). First, in S50, the current allowable value AMe is calculated. The current allowable value AMe is the allowable value of the methane emission amount allowed from the previous process to the current process of this routine, and is obtained by multiplying the methane emission target value Tme [mg / km] per driving distance of the vehicle V by the driving distance [km] of the vehicle V from the previous process to the current process of this routine. In the present embodiment, the current allowable value AMe [mg] is calculated by multiplying the methane emission target value Tme [mg / km] by the vehicle speed SPD [km / h] and the time interval Δt [h] between the previous process and the current process (AMe = Tme × SPD × Δt).

[0068] In the subsequent S51, the current methane emission allowance value AMe is added to the previous methane emission allowance value ΣAMe to calculate the (current) methane emission allowance value ΣAMe [mg], and the process proceeds to S52.

[0069] In S52, it is determined whether the flag Fm is 1. When the flag Fm is 1, an affirmative determination is made and the current routine ends. When the flag Fm is 0, a negative determination is made and the process proceeds to S53.

[0070] In S53, it is determined whether the methane emission amount ΣMe is less than or equal to the methane emission allowance value ΣAMe. When the methane emission amount ΣMe is less than or equal to the methane emission allowance value ΣAMe (ΣMe ≦ ΣAMe), an affirmative determination is made and the current routine ends. When the methane emission amount ΣMe exceeds the methane emission allowance value ΣAMe (ΣMe > ΣAMe), a negative determination is made and the process proceeds to S54.

[0071] In S54, after setting the flag Fm to 1, the process proceeds to S55, where the methane emission allowance value ΣAMe is reset to 0 [mg], and the current routine ends. In this way, when the methane emission amount ΣMe calculated in S14 (Figure 3) exceeds the methane emission allowance value ΣAMe during the execution of the rich purge, the flag Fm is set to 1.

[0072] Referring to Figure 3, when the flag Fm is set to 1, a negative determination is made in S12, the rich purge is prohibited, and the process proceeds to S16. In S16, after resetting the methane emission amount ΣMe to 0 [mg], the process proceeds to S17.

[0073] In S17, it is determined whether it is possible to perform a temperature-rising purge. For example, when the temperature Tf of the SCRDPF70 is equal to or higher than the activation temperature of the catalyst, it is determined that it is possible to perform a temperature-rising purge. If it is determined in S17 that it is possible to perform a temperature-rising purge, the process proceeds to S18. If it is not possible to perform a temperature-rising purge, a negative determination is made in S17 and the process proceeds to S20.

[0074] In S18, a temperature-rising purge is executed, the flag Fr is set to 0, and the flag Ft is set to 1. If a rich purge was executed during the previous process, the rich purge is terminated. The temperature-rising purge may increase the temperature of the exhaust gas and warm up the NSR catalyst 70 by controlling the throttle valve 26 to reduce the intake air amount Ga and retard the main injection timing. Further, in addition to reducing the intake air amount Ga and retarding the main injection timing, a post-injection that does not make the air-fuel ratio of the exhaust gas rich may be performed at an injection timing retarded more than when the rich purge is executed. Further, in addition to or instead of these, the temperature of the exhaust gas may be increased and the NSR catalyst 70 may be warmed up by adding fuel from the fuel addition valve 90. After the process of S18, the routine proceeds to S15.

[0075] When the NOx storage capacity of the NSR catalyst 70 has fully recovered, the NOx storage amount ΣNOx becomes equal to or less than the purge end threshold value Df, and in S42 (FIG. 4), when the flag Fp is set to 0, an affirmative determination is made in S15 and the routine proceeds to S19.

[0076] In S19, after setting the flag Fm to 0, the routine proceeds to S20. In S20, if a rich purge is being executed, the rich purge is terminated. Also, if a temperature-rising purge is being executed, the temperature-rising purge is terminated. Further, after setting the flag Fr to 0 and the flag Ft to 0, the current routine is terminated.

[0077] FIG. 6 is a diagram for explaining the operation of the present embodiment. In FIG. 6, the horizontal axis represents time, (A) represents the change in the running distance, (B) represents the change in the NOx storage amount ΣNOx, (C) represents the change in the flag Fp, (D) represents the change in the flag Fr, (E) represents the change in the flag Ft, (F) represents the change in the methane emission allowance value ΣAMe and the methane emission amount ΣMe, and (G) represents the change in the flag Fm.

[0078] Referring to FIG. 6, when the trip of vehicle V starts at time t0, as shown in FIG. 6(A), the travel distance increases with the passage of time. As the travel distance increases and time elapses, as shown in FIG. 6(B), the NOx storage amount ΣNOx increases. Also, as the travel distance increases, as shown in FIG. 6(F), the methane emission allowance ΣAMe increases.

[0079] At time t1, when the NOx storage amount ΣNOx becomes equal to or greater than the purge request threshold Dc (positive determination in FIG. 4: S33), as shown in FIG. 6(C), the flag Fp is set to 1 (FIG. 4: S34). At time t1, when the flag Fp is set to 1 and the operation state is such that rich purge is possible (positive determination in FIG. 3: S11), rich purge is started, and as shown in FIG. 6(D), the flag Fr is set to 1 (FIG. 3: S13).

[0080] When the flag Fr is set to 1, the NOx emission amount Nr1 is calculated, and as shown in FIG. 6(B), the NOx storage amount ΣNOx decreases (FIG. 4: S36, S37). Also, when rich purge is started at time t1, the methane amount Me is calculated, and as shown in FIG. 6(F), the methane emission amount ΣMe increases (FIG. 3: S14).

[0081] When the methane emission amount ΣMe increases and at time t2, the methane emission amount ΣMe exceeds the methane emission allowance ΣAMe (negative determination in FIG. 5: S53), as shown in FIG. 6(G), the flag Fm is set to 1 (FIG. 5: S54).

[0082] At time t2, when the methane emission amount ΣMe exceeds the methane emission allowance ΣAMe (negative determination in FIG. 5: S53), as shown in FIG. 6(F), the methane emission allowance ΣAMe is once reset to 0 [mg] (FIG. 5: S55), and then the methane emission allowance ΣAMe increases with the increase in the travel distance.

[0083] When the flag Fm is set to 1, the methane emission ΣMe is reset to 0 [mg] (Fig. 3: S16). Therefore, as shown in Fig. 6(F), at time t2, the methane emission ΣMe becomes 0 [mg], and thereafter, until the rich purge is started (until the flag Fr is set to 1), the methane emission ΣMe is maintained at 0 [mg].

[0084] When the flag Fm is set to 1, the rich purge is prohibited (positive determination in Fig. 3: S12), and when the operating state allows the temperature increase purge (positive determination in Fig. 3: S17), the temperature increase purge is started. As shown in Fig. 6(E), the flag Ft is set to 1 (Fig. 3: S18). Also, as shown in Fig. 6(D), the flag Fr is set to 0 (Fig. 3: S18).

[0085] When the flag Fr is set to 0 and the flag Ft is set to 1, the NOx emission amount Nr2 is calculated, and as shown in Fig. 6(B), the NOx storage amount ΣNOx decreases (Fig. 4: S39, S40). When the NOx storage amount ΣNOx decreases and at time t3, the NOx storage amount ΣNOx becomes less than or equal to the purge end threshold Df (positive determination in Fig. 4: S41), as shown in Fig. 6(C), the flag Fp is set to 0 (Fig. 3: S42).

[0086] When the flag Fp is set to 0, a positive determination is made in S15 (Fig. 3), so the flag Fm is set to 0 (Fig. 3: S19), the rich purge and the temperature increase purge are ended, and the flags Fr and Ft are set to 0 (Fig. 3: S20).

[0087] At time t3, when the flag Fp is set to 0, a negative determination is made in S30 (Fig. 4), so the NOx emission amount Ng is calculated. As shown in Fig. 6(B), as the driving distance increases, the NOx storage amount ΣNOx increases. When the NOx storage amount ΣNOx increases and at time t4, the NOx storage amount ΣNOx becomes greater than or equal to the purge request threshold Dc, the flag Fp is set to 1. From time t4 to time t6, the same processing as that from time t1 to time t3 is performed.

[0088] According to this embodiment, when the NOx storage amount ΣNOx of the NSR catalyst 70 becomes equal to or greater than the purge requirement threshold value Dc, a rich purge is executed by making the air-fuel ratio of the exhaust gas rich with the fuel injected from the injector 14 into the combustion chamber, and the NOx stored in the NSR catalyst 70 is released and reduced. The engine ECU 100 calculates the methane emission amount ΣMe, which is the amount of methane discharged from the combustion chamber, during the execution of the rich purge. When the methane emission amount ΣMe exceeds the methane emission allowable value ΣAMe, the flag Fm is set to 1 and the rich purge is prohibited. Since the rich purge is prohibited when the methane emission amount ΣMe exceeds the methane emission allowable value ΣAMe, the amount of methane discharged into the atmosphere during the execution of the rich purge can be controlled to an appropriate amount.

[0089] In this embodiment, since the methane emission allowable value ΣAMe is calculated based on the driving distance of the vehicle V, it becomes possible to appropriately manage the amount of methane discharged into the atmosphere in the same way as the regulation of the amount of carbon dioxide (CO2) discharged in the exhaust gas. Further, when the methane emission amount ΣMe exceeds the methane emission allowable value ΣAMe, the methane emission amount ΣMe and the methane emission allowable value ΣAMe are reset to 0, so that an overflow of the register that holds the calculated values of the methane emission amount ΣMe and the methane emission allowable value ΣAMe can be suppressed.

[0090] In this embodiment, when the flag Fm is set to 1 and the rich purge is prohibited, a temperature-rising purge is executed until the NOx storage amount ΣNOx becomes equal to or less than the purge end threshold value Df, the temperature of the NSR catalyst 70 is increased, NOx is released from the NSR catalyst 70, and the NOx storage capacity of the NSR catalyst 70 is restored. Since the NOx released by the temperature-rising purge is reductively purified by the SCRDPF 72 provided downstream of the NSR catalyst 70, it is possible to suppress the NOx released from the NSR catalyst 70 from being discharged into the atmosphere.

[0091] In the above embodiment, the SCRDPF 72 and the SCR catalyst 74 are provided in the exhaust passage 52 downstream of the NSR catalyst 70. However, only the SCR catalyst 74 may be provided downstream of the NSR catalyst 70. Further, a configuration may be adopted in which a DPF is provided downstream of the NSR catalyst 70 and an SCR catalyst is provided in the flow of the DPF.

[0092] The embodiments disclosed this time should be considered as illustrative in all respects and not restrictive. The scope of the present disclosure is shown not by the above description but by the scope of the claims, and it is intended that all modifications within the meaning and scope equivalent to the scope of the claims are included.

Explanation of Reference Numerals

[0093] 1 Engine, 2 Torque converter, 3 Automatic transmission, 4 Differential gear, 5 Rear wheels, 10 Engine body, 12 Cylinder, 14 Injector, 20 Intake passage, 22 Air cleaner, 24 Intercooler, 26 Throttle valve, 28 Intake manifold, 30 Turbocharger, 32 Compressor, 34 Turbine, 40 Fuel tank, 41 Feed pump, 42 High-pressure fuel pump, 43 Fuel passage, 44 Common rail, 45 Fuel passage, 50 Exhaust manifold, 52 Exhaust passage, 60 EGR passage, 62 EGR cooler, 64 EGR valve, 70 NSR catalyst, 72 SCRDPF, 74 SCR catalyst, 80 AdBlue tank, 82, 83 AdBlue addition valves, 90 Fuel addition valve, 100 Engine ECU, 101 CPU, 102 Memory, 111 Vehicle speed sensor, 112 Engine speed sensor, 113 Accelerator pedal sensor, 114 Airflow meter, 115 NSR catalyst temperature sensor, 116 SCRDPF temperature sensor, 117, 118 NOx sensors, 119 A / F sensor, 200 Transmission ECU.

Claims

1. A NOx storage reduction catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas is rich, A fuel injection valve that injects fuel into the combustion chamber of an internal combustion engine, A control device, and an exhaust gas purification system for an internal combustion engine comprising: The control device is configured to: When the NOx stored in the NOx storage reduction catalyst reaches or exceeds a purge request threshold value, make the air-fuel ratio of the exhaust gas rich by the fuel injected from the fuel injection valve into the combustion chamber, and execute a rich purge to release and reduce the NOx stored in the NOx storage reduction catalyst; During execution of the rich purge, calculate the methane emission amount, which is the amount of methane discharged from the combustion chamber, and prohibit the rich purge when the methane emission amount exceeds a methane emission allowable value. An exhaust gas purification system for an internal combustion engine.

2. The internal combustion engine is mounted on a vehicle, The control device is configured to: Calculate the methane emission allowable value based on the driving distance of the vehicle. The exhaust gas purification system for an internal combustion engine according to Claim 1.

3. The control device is configured to: When the methane emission amount exceeds the methane emission allowable value, set the methane emission amount and the methane emission allowable value to initial values. The exhaust gas purification system for an internal combustion engine according to Claim 2.

4. A selective reduction type NOx catalyst is provided in the exhaust passage downstream of the NOx storage reduction catalyst, The control device is configured to: When the methane emission amount exceeds the methane emission allowable value and the rich purge is prohibited during execution of the rich purge, raise the temperature of the NOx storage reduction catalyst and execute a temperature-raising purge to release the NOx stored in the NOx storage reduction catalyst. The exhaust gas purification system for an internal combustion engine according to any one of Claims 1 to 3.

5. The control device is configured to: When the NOx stored in the NOx storage reduction catalyst reaches or falls below a purge end threshold value, end the execution of the rich purge and the temperature-raising purge. The exhaust gas purification system for an internal combustion engine according to Claim 4.

6. A NOx storage reduction catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas is rich, A fuel injection valve that injects fuel into the combustion chamber of an internal combustion engine, and an exhaust gas purification method for an internal combustion engine comprising: When the NOx stored in the NOx storage reduction catalyst reaches or exceeds a purge request threshold value, a step of executing a rich purge in which the air-fuel ratio of the exhaust gas is made rich by the fuel injected from the fuel injection valve into the combustion chamber to release and reduce NOx; A method for purifying exhaust gas of an internal combustion engine, including: calculating a methane emission amount, which is the amount of methane discharged from the combustion chamber during execution of the rich purge; and prohibiting the rich purge when the methane emission amount exceeds a methane emission allowable value.

7. The internal combustion engine is mounted on a vehicle, The exhaust gas purification method, The method for purifying exhaust gas of an internal combustion engine according to claim 6, further comprising a step of calculating the methane emission allowable value based on the driving distance of the vehicle.

8. The internal combustion engine further includes a selective reduction type NOx catalyst in an exhaust passage downstream of the NOx storage reduction catalyst, The exhaust gas purification method, The method for purifying exhaust gas of an internal combustion engine according to claim 6 or claim 7, further comprising a step of executing a temperature-raising purge in which the temperature of the NOx storage reduction catalyst is raised to release NOx when the rich purge is prohibited in the step of prohibiting the rich purge.

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