JP7704062B2 - Exhaust gas purification system for internal combustion engine and exhaust gas purification method for internal combustion engine - Google Patents

Description

This disclosure relates to an exhaust purification system for an internal combustion engine and an exhaust purification method for an internal combustion engine, and in particular to an exhaust purification system and an exhaust purification method for an internal combustion engine equipped with a NOx storage reduction catalyst.

Internal combustion engines equipped with NOx storage reduction catalysts (hereinafter also referred to as NSR catalysts) are known to purify NOx (nitrogen oxides) contained in exhaust gas. NOx storage reduction catalysts (hereinafter also referred to as NSR catalysts) store NOx when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is lean, and release and reduce the stored NOx when the air-fuel ratio of the exhaust gas becomes rich.

When making the air-fuel ratio of the exhaust gas rich in order to reduce the NOx stored in the NSR catalyst, it is known to perform a so-called post-injection, in which fuel is injected into the combustion chamber during the expansion stroke of the internal combustion engine separately from the main injection. For example, Japanese Patent Application Laid-Open No. 2004-132262 (Patent Document 1) claims 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.

JP 2004-132262 A

When post-injection is performed to make the exhaust gas air-fuel ratio rich in order to release and reduce the NOx stored in the NSR catalyst, methane (CH4) is generated during fuel reforming and is discharged from the combustion chamber. Methane has a low ability to reduce NOx and is difficult to purify by catalysts. For this reason, when post-injection is used to make the exhaust gas air-fuel ratio rich and release and reduce the NOx stored in the NSR catalyst, methane is released into the atmosphere.

Although methane is not harmful, it is a greenhouse gas, so it is desirable to control its emissions.

The purpose of this disclosure is to control the amount of methane emitted when fuel is injected to enrich the air-fuel ratio of the exhaust gas in order to release and reduce the NOx stored in the NSR catalyst.

The exhaust purification system for an internal combustion engine disclosed herein is an exhaust purification system for an internal combustion engine that includes 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 reaches or exceeds a purge request threshold, the control device performs a rich purge that makes the air-fuel ratio of the exhaust gas rich by fuel injected from the fuel injection valve into the combustion chamber and releases and reduces the NOx stored in the NOx storage reduction catalyst, calculates the methane emission amount, which is the amount of methane discharged from the combustion chamber when the rich purge is performed, and prohibits the rich purge when the methane emission amount exceeds the methane emission allowable value.

According to this configuration, when the NOx stored in the NOx storage reduction catalyst reaches or exceeds the purge request threshold, the control device of the exhaust purification system of the internal combustion engine performs a rich purge by making the air-fuel ratio of the exhaust gas rich by fuel injected from the fuel injection valve into the combustion chamber, and releases and reduces the NOx stored in the NOx storage reduction catalyst. When a rich purge is performed, 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. Since the rich purge is prohibited when the methane emission amount exceeds the methane emission allowable value, the amount of methane discharged into the atmosphere when a rich purge is performed can be controlled to an appropriate amount.

Preferably, the internal combustion engine is mounted on a vehicle, and the control device may calculate the methane emission allowable value based on the vehicle's travel distance. In addition, 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.

With this configuration, the methane emission allowable value is calculated based on the vehicle's travel distance, making it possible to appropriately manage methane emissions into the atmosphere in a manner similar to the regulations governing carbon dioxide (CO2) emissions in exhaust gas. In addition, when the methane emission amount exceeds the methane emission allowable value, the methane emission amount and the methane emission allowable value are set to initial values (which may be "0", for example), preventing overflow of the register that holds the calculated methane emission amount and the methane emission allowable value.

Preferably, a selective reduction type NOx catalyst is provided in the exhaust passage downstream of the NOx storage reduction catalyst, and the control device may perform a temperature increase purge by increasing the temperature of the NOx storage reduction catalyst to release the NOx stored in the NOx storage reduction catalyst if the amount of methane emissions exceeds the methane emission allowable value during rich purge and the control device prohibits rich purge.

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 control device prohibits rich purge, it executes a temperature increase purge to increase 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.

Preferably, the control device may terminate the execution of the rich purge and temperature increase purge when the NOx stored in the NOx storage reduction catalyst falls below the purge end threshold.

With this configuration, a rich purge or temperature-raising purge is performed until the NOx stored in the NOx catalyst falls below the purge end threshold, allowing sufficient NOx to be released from the NOx storage reduction catalyst, and the NOx storage capacity of the NOx storage reduction catalyst is effectively restored.

The disclosed exhaust gas purification method for an internal combustion engine includes 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 for an internal combustion engine includes the steps of: when the NOx stored in the NOx storage reduction catalyst reaches or exceeds a purge request threshold, performing a rich purge in which the air-fuel ratio of the exhaust gas is made rich by fuel injected from the fuel injection valve into the combustion chamber and the NOx is released and reduced; and calculating the amount of methane emissions, which is the amount of methane emitted from the combustion chamber when the rich purge is performed, and prohibiting the rich purge when the amount of methane emissions exceeds the methane emissions allowable value.

According to this exhaust purification method, when the NOx stored in the NOx storage reduction catalyst reaches or exceeds the purge request threshold, a rich purge is performed by making the air-fuel ratio of the exhaust gas rich with fuel injected from the fuel injection valve into the combustion chamber in order to release and reduce the NOx stored in the NOx storage reduction catalyst. When a rich purge is performed, 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, since the rich purge is prohibited when the methane emission amount exceeds the methane emission allowable value, the amount of methane discharged into the atmosphere can be controlled to an appropriate amount.

Preferably, the internal combustion engine is mounted on a vehicle, and the exhaust purification method may further include a step of calculating the methane emission allowable value based on the mileage of the vehicle.

This exhaust purification method calculates the allowable methane emissions based on the vehicle's travel distance, making it possible to appropriately manage methane emissions into the atmosphere in a manner similar to the regulations on carbon dioxide (CO2) emissions in methane exhaust gas.

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 purification method may further include a step of performing a temperature increase purge that increases 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.

According to this exhaust purification method, when rich purge is prohibited, a temperature increase purge is performed 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.

According to the present disclosure, the NOx stored in the NSR catalyst is released and reduced, so that the amount of methane emitted when fuel is injected into the combustion chamber to enrich the air-fuel ratio of the exhaust gas can be controlled to an appropriate amount.

1 is a diagram showing an overall configuration of a vehicle according to an embodiment of the present invention; 1 is a schematic configuration diagram of an engine 1. FIG. 3 is a flowchart showing a process of NOx emission control executed by an engine ECU 100. 4 is a flowchart showing an example of a NOx occlusion amount calculation process executed by an engine ECU 100. 4 is a flowchart showing an example of a methane emission allowable value calculation process executed by an engine ECU 100. 1A to 1G are diagrams illustrating the operation of this embodiment.

Below, an embodiment of the present disclosure will be described with reference to the drawings. In the following description, the same or equivalent parts in the drawings will be given the same reference numerals, and their description may not be repeated.

Figure 1 shows the overall configuration of a vehicle according to this embodiment. The vehicle V includes an engine 1, a torque converter 2, and an automatic transmission 3.

In this embodiment, the engine 1 is a compression ignition type internal combustion engine (diesel engine) equipped with an exhaust 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 is equipped with a pump impeller, a turbine runner, a stator, and a lock-up clutch, which are 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 multiple 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 the driving wheels, via a drive shaft. The vehicle V is a rear-wheel drive vehicle, but may be a front-wheel drive vehicle or a four-wheel drive vehicle.

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.

Figure 2 is a schematic diagram of the engine 1. The engine 1 is a compression ignition type internal combustion engine (diesel engine) equipped with an exhaust purification system, and is an internal combustion engine in which fuel is injected from a fuel injection valve (injector) 14 into a combustion chamber formed in a cylinder (cylinder) 12 of the engine body 10, and compression autoignition is performed. In this 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. Fresh air from which foreign matter has been removed by the air cleaner 22 is supercharged (compressed) by a compressor 32 of a turbocharger 30, cooled by the intercooler 24, supplied to an intake manifold 28, and supplied to each combustion chamber from an intake port.

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 through a fuel passage 43 to a common rail 44. The high-pressure fuel stored in the common rail 44 is injected from the injector 14 into the combustion chamber (inside the cylinder).

The exhaust gas discharged from the combustion chamber is collected in the exhaust manifold 50 and released to the outside air via the exhaust passage 52. A portion of the exhaust gas is returned to the intake manifold 28 via an EGR (Exhaust Gas Recirculation) passage 60. An EGR cooler 62 and an EGR valve 64 are provided in the EGR passage 60.

In the exhaust passage 52, from the upstream side, there are provided the 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.

The NSR catalyst 70 is a catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean (excess oxygen), 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 catalyst in which barium (Ba) is supported as a NOx storage component and a platinum group metal is supported as a catalytic component on a support made of cordierite. The NSR catalyst 70 may also be a NOx storage reduction type three-way catalyst in which an alkaline substance is added as a NOx storage material to a three-way catalyst.

The SCRDPF72 is a catalyst that combines a filter function that captures particulate matter (PM) in the exhaust gas and purifies the exhaust gas by appropriately burning and removing the captured PM, with a function that reduces and purifies NOx in the exhaust gas. For example, it may be a honeycomb filter made of cordierite or silicon carbide that supports copper (Cu) ion-exchanged zeolite as a catalyst. The SCR catalyst 74 is a catalyst that reduces and purifies NOx in the exhaust gas, and may be a carrier made of cordierite that supports copper (Cu) ion-exchanged zeolite as a catalyst.

The SCRDPF 72 and the SCR catalyst 74 use ammonia (NH3) as a reducing agent to achieve a high NOx purification rate. The ammonia used as a reducing agent is generated by hydrolysis of urea water supplied to the exhaust passage 52 upstream of the SCRDPF 72 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 SCRDPF 72, and a urea addition valve 83 is provided in the exhaust passage 52 upstream of the SCR catalyst 74. When reducing and purifying NOx flowing into the SCRDPF 72 and the SCR catalyst 74, urea water pumped from a 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.

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 by the feed pump 41 via the fuel passage 45, and when the fuel addition valve 90 opens, fuel is added (injected) into the exhaust passage 52.

When the amount of PM accumulated in the SCRDPF 72 reaches or exceeds a predetermined value, fuel is added (injected) from the fuel addition valve 90 into the exhaust passage 52, the temperature of the SCRDPF 72 is raised, and the accumulated PM is burned off. When fuel is added from the fuel addition valve 90, the catalytic components of the NSR catalyst 70 function as an oxidation catalyst, the fuel generates heat (burns), the temperature of the exhaust gas flowing into the SCRDPF 72 rises, and the temperature of the SCRDPF 72 rises.

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

Various sensors whose signals are input to the engine ECU 100 include, for example, a vehicle speed sensor 111, an engine speed sensor 112, an accelerator pedal sensor 113, an air flow meter 114, an NSR catalyst temperature sensor 115, and an SCRDPF temperature sensor 116. The engine speed sensor 112 detects the rotation speed NE of the engine 1. The accelerator pedal sensor 113 detects the amount of accelerator pedal operation by the user (hereinafter also referred to as "accelerator opening") AP. The air flow meter 114 detects the intake amount (intake 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 SCRDPF 72.

The engine 1 is a diesel engine, and even at full load, the excess air ratio is 1.5 to 2, and the air-fuel ratio of the exhaust gas is lean. Therefore, in this 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 NOx from being discharged into the atmosphere. Since the NOx storage amount of the NSR catalyst 70 is limited, 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. Therefore, before the amount of stored NOx exceeds the allowable value, the air-fuel ratio of the exhaust gas is made rich (reducing atmosphere), and the NOx stored in the NSR catalyst 70 is released and reduced, thereby recovering the NOx storage capacity of the NSR catalyst 70. In addition, making the air-fuel ratio of the exhaust gas rich to release and reduce the NOx stored in the NSR catalyst 70 is also called rich purge.

In addition, when the temperature Tc of the NSR catalyst 70 increases, the NOx storage capacity of the NSR catalyst 70 decreases, and the allowable amount of NOx storage decreases. Therefore, it is possible to release the NOx stored in the NSR catalyst 70 by raising the temperature Tc of the NSR catalyst 70 (without making the air-fuel ratio of the exhaust gas rich) and lowering the allowable amount of NOx storage. Hereinafter, raising the temperature Tc of the NSR catalyst 70 and releasing the NOx stored in the NSR catalyst 70 is also referred to as temperature-raising purge.

The SCRDPF 72 reduces and purifies the NOx released from the NSR catalyst 70 by the temperature rise purge. A 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. The engine ECU 100 calculates the amount of urea water to be added from the urea addition valve 82 based on the NOx concentration of the exhaust gas flowing into the SCRDPF 72 and the exhaust gas amount (intake air amount Ga), and reduces and purifies the NOx that has flowed into the SCRDPF 72 by adding urea from the urea addition valve 82. In this way, the NOx that flows into the SCRDPF 72 from the NSR catalyst 70 (regardless of the NOx released from the NSR catalyst 70 by the temperature rise purge) is reduced and purified by the SCRDPF 72 using the urea water added from the urea addition valve 82.

A NOx sensor 118 is provided in the exhaust passage 52 on the inlet side (upstream side) of the SCR catalyst 74, and the NOx that flows from the SCRDPF 72 into the SCR catalyst 74 is similarly reduced and purified in the SCR catalyst 74 using urea water added from the urea addition valve 83.

A rich purge is performed to release and reduce the NOx stored in the NSR catalyst 70. When a rich purge is performed, 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 a post-injection is performed after the main injection of fuel injected from the injector 14. By performing a post-injection, the air-fuel ratio of the exhaust gas is made rich without increasing the output torque.

The engine ECU 100 calculates the fuel injection amount Qf and the fuel injection timing based on the accelerator opening AP and the engine speed NE. The fuel injection amount Qf is preset as a map with the accelerator opening AP and the engine speed NE as parameters. In this embodiment, "pilot injection", "pre-injection" and "main injection" are performed, and the fuel injection timing of each injection is preset as a map with the accelerator opening AP and the engine speed NE as parameters. The fuel injection amount Qf and each fuel injection timing are calculated from each map based on the accelerator opening AP and the engine speed NE. The calculated fuel injection amount 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.

When performing a rich purge, the throttle valve 26 is controlled to reduce the intake air amount Ga, and the post injection amount Qp is calculated based on the intake air amount Ga and the fuel injection amount 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 amount required to burn the total fuel injection amount (Qf + Qp) obtained by adding the fuel injection amount Qf and the post injection amount Qp is set to G0, the post injection amount Qp required to make the air excess ratio λ expressed as λ = Ga / G0 a predetermined value smaller than 1 is calculated. By post-injecting the post injection amount Qp in addition to the fuel injection amount 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 that detects the air-fuel ratio of the exhaust gas is provided in the exhaust passage 52 upstream (inlet side) of the NSR catalyst 70, and when rich purge is performed, the post injection amount 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 that can simultaneously detect the air-fuel ratio and the NOx concentration.

In engine 1, which uses diesel as fuel, very little methane is emitted from the combustion chamber during normal operation, but when a rich purge is performed and post-injection is performed to make the air-fuel ratio of the exhaust gas rich, more methane is emitted than during normal operation. The reason for this is unclear, but it is presumed that methane is produced when the post-injected fuel is reformed in the combustion chamber in a rich atmosphere. Although methane is not harmful, it is a greenhouse gas, so it is desirable to control its emissions.

In this embodiment, when a rich purge is performed and the amount of methane discharged from the combustion chamber exceeds an allowable value, the rich purge is prohibited (stopped), thereby controlling the amount of methane discharged by the rich purge to an appropriate level.

Figure 3 is a flowchart showing the process of NOx release control executed by the engine ECU 100. This flowchart is repeatedly processed at predetermined intervals while the engine 1 is operating (between when an ignition switch (not shown) is turned on and when it is turned off). In step (hereinafter, step is abbreviated as "S") 10, it is determined whether or not flag Fp is 1. Flag Fp is a flag that is set to 1 when it is necessary to release NOx stored in the NSR catalyst 70 and restore the NOx storage capacity of the NSR catalyst 70, and is set by the process shown in Figure 4.

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

In S31, the amount of NOx emissions Ng from engine 1 is calculated based on the accelerator opening AP and engine speed NE. For example, the amount of NOx emitted from engine 1 per unit time is determined in advance by experiments or the like using accelerator opening AP and engine speed NE as parameters, mapped, and stored in memory. The amount of NOx emitted per unit time read from this map is multiplied by the time interval Δt between the previous processing and the current processing to calculate the amount of NOx emissions Ng. Note that the amount of NOx emissions Ng may also be calculated using the fuel injection amount Qf and engine speed NE as parameters.

In the next step 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.

In S33, it is determined whether the NOx occlusion amount ΣNOx is equal to or greater than the purge request threshold Dc. The purge request threshold Dc is a value that, if the NOx occlusion amount ΣNOx exceeds, there is a risk that NOx will not be occluded in the NSR catalyst 70 and will be discharged downstream of the NSR catalyst 70, and is set by experiment, etc. If the NOx occlusion amount ΣNOx is equal to or greater than the purge request threshold Dc, a positive determination is made and the process proceeds to S34, where the flag Fp is set to 1 and the current routine is terminated. If the NOx occlusion amount ΣNOx is smaller than the purge request threshold Dc, a negative determination is made and the current routine is terminated.

If the flag Fp is 1 and a positive determination is made in S30, 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 when a rich purge is performed. If the flag Fr is 1, a positive determination is made in S35 and the process proceeds to S36.

In S36, the NOx emission amount Nr1 is calculated based on the post injection amount Qp and the engine rotation speed NE. The NOx emission amount Nr1 is the amount of NOx released and reduced from the NSR catalyst 70 when the exhaust gas air-fuel ratio becomes rich (reducing atmosphere). For example, the amount of NOx released and reduced from the NSR catalyst 70 per unit time is determined in advance by experiments or the like using the post injection amount Qp and the engine rotation speed NE as parameters, mapped, and stored in 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 processing and the current processing. 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 rotation speed NE, etc.

In the next step S37, the NOx release 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.

If a rich purge is not being performed and flag Fr is 0, a negative determination is made in S35 and the routine proceeds to S38. In S38, it is determined whether flag Ft is 1. Flag Ft is a flag that is set to 1 when a temperature increase purge is being performed. If flag Ft is 1, a positive determination is made in S38 and the routine proceeds to S39. If flag Ft is 0, a negative determination is made in S38 and the routine ends.

In S39, the amount of NOx released Nr2 is calculated based on the temperature Tc of the NSR catalyst 70. The amount of NOx released Nr2 is the amount of NOx released from the NSR catalyst 72 when the temperature of the NSR catalyst 72 is raised. For example, the amount of NOx released from the NSR catalyst 70 per unit time is determined in advance by experiments or the like using the temperature Tc as a parameter, mapped, and stored in memory. The amount of NOx released 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 processing and the current processing.

In the next step S40, the NOx release 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.

In S41, it is determined whether the NOx occlusion amount ΣNOx is equal to or less than the purge end threshold Df. The purge end threshold Df is the value of the NOx occlusion amount ΣNOx when the NOx occlusion capacity of the NSR catalyst 70 is fully restored after the NOx occlusion amount ΣNOx has been released from the NSR catalyst 70, and may be, for example, 0. If the NOx occlusion amount ΣNOx is equal to or less than the purge end threshold Df, a positive determination is made and the process proceeds to S42, where the flag Fp is set to 0 and the current routine is terminated. If the NOx occlusion amount ΣNOx is greater than the purge end threshold Df, a negative determination is made and the current routine is terminated.

In this way, when the NOx occlusion amount ΣNOx becomes equal to or greater than the purge request threshold Dc, the flag Fp is set to 1. Also, when the NOx occlusion amount ΣNOx becomes equal to or less than the purge end threshold Df due to a rich purge or a temperature rise purge, the flag Fp is set to 0.

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

In S11, it is determined whether or not a rich purge can be performed. For example, based on the engine rotation speed NE and the fuel injection amount Qf, it is determined whether or not the operating range of the engine 1 is an operating range 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 range of the engine 1 is an operating range 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 a rich purge can be performed. If it is determined in S11 that a rich purge can be performed, the process proceeds to S11. If a rich purge cannot be performed, a negative determination is made in S11 and the process proceeds to S17.

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

In S13, a rich purge is performed, flag Fr is set to 1, and flag Ft is set to 0. As described above, the rich purge controls the throttle valve 26 to reduce the intake air amount Ga and performs post injection to make the exhaust gas air-fuel ratio rich. If a temperature increase purge was performed during the previous processing, the temperature increase purge is terminated.

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

In S15, it is determined whether or not the flag Fp is 0. If the NOx storage capacity of the NSR catalyst 70 has not been fully restored, the flag Fp is 1 and a negative determination is made, and the process returns to S11 to continue releasing the NOx stored in the NSR catalyst 70.

Figure 5 is a flowchart showing the methane emission tolerance calculation process executed by the engine ECU 100. This flowchart is repeatedly processed at predetermined intervals while the engine 1 is operating (between when the ignition switch, not shown, is turned on and off). First, in S50, the current tolerance value AMe is calculated. The current tolerance value AMe is the tolerance value of the methane emission amount allowed from the previous processing of this routine to the current processing, and is calculated 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 processing of this routine to the current processing. In this embodiment, the current tolerance 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 processing and the current processing (AMe = Tme x SPD x Δt).

In the next step S51, the current methane emission allowable value AMe is added to the previous methane emission allowable value ΣAMe to calculate the current methane emission allowable value ΣAMe [mg], and the process proceeds to S52.

In S52, it is determined whether flag Fm is 1. If flag Fm is 1, the determination is affirmative and the routine ends. If flag Fm is 0, the determination is negative and the routine proceeds to S53.

In S53, it is determined whether the methane discharge amount ΣMe is equal to or less than the methane discharge allowable value ΣAMe. If the methane discharge amount ΣMe is equal to or less than the methane discharge allowable value ΣAMe (ΣMe≦ΣAMe), a positive determination is made and the current routine is terminated. If the methane discharge amount ΣMe exceeds the methane discharge allowable value ΣAMe (ΣMe>ΣAMe), a negative determination is made and the routine proceeds to S54.

In S54, the flag Fm is set to 1, and then the routine proceeds to S55, where the methane emission allowable value ΣAMe is reset to 0 [mg], and the current routine ends. In this way, when the methane emission amount ΣMe calculated in S14 (FIG. 3) during rich purge exceeds the methane emission allowable value ΣAMe, the flag Fm is set to 1.

Referring to FIG. 3, when flag Fm is set to 1, a negative determination is made in S12, rich purge is prohibited, and the process proceeds to S16. In S16, the methane discharge amount ΣMe is reset to 0 [mg], and then the process proceeds to S17.

In S17, it is determined whether or not a temperature increase purge can be performed. For example, if the temperature Tf of the SCRDPF 70 is equal to or higher than the activation temperature of the catalyst, it is determined that a temperature increase purge can be performed. If it is determined in S17 that a temperature increase purge can be performed, the process proceeds to S18. If a temperature increase purge cannot be performed, a negative determination is made in S17 and the process proceeds to S20.

In S18, a temperature increase purge is performed, and flag Fr is set to 0 and flag Ft is set to 1. If a rich purge was performed in the previous process, the rich purge is terminated. The temperature increase purge may be performed by controlling the throttle valve 26 to reduce the intake air amount Ga and retard the main injection timing to increase the temperature of the exhaust gas and increase the temperature of the NSR catalyst 70. In addition to reducing the intake air amount Ga and retarding the main injection timing, a post injection may be performed at an injection timing retarded from that when the rich purge was performed, so that the air-fuel ratio of the exhaust gas does not become rich. In addition to or instead of these, fuel may be added from the fuel addition valve 90 to increase the temperature of the exhaust gas and increase the temperature of the NSR catalyst 70. After the process of S18, proceed to S15.

When the NOx storage capacity of the NSR catalyst 70 is sufficiently restored, the NOx storage amount ΣNOx becomes equal to or less than the purge end threshold Df, and the flag Fp is set to 0 in S42 (FIG. 4), a positive determination is made in S15 and the process proceeds to S19.

In S19, the flag Fm is set to 0, and the routine proceeds to S20. In S20, if a rich purge is being performed, the rich purge is terminated. Also, if a temperature increase purge is being performed, the temperature increase purge is terminated. Furthermore, the flag Fr is set to 0, and the flag Ft is also set to 0, and the routine is then terminated.

Figure 6 is a diagram explaining the operation of this embodiment. In Figure 6, the horizontal axis is time, (A) represents the progress of the mileage, (B) represents the progress of the NOx storage amount ΣNOx, (C) represents the progress of the flag Fp, (D) represents the progress of the flag Fr, (E) represents the progress of the flag Ft, (F) represents the progress of the methane emission allowable value ΣAMe and the methane emission amount ΣMe, and (G) represents the progress of the flag Fm.

Referring to FIG. 6, when the trip of vehicle V starts at time t0, the mileage increases over time as shown in FIG. 6(A). As the mileage increases and time passes, the NOx storage amount ΣNOx increases as shown in FIG. 6(B). In addition, as the mileage increases, the methane emission allowable value ΣAMe increases as shown in FIG. 6(F).

At time t1, when the NOx occlusion amount ΣNOx becomes equal to or greater than the purge request threshold Dc (positive determination in FIG. 4: S33), flag Fp is set to 1 as shown in FIG. 6(C) (FIG. 4: S34). When flag Fp is set to 1 at time t1, if the engine is in an operating state where a rich purge is possible (positive determination in FIG. 3: S11), a rich purge is started, and flag Fr is set to 1 as shown in FIG. 6(D) (FIG. 3: S13).

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

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

At time t2, when the methane emission amount ΣMe exceeds the methane emission allowable value ΣAMe (negative judgment in FIG. 5: S53), as shown in FIG. 6 (F), the methane emission allowable value ΣAMe is temporarily reset to 0 [mg] (FIG. 5: S55), and then the methane emission allowable value ΣAMe increases as the mileage increases.

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

When flag Fm is set to 1, rich purge is prohibited (positive determination in S12 in FIG. 3), and if the operating state allows for temperature increase purge (positive determination in S17 in FIG. 3), temperature increase purge is started, and flag Ft is set to 1 (S18 in FIG. 3) as shown in (E) in FIG. 6. Also, flag Fr is set to 0 (S18 in FIG. 3) as shown in (D) in FIG. 6.

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

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

When flag Fp is set to 0 at time t3, a negative determination is made in S30 (FIG. 4), so that the NOx emission amount Ng is calculated, and as shown in FIG. 6(B), the NOx occlusion amount ΣNOx increases with increasing travel distance. When the NOx occlusion amount ΣNOx increases and reaches or exceeds the purge request threshold value Dc at time t4, flag Fp is set to 1. From time t4 to time t6, the same processing as from time t1 to time t3 is carried out.

According to this embodiment, when the NOx storage amount ΣNOx of the NSR catalyst 70 becomes equal to or greater than the purge request threshold Dc, the air-fuel ratio of the exhaust gas is made rich by the fuel injected from the injector 14 into the combustion chamber, and a rich purge is performed, releasing and reducing the NOx stored in the NSR catalyst 70. The engine ECU 100 calculates the methane emission amount ΣMe, which is the amount of methane discharged from the combustion chamber when a rich purge is performed, and 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. When the methane emission amount ΣMe exceeds the methane emission allowable value ΣAMe, the rich purge is prohibited, so that the amount of methane discharged into the atmosphere when a rich purge is performed can be controlled to an appropriate amount.

In this embodiment, the methane emission allowable value ΣAMe is calculated based on the travel distance of the vehicle V, so that methane emissions into the atmosphere can be appropriately managed in a manner similar to the regulations on carbon dioxide (CO2) emissions in exhaust gas. In addition, 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 the overflow of the register that holds the calculated values of the methane emission amount ΣMe and the methane emission allowable value ΣAMe can be prevented.

In this embodiment, when flag Fm is set to 1 and rich purge is prohibited, a temperature increase purge is performed until the NOx storage amount ΣNOx becomes equal to or less than the purge end threshold Df, raising the temperature of the NSR catalyst 70 and releasing NOx from the NSR catalyst 70, thereby recovering the NOx storage capacity of the NSR catalyst 70. The NOx released by the temperature increase purge is reduced and purified by the SCRDPF 72 provided downstream of the NSR catalyst 70, so that the NOx released from the NSR catalyst 70 can be prevented from being released into the atmosphere.

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. Alternatively, a DPF may be provided downstream of the NSR catalyst 70, and an SCR catalyst may be provided in the flow of the DPF.

The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is indicated by the claims rather than the above description, and is intended to include all modifications within the meaning and scope of the claims.

REFERENCE SIGNS LIST 1 engine, 2 torque converter, 3 automatic transmission, 4 differential gear, 5 rear wheel, 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 SCR catalyst, 72 SCRDPF, 74 SCR catalyst, 80 urea water tank, 82, 83 urea addition valve, 90 fuel addition valve, 100 engine ECU, 101 CPU, 102 Memory, 111 vehicle speed sensor, 112 engine speed sensor, 113 accelerator pedal sensor, 114 air flow meter, 115 NSR catalyst temperature sensor, 116 SCRDPF temperature sensor, 117, 118 NOx sensor, 119 A/F sensor, 200 transmission ECU.

Claims (8)

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 a combustion chamber of an internal combustion engine;
An exhaust gas purification system for an internal combustion engine comprising:
The control device includes:
When the NOx stored in the NOx storage reduction catalyst becomes equal to or greater than a purge request threshold, a rich purge is executed 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, and the NOx stored in the NOx storage reduction catalyst is released and reduced;
An exhaust purification system for an internal combustion engine, which calculates a methane emission amount, which is the amount of methane emitted from the combustion chamber when the rich purge is performed, and prohibits the rich purge when the methane emission amount exceeds a methane emission allowable value. The internal combustion engine is mounted on a vehicle,
The control device includes:
2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the allowable methane emission value is calculated based on a traveling distance of the vehicle. The control device includes:
3. The exhaust gas purification system for an internal combustion engine according to claim 2, wherein when the methane emission amount exceeds the methane emission allowable value, the methane emission amount and the methane emission allowable value are set to initial values. a selective reduction type NOx catalyst is provided in an exhaust passage downstream of the NOx storage reduction catalyst,
The control device includes:
4. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein, when the rich purge is performed, if the methane emission amount exceeds the methane emission allowable value and the rich purge is prohibited, a temperature increase purge is performed to increase the temperature of the NOx storage reduction catalyst and release the NOx stored in the NOx storage reduction catalyst. The control device includes:
5. The exhaust gas purification system for an internal combustion engine according to claim 4, wherein execution of the rich purge and the temperature increase purge is terminated when the amount of NOx stored in the NOx storage reduction catalyst becomes equal to or less than a purge end threshold value. 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 method for purifying exhaust gas from an internal combustion engine including a fuel injection valve that injects fuel into a combustion chamber of the internal combustion engine, comprising:
a step of performing a rich purge in which, when the NOx stored in the NOx storage reduction catalyst becomes equal to or greater than a purge request threshold, an air-fuel ratio of the exhaust gas is made rich by fuel injected from the fuel injection valve into the combustion chamber, and NOx is released and reduced;
a step of calculating a methane emission amount, which is an amount of methane emitted from the combustion chamber when the rich purge is performed, and prohibiting the rich purge when the methane emission amount exceeds a methane emission allowable value. The internal combustion engine is mounted on a vehicle,
The exhaust gas purification method includes:
7. The method for purifying exhaust gas from an internal combustion engine according to claim 6, further comprising the step of calculating the allowable methane emission value based on a traveling distance of the vehicle. 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 includes:
8. The exhaust gas purification method for an internal combustion engine according to claim 6, further comprising the step of performing a temperature raising purge in which a 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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