JP5070964B2 - NOx purification system and control method of NOx purification system - Google Patents

Description

  The present invention relates to a NOx purification system including a NOx storage reduction catalyst that purifies NOx (nitrogen oxide) in exhaust gas of an internal combustion engine, and a control method for the NOx purification system.

  Emission regulations for PM (particulate matter), NOx, CO (carbon monoxide), HC (hydrocarbon), etc. discharged from diesel engines have been strengthened year by year. With the tightening of regulations, it has become difficult to meet the regulation values only by improving the engine. Therefore, a technique has been adopted in which an exhaust gas aftertreatment device is attached to the exhaust passage of the engine to reduce these substances discharged from the engine.

  Under such circumstances, various studies and proposals have been made on NOx catalysts for reducing and removing NOx (nitrogen oxides) from exhaust gases of internal combustion engines such as diesel engines and some gasoline engines and various combustion devices. ing. One of them is a NOx occlusion reduction type catalyst as a NOx reduction catalyst for diesel engines. By using this NOx occlusion reduction type catalyst, NOx in the exhaust gas can be effectively purified.

This NOx occlusion reduction type catalyst is formed of a monolith honeycomb or the like, and a large number of polygonal cells are formed on a carrier of structural material formed of cordierite, silicon carbide (SiC) or ultrathin plate stainless steel of this monolith honeycomb. Configured. A porous catalyst coat layer serving as a catalyst support layer formed of alumina (Al ₂ O ₃ ), zeolite, silica, various oxides or the like is provided on the wall surface of the cell. A catalytic noble metal (catalytically active metal) having an oxidation function and a NOx storage agent (NOx storage material: NOx storage material: NOx absorbent) having a NOx storage function are supported on the surface of the catalyst coat layer. The catalyst noble metal is formed of platinum (Pt) or the like, and the NOx storage agent is an alkali metal such as potassium (K), sodium (Na), lithium (Li), or cesium (Cs), barium (Ba), calcium ( It is formed from some of alkaline earth metals such as Ca) and rare earths such as lanthanum (La) and yttrium (Y).

  Thus, three functions of NOx occlusion, NOx release, and NOx purification are exhibited depending on the oxygen concentration in the exhaust gas. That is, this NOx occlusion reduction type catalyst occludes NOx in the NOx occlusion agent during normal operation, and when the occlusion capacity approaches saturation, the air-fuel ratio of the inflowing exhaust gas is made the rich air-fuel ratio and occluded when appropriate. While releasing NOx, the released NOx is reduced by the ternary function of the catalytic noble metal.

More specifically, when the exhaust gas air-fuel ratio is in a lean air-fuel ratio state in which oxygen (O ₂ ) is contained in the exhaust gas of a normal diesel engine, lean burn gasoline engine, etc., Nitrogen monoxide (NO) discharged from the engine is oxidized into nitrogen dioxide (NO ₂ ) by the oxygen contained therein by the oxidation catalytic function of the catalytic noble metal. The nitrogen dioxide is stored in the form of chloride such as nitrate in a NOx storage agent such as barium having a NOx storage function to purify NOx.

  However, if this state is continued, the NOx occlusion agent having the NOx occlusion function is all changed to nitrate and loses the NOx occlusion function. Therefore, by changing the operating conditions of the engine, or by injecting a reducing agent such as fuel into the exhaust passage, oxygen is not present in the exhaust gas, and carbon monoxide (CO), hydrocarbon (HC), etc. An exhaust gas having a high concentration of reducing agent and a high exhaust temperature, that is, a rich combustion exhaust gas (rich spike gas) is produced and sent to the catalyst.

When the exhaust gas has no oxygen, the reducing agent concentration is high, and the exhaust gas temperature is raised to a rich air-fuel ratio, the NOx occluded nitrate releases nitrogen dioxide and returns to the original barium or the like. Since the released nitrogen dioxide does not contain oxygen in the exhaust gas, carbon monoxide, hydrocarbons, and hydrogen (H ₂ ) in the exhaust gas are used as a reducing agent on a catalytic noble metal such as platinum having an oxidizing function. , Converted into water, carbon dioxide (CO ₂ ), and nitrogen (N ₂ ) for purification.

  Therefore, in a NOx purification system equipped with a NOx occlusion reduction catalyst, when the NOx occlusion capacity becomes close to saturation, the stored NOx is released and purified to regenerate the catalyst, so that the fuel is increased from the stoichiometric air-fuel ratio. Therefore, it is necessary to supply the catalyst with high-temperature reducing composition exhaust gas in which the air-fuel ratio of the exhaust gas is made rich and the oxygen concentration of the inflowing exhaust gas is reduced. By performing rich control for recovering the NOx storage capacity, the absorbed NOx is released, and a NOx regeneration operation is performed in which the released NOx is reduced by a noble metal catalyst.

  In the NOx purification system of the prior art, in an operation state where the exhaust temperature is about 250 ° C. or less with a low load, the NOx occlusion reduction type catalyst is not activated and occluded even if NOx regeneration control by rich combustion is performed. Only NOx is desorbed and NOx purification does not occur. Therefore, NOx purification cannot be performed in a low temperature range, and the NOx purification rate is extremely reduced. Therefore, in the low load region, there is a problem that NOx is saturated and the NOx occlusion reduction type catalyst flows out downstream (NOx slip).

Further, when the load increases from this low load region and the temperature of the exhaust gas rises, the NOx that has been occluded and adsorbed from the occlusion agent or the catalyst surface is released at once due to this temperature rise. Due to the shortage and the high O ₂ concentration in the exhaust gas, there is a problem that NOx purification becomes insufficient and the NOx purification performance is extremely deteriorated.

  In addition, even in the middle temperature range and high temperature range where catalytic activity is occurring, the engine operating conditions are switched to generate rich exhaust gas with extremely low oxygen concentration at high temperatures in order to release and purify NOx stored in the catalyst. Thus, by supplying this exhaust gas to the catalyst, NOx release and reduction purification are performed in the catalyst. In this case, in a diesel engine that is not good at generating rich exhaust gas, problems such as extreme deterioration of fuel consumption, deterioration of smoke and PM, and deterioration of durability due to this high concentration smoke generation operation, Furthermore, there is a problem that the drivability of the car is also deteriorated due to inappropriate combustion. Further, in the case of a diesel engine, about several percent of oxygen remains in the rich exhaust gas during NOx regeneration, so the NOx purification rate during NOx regeneration is also reduced.

  In order to solve the problems of extreme deterioration of fuel consumption due to addition of the reducing agent by switching of the operating conditions, smoke and PM, the fuel addition means and the plasma reformer are provided upstream of the NOx storage reduction type catalyst. By providing a bypass passage, the fuel is decomposed into hydrogen and carbon monoxide by plasma generated by discharge into the exhaust gas, and the oxygen in the exhaust gas in the low-temperature region is decomposed by this highly reactive hydrogen and carbon monoxide. Has been proposed to reduce the concentration of oxygen in the exhaust gas and release NOx, and to efficiently reduce this NOx to nitrogen by highly reactive hydrogen and carbon monoxide. (For example, refer to Patent Document 1).

However, in order to solve the above-mentioned problem when rich exhaust gas for regeneration of catalyst NOx is generated by changing operating conditions in a lean combustion engine, in this exhaust purification device, a catalyst is added by fuel addition and plasma discharge to generate rich exhaust gas. The production and supply of effective reducing gas. In this case, the fuel addition and the exhaust gas that is plasma-discharged are performed in a flowing state, and therefore, a sufficient rich reducing gas cannot be generated. In addition, the O ₂ concentration does not sufficiently decrease, and the rich exhaust gas is initially in the rich fuel exhaust state and becomes deep rich exhaust gas in the latter half, which is contrary to the supply rich exhaust gas condition in which the catalyst can efficiently regenerate NOx.
JP 2006-316654 A

The present invention has been made to solve the above-described problems, and its object is to restore the NOx occlusion ability of the NOx occlusion reduction type catalyst without changing the operation state of the internal combustion engine and to decompose it by plasma. The exhaust gas containing a large amount of oxidized HC, CO, H ₂ can improve the low temperature activity, improve the NOx purification performance from the low temperature range, improve the durability of the internal combustion engine, improve the fuel consumption, It is possible to prevent deterioration of drivability, and in particular, by supplying a rich rich exhaust gas in which the O ₂ concentration at the initial stage of NOx regeneration is extremely reduced, the temperature of the catalyst increases due to the release of residual oxygen at the initial stage of NOx regeneration. An object of the present invention is to provide a NOx purification system and a control method for the NOx purification system that can sufficiently cope with the release of NOx.

In the NOx purification system for achieving the above object, NOx is occluded in the exhaust passage of the internal combustion engine when the air-fuel ratio of the exhaust gas is lean, and occluded when it is rich. In a NOx purification system including a NOx storage reduction catalyst that releases and reduces NOx, and a control device that performs NOx regeneration control for recovering the NOx storage capacity of the NOx storage reduction catalyst,
A reducing agent addition apparatus for providing a part of the exhaust passage on the upstream side of the NOx occlusion reduction type catalyst to a first branch exhaust passage and a second branch exhaust passage which are arranged in parallel and for adding a reducing agent to the first branch exhaust passage. A plasma generator for generating plasma in the first branch exhaust passage, and a control valve for opening and closing a downstream portion of the first branch exhaust passage, and the control device includes a NOx storage reduction catalyst. When it is determined that the NOx storage capacity needs to be restored, the first branch exhaust passage is closed by closing the control valve, and the reducing agent is added to the first branch exhaust passage by the reducing agent addition device. The plasma generator is configured to generate plasma in the first branch exhaust passage .

In the NOx purification system, the control device opens the control valve after a lapse of a predetermined time after the plasma is generated in the first branch exhaust passage by the plasma generator . The branch exhaust passage is opened, and the addition of the reducing agent and the generation of the plasma are continued until it is determined that the NOx storage capacity has been recovered to a predetermined range.

And, the control method of the NOx purification system for achieving the above object is the case where the exhaust passage of the internal combustion engine stores NOx when the air-fuel ratio of the exhaust gas is lean, and the rich state The NOx occlusion reduction type catalyst that releases and reduces the NOx occluded in the NOx occlusion reduction catalyst is provided, and a part of the exhaust passage on the upstream side of the NOx occlusion reduction type catalyst is divided into a first branch exhaust passage and a second branch exhaust passage in parallel. A reducing agent addition device for adding a reducing agent to the first branch exhaust passage, a plasma generator for generating plasma in the first branch exhaust passage, and opening and closing a downstream portion of the first branch exhaust passage In a control method of a NOx purification system, comprising: a control valve that performs control, and a control device that performs NOx regeneration control for recovering the NOx storage capacity of the NOx storage reduction catalyst. When it is determined that the NOx storage capacity of the Ox storage reduction catalyst needs to be recovered, the first branch exhaust passage is closed by closing the control valve, and the first branch exhaust passage is closed by the reducing agent addition device. A reducing agent is added to the plasma, and plasma is generated in the first branch exhaust passage by the plasma generator, and after the elapse of a predetermined time, the first branch exhaust passage is opened by opening the control valve to store NOx. The control method is characterized in that the addition of the reducing agent and the generation of the plasma are continued until it is determined that the capacity has recovered to a predetermined range.

  In the present invention, in the NOx regeneration control, the exhaust passage on the upstream side of the NOx occlusion reduction type catalyst is divided into two parts, the first and second branch exhaust passages. A reducing agent is injected, and the reducing agent is oxidized at a low temperature by plasma. Thereby, NOx purge and purification of the NOx storage reduction catalyst, that is, NOx regeneration, can be performed without changing the operating condition of the engine. In particular, since a reducing agent activated by plasma can be used, NOx regeneration can be performed even in a low temperature range, the reaction efficiency of the reducing agent can be improved, and the amount of reducing agent flowing out without reacting during NOx regeneration can be reduced. , Deterioration of fuel consumption due to NOx regeneration can be suppressed.

  At the start of this NOx regeneration, the downstream portion of the first branch exhaust passage is closed for a predetermined time (for example, about 1.0 s to 1200 s: the time during which the NOx storage reduction catalyst (LNT catalyst) is stored). As a result, the oxygen concentration in the exhaust gas staying in the first branch exhaust passage is reduced to 0%, and an exhaust gas in which a large amount of a partial oxidation-reduction agent having high reaction activity coexists is produced. When the NOx occlusion reduction type catalyst is saturated with NOx, the downstream portion of the first branch exhaust passage is opened, and the exhaust gas coexisting in a large amount with this partial redox agent is supplied to the NOx occlusion reduction type catalyst and occluded. Release the NOx and reduce purification. As a result, it is possible to efficiently perform the treatment of oxygen adsorbed and stored in the catalyst released in the early stage of NOx regeneration and the treatment of a large amount of NOx.

That is, in the initial stage of NOx regeneration, O ₂ is adsorbed on the catalyst surface. When the catalyst is in an oxidized state and HC, CO, and H ₂ are supplied, the temperature of the catalyst rises due to the oxidation reaction, and a large amount of NOx is released. Therefore, an excessive reducing agent is required.

Also, after opening the downstream portion of the first branch exhaust passage, the reducing agent injection and plasma generation in the first branch exhaust passage are continued, the reducing agent is oxidized at a low temperature by plasma, and the activated reducing agent is removed. In the shallow rich state, NOx is released from the NOx occlusion reduction catalyst and the released NOx is reduced and purified. Further, by making the shallow rich state in the latter half, residual gases of HC, CO, and H ₂ are oxidized to prevent slipping. Thereby, reduction purification according to the amount of NOx released from the NOx occlusion reduction catalyst after releasing a large amount of NOx can be efficiently performed, and deterioration of fuel consumption can be prevented.

According to the NOx purification system and the control method of the NOx purification system according to the present invention, the reducing agent is directly injected into the exhaust passage without changing the operating condition of the internal combustion engine, and the reducing agent is decomposed and generated by generation of plasma. Since it is reformed and supplied to the NOx occlusion reduction type catalyst, this reformed reducing agent can reduce the oxygen concentration even in a low temperature range and can efficiently reduce the released NOx to nitrogen. . Gas oil is decomposed by plasma over a sufficient period of time, so that O ₂ is consumed (to O ₂ concentration 0 ppm) and catalytic activity is generated from a low temperature region by the generation of decomposition HC, CO, H _2. The purification rate can be improved.

  Accordingly, NOx purification performance can be improved from a low temperature range, and NOx regeneration of the NOx storage reduction catalyst and NOx released during NOx regeneration are reduced and purified without changing the operating conditions of the internal combustion engine. Therefore, it is possible to improve the durability of the internal combustion engine, improve the fuel consumption, and further prevent the drivability from deteriorating.

In particular, in the initial stage of NOx regeneration, the exhaust gas is supplied with exhaust gas that is extremely reduced in oxygen concentration and rich in reducing agent. In addition, the degree of decrease in oxygen concentration and reducing agent concentration are extremely large in the initial stage, and thereafter oxygen is reduced. Since the degree of concentration reduction and the reducing agent concentration can be gradually reduced to a predetermined value, it is possible to prevent NOx slip in the rich initial stage and to prevent HC, CO, H ₂ slip in the latter half of the rich condition. , Improve the purification rate.

In the present invention provided with a control valve, the diaphragm of the downstream portion of the first branch exhaust passage, for performing the consumption of degradation coexistence of O ₂ added fuel was quiescent exhaust gas flow, necessary and sufficient and reliable enough from O ₂ consumption The rich exhaust gas in which the reducing agent is present can be generated, and the concentration distribution of the supplied rich exhaust gas can be generated ideally, and the rich exhaust gas that is initially overconcentrated and shallow in the latter half can be generated.

  Accordingly, it is possible to sufficiently cope with oxygen release and a large amount of NOx release at the initial stage of NOx regeneration, and NOx release and reduction can be efficiently performed thereafter, so that the NOx purification rate can be improved.

  Hereinafter, an NOx purification system and a control method of the NOx purification system according to an embodiment of the present invention will be described with reference to the drawings. The present invention is applied to an entire lean combustion engine represented by a diesel engine. Here, a diesel engine will be described as an example. The rich state of exhaust gas here does not necessarily need to be richly burned in the cylinder, but the “air amount” and “fuel amount (in the cylinder) supplied to the exhaust gas flowing into the NOx storage reduction catalyst. The ratio of "combusted) + reducing agent (usually using a part of fuel)" is in a state close to the stoichiometric air-fuel ratio or in a rich state in which the amount of fuel is greater than the stoichiometric air-fuel ratio.

  FIG. 1 shows a configuration of a NOx purification system 1 according to an embodiment of the present invention. In this NOx purification system 1, a NOx occlusion reduction type catalyst (exhaust gas purification device: catalytic converter) 20 is disposed in an exhaust passage 3 of an engine (internal combustion engine) E.

The NOx occlusion reduction type catalyst 20 is provided with a catalyst coating layer made of aluminum oxide (Al ₂ O ₃ ), titanium oxide (TiO) or the like on a monolith catalyst formed of cordierite or silicon carbide (SiC) ultra-thin plate stainless steel. The catalyst coat layer is configured to carry a catalyst metal such as platinum (Pt) or palladium (Pd) and a NOx storage material (NOx storage material) such as barium (Ba). The monolith catalyst structural material carrier has a large number of cells, and the catalyst coat layer provided on the inner wall of the cells has a large surface area to increase the contact efficiency with the exhaust gas. .

  In the NOx occlusion reduction type catalyst 20, when the oxygen concentration is in the exhaust gas state (lean air-fuel ratio state), the NOx occlusion material occludes NOx in the exhaust gas, thereby purifying NOx in the exhaust gas, When the oxygen concentration is low or zero (0%) in the exhaust gas state (rich air-fuel ratio state), the stored NOx is released and the released NOx is reduced by the catalytic action of the catalytic metal, thereby returning to the atmosphere. Prevent NOx outflow.

  A control device (ECU: engine control unit) 30 that performs overall control of the operation of the engine E and also performs recovery control of the NOx purification ability of the NOx storage reduction catalyst 20 is provided. An upstream NOx concentration sensor 21, an upstream excess air ratio (λ) sensor (oxygen concentration sensor) 22, and an upstream exhaust gas temperature sensor 23 are disposed on the upstream side (inlet side) of the NOx storage reduction catalyst 20. A downstream exhaust gas temperature sensor 24, a downstream NOx concentration sensor 25, and a binary λ sensor (oxygen concentration sensor) 26 are disposed downstream of the NOx storage reduction catalyst 20. The outputs of these sensors are input to a control unit (ECU) 30. The binary λ sensor 26 is a sensor whose output characteristics change greatly when the theoretical air-fuel ratio λ = 1.

  The control unit (ECU) 30 is a digital computer having a configuration in which a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), an input port, and an output port are connected in a bidirectional path. In this embodiment, the engine combustion control, plasma control, NOx occlusion reduction type catalyst control, exhaust throttle valve control, intake throttle valve control are performed. Also do.

  In order to perform these controls, the control device 30 is further supplied with detection values (output signals) from the load sensor 28 from the accelerator opening, the crank angle sensor 29 provided on the engine crankshaft, and the like. The control device 30 outputs a signal for controlling the intake throttle valve (intake throttle valve) 8 of the engine E, the EGR valve 12, the fuel injection valve 13 of the common rail electronic control fuel injection device for fuel injection, and the like.

  In this NOx purification system 1, the intake air A sucked into the engine E passes through the air purifier 5 and the mass air flow sensor (MAF sensor) 6 in the intake passage 2 and is compressed and pressurized by the compressor of the turbocharger 7. Further, it is cooled by an intercooler (not shown), the amount thereof is adjusted by the intake throttle valve 8, and enters the cylinder from the intake manifold. The mass air flow sensor 6 measures the amount of intake air, and a voltage corresponding to the flow rate of the intake air A is input to a control device (ECU) 30. The intake throttle valve 8 is controlled by a control signal from a control unit (ECU) 30. The fuel pressurized by the fuel pump 10 from the fuel tank 9 is injected into the air A sucked into the cylinder through the common rail and the fuel injection valve 13 and burned.

  The exhaust gas G generated in the cylinder by this combustion exits from the exhaust manifold to the exhaust passage 3, drives the turbine of the turbocharger 7, and then passes through the NOx occlusion reduction catalyst 20 to be purified. Then, it is discharged into the atmosphere through a silencer (not shown). Part of the exhaust gas G is cooled as EGR gas Ge through the high-efficiency EGR cooler 11 in the EGR passage 4, and the amount thereof is adjusted by the EGR valve 12 installed on the outlet side of the EGR passage 4. It is recirculated to the intake manifold on the downstream side of the throttle valve 8. In this embodiment, a large amount of EGR gas can be recirculated.

  In the present invention, the exhaust passage 3 is branched upstream of the NOx storage reduction catalyst 20 and divided into a first branch exhaust passage 3a and a second branch exhaust passage 3b, and the first branch exhaust passage 3a The second branch exhaust passage 3 b is joined on the upstream side of the NOx storage reduction catalyst 20. That is, a part of the exhaust passage 3 on the upstream side of the NOx storage reduction catalyst 20 is divided into a first branch exhaust passage 3a and a second branch exhaust passage 3b which are arranged in parallel.

  The first branch exhaust passage 3 a is provided with a plasma generator 32 and an exhaust throttle valve 33 connected to a high-voltage power supply 31. The exhaust throttle valve 33 is a control valve that opens and closes the downstream portion of the first branch exhaust passage 3a. The first branch exhaust passage 3a is configured to join the second branch exhaust passage 3b downstream of the exhaust throttle valve 33. The NOx occlusion reduction type catalyst 20 is disposed downstream of this junction. Further, the reducing agent addition valve 34 for adding a part of the fuel to the exhaust gas in the first branch exhaust passage 3a is located at a position facing the upstream of the first branch exhaust passage 3a as shown in FIG. It arrange | positions in the upstream in the exhaust passage 3a. The high-voltage power supply 31, the exhaust throttle valve 33, and the reducing agent addition valve 34 are controlled by control signals from a control device (ECU) 30.

  Next, a control method in the NOx purification system 1 will be described. In this control method, in the normal operation of the diesel engine E, lean operation in a lean combustion state is performed. In this lean air-fuel ratio state, NOx in the exhaust gas is stored in the NOx storage reduction catalyst 20 and the exhaust gas is purified. The When NOx is occluded in the NOx occlusion reduction type catalyst 20 and the NOx occlusion capacity becomes close to saturation, it is determined whether or not the NOx occlusion amount threshold value of the catalyst determined by the preliminary test has been reached, and the threshold value has been reached. First, NOx regeneration control for releasing and reducing NOx to purify is performed, and NOx purification performance is maintained.

  In the present invention, this NOx regeneration control is performed as follows according to the control flow illustrated in FIG. The control flow shown in FIG. 2 is called and executed when it is necessary to determine whether or not the NOx regeneration control of the NOx storage reduction catalyst 20 is necessary from the advanced control flow for controlling the overall operation of the engine E. Are shown as being repeatedly executed. The control flow of FIG. 2 starts with the advanced control flow when the operation of the engine E is started, and ends with the completion of the operation of the engine E and the advanced control flow.

  When the control flow of FIG. 2 is called from the advanced control flow and starts, the NOx occlusion amount Va occluded in the NOx occlusion reduction type catalyst 20 is estimated in step S11. In the next step S12, the NOx occlusion amount Va is checked to determine whether or not it is larger than a predetermined start determination amount (threshold value) Vac. This determination is a determination of whether or not to start NOx regeneration control, and the well-known determination of the start of NOx regeneration control can be used.

  For example, the NOx occlusion amount Va is estimated by calculating the NOx amount exhausted from the operating state of the engine E, or by detecting the NOx occlusion reduction type catalyst 20 from the detected value of the upstream NOx concentration sensor 21 and the exhaust gas amount. The inflowing NOx inflow amount is calculated, and the NOx outflow amount flowing out from the NOx occlusion reduction type catalyst 20 is calculated from the detection value of the downstream NOx concentration sensor 25 and the exhaust gas amount. The amount stored in the NOx storage reduction catalyst 20 is calculated from the difference between the NOx inflow amount and the NOx outflow amount, and this amount is integrated over time to obtain the NOx storage amount. The exhaust gas amount can be calculated from the intake air amount and the fuel injection amount.

  If it is determined in step S12 that the NOx occlusion amount Va is equal to or less than the predetermined start determination amount Vac, it is determined that the NOx regeneration control is not started and the routine proceeds to step S13 to continue the normal operation of the lean air-fuel ratio operation. This normal operation is performed for a predetermined time (a time related to the NOx occlusion amount check interval), and the process returns.

  Further, if the NOx occlusion amount Va is larger than the predetermined start determination amount Vac in the determination in step S12, it is determined that the NOx regeneration control is started, the process proceeds to step S20, and NOx regeneration control is performed. This NOx regeneration control is not the rich air-fuel ratio control of the prior art, but is performed as follows in the present invention.

  First, in step S21, the exhaust throttle valve 33 installed in the downstream portion of the plasma generator 32 is closed. As a result, the entire amount of exhaust gas flows through the second branch exhaust passage 3b. At substantially the same time, in the next step S22, the high-voltage power supply 31 and the plasma generator 32 are turned on to operate, and plasma is generated in the first branch exhaust passage 3a where the flow of exhaust gas is interrupted. At the same time, the reducing agent addition valve 34 is turned on to open the reducing agent addition valve 34, and the exhaust pipe reducing agent is added. Thereby, with the exhaust throttle valve 33 closed, the exhaust gas stays in the first branch exhaust passage 3a, and hydrocarbon (HC) of the reducing agent (a part of the fuel) injected into the exhaust gas is plasma. Are decomposed and modified. By the hydrocarbon (HC) activated by the decomposition and reforming, oxygen remaining in the rich exhaust gas is carbonized by plasma activation in the first branch exhaust passage 3a closed by the exhaust throttle valve 33. Since it is consumed by reacting with hydrogen, exhaust gas having an oxygen concentration of zero is generated, so that the downstream tip side becomes deep rich exhaust gas and the upstream side becomes relatively shallow rich exhaust gas.

  The generation of the plasma and the addition of the exhaust pipe reducing agent in step S22 are performed for a predetermined time (for example, 1 s to 1200 s) set in advance by an experiment or the like, the process proceeds to the next step S23, and the exhaust throttle valve 33 is instantaneously opened. . By opening the exhaust throttle valve 33, the exhaust gas flow path returns to the original first branch exhaust passage 3a, so that the rich exhaust gas that has stopped so far is supplied to the NOx storage reduction catalyst 20. Even after the exhaust throttle valve 33 is opened, plasma discharge and exhaust pipe reducing agent addition continue.

  After the exhaust throttle valve 33 is opened in step S23, in step S24, the exhaust pipe reducing agent addition adjustment control is performed for a predetermined time (time related to the NOx release amount check interval). Go to check NOx release amount Vb. In this adjustment control of the exhaust pipe reducing agent addition, the valve opening degree of the reducing agent addition valve 34 is controlled while monitoring the detection value of the upstream excess air ratio (λ) sensor 22, and a predetermined rich air-fuel ratio is set. The reducing agent addition amount of the exhaust pipe reducing agent is adjusted so that the excess air ratio (λ) (for example, about 1.00 to 0.97 in terms of excess air ratio) is obtained. That is, if the detected λm value is larger than the target λc value, the reducing agent addition amount is decreased, and if the detected λm value is smaller than the target λc value, the reducing agent addition amount is increased.

  The check of the NOx release amount Vb in step S25 is for determining whether or not NOx regeneration has been completed, in other words, whether or not the NOx regeneration control can be terminated, and uses a conventional determination method. be able to.

  For example, the amount of NOx released from the NOx occlusion reduction catalyst 20 is obtained in advance through experiments or the like based on the control for initial NOx regeneration in steps S21 to S23 and the control for NOx regeneration in step S24. It is set by map data or the like corresponding to the operating state of the engine and input to the control device 30. The amount of NOx released is calculated from the engine operating state at the time of regeneration with reference to these map data, and the amount of NOx released is integrated over time to calculate the amount of NOx released.

  In the check of the NOx release amount Vb in step S25, if the NOx release amount Vb is equal to or less than the predetermined end determination amount (threshold value) Vbc considering the NOx occlusion amount Va, it is still necessary to continue the NOx regeneration. Then, the process returns to step S24, and the exhaust pipe reducing agent addition adjustment control is performed. The adjustment control of the exhaust pipe reducing agent addition in step S24 is performed until the NOx release amount Vb becomes larger than the predetermined end determination amount Vbc in step S25.

  In the check of the NOx release amount Vb in step S25, if the NOx release amount Vb is larger than the predetermined end determination amount Vbc, the NOx release may be completed and the NOx regeneration may be ended. The NOx regeneration end work is performed. In this NOx regeneration end operation, the exhaust pipe reducing agent addition is stopped and plasma generation is stopped. Thereby, the NOx regeneration control in step S20 ends.

After the end of the NOx regeneration, the process returns to the advanced control flow. Although depending on the NOx occlusion reduction type catalyst, for example, while the predetermined time during the closing of the exhaust throttle valve 33 is within the lean condition, the addition of the reducing agent and the generation of plasma The NOx regeneration time that is the period is about 1 s to 30 s, and the lean operation time from the end to the start of NOx regeneration control is about 1 s to 1200 s.

  According to this control method, in the initial stage of NOx regeneration, the exhaust throttle valve 33 in step S23 is opened, and the rich rich exhaust gas generated in the first branch exhaust passage 3a is immediately discharged into the NOx storage reduction catalyst. Therefore, NOx released in large quantities by NOx regeneration can also be efficiently reduced. This deep rich exhaust gas has a substantially zero oxygen concentration and a large amount of hydrocarbon as a reducing agent, and is suitable for NOx reduction. In addition, since the hydrocarbon serving as the reducing agent is modified by plasma, the released NOx can be reduced efficiently. Therefore, the NOx purification rate in the initial stage of this NOx regeneration can be remarkably improved.

  Further, after that, the exhaust gas can be caused to flow into the shallow rich state by the adjustment control of the exhaust pipe reducing agent addition in step S24, and the reducing agent supplied into the exhaust gas. Since the hydrocarbon to be modified by plasma is activated, the activated hydrocarbon can efficiently consume oxygen and efficiently reduce the released NOx with a small amount of reducing agent added. . Further, since this shallow rich exhaust gas, hydrocarbons are oxidized while passing through the NOx occlusion reduction catalyst 20, and the NOx occlusion reduction catalyst 20 is heated by this oxidation reaction heat. The activation of NOx is promoted, and the NOx purification performance is improved.

In these NOx regeneration controls, carbon monoxide (CO), hydrogen (H) that has passed through the plasma and was activated even in a temperature range of 300 ° C. or lower, which is a low temperature range in which catalytic activity does not occur with a normal light oil reducing agent HC. ₂ ) Since exhaust gas having a large amount of components such as lower hydrocarbons is allowed to flow into the NOx occlusion reduction type catalyst 20, the catalytic activity is improved from a low temperature range and a high NOx purification rate is obtained.

Therefore, according to the control method of the NOx purification system and the NOx purification system, the reducing agent is directly injected into the exhaust passage 3a without changing the operating condition of the engine E, and the reducing agent is decomposed by generation of plasma. In addition, since the reformed and supplied NOx occlusion reduction type catalyst 20 is supplied, the reformed reducing agent can reduce the oxygen concentration even in a low temperature range and efficiently convert the released NOx into nitrogen. It can be reduced. Since fuel (light oil) is decomposed by plasma over a sufficient period of time, O ₂ is consumed and O ₂ concentration is reduced to 0 ppm, and catalytic activity is generated from a low temperature range by generating decomposed HC, CO, and H _2. The purification rate can be improved.

  As a result, the NOx purification performance can be improved from a low temperature range, and NOx regeneration of the NOx occlusion reduction type catalyst 20 and NOx released during NOx regeneration can be reduced and purified without changing the operating conditions of the engine E. Therefore, the durability of the engine E can be improved, the fuel consumption can be improved, and the drivability can be prevented from deteriorating. In particular, a system that does not deteriorate NOx regeneration and engine operating conditions at low temperatures can be realized.

In particular, in the initial stage of NOx regeneration, the exhaust gas is supplied with exhaust gas that is extremely reduced in oxygen concentration and rich in reducing agent. In addition, the degree of decrease in oxygen concentration and reducing agent concentration are extremely large in the initial stage, and thereafter oxygen is reduced. Since the degree of concentration reduction and the reducing agent concentration can be gradually reduced to a predetermined value, it is possible to prevent NOx slip in the rich initial stage and to prevent HC, CO, H ₂ slip in the latter half of the rich condition. , Improve the purification rate.

  In addition, an exhaust throttle valve (control valve) 33 is provided to throttle the downstream portion of the first branch exhaust passage 3a and stop the exhaust gas flow to decompose the added fuel and consume residual oxygen. It is possible to generate rich exhaust gas that contains consumption and certain necessary and sufficient reducing agent, and it is possible to ideally generate the concentration distribution of the supply rich exhaust gas, and it is possible to generate rich exhaust gas that is initially excessively concentrated and shallow in the latter half.

  Accordingly, it is possible to sufficiently cope with oxygen release and a large amount of NOx release at the initial stage of NOx regeneration, and NOx release and reduction can be efficiently performed thereafter, so that the NOx purification rate can be improved.

It is a figure which shows the structure of the NOx purification system of embodiment which concerns on this invention. It is a figure which shows an example of the control flow for enforcing the control method of the NOx purification system of embodiment which concerns on invention.

Explanation of symbols

E engine (internal combustion engine)
DESCRIPTION OF SYMBOLS 1 NOx purification system 3 Exhaust passage 3a 1st branch exhaust passage 3b 2nd branch exhaust passage 20 Occlusion reduction type catalyst (exhaust gas purification apparatus: catalytic converter)
30 control unit (ECU)
31 High-voltage power supply 32 Plasma generator 33 Exhaust throttle valve 34 Reducing agent addition valve A Intake air G Exhaust gas Gc Purified exhaust gas

Claims (3)

The exhaust passage of the internal combustion engine is provided with a NOx occlusion reduction type catalyst that occludes NOx when the air-fuel ratio of the exhaust gas is lean, and releases and reduces NOx occluded when it is rich. In the NOx purification system comprising a control device for performing NOx regeneration control for recovering the NOx storage capacity of the NOx storage reduction catalyst,
A reducing agent addition apparatus for providing a part of the exhaust passage on the upstream side of the NOx occlusion reduction type catalyst to a first branch exhaust passage and a second branch exhaust passage which are arranged in parallel and for adding a reducing agent to the first branch exhaust passage. And a plasma generator for generating plasma in the first branch exhaust passage, and a control valve for opening and closing a downstream portion of the first branch exhaust passage ,
When the control device determines that the NOx storage capacity of the NOx storage reduction catalyst needs to be restored, the control valve closes the first branch exhaust passage by closing the control valve, and the reducing agent addition device A NOx purification system , wherein a reducing agent is added to the first branch exhaust passage, and plasma is generated in the first branch exhaust passage by the plasma generator . The control device opens the first branch exhaust passage by opening the control valve after a lapse of a predetermined time after the plasma is generated in the first branch exhaust passage by the plasma generator , and the NOx storage capacity 2. The NOx purification system according to claim 1, wherein the addition of the reducing agent and the generation of the plasma are continued until it is determined that has recovered to a predetermined range. The exhaust passage of the internal combustion engine is provided with a NOx occlusion reduction type catalyst that occludes NOx when the air-fuel ratio of the exhaust gas is lean, and releases and reduces NOx occluded when it is rich. A part of the exhaust passage on the upstream side of the NOx occlusion reduction catalyst is divided into a first branch exhaust passage and a second branch exhaust passage which are provided in parallel, and a reducing agent addition device which adds a reducing agent to the first branch exhaust passage. A plasma generator for generating plasma in the first branch exhaust passage, a control valve for opening and closing a downstream portion of the first branch exhaust passage, and a NOx storage capacity of the NOx storage reduction catalyst for recovering In a control method of a NOx purification system including a control device that performs NOx regeneration control,
When it is determined that the NOx occlusion capacity of the NOx occlusion reduction catalyst needs to be restored, the first branch exhaust passage is closed by closing the control valve, and the first branch exhaust is made by the reducing agent addition device. A reducing agent is added to the passage, plasma is generated in the first branch exhaust passage by the plasma generator, and after a predetermined time has elapsed, the first branch exhaust passage is opened by opening the control valve, and NOx The control method of the NOx purification system, wherein the addition of the reducing agent and the generation of the plasma are continued until it is determined that the storage capacity has recovered to a predetermined range.

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