JP4165085B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an engine exhaust purification device.
[0002]
[Prior art]
  As a catalyst for purifying engine exhaust gas, in an oxygen-excess atmosphere with high oxygen concentration of exhaust gas, NOx in the exhaust gas is absorbed by NOx absorbent such as Ba and the oxygen concentration of exhaust gas decreases (theoretical) The NOx (nitrogen oxide) absorbed when the air-fuel ratio is approached or when the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio is released and moved onto the noble metal, and this is reduced to the reducing gas (HC) in the exhaust gas. (Hydrocarbons, CO, etc.) and N₂A NOx absorption catalyst that reduces and purifies is known.
[0003]
  For example, Japanese Patent Application Laid-Open No. 11-223123 discloses a NOx reduction catalyst that reduces and purifies NOx in an exhaust gas in an oxygen-excess atmosphere where the oxygen concentration of the exhaust gas is high in order to purify the exhaust gas of a lean burn gasoline engine. It is described that the NOx absorption catalyst is arranged such that the former is on the upstream side in the exhaust passage and the latter is on the downstream side. As the NOx reduction catalyst, a catalyst in which at least one of copper, cobalt, iron or nickel and a noble metal such as platinum are supported on zeolite is disclosed.
[0004]
  That is, the technique described in the above publication uses a NOx absorption catalyst having a high NOx purification rate when the exhaust gas temperature is relatively low (250 to 400 ° C.) and NOx when the exhaust gas temperature is high (400 ° C. or more). By combining a NOx reduction catalyst with a high purification rate, the temperature range in which NOx in the exhaust gas can be purified is expanded to the high temperature side.
[0005]
[Problems to be solved by the invention]
  However, in the case of a diesel engine, although the exhaust gas temperature generally rises to about 350 ° C. or 400 ° C. at high speeds exceeding the vehicle traveling speed of 60 km / h, the exhaust gas temperature at low speeds below 60 km / h is 150 to It is about 200 ° C. and low. Further, even in a gasoline engine, the exhaust gas temperature at the start of the engine is low. On the other hand, the NOx reduction catalyst described in the above publication has a high NOx purification rate at high temperatures, but deteriorates NOx purification characteristics at low temperatures. On the other hand, even if the NOx absorption catalyst is effective at a lower temperature than the NOx reduction catalyst, the effective temperature range is about 250 to 400 ° C., and the exhaust gas temperature is low as in a diesel engine. It is not valid.
[0006]
  The subject of this invention is expanding the temperature range which can purify NOx effectively to the low temperature side.
[0007]
[Means for Solving the Problems]
  Therefore, in the present invention, the NOx absorption catalyst is combined with a NOx reduction catalyst having excellent NOx purification activity at a low temperature.
[0008]
  That is, the invention according to claim 1 is disposed in the exhaust passage of the engine and absorbs NOx in the exhaust gas in an oxygen-excess atmosphere where the oxygen concentration of the exhaust gas is high, and absorbs it when the oxygen concentration of the exhaust gas decreases. NOx absorption catalyst for reducing and purifying NOx that has been released,
  A NOx reduction catalyst that is disposed in the exhaust passage upstream of the NOx absorption catalyst and reduces and purifies NOx in the exhaust gas in the oxygen-excess atmosphere;
  In an engine exhaust purification device comprising a reducing agent increasing means for increasing the reducing agent in the exhaust gas supplied to the NOx reduction catalyst,
  The NOx absorption catalyst has a higher NOx purification performance when it is in a predetermined first temperature range than when it is in a second temperature range lower than the first temperature range,
  The NOx reduction catalyst has a higher NOx purification performance when in the second temperature range than when in the first temperature range.The
The reducing agent increasing means releases NOx from the NOx absorption catalyst when the NOx amount absorbed by the NOx absorption catalyst is equal to or higher than a predetermined value and the temperature of the NOx absorption catalyst is higher than a predetermined temperature. As described above, the reducing agent in the exhaust gas is increased by the first degree, and when the temperature of the NOx absorption catalyst is equal to or lower than the predetermined temperature, the reducing agent is increased by the second degree lower than the first degree.It is characterized by that.
[0009]
  Therefore, in the second temperature range where the exhaust gas temperature is low, NOx in the exhaust gas is reduced and purified by the NOx reduction catalyst, and the reducing agent in the exhaust gas is increased as necessary to promote the NOx reduction and purification reaction. In the first temperature range where the exhaust gas temperature is high, the amount of reducing agent in the exhaust gas is intermittently increased, so that NOx absorption / release reduction reaction by the NOx absorption catalyst can be utilized to purify NOx. It can be applied to diesel engines and lean burn gasoline engines whose exhaust gas temperature is generally low, and NOx in exhaust gas can be efficiently purified over a relatively wide temperature range from cold start to high-speed driving mode. .
[0010]
Further, when the amount of NOx absorbed by the NOx absorption catalyst is equal to or higher than a predetermined value, when the temperature of the NOx absorption catalyst is higher than the predetermined temperature, the degree of increase of the reducing agent is high. The NOx absorption capacity of the NOx absorption catalyst can be recovered by reducing the amount of the NOx absorption catalyst. On the other hand, when the temperature of the NOx absorption catalyst is equal to or lower than the predetermined temperature, the degree of increase of the reducing agent is low. Therefore, the increased reducing agent promotes the reduction and purification reaction of NOx with the NOx reduction catalyst, and the reaction heat causes The temperature of the exhaust gas flowing to the side can be raised to raise the temperature of the NOx absorption catalyst to a temperature suitable for the reduction and purification of the released NOx. Therefore, it is possible to reduce the amount of NOx that is released without being purified, and to avoid waste of the reducing agent, that is, deterioration of fuel consumption.
[0011]
  As the NOx absorption catalyst, an inorganic porous material such as alumina carrying a catalyst metal and a NOx absorption material can be employed. As the catalyst metal, a noble metal or other transition metal can be adopted, and as the noble metal, for example, Pt, Rh, Ir or the like can be adopted. As the NOx absorbent, compounds (carbonates or oxides) such as alkali metals, alkaline earth metals, and rare earth metals can be employed.
[0012]
  As the NOx reduction catalyst, a catalyst in which a catalytic metal is supported on an inorganic porous material such as zeolite can be employed. As the catalyst metal, a noble metal or other transition metal can be adopted, and as the noble metal, for example, Pt, Rh, Ir or the like can be adopted.
[0013]
  For example, a NOx absorption catalyst in which Pt and Ba are supported on γ-alumina has a high NOx purification performance at 250 to 400 ° C. (first temperature range), and a low NOx purification performance at a second temperature range lower than that. In the NOx reduction catalyst in which Pt is supported on zeolite, the NOx purification performance is high at 150 to 250 ° C. (second temperature range), and the NOx purification performance is low in the first temperature range.
[0014]
  As the reducing agent increasing means, the operation of the fuel injection valve that supplies fuel to the engine combustion chamber is controlled to increase the main injection amount, to perform the post injection after the main injection, or to increase the post injection amount, Or a method of combining these with EGR (exhaust gas recirculation) control, or a method of directly supplying a reducing agent to the exhaust passage.
[0015]
  The oxygen-excess atmosphere is, for example, an atmosphere having an oxygen concentration of 4 to 5% to 20% (A / F = 16 to 22 or A / F = 18 to 50 in an air-fuel ratio), and the oxygen concentration of exhaust gas is reduced. For example, when it is 2% or less, or 0.5% or less.
[0016]
  The invention according to claim 2 is the engine exhaust gas purification apparatus according to claim 1,
  The NOx reduction catalyst is characterized in that Pt is supported on zeolite.
[0017]
  Therefore, it is advantageous for NOx purification when the exhaust gas temperature is low.
[0018]
  The invention according to claim 3 is the engine exhaust gas purification apparatus according to claim 2,
  The NOx reduction catalyst is supported on a carrier, and the upstream portion of the carrier in the exhaust gas flow direction has a larger amount of Pt supported than the downstream portion.
[0019]
  That is, the activation temperature of the NOx reduction catalyst depends on the amount of Pt supported, and the activity increases at lower temperatures as the amount of Pt supported increases. In the case of the present invention, the NOx reduction catalyst has a low amount of Pt carrying amount at the upstream part and the downstream part of the carrier, and the upstream part has a higher Pt carrying amount. The downstream portion exhibits high-temperature activity, and therefore NOx can be reduced and purified over a wide temperature range as a whole.
[0020]
  As a result, even when the exhaust gas temperature is low, the NOx reduction reaction easily proceeds at the upstream site where the amount of Pt supported is large. Therefore, the temperature of the NOx reduction catalyst as a whole can be increased by the heat of reaction at that time. The exhaust gas temperature flowing downstream from the NOx reduction catalyst rises. Therefore, when the NOx absorption amount of the NOx absorption catalyst increases, even if the catalyst temperature does not rise to a temperature suitable for NOx reduction purification, the reducing agent increasing means causes the upstream NOx reduction catalyst to When the amount of the reducing agent supplied is increased, the temperature of the downstream NOx absorption catalyst can be quickly raised, which is advantageous for the reduction and purification of NOx released from the NOx absorption catalyst.
[0021]
  For example, the amount of Pt supported per liter of the carrier in the upstream region may be 3 to 6 g, and the amount of Pt supported in the downstream region may be 0.3 g or more and less than 3 g.
[0022]
  According to a fourth aspect of the present invention, there is provided an engine exhaust purification apparatus according to the first aspect,
  A diesel particulate filter is disposed between the NOx reduction catalyst and the NOx absorption catalyst.
[0023]
  Therefore, soot contained in the exhaust gas of the diesel engine can be removed. Thus, the exhaust gas temperature can be increased by the catalytic reaction heat in the NOx reduction catalyst disposed upstream of the diesel particulate filter, and the diesel particulate filter can be regenerated.
[0024]
  The invention according to claim 5 is the engine exhaust gas purification apparatus according to claim 1,
  The NOx reduction catalyst is coated on the cell surface of a diesel particulate filter.
[0025]
  Therefore, the soot contained in the exhaust gas of the diesel engine can be removed, and the temperature of the diesel particulate filter can be raised by the catalytic reaction heat in the NOx reduction catalyst, and the regeneration can be achieved.
[0026]
  The invention according to claim 6 is the engine exhaust gas purification apparatus according to claim 1,
  The NOx absorption catalyst is coated on the cell surface of a diesel particulate filter.
[0027]
  Therefore, the soot contained in the exhaust gas of the diesel engine can be removed, and the temperature of the diesel particulate filter is increased by the reaction heat when the NOx released from the NOx absorption catalyst is reduced and purified. Can be planned.
[0028]
【The invention's effect】
  According to the first aspect of the present invention, in the exhaust emission control device for an engine provided with the NOx absorption catalyst, the NOx reduction catalyst, and the reducing agent increasing means, the NOx absorption catalyst is in a predetermined first temperature range. The NOx purification performance is higher than when it is in the second temperature range lower than the first temperature range, and the NOx reduction catalyst is more NOx purification performance when it is in the second temperature range than when it is in the first temperature range. Therefore, it can be applied to diesel engines and lean burn gasoline engines whose exhaust gas temperature is generally low to efficiently purify NOx in exhaust gas in a relatively wide temperature range from cold start to high-speed driving mode. InIn addition, when the amount of NOx absorbed by the NOx absorption catalyst is greater than or equal to a predetermined value, when the catalyst temperature is low, the amount of reducing agent in the exhaust gas is increased by a lower second degree. The NOx absorption catalyst can be raised to a temperature suitable for the reduction and purification of released NOx by increasing the temperature of the exhaust gas flowing downstream by the reaction heat while promoting the reduction and purification reaction of NOx. If the temperature is high In this case, the amount of reducing agent in the exhaust gas is increased at a high first degree, so that NOx can be quickly released from the catalyst to reduce and purify it, and the NOx absorption capacity of the catalyst can be recovered and released. The amount of NOx discharged without being purified can be reduced, and deterioration of fuel consumption can be avoided.
[0029]
  According to the invention according to claim 2, in the engine exhaust gas purification apparatus according to claim 1, when the NOx reduction catalyst is formed by supporting Pt on zeolite, the exhaust gas temperature is low. This is advantageous for NOx purification.
[0030]
  According to a third aspect of the present invention, in the engine exhaust gas purification apparatus according to the second aspect, the NOx reduction catalyst is supported on a carrier and is disposed at an upstream portion of the carrier in the exhaust gas flow direction. Since the amount of Pt supported on the downstream side is larger than that on the downstream side, NOx can be reduced and purified in a wide temperature range, and when the amount of NOx absorbed by the NOx absorption catalyst increases, This is also advantageous in rapidly raising the temperature of the reduction catalyst and increasing the temperature of the NOx absorption catalyst.
[0031]
  According to the fourth aspect of the present invention, in the engine exhaust gas purification apparatus according to the first aspect, the diesel particulate filter is disposed between the NOx reduction catalyst and the NOx absorption catalyst. Soot contained in the exhaust gas of the engine can be removed, and the regeneration of the diesel particulate filter is also advantageous.
[0032]
  According to the fifth aspect of the present invention, in the engine exhaust gas purification apparatus according to the first aspect, the NOx reduction catalyst is coated on the cell surface of the diesel particulate filter. The soot contained therein can be removed, and it is advantageous for regeneration of the diesel particulate filter.
[0033]
  According to the sixth aspect of the present invention, in the engine exhaust gas purification apparatus according to the first aspect, the NOx absorption catalyst is coated on the cell surface of the diesel particulate filter. The soot contained therein can be removed, and it is advantageous for regeneration of the diesel particulate filter.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0035]
<Configuration of exhaust purification device>
  FIG. 1 shows a portion of one cylinder 2 of a multi-cylinder diesel engine 1 mounted on a vehicle. In the engine 1, 3 is a combustion chamber, 4 is a fuel injection valve that directly injects fuel into the combustion chamber 3, 5 is an intake passage, and 6 is an exhaust passage. In the exhaust passage 6, a lean NOx reduction catalyst 7 and a NOx absorption catalyst 8 are provided with the former disposed upstream in the exhaust flow direction and the latter disposed downstream thereof. An EGR (exhaust gas recirculation, hereinafter the same) passage 9 extends from the exhaust passage 6 upstream of the NOx reduction catalyst 7 toward the intake passage 5, and an EGR valve 10 is provided in the passage 9. The fuel injection valve 4 and the EGR valve 10 are controlled by an ECU (engine control means) 11 according to the operating state of the engine.
[0036]
  The NOx reduction catalyst 7 is formed by supporting Pt on zeolite (MFI), and is coated on the cell surface of a cordierite honeycomb carrier. Further, the amount of Pt supported on the carrier differs between the upstream portion 7a and the downstream portion 7b of the carrier, and the upstream portion 7a is larger than the downstream portion 7b. In such a NOx reduction catalyst 7, Pt functions to reduce and purify NOx in the exhaust gas with a reducing agent (HC, CO, etc.) even in an oxygen-excess atmosphere where the oxygen concentration of the exhaust gas is high.
[0037]
  The NOx absorption catalyst 8 is formed by supporting Pt and Ba as a NOx absorbent on γ-alumina, and is coated on the cell surface of a cordierite honeycomb carrier. In such a NOx absorption catalyst 8, Ba absorbs NOx in the exhaust gas in an oxygen-excess atmosphere where the oxygen concentration of the exhaust gas is high, and the absorbed Ba absorbs NOx when the oxygen concentration of the exhaust gas decreases. The released NOx is reduced and purified by Pt by the reducing agent in the exhaust gas.
[0038]
<Performance of NOx reduction catalyst and NOx absorption catalyst>
  FIG. 2 shows the result of examining the NOx purification characteristics by attaching the NOx reduction catalyst to the exhaust pipe of an actual vehicle engine and using the EU mode (a mode in which ECER15 which is the low speed mode and EUDC which is the high speed mode). The capacity of the honeycomb carrier of this NOx reduction catalyst is 2.5 L, the zeolite (MFI) loading per 1 L of this carrier is 300 g, and the Pt loading is 4.2 g. Note that the amount of Pt supported was uniform throughout the carrier.
[0039]
  In the low-speed mode from the start of the test to 800 seconds, the integrated NOx purification rate (NOx purification rate from the start of the test to each measurement point) is around 65%, and the catalyst inlet temperature (exhaust gas temperature at the catalyst inlet, below) , The same)) is 150-200 ° C. However, when 800 seconds have passed and the high-speed mode is entered, the catalyst inlet temperature rises and the integrated NOx purification rate decreases, and after 1000 seconds from the start of the test, the catalyst inlet temperature increases from 250 ° C. to 400 ° C. The integrated NOx purification rate finally decreases to nearly 40%.
[0040]
  From this, the NOx reduction catalyst 7 shows a high NOx purification rate when the catalyst inlet temperature is low (second temperature range) of about 150 to 250 ° C., but when it exceeds 250 ° C., the NOx purification rate rapidly decreases. It can be said that.
[0041]
  FIG. 3 shows the result of a bench test of the relationship between the NOx reduction rate by the NOx absorption catalyst 8 and the catalyst inlet temperature. The capacity of the honeycomb carrier of the NOx absorption catalyst 8 is 1.3 L. The amount of γ-alumina supported per 1 L of this carrier is 160 g, similarly the amount of Pt supported is 4.0 g, and the amount of Ba supported is 30 g. In the test, the fuel injection amount was intermittently increased so that λ (excess air ratio) = 1. In the figure, the NOx reduction rate is low when the catalyst inlet temperature is 220 ° C., but a high NOx reduction rate is shown at 250 to 400 ° C. (first temperature range).
[0042]
  From the above results, it can be seen that when the NOx reduction catalyst 7 and the NOx absorption catalyst 8 are combined as shown in FIG. 1, a high NOx purification rate can be expected over a wide range of the catalyst inlet temperature of 150 to 400 ° C. .
[0043]
<Fuel injection control (reducing agent increasing means)>
  The NOx reduction catalyst 7 has high NOx purification performance when there is a large amount of reducing agent in the exhaust gas. Further, the NOx absorption catalyst 8 needs to release NOx by reducing the oxygen concentration of the exhaust gas when the NOx absorption amount in the oxygen-excess atmosphere increases. Hereinafter, fuel injection control for that purpose (in other words, reducing agent increase control) will be described.
[0044]
  The memory of the ECU 11 stores a target torque map in which the optimum value of the target torque is experimentally determined and recorded with respect to changes in the accelerator opening (engine load) and the engine speed, and the target torque, intake air amount, and rotation. A fuel injection amount map in which the optimum fuel injection amount Qm determined experimentally according to the change in the number is recorded electronically.
[0045]
  Normally, the target torque is obtained from the map based on the accelerator opening and the engine speed, and the fuel injection amount Qm is obtained from the map based on the target torque, the engine speed and the intake air amount, and the fuel injection of each cylinder is obtained. The excitation time (valve opening time) of the valve 4 is determined. Thus, fuel of the fuel injection amount Qm is basically injected in the vicinity of the top dead center of the compression stroke of each cylinder 2 (hereinafter referred to as main injection), and the engine 1 has a considerably lean air-fuel ratio in the combustion chamber 3 (oxygen). It is operated in a state where the concentration is 20% or more.
[0046]
  Further, when the NOx reduction catalyst 7 is in the second temperature range suitable for NOx reduction, the fuel injection amount is increased, and a part of the fuel injection amount is injected in the expansion stroke or the exhaust stroke after the main injection. The amount of reducing agent in the exhaust gas is increased to promote the reduction of NOx.
[0047]
  Further, the NOx absorption amount of the NOx absorption catalyst 8 is estimated based on the operating time of the engine, and when the estimated value becomes larger than a predetermined value and a decrease in NOx absorption performance is expected, the fuel injection amount is increased. The exhaust gas reducing agent is increased by post-injecting a part thereof. The degree of increase of the reducing agent varies depending on the temperature of the NOx absorption catalyst 8.
[0048]
  That is, when the temperature of the NOx absorption catalyst 8 is in the first temperature range suitable for NOx reduction, the atmosphere corresponding to λ = 1 is obtained by increasing the EGR rate while setting the amount of reducing agent to the first degree. When NOx is released from the catalyst 8 and the temperature is equal to or lower than the first temperature range (second temperature range), the catalyst in the NOx reduction catalyst 7 is set to a second degree with a small amount of reducing agent. The reaction is promoted, and the temperature of the NOx absorption catalyst 8 is raised using the reaction heat.
[0049]
  Thus, means for detecting the accelerator opening, the engine speed, the intake air amount, the temperature of the NOx reduction catalyst 7, and the temperature of the NOx absorption catalyst 8 are provided for the fuel injection control.
[0050]
  Hereinafter, the fuel injection control will be described specifically according to the flow shown in FIG.
[0051]
  In step S1 after the start, various data such as the accelerator opening are input, and in step S2, the NOx absorption amount NOa is estimated. In the subsequent step S3, it is determined whether or not the NOx absorption amount NOa exceeds a predetermined value, that is, whether or not the NOx absorption capacity of the NOx absorption catalyst 8 is reduced. When the NOx absorption amount NOa does not exceed the predetermined value, normal fuel injection control is performed.
[0052]
  That is, the process proceeds to step S4, where the basic injection amount Qb and the basic injection timing Ib are set according to the operating state of the engine. In the subsequent step S5, the basic injection amount Qb is set as the main injection amount Qm, and the main injection timing Im is set. A basic injection timing Ib is set. In subsequent step S6, it is checked whether or not the temperature TNR of the NOx reduction catalyst 7 is in the second temperature range (TNR1 <TNR <TNR2). The acceleration post-injection amount Qpr is set, and the post-injection timing Ipr for NOx reduction promotion is set as the post-injection timing Ip. The process proceeds to step S8, and main injection and post-injection are executed.
[0053]
  The basic injection timing Ib is basically set near the top dead center of the compression stroke. However, if the engine water temperature or the engine speed is different, the fuel spray ignition delay time will be different. The higher the number, the faster it is set. The post-injection timing Ip is set to advance as the engine load increases in the range of ATDC 30 to 90 ° CA, for example.
[0054]
  When the NOx absorption amount NOa is greater than or equal to the predetermined value NOao in step S3, the process proceeds to step S9 to check whether or not the reducing agent increase flag F = 1. If F = 0, the process proceeds to step S10, F = 1, and the process proceeds to step S11 to check whether the temperature TNT of the NOx absorption catalyst 8 is equal to or higher than a predetermined value TNTo. That is, it is a determination as to whether or not the NOx released by the NOx absorption catalyst 8 is in a first temperature range where reduction and purification can be performed.
[0055]
  When the temperature TNT of the NOx absorption catalyst 8 exceeds the predetermined value TNTo, NOx release control is performed. That is, it progresses to step S12 and the 1st timer Ta for increasing a reducing agent by the 1st degree is incremented. In subsequent step S13, it is determined whether or not the first timer Ta is longer than a predetermined timer time Tao (for example, a value corresponding to 0.5 to 5 seconds). If not, the process proceeds to step S14 and the main fuel injection amount is determined. The injection amount Qλ is set as Qm for λ for increasing the reducing agent in the first degree (for λ, it means that the exhaust gas corresponds to λ = 1. The same applies hereinafter). At the same time, the injection timing Iλ for λ is set as the main injection timing Im. In the subsequent step S15, the λ post-injection amount Qpλ is set as the post-injection amount Qp, the post-injection timing Ipλ for λ is set as the post-injection timing Ip, and main injection and post-injection are executed (step S8). In this case, control for increasing the EGR rate is performed at the same time. Therefore, the main injection amount Qλ, the post-injection amount Qpλ, and the EGR rate increase, resulting in an exhaust gas corresponding to λ = 1.
[0056]
  On the other hand, if the temperature TNT of the NOx absorption catalyst 8 is equal to or lower than the predetermined value TNTo in step S11, the temperature increase control of the NOx absorption catalyst 8 is performed. That is, the process proceeds to step S16 to set the timer time Tbo for increasing the reducing agent at the second degree according to the current temperature TNT of the NOx absorption catalyst 8, and at the subsequent step S17, the reducing agent at the second degree is set. The second timer Tb for increasing is incremented. As shown in FIG. 5, the timer time Tbo is preset and stored in the memory so as to decrease as the temperature TNT increases.
[0057]
  In the following step S18, it is determined whether or not the second timer Tb is larger than the timer time Tbo. If not, the process proceeds to step S19 to increase the reducing agent by a second degree as the main injection amount Qm of the fuel, and the NOx absorption catalyst. The temperature raising injection amount Qu for raising the temperature of 8 is set, and the temperature raising injection timing Iu is set as the main injection timing Im. In the subsequent step S20, the post-temperature increase injection amount Qpu is set as the post-injection amount Qp, and the post-temperature increase injection timing Ipu is set as the post-injection timing Ip. . Here, the relationship between the injection amounts is (Qλ + Qpλ)> (Qu + Qpu)> (Qb + Qpr).
[0058]
  When the second timer Tb exceeds the timer time Tbo in step S18, the process proceeds to step S21, the second timer Tb is returned to zero, and the process proceeds to NOx release control (proceeds to step S12).
[0059]
  When the first timer Ta exceeds the timer time Tao in step S13, the process proceeds to step S22 to end the NOx release control, the first timer Ta is returned to zero, the NOx absorption amount NOa is reset, and F = 1 shifts to normal control (proceeds to step S4).
[0060]
  When F = 1 in step S9, the reducing agent increase control is in progress, and the process proceeds to step S23 to check whether the first timer Ta is being counted. If the counting is in progress, the process proceeds to step S12 to continue the NOx release control.
[0061]
  Therefore, in the case of the fuel injection control (reducing agent increase control) as described above, when the NOx absorption amount NOa of the NOx absorption catalyst 8 is small, the fuel injection control corresponding to the normal engine operating state is performed. In this normal control, if the temperature of the NOx reduction catalyst 7 is in the second temperature range (TNR1 <TNR <TNR2), the amount of reducing agent in the exhaust gas is increased by supplying the post-injection fuel Qpr. Thus, NOx reduction purification is efficiently performed.
[0062]
  Further, even if the exhaust gas temperature rises and the NOx reduction and purification efficiency of the NOx reduction catalyst 7 decreases, NOx is absorbed by the NOx absorption catalyst 8, so that a large amount of unpurified NOx is discharged into the atmosphere. It is prevented.
[0063]
  When the NOx absorption amount NOa of the NOx absorption catalyst 8 exceeds the predetermined value NOao, when the temperature TNT of the NOx absorption catalyst 8 is equal to or lower than TNTo (not yet a temperature suitable for NOx release and reduction purification), Since the reducing agent in the exhaust gas is increased by the second degree by supplying the injected fuel Qpu, the NOx reduction purification (HC oxidation) reaction in the NOx reduction catalyst 7 is promoted, and the temperature of the catalyst 7 rises. . As a result, the temperature of the exhaust gas flowing from the catalyst to the downstream side increases, and the NOx absorption catalyst 8 can be raised at an early temperature.
[0064]
  The temperature increase control time is monitored by the second timer Tb, and the timer time Tbo is set according to the temperature of the NOx absorption catalyst 8 at the start of the temperature increase control, so that the temperature increase is insufficient at the start of the NOx release control. In addition to avoiding excessive temperature rise, it is advantageous for reducing and purifying the released NOx, and it is possible to raise the temperature to the optimum temperature in the minimum necessary time, which is advantageous for preventing discharge of unpurified NOx.
[0065]
  In the NOx release control, since the EGR rate is increased together with the increase in the post-injection amount (increase in the first degree of the reducing agent), it is not necessary to increase the degree of increase in the post-injection amount, which is advantageous for improving fuel efficiency. .
[0066]
<Other embodiments>
  As shown in FIG. 6, a DPF (diesel particulate filter) 12 can be disposed in the exhaust passage 6 between the NOx reduction catalyst 7 and the NOx absorption catalyst 8. In this way, the soot contained in the exhaust gas can be removed, and the exhaust gas temperature can be increased by the catalytic reaction heat in the NOx reduction catalyst 7 to regenerate the DPF 12.
[0067]
  As shown in FIG. 7, when the NOx reduction catalyst 7 is coated on the cell surface of the DPF 12, soot contained in the exhaust gas can be removed, and the DPF 12 can be removed by the heat of catalytic reaction of the NOx reduction catalyst 7. The regeneration can be achieved by raising the temperature.
[0068]
  As shown in FIG. 8, if the NOx absorption catalyst 8 is coated on the cell surface of the DPF 12, soot contained in the exhaust gas can be removed and NOx released from the NOx absorption catalyst 8 is reduced. Regeneration can be achieved by increasing the temperature of the DPF 12 by the reaction heat at the time of purification.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an exhaust emission control device for an engine according to an embodiment of the present invention.
FIG. 2 is a graph showing the result of evaluating the relationship between the catalyst inlet gas temperature and the NOx purification rate of the NOx reduction catalyst according to the same embodiment with an actual vehicle.
FIG. 3 is a graph showing the results of a bench test that evaluates the relationship between the catalyst inlet gas temperature and the NOx reduction rate of the NOx absorption catalyst according to the same embodiment.
FIG. 4 is a flowchart of fuel injection control according to the embodiment.
FIG. 5 is a graph showing a relationship between a timer time for increasing the reducing agent in the second embodiment by a second degree and the temperature of the NOx absorption catalyst;
FIG. 6 is a configuration diagram of an engine exhaust purification device according to another embodiment of the present invention.
FIG. 7 is a configuration diagram of an engine exhaust purification device according to still another embodiment of the present invention.
FIG. 8 is a configuration diagram of an exhaust emission control device for an engine according to still another embodiment of the present invention.
[Explanation of symbols]
    1 Diesel engine
    2-cylinder
    3 Combustion chamber
    4 Fuel injection valve
    5 Intake passage
    6 Exhaust passage
    7 NOx reduction catalyst
    8 NOx absorption catalyst
    9 EGR passage
  10 EGR valve
  11 ECU (engine control means)
  12 DPF

Claims (6)

In an oxygen-excess atmosphere where the exhaust gas has a high oxygen concentration, the NOx in the exhaust gas is absorbed, and when the oxygen concentration in the exhaust gas decreases, the absorbed NOx is released and reduced and purified. A NOx absorption catalyst;
A NOx reduction catalyst that is disposed in the exhaust passage upstream of the NOx absorption catalyst and reduces and purifies NOx in the exhaust gas in the oxygen-excess atmosphere;
In an engine exhaust purification device comprising a reducing agent increasing means for increasing the reducing agent in the exhaust gas supplied to the NOx reduction catalyst,
The NOx absorption catalyst has a higher NOx purification performance when it is in a predetermined first temperature range than when it is in a second temperature range lower than the first temperature range,
The NOx reduction catalyst, when in the second temperature range is rather NOx purification performance is higher than when it is in the first temperature range,
The reducing agent increasing means releases NOx from the NOx absorption catalyst when the NOx amount absorbed by the NOx absorption catalyst is equal to or higher than a predetermined value and the temperature of the NOx absorption catalyst is higher than a predetermined temperature. As described above, the reducing agent in the exhaust gas is increased in a first degree, and when the temperature of the NOx absorption catalyst is equal to or lower than the predetermined temperature, the reducing agent is increased in a second degree lower than the first degree. An exhaust gas purification device for an engine. The engine exhaust gas purification apparatus according to claim 1,
The exhaust gas purification apparatus for an engine, wherein the NOx reduction catalyst is formed by supporting Pt on zeolite. In the exhaust emission control device for an engine according to claim 2,
The engine NOx reduction catalyst is supported on a carrier, and the upstream portion of the carrier in the exhaust gas flow direction has a larger amount of Pt supported than the downstream portion. The engine exhaust gas purification apparatus according to claim 1,
An exhaust emission control device for an engine, wherein a diesel particulate filter is disposed between the NOx reduction catalyst and the NOx absorption catalyst. The engine exhaust gas purification apparatus according to claim 1,
An exhaust gas purification apparatus for an engine, wherein the NOx reduction catalyst is coated on a cell surface of a diesel particulate filter. The engine exhaust gas purification apparatus according to claim 1,
The exhaust gas purification apparatus for an engine, wherein the NOx absorption catalyst is coated on a cell surface of a diesel particulate filter.

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