JP2005090310A - NOx CLEANING DEVICE - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress to the minimum generation of N<SB>2</SB>O in an NOx cleaning device provided with an NOx catalyst to remove NOx. <P>SOLUTION: This device is provided with the NOx catalyst 25 to remove NOx when controlling for richening that an air-fuel ratio of an internal atmosphere is made to be a stoichiometric air-fuel ratio or richen than the stoichimetric air-fuel ratio. Execution of control for richening is prohibited when estimated that an NOx cleaning factor by the NOx catalyst when controlling for richening is low and a generation rate of N<SB>2</SB>O generated by the NOx catalyst is high. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a NOx purification device.

  Patent Document 1 discloses means for reducing and purifying nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine (in Patent Document 1, reference numeral 125 is attached and expressed as “nitrogen oxide absorbent”. In the following, an exhaust emission control device including a “NOx absorbent” is disclosed. According to Patent Document 1, when the temperature of the NOx absorbent is equal to or higher than a certain temperature (referred to as “Regeneration Allowable Temperature” in Patent Document 1 and hereinafter also referred to as “Regeneration Allowable Temperature”), NOx. When the regeneration operation of the absorbent (according to Patent Document 1, this may cause the internal combustion engine to perform rich combustion), NOx may be released from the NOx absorbent to the atmosphere.

  According to Patent Document 1, this means that even when the NOx absorbent regeneration operation is started, the NOx absorbent is absorbed during the NOx absorbent regeneration operation even if the NOx absorbent temperature is below the allowable regeneration temperature. If the temperature of the agent becomes equal to or higher than the regeneration allowable temperature, it means that NOx is released from the NOx absorbent to the atmosphere.

  Therefore, in Patent Document 1, when it is predicted that the temperature of the NOx absorbent is equal to or higher than the regeneration allowable temperature, only when the amount of NOx absorbed in the NOx absorbent is greater than a certain amount. The regeneration operation of the NOx absorbent is performed. In other words, when it is predicted that the temperature of the NOx absorbent is equal to or higher than the regeneration allowable temperature, the regeneration operation of the NOx absorbent is not performed if the amount of NOx absorbed in the NOx absorbent is small. .

  According to this, when it is predicted that the temperature of the NOx absorbent becomes higher than the allowable regeneration temperature during the regeneration operation of the NOx absorbent (as described above, this includes NOx absorbed in the NOx absorbent. NOx absorbent regeneration operation is not performed under the condition that the amount of NOx is small), it is avoided that NOx is released from the NOx absorbent to the atmosphere during the NOx absorbent regeneration operation. The That is, by predicting in advance the behavior of the temperature of the NOx absorbent during the regeneration operation of the NOx absorbent, it is reliably avoided that NOx is released from the NOx absorbent to the atmosphere.

International Publication No. 97/16632 Pamphlet JP 2002-54470 A JP 11-107811 A Japanese Patent Laid-Open No. 2000-240431

By the way, a NOx purification device having a NOx catalyst for purifying NOx is known. In such a NOx purification device, N ₂ O may be generated along with the reduction and purification of NOx. Since N ₂ O is a gas having a greenhouse effect like CO ₂ , it is preferable to suppress its generation as much as possible. Then, in such NOx purification device, even when attempting to suppress as much as possible the generation of N 2 _O, to predict the parameter values relating to the generation rate of the N _{2 O} when the NOx was reduced and purified by the NOx catalyst in advance, It is preferable to determine whether or not to purify NOx by the NOx catalyst according to the predicted parameter value from the viewpoint of suppressing the generation of N ₂ O in the NOx purifying device as much as possible.

Accordingly, an object of the present invention is to suppress as much as possible the generation of N ₂ O in a NOx purification device including a NOx catalyst that purifies NOx.

In order to solve the above-mentioned problem, in the first invention, the NOx having a NOx catalyst that purifies NOx when rich control is performed to make the air-fuel ratio of the internal atmosphere the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. In the purifying apparatus, when the NOx purification rate by the NOx catalyst when the enrichment control is performed is low and the generation rate of N ₂ O produced by the NOx catalyst is predicted to be high, the enrichment control is executed. Enrichment control means for prohibiting.
In the second invention, in the first invention, the NOx purification rate by the NOx catalyst when the enrichment control is performed is expected to be low, and the production rate of N ₂ O produced by the NOx catalyst is expected to be high. Means for raising the temperature of the NOx catalyst.
In order to solve the above-described problem, in the third aspect of the invention, the NOx having a NOx catalyst that purifies NOx when rich control is performed to make the air-fuel ratio of the internal atmosphere the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. In the purification apparatus, when the enrichment control is currently performed, the NOx purification rate by the NOx catalyst is high and the production rate of N ₂ O produced by the NOx catalyst is high or the NOx purification rate is low. and although the N _{2 O} production rate is expected to be low, when the NOx purification rate is expected to be low and the N _{2 O} formation rate increases after permits the execution of the rich control, whereas, after the When it is predicted that the NOx purification rate is high and the N ₂ O generation rate is expected to be low, a enrichment control means for prohibiting execution of the enrichment control is provided.
In a fourth invention, in the first or third invention, the enrichment control means has a high NOx purification rate by the NOx catalyst when the enrichment control is performed, and N ₂ produced by the NOx catalyst. When it is predicted that the O generation rate is low, execution of the enrichment control is permitted.
According to a fifth aspect, in the third aspect, the enrichment control means has a low NOx purification rate by the NOx catalyst when the enrichment control is performed, and the N ₂ O produced by the NOx catalyst is low. When it is predicted that the generation rate is high, execution of the enrichment control is prohibited.
In the sixth aspect, in the fifth aspect, the NOx purification rate by the NOx catalyst when the enrichment control is performed is expected to be low, and the generation rate of N ₂ O produced by the NOx catalyst is expected to be high. Means for raising the temperature of the NOx catalyst.
In the seventh invention, in any one of the first to sixth inventions, the NOx purification rate by the NOx catalyst and the production rate of N ₂ O produced by the NOx catalyst when the enrichment control is performed. Are estimated using the temperature of the NOx catalyst and the amount of NOx held in the NOx catalyst as parameters.

Generally, N ₂ O may be generated along with the reduction and purification of NOx. Here, the N ₂ O production rate is preferably low. According to the first invention, when it is predicted that the NOx purification rate is low and the N ₂ O production rate is high, NOx purification by the NOx catalyst is not performed. Therefore, according to this, so that the generation of N _{2 O} in the NOx purification device can be suppressed.
According to the third aspect of the present invention, if NOx purification using a NOx catalyst is performed at the present time, the NOx purification rate is high but the N ₂ O production rate is expected to be high, but thereafter the NOx purification rate is high. The N ₂ O production rate is expected to be low until, or the N ₂ O production rate is low but the NOx purification rate is also expected to be low, but thereafter the N ₂ O production rate remains low and the NO x production rate is low. When it is expected to be high, NOx purification by the NOx catalyst is not performed. Therefore, according to this, so that the generation of N _{2 O} in the NOx purification device can be suppressed.
On the other hand, according to the third invention, when the performed NOx purification by the NOx catalyst at this time, but although the NOx purification rate is higher N _{2 O} formation rate is also expected to be high, then, N 2 _O formation rate is When it is predicted that the NOx purification rate will not be lowered or the N ₂ O production rate is low, the NOx purification rate is also expected to be low, but after that, the NOx purification rate is not increased and N When it is predicted that the ₂ O production rate will increase, NOx purification by the NOx catalyst is performed. Therefore, according to this, so that the generation of N _{2 O} in the NOx purification device can be suppressed.

  FIG. 1 shows a four-stroke compression ignition type internal combustion engine. In FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve, 8 is an intake port, and 9 is Exhaust valves 10 represent exhaust ports, respectively. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11. The surge tank 12 is connected to an outlet portion of a compressor 16 of a supercharger (for example, an exhaust turbocharger) 15 via an intake duct 13 and an intercooler 14. An inlet portion of the compressor 16 is connected to an air cleaner 18 via an air suction pipe 17. A throttle valve 20 driven by a step motor 19 is disposed in the air suction pipe 17. A mass flow rate detector 21 for detecting the mass flow rate of the intake air is disposed in the air suction pipe 17 upstream of the throttle valve 20.

  On the other hand, the exhaust port 10 is connected to an inlet portion of the exhaust turbine 23 of the exhaust turbocharger 15 via an exhaust manifold 22. The outlet of the exhaust turbine 23 is connected via an exhaust pipe 24 to a catalytic converter 26 containing a catalyst 25 (details will be described later) for purifying NOx. An air-fuel ratio sensor (hereinafter referred to as “upstream air-fuel ratio sensor”) 27 a is disposed in the exhaust manifold 22 on the upstream side of the catalytic converter 26. On the other hand, an air-fuel ratio sensor (hereinafter referred to as “downstream air-fuel ratio sensor”) 27 b is disposed in the exhaust pipe 28 on the downstream side of the catalytic converter 26. The exhaust pipe 24 is provided with a hydrocarbon addition valve 37 for injecting hydrocarbons (for example, the same fuel injected from the fuel injection valve 6) and adding hydrocarbons to the exhaust gas. Yes.

  The exhaust pipe 28 connected to the outlet of the catalytic converter 26 and the air suction pipe 17 downstream of the throttle valve 20 are connected to each other via an exhaust gas recirculation (hereinafter referred to as “EGR”) passage 29. An EGR control valve 31 driven by a step motor 30 is disposed in the EGR passage 29. An intercooler 32 for cooling exhaust gas (hereinafter also referred to as “EGR gas”) flowing in the EGR passage 29 is disposed in the EGR passage 29. In the example shown in FIG. 1, the engine cooling water is guided into the intercooler 32, and the EGR gas is cooled by the engine cooling water.

  On the other hand, the fuel injection valve 6 is connected to a fuel reservoir (so-called common rail) 34 via a fuel supply pipe 33. Fuel is supplied into the common rail 34 from an electrically controlled fuel pump 35 with variable discharge amount. The fuel supplied into the common rail 34 is supplied to the fuel injection valve 6 through each fuel supply pipe 33. A fuel pressure sensor 36 for detecting the fuel pressure in the common rail 34 is attached to the common rail 34. The discharge amount of the fuel pump 35 is controlled based on the output signal of the fuel pressure sensor 36 so that the fuel pressure in the common rail 34 becomes the target fuel pressure.

  The electronic control unit 40 is composed of a digital computer, and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45, and an output A port 46 is provided. The output signal of the mass flow rate detector 21 is input to the input port 45 via the corresponding AD converter 47. Output signals of the upstream air-fuel ratio sensor 27a, the downstream air-fuel ratio sensor 27b, and the fuel pressure sensor 36 are also input to the input port 45 via the corresponding AD converters 47, respectively. A load sensor 51 that generates an output voltage proportional to the depression amount L of the accelerator pedal 50 is connected to the accelerator pedal 50. The output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. The input port 45 is connected to a crank angle sensor 52 that generates an output pulse every time the crankshaft rotates, for example, 30 °. On the other hand, the output port 46 is connected to the fuel injection valve 6, the throttle valve control step motor 19, the EGR control valve control step motor 30, the fuel pump 35, and the hydrocarbon addition valve 37 via a corresponding drive circuit 48. Is done.

  Next, the NOx catalyst 25 will be described. The NOx catalyst 25 uses, for example, alumina as a carrier, and an alkali metal (for example, potassium (K), sodium (Na), lithium (Li), cesium (Cs)), alkaline earth (for example, barium) on the carrier. (Ba), calcium (Ca)), rare earth (for example, lanthanum (La), yttrium (Y)) and a noble metal such as platinum (Pt) are supported.

  The NOx catalyst 25 absorbs and holds NOx when the air-fuel ratio of the atmosphere inside it is leaner than the stoichiometric air-fuel ratio. On the other hand, the NOx catalyst 25 releases the held NOx when the air-fuel ratio of the atmosphere inside thereof is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. Further, the NOx catalyst 25 similarly has NOx (in addition to NOx released from the NOx catalyst 25) when the air-fuel ratio in the atmosphere is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. , NOx newly coming in the NOx catalyst 25 is included). Here, the air-fuel ratio of the exhaust gas is determined based on the amount of fuel injected from the fuel injection valve 6 (when the fuel can be added to the exhaust gas discharged from the combustion chamber 5 on the upstream side of the NOx catalyst 25). Includes the amount of air sucked into the combustion chamber 5 with respect to the amount of the added fuel (on the upstream side of the NOx catalyst 25, air is added to the exhaust gas discharged from the combustion chamber 5). If it can be defined, the air / fuel ratio of the atmosphere in the NOx catalyst 25 matches the air / fuel ratio of the exhaust gas.

  Accordingly, while the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 is leaner than the stoichiometric air-fuel ratio, the NOx catalyst 25 absorbs and holds NOx in the inflowing exhaust gas. In the present embodiment, when the internal combustion engine is in a normal operating state, the air-fuel ratio of the exhaust gas arriving at the NOx catalyst 25 is leaner than the stoichiometric air-fuel ratio, so as long as the internal combustion engine is in a normal operating state. The NOx catalyst 25 continues to absorb and hold NOx in the exhaust gas.

  However, when the total amount of NOx absorbed in the NOx catalyst 25 (hereinafter referred to as “NOx occlusion amount”) exceeds a limit value (that is, the amount of NOx that can be absorbed to the maximum by the NOx catalyst 25), NOx. Since the catalyst 25 can no longer absorb NOx, in this case, the NOx flows out downstream of the NOx catalyst 25. However, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 becomes the stoichiometric air-fuel ratio or becomes richer than the stoichiometric air-fuel ratio, the NOx catalyst 25 releases the retained NOx and the released NOx and NOx catalyst. NOx newly flowing into the exhaust gas 25 is reduced and purified using the fuel (that is, hydrocarbons) in the exhaust gas as a reducing agent. Therefore, if the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 is made the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio before the NOx occlusion amount exceeds the limit value, it is absorbed and held by the NOx catalyst 25. Since NOx is reduced and purified, according to this, the outflow of NOx to the downstream side of the NOx catalyst 25 is avoided.

  Next, an embodiment of the present invention of control for reducing and purifying NOx held in the NOx catalyst 25 (hereinafter referred to as “NOx purification control”) will be described. As described above, there is a limit to the amount of NOx that the NOx catalyst 25 can absorb and hold to the maximum. Therefore, in the present embodiment, first, the NOx occlusion amount exceeds the limit value (actually, it is a value smaller than the limit value for reasons described later and is referred to as “determination value” in the following description). Therefore, it is determined that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 should be the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. Here, the NOx occlusion amount can be calculated by integrating the amount of NOx generated in the combustion chamber 5, and the amount of NOx generated in the combustion chamber 5 depends on the engine speed and the required load. And can be expressed in the form of a function. Therefore, in the present embodiment, the amount of NOx generated in the combustion chamber 5 (hereinafter referred to as “NOx generation amount”) is obtained by experiments or the like, and this NOx generation amount Anox is a function of the engine speed N and the required load L. Therefore, it is stored in advance in the ROM 42 in the form of a map as shown in FIG. During engine operation, the NOx generation amount Anox is read from the map based on the engine speed N and the required load L at every predetermined time (for example, for each combustion stroke), and these NOx generation amounts Anox are integrated. Thus, the NOx occlusion amount is calculated.

  Furthermore, in this embodiment, when it is determined that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 should be the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, the exhaust gas actually flows into the NOx catalyst 25. It is determined whether or not control (hereinafter referred to as “riching control”) that makes the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio may be executed (whether or not to execute the enrichment control). . Here, if it is determined that the enrichment control may be performed, the enrichment control is performed. On the other hand, if it is determined that the enrichment control should not be performed, the enrichment control is not performed, and the execution of the enrichment control is postponed until it is determined that the enrichment control may be performed. As a specific method for setting the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 to the stoichiometric air-fuel ratio or to make it richer than the stoichiometric air-fuel ratio, for example, a predetermined amount (this is obtained from the hydrocarbon addition valve 37 into the exhaust gas). There is a method of injecting hydrocarbons in such a manner that the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or an amount that can be made richer than the stoichiometric air-fuel ratio.

  Next, a method for determining whether or not to execute the enrichment control will be described. The NOx reduction purification efficiency (hereinafter referred to as “NOx purification rate”) by the NOx catalyst 25 depends on the NOx occlusion amount, and the NOx occlusion amount Anoxt exceeds a certain amount A1 as shown by the solid line in FIG. The NOx purification rate Rnox is substantially constant at a high value Rnoxh, but when the NOx occlusion amount Anoxt exceeds a certain amount A1, the NOx purification rate Rnox decreases as the NOx occlusion amount Anoxt increases, and the NOx occlusion amount Anoxt. Exceeds a certain amount A2, the NOx purification rate Rnox becomes substantially constant at a low value Rnoxl. From the viewpoint of reducing and purifying NOx efficiently and sufficiently, since it is preferable to determine whether or not to execute the enrichment control according to the NOx purification rate, in this embodiment, the NOx purification rate related to the NOx occlusion amount is This is adopted as one parameter for determining whether to execute the enrichment control.

Further, N ₂ is mainly generated as NOx is reduced and purified by the NOx catalyst 25, but N ₂ O may also be generated at a constant rate. Here, since N ₂ O is a gas having a greenhouse effect similar to CO ₂ , the N ₂ O production rate is preferably as small as possible (in other words, the N ₂ production rate is as high as possible). . Here, the N ₂ O production rate depends on the temperature of the NOx catalyst 25 (hereinafter referred to as “catalyst temperature”), and as shown by the solid line in FIG. 4, until the catalyst temperature T exceeds a certain temperature T1. The N ₂ O production rate Rn ₂ o is substantially constant at a high value Rn ₂ oh, but when the catalyst temperature T exceeds a certain temperature T1, the N ₂ O production rate Rn ₂ o increases as the catalyst temperature T increases. When the catalyst temperature T decreases and exceeds a certain temperature T2, the N ₂ O production rate Rn ₂ o becomes substantially constant at a low value Rn ₂ ol. From the viewpoint of maintaining the N ₂ O production rate at a low level, it is preferable to determine whether or not to execute the enrichment control in accordance with the N ₂ O production rate. In this embodiment, the N ₂ O production rate further relates to the catalyst temperature. The N ₂ O generation rate is employed as one parameter for determining whether to execute the enrichment control.

In summary, in this embodiment, the NOx purification rate related to the NOx occlusion amount and the N ₂ O generation rate related to the catalyst temperature are adopted as parameters for determining whether or not to execute the enrichment control. . Specifically, as shown in FIG. 5, a first lower limit value Rmin1 and a second lower limit value Rmin2 are set for the NOx purification rate Rnox. On the other hand, as shown in FIG. 6, a first upper limit value Rmax1 and a second upper limit value Rmax2 are set for the N ₂ O production rate.

In FIG. 5, a region where the NOx purification rate Rnox is higher than the first lower limit value Rmin1 is represented by “X1”, and the NOx purification rate Rnox is lower than the first lower limit value Rmin1 and is lower than the second lower limit value Rmin2. The region where the NOx purification rate Rnox is lower than the second lower limit value Rmin2 is represented by “X3”. Further, in FIG. 6, a region where the N ₂ O generation rate Rn ₂ o is lower than the first upper limit value Rmax1 is represented by “Y1”, and the N ₂ O generation rate Rn ₂ o is higher than the first upper limit value Rmax1 and A region lower than the second upper limit value Rmax2 is represented by “Y2”, and a region where the N ₂ O generation rate Rn ₂ o is higher than the second upper limit value Rmax2 is represented by “Y3”.

In this embodiment, when the NOx purification rate Rnox is in the region X1 and the N ₂ O generation rate Rn ₂ o is in the region Y1, execution of the enrichment control is permitted. Therefore, in this case, since the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, NOx is reduced and purified at the NOx catalyst 25 at a high purification rate, The N ₂ O production rate accompanying the reduction and purification of NOx can be kept low.

On the other hand, in this embodiment, when the NOx purification rate Rnox is in the region X3 and the N ₂ O generation rate Rn ₂ o is in the region Y3, execution of the enrichment control is prohibited. Accordingly, in this case, NOx reduction purification is not performed in the NOx catalyst 25, and N ₂ O is not generated.

On the other hand, in the present embodiment, when neither the NOx purification rate Rnox nor the N ₂ O generation rate Rn ₂ o satisfies the above-described conditions, whether to execute the enrichment control is determined as follows. That is, when the NOx purification rate and the N ₂ O production rate are considered together, the NOx purification rate is relatively short in the future (this is within the time determined within a range where the NOx occlusion amount does not exceed the limit value). Is expected to be higher than the current NOx purification rate and the N ₂ O production rate is expected to be lower than the current N ₂ O production rate, the execution of the enrichment control is prohibited. When the NOx purification rate becomes higher than the current level and the N ₂ O generation rate becomes lower than the current level, the execution of the enrichment control is permitted. According to this, since the enrichment control is executed when the NOx purification rate becomes high and the N ₂ O production rate becomes low, the NOx purification rate is maintained high and the N ₂ O production rate as a whole. Is preferable from the viewpoint of maintaining low.

On the other hand, when it is predicted that the NOx purification rate will be lower than the present and the N ₂ O production rate will be higher than the present in the relatively near future, the execution of the enrichment control is permitted. According to this, since the enrichment control is executed before the NOx purification rate becomes low and the N ₂ O production rate becomes high, as a whole, the NOx purification rate is maintained high and the N ₂ O production rate is reduced. It is preferable from the viewpoint of keeping it low.

Whether to execute the enrichment control described above will be described with reference to FIG. In FIG. 7, vertical columns X1, X2, and X3 correspond to the respective regions (hereinafter also referred to as “NOx purification rate regions”) X1, X2, and X3 defined with reference to FIG. Y3 corresponds to each region (hereinafter referred to as “N ₂ O generation rate region”) Y1, Y2, Y3 defined with reference to FIG. In FIG. 7, A to I indicate the overall purification performance states (hereinafter referred to as “performance states”) determined by combining the regions X1 to X3 and the regions Y1 to Y3. Moreover, in FIG. 7, the arrow has shown the direction where a performance state changes. In FIG. 7, the symbol O shown on the start point side of the arrow means that the execution of the enrichment control is permitted when the performance state is predicted to change according to the arrow in the near future. This means that the execution of the enrichment control is prohibited when the performance state is expected to change according to the arrow in the near future (that is, the execution of the enrichment control is postponed).

  For example, in FIG. 7, when the current performance state is in the state E, if it is predicted that the performance state will shift to the state D in the near future, execution of the enrichment control is prohibited. That is, when it is predicted that the performance state will shift from the worse state E to the better state D in the near future, the execution of the enrichment control is prohibited. Then, after the performance state transitions from state E to state D, if the performance state is predicted to transition from state D to state G (or state E) in the near future, execution of enrichment control is permitted. Is done. However, as can be seen from FIG. 7, after the performance state becomes state D, the performance state does not shift to state A (that is, the performance state does not shift to a better state). ), Can only transition to state G or state E (ie, the performance state can only move to a worse state). This is because the NOx occlusion amount increases unless the enrichment control is executed, and the NOx purification rate region moves only in the direction from the region X1 toward the region X3. Therefore, the execution of the enrichment control may be permitted at the same time as the performance state shifts from the state E to the state D. In any case, according to this, the enrichment control is executed in a better performance state.

  On the other hand, when the current performance state is in state E, if it is predicted that the performance state will shift to state F (or state H) in the near future, execution of enrichment control is permitted. In other words, if it is predicted that the performance state will shift from the better state E to the bad state F (or state H) in the near future, execution of the enrichment control is permitted. In this case, the enrichment control is executed while the performance state is in the state E. According to this, the enrichment control is executed in a better performance state.

  In FIG. 7, an arrow from region G to region D, an arrow from region D to region A, an arrow from region H to region E, an arrow from region E to region B, and an arrow from region I to region F As described above, the NOx occlusion amount is increased unless the enrichment control is executed, and the NOx purification rate region is only in the direction from X1 to X3. This is because it does not migrate. In FIG. 7, the absence of an arrow from region A to region D and an arrow from region A to region B indicates that enrichment control is permitted when the performance state is in region A, and therefore enrichment is performed. This is because control is executed (that is, when the performance state is in state A, it is not determined to which state the performance state will shift in the near future).

  Next, the control described above will be described with reference to FIG. In FIG. 8, the horizontal axis t represents time, the vertical axis Anoxt represents the NOx occlusion amount, and E, B, and A represent performance states defined with reference to FIG. In the example illustrated in FIG. 8, when the performance state is in the state E, the NOx occlusion amount Anoxt is determined to be the determination value α (this is the NOx occlusion amount for which it is determined that the enrichment control should be executed) at time t1. Which is smaller than the amount of NOx that the NOx catalyst 25 can store at the maximum). Here, it is determined whether or not to execute the enrichment control. In the example shown in FIG. 8, the performance state is expected to shift to state B in the near future. Therefore, as can be seen from FIG. 7, in this case, the execution of the enrichment control is permitted, and thus the enrichment control is performed. According to this, NOx is reduced and purified by the NOx catalyst 25. Therefore, the NOx occlusion amount Anoxt decreases at a stretch, and the performance state shifts from the state E to the state B.

  Further, when the performance state is in the state B, the NOx occlusion amount Anoxt reaches the determination value α at time t2. Here, it is determined whether or not to execute the enrichment control. In the example shown in FIG. 8, the performance state is expected to shift to state A in the near future. Therefore, as can be seen from FIG. 7, in this case, the execution of the enrichment control is prohibited, and therefore the enrichment control is not performed. For this reason, the NOx occlusion amount Anoxt gradually increases. At time t3, the performance state shifts to the state A, and at the same time, the execution of the enrichment control is permitted and the enrichment control is performed. According to this, since NOx is reduced and purified by the NOx catalyst 25, the NOx occlusion amount Anoxt decreases at a stretch.

  As can be seen from the above description, before the execution of the enrichment control is permitted (that is, while the execution of the enrichment control is postponed), the NOx occlusion amount is the limit value (this is the NOx catalyst). 25 is the maximum amount of NOx that can be occluded). Even in this case, if the execution of the enrichment control is prohibited (that is, postponed), the NOx catalyst 25 can no longer occlude NOx, and NOx flows out of the NOx catalyst 25. . Therefore, in the above-described embodiment, when the NOx occlusion amount reaches the limit value before the execution of the enrichment control is permitted, the execution of the enrichment control is permitted regardless of the performance state. Control is performed. According to this, the outflow of NOx from the NOx catalyst 25 is suppressed.

In the example shown in FIG. 8, the determination value α is constant regardless of the temperature of the NOx catalyst 25 (catalyst temperature). However, as described above, the N ₂ O production rate tends to be lower as the catalyst temperature is higher. Therefore, even if the determination value α is increased as the catalyst temperature is higher (that is, the NOx occlusion amount is increased). ), And the N ₂ O production rate is kept low. Therefore, in the above-described embodiment, the determination value α may be determined according to the catalyst temperature. Specifically, the determination value α may be increased as the catalyst temperature increases, and conversely, the determination value α may be decreased as the catalyst temperature decreases.

  In the example shown in FIG. 8, the performance state is constant regardless of which state the performance state shifts to in the near future. However, in the above-described embodiment, the determination value α is determined according to what state the performance state will shift to in the near future, and when the NOx occlusion amount Anoxt reaches the determination value α, the enrichment control is executed. You may make it permit. Specifically, if the performance state is predicted to shift to a better state in the near future, the determination value α is increased so that the execution of the enrichment control is postponed until the performance state shifts to the better state. On the contrary, if it is predicted that the performance state will shift to a worse state in the near future, the determination value α should be reduced so that the enrichment control is executed before the performance state shifts to the worse state. You may make it do. According to this, in the example shown in FIG. 8, when the performance state is in the state B, it is predicted that the performance state will shift to the better state A in the near future. Until the performance state shifts to a better state A, the execution of the enrichment control is postponed.

Various methods are conceivable as a method for predicting which state the performance state will shift to. One example is as follows. As described above, the performance state is a state determined from the NOx purification rate regions X1 to X3 and the N ₂ O generation rate regions Y1 to Y3. Therefore, if the NOx purification rate and N ₂ O generation rate in the near future are known, the performance state in the near future will be known. Here, the NOx purification rate depends on the NOx occlusion amount, and the N ₂ O production rate depends on the catalyst temperature. Therefore, if the NOx occlusion amount and catalyst temperature in the near future are known, the performance state in the near future will be known. Here, the NOx occlusion amount depends on the engine speed and the required load, and the catalyst temperature also depends on the engine speed and the required load. Therefore, after all, if the engine speed and required load in the near future are known, the performance state in the near future will be known.

Here, since the internal combustion engine is normally used for running the vehicle, the engine speed and the required load depend on the road conditions on which the vehicle runs. That is, as a general tendency, it can be said that the engine speed and required load are higher when the vehicle is traveling on a highway than when the vehicle is traveling on a general road, and the vehicle travels downhill. It can be said that the engine speed and the required load are higher when traveling uphill than when traveling. Here, recently, a so-called navigation system that provides map information has been used in vehicles. The map information provided from this navigation system naturally includes road information. Therefore, if this road information is used, it is possible to know what road condition the vehicle will travel from after the current vehicle travel position. Then, if the engine speed and the required load in the near future are predicted based on the road conditions thus known, the NOx purification rate and the N ₂ O are calculated based on the predicted engine speed and the required load. The generation rate can be known, and as a result, the state of the performance state can be predicted in the near future. That is, in the present embodiment, as a performance state transition prediction method, a method based on information on future road conditions obtained from the navigation system can be used.

Also, the engine speed and required load in the near future can be predicted from the driving pattern of the person driving the vehicle. That is, usually, the driver of one vehicle is often the same person, and each driver has a certain tendency to drive. Therefore, the driving pattern of the vehicle (that is, the engine speed and the change pattern of the required load) is stored as a history, and the engine speed and the required load in the near future are predicted based on this history, and the predicted engine Based on the rotation speed and the required load, the NOx purification rate and the N ₂ O production rate can be known, and as a result, it is possible to predict what the performance state will be in the near future. That is, in the present embodiment, as a performance state transition prediction method, a method based on information on the future engine speed and required load obtained from the operation history can be used.

By the way, as shown in FIG. 9, the NOx purification rate peaks when the temperature of the NOx catalyst 25 itself (catalyst temperature) is at a certain temperature. That is, the NOx purification rate decreases at a temperature at which the NOx purification rate reaches a peak (hereinafter referred to as “optimum temperature”) Tm as the catalyst temperature decreases, and the NOx purification rate decreases as the catalyst temperature increases. To do. Therefore, in consideration of the catalyst temperature, the same relationship between the NOx occlusion amount Anoxt and the NOx purification rate Rnox as shown in FIG. 5 is as shown in FIG. That is, in FIG. 10 shows the relationship between the NOx occlusion amount Anoxt and NOx purification rate Rnox when solid line denoted by reference numeral Tm catalyst temperature is the optimum temperature, the solid line denoted by reference numeral _{T 1} is the temperature of the catalyst The relationship between the NOx occlusion amount Anoxt and the NOx purification rate Rnox when the temperature is lower than the optimum temperature is shown. The NOx purification rate Rnox is a temperature T ₁ at which the catalyst temperature is lower than the optimum temperature Tm. Sometimes the catalyst temperature is smaller as a whole than when the catalyst temperature is the optimum temperature Tm. Therefore, when the NOx occlusion amount Anoxt is, for example, the illustrated value A3, when the catalyst temperature is the temperature Tm, the NOx purification rate Rnox is in the region X1, whereas the NOx occlusion amount Anox is the same value A3. in some cases, when the catalyst temperature is temperatures _{T 1} is, NOx purification rate Rnox is in the region X2.

  Therefore, in the above-described embodiment, it may be determined in which region X1, X2, X3 the NOx purification rate Rnox is in consideration of the catalyst temperature. In this case, there is an advantage that the overall NOx purification performance can be maintained higher.

Further, the N ₂ O production rate varies depending on a method (hereinafter referred to as “riching method”) in which the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 25 in the enrichment control is made the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. . For example, as the enrichment method, a predetermined amount of exhaust gas discharged from the combustion chamber 5 (this is an amount that can make the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio). A method of injecting hydrocarbons (for example, the same fuel injected from the fuel injection valve 6) (this is also a method adopted in the above-described embodiment, and hereinafter referred to as “fuel addition method”); When the internal combustion engine is performing combustion other than low-temperature combustion, which will be described later, a predetermined amount (also referred to as the above-described predetermined amount) from the fuel injection valve 6 in the later stage of the combustion stroke of the internal combustion engine (also called an expansion stroke) A fuel injection valve 6 in a later stage of the combustion stroke of the internal combustion engine when the internal combustion engine is performing low-temperature combustion, which will be described later, and a method of injecting a fuel of the same amount) (hereinafter referred to as “post fuel injection method”) To a predetermined amount ( Re also and a method for injecting fuel likewise amount defined) and a predetermined amount described above (hereinafter referred to as "low-temperature combustion injection method").

Here, in consideration of such a enrichment method, the same relationship between the catalyst temperature T and the N ₂ O production rate Rn ₂ o as shown in FIG. 6 is shown in FIG. That is, in FIG. 11, the solid line marked with Rich 1 indicates the catalyst temperature T and the N ₂ O production rate Rn ₂ o when the fuel addition method is performed, and the solid line marked with Rich ₂ indicates the post fuel injection. The catalyst temperature T and the N ₂ O production rate Rn ₂ o when the method is performed are shown, and the solid line with the symbol Rich 3 indicates the catalyst temperature T and the N ₂ O production rate when the low-temperature combustion injection method is performed. Although the relationship with Rn ₂ o is shown, the N ₂ O production rate Rn ₂ o is generally higher when the post fuel injection method is performed than when the low temperature combustion injection method is performed, Overall, the fuel addition method is higher than the post fuel injection method.

Which enrichment method is executed, in other words, which enrichment method can be executed depends on the operating state of the internal combustion engine. That is, from the viewpoint of maintaining high performance of the internal combustion engine, which enrichment method should be executed is determined according to the operating state of the internal combustion engine. Also in the embodiment of the present invention, which enrichment method should be executed is determined from such a viewpoint, and in the above-described embodiment, the N ₂ O production rate Rn _{2 is} determined according to the enrichment method. You may make it determine in which area | region Y1, Y2, Y3 o exists. In this case, as a whole, there is an advantage that the NOx purification rate can be further increased and the N ₂ O production rate can be further decreased.

  Finally, the low temperature combustion described above will be described. FIG. 12 shows the opening of the throttle valve 20 (hereinafter referred to as “throttle” when the engine is operated at a low load (that is, when the internal combustion engine is operated at a relatively low engine speed and a relatively low required load)). The air-fuel ratio A / F (the horizontal axis in FIG. 12) is changed by changing the EGR rate (that is, the ratio of the amount of EGR gas to the total amount of gas sucked into the combustion chamber 5). The experiment example which showed the change of the output torque when changing, and the change of discharge | emission amount of smoke, HC, CO, NOx is represented. As can be seen from FIG. 12, in this experimental example, the EGR rate increases as the air-fuel ratio A / F decreases. When the air-fuel ratio A / F is less than the theoretical air-fuel ratio (≈14.6), the EGR rate is 65% or more.

  As shown in FIG. 12, when the air-fuel ratio A / F is reduced by increasing the EGR rate, the EGR rate becomes around 40%, and when the air-fuel ratio A / F becomes about 30, the smoke The amount of generation begins to increase. Next, when the EGR rate is further increased and the air-fuel ratio A / F is decreased, the amount of smoke generated increases rapidly and reaches a peak. Next, when the EGR rate is further increased and the air-fuel ratio A / F is decreased, this time, the smoke rapidly decreases, the EGR rate is set to 65% or more, and when the air-fuel ratio A / F is close to 15.0, the smoke is reduced. Nearly zero. That is, almost no soot is generated. At this time, the output torque of the engine slightly decreases, and the amount of NOx generated becomes considerably low. On the other hand, at this time, the generation amount of HC and CO starts to increase.

  FIG. 13A shows a change in the combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is near 21 and the amount of smoke generated is the largest. On the other hand, FIG. 13B shows a change in the combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is around 18 and the amount of smoke generated is almost zero. As can be seen by comparing FIG. 13A and FIG. 13B, in the case shown in FIG. 13B where the amount of smoke generated is almost zero, the amount of smoke generated is large. It can be seen that the combustion pressure is lower than in the case shown in FIG.

  From the experimental results shown in FIGS. 12 and 13, the following can be said. That is, first of all, when the air-fuel ratio A / F is 15.0 or less and the amount of smoke generated is almost zero, the amount of NOx generated decreases considerably as shown in FIG. That the amount of NOx generated has decreased means that the combustion temperature in the combustion chamber 5 has decreased. Therefore, when almost no soot is generated, it can be said that the combustion temperature in the combustion chamber 5 is low. The same can be said from FIG. That is, in the state shown in FIG. 13 (B) where almost no soot is generated, the combustion pressure is low. Therefore, at this time, the combustion temperature in the combustion chamber 5 is low.

  Secondly, when the amount of smoke generated (that is, the amount of soot generated) becomes substantially zero, the HC and CO emissions increase as shown in FIG. This means that the hydrocarbons are discharged without growing up to the soot. That is, linear hydrocarbons and aromatic hydrocarbons as shown in FIG. 14 contained in the fuel are thermally decomposed to form soot precursors when the temperature is raised in a state of lack of oxygen, Next, a soot composed mainly of a solid in which carbon atoms are assembled is generated. In this case, the actual soot formation process is complicated, and it is not clear what form the soot precursor will take, but in any case, the hydrocarbons shown in FIG. It will grow up to heels through the body. Therefore, as described above, when the amount of soot generated becomes almost zero, as shown in FIG. 12, the amount of HC and CO emissions increases. At this time, HC is the precursor of soot (or its Hydrocarbon in the previous state).

  Summarizing these considerations based on the experimental results shown in FIGS. 12 and 13, when the combustion temperature in the combustion chamber 5 is low, the amount of soot generated becomes almost zero, and at this time, the soot precursor (or The hydrocarbon in the previous state) is discharged from the combustion chamber 5. As a result of repeated experimental research in more detail, if the temperature of the fuel in the combustion chamber 5 and the surrounding gas are below a certain temperature, the soot growth process stops halfway (ie, It has been found that soot is generated when the temperature of the fuel in the combustion chamber 5 and the surrounding temperature rise above a certain temperature.

  By the way, the fuel and its surrounding temperature (that is, the above-mentioned certain temperature) when the hydrocarbon generation process is stopped in the state of soot precursor are various factors such as the type of fuel and the compression ratio of the air-fuel ratio. However, this certain temperature has a deep relationship with the amount of NOx generated. Therefore, this certain temperature is determined to some extent from the amount of NOx generated. Can be prescribed. That is, as the EGR rate increases, the temperature of the fuel during combustion and the surrounding gas temperature decrease, and the amount of NOx generated decreases. At this time, when the amount of NOx generated is around 10 p.p.m. (or less), soot is hardly generated. Therefore, the above-mentioned certain temperature substantially coincides with the temperature when the amount of NOx generated is around 10 p.p.m. (or less).

  Once soot is produced, it cannot be purified by post-treatment using a catalyst having an oxidizing function. On the other hand, the soot precursor (or the hydrocarbon in the previous state) can be easily purified by post-treatment using a catalyst having an oxidation function (the NOx catalyst 25 also has an oxidation function). it can. Considering the post-treatment with a catalyst having an oxidation function in this way, hydrocarbons are discharged from the combustion chamber 5 as a soot precursor (or a state before the soot), or discharged from the combustion chamber 5 in the form of soot. There is a huge difference in whether or not The new combustion system employed in the present invention allows hydrocarbons to be discharged from the combustion chamber 5 in the form of soot precursor (or the previous state) without generating soot in the combustion chamber 5; This hydrocarbon is oxidized by a catalyst having an oxidation function.

  Now, in order to stop the growth of hydrocarbons in a state before the soot is generated, the temperature of the fuel during combustion in the combustion chamber 5 and the gas temperature around it are referred to as the temperature at which soot is generated (hereinafter referred to as “soot generation temperature” Need to be controlled at a lower temperature than In this case, in order to suppress the temperature of the fuel and the surrounding gas, it has been found that the endothermic action of the gas around the fuel when the fuel burns has a great influence.

  That is, if there is only air around the fuel, the evaporated fuel immediately reacts with oxygen in the air and burns. In this case, the temperature of the air away from the fuel does not increase so much, and only the temperature around the fuel becomes extremely high locally. That is, at this time, the air away from the fuel hardly performs the endothermic action of the combustion heat of the fuel. In this case, since the combustion temperature is extremely high locally, the unburned hydrocarbons that have received this heat of combustion produce soot.

  On the other hand, when fuel is present in a mixed gas of a large amount of inert gas and a small amount of air, the situation is slightly different. In this case, the evaporated fuel diffuses around and reacts with oxygen mixed in the inert gas and burns. In this case, since the combustion heat is absorbed by the surrounding inert gas, the combustion temperature does not rise so much. That is, the combustion temperature can be kept low. That is, in order to suppress the combustion temperature, the presence of the inert gas plays an important role, and the combustion temperature can be kept low by the endothermic action of the inert gas.

In this case, in order to suppress the temperature of the fuel and the surrounding gas to a temperature lower than the soot generation temperature, an amount of inert gas that can absorb a sufficient amount of heat is required. Therefore, if the amount of fuel increases, the amount of inert gas required increases accordingly. In this case, the greater the specific heat of the inert gas, the stronger the endothermic action. Therefore, the inert gas is preferably a gas having a large specific heat. In this respect, since CO ₂ and EGR gas have a relatively large specific heat, it can be said that it is preferable to use EGR gas as the inert gas.

  FIG. 15 shows the relationship between the EGR rate and smoke when the EGR gas is used as the inert gas and the cooling degree of the EGR gas is changed. That is, in FIG. 15, a curve A shows a case where the EGR gas is strongly cooled and the EGR gas temperature is maintained at about 90 ° C., and a curve B shows a case where the EGR gas is cooled by a small cooling device. The curve C shows a case where the EGR gas is not forcibly cooled.

  As shown by the curve A in FIG. 15, when the EGR gas is strongly cooled, the amount of soot generated peaks when the EGR rate is slightly lower than 50%. In this case, the EGR rate is approximately 55%. If it is more than a percentage, almost no wrinkles occur. On the other hand, as shown by curve B in FIG. 15, when the EGR gas is cooled a little, the amount of soot generated peaks when the EGR rate is slightly higher than 50%. In this case, the EGR rate is almost If it is 65% or more, almost no wrinkle is generated. Further, as shown by the curve C in FIG. 15, when the EGR gas is not forcibly cooled, the amount of generation of soot reaches a peak when the EGR rate is around 55%. In this case, the EGR rate If the value is set to approximately 70% or more, almost no soot is generated.

  FIG. 15 shows the amount of smoke generated when the engine load is relatively high. When the engine load is reduced, the EGR rate at which the amount of soot reaches a peak slightly decreases and soot hardly occurs. The lower limit of the EGR rate also decreases slightly. Thus, the lower limit of the EGR rate at which soot hardly occurs changes according to the degree of cooling of the EGR gas and the engine load.

  FIG. 16 shows the mixed gas of EGR gas and air necessary for making the temperature of the fuel and the surrounding gas lower than the soot generation temperature when EGR gas is used as the inert gas. The amount, the ratio of the amount of air in the mixed gas, and the ratio of the amount of EGR gas in the mixed gas are shown. In FIG. 16, the vertical axis indicates the total intake gas amount sucked into the combustion chamber 5, and the chain line Y indicates the total intake gas that can be sucked into the combustion chamber 5 when supercharging is not performed. The abscissa indicates the required load.

  In FIG. 16, the ratio of air (that is, the amount of air in the mixed gas) indicates the amount of air necessary for completely burning the injected fuel. That is, in the case shown in FIG. 16, the ratio between the air amount and the injected fuel amount is the stoichiometric air-fuel ratio. On the other hand, in FIG. 16, the ratio of EGR gas (that is, the amount of EGR gas in the mixed gas) is lower than the temperature at which soot is formed in the fuel and the surrounding gas when the injected fuel is burned. The minimum amount of EGR gas necessary for the temperature is shown. The amount of EGR gas is approximately 55% or more when expressed in terms of EGR rate, and is 70% or more in the example shown in FIG. That is, when the ratio of the amount of air and the amount of EGR gas in the total amount of intake gas sucked into the combustion chamber 5 (shown by the solid line X in FIG. 16) is set as shown in FIG. The temperature of the fuel and the surrounding gas is lower than the soot generation temperature, and so no soot is generated. Further, the amount of NOx generated at this time is about 10 p.p.m. (or less), and therefore the amount of NOx generated is extremely small.

  If the amount of fuel injection increases, the amount of heat generated when the fuel burns increases, so in order to maintain the temperature of the fuel and the surrounding gas at a temperature lower than the soot generation temperature, the absorption of heat by the EGR gas The amount must be increased. Therefore, as shown in FIG. 16, the amount of EGR gas must be increased as the amount of injected fuel increases. That is, the amount of EGR gas needs to be increased as the required load increases.

  By the way, when supercharging is not performed, the upper limit of the total intake gas amount sucked into the combustion chamber 5 (solid line X in FIG. 16) is Y. Therefore, in FIG. 16, in the region where the required load is larger than Lo, the air-fuel ratio cannot be maintained at the stoichiometric air-fuel ratio unless the ratio of EGR gas is reduced as the required load increases. In other words, when supercharging is not performed and an attempt is made to maintain the air-fuel ratio at the stoichiometric air-fuel ratio in a region where the required load is larger than Lo, the EGR rate decreases as the required load increases. Thus, in a region where the required load is larger than Lo, the temperature of the fuel and the surrounding gas cannot be maintained at a temperature lower than the soot generation temperature.

  However, as shown in FIG. 1, when the EGR gas is recirculated to the inlet side of the supercharger (that is, in the air suction pipe 17 of the exhaust turbocharger 15) via the EGR passage 29, the required load is reduced from Lo. In the larger region, the EGR rate can be maintained at 55% or higher (eg, 70%), and thus the temperature of the fuel and the surrounding gas can be maintained at a temperature lower than the soot production temperature. . That is, if the EGR gas is recirculated so that the EGR rate in the air suction pipe 17 becomes, for example, 70%, the EGR rate of the intake gas boosted by the compressor 16 of the exhaust turbocharger 15 also becomes 70%. Thus, the temperature of the fuel and the surrounding gas can be maintained at a temperature lower than the soot production temperature up to a limit that can be increased by the compressor 16. Therefore, the operating range of the engine that can cause low temperature combustion can be expanded. When the EGR rate is set to 55% or more in the region where the required load is larger than Lo, the EGR control valve 31 is fully opened and the throttle valve 20 is slightly closed.

  As described above, FIG. 16 shows the case where the fuel is burned under the stoichiometric air-fuel ratio, but even if the air amount is smaller than the air amount shown in FIG. However, the amount of NOx generated can be reduced to around 10 ppm (or less) while preventing the generation of soot, and the amount of air can be made larger than the amount of air shown in FIG. (That is, even if the average value of the air-fuel ratio is made lean from 17 to 18), the generation amount of NOx can be reduced to around 10 p.pm (or less) while preventing the generation of soot. That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but since the combustion temperature is suppressed to a low temperature, the excess fuel does not grow to soot, and soot is generated. Absent. At this time, only a very small amount of NOx is generated. On the other hand, even when the average air-fuel ratio is lean (or when the air-fuel ratio is the stoichiometric air-fuel ratio), a small amount of soot is generated if the combustion temperature is high, but in the present invention, the combustion temperature is suppressed to a low temperature. As a result, no soot is produced and NOx is generated only in a very small amount.

  Thus, when low-temperature combustion is performed, soot is not generated regardless of the air-fuel ratio, that is, whether the air-fuel ratio is rich, the stoichiometric air-fuel ratio, or the average air-fuel ratio is lean. In addition, the amount of NOx generated is extremely small. Therefore, considering the improvement of the fuel consumption rate, it can be said that it is preferable to make the average air-fuel ratio lean at this time.

  As can be seen from the above, since the amount of EGR gas in the fuel chamber is large and therefore the amount of air in the fuel chamber is small when low-temperature combustion is performed, the above-described low-temperature combustion injection method is adopted. When the enrichment control is performed, it is relatively easy (ie, no large amount of soot is generated, no large amount of NOx is generated, and the fuel injection amount is simply increased by a relatively small amount). The air-fuel ratio can be made rich.

  Incidentally, when low temperature combustion is performed, the temperature of the fuel and the surrounding gas is lowered, but the temperature of the exhaust gas is increased. This will be described with reference to FIG. The solid line in FIG. 17 (A) shows the relationship between the average gas temperature Tg in the combustion chamber when low-temperature combustion is performed and the crank angle, and the broken line in FIG. 17 (A) is when normal combustion is performed. The relationship between the average gas temperature Tg in the combustion chamber and the crank angle is shown. Also, the solid line in FIG. 17B shows the relationship between the fuel and the surrounding gas temperature Tf and the crank angle when low-temperature combustion is performed, and the broken line in FIG. The relationship between the fuel and the surrounding gas temperature Tf when broken and the crank angle is shown.

  When low-temperature combustion is performed, the amount of EGR gas is larger than when normal combustion is performed. Therefore, as shown in FIG. 17A, before compression top dead center, that is, during the compression stroke. The average gas temperature Tg during low-temperature combustion indicated by a solid line is higher than the average gas temperature Tg during normal combustion indicated by a broken line. At this time, as shown in FIG. 17B, the fuel and the surrounding gas temperature Tf are substantially equal to the average gas temperature Tg.

  Next, combustion starts near the compression top dead center. In this case, when low-temperature combustion is performed, as shown by the solid line in FIG. It will not be high. On the other hand, when normal combustion is performed, since a large amount of oxygen exists around the fuel, as shown by the broken line in FIG. Extremely high. In this way, when normal combustion is performed, the temperature of the gas and the surrounding gas Tf is considerably higher than when low-temperature combustion is performed, but the temperature of the other gases that occupy the majority is It is lower when normal combustion is performed than when low temperature combustion is performed. Therefore, as shown in FIG. 17A, the average in the combustion chamber near the compression top dead center The gas temperature Tg is higher when low-temperature combustion is performed than when normal combustion is performed. As a result, as shown in FIG. 17A, the average gas temperature in the combustion chamber after completion of combustion is higher when low-temperature combustion is performed than when normal combustion is performed. Thus, when the low temperature combustion is performed, the temperature of the exhaust gas becomes high.

1 is an overall view of an internal combustion engine to which the present invention is applied. 3 is a diagram showing a map for calculating a NOx generation amount Anox from an engine speed N and a required load L. FIG. It is a figure which shows the relationship between NOx occlusion amount Anoxt and NOx purification rate Rnox. It is a diagram showing a relationship between the catalyst temperature T and _{N 2} O formation rate Rn ₂ o. It is a figure which shows the relationship between NOx occlusion amount Anoxt and NOx purification rate Rnox, Comprising: Here, three area | regions X1, X2, and X3 are formed by 1st lower limit value Rmin1 and 2nd lower limit value Rmin2. Yes. A diagram showing the relationship between the catalyst temperature T and _{N 2} O formation rate Rn ₂ o, where, by a first upper limit value Rmax1 the second upper limit value Rmax 2, the three regions Y1, Y2, Y3 Is formed. It is a figure for demonstrating the permission of execution of enrichment control. It is a figure which shows an example of the enrichment control according to this invention. It is a figure which shows the relationship between the catalyst temperature T and NOx purification rate Rnox. FIG. 6 is a diagram similar to FIG. 5, showing the relationship between the NOx occlusion amount Anoxt and the NOx purification rate Rnox. It is a view similar to FIG. 6 is a diagram showing the relationship between the catalyst temperature T and N _{2 O} formation rate Rn 2 _o. It is a figure which shows the relationship between air fuel ratio A / F, throttle opening, etc. FIG. 4A and 4B are diagrams showing a change in combustion pressure, where FIG. 5A shows a change in combustion pressure when the air-fuel ratio is near 21 and FIG. 5B shows a change in combustion pressure when the air-fuel ratio is near 18. ing. It is a figure which shows the composition of hydrocarbon. It is a figure which shows the relationship between an EGR rate and the generation amount of smoke. It is a figure which shows the relationship between a request | requirement load and the total intake gas amount. (A) shows the relationship between the crank angle when low-temperature combustion is performed and when normal combustion is performed, and the average gas temperature Tg in the fuel chamber, and (B) is normal and when low-temperature combustion is performed. The relationship between the crank angle and the gas temperature Tf of the fuel and its surroundings when the combustion of is performed is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Engine body 4 ... Piston 5 ... Combustion chamber 6 ... Fuel injection valve 7 ... Intake valve 9 ... Exhaust valve 25 ... NOx catalyst 37 ... Hydrocarbon addition valve

Claims (7)

When the enrichment control is performed so that the air-fuel ratio of the internal atmosphere is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, in the NOx purification device having a NOx catalyst that purifies NOx, when the enrichment control is performed Characterized by comprising a enrichment control means for prohibiting the execution of the enrichment control when the NOx purification rate by the NOx catalyst is low and the production rate of N ₂ O produced by the NOx catalyst is predicted to be high. NOx purification device. Means for increasing the temperature of the NOx catalyst when the NOx purification rate by the NOx catalyst when the enrichment control is performed is low and when the production rate of N ₂ O produced by the NOx catalyst is predicted to be high The NOx purification device according to claim 1, further comprising: When the enrichment control is performed so that the air-fuel ratio of the internal atmosphere is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, the above enrichment control is performed at the present time in the NOx purification device including the NOx catalyst that purifies NOx. It is expected that the NOx purification rate by the NOx catalyst is high and the production rate of N ₂ O produced by the NOx catalyst is high, or the NOx purification rate is low and the N ₂ O production rate is low. When the NOx purification rate is predicted to be low and the N ₂ O generation rate is expected to be high later, execution of the enrichment control is permitted. On the other hand, the NOx purification rate is high and the N ₂ O generation rate is A NOx purification device comprising a enrichment control means for prohibiting execution of the enrichment control when it is predicted to be low. When the enrichment control means is expected to have a high NOx purification rate by the NOx catalyst when the enrichment control is performed and a low production rate of N ₂ O produced by the NOx catalyst, the enrichment control means The NOx purification device according to claim 1 or 3, wherein execution of control is permitted. When the enrichment control means is predicted to have a low NOx purification rate by the NOx catalyst when the enrichment control is performed and a high production rate of N ₂ O produced by the NOx catalyst, the enrichment control means The NOx purification device according to claim 3, wherein execution of control is prohibited. Means for increasing the temperature of the NOx catalyst when the NOx purification rate by the NOx catalyst when the enrichment control is performed is low and when the production rate of N ₂ O produced by the NOx catalyst is predicted to be high The NOx purification device according to claim 5, further comprising: The NOx purification rate by the NOx catalyst and the production rate of N ₂ O produced by the NOx catalyst when the enrichment control is performed are the temperature of the NOx catalyst and the amount of NOx held in the NOx catalyst. The NOx purification device according to any one of claims 1 to 6, wherein the NOx purification device is estimated as a parameter.

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