EXHAUST GAS PURIFICATION DEVICE

TECHNICAL FIELD

The present invention relates to exhaust gas purification devices for purifying exhaust gas emitted from an engine, and more particularly, to an exhaust gas purification device provided with a NO_xadsorption catalyst.

BACKGROUND ART

An exhaust gas purification device has been known wherein a NO_xadsorption catalyst which, in an oxidizing atmosphere, adsorbs NO_x(nitrogen oxides) contained in the exhaust gas and, in a reducing atmosphere, releases the adsorbed NO_xfor reduction, is arranged in the exhaust passage of the engine to remove NO_xfrom the exhaust gas.

Meanwhile, fuel and lubricating oil for engines contain sulfur components, and the sulfur components are also emitted, in the form of SO_x(sulfur oxides), together with the exhaust gas from the engine. The SO_xalso is adsorbed to the NO_xadsorption catalyst through a mechanism similar to the NO_xadsorption mechanism, and as the SO_xadsorption amount increases, lowering of the NO_xadsorption capacity, or the so-called sulfur poisoning, occurs.

To make the catalyst recover from such sulfur poisoning, that is, to carry out S purge, a technique has been known in which HC (hydrocarbon) is intermittently added to the exhaust gas so as to raise the temperature of the NO_xadsorption catalyst and also to create a reducing atmosphere (hereinafter such addition of HC is referred to as the “rich spike”). The S purge is disclosed, for example, in Unexamined Japanese Patent Publication No. 2004-251172 (hereinafter referred to as “Patent Document 1”).

When performing the S purge, it is necessary that the temperature of the NO_xadsorption catalyst should be raised to a high temperature in the vicinity of 700° C., for example, and also that a reducing atmosphere should be created around the NO_xadsorption catalyst. By merely carrying out the rich spike to supply HC, however, it is difficult to stably carry out the temperature increase as well as the creation of reducing atmosphere. Especially, in diesel engines and lean-burn engines, a large amount of oxygen is contained in the exhaust gas, and when the S purge is initiated by performing the rich spike to supply HC to the exhaust gas, the HC rapidly reacts with oxygen locally in the exhaust gas, giving rise to the problem that the temperature of the NO_xadsorption catalyst rises to an excessive level.

If the amount of HC supplied by carrying out the rich spike is decreased in order to prevent such excessive temperature rise, it takes a longer time to raise the temperature of the NO_xadsorption catalyst to a level where the S purge can be performed, delaying the start of the S purge. A problem also arises in that even after the S purge is initiated, a satisfactory reducing atmosphere fails to be created around the NO_xadsorption catalyst, requiring a long time to complete the S purge.

In the exhaust gas purification device disclosed in Patent Document 1, the NO_xadsorption catalyst is carried on a particulate filter. When the S purge is restarted after being suspended, the temperature of the particulate filter is raised to regenerate the filter and then the temperature is further increased to carry out the S purge. Also in such cases where the temperature of the NO_xadsorption catalyst is raised stepwise, the exhaust temperature needs to be increased to about 600° C. in order to regenerate the particulate filter. If the rich spike for the S purge is initiated in such a state, the HC supplied by the rich spike rapidly burns on the high-temperature catalyst, causing excessive temperature rise of the catalyst and also making it difficult to keep the amount of HC in the exhaust gas at a suitable level necessary for the S purge.

Thus, with the conventional exhaust gas purification device, it is difficult to stably carry out both the temperature increase of the NO_xadsorption catalyst and the creation of reducing atmosphere for the S purge.

DISCLOSURE OF THE INVENTION

The present invention was made to solve the above problems, and an object thereof is to provide an exhaust gas purification device capable of stable and efficient recovery of a NO_xadsorption catalyst from sulfur poisoning, without causing excessive temperature rise of the NO_xadsorption catalyst.

To achieve the object, the present invention provides an exhaust gas purification device comprising: a NO_xadsorption catalyst arranged in an exhaust passage of an engine, for adsorbing, in an oxidizing atmosphere, NO_xcontained in an exhaust gas and for releasing and reducing, in a reducing atmosphere, the adsorbed NO_x; HC supply means for adding HC to the exhaust gas flowing to the NO_xadsorption catalyst; and control means for causing the HC supply means to supply HC to raise temperature of the NO_xadsorption catalyst and also causing the HC supply means to supply HC by rich spike to create the reducing atmosphere and thus carry out S purge of the NO_xadsorption catalyst, wherein the control means controls the HC supply means by first causing the HC supply means to supply an amount of HC required to raise the temperature of the NO_xadsorption catalyst to a second temperature which is derived by subtracting a temperature rise caused by the rich spike from a first temperature predetermined as a temperature necessary for the S purge, to raise the temperature of the NO_xadsorption catalyst, and then additionally performing the rich spike while continuing the HC supply.

In the exhaust gas purification device of the present invention, prior to the start of the S purge of the NO_xadsorption catalyst by means of the rich spike, an amount of HC required to maintain the NO_xadsorption catalyst at the second temperature, which is derived by subtracting the temperature rise caused by the rich spike for creating a reducing atmosphere around the NO_xadsorption catalyst from the first temperature predetermined as a temperature for recovering the catalyst from sulfur poisoning, is added to the exhaust gas flowing to the NO_xadsorption catalyst. Accordingly, even in a situation where the HC supply for creating a reducing atmosphere around the NO_xadsorption catalyst has just been started and thus the exhaust gas still has a large content of oxygen, the temperature of the NO_xadsorption catalyst is prevented from rising to an excessive level due to the reaction of the supplied HC with oxygen.

Also, the S purge is carried out by additionally supplying an amount of HC necessary for the rich spike to create a reducing atmosphere around the NO_xadsorption catalyst while the amount of HC required to raise the temperature of the NO_xadsorption catalyst to the second temperature, which is obtained by subtracting the temperature rise caused by the rich spike from the first temperature, is continuously supplied. Consequently, during the execution of the S purge by means of the rich spike, the temperature of the NO_xadsorption catalyst can be easily kept at the first temperature, which is the temperature necessary for the catalyst to recover from sulfur poisoning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an entire configuration of an exhaust gas purification device according to one embodiment of the present invention;

FIG. 2 is a flowchart illustrating S purge control executed in the exhaust gas purification device of FIG. 1; and

FIG. 3 shows how an HC supply amount, an excess air ratio of exhaust gas flowing to a NO_xadsorption catalyst, and an outlet-side exhaust temperature of the NO_xadsorption catalyst change with the lapse of time during the execution of the S purge control shown in FIG. 2.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a system configuration of a four-cylinder diesel engine (hereinafter referred to as the “engine”) to which an exhaust gas purification device according to the embodiment of the invention is applied. Referring first to FIG. 1, the construction of the exhaust gas purification device according to the present invention will be described.

The engine 1 has a high-pressure accumulator (hereinafter referred to as the “common rail”) 2 provided in common for the cylinders. High-pressure light oil as fuel supplied from a fuel injection pump (not shown) and stored in the common rail 2 is supplied to injectors 4 associated with the respective cylinders and injected from the injectors 4 into the corresponding cylinders.

An intake passage 6 has a turbocharger 8 arranged therein. Intake air introduced from an air cleaner, not shown, into the intake passage 6 flows into a compressor 8a of the turbocharger 8, and the intake air turbocharged by the compressor 8a is guided through an intercooler 10 and an intake air control valve 12 to an intake manifold 14. An intake air flow sensor 16 for detecting the flow rate of intake air flowing into the engine 1 is arranged in the intake passage 6 on the upstream side of the compressor 8a.

Exhaust ports (not shown), through which exhaust gas is discharged from the respective cylinders of the engine 1, are connected through an exhaust manifold 18 to an exhaust pipe (exhaust passage) 20. The exhaust manifold 18 and the intake manifold 14 communicate with each other through an EGR passage 24 provided with an EGR valve 22.

The exhaust pipe 20 extending from a turbine 8b of the turbocharger 8 is connected through an exhaust throttle valve 26 to an exhaust after-treatment device 28. The rotary shaft of the turbine 8b is coupled to the rotary shaft of the compressor 8a, so that as the exhaust gas flowing through the exhaust pipe 20 acts upon the turbine 8b, the compressor 8a is driven by the turbine 8b.

The exhaust after-treatment device 28 comprises an upstream-side casing 30 and a downstream-side casing 34 which is located downstream of the upstream-side casing 30 and communicates with same through a communication passage 32.

The upstream-side casing 30 contains a NO_xadsorption catalyst 36, as well as a particulate filter (hereinafter referred to as the “filter”) 38 arranged downstream of the NO_xadsorption catalyst 36.

The NO_xadsorption catalyst 36 has the function of adsorbing NO_xcontained in the exhaust gas when exposed to an oxidizing atmosphere in which the oxygen concentration of the introduced exhaust gas is high, and releasing and reducing the adsorbed NO_xwhen exposed to a reducing atmosphere in which the oxygen concentration of the introduced exhaust gas is low and reducing components such as HC and CO (carbon monoxide) are contained in the exhaust gas.

The filter 38 comprises a honeycomb-type ceramic substrate and includes a large number of passages communicating the upstream and downstream sides of the filter with each other. The upstream- and downstream-side openings of the passages are alternately closed so that particulates contained in the exhaust gas may be arrested by the filter, thereby purifying the exhaust gas emitted from the engine 1.

NO_xin the exhaust gas that failed to be adsorbed by the NO_xadsorption catalyst 36 because of the limit of its NO_xadsorption capacity flows into the filter 38 and acts as an oxidizing agent for the particulates caught and deposited on the filter 38. Namely, the NO_xflowing into the filter 38 oxidizes the particulates and removes them from the filter 38, thereby continuously regenerating the filter 38. The resultant matter, that is, N₂, is discharged into the air.

An exhaust temperature sensor 40 for detecting the exhaust temperature Tc at the outlet side of the NO_xadsorption catalyst 36 is arranged in the upstream-side casing 30 at a location downstream of the catalyst 36. Also, an upstream pressure sensor 42 is arranged upstream of the filter 38 to detect the exhaust pressure on the upstream side of the filter 38, and a downstream pressure sensor 44 is arranged downstream of the filter 38 to detect the exhaust pressure on the downstream side of the filter 38.

The downstream-side casing 34 contains a post-stage oxidation catalyst 46. The post-stage oxidation catalyst 46 has the function of oxidizing HC and CO that remain in the exhaust gas without being removed by the NO_xadsorption catalyst 36. Also, the post-stage oxidation catalyst 46 has the function of oxidizing HC separated from the filter 38 due to increase in temperature during forced regeneration, described later, of the filter 38, as well as the function of oxidizing CO produced as a result of the burning of particulates during the forced regeneration of the filter 38 and allowing the resultant matter, namely, CO₂, to be discharged into the air.

At a portion of the exhaust pipe 20 between the exhaust throttle valve 26 and the exhaust after-treatment device 28 is arranged a fuel addition valve (HC supply means) 48 which is adapted to be supplied with fuel from the fuel injection pump (not shown) to inject fuel into the exhaust pipe 20. Thus, the fuel addition valve 48 adds fuel to the exhaust gas flowing toward the NO_xadsorption catalyst 36, whereby a reducing atmosphere is created around the catalyst 38. Consequently, the NO_xadsorbed to the NO_xadsorption catalyst 38 is released and reduced.

Fuel injection from the fuel addition valve 48 into the exhaust gas is also carried out during the forced regeneration of the filter 38, as described later, in order to raise the temperature of the filter 38.

An ECU (control means) 50 is a control device for performing integrated control including the operation control of the engine 1 and comprises a CPU, memories, timer-counters, etc. The ECU calculates various control values and controls various devices in accordance with the calculated control values.

To collect information necessary for various control actions, the input side of the ECU 50 is connected with various sensors including, besides the aforementioned intake air flow sensor 16, exhaust temperature sensor 40, upstream pressure sensor 42 and downstream pressure sensor 44, a rotational speed sensor 52 for detecting the engine rotation speed, and an accelerator position sensor 54 for detecting the amount of depression of the accelerator pedal. The output side of the ECU 50 is connected to various devices controlled in accordance with respective calculated control values, such as the injectors 4 associated with the respective cylinders, the intake air control valve 12, the EGR valve 22, the exhaust throttle valve 26, and the fuel addition valve 48.

The ECU 50 also takes charge of the calculation of a fuel amount to be supplied to the individual cylinders of the engine 1, as well as the control of fuel supply from the injectors 4 according to the calculated fuel supply amount. The fuel supply amount (main injection amount) required to operate the engine 1 is determined by being read out from a pre-stored map on the basis of the engine rotation speed detected by the rotational speed sensor 52 and the accelerator position detected by the accelerator position sensor 54. The amount of fuel supplied to the individual cylinders is adjusted by means of the valve opening time of the injectors 4. The injectors 4 are opened for a time period corresponding to the determined fuel amount to effect the main injection of fuel into the respective cylinders, thereby supplying the engine 1 with the amount of fuel necessary for the engine operation.

Also, the ECU 50 controls the forced regeneration of the filter 38. The particulates deposited on the filter 38 are oxidized and removed by the continuous regeneration of the filter 38 utilizing the reaction of the particulates with the NO₂flowing into the filter 38 through the NO_xadsorption catalyst 36. However, it is sometimes the case that the continuous regeneration fails to fully remove the deposited particulates by oxidation. If the particulates are left unremoved, an excessive amount of particulates is accumulated in the filter 38, possibly clogging the filter 38. Accordingly, the filter 38 is forcedly regenerated as needed depending on the degree of accumulation of particulates in the filter 38.

Specifically, if it is judged based on, for example, the detected values of the upstream pressure sensor 42, the downstream pressure sensor 44 and the intake air flow sensor 16, that the amount of particulates accumulated in the filter 38 has reached a predetermined amount, forced regeneration control is initiated.

In the forced regeneration control, the intake air control valve 12 and the exhaust throttle valve 26 are operated in the closing direction to raise the exhaust temperature, and also fuel is injected from the fuel addition valve 48 into the exhaust gas to raise the temperature of the filter 38 up to a level at which the particulates can be burned up. Specifically, the HC supplied from the fuel addition valve 48 flows to the NO_xadsorption catalyst 36, where the exhaust gas is heated due to the oxidation of the HC, and the resultant high-temperature exhaust gas flows into the filter 38. The particulates accumulated in the filter 38 are burned by the high-temperature exhaust gas, whereby the filter 38 is forcedly regenerated.

In addition, the ECU 50 performs control to properly remove NO_xby means of the NO_xadsorption catalyst 36. The engine 1 is a diesel engine, and thus, lean-burn operation is carried out in most of the engine operating region. Consequently, the oxygen concentration of the exhaust gas is high and NO_xcontained in the exhaust gas is adsorbed to the NO_xadsorption catalyst 36. If the NO_x-adsorbing state of the NO_xadsorption catalyst 36 continues for a long time, the NO_xstorage capacity of the NO_xadsorption catalyst 36 becomes saturated, causing the possibility that the NO_xin the exhaust gas fails to be adsorbed by the NO_xadsorption catalyst 36 and is emitted directly into the air.

To prevent the saturation of the NO_xadsorption capacity, the ECU 50 controls the fuel addition valve 48 so as to inject fuel into, and hence add HC to the exhaust gas at predetermined intervals of time, for example, thereby creating a reducing atmosphere around the NO_xadsorption catalyst 36 and allowing the NO_xadsorbed to the catalyst 36 to be released and reduced.

The fuel and lubricating oil used in the engine 1 equipped with the aforementioned exhaust gas purification device contain sulfur components, and the sulfur components are discharged, in the form of SO_x, from the engine 1 together with the exhaust gas. The SO_xcontained in the exhaust gas is adsorbed to the NO_xadsorption catalyst 36 by a mechanism similar to the NO_xadsorption mechanism, and as the SO_xadsorption amount increases, reduction of the NO_xadsorption capacity of the NO_xadsorption catalyst 36, namely, sulfur poisoning, occurs. Also in cases where the sulfur poisoning is left unattended, the NO_xremoval efficiency of the NO_xadsorption catalyst 36 lowers, causing the possibility that the NO_xin the exhaust gas is emitted directly into the air without being adsorbed by the NO_xadsorption catalyst 36.

Accordingly, in the exhaust gas purification device provided with the NO_xadsorption catalyst 36, recovery of the catalyst 36 from the sulfur poisoning, that is, S purge, is carried out when necessary. Specifically, the SO_xadsorption amount of the NO_xadsorption catalyst 36 is estimated from the fuel consumption amount and operation time of the engine 1, for example, and if the estimated SO_xadsorption amount is larger than a predetermined value, the ECU 50 executes S purge control. In the S purge control, the temperature of the NO_xadsorption catalyst 36 is raised to about 700° C., and it is also necessary that a reducing atmosphere be created around the NO_xadsorption catalyst 36.

A routine for the S purge control is started when it is concluded by an S purge control discrimination routine, not shown, that the estimated SO_xadsorption amount is larger than the predetermined value and thus that the S purge is required. The S purge control routine is executed at predetermined control intervals, following the procedure shown in the flowchart of FIG. 2.

Upon start of the S purge control, it is first determined in Step S2 whether or not the value of a flag F1 is equal to “1”. The flag F1 indicates whether to permit the HC supply by the rich spike for the S purge, and the value “1” means that the rich spike is permitted. The initial value of the flag F1 is “0”; therefore, in the first control cycle just after the start of the S purge control, the process proceeds to Step S4.

In Step S4, it is determined whether or not the value of a flag Fa is “1”. The flag Fa indicates whether a timer A, described later, has started counting or not, and the value “1” means that the timer A has started counting. The initial value of the flag Fa is “0”, and therefore, the process proceeds to Step S6.

In Step S6, the timer A is started, and then in Step S8, the value of the flag Fa is set to “1” correspondingly with the start of the timer A.

Subsequently, in Step S10, it is determined whether or not the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36, detected by the exhaust temperature sensor 40, is higher than or equal to a predetermined temperature T2.

The predetermined temperature T2 is corresponding to an outlet-side exhaust temperature of the NOx adsorption catalyst 36 in a condition where a temperature of the NO_xadsorption catalyst 36 is equal to a temperature (second temperature) which is obtained by subtracting a temperature rise that is expected to be caused by the HC supply for the rich spike from a temperature (first temperature) necessary for the S purge. Accordingly, if it is judged in Step S10 that the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 is higher than or equal to the predetermined temperature T2, then it means that the temperature of the NO_xadsorption catalyst 36 is found to be higher than or equal to the second temperature.

The first temperature represents a temperature necessary to carry out the S purge and is set to a temperature higher by several tens of degrees Celsius than a lower-limit temperature at and above which the S purge of the NO_xadsorption catalyst 36 can be effected. The first and second temperatures are set to respective appropriate values taking account of the characteristics of the NO_xadsorption catalyst 36 and engine 1. In this embodiment, the first and second temperatures are set, for example, to 700° C. and 500° C., respectively.

In accordance with the result of the comparison in Step S10 between the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 detected by the exhaust temperature sensor 40 and the predetermined temperature T2, Step S12 is executed if the temperature of the NO_xadsorption catalyst 36 is judged to be lower than the second temperature, or Step S14 is executed if the temperature of the NO_xadsorption catalyst 36 is judged to be higher than or equal to the second temperature.

In Steps S12 and S14, the opening/closing of the fuel addition valve 48 is controlled so that HC may be added to the exhaust gas in an amount read out from a pre-stored map based on the engine rotation speed detected by the rotational speed sensor 52, the amount of depression of the accelerator pedal detected by the accelerator position sensor 54 and the like. The map stores HC supply amounts required to raise the temperature of the NO_xadsorption catalyst 36 to the second temperature. Specifically, where Step S12 is executed, a large-amount map storing relatively large HC supply amounts is used because the temperature of the NO_xadsorption catalyst 36 is lower than the second temperature. On the other hand, where Step S14 is executed, a small-amount map storing relatively small HC supply amounts is used since the temperature of the NO_xadsorption catalyst 36 is higher than or equal to the second temperature.

The process then proceeds to Step S16, where it is determined whether or not the time to counted by the timer A, started in Step S6, has reached a predetermined time t1. In the initial stage after the start of the S purge control, the predetermined time t1 is not yet reached by the time ta, so that the present control cycle ends.

In the next control cycle, the value of the flag F1 is still “0”. Accordingly, the process proceeds from Step S2 to Step S4, and since the value of the flag Fa has been set to “1”, the process proceeds from Step S4 directly to Step S10.

In Step S10, it is determined whether or not the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 is higher than or equal to the predetermined temperature T2, as mentioned above, to thereby determine whether or not the temperature of the NO_xadsorption catalyst 36 is higher than or equal to the second temperature.

If it is judged that the temperature of the NO_xadsorption catalyst 36 is still lower than the second temperature, the HC supply is carried out using the large-amount map, in Step S12. On the other hand, if it is judged that the temperature of the NO_xadsorption catalyst 36 is higher than or equal to the second temperature, the HC supply is performed using the small-amount map, in Step S14.

In this manner, the HC supply is repeatedly performed in Step S12 or S14 at the control intervals, so that the temperature of the NO_xadsorption catalyst 36 is elevated to the second temperature or thereabout.

FIG. 3 shows changes with time of the amount of HC supplied from the fuel addition valve 48, the excess air ratio of the exhaust gas supplied to the NO_xadsorption catalyst 36, and the outlet-side exhaust temperature of the NO_xadsorption catalyst 36.

Upon start of the S purge control, the HC supply is performed in Step S12 or S14 in FIG. 2. Since it takes some time for the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 to reach the predetermined temperature T2, the HC supply using the large-amount map is carried out in Step S12.

The HC thus supplied is oxidized on the NO_xadsorption catalyst 36, so that the temperature of the NO_xadsorption catalyst 36 rises. When the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 becomes equal to or higher than the predetermined temperature T2, the HC supply is switched and HC is supplied using the small-amount map in Step S14. Each time the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 crosses the predetermined temperature T2 thereafter, the HC supply is switched between the one using the large-amount map (Step S12) and the one using the small-amount map (Step S14). Supplying HC in this manner serves to keep the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 at around the predetermined temperature T2.

At this time, the excess air ratio of the exhaust gas flowing in the NO_xadsorption catalyst 36 is lower than that before the start of the control because of the HC supply carried out in Step S12 or S14, and thus the oxygen concentration has become low, but the catalyst is still in an oxidizing atmosphere.

The timer A keeps counting while the temperature of the NO_xadsorption catalyst 36 is maintained at around the second temperature, and if it is judged in Step S16 in FIG. 2 that the time to counted by the timer A has reached the predetermined time t1, the process proceeds to Step S18 to set the value “1” for the flag F1, whereupon the control cycle ends.

Thus, before the predetermined time t1 elapses after the start of the S purge control, only the HC supply in an amount required to raise the temperature of the NO_xadsorption catalyst 36 to the second temperature is carried out in Step S12 or S14, and the HC supply by the rich spike is not performed.

Immediately after the S purge control is initiated, the oxygen concentration around the catalyst is high and the HC supplied in Step S12 or S14 reacts with oxygen on the catalyst, so that rapid temperature rise occurs locally in the NO_xadsorption catalyst 36. However, since HC is supplied at this time only in an amount necessary to raise the temperature of the NO_xadsorption catalyst 36 to the second temperature and also since the HC supply by the rich spike is not performed, the temperature of the catalyst 36 does not rise to an excessive level.

Uneven temperature distribution of the NO_xadsorption catalyst 36 caused by the local temperature rise becomes uniform over the entire area with the lapse of time, and the predetermined time t1 is set to be long enough to allow the temperature distribution to become uniform. In this manner, before the predetermined time t1 elapses after the start of the S purge control, only the HC supply of Step S12 or S14 is carried out and the HC supply by the rich spike is not performed, whereby the temperature of the NO_xadsorption catalyst 36 is raised to the second temperature or thereabout while at the same time the uneven temperature distribution of the catalyst 36 caused by local temperature rise is made uniform over the entire area of the NO_xadsorption catalyst 36.

In the control cycle executed after the value of the flag F1 is set to “1” in Step S18 and thus the HC supply by the rich spike is permitted, the process proceeds from Step S2 to Step S20.

In Step S20, it is determined whether or not the value of a flag Fb is “1”. The flag Fb indicates whether a timer B, described later, has started counting or not, and the value “1” means that the timer B has started counting. The initial value of the flag Fb is “0”, and accordingly, the process proceeds to Step S22.

In Step S22, the timer B is started, and then in Step S24, the value of the flag Fb is set to “1” correspondingly with the start of the timer B.

Subsequently, in Step S26, it is determined whether or not the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36, detected by the exhaust temperature sensor 40, is higher than or equal to a predetermined temperature T1.

The predetermined temperature T1 is corresponding to an outlet-side exhaust temperature of the NO_xadsorption catalyst 36 in a condition where a temperature of the NO_xadsorption catalyst 36 is equal to the first temperature necessary to carry out the S purge. Accordingly, if it is judged in Step S26 that the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 is higher than or equal to the predetermined temperature T1, then it means that the temperature of the NO_xadsorption catalyst 36 is found to be higher than or equal to the first temperature.

In accordance with the result of the comparison in Step S26 between the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 detected by the exhaust temperature sensor 40 and the predetermined temperature T1, Step S28 is executed if the temperature of the NO_xadsorption catalyst 36 is judged to be lower than the first temperature, or Step S30 is executed if the temperature of the NO_xadsorption catalyst 36 is judged to be higher than or equal to the first temperature.

In Steps S28 and S30, the opening/closing of the fuel addition valve 48 is controlled so that HC may be added to the exhaust gas in an amount read out from the map, used in Step S12 or S14, on the basis of the engine rotation speed detected by the rotational speed sensor 52, the amount of depression of the accelerator pedal detected by the accelerator position sensor 54 and the like. Specifically, where Step S28 is executed, the large-amount map is used because the temperature of the NO_xadsorption catalyst 36 is lower than the first temperature. On the other hand, where Step S30 is executed, the small-amount map is used since the temperature of the NO_xadsorption catalyst 36 is higher than or equal to the first temperature.

After the HC supply is thus carried out in Step S28 or S30, HC is additionally supplied from the fuel addition valve 48 to perform the rich spike, in Step S32. The HC supply by the rich spike is executed in addition to the HC supply of Step S28 or S30, by additionally opening the fuel addition valve 48 only for a predetermined period. The intervals of the rich spike are varied in accordance with the operating condition of the engine 1 and the like.

In this embodiment, therefore, the HC supply by the rich spike is not effected every time Step S32 is executed in the individual control cycles, but is effected only when the current control cycle matches the timing for performing the rich spike.

Since the amount of HC required to raise the temperature of the NO_xadsorption catalyst 36 to the second temperature is supplied in Step S28 or S30, the HC supply by the rich spike, carried out in Step S32, allows the temperature of the NO_xadsorption catalyst 36 to be further increased above the second temperature due to the oxidation of the additionally supplied HC and also creates a reducing atmosphere around the NO_xadsorption catalyst 36.

The process then proceeds to Step S34, where it is determined whether or not the time tb counted by the timer B, started in Step S22, has reached a predetermined time t2. The predetermined time tb is set to a time period within which the NO_xadsorption catalyst 36 can be fully recovered from sulfur poisoning by carrying out the rich spike. While the predetermined time t2 is not reached by the time tb counted by the timer B, the control cycle ends after the execution of Step S34.

In the next and succeeding control cycles, the value of the flag F1 still remains at “1”, and accordingly, the process proceeds to Step S20, where it is determined whether or not the value of the flag Fb is “1”. Since the value of the flag Fb was set to “1” in Step S24 when the timer B was started, the process proceeds from Step S20 directly to Step S26.

In Step S26, it is determined whether or not the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 is higher than or equal to the predetermined temperature T1, as stated above, to thereby determine whether or not the temperature of the NO_xadsorption catalyst 36 is higher than or equal to the first temperature.

If it is judged that the temperature of the NO_xadsorption catalyst 36 is still lower than the first temperature, the HC supply is carried out using the large-amount map, in Step S28. On the other hand, if it is judged that the temperature of the NO_xadsorption catalyst 36 is higher than or equal to the first temperature, the HC supply is performed using the small-amount map, in Step S30.

Then, in Step S32, the HC supply by the rich spike is carried out.

The amount of HC supplied in Step S28 or S30 is determined from the map used in Step S12 or S14 and is set to an amount required to raise the temperature of the NO_xadsorption catalyst 36 to the second temperature, as mentioned above. The second temperature is derived by subtracting the temperature rise, which is expected to be caused when the HC supply by the rich spike is carried out, from the first temperature necessary for the S purge. Consequently, by performing the HC supply in Step S28 or S30 as well as the HC supply by the rich spike in Step S32, it is possible to maintain the temperature of the NO_xadsorption catalyst 36 at around the first temperature necessary for the S purge.

In this case, the temperature of the NO_xadsorption catalyst 36 is adjusted by switching between the HC supply using the large-amount map in Step S28 and the HC supply using the small-amount map in Step S30, taking into account the temperature rise of the NO_xadsorption catalyst 36 caused by the rich spike, as mentioned above. The temperature of the NO_xadsorption catalyst 36 can therefore be easily kept at the first temperature.

FIG. 3 illustrates how the amount of HC supplied from the fuel addition valve 48, the excess air ratio of the exhaust gas supplied to the NO_xadsorption catalyst 36 and the outlet-side exhaust temperature of the NO_xadsorption catalyst 36 change with the lapse of time at this time.

Once the time ta counted by the timer A reaches the predetermined time t1 after the start of the S purge control, the HC supply is performed in Step S28 or S30 shown in the flowchart of FIG. 2 and also the HC supply by the rich spike is carried out in Step S32, so that the outlet-side exhaust temperature of the NO_xadsorption catalyst 36 further increases from the predetermined temperature T2. However, the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 does not instantly rise to the predetermined temperature T1 but remains below T1 for some time, and therefore, the HC supply using the large-amount map is performed in Step S28.

When the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 becomes equal to or higher than the predetermined temperature T1, that is, when the temperature of the NO_xadsorption catalyst 36 becomes equal to or higher than the first temperature, the HC supply is switched and HC is supplied using the small-amount map, in Step S30. Each time the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 crosses the predetermined temperature T1 thereafter, that is, each time the temperature of the NO_xadsorption catalyst 36 crosses the first temperature thereafter, the HC supply is switched between the one using the large-amount map in Step S28 and the one using the small-amount map in Step S30. Also, at this time, the HC supply by the rich spike is carried out in Step S32, in addition to the HC supply in Step S28 or S30. By supplying HC in this manner, it is possible to keep the outlet-side exhaust temperature Tc of the NO_xadsorption catalyst 36 at around the predetermined temperature T1, and thus to keep the temperature of the NO_xadsorption catalyst 36 at around the first temperature necessary for the S purge.

When the rich spike is carried out, the excess air ratio of the exhaust gas around the NO_xadsorption catalyst 36 temporarily significantly drops because of the HC supplied by the rich spike, and therefore, the oxygen concentration lowers, creating a reducing atmosphere around the catalyst. As a result, the SO_xadsorbed to the NO_xadsorption catalyst 36 is released, so that the catalyst 36 recovers from sulfur poisoning.

In this manner, the S purge of the NO_xadsorption catalyst is carried out by the rich spike in Step S32, and if the time counted by the timer B reaches the predetermined time t2, it is judged that the S purge of the NO_xadsorption catalyst 36 is completed, whereupon the process proceeds to Step S36. In Step S36, the values of the flags F1, Fa and Fb, all used for the S purge control, are set to “0”, and then in Step S38, the timers A and B are reset, whereupon the control cycle ends. Also, this S purge control routine is terminated by the S purge control discrimination routine, not shown.

As described above, when the S purge of the NO_xadsorption catalyst 36 is needed, the HC supply by the rich spike is not instantly started, but HC is supplied first from the fuel addition valve 48 in such an amount that the temperature of the NO_xadsorption catalyst 36 is raised to the second temperature derived by subtracting the temperature rise, which is expected to be caused by the HC supply by the rich spike, from the first temperature necessary for the S purge. Accordingly, while the oxygen concentration is still high immediately after the start of the S purge control, the temperature of the NO_xadsorption catalyst 36 can be prevented from rising to an excessive level. Also, the temperature of the NO_xadsorption catalyst 36 does not rise to a level far above the first temperature immediately after the rich spike is initiated to produce a reducing atmosphere around the NO_xadsorption catalyst 36.

Since the temperature of the NO_xadsorption catalyst 36 is prevented from rising to an excessive level, the HC supply amount need not be restrained, unlike the case where the rich spike is performed as soon as the S purge control is initiated, and the NO_xadsorption catalyst 36 can be brought earlier to a state where the S purge can be executed.

Also, even if rapid temperature rise occurs locally in the NO_xadsorption catalyst due to oxygen present around the catalyst at the start of the S purge control, such uneven temperature distribution is made uniform over the entire area of the NO_xadsorption catalyst 36 because the HC supply by the rich spike is not effected until the predetermined time t1 passes after the start of the S purge control. Because of the uniform temperature distribution, the temperature of the NO_xadsorption catalyst 36 does not rise to an excessive level when the HC supply by the rich spike is carried out thereafter.

Further, after the HC supply by the rich spike is started, the amount of HC required to raise the temperature of the NO_xadsorption catalyst 36 to the second temperature is continuously supplied, and in addition to this HC supply, the HC supply by the rich spike is effected. The second temperature is derived by subtracting the temperature rise, which is expected to be caused by the HC supply by the rich spike, from the first temperature necessary for the S purge. Thus, by adjusting the amount of the HC supply on which the amount of the HC supply by the rich spike is added, it is possible to easily keep the temperature of the NO_xadsorption catalyst 36 at the first temperature necessary for the S purge.

While the exhaust gas purification device according to the embodiment of the present invention has been described, it is to be noted that the invention is not limited to the foregoing embodiment alone.

For example, in the above embodiment, the fuel addition valve 48 is used as the HC supply means. Instead of using the fuel addition valve 48, post-injection of fuel into the individual cylinders of the engine 1 may be executed following the main injection, thereby increasing the HC content in the exhaust gas. In this case, the injectors 4 associated with the respective cylinders serve as the HC supply means of the present invention.

Also, in the embodiment, whether the temperature of the NO_xadsorption catalyst 36 has reached the first/second temperature or not is determined on the basis of the outlet-side exhaust temperature Tc of the catalyst 36 detected by the exhaust temperature sensor 40. Alternatively, a temperature sensor may be provided on the carrier of the NO_xadsorption catalyst 36 to directly detect the temperature of the catalyst 36, or the inlet-side exhaust temperature of the NO_xadsorption catalyst 36 may be detected to estimate the temperature of the catalyst 36 from the detected temperature.

However, the foregoing embodiment, in which the temperature of the NO_xadsorption catalyst 36 is determined from the outlet-side exhaust temperature Tc of the catalyst 36, is advantageous in that the detected temperature is less affected by local temperature variations that occur in other locations such as at the inlet or in the interior of the NO_xadsorption catalyst 36, in comparison with the case where the temperature sensor is provided on the other position of the NO_xadsorption catalyst 36.

Also, in the above embodiment, the S purge control is started when the SO_xadsorption amount of the NO_xadsorption catalyst 36, which is estimated from the fuel consumption amount, operation time of the engine 1 and the like, becomes equal to or larger than the predetermined value, but the condition for starting the S purge control is not limited to such condition alone. For example, the S purge control may be initiated at predetermined intervals of time in the course of the operation of the engine 1, or a NO_xsensor may be arranged downstream of the NO_xadsorption catalyst 36 so that the S purge control may be started when the NO_xcontent in the exhaust gas, detected by the NO_xsensor, is equal to or larger than a predetermined amount.

Furthermore, in the embodiment, the S purge control is terminated on the basis of the time elapsed from the start of the HC supply by the rich spike, but the condition for terminating the S purge control is not limited to such condition alone. For example, also in this case, a NO_xsensor may be arranged downstream of the NO_xadsorption catalyst 36, and if the value detected by the NO_xsensor after the start of the S purge control becomes equal to or lower than a predetermined value, the S purge control may be terminated on the assumption that the NO_xadsorption catalyst 36 has recovered its NO_xremoval function. Also, the time period upon lapse of which the S purge control is terminated may be varied depending on the operating condition of the engine 1.

In the foregoing embodiment, moreover, the exhaust after-treatment device 28 is constituted by the upstream- and downstream-side casings 30 and 34 separate from each other but may comprise a single casing.

The exhaust gas purification device of the above embodiment is applied to a diesel engine, but the engine type is not particularly limited. The present invention is applicable to any type of engine insofar as the engine is provided with a NO_xadsorption catalyst and HC supply means for supplying HC to the NO_xadsorption catalyst.

EXHAUST GAS PURIFICATION DEVICE

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