Exhaust gas purification system and exhaust gas purification method

Abstract

When a catalyst temperature of a catalyst device is at or below a lower limit air-fuel ratio richness control is prohibited. When a first timing, where an estimated value of a NOx storage amount has reached an enrichment start threshold value, and a second timing, based on a set interval time in an enrichment interval time map, are both satisfied, the control is started. The second timing is corrected by multiplying the set interval time by an enrichment interval correction coefficient preset based on the catalyst temperature and a storage ratio of the estimated value of the NOx storage amount to an enrichment start threshold value of the NOx storage amount. The frequency of the air-fuel ratio richness control of a catalyst device configured to recover a purification capacity of a catalyst is reduced, and the catalyst temperature is raised while preventing white smoke development and hydrocarbon slip, to thereby achieve improvement in exhaust gas composition and improvement in fuel efficiency.

Claims

1. An exhaust gas purification system, comprising: a catalyst device provided in an exhaust passage of an internal combustion engine and configured to recover a purification capacity of a catalyst by an air-fuel ratio richness control; and an electronic controller programmed to execute control of the air-fuel ratio richness control by— prohibiting the air-fuel ratio richness control, when a catalyst temperature is at or below a preset lower limit catalyst temperature, starting the air-fuel ratio richness control, when a first timing where an estimated value of an NOx storage amount has reached a preset enrichment start threshold value and a second timing based on a set interval time set in an enrichment interval time map are both satisfied, presetting an enrichment interval correction coefficient based on the catalyst temperature of the catalyst device and on a storage ratio of the estimated value of the NOx storage amount to an enrichment start threshold value of the NOx storage amount, and correct the second timing by multiplying the set interval time by the enrichment interval correction coefficient, and setting the enrichment interval correction coefficient to be larger as the catalyst temperature gets lower, and to be larger as the storage ratio gets lower.

2. An exhaust gas purification system, comprising: a catalyst device provided in an exhaust passage of an internal combustion engine and configured to recover a purification capacity of a catalyst by an air-fuel ratio richness control; and an electronic controller programmed to execute control of the air-fuel ratio richness control by— prohibiting the air-fuel ratio richness control, when a catalyst temperature is at or below a preset lower limit catalyst temperature, starting the air-fuel ratio richness control, when a first timing where an estimated value of an NOx storage amount has reached a preset enrichment start threshold value and a second timing based on a set interval time set in an enrichment interval time map are both satisfied, finishing the air-fuel ratio richness control, when a continuation time for which the rich state is continued after the starting of the air-fuel ratio richness control satisfies both a third timing where an estimated value of an NOx reduction amount has reached an enrichment finish threshold value and a fourth timing where a set continuation time specified in an enrichment continuation time map has elapsed, and correcting the fourth timing by multiplying the set continuation time by a continuation time correction coefficient set according to the catalyst temperature.

3. An exhaust gas purification method, in which a purification capacity of a catalyst in a catalyst device provided in an exhaust passage of an internal combustion engine is recovered by an air-fuel ratio richness control, the method comprising: prohibiting the air-fuel ratio richness control, when a catalyst temperature is at or below a preset lower limit catalyst temperature; starting the air-fuel ratio richness control, when a first timing where an estimated value of an NOx storage amount has reached a preset enrichment start threshold value and a second timing based on a set interval time set in an enrichment interval time map are both satisfied; correcting the second timing by multiplying the set interval time by an enrichment interval correction coefficient preset based on the catalyst temperature of the catalyst device and on a storage ratio of the estimated value of the NOx storage amount to an enrichment start threshold value of the NOx storage amount; and setting the enrichment interval correction coefficient to be larger as the catalyst temperature gets lower, and to be larger as the storage ratio gets lower.

4. An exhaust gas purification method, in which a purification capacity of a catalyst in a catalyst device provided in an exhaust passage of an internal combustion engine is recovered by an air-fuel ratio richness control, the method comprising: prohibiting the air-fuel ratio richness control, when a catalyst temperature is at or below a preset lower limit catalyst temperature; starting the air-fuel ratio richness control, when a first timing where an estimated value of an NOx storage amount has reached a preset enrichment start threshold value and a second timing based on a set interval time set in an enrichment interval time map are both satisfied; finishing the air-fuel ratio richness control, when a continuation time for which the rich state is continued after the starting of the air-fuel ratio richness control satisfies both a third timing where an estimated value of a NOx reduction amount has reached an enrichment finish threshold value and a fourth timing where a set continuation time specified in an enrichment continuation time map has elapsed; and correcting the fourth timing by multiplying the set continuation time by a continuation time correction coefficient set according to the catalyst temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view showing a configuration of an exhaust gas purification system of an embodiment according to the present invention.

(2) FIG. 2 is a graph schematically showing the enrichment interval correction coefficient based on a catalyst temperature and on a storage ratio of an estimated value of a NOx storage amount to an enrichment start threshold value of the NOx storage amount used in an exhaust gas purification method of the embodiment according to the present invention.

(3) FIG. 3 is a graph schematically showing a continuation time correction coefficient against the catalyst temperature used in the exhaust gas purification method of the embodiment according to the present invention.

(4) FIG. 4 is a graph schematically showing a time series of an estimated catalyst temperature, a NOx storage amount, a NOx storage amount threshold value (enrichment start threshold value), and rich flags, after ignition of an engine, for describing a problem of a conventional technology.

DETAILED DESCRIPTION

(5) Hereinafter, an exhaust gas purification system and an exhaust gas purification method of an embodiments according to the present invention are described with reference to the drawings. As shown in FIG. 1, an engine (internal combustion engine) 10 equipped with an exhaust gas purification system 1 of a first embodiment of the present invention includes an engine main body 11, an intake passage 13, and an exhaust passage 15.

(6) In the intake passage 13 connected to an intake manifold 12 of the engine main body 11, an air cleaner 17, a compressor 18b of a turbocharger 18, and an intercooler 19 are provided in this order from an upstream side. Meanwhile, in the exhaust passage 15 connected to an exhaust manifold 14 of the engine main body 11, a turbine 18a of the turbocharger 18 is provided in this order from the upstream side.

(7) In addition, the engine 10 is equipped with an exhaust gas recirculation (“EGR”) system 20 and the exhaust gas purification system 1 including an exhaust gas purification device 31 provided in the exhaust passage 15.

(8) The EGR system 20 includes an EGR passage 21 connecting the intake manifold 12 and the exhaust manifold 14 to each other, and also includes an EGR cooler 22 and an EGR valve 23 provided in this EGR passage 21 in this order from the upstream side.

(9) This EGR cooler 22 is a device configured to perform heat exchange between EGR gas Ge and engine coolant water W. The EGR gas Ge is cooled by this heat exchange to reduce the volume of the EGR gas Ge, so that the air intake efficiency is improved.

(10) On the other hand, the exhaust gas purification system 1 includes the exhaust gas purification device 31 disposed in the exhaust passage 15 and configured to perform a treatment for purifying NOx (nitrogen oxides), PM (particulate matter), and the like contained in exhaust gas G generated by the combustion reaction in the engine main body 11. The exhaust gas Gc subjected to the purification treatment is released to the atmosphere through a muffler (not illustrated) or the like. This exhaust gas purification device 31 includes a combination of a NOx storage reduction-type catalyst device (LNT: catalyst device) 31a, an oxidation catalyst device (DOC) (not illustrated), a selective reduction-type catalyst device (SCR) (not illustrated), and the like.

(11) When a vehicle travels normally, i.e., when the air-fuel ratio of the exhaust gas G is in a lean state, this NOx storage reduction-type catalyst device 31a oxidizes NO contained in the exhaust gas G to NO.sub.2, and stores the NO.sub.2. When the amount of NOx stored approaches a storage limit, an air-fuel ratio richness control for placing the air-fuel ratio of the exhaust gas G in a rich state is performed to release the amount of NOx stored and also reduce the released NOx.

(12) To place the air-fuel ratio of the exhaust gas G in a rich state in the air-fuel ratio richness control, post injection is conducted based on in-cylinder fuel injection, or fuel F is directly injected into the exhaust gas G from a fuel injection device 32 provided in the exhaust passage 15. In this manner, the amount of HCs in the exhaust gas G is increased temporarily, and the HCs are combusted with oxygen in the exhaust gas G to place the exhaust gas G in a rich state.

(13) In addition, a controlling device 41 configured to perform the air-fuel ratio richness control on the NOx storage reduction-type catalyst device 31a is provided. This controlling device 41 is generally integrated in an entire system-controlling device 40 configured to control the entirety of the engine 10 or the entirety of a vehicle on which the engine 10 is mounted.

(14) In the present invention, the controlling device 41 is configured as follows. Specifically, when the catalyst temperature T is at or below a preset lower limit catalyst temperature Tc, the air-fuel ratio richness control is prohibited. With this, when a first timing ti1 where an estimated value V of the NOx storage amount has reached a preset enrichment start threshold value Vc and a second timing ti2 based on a set interval time (enrichment start time threshold value) tic set in an enrichment interval time map M1 are both satisfied, the air-fuel ratio richness control is started.

(15) In addition, an enrichment interval correction coefficient α is preset based on the catalyst temperature T of the NOx storage reduction-type catalyst device 31a and on a storage ratio R (=V/Vc) of the estimated value V of the NOx storage amount to the enrichment start threshold value Vc of the NOx storage amount, and the second timing ti2 is corrected by multiplying the set interval time tic by the enrichment interval correction coefficient α. In other words, ti2=tic×α. The enrichment interval correction coefficient α is set to be smaller as the catalyst temperature T gets lower, and set to be larger as the storage ratio R gets smaller, as exemplified in FIG. 2.

(16) In addition, when a continuation time to for which the rich state is continued after the start of the air-fuel ratio richness control satisfies both a third timing tea where the estimated value V of the NOx reduction amount has reached an enrichment finish threshold value Ve and a fourth timing te4 where a set continuation time tec specified in an enrichment continuation time map M2 has elapsed, the air-fuel ratio richness control is finished. In addition, the fourth timing te4 is corrected by multiplying the set continuation time tec by a continuation time correction coefficient β set according to the catalyst temperature T. In other words, te4=tec×β. As exemplified in. FIG. 3, the continuation time correction coefficient β is set to be larger as the catalyst temperature T gets lower.

(17) Next, an exhaust gas purification method performed in the above-described exhaust gas purification system 1 is described. The exhaust gas purification method of the embodiment of the present invention is an exhaust gas purification method, in which a purification capacity of a catalyst in a NOx storage reduction-type catalyst device 31a provided in an exhaust passage 15 of an engine 10 is recovered by an air-fuel ratio richness control. In the exhaust gas purification method, the air-fuel ratio richness control is prohibited, when a catalyst temperature T is at or below a preset lower limit catalyst temperature Tc.

(18) With this, when the first timing ti1 where the estimated value V of the NOx storage amount has reached the preset enrichment start threshold value Vc and the second timing ti2 based on the set interval time tic set in an enrichment interval time map M1 are both satisfied, the air-fuel ratio richness control is started. In addition, the second timing ti2 is corrected by multiplying the set interval time tic by an enrichment interval correction coefficient α. Here, the enrichment interval correction coefficient α is preset based on the catalyst temperature T of the NOx storage reduction-type catalyst device 31a and on a storage ratio R (=V/Vc) of the estimated value V of the NOx storage amount to the enrichment start threshold value Vc of the NOx storage amount, and is set to be larger as the catalyst temperature T gets lower, and to be larger as the storage ratio R gets smaller, as exemplified in FIG. 2. In other words, ti2=tic×α.

(19) In addition, when the continuation time to for which the rich state is continued after the start of the air-fuel ratio richness control satisfies both the third timing te3 where the estimated value V of the NOx reduction amount has reached the enrichment finish threshold value Ve and the fourth timing te4 where the set continuation time tec specified in an enrichment continuation time map M2 has elapsed, the air-fuel ratio richness control is finished. In addition, the fourth timing te4 is corrected by multiplying the set continuation time tec by a continuation time correction coefficient β set according to the catalyst temperature T, specifically, set to be larger as the catalyst temperature T gets lower, as exemplified in FIG. 3. In other words, te4=tec×β.

(20) According to the exhaust gas purification system 1 and the exhaust gas purification method configured as described above, the enrichment interval correction coefficient α based on the catalyst temperature T of the NOx storage reduction-type catalyst device 31a and on the storage ratio R (=V/Vc) of the estimated value V of the NOx storage amount to the enrichment start threshold value Vc is introduced in the exhaust gas purification system 1 in which the purification capacity of the NOx storage reduction-type catalyst supported in the NOx storage reduction-type catalyst device 31a is recovered by the air-fuel ratio richness control. This introduction makes it possible to perform the air-fuel ratio richness control at an enrichment interval ti optimized based on mutual relationships among the estimated value V of the NOx storage amount, the catalyst temperature T, and the enrichment interval ti, without performing determinations related to the estimated value V of the NOx storage amount, the catalyst temperature T each independently.

(21) Moreover, the introduction of the continuation time correction coefficient β set according to the catalyst temperature T makes it possible to optimize the continuation time te, for which the rich state is continued, according to the catalyst temperature T of the NOx storage reduction-type catalyst device 31a and the reaction rate of the catalyst which depends on the catalyst temperature T, so that the air-fuel ratio richness control can be performed more appropriately.

(22) Consequently, more appropriate determination criteria according to the operation state of the engine 10 can be provided for determining the start or finish of the air-fuel ratio richness control in the exhaust gas purification system 1 including the NOx storage reduction-type catalyst device 31a configured to recover the purification capacity of the catalyst by the air-fuel ratio richness control. This makes it possible to reduce the frequency of the air-fuel ratio richness control and raise the catalyst temperature T, while preventing the white smoke development and the HC slip. As a result, improvement in exhaust gas composition and improvement in fuel efficiency can be achieved.