An exhaust emission purification apparatus and method for an internal combustion engine

Abstract: An exhaust emission gas purification apparatus for an internal combustion engine is disclosed, in which a proper purification rate can be maintained even at a transient time by suppressing an excessive temperature increase of a lean NO x catalyst. In order to improve the purification efficiency of a lean NO x catalyst 307, diesel fuel constituting a hydrocarbon material capable of reducing the NO x is supplied by way of a diesel fuel injection hole 311. A basic amount of diesel fuel to be supplied is determined on the basis of a space velocity SV providing a quantity of state representing the operating conditions of the catalyst calculated from the intake air amount detected by an air flowmeter 302 and the catalyst temperature detected by an outlet exhaust gas temperature sensor 322. The basis amount of diesel fuel to be supplied is corrected based on the difference between the catalyst temperature and the inlet exhaust gas temperature detected by the inlet exhaust gas temperature sensor 321. The purification rate of the catalyst can thus be prevented from being deteriorated at a transient time.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust emission purification apparatus for an internal combustion engine or, in particular, to an exhaust emission purification apparatus for an internal combustion engine using a lean NO x catalyst. 2. Description of the Related Art A three-way catalyst for purifying carbon monoxide, hydrocarbon and nitrogen oxides at the same time has found application as an exhaust emission purification apparatus for an internal combustion engine. For the exhaust emission gas to be efficiently purified by a three-way catalyst, however, it is necessary that the air-fuel ratio of the mixture supplied to the internal combustion engine is maintained at substantially a stoichiometric air-fuel ratio. However, in the case where a three-way catalyst is used for a lean-burn internal combustion engine in which the air-fuel ratio of the mixture is set lean while the vehicle is running normally in order to reduce the fuel consumption, the carbon monoxide and hydrocarbon can be removed but the nitrogen oxide cannot be removed. For this reason, a lean NO x catalyst using zeolite ion-exchanged with a transition metal has been developed and finds practical applications. When NO x is to be purified by the lean NO x catalyst, the presence of a hydrocarbon is essential, and it is necessary to supply a hydrocarbon upstream of the lean NO x catalyst. Because the hydrocarbon thus supplied generates heat when being oxidized in the lean NO x catalyst, the temperature of the lean NO x catalyst increases excessively and sometimes deviates from a temperature range (window) in which the catalyst is to be operated. In an attempt to solve this problem, document JP-A-04-214919 proposes an exhaust emission purification apparatus for internal combustion engines, in which the amount of hydrocarbon to be supplied is controlled on the basis of the state variables (including the lean NO x catalyst temperature, the space velocity representing the exhaust gas flow rate through the lean NO x catalyst, or the engine speed and the accelerator opening) representing the operating conditions of the lean NO x catalyst. Further, JP-A-08-049526 discloses an exhaust emission gas purification apparatus and method in which the supply of hydrocarbon is decreased or stopped when the temperature at the outlet or center of a lean NO x catalyst exceeds a predetermined temperature. Finally, document EP-A-0 498 598 discloses an exhaust emission gas purification system according to the preambles of claims 1 and 8, respectively. A long-term thermal degradation of a lean NO x catalyst is compensated by calculating the catalyst degradation extent on the basis of a temperature difference between an exhaust gas temperature detected by temperature sensors at the inlet and the outlet of the lean NO x catalyst and by providing a proper hydrocarbon supply amount which prevents the temperature of the catalyst from deviating outside a temperature window for achieving a proper purification rate. The larger the catalyst degradation extent is, the more hydrocarbon is supplied to the catalyst to change the catalyst temperature to a higher temperature side. In this way, the catalyst can be operated with a considerably high NO x purification rate even after the catalyst has been thermally degraded. SUMMARY OF THE INVENTION According to the state of the art, however, the amount of hydrocarbon to be supplied corresponds to state variables which are substantially steady. Under transient operating conditions such as accelerating or decelerating conditions, a catalyst temperature deviation to outside the desired temperature window and a deterioration of the purification rate are unavoidable. Fig. 1 is a diagram for explaining the above-mentioned problem, in which the abscissa represents the time and the ordinate the temperature. Specifically, in the steady state at and before time t 1 , the catalyst temperature T c is higher than the exhaust gas emission temperature T gi at the catalyst inlet due to heat generated by the oxidation of the supplied hydrocarbon. When the acceleration starts at time t 1 , the inlet exhaust gas temperature T gi rises, so that the lean NO x catalyst begins to be heated after t 2 when the inlet exhaust gas temperature T gi exceeds the catalyst temperature T c . Due to the thermal capacity of the lean NO x catalyst, however, the catalyst temperature T c does not rise immediately, but begins to rise only at t 3 , and deviates from the temperature window at the time t 4 . The same problem occurs also in the deceleration condition, when the catalyst temperature T c begins to drop and deviates from the temperature window some time after the deceleration starts. The present invention has been developed in view of the above-mentioned problem, and the object thereof is to provide an exhaust emission gas purification apparatus and method for an internal combustion engine capable of maintaining a proper purification rate even under transient operating conditions of the internal combustion engine. The above object is achieved by an exhaust emission gas purification apparatus as set out in claim 1 and by an exhaust emission gas purification method as set out in claim 8. In this apparatus and method, the basic amount of hydrocarbon supplied for reducing NO x is corrected when the temperature difference between the inlet exhaust gas temperature and the catalyst temperature deviates from a predetermined value corresponding to the amount of heat generated by the basic amount of hydrocarbon. Even in the case where the inlet exhaust gas temperature undergoes a transient fluctuation, therefore, an excessive temperature fluctuation of the lean NO x catalyst can be suppressed, thereby preventing the lean NO x catalyst temperature from deviating outside a desired window set for achieving a proper purification rate. A diesel fuel injection hole 311 is inserted in the exhaust pipe 306 upstream of the exhaust gas purification apparatus 307. The diesel fuel stored in a diesel fuel tank 312, pressured by a diesel fuel pump 313 and functioning as a NO x reducing agent is injected into the exhaust gas. The injected amount of the diesel fuel thus is controlled by adjusting the opening of a solenoid valve 314. An inlet exhaust gas temperature sensor 321 and an outlet exhaust gas temperature sensor 322 for detecting the temperature of the exhaust gas are installed at the inlet and the output, respectively, of the exhaust gas purification apparatus 307. According to this embodiment, it is difficult to detect the temperature of the lean NO x catalyst directly, and therefore the temperature detected by the outlet exhaust gas temperature sensor 322 is used as a catalyst temperature. The exhaust gas purification apparatus for the internal combustion engine described above is controlled by a controller 33 constituting a microcomputer system. The controller 33 includes a bus 331, a CPU 332, a memory 333, an output interface 334 and an input interface 335 built around the bus 331. The input interface 335 is connected with an internal combustion engine speed sensor 304, an air flowmeter 302, an inlet exhaust gas temperature sensor 321 and an outlet exhaust gas temperature sensor 322. The outlet interface 334 is connected with the solenoid valve 314. Fig. 3 is a flowchart showing a diesel fuel supply amount control routine stored in the memory 33 and executed in the CPU 332. Step 402 fetches the intake air amount Q a detected by the air flowmeter 302, the inlet exhaust gas temperature T gi detected by the inlet exhaust gas temperature sensor 321 and the outlet exhaust gas temperature T go detected by the outlet exhaust gas temperature sensor 322. Step 404 calculates the space velocity SV which is a state variable representing the operating conditions of the exhaust gas purification apparatus 307 on the basis of the intake air amount Q a . SV ← SV (Q a ) Then, step 406 calculates the basic diesel fuel supply amount HC base as a function of the outlet exhaust gas temperature T go used in place of the catalyst temperature and the space velocity SV. HC base ← HC base (SV, T go ) Fig. 4 is a map for determining the basic diesel fuel supply amount HC base , in which the abscissa represents the space velocity SV, the ordinate the outlet exhaust gas temperature T go and the contour lines the basic diesel fuel supply amount HC base . The basic diesel fuel supply amount HC base increases progressively from HC base0 toward HC base3 . Further, Fig. 4 shows cross sections taken in lines X 1 -X 1 , X 2 -X 2 , X 3 -X 3 . Specifically, the basic diesel fuel supply amount HC base assumes an angular shape in the window area with the supply amount which is highest at the center and reduced progressively toward the boundaries. Upon deviation from the window, the supply amount becomes zero. In other words, the height of each cross section represents the basic diesel fuel supply amount HC base . More specifically, the basic diesel fuel supply amount HC base has an angular shape which is progressively higher toward the upper right (in the direction of arrow Y). In this way, the basic diesel fuel supply amount HC base increases. Step 408 calculates the temperature difference ΔT between the inlet exhaust gas temperature T gi and the outlet exhaust gas temperature T go . Step 410 calculates the correcting diesel fuel supply amount ΔHC as a function of the temperature difference ΔT. ΔT ← T gi - T go ΔHC ← ΔHC (ΔT) Fig. 5 is a map for determining the correcting diesel fuel supply amount ΔHC, in which the abscissa represents the temperature difference ΔT and the ordinate the correcting diesel fuel supply amount ΔHC. Specifically, in order to set the correcting diesel fuel supply amount ΔHC to zero in a steady state, the abscissa is crosses the abscissa at a point "a" representing the state in which the outlet exhaust gas temperature T go is higher than the inlet exhaust gas temperature T gi by the amount of heat generated by the basic diesel fuel supply amount HC base . The larger the temperature difference ΔT becomes, i.e., the more the lean NO x catalyst is heated due to the rise of the inlet exhaust gas temperature T gi , the more the correcting diesel fuel supply amount ΔHC is reduced (the absolute value is increased in the negative direction) to suppress the temperature increase of the lean NO x catalyst. In the case where the inlet exhaust gas temperature T gi drops, on the other hand, the correcting diesel fuel supply amount ΔHC is increased in order to increase the temperature of the lean NO x catalyst. Note, because the outlet exhaust gas temperature is increased by the oxidizing heat of the supplied diesel fuel, an immediate decreasing correction is not required when the inlet exhaust gas temperature drops under the lower limit temperature of the window. Namely, diesel fuel supply amount may be corrected after the temperature difference ΔT becomes smaller than the temperature which corresponds to 2×HC base . Fig. 5 shows a linear function. Other functions can alternatively be used as far as the relation is held that with the increase in the temperature difference ΔT, the correcting diesel fuel supply amount ΔHC decreases, while with the decrease in the temperature difference ΔT, the correcting diesel fuel supply amount ΔHC increases. Step 412 calculates the diesel fuel supply amount HC by adding the basic diesel fuel supply amount HC base to the correcting diesel fuel supply amount ΔHC. Step 414 supplies a pulse train having a duty factor corresponding to the diesel fuel supply amount HC to the solenoid valve 314 through the output interface 334, and thus controls. the amount of the diesel fuel injected from the diesel fuel injection hole 311. Fig. 6A and 6B are diagrams for explaining the effects of the present invention, in which the abscissa represents the time, and the ordinate the temperature (upper part) and the diesel fuel supply amount (lower part). In the upper part of the graph, the solid line represents the inlet exhaust gas temperature T gi and the dashed line the catalyst temperature T c . In the two graphs, the thick dashed line represents an application of the present invention, and the thin dashed line the case in which the present invention is not applicable. Assume that the engine is operating at point A (Fig. 4) where the space velocity is SV 1 and the outlet exhaust gas temperature is T go (i.e., the catalyst temperature is T c ) in steady state at or before time point t 1 . First, consider the case where the invention is not applicable, i.e., the case where only the basic diesel fuel supply amount is used. The condition involved is described below. At t 1 when acceleration starts, the intake air amount increases and so does the space velocity. In spite of this, the catalyst temperature T c remains substantially unchanged due to the thermal capacity of the lean NO x catalyst. As a result the operation point moves rightward from point A so that, as shown by the thin dashed line in the lower part of the graph, the amount of diesel fuel supplied steadily increases and reaches a maximum point at time t 2 when the inlet exhaust gas temperature T gi reaches a maximum. In this way, the amount of heat received by the lean NO x catalyst increases not only with the temperature increase of the exhaust gas but with the increase in the amount of diesel fuel supplied. As shown by the thin dashed line in the upper part of the graph, therefore, the catalyst temperature T c gradually increases to such an extent that it deviates from the window at time t 3 . According to the present invention, in contrast, with the increase of the inlet exhaust gas temperature T gi from time point t 1 , the temperature difference ΔT between the inlet exhaust gas temperature T gi and the outlet exhaust gas temperature T go increases. As shown by the thick dashed line in the lower part of the graph, therefore, the correcting diesel fuel supply amount ΔH suppresses the increase in the diesel fuel supply amount. Consequently, in spite of the fact that the amount of heat received by the lean NO x catalyst increases with the temperature of the exhaust gas, the diesel fuel supply amount is suppressed, so that the heat generated by the NO x reduction due to the diesel fuel is rather reduced. As shown by the thick dashed line in the upper part of the graph, therefore, the catalyst temperature T c is prevented from deviating from the window. The basic diesel fuel supply amount HC base , which is determined from the map of the catalyst temperature represented by the outlet exhaust gas temperature T go and the space velocity SV according to the present embodiment, can alternatively be determined from the map of the intake air amount and the rotational speed of the internal combustion engine. In such a case, the speed of the internal combustion engine is detected by the rpm sensor 304. Note, this invention can be applied when the vehicle is running under steady state condition and the inlet exhaust gas temperature and the catalyst temperature are within the window. Further, although the diesel fuel is used as a hydrocarbon for reducing NO x in the above-mentioned embodiment, other hydrocarbon materials such as alcohol can be used with equal effect.

Claims: An exhaust emission gas purification apparatus for an internal combustion engine, comprising: a lean NO x catalyst (307) arranged in the exhaust pipe (306) of an internal combustion engine (30); a hydrocarbon supply means (311) for supplying hydrocarbon into the exhaust gas for reducing NO x upstream of said NO x catalyst; a state variables detection means (302, 304, 321, 322) for detecting state variables (Q a , T gi , T c ) representing the operating conditions of said lean NO x catalyst; a basic hydrocarbon supply amount determining means (33) for determining the basic amount of hydrocarbon (HC base ) supplied from said hydrocarbon supply means (311) in accordance with the state variables detected by said state variables detection means; and a temperature difference detection means (321, 322) for detecting the difference (ΔT) between the exhaust gas temperature (T gi ) at the inlet of said lean NO x catalyst and the temperature (T c ) of the lean NO x catalyst which are included in the state variables detected by said state variables detection means, characterized by a hydrocarbon supply amount correction means (33) for correcting the basic amount of hydrocarbon (HC base ) determined by said basic hydrocarbon supply amount determining means when the temperature difference (ΔT) detected by said temperature difference detection means (321, 322) deviates from a predetermined value (a; 2a) corresponding to an amount of heat generated by the basic amount of hydrocarbon (HC base ) supplied from said hydrocarbon supply means (311) so as to prevent the temperature (T c ) of said lean NO x catalyst from deviating outside a temperature window set for achieving a proper purification rate. An exhaust emission gas purification apparatus for internal combustion engines according to claim 1, wherein said predetermined value (a; 2a) is greater when the exhaust gas temperature (T gi ) at the inlet of said lean NO x catalyst drops. An exhaust emission gas purification apparatus for internal combustion engines according to claim 1 or 2, wherein said hydrocarbon supply amount correction means (33) uses a linear function for correcting the basic amount of hydrocarbon (HC base ) in accordance with the temperature difference (ΔT). An exhaust emission gas purification apparatus for internal combustion engines according to claim 1, wherein said state variables detection means includes an intake air amount detection means (302) for detecting the intake air amount (Q a ) of the internal combustion engine, and a lean catalyst temperature detection means (322) for detecting the temperature (T c ) of said lean NO x catalyst. An exhaust emission gas purification apparatus for internal combustion engines according to claim 4, wherein said lean catalyst temperature detection means (322) detects the exhaust gas temperature (T go ) at the outlet of said lean NO x catalyst. An exhaust emission gas purification apparatus for internal combustion engines according to claim 1, wherein said state variables detection means includes an intake air amount detection means (302) for detecting the intake air amount (Q a ) of the internal combustion engine, and an engine speed detection means (304) for detecting the rotational speed of the internal combustion engine. An exhaust emission gas purification apparatus for internal combustion engines according to claim 1, wherein said temperature difference detection means (321, 322) detects the difference (ΔT) in the exhaust gas temperature between the inlet and the outlet of said lean NO x catalyst. An exhaust emission gas purification method for internal combustion engines, comprising the steps of: supplying hydrocarbon into the exhaust gas for reducing the NO x upstream of a lean NO x catalyst (307) arranged in the exhaust pipe (306) of an internal combustion engine (30); detecting state variables (Q a , T gi , T c ) representing the operating conditions of said lean NO x catalyst; and determining the basic amount of hydrocarbon (HC base ) supplied in said hydrocarbon supply step in accordance with the state variables detected in said state variables detection step; and detecting the difference (ΔT) between the temperature of the exhaust gas (T gi ) at the inlet of said lean NO x catalyst and the temperature (T c ) of said lean NO x catalyst which are included in the state variables detected at said state variables detection step, characterized by correcting the basic amount of hydrocarbon (HC base ) determined in said basic hydrocarbon supply amount determining step when the temperature difference (ΔT) detected in said temperature difference detection step deviates from a predetermined value (a; 2a) corresponding to an amount of heat generated by the basic amount of hydrocarbon (HC base ) supplied in said hydrocarbon supply step so as to prevent the temperature (T c ) of said lean NO x catalyst from deviating outside a temperature window set for achieving a proper purification rate. An exhaust emission gas purification method for internal combustion engines according to claim 8, wherein said predetermined value (a; 2a) is greater when the exhaust gas temperature (T gi ) at the inlet of said lean NO x catalyst drops. An exhaust emission gas purification method for internal combustion engines according to claim 8 or 9, wherein said hydrocarbon supply amount correction step uses a linear function for correcting the basic amount of hydrocarbon (HC base ) in accordance with the temperature difference (ΔT). An exhaust emission gas purification method for internal combustion engines according to claim 8, wherein said state variables detection step includes the steps of detecting the intake air amount (Q a ) of the internal combustion engine, and detecting the temperature (T c ) of said lean NO x catalyst. An exhaust emission gas purification method for internal combustion engines according to claim 11, wherein in said lean catalyst temperature detection step the exhaust temperature (T go ) is detected at the outlet of said lean NO x catalyst. An exhaust emission gas purification method for internal combustion engines according to claim 8, wherein said state variables detection step includes the steps of detecting the intake air amount (Q a ) of the internal combustion engine, and detecting the rotational speed of the internal combustion engine. An exhaust emission gas purification method for internal combustion engines according to claim 8, wherein in said temperature difference detection step the difference (ΔT) in the exhaust gas temperature is detected between the inlet and the outlet of said lean NO x catalyst.

Inventors: KIBE KAZUYA, KIBE, KAZUYA
Applicants: DENSO CORP, TOYOTA MOTOR CO LTD, DENSO CORPORATION, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1997
Main classification: F01N3
IPC codes: F01N3, B01D53, F01N9