An exhaust gas purification device for an engine

Abstract: In the present invention, the exhaust gas from the engine (1) is divided into a first (4a) and a second branch (4b) exhaust passages after it passes through a three-way reducing and oxidizing catalyst (11), and the two branch exhaust passages merge into an exhaust gas outlet passage (5). In the first branch exhaust passage (4a), an oxidizing catalyst (13) is disposed, and in the exhaust gas outlet passage (5), a denitrating and oxidizing catalyst (15) is disposed. NO X in the exhaust gas from the engine is all converted to N₂ and NH₃ by the three-way reducing and oxidizing catalyst (11) and a part of the NH₃ generated by the three-way catalyst flows into the first branch exhaust passage (4a) and is converted to NO X again by the oxidizing catalyst (13). The amount of NO X produced by the oxidizing catalyst (13) and the amount of NO X flowing through the second branch exhaust passage (4b) is determined by the flow distribution ratio of the first and the second branch exhaust passages. In this invention, the flow ratio is determined in such a manner that the amount of the NO X produced by the oxidizing catalyst (13) and the amount of the NH₃ passing through the second branch exhaust passage (4b) are stoichiometric to produce N₂ and H₂O. Therefore, when these exhaust gases flow into the denitrating and oxidizing catalyst (15) after they mix with each other in the exhaust gas outlet passage (5), all the NO X and the NH₃ are converted to N₂ and H₂O by the denitrating and oxidizing catalyst (15) without producing any surplus NO X or NH₃.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the present invention relates to a device which is capable of effectively removing HC, CO and NO X components from the exhaust gas. 2. Description of the Related Art An exhaust gas purification device utilizing a three-way reducing and oxidizing catalyst is commonly used for removing HC, CO and NO X components from the exhaust gas of an internal combustion engine (in this specification, the term NO X means a nitrogen oxide such as NO, NO 2 , N 2 O and N 3 O 4 , in general). The three-way reducing and oxidizing catalyst is capable of oxidizing HC and CO components, and reducing NO X components in the exhaust gas, when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. Namely, the three-way reducing and oxidizing catalyst is capable of removing these harmful components from exhaust gas simultaneously when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. However, even when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, the three-way reducing and oxidizing catalyst cannot remove 100% of HC, CO, and NO X components in the exhaust gas, and a small amount of these components pass through the catalyst. Further, the ability of the three-way reducing and oxidizing catalyst for oxidizing HC and CO components becomes lower when the air-fuel ratio of the exhaust gas becomes rich (i.e., when the air-fuel ratio becomes lower than the stoichiometric air-fuel ratio). However, the ability of the three-way reducing and oxidizing catalyst for reducing NO X components also becomes lower when the air-fuel ratio of the exhaust gas becomes lean (i.e., as the air-fuel ratio becomes higher than the stoichiometric air-fuel ratio). Therefore, when the air-fuel ratio of the exhaust gas deviates from the stoichiometric air-fuel ratio, such as in a transient operation of the engine, the amount of HC and CO, or NO X passing through the catalyst increases. Though HC and CO components, even when they pass through the three-way, reducing and oxidizing catalyst, can be removed by disposing an oxidizing catalyst downstream of the three-way reducing and oxidizing catalyst, it is difficult to remove the NO X components, especially a small amount of NO X components, from the exhaust gas. Therefore, when the three-way reducing and oxidizing catalyst is used, it is difficult to remove NO X components completely from the exhaust gas of the engine. To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 4-365920 discloses an exhaust gas purification device utilizing a denitrating reaction. When the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio, the three-way reducing and oxidizing catalyst converts a portion of the NO X in the exhaust gas to an NH 3 component while reducing most of the NO X components in the exhaust gas and converting it into an N 2 component. It is known that NH 3 reacts with NO X and produce N 2 and H 2 O by the denitrating reaction. In JPP'920, a multiple-cylinder internal combustion engine is used, and a group of cylinders of the engine are operated at a rich air-fuel ratio while other cylinders are operated at a lean air-fuel ratio. Further, a three-way reducing and oxidizing catalyst, having a high capability for converting NO X to NH 3 , is disposed in an exhaust gas passage connected to the rich air-fuel ratio cylinders. After it flows through the three-way reducing and oxidizing catalyst, the exhaust gas from the rich air-fuel ratio cylinders mixes with the exhaust gas from the lean air-fuel ratio cylinders. Since, when the exhaust gas from the rich air-fuel ratio cylinders flows through the three-way reducing and oxidizing catalyst, a portion of NO X component in the exhaust gas is converted to an NH 3 component, the exhaust gas downstream of the three-way reducing and oxidizing catalyst contains a relatively large amount of an NH 3 component. On the other hand, the exhaust gas from the lean air-fuel ratio cylinders contains a relatively large amount of NO X . Therefore, by mixing the exhaust gas from the three-way reducing and oxidizing catalyst and the exhaust gas from the lean air-fuel ratio cylinders, the NH 3 component in the exhaust gas from the three-way reducing and oxidizing catalyst reacts with the NO X component in the exhaust gas from the lean air-fuel ratio cylinder, and NH 3 component and NO X component produce N 2 and H 2 O by a denitrating reaction. Thus, according to the device in JPP'920, NO X in the exhaust gas is removed from the exhaust gas. In the device in JPP'920, though all of NO X can, theoretically, be removed from the exhaust gas, it is necessary to control the amount of NH 3 and NO X in the exhaust gas precisely in such a manner that the NH 3 and NO X react each other to produce N 2 and H 2 O without causing surplus NH 3 or NO 2 . For example, NO X generated by the internal combustion engine is mainly composed of NO, and the NO reacts with NH 3 to produce N 2 and H 2 O by the following denitrating reaction. 4 NH 3 + 4 NO + O 2 → 4 N 2 + 6 H 2 O To convert all of NO and NH 3 to N 2 and H 2 O and to prevent surplus NH 3 and NO from being discharged to atmosphere, it is necessary to adjust the amounts of NH 3 and NO X (in this case, NO) so that the amounts of NH 3 and NO X become precisely the stoichiometrical ratio to produce N 2 and NH 3 . However, in the device in JPP'920, since NH 3 is generated by converting the NO X in the exhaust gas from the cylinders operated on a rich air-fuel ratio, and NO X is generated by the cylinders operated on a lean air-fuel ratio, the amount of NH 3 generated by the three-way reducing and oxidizing catalyst and the amount of NO X generated by the cylinders operated on a lean air-fuel ratio must be precisely controlled to maintain stoichiometry. Since the amounts of NO X generated by the cylinders varies in accordance with engine operating conditions such as engine load and engine speed, it is required to control the air-fuel ratio of the respective cylinders very precisely in accordance with the engine operating conditions to control the required ratio of the amounts of NO X and NH 3 . In the actual operation of the engine, it is almost impossible to control the air-fuel ratio to such a precision in all of the operating conditions of the engine. Therefore, in the device in JPP'920, it is difficult to maintain the stoichiometry of NH 3 and NO X , and surplus NH 3 or NO X are discharged to atmosphere in certain engine operating conditions. Further, from EP-A-0 510 498, there is known a multistage catalytic reactor. SUMMARY OF THE INVENTION In view of the problems in the related art, the object of the present invention is to provide an exhaust gas purification device for an engine which is capable of efficiently removing HC, CO, and NO X components in the exhaust gas, and especially the NO X component from the exhaust gas of the engine. The above object is achieved with an exhaust gas purification device having the features of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the description, as set forth hereinafter with reference to the accompanying drawings in which: Fig. 1 is a drawing schematically illustrating an embodiment of the present invention when applied to an automobile engine; Fig. 2 is a graph showing typical changes in conversion rates of HC, CO and NO X of a three-way reducing and oxidizing catalyst in accordance with a change in the air-fuel ratio of the exhaust gas; Fig. 3 is a graph showing typical changes in the rate of the amounts of NO 2 in the NO X generated by a oxidizing catalyst in accordance with changes in the temperature of the catalyst and the air-fuel ratio of the exhaust gas; Fig. 4 is a schematic drawing showing an example of the performance of the exhaust gas purification device according to the embodiment in Fig. 1; Fig. 5 is a schematic drawing showing another example of the performance of the exhaust gas purification device according to the embodiment in Fig. 1; Fig. 6 is a drawing schematically illustrating an embodiment of the present invention; Fig. 7 is a drawing schematically illustrating an embodiment of the present invention; Fig. 8 is a drawing schematically illustrating an embodiment of the present invention; Fig. 9 is a drawing showing an example of the result obtained by an experiment using the device in Fig. 1; Fig. 10 is a drawing showing an example of the result obtained by an experiment using the device in Fig. 5; Fig. 11 is a drawing schematically illustrating another embodiment of the present invention; Fig. 12 is a drawing schematically illustrating another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 schematically illustrates an embodiment of the exhaust gas purification device according to the present embodiment. In Fig. 1, reference numeral 1 designates a multiple-cylinder type internal combustion engine and 2 designates an intake air passage of the engine 1. Numeral 3 is an exhaust manifold which connects exhaust ports of the respective cylinder of the engine 1 to an exhaust gas passage 4. A three-way reducing and oxidizing catalyst 11 is disposed on the exhaust gas passage 4. In this embodiment, the exhaust gas passage 4 diverges into two branch exhaust passages 4a and 4b. In the first branch exhaust passage 4a, an oxidizing catalyst 13 is provided, and in the second branch exhaust passage 4b, a resistance body 17, which generates approximately the same flow resistance as the oxidizing catalyst 13, is disposed. The oxidizing catalyst 13 and the resistance body 17 are explained later in detail. The first branch exhaust passage 4a and the second branch exhaust passage 4b are merged into an exhaust gas outlet passage 5, and a denitrating and oxidizing catalyst 15 is disposed on the exhaust gas outlet passage 5. The exhaust gas outlet passage 5 opens to atmosphere via a muffler (not shown) disposed in the exhaust gas outlet passage downstream of the denitrating and oxidizing catalyst 15. In Fig. 1, numeral 21 shows a secondary air supplying unit which injects air into the first branch exhaust passage 4a at the portion upstream of the oxidizing catalyst 13. The secondary air supply unit 21 includes a pressurized air source 21c such as an air pump, an air injection nozzle 21a which injects the air from the air source 21 into the branch exhaust passage 4a, and a flow control valve 21b which adjusts the injection rate of the air from the nozzle 21a in accordance with a control signal from an engine control circuit 30. The engine control circuit 30 consists of, for example, a micro computer which comprises a RAM, a ROM, a CPU and input and output ports connected each other by a bi-directional bus. The control circuit 30 performs basic control of the engine 1 such as a fuel injection control and an air-fuel ratio control. In addition to that, the control circuit 30 controls the injection rate of the secondary air by controlling the flow control valve 21b of the secondary air supply unit 21 in this embodiment. In order to perform these controls, various signals representing the operating conditions of the engine 1 are fed to the input ports of the control circuit 30. These signals are, for example, an inlet air signal from an air-flow meter (not shown) disposed on the intake air passage which represents a flow rate of the inlet air supplied to the engine 1, an engine speed signal from an engine speed sensor (not shown) disposed on a crankshaft of the engine 1, and an air-fuel ratio signal from the air-fuel ratio sensor 31 disposed on the exhaust gas passage 4 at the portion upstream of the three-way reducing and oxidizing catalyst 11. The air-fuel ratio signal from the sensor 31 represents an air-fuel ratio of the exhaust gas flowing into the three-way reducing and oxidizing catalyst 11. In this specification, an air-fuel ratio of the exhaust gas at a certain point is defined by a ratio of the air and the fuel which are supplied to the engine or exhaust passages upstream of the point. Therefore, when no air or fuel is supplied in the exhaust passages upstream of the considered point, the air-fuel ratio of the exhaust gas at the point becomes the same as the operating air-fuel ratio of the engine (i.e., the air-fuel ratio of the air-fuel mixture supplied to combustion chambers of the engine). Further, the output port of the control circuit 30 is connected to fuel injection valves (not shown) which inject a pressurized fuel to the inlet ports of the respective cylinder of the engine 1, and the flow control valve 21b, to perform the fuel injection control and the secondary air supply control. In this embodiment, the control circuit 30 controls the air-fuel ratio of the engine to be slightly lower than the stoichiometric air-fuel ratio in such a manner that, for example, λ (an excess air ratio) falls in the range between 0.98 and 0.995. The control circuit 30 calculates the basic fuel injection amount TAUP based on the inlet air signal and the engine speed signal. For example, the control circuit calculates the amount of inlet air supplied to the engine per one revolution of the engine (Q/N) from the amount of inlet air Q and the engine speed N, and calculates the basic fuel injection amount TAUP by a formula TAUP = (Q/N) × α . The basic fuel injection amount TAUP is an amount of the fuel required to obtain the stoichiometric air-fuel ratio (i.e., λ = 1.0). Further, the control circuit 30 calculates the actual fuel injection amount TAU by a formula TAU = TAUP × (1/λ T ) × FAF. λ T is a target value of the excess air ratio, and is set at the value λ T = 0.98 to 0.995 as explained before. FAF is a air-fuel ratio correction factor which is determined based on the air-fuel ratio signal from the air-fuel ratio sensor 31. In this embodiment, a linear air-fuel ratio sensor which outputs a signal corresponding to the air-fuel ratio of the exhaust gas in a wide air-fuel ratio range is used for the air-fuel ratio sensor 31. The control circuit 30 calculates an actual excess air ratio λ of the exhaust gas based on the air-fuel ratio signal from the air-fuel ratio sensor 31, and determines the value of the correction factor FAF by a feedback control based on the values of λ and λ T , in such a manner that λ becomes equal to λ T . By this air-fuel ratio control, the air-fuel ratio (the excess air ratio) of the engine is precisely controlled at the target value. The value of the correction factor FAF is determined by, for example, PID (proportional, integral and derivative) process based on the amount of the deviation of λ from the target value λ T , or other known type feedback control. Further, in order to assure that all of NO X from the engine is converted to N 2 and NH 3 , it is necessary to maintain the air-fuel ratio of the exhaust gas lower than the stoichiometric air-fuel ratio. Therefore, it is preferable to set a lower limit value for the FAF to prevent the air-fuel ratio of exhaust gas from becoming higher than the stoichiometric air-fuel ratio. The three-way reducing and oxidizing catalyst 11 uses, for example, a honeycomb type substrate made of cordierite, and an alumina layer which acts as a carrier for the catalyst is coated on the surface of the substrate. On this carrier, precious metals such as platinum Pt, rhodium Rh, and palladium Pd are carried. Fig. 2 shows changes in conversion rates of HC, CO and NO X of the three-way reducing and oxidizing catalyst 11 in accordance with the change in the air-fuel ratio of the exhaust gas. As shown in Fig. 2, the three-way reducing and oxidizing catalyst converts HC, CO, NO X in the exhaust gas at high efficiency when the air-fuel ratio of exhaust gas is the stoichiometric air-fuel ratio (i.e., λ = 1.0). The conversion rates of HC and CO become higher than that of stoichiometric air-fuel ratio when the air-fuel ratio becomes lean (λ > 1.0). Conversely, the conversion rate of NO X becomes higher than that of stoichiometric air-fuel ratio when the air-fuel ratio becomes rich (λ < 1.0). As stated before, most of the NO X in the exhaust gas from the engine 1 consists of NO. When λ is smaller than 1.0 (i.e., when the air-fuel ratio of the exhaust gas is rich), most part of this NO is converted by the three-way reducing and oxidizing catalyst 11 by reducing reactions 2 CO + 2 NO → N 2 + 2 CO 2 , and 2 H 2 + 2 NO → N 2 + 2 H 2 O. However, a small part of NO is converted to NH 3 by the reaction 5 H 2 + 2 NO → 2 NH 3 + 2 H 2 O. The conversion rate of NO to N 2 becomes higher as the amount of rhodium Rh contained in the three-way reducing and oxidizing catalyst increases, and the conversion rate of NO to NH 3 becomes lower accordingly. As explained later, this embodiment uses an oxidizing catalyst 13 and the denitrating and oxidizing catalyst 15 disposed at the portions downstream of the three-way reducing and oxidizing catalyst for converting NO and NH 3 in the exhaust gas. Therefore, it is preferable to maintain the amounts of NO and NH 3 processed by the catalysts 13 and 15 as small as possible to increase an overall conversion rate of NO X . Considering the above, the three-way reducing and oxidizing catalyst 11 in this embodiment contains a relatively large amount of rhodium Rh as the catalytic component to reduce the amount of NH 3 and NO X in the exhaust gas after it passes through the three-way reducing and oxidizing catalyst 11. A line (A) in Fig. 2 shows a change in the amount of NH 3 generated by the three-way reducing and oxidizing catalyst 11 in accordance with the change in λ of exhaust gas. The amount of NH 3 increases as the air-fuel ratio becomes lower than the stoichiometric ratio. However, when the air-fuel ratio is slightly lower than the stoichiometric air-fuel ratio (i.e., when λ = 0.98 to 0.995), the amount of NH 3 generated by the three-way reducing and oxidizing catalyst is relatively small, and all the NO X is converted to N 2 and a small amount of NH 3 . Thus, the exhaust gas downstream of the three-way reducing and oxidizing catalyst 11 in this embodiment contains a small amount of NH 3 and does not contain NO X at all. At the portion downstream of the three-way reducing and oxidizing catalyst 11, the exhaust gas is diverged into two branch exhaust passages 4a and 4b. In this embodiment, the flow resistances of the first and the second branch exhaust passages are selected in such a manner that the ratio of the amounts of the exhaust gases flowing into the first and the second branch exhaust passages 4a and 4b becomes a:b. The settings of the flow distribution ratio a:b is explained later. In this embodiment, by setting the flow resistances of the first and the second branch exhaust passages at a predetermined value, the exhaust gas flow downstream of the three-way reducing and oxidizing catalyst can be divided by a required ratio without complicated flow controls. The exhaust gas flows into the first branch exhaust passage 4a, then flows into the oxidizing catalyst 13. Before it flows into the oxidizing catalyst 13, secondary air is supplied to the exhaust gas so that the air-fuel ratio of the exhaust gas is adjusted to be higher than the stoichiometric air-fuel ratio. The oxidizing catalyst 13 uses a substrate made of cordierite similar to that of the three-way reducing and oxidizing catalyst 11, and oxidizing catalytic components such as platinum Pt, palladium Pd are carried on the alumina layer. The oxidizing catalyst 13 oxidizes the HC and CO component in the exhaust gas and converts them to H 2 O and CO 2 . Further, the oxidizing catalyst 13 oxidizes the NH 3 component in the exhaust gas and converts it to NO, NO 2 , and H 2 O. By oxidizing reactions of 2 NH 3 + 7/2 O 2 → 2 NO 2 + 3 H 2 O, and 4 NH 3 + 5 O 2 → 4 NO + 6 H 2 O. Thus, all of the NH 3 in the exhaust gas flowing into the first branch exhaust passage 4a is converted to NO X (in this case, NO and NO 2 ). The ratio of the amounts of NO and NO 2 generated by the conversion of NH 3 by the oxidizing catalyst changes according to the temperature of the oxidizing catalyst 13 and the air-fuel ratio of the exhaust gas flowing into the catalyst 13. Fig. 3 shows a typical change in the ratio of the NO and the NO 2 in the NO X components converted from NH 3 by the oxidizing catalyst 13. The vertical axis in Fig. 3 represents a ratio of the amount of the NO 2 component in the total amount of NO X converted by the oxidation of NH 3 and the horizontal axis represents the temperature of the oxidizing catalyst 13. Further, the solid line in Fig. 3 represents the ratio of NO 2 component in the total amount on NO X when the air-fuel ratio of the exhaust gas flowing into the catalyst 13 corresponds to λ = 1.02, and the dotted line represents the ratio of NO 2 component in the total amount on NO X when the air-fuel ratio corresponds to λ = 1.05. As shown in Fig. 3, when the temperature T C of the oxidizing catalyst 13 is low (for example, T C < T 1 ), both NO and NO 2 components are produced by the oxidation of NH 3 , and the ratio of NO 2 in the produced NO X increases as the temperature T C becomes higher. When the temperature T C is in a medium region (for example, T 1 < T C < T 2 in case of λ = 1.02), the ratio of NO 2 becomes 100%, i.e., only NO 2 is produced. The ratio of NO in the NO X generated by the oxidation of NH 3 is the value obtained by subtracting the ratio of NO 2 from 100%. On the other hand, when the temperature T C becomes higher, the ratio of NO 2 decreases as the temperature T C increases, and in the high temperature region of T C (for example, T C > T 3 in case of λ = 1.02), the ratio of NO 2 becomes 0, i.e., when the temperature T C is in the high temperature region, all of NH 3 flowing into the oxidizing catalyst 13 is converted to NO. The temperatures T 1 , T 2 , and T 3 vary according to the type of the oxidizing catalyst. However in general, T 2 is between 100°C and 200°C, and T 3 is between 350°C and 400°C. Further, even if the temperature is the same, the ratio of NO and NO 2 in the NO X changes according to the air-fuel ratio of the exhaust gas flowing into the oxidizing catalyst 13. When the air-fuel ratio is higher, the ratio of NO 2 becomes higher in the high temperature region of T C . For example, when λ = 1.05, the ratio of NO 2 becomes higher than that when λ = 1.02 in the range T C > T 3 , as shown by the dotted line in Fig. 3. On the other hand, in the medium and low temperature ranges, the ratio of NO 2 is determined only by the temperature T C regardless of the air-fuel ratio. The reason why the ratio of NO 2 becomes higher as the air-fuel ratio becomes higher in the high temperature region of T C , is that the conversion ratios of NO and NO 2 from NH 3 are determined by a chemical equilibrium in the high temperature region because the reaction proceeds rapidly in the high temperature region. Therefore, when the concentration of O 2 in the exhaust gas becomes higher (i.e., when the air-fuel ratio of exhaust gas becomes higher), a larger amount of NO 2 is produced by the oxidation of NH 3 . On the other hand, since the speeds of the chemical reactions are low in the low temperature region, the conversion ratios of NO and NO 2 from NH 3 are determined by the speeds of chemical reactions and the concentration of the O 2 in the exhaust gas becomes less effective. Therefore the conversion rates are determined by the temperature TC in the low temperature region regardless of the air-fuel ratio of the exhaust gas. In order to convert all NH 3 component in the exhaust gas flowing into the oxidizing catalyst 13 to NO X , such as NO and NO 2 , it is necessary to maintain the air-fuel ratio of the exhaust gas higher than stoichiometric air-fuel ratio as much as possible. Further, in order to convert the HC, CO components in the exhaust gas to H 2 O, CO 2 by the denitrating and oxidizing catalyst 15 on the exhaust gas outlet passage 5, it is necessary to maintain the air-fuel ratio of the mixture of the exhaust gases from the first and the second branch exhaust passages 4a, 4b higher than the stoichiometric air-fuel ratio. Therefore, it is preferable to set the amount of secondary air to the branch exhaust passage 4a as large as possible. However, if the excessive secondary air is supplied to the branch exhaust passage 4a, the oxidizing catalyst 13 is cooled by the secondary air and the oxidation reaction of NH 3 in the catalyst 13 becomes lower. Therefore, the amount of the secondary air supplied from the unit 21 in this embodiment is adjusted in such a manner that the excess air ratio of the exhaust gas flowing into the oxidizing catalyst 13 is around λ = 1.05. Namely, the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst in this embodiment is adjusted to be relatively high, and the conversion ratio from NH 3 to NO 2 are relatively high even in the high temperature region (refer to Fig. 3.) The control circuit 30 calculates the amount of secondary air required for maintaining the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13 at 1.05 based on the inlet air amount Q and the target excess air ratio λ T of the air-fuel ratio control. Further, the control circuit 30 controls the flow control valve 21b so that the calculated amount of the secondary air is injected from the nozzle 21a. Thus, the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13 is maintained at λ = 1.05 regardless of the engine operating conditions. Further, if an air-fuel ratio sensor similar to the sensor 31 is disposed on the first branch exhaust passage 4a downstream of the oxidizing catalyst 13, the amount of the secondary air can be feedback controlled by the air-fuel ratio of the exhaust gas flowing through the oxidizing catalyst 13. In this case, a precise control of the air-fuel ratio of exhaust gas flowing into the oxidizing catalyst 13 and the ratio of NO and NO 2 downstream of the oxidizing catalyst 13 can be achieved. As explained above, all of the NH 3 component in the exhaust gas flowing into the first branch exhaust passage 4a is converted to NO X components by the oxidizing catalyst 13. On the other hand, the exhaust gas flowing into the second branch exhaust passage 4b passes through the resistance body 17 and flows into the exhaust gas outlet passage 5 without any change in its composition. Therefore, after the exhaust gases from the first and the second branch exhaust passage mix with each other in the exhaust gas outlet passage 5, the mixture contains b/(a + b) of the total amount of the NH 3 produced by the three-way reducing and oxidizing catalyst 11, i.e., the amount of the NH 3 in the exhaust gas outlet passage 5 is determined by the flow distribution ratio a:b between the first and the second branch exhaust passages. Further, since the amount (moles) of the NO X produced by the oxidizing catalyst 13 is the same as the amount (moles) of the NH 3 converted by the oxidizing catalyst 13, the exhaust gas flowing into the exhaust gas outlet passage 5 from the first branch exhaust passage 4a contains NO X corresponds to a/(a + b) of the total amount of the NH 3 produced by the three-way reducing and oxidizing catalyst 11. Therefore, the ratio of the amounts (moles) of NO X and NH 3 in the exhaust gas mixture in the exhaust gas outlet passage 5 becomes a:b, i.e., same as the flow distribution ratio between the first and the second branch exhaust passages. The excess air ratio of the exhaust gas flowing into the exhaust gas outlet passage 5 from the first branch exhaust passage 4a is maintained at λ = 1.05, and this exhaust gas mixes with the exhaust gas from the second branch exhaust passage 4b which has the excess air ratio of λ = 0.98 to 0.995. Therefore, the excess air ratio of the mixture of the exhaust gas in the exhaust gas outlet passage 5 becomes lean (for example, λ = 1.01 to 1.02). Thus, an exhaust gas of lean air-fuel ratio containing NO X and NH 3 of a:b in molar ratio flows into the denitrating and oxidizing catalyst 15. The denitrating and oxidizing catalyst 15 uses a substrate made of cordierite similar to that of the three-way reducing and oxidizing catalyst 11, and catalytic components such as vanadium oxide V 2 O 5 , and titania TiO 2 is carried on the alumina layer. The denitrating and oxidizing catalyst 15 oxidizes HC, CO component by the oxidizing reactions 4 C m H n + (4m + n) O 2 → 4m CO 2 + 2n H 2 O, and 2 CO + O 2 → 2 CO 2 . Further, the denitrating and oxidizing catalyst oxidizes NH 3 component by the denitrating reactions 8 NH 3 + 6 NO 2 → 12 H 2 O + 7 N 2 , and 4 NH 3 + 4 NO + O 2 → 6 H 2 O + 4 N 2 . In order to convert all the NH 3 and NO X components in the exhaust gas by the above denitrating reactions, the molar ratio of the NH 3 and NO X (NO and NO 2 ) must be strictly maintained at a stoichiometric molar ratio. In this embodiment, the molar ratio of NO X and NH 3 in the exhaust gas flowing into the denitrating and oxidizing catalyst 15 is maintained at a:b. However, the molar ratio of NO and NO 2 in the total amount of NO X is changes according to the temperature T C of the oxidizing catalyst 13 and the air-fuel ratio of exhaust gas flowing into the oxidizing catalyst 13. Though the air-fuel ratio of the exhaust gas is kept at constant (i.e., at λ = 1.05) at the inlet of oxidizing catalyst 13 in this embodiment, the temperature T C changes according to the exhaust gas temperature. Therefore, if the flow distribution ratio a:b is set at a fixed value, it is difficult to maintain the molar ratio of NO X and NH 3 in the exhaust gas outlet passage 5 stoichiometrical, and surplus NO X or NH 3 is produced by the denitrating and oxidizing catalyst 15. For example, when the temperature T C of the oxidizing catalyst 13 is high, all the NH 3 component in the exhaust gas flowing into the first branch exhaust passage 4a is converted to NO by the oxidizing catalyst 13. In this case, since one mole of NO and one mole of NH 3 are stoichiometrical to produce N 2 and H 2 O, the flow distribution ratio a:b (i.e., in this case, molar ratio of NO and NH 3 ) must be set 1:1. On the other hand, when the temperature T C is in the medium region (for example, T 1 ≤ T C ≤ T 2 ' in Fig. 3), all the NH 3 component in the exhaust gas flowing into the first branch exhaust passage 4a is converted to NO 2 by the oxidizing catalyst 13. Since one mole of NO 2 and 3/4 mole of NH 3 are stoichiometrical to produce N 2 and H 2 O, it is necessary to set the flow distribution ratio of the first and the second branch exhaust passage a:b (i.e., molar ratio of NO 2 and NH 3 ) at 3:4 in this case. However, when the engine operating conditions are constant, the exhaust gas temperature and the temperature T C of the oxidizing catalyst 13 becomes also constant. In such a case, the ratio of NO and NO 2 produced by the oxidizing catalyst 13 becomes also constant, and the flow distribution ratio a:b may be set at a fixed value. Therefore, first, the embodiments in which the flow distribution ratio a:b can be set at a fixed value are explained. As a first example, the case in which the engine 1 is operated on a relatively low load conditions in the most of operations is considered. In this case, the temperature T C of the oxidizing catalyst 13 is always medium, i.e., between T 1 and T 2 ' in Fig. 3. When the temperature T C is between T 1 and T 2 ', all the NH 3 component is converted to NO 2 by the oxidizing catalyst 13. Therefore, the flow distribution of the first and the second branch exhaust passage is set at a fixed value of 3:4. Namely, in this case, 3/7 of the exhaust gas from the three-way reducing and oxidizing catalyst 11 flows into the first branch exhaust passage 4a and 4/7 of the exhaust gas flows into the second branch exhaust passage 4b. Since the NH 3 in the exhaust gas flows into the first branch exhaust passage is converted to equimolar amount of NO 2 , the molar ratio of NO 2 and NH 3 in the exhaust gas mixture in the exhaust gas outlet passage 5 becomes precisely 3:4, i.e., the stoichiometric molar ratio to produce N 2 and H 2 O. Therefore, the NO 2 and the NH 3 in the exhaust gas mixture react each other in the denitrating and oxidizing catalyst 15 and are converted to N 2 and H 2 O without producing any surplus NO 2 and NH 3 . Also, since the air-fuel ratio of exhaust gas mixture flowing into the denitrating and oxidizing catalyst 15 is maintained higher than the stoichiometric air-fuel ratio, HC and CO components in the exhaust gas mixture are oxidized by the denitrating and oxidizing catalyst 15 and converted to H 2 O and CO 2 . Therefore, the exhaust gas downstream of the denitrating and oxidizing catalyst 15 is substantially free from HC, CO, NO X or NH 3 components. Fig. 4 is a simplified schematic drawing for explaining the operation of the above embodiment. In Fig. 4, the same reference numerals as those in Fig. 1 designate the same elements. In Fig. 4, the exhaust gas discharged from the engine 1 has an excess air ratio of λ = 0.98 to 0.995, and contains HC, CO and NO X (NO) in relatively large amounts (Fig. 4, portion A). After passing through the three-way reducing and oxidizing catalyst, most of HC and CO components are removed from the exhaust gas, and all the NO X components are converted to N 2 and NH 3 (Fig. 4, portion B). Downstream of the three-way reducing and oxidizing catalyst 11, the exhaust gas diverges into the first and the second branch exhaust passage at the flow distribution ratio of 3:4. Then the secondary air is supplied to the exhaust gas in the first branch exhaust passage 4a, and the excess air ratio of the exhaust gas becomes λ = 1.05 (Fig. 4, portion C). Then the exhaust gas flows through the oxidizing catalyst 13 where NH 3 in the exhaust gas is oxidized and converted to NO 2 (Fig. 4, portion D). On the other hand, the composition of the exhaust gas flowing through the second branch exhaust passage 4b is not changed (Fig. 4, portion E). Therefore, when the exhaust gases from the first and the second branch exhaust passages mix with each other in the exhaust gas outlet passage 5, the molar ratio of NO 2 and NH 3 in the exhaust gas mixture becomes 3:4, i.e., exactly the stoichiometric molar ratio, and the excess air ratio of the mixture becomes λ = 1.01 to 1.02 (Fig. 4, portion F). This exhaust gas mixture flows into the denitrating and oxidizing catalyst 15 where NO 2 and NH 3 reacts each other to produce N 2 and H 2 O without any surplus NO 2 and NH 3 . Thus the exhaust gas downstream of the denitrating and oxidizing catalyst practically does not contain any HC, CO, NO X or NH 3 components. Now, the reason why the resistance body 17 is disposed on the second branch exhaust passage 4b is explained. The resistance body 17 consists of a dummy catalyst which uses a honeycomb type substrate the same as that of the oxidizing catalyst 13 on the first branch exhaust passage 4a, but no catalyst is attached thereon. The resistance body 17 is used for generating approximately the same flow resistance as that of the oxidizing catalyst 13. If only the flow distribution ratio is to be set to a predetermined value, it may be sufficient to adjust the cross sectional areas of the branch exhaust passages 4a and 4b, and it is not necessary to dispose the resistance body 17 on the second branch exhaust passage 4b. However, in the actual operations of the engine 1, pulsation of the exhaust gas occurs and, due to this pulsation, a recirculating flow of the exhaust gas as shown by an arrow R in Fig. 1 occurs between the branch exhaust passages 4a and 4b if the characteristics of the flow resistances in the passages 4a and 4b are not the same. If the recirculation flow R occurs, the molar ratio of NO 2 and NH 3 in the exhaust gas mixture flowing into the denitrating and oxidizing catalyst 15 can not be maintained at the stoichiometric molar ratio. Namely, if the resistance body 17 is not provided on the second branch exhaust passage 4b, the pulsation of the exhaust gas in the second branch exhaust passage 4a reaches the merging point of passage 4a and 4b without attenuation, while the pulsation of exhaust gas in the first branch exhaust passage 4a is attenuated by the oxidizing catalyst 13. Further, the phase of the pulsation reaching the merging point through the first branch exhaust passage 4a becomes different from the phase of the pulsation reaching the merging point through the second branch exhaust passage 4b due to the existence of the oxidizing catalyst 13, and the recirculation flow R occurs near the merging point due to the differences in the phases and the intensities of the pulsation. However, in this embodiment, since the resistance body 17 is provided on the second branch exhaust passage 4b, the characteristics of the flow resistance of the second branch exhaust passage 4b becomes substantially the same as that of the first branch exhaust passage 4a, i.e., the phases and the intensities of the pulsation of the exhaust gases reaching the merging point become substantially the same. Therefore, a recirculating flow does not occur in this embodiment. Since the purpose for providing the resistance body 17 is to maintain the attenuation and the phase of the pulsation of the exhaust gas flows through the second branch exhaust passage 4b as the same as those of the exhaust gas flows through the first branch exhaust passage 4a, it is not necessary to adjust the flow resistance of the resistance body 17 to be exactly the same as that of the oxidizing catalyst 13. Therefore, the resistance body 17 may have approximately the same resistance as the oxidizing catalyst 13. Next, the case in which the engine 1 is operated on high load conditions in the most operation is considered. In this case, the temperature T C of the oxidizing catalyst 13 becomes high (for example, higher than T 3 ' in Fig. 3). In this case, all the NH 3 component in the exhaust gas is converted to NO by the oxidizing catalyst 13. Therefore, the flow distribution ratio a:b is set at 1:1, and 1/2 of the exhaust gas from the three-way reducing and oxidizing catalyst 11 flows into the first branch exhaust passage 4a where the NH 3 in the exhaust gas is converted to equimolar amount of NO. Namely, 1/2 of the NH 3 produced by the three-way reducing and oxidizing catalyst 11 is converted to equimolar amount of NO. Thus, the molar ratio of NO and NH 3 in the exhaust gas mixture in the exhaust gas outlet passage 5 becomes exactly 1:1. Fig. 5 is a similar schematic drawing which shows the compositions of the exhaust gas in this case. As seen from Fig. 5, the exhaust gas downstream of the denitrating and oxidizing catalyst is also substantially free from HC, CO, NO X or NH 3 in this case. Next, another embodiment of the present invention is explained. In the embodiment in Fig. 1, the flow distribution ratio a:b of the first and the second branch exhaust passage is set at a fixed value. As explained before, the ratio of NO and NO 2 generated by the oxidizing catalyst 13 varies as the temperature T C of the oxidizing catalyst 13 and the excess air ratio λ of the exhaust gas change. Therefore, if the flow distribution ratio a:b is fixed, it is difficult to keep the ratio of the amounts of NH 3 and NO 2 at stoichiometric ratio when the operating condition of the engine changes. In the embodiment explained below, the flow distribution ratio a:b is changed according to the temperature T C of the oxidizing catalyst 13 and the excess air ratio of the exhaust gas so that the ratio of NH 3 and NO X in the exhaust gas flowing into the denitrating and oxidizing catalyst 15 is maintained at the stoichiometrical ratio. Fig. 6 is a schematic drawing similar to Fig. 1 which illustrates the present embodiment. However, this embodiment is different from the embodiment in Fig. 1 in that the flow distribution ratio of the first and the second branch exhaust passages 4a and 4b is set at the fixed value of 1:1, and a variable throttle valve 51 is provided at the inlet of the first branch exhaust passage 4a. The variable throttle valve 51 is actuated by an actuator 52 of an appropriate type, such as a vacuum actuator or a stepper motor, which adjusts the flow distribution ratio of the first and the second branch exhaust passages 4a and 4b by throttling the exhaust gas flowing into the first branch exhaust passage 4a. In this embodiment, the variable throttle valve 51 is capable of adjusting the flow distribution ratio a:b from 3:4 (when the variable throttle valve 51 fully closes) to 1:1 (when the variable throttle valve 51 fully opens). In this embodiment, the flow distribution ratio a:b (i.e., the degree opening of the variable throttle valve 51) is determined on the following basis. Assuming that N moles of NH 3 is produced by the three-way reducing and oxidizing catalyst 11, the amount of NH 3 in the exhaust gas flowing into and oxidized by the oxidizing catalyst 13 on the second branch exhaust passage 4a becomes N × a/(a + b) moles when the flow distribution ratio is a:b. In the oxidizing catalyst 13, one mole of NO 2 or NO is produced by the oxidation reactions 2 NH 3 + 7/2 O 2 → 2 NO 2 + 3 H 2 O, and 4 NH 3 + 5 O 2 → 4 NO + 6 H 2 O. Therefore, the total N × a/(a + b) moles of NO 2 and NO are produced from N × a/(a + b) moles of NH 3 by the oxidizing catalyst 13 on the branch exhaust passage 4a. Further, assuming that the ratio of NO 2 in the total NO X is X, the moles of the NO 2 and NO produced by the oxidizing catalyst 13 become: X × N × a/(a + b) [NO 2 ], and (1 - X) × N × a/(a + b) [NO]. The moles of NH 3 reacting with these NO 2 and NO to stoichiometrically produce N 2 and H 2 O are, (4/3) × X × N × a/(a + b) [NO 2 ], and 1 × (1 - X) × N × a/(a + b) [NO], respectively. Therefore, the total moles of NH 3 required is, {(4 X/3) + (1 - X)} × N × a/(a + b) = (1 + X/3) × N × a/(a + b). In order to supply this amount of NH 3 through the second branch exhaust passage 4b, the exhaust gas flowing into the second branch exhaust passage 4b must contain the (1 + X/3) × N × a/(a + b) moles of NH 3 . Since the flow distribution ratio of the first and the second branch exhaust passages 4a and 4b is a:b, N × b/(a + b) moles of the NH 3 produced by the three-way reducing and oxidizing catalyst 11 flows into the second branch exhaust passage 4b. Therefore, (1 + X/3) × N × a/(a + b) must be equal to N × b(a + b). Thus, (1 + X/3) × N × a/(a + b) = N × b/(a + b). In order to satisfy this relationships, b must be equal to (1 + X/3) × a , i.e., b = (1 + X/3) × a . This means that when the ratio of NO 2 in the NO x produced by the oxidizing catalyst 13 is X, all NH 3 and NO 2 are converted to N 2 and H 2 O by the denitrating and oxidizing catalyst 15 without any surplus NH 3 and NO 2 , only if the flow distribution ratio a:b is 1:(1 + X/3). In this embodiment, the control circuit 30 detects the temperature T C of the oxidizing catalyst 13, and further calculates the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13 based on the amount of the air supplied to the first branch exhaust passage 4a from the secondary air supply unit 21. Then, the control circuit 30 determines the ratio X of NO 2 in the NO X produced by the oxidizing catalyst 13 from Fig. 3. After determining the ratio X, the control circuit 30 controls the variable throttle valve 51 in such a manner that the flow resistance of the first branch exhaust passage 4a becomes (1 + X/3) times of the flow resistance of the second branch exhaust passage 4b. In this embodiment, the relationships between the degree of opening of the variable throttle valve 51 and the flow resistance of the first branch exhaust passage 4a is previously obtained, for example, by experiment and stored in the ROM of the control circuit 30. Since the flow resistance of the second branch exhaust passage 4b is constant, the required value of the degree of opening of the variable throttle valve 51 to achieve the flow distribution ratio of 1:(1 + X/3) is easily obtained using this relationships. After the required degree of opening of the variable throttle valve 51 is obtained, the control circuit 30 controls the actuator 52 so that the variable throttle valve 51 is adjusted to the required degree of opening. Thus, according to this embodiment, NH 3 and NO X react with each other in the denitrating and oxidizing catalyst 15 without producing any surplus NH 3 and NO X , and are converted to N 2 and H 2 O even when the temperature T C of the oxidizing catalyst 13 and the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13 change. The temperature T C of the oxidizing catalyst 13 may be detected directly by disposing a temperature sensor in the substrate of the oxidizing catalyst 13. However, since the temperature T C of the catalyst 13 changes according to the exhaust gas temperature, the temperature T C may be detected indirectly from the exhaust gas temperature. In this case, the relationships between the exhaust gas temperature and the temperature T C of the catalyst 13 is previously obtained, for example, by experiment, and the T C is determined from the exhaust gas temperature based on this relationships. Further, the exhaust gas temperature is determined by the load conditions of the engine. Therefore, the exhaust gas temperature may be determined indirectly from the engine load conditions. In this case, the relationships between the engine load (such as the amount of inlet air per one revolution of the engine and the engine speed) and the exhaust gas temperature is previously obtained, for example, by experiment and stored in the ROM of the control circuit 30. Fig. 7 shows a compound type converter which may be used for accommodating the oxidizing catalyst 13 and the resistance body 17 in the embodiments illustrated by Figs. 1 and 6. In the embodiments in Figs. 1 and 6, two branch exhaust passages 4a and 4b are required to accommodate the oxidizing catalyst 13 and the resistance body 17, respectively. However, this arrangement is rather complicated and requires relatively large space for installation. The compound type converter in Fig. 7 provides more simple and compact arrangement for achieving the same functions as the arrangements in Figs. 1 and 6. In Fig. 7, the compound type converter 40 comprises a housing 41. The housing 41 is provided with an exhaust gas inlet 43 which is connected to the exhaust gas passage 4, and an exhaust gas outlet which is connected to the exhaust gas outlet passage 5. In the housing 41, a partitioning plate 47 is disposed. The partitioning plate defines two passages 41a and 41b both connecting the exhaust gas inlet 43 and the exhaust gas outlet 45 in parallel. Namely, the exhaust gas flowing into the housing 41 from the exhaust gas inlet 43 diverges into the two flow passages 41a and 41b, and after passing through the passages 41a and 41b, the exhaust gas merges to the exhaust gas outlet passage 5. Further, the nozzle 21a of the secondary air supply unit 21 is disposed at the inlet of the exhaust gas passage 41a. In this embodiment, the oxidizing catalyst 13 is disposed on the exhaust gas passage 41a in the housing 41, and the resistance body 17 is disposed on the exhaust gas passage 41b in the same housing 41. Further, a variable throttle valve 54 for adjusting the amount of exhaust gas flowing into the exhaust gas passage 41a can be disposed at the inlet of the exhaust gas passage 41a as shown by a dotted line in Fig. 7. By using the compound type converter 40 as shown in Fig. 7, it is not necessary to provide branch exhaust passages in the exhaust gas system, and accordingly, the arrangement of the exhaust gas system becomes simple and compact. Next, another embodiment of the present invention is explained with reference to Fig. 8. In the embodiments explained above, the exhaust gas flow is divided into two flow passages in order to obtain the required molar ratio of NH 3 and NO 2 at the denitrating and oxidizing catalyst. Though the above embodiments are advantageous in that the required molar ratio of NH 3 and NO 2 is obtained easily and precisely by dividing the exhaust gas flow mechanically, these embodiments have disadvantages that they require the branch exhaust passages or compound type converter to divide the exhaust gas flow in a predetermined ratio. The embodiment explained below provides an arrangement in which the required molar ratio of NH 3 and NO 2 at the denitrating and oxidizing catalyst is obtained without dividing the exhaust gas flow. Fig. 8 shows a schematic drawing which illustrates the present embodiment. In Fig. 8, the same reference numerals as those in Fig. 1 designate elements similar to those in Fig. 1. As shown in Fig. 8, branch exhaust passages are not provided on the exhaust gas system, i.e., the three-way reducing and oxidizing catalyst 11, oxidizing catalyst 13 and denitrating and oxidizing catalyst 15 are connected in series in this order, and the air injection nozzle 21a of the secondary air supply unit 21 is disposed on an exhaust gas passage 4c which connects the three-way reducing and oxidizing catalyst 11 and the oxidizing catalyst 13. Further, another air supply unit 22 which injects air to the exhaust gas outlet passage 5 connecting the oxidizing catalyst 13 and the denitrating and oxidizing catalyst 15 is provided in this embodiment. The air supply unit 22 has a similar construction as the secondary air supply unit 21 and includes a pressurized air source 22c, a flow control valve 22b actuated by a control signal from the control circuit 30 and an air injection nozzle 22a disposed on the exhaust gas outlet passage 5. In this embodiment, the three-way reducing and oxidizing catalyst 11 also contains a relatively large amount of rhodium Rh as a catalytic component and converts most of NO component in the exhaust gas from the engine 1 to an N 2 component. Further, the three-way reducing and oxidizing catalyst 11 converts the remaining small amount of NO component in the exhaust gas to an NH 3 component. Since all the exhaust gas from the three-way reducing and oxidizing catalyst 11 flows into the oxidizing catalyst 13 in this embodiment, the molar ratio of NH 3 and NO X downstream of the catalyst 13 is controlled by adjusting the conversion ratio of NH 3 to NO X in the oxidizing catalyst 13. As explained before, the oxidizing catalyst 13 converts NH 3 to NO 2 and NO by the oxidation reaction 2 NH 3 + 7/2 O 2 → 2 NO 2 + 3 H 2 O, and 4 NH 3 + 5 O 2 → 4 NO + 6 H 2 O. Therefore, if the amount of O 2 in the exhaust gas is not sufficient to oxidize all the NH 3 in the exhaust gas, a part of NH 3 in the exhaust gas passes through the oxidizing catalyst 13 without being oxidized. Further, the ratio of the amount of NH 3 passing through the catalyst 13 to the amount of NH 3 flowing into the catalyst 13 (i.e., conversion rate of NH 3 ) is determined by the excess air ratio λ of the exhaust gas flowing into the catalyst 13. Therefore the ratio of NH 3 and NO 2 in the exhaust gas downstream of the oxidizing catalyst 13 can be controlled by changing the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13. Also, the ratio of NO 2 and NO in the NO X produced by the oxidizing catalyst 13 is determined by the temperature T C of the oxidizing catalyst 13 as explained in Fig. 3. Therefore, by adjusting the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13 in accordance with the temperature T C of the oxidizing catalyst 13, the amounts of NH 3 passing through the catalyst 13 and NO X produced by the catalyst 13 (NO 2 and NO) can be adjusted in such a manner that the amounts of NH 3 and NO X are stoichiometrical to produce N 2 and H 2 O. In this embodiment, the control circuit 30 adjusts the amount of the secondary air supplied from the unit 21 in accordance with the temperature T C of the oxidizing catalyst 13 and adjusts the ratio a:b of the amount of NH 3 passes through the catalyst 13 and the amount of NO X (NO 2 and NO) in the exhaust gas downstream of the catalyst 13 so that the ratio a:b becomes stoichiometric. The excess air ratio λ of exhaust gas required to obtain the stoichiometric molar ratio of NH 3 and NO X varies according to types and temperatures of oxidizing catalyst 13. Therefore, it is preferable to determine the required excess air ratio λ, for example, by experiment using the actual oxidizing catalyst under various conditions of temperature and excess air ratio of exhaust gas. For example, when the excess air ratio λ of the exhaust gas flowing into the catalyst 13 is 1.0 and the temperature T C of the catalyst 13 is relatively high, the oxidizing catalyst 13 in this embodiment converts a part of NH 3 to NO only, and the ratio of the NH 3 passing through the catalyst 13 and the NO produced by the catalyst 13 becomes 1:1. For example, in Fig. 8, the exhaust gas from the engine 1 flows through the three-way reducing and oxidizing catalyst 11, and all the NH 3 component in the exhaust gas is converted to N 2 and NH 3 by the three-way reducing and oxidizing catalyst 11. Then, the excess air ratio λ of the exhaust gas downstream of the catalyst is adjusted to 1.0 by the secondary air supplied from the secondary air supply unit 21, and this exhaust gas flows into the oxidizing catalyst 13 of temperature T C . As explained before, the ratio of the amounts of NH 3 and NO X (in this case, NO) is determined by the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13. This means that the amount of the secondary air supplied from the secondary air supply unit 21 must be controlled as precisely as possible. Therefore, it is preferable to dispose an air-fuel ratio sensor similar to the sensor 31 on the exhaust gas outlet passage 5 at the portion downstream of the oxidizing catalyst 13, and to feedback control the amount of the secondary air based on the output of the air-fuel ratio sensor so that the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13 becomes the required value. By controlling the excess air ratio λ of the exhaust gas flowing into the oxidizing catalyst 13, the exhaust gas downstream of the catalyst 13 contains NH 3 and NO, and the molar ratio of the NH 3 and NO becomes exactly 1:1. Before flowing into the denitrating and oxidizing catalyst 15, air is supplied to this exhaust gas from the air supply unit 22. Due to the air supplied from the unit 22, the excess air ratio λ of the exhaust gas flowing into the denitrating and oxidizing catalyst is adjusted so that the λ becomes larger than 1.0 (i.e., a lean air-fuel ratio). Therefore, NH 3 and NO in the exhaust gas react to produce N 2 and H 2 O without causing any surplus NH 3 and NO, and the exhaust gas downstream of the denitrating and oxidizing catalyst 15 also becomes free from HC, CO and NO X or NH 3 in this embodiment. In Fig. 8, precise control of the amount of the air supplied from the air supply unit 22 is not required. It is sufficient to control the amount of air in an appropriate range in which the air-fuel ratio of the exhaust gas becomes lean and the denitrating and oxidizing catalyst is not cooled excessively. In this embodiment, the amount of the air supplied from the air supply unit 22 is controlled in such a manner that the excess air ratio λ of the exhaust gas flowing into the denitrating and oxidizing catalyst 15 is maintained around 1.02. As explained above, the ratio of NO 2 and NO in the NO X contained in the exhaust gas flowing into the denitrating and oxidizing catalyst 15 can be controlled by merely adjusting the amount of the secondary air supplied to the exhaust gas upstream of the oxidizing catalyst 13. Therefore, it becomes possible to remove HC, CO, and NO X components in the exhaust gas simultaneously without providing the branch exhaust passages in the exhaust gas system. Figs. 9 and 10 show the results of experiments in which the exhaust gas purification devices in the above embodiments are applied to actual internal combustion engines. Figs. 9 and 10 corresponds to the experiments of the devices corresponding to the embodiment in Fig. 1 and the embodiment in Fig. 8, respectively. In these experiments, a 2,200 cc engine is used, at an engine speed of 1,600 rpm and an output torque of 40 Nm. In the experiments, the temperature T C is maintained at about 325°C, i.e., NH 3 in the exhaust gas is converted to N 2 and NO. Further, the excess air ratio λ of the engine is feedback controlled at λ = 1.02. In Figs. 9 and 10, the symbols (I) to (VI) correspond to the portions indicated by the same symbols (I) to (VI) in Figs. 1 and 8. For example, the exhaust gas from the engine contains NO X of 2,000 ppm and large amounts of HC and CO at the outlet of the engine (Fig. 9, (I)) in the case of the embodiment in Fig. 1. At the outlet of the three-way reducing and oxidizing catalyst 11, all of NO X in the exhaust gas is converted to N 2 and NH 3 , and the amounts of HC and CO are also reduced (Fig. 9, (II)). The exhaust gas downstream of the three-way reducing and oxidizing catalyst 11 diverges into the branch exhaust passages 4a and 4b, and 3/4 of the total amount of the exhaust gas flows into the oxidizing catalyst 13 (Fig. 9, (III)). Since the exhaust gas is diluted by the secondary air, the concentrations of the respective components are lowered at the inlet of oxidizing catalyst 13. In the oxidizing catalyst 13, all NH 3 in the exhaust gas (95 ppm) is oxidized by the catalyst, and same amount of NO 2 (95 ppm) is produced (Fig. 9, (IV)). This exhaust gas is then mixed with the exhaust gas passing through the branch exhaust passage 4b which contains NH 3 , and forms the mixture of exhaust gas containing NH 3 and NO 2 having a molar ratio of 4:3 (Fig. 9, (V)). The air-fuel ratio of the mixture of the exhaust gas is maintained on a lean side of the stoichiometric air-fuel ratio (λ = 1.02). After passing through the denitrating and oxidizing catalyst 15, almost all of the HC, CO, and NO X components as well as NH 3 component are removed from the exhaust gas (Fig. 9, (VI)). Further, as seen from Fig. 10, similar results are obtained at the outlet of the denitrating and oxidizing catalyst 15 also in the embodiment in Fig. 8. It will be understood that, according to the embodiments explained above, all NO component produced by the engine is first converted to N 2 and NH 3 , and a part of this NH 3 is converted to NO X again. Therefore, even if the amount of NO produced by the engine is changed, the molar ratio of NH 3 and NO X can be maintained at a predetermined ratio easily and precisely. Thus, it becomes possible to convert all NH 3 and NO X in the exhaust gas to N 2 and H 2 O by the denitrating and oxidizing catalyst without producing any surplus NH 3 and NO X . Next, another embodiment of the present invention is explained with reference to Fig. 11. The embodiment in Fig. 11 is different from the embodiment in Fig. 1 in that a NH 3 decomposing catalyst 61 is further provided on the exhaust gas passage downstream of the denitrating and oxidizing catalyst 15. The reference numerals in Fig. 11 which are the same as those in Fig. 1 indicate similar elements as shown in Fig. 1. Further, the flow distribution ratio of the first and the second branch exhaust passages 4a and 4b are set at the fixed value of 3:4 in this embodiment. The NH 3 decomposing catalyst 61 has a substitute made of, for example, zeolite ZSM5, and catalytic components such as copper Cu and platinum Pt are coated on the substrate. The NH 3 decomposing catalyst 61 oxidizes NH 3 under the presence of oxygen, and decomposes NH 3 into N 2 and NH 3 without producing NO X . In this embodiment, since the flow distribution ratio of branch exhaust passages 4a and 4b is fixed to 3:4, the molar ratio of NH 3 and NO X in the exhaust gas flowing into the denitrating and oxidizing catalyst 15 becomes stoichiometric only when all the NH 3 in the exhaust gas is converted to NO 2 by the oxidizing catalyst 13. However, if NH 3 is converted to NO 2 and NO by the oxidizing catalyst 13, for example, due to the change of the temperature T C of the oxidizing catalyst 13, surplus NH 3 is produced by the denitrating and oxidizing catalyst 15. As explained before, 1 mole of NO reacts with only 1 mole of NH 3 while 1 mole of NO 2 reacts with 4/3 mole of NH 3 . Therefore, since the flow distribution ratio of the passages 4a and 4b is fixed to 3:4 in this embodiment, the amounts of NO X flowing into the denitrating and oxidizing catalyst 15 become insufficient to react with all the NH 3 if NO, as well as NO 2 , is produced by the oxidizing catalyst 13, and surplus NH 3 is discharged from the denitrating and oxidizing catalyst 15. Even in this case, it may be possible to oxidize the surplus NH 3 using an oxidizing catalyst disposed downstream of the denitrating and oxidizing catalyst 15. However, if an oxidizing catalyst is used, NH 3 is rather converted to NO X again by the oxidizing catalyst, and the amount of NO X discharged to atmosphere increases. In this embodiment, this problem is solved by providing the NH 3 decomposing catalyst 61 on the exhaust gas passage downstream of the denitrating and oxidizing catalyst 15. Since the NH 3 discharged from the denitrating and oxidizing catalyst 15 is decomposed to N 2 and H 2 O by the NH 3 decomposing catalyst 61 without producing NO X , the exhaust gas downstream of the NH 3 decomposing catalyst 61 can be kept free from NO X even when NO is produced by the oxidizing catalyst 13. Therefore, though the flow distribution ratio of the passages 4a and 4b are set at a fixed value in this embodiment, NO X can be removed from the exhaust gas over a wide temperature range of the oxidizing catalyst 13. In this embodiment, if the amount of the secondary air from the secondary air supply unit 21 deviates from the target value, NO is also produced by the oxidizing catalyst 13, and surplus NH 3 will be produced by the denitrating and oxidizing catalyst 15. However by disposing the NH 3 decomposing catalyst 61 at downstream of the denitrating and oxidizing catalyst 15, the surplus NH 3 can be decomposed to N 2 and H 2 O by the NH 3 decomposing catalyst 61 also in this case, and completely removed from the exhaust gas. Fig. 12 shows an embodiment in which the NH 3 decomposing catalyst 61 is applied to the embodiment in Fig. 8. In this embodiment, the amount of the secondary air from the secondary air supply unit is set at a fixed value required for oxidizing catalyst 13 to convert 3/7 of total NH 3 to NO X . Though NO is produced by the oxidizing catalyst 13 when the temperature of the oxidizing catalyst 13 changes, the surplus NH 3 produced by the denitrating and oxidizing catalyst 15 is decomposed to N 2 and H 2 O by the NH 3 decomposing catalyst 61. Although the engines are operated at a rich air-fuel ratio in the above embodiments, the present invention can be also applied to an engine operated at a lean air-fuel ratio. In this case, the air-fuel ratio of the exhaust gas is adjusted by supplying an air-fuel ratio adjusting agent such as a fuel or gaseous or liquid hydrocarbon in such a manner that the air-fuel ratio of the exhaust gas flowing into the three-way reducing and oxidizing catalyst becomes slightly lower than the stoichiometric air-fuel ratio (for example, λ = 0.995).

Claims: An exhaust gas purification device for an internal combustion engine (1) comprising: a three-way reducing and oxidizing catalyst (11) disposed in the exhaust gas passage (4) of said engine for oxidizing HC, CO components and reducing NO X components whereby NH 3 is also generated; a components ratio determining means disposed in said exhaust gas passage and including an oxidizing catalyst (13) disposed downstream of said three-way catalyst (11) for converting a portion of the generated NH 3 component into NO X components, wherein the amount of NO X components generated and the amount of the remaining NH 3 component are adjusted in such a manner that the amount of said NO X components and the amount of said remaining NH 3 component are stoichiometrical to produce N 2 and H 2 O components; a denitrating and oxidizing catalyst (15) disposed in the exhaust gas passage downstream of the components ratio determining means for causing the NH 3 component to react with the NO X components generated by the components ratio determining means to produce N 2 and H 2 O components. A device according to claim 1, wherein the NO X components generated by the conversion is essentially composed of NO 2 , and wherein the ratio of the amounts of said NO 2 and NH 3 generated by the conversion is 3:4 in molar ratio. A device according to claim 1, wherein the NO X components generated by the conversion is essentially composed of NO, and wherein the ratio of the amounts of said NO 2 and NH 3 generated by the conversion is 1:1 in molar ratio. A device according to claim 1, wherein the NO X components generated by the conversion is composed of a mixture of NO 2 and NO, and wherein the ratio of total moles of said NO 2 and NO and the moles of the NH 3 is larger than ¾ and smaller than 1. A device according to claim 1, wherein said oxidizing catalyst converts 3/7 of the amount of the NH 3 component into an NO 2 component. A device according to claim 1, wherein said oxidizing catalyst converts ½ of the amount of the NH 3 component into an NO component. A device according to claim 1, wherein said oxidizing catalyst converts between 3/7 and ½ of the amount of the NH 3 component into NO 2 and NO components. A device according to claim 1, wherein said components ratio determining means comprises; a first branch exhaust passage (4a) and a second branch exhaust passage (4b) which divide the exhaust gas flow containing the NH 3 component; said oxidizing catalyst (13) being disposed on said first branch exhaust passage (4a) for converting substantially all of the NH 3 component in the exhaust gas flow through the first branch exhaust passage (4a) into NO X components; and an exhaust gas outlet passage (5) merging the first (4a) and the second (4b) branch exhaust passage, where the exhaust gas flowing through the first branch exhaust passage (4a) and the exhaust gas flowing through the second branch exhaust passage (4b) mix with each other; wherein the amount of the exhaust gas flowing through the first branch exhaust passage (4a) and the amount of the exhaust gas flowing through the second branch exhaust passage (4b) are adjusted in such a manner that the amount of the NO X components generated by the oxidizing catalyst (13) and the amount of the NH 3 component in the exhaust gas flowing through the second branch exhaust passage (4b) are stoichiometrical to produce N 2 and H 2 O components. A device according to claim 8, wherein the oxidizing catalyst (13) converts substantially all of the NH 3 components in the exhaust gas flowing through the first branch exhaust passage (4a) into an NO 2 component, and wherein the ratio of the amount of the exhaust gas flowing through the first branch exhaust passage (4a) and the amount of the exhaust gas flowing through the second branch exhaust passage (4b) is 3:4. A device according to claim 8, wherein the oxidizing catalyst (13) converts substantially all of the NH 3 components in the exhaust gas flowing through the first branch exhaust passage (4a) into an NO component, and wherein the ratio of the amount of the exhaust gas flowing through the first branch exhaust passage (4a) and the amount of the exhaust gas flowing through the second branch exhaust passage (4b) is 1:1. A device according to claim 8, wherein the oxidizing catalyst (13) converts substantially all of the NH 3 components in the exhaust gas flowing through the first branch exhaust passage (4a) into NO 2 and NO components, and wherein the ratio of the amount of the exhaust gas flowing through the first branch exhaust passage (4a) and the amount of the exhaust gas flowing through the second branch exhaust passage (4b) is larger than 3:4 and smaller than 1:1. A device according to claim 8, further comprising a flow resistance means (17) disposed on the second branch exhaust passage (4b) for generating a flow resistance substantially the same as the flow resistance generated by said oxidizing catalyst (13) in the first branch exhaust passage (4a). A device according to claim 11, further comprising a flow resistance means (17) disposed in the second branch exhaust passage (4b) for generating a flow resistance substantially the same as the flow resistance generated by said oxidizing catalyst (13) in the first branch exhaust passage (4a). A device according to claim 8, further comprising a variable means for adjusting the amount of the NH 3 converted by said oxidizing catalyst (13) in the range between 3/7 and ½ of the amount of the NH 3 generated by said three-way reducing and oxidizing catalyst (11) in accordance with the temperature of said oxidizing catalyst (13). A device according to claim 8, further comprising a variable means for adjusting the ratio of the amount of exhaust gas flowing through the first branch exhaust passage (4a) and the amount of exhaust gas flowing through the second branch exhaust passage (4b) in the range between ¾ and 1, in accordance with the temperature of said oxidizing catalyst (13). A device according to claim 8, further comprising a variable means for adjusting the ratio of the amount of exhaust gas flowing through the first branch exhaust passage (4a) and the amount of exhaust gas flowing through the second branch exhaust passage (4b) in the range between ¾ and 1, in accordance with an air-fuel ratio of the exhaust gas flowing into said oxidizing catalyst (13). A device according to claims 1 to 16, wherein said engine is operated at an air-fuel ratio lower than the stoichiometric air-fuel ratio.

Inventors: IGARASHI KOUHEI, ITOU TAKAAKI, KINUGASA YUKIO, TAKAOKA TOSHIFUMI, YAEGASHI TAKEHISA, IGARASHI, KOUHEI, ITOU, TAKAAKI, KINUGASA, YUKIO, TAKAOKA, TOSHIFUMI, YAEGASHI, TAKEHISA
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1995
Main classification: B01D53
IPC codes: F01N3, F01N7, F02D41, B01D53