KR101091628B1 - Exhaust system - Google Patents

Abstract

Exhaust system according to an embodiment of the present invention, the exhaust line through which the exhaust gas discharged from the engine passes, the nitrogen oxide purification catalyst which is installed in the exhaust line to reduce the nitrogen oxide contained in the exhaust gas, the nitrogen oxide purification catalyst Injector installed at the front end of the nitrogen oxide purification catalyst to further inject the fuel for generating a reducing agent in order to desorb the reduced nitrogen oxide to remove and remove the stored nitrogen oxide, and if it is determined that the nitrogen oxide purification catalyst deteriorated, set operating conditions In the engine further comprises a control unit for performing a rapid temperature increase to increase the temperature of the nitrogen oxide purification catalyst by further fuel injection.

Therefore, when it is determined that the deterioration degree of the nitrogen oxide purification catalyst or the fuel decomposition catalyst is higher than the set value, the purification efficiency of the catalyst is quickly normalized by rapidly increasing the temperature under the set operating conditions.

Secondary injection, injector, NOx, fuel decomposition catalyst, DPF, DOC, nitrogen oxide, purification catalyst

Description

Exhaust system {EXHAUST SYSTEM}

The present invention relates to an exhaust system, and more particularly to an exhaust system for reducing nitrogen oxide contained in the exhaust gas.

In general, the exhaust gas discharged from the engine through the exhaust manifold is guided to a catalytic converter formed in the middle of the exhaust pipe and purified, and the noise is attenuated while passing through the muffler, and then released into the atmosphere through the tail pipe.

The catalytic converter treats pollutants contained in the exhaust gas. And a soot filter for trapping particulate matter (PM) contained in the exhaust gas is mounted on the exhaust pipe.

Selective Catalytic Reduction (SCR) equipment is a type of such catalytic converter. Selective Catalytic Reduction (SCR) devices are termed selective catalytic reduction in the sense that reducing agents such as urea, ammonia, carbon monoxide and hydrocarbons (HC) react better with nitrogen oxides in oxygen and nitrogen oxides. do.

When using a hydrocarbon (HC) in the internal combustion engine equipped with such a selective catalytic reduction device as a reducing agent, fuel was continuously injected in order to supply hydrocarbons according to the amount of nitrogen oxide contained in the exhaust gas. Thus, slip of hydrocarbon occurs and fuel economy deteriorates.

In addition, when the reducing agent was continuously supplied, the oxidation / reduction reaction also occurred continuously. Therefore, the durability of the catalyst is deteriorated by the heat of oxidation generated during the oxidation / reduction reaction.

Accordingly, the present invention has been made to solve the above problems, and an object of the present invention is to provide an exhaust system which performs control for improving the purification efficiency according to deterioration of the catalyst.

Exhaust system according to an embodiment of the present invention for achieving this object, an exhaust line through which the exhaust gas discharged from the engine passes, a nitrogen oxide purification catalyst installed in the exhaust line to reduce the nitrogen oxide contained in the exhaust gas, The injector installed at the front end of the nitrogen oxide purification catalyst, and the nitrogen oxide purification catalyst deteriorated to further inject the fuel for generating a reducing agent in order to desorb the nitrogen oxide stored in the nitrogen oxide purification catalyst to reduce and remove it. If determined, the control unit for performing a rapid temperature increase to increase the temperature of the nitrogen oxide purification catalyst by additionally injecting fuel from the engine under the set operating conditions.

The control unit may determine that the fuel decomposition catalyst or the nitrogen oxide purification catalyst is deteriorated when the temperature difference between the front and rear ends of the fuel decomposition catalyst or the nitrogen oxide purification catalyst is less than or equal to a predetermined value after the fuel is additionally injected from the injector. .

The control unit, after the fuel is further injected in the injector, the oxygen concentration difference between the front and rear ends of the nitrogen oxide purification catalyst is less than the set value, it is determined that the nitrogen oxide purification catalyst deteriorated.

The set operating conditions include that the coolant temperature or the oil temperature is higher than the set temperature when the engine is restarted and the shear temperature of the nitrogen oxide purification catalyst is lower than or equal to the set temperature.

When the temperature of the front end of the nitrogen oxide purification catalyst reaches a set temperature, the engine stops injecting further fuel.

If the temperature of the front end portion of the nitrogen oxide purification catalyst during the operation falls below the set temperature, the control unit further injects fuel from the engine to rapidly increase the temperature.

A fuel decomposition catalyst for converting the fuel further injected from the injector into a reducing agent and raising the temperature of the rear stage by an oxidation reaction is further provided between the injector and the nitrogen oxide purification catalyst, and the nitrogen oxide purification catalyst is an exhaust gas. Part of the nitrogen oxide contained in the reduced by using unburned fuel or hydrocarbon, and the other part of the nitrogen oxide is stored by diffusing the inside, the nitrogen stored in the nitrogen oxide purification catalyst using the reducing agent formed in the fuel decomposition catalyst The oxide is desorbed and reduced.

A catalyst filtration filter installed between the fuel decomposition catalyst and the nitrogen oxide purification catalyst to collect and remove particulate matter in exhaust gas, and the particulate matter collected in the catalyst filtration filter by detecting a pressure difference between front and rear ends of the catalyst filtration filter. It further comprises a differential pressure sensor for sensing the amount of material.

The controller determines that the fuel decomposition catalyst or the nitrogen oxide purification catalyst is deteriorated, and when the set operating condition is satisfied, the controller performs rapid temperature increase to increase the temperature of the nitrogen oxide purification catalyst by additionally injecting fuel from the engine.

The control unit determines that the fuel decomposition catalyst is deteriorated if the temperature difference between the front and rear ends of the fuel decomposition catalyst is less than or equal to a set value after the fuel has been further injected in the injector.

As described above, according to the present invention, if it is determined that the degree of deterioration of the nitrogen oxide purification catalyst or the fuel decomposition catalyst is higher than the set value, the purification efficiency of the catalyst is quickly normalized by rapidly increasing the temperature under the set operating conditions.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic structural diagram of an exhaust system according to an embodiment of the present invention.

Referring to FIG. 1, the exhaust system includes an engine 100, a controller 110, an injector 120, a fuel decomposition catalyst 130, a catalyst filtration filter 140, an exhaust line 150, and a nitrogen oxide purification catalyst 160. ), The first oxygen sensor 170a, the second oxygen sensor 170b, the first temperature sensor 180a, the second temperature sensor 180b, the third temperature sensor 180c, the fourth temperature sensor 180d, And a differential pressure sensor 190.

The engine 100 is provided with a separate injector (not shown) for injecting fuel into the cylinder or injecting fuel into the intake air.

Combustion gas discharged from the engine 100 is discharged through the exhaust line 150, and the injector 120, the fuel decomposition catalyst 130, and the catalyst filtration filter 140 are provided in the exhaust line 150. The nitrogen oxide purification catalyst 160 is sequentially disposed.

In the exhaust line 150, a first oxygen sensor 170a and a first temperature sensor 180a are disposed between the engine 100 and the injector 120, and the fuel decomposition catalyst 130 and the catalyst filtration. The second temperature sensor 180b is disposed between the filters 140.

In addition, the third temperature sensor 180c and the fourth temperature sensor 180d are disposed at the inlet side and the outlet side of the nitrogen oxide purification catalyst 160, and the second oxygen sensor 170b is formed of the first oxygen sensor 170b. 4 is disposed downstream of the temperature sensor 180d.

The differential pressure sensor 190 detects a pressure difference between the front and rear ends of the catalyst filtration filter 140.

Signals detected by the first and second oxygen sensors 170a and 170b, the first, second, third and fourth temperature sensors 180a, 180b, 180c and 180d, and the differential pressure sensor 190 may be controlled by the controller 110. ), The injector 120 and the engine 100 are controlled according to the transmitted signal.

The nitrogen oxide purification catalyst 160 occludes nitrogen oxides (NOx) included in the exhaust gas discharged from the engine 100, and the fuel decomposition catalyst 130 is formed by the fuel additionally injected from the injector 120. It is desorbed and reduced by using a reducing agent produced in).

That is, in order to desorb and reduce the nitrogen oxide stored in the nitrogen oxide purification catalyst 160, fuel is further injected using the injector 120 installed in the exhaust line 150, and the fuel decomposition catalyst 130 ) Produces reducing agents such as HC, CO, H2 and the like. The nitrogen oxide purification catalyst 160 is regenerated using this reducing agent.

On the other hand, in order to solve the problem that the deterioration rate (deterioration rate) of the nitrogen oxide purification catalyst 160 or the fuel decomposition catalyst 130 increases as the engine operation time elapses, thereby reducing the purification efficiency. In an embodiment, the nitrogen oxide purification catalyst 160 or the fuel decomposition catalyst 130 by rapidly increasing the temperature of the exhaust gas in accordance with the operating conditions upon restarting, the nitrogen oxide purification catalyst 160 or the The function of the fuel decomposition catalyst 130 is initially normalized.

Referring to FIG. 1, the catalytic filtration filter 140 collects particulate matter (PM) included in exhaust gas and removes the particulate matter under a set condition.

The controller 110 may control to regenerate the catalyst filtration filter 140 when the pressure difference measured by the differential pressure sensor 190 is greater than or equal to a set value. In this case, the soot trapped in the catalyst filtration filter 140 may be combusted by post-injection in the combustion chamber or post-injection of fuel in the injector 120.

The ratio of nitrogen oxide stored in the nitrogen oxide purification catalyst 160 and HC in the exhaust gas is set in the map data, and the controller 110 compares the ratio of HC to NOx and the value set in the map data under actual operating conditions. If the value is less than the set value, the injector 120 is operated to further inject fuel into the exhaust gas.

The fuel decomposition catalyst 130 breaks down the chain ring of the carbon compound contained in the fuel through a catalytic reaction. That is, the fuel decomposition catalyst 130 breaks down the connecting ring constituting the hydrocarbon through the thermal cracking function of decomposing the fuel. Here, thermal cracking is performed through the following procedure.

C ₁₆ H ₃₄ → 2n-C ₈ H17 * → nC ₆ H ₁₃ * → nC ₄ H ₉ * → C ₂ H ₅ * → C ₂ H ₄

C ₁₆ H ₃₄ → 8C ₂ H ₄ + H ₂ , * denotes a radical.

In addition, the fuel decomposition catalyst 130 converts some of the hydrocarbons into hydrocarbons combined with oxygen to activate fuel injected from the injector 120.

In addition, the fuel decomposition catalyst 130 converts the fuel evaporated and ejected into the droplet state into a highly reactive reducing agent, and also reduces the concentration of oxygen and raises the temperature of the exhaust gas by oxidation. As the catalyst component, Pt, Pd, and Rh are used.

A nitrogen oxide detection sensor may be provided at a rear end of the nitrogen oxide purification catalyst 160, and the nitrogen oxide detection sensor may detect an amount of nitrogen oxide in the exhaust gas and transmit a signal thereof to the controller.

On the other hand, in the embodiment of the present invention, instead of using the nitrogen oxide detection sensor, it is preferable to predict the storage amount from the map determined by the experimental value.

The controller 110 desorbs and removes the nitrogen oxide stored in the nitrogen oxide purification catalyst 160 by controlling the additional injection amount and the additional injection timing of the fuel based on the signals and the map data detected by the sensors.

As an example, the controller 110 controls to further inject fuel when the nitrogen oxide stored in the nitrogen oxide purification catalyst 160 is greater than or equal to a set value.

Here, the control unit 110 controls so that the ratio of the hydrocarbon (HC) to the nitrogen oxide (NOx) is more than the set ratio so that the nitrogen oxide stored in the nitrogen oxide purification catalyst 160 is easily removed. The set ratio may be eight.

2 is a control flowchart of an exhaust system according to an embodiment of the present invention.

Referring to FIG. 2, the exhaust system includes step 0 (S00), step 1 (S01), step 2 (S02), step 3 (S03), step 4 (S04), and step 5 (S05). Then, control associated with the sixth step S06 and the seventh step S07 is performed.

In the 0th step S00, start-up is detected and control starts. In the first step S01, an operating condition is sensed. The operating condition includes an intake air amount, a fuel injection amount, a rotation speed, an EGR rate, a torque, a cooling water temperature, an oil temperature, and an outside air temperature.

In the second step S02, catalyst regeneration conditions are sensed. The catalyst regeneration conditions include regeneration conditions of the nitrogen oxide purification catalyst 160, regeneration conditions of the catalyst filtration filter 140, or desulfurization conditions of the nitrogen oxide purification catalyst 160.

In the second step (S02), if the regeneration conditions of the nitrogen oxide purification catalyst 160 and the catalyst filtration filter 140 is satisfied, in the third step (S03), the exhaust line 150 is installed The injector 120 is activated to further inject fuel.

When the fuel is further injected, the deterioration degree of the nitrogen oxide purification catalyst 160 or the fuel decomposition catalyst 130 is determined in the fourth step S04.

A method of determining the degree of deterioration of the nitrogen oxide purification catalyst 160 will be described in detail. After the fuel is further injected, the first, second, third and fourth temperature sensors 180a, 180b, 180c, and 180d are performed. The difference in temperature between the front and rear ends of the fuel decomposition catalyst 130 and the nitrogen oxide purification catalyst 160 is calculated by using a). Then, when the temperature difference is smaller than the set value, it is determined that the fuel decomposition catalyst 130 and the nitrogen oxide purification catalyst 160 are deteriorated.

In addition, in the embodiment of the present invention, deterioration of the fuel decomposition catalyst 130 or the nitrogen oxide purification catalyst 160 using the first and second oxygen sensors 170a and 170b installed in the exhaust line 150. To determine the degree, the first and second oxygen sensor (170a, 170b) detects the concentration of oxygen, when the difference is less than the set value, the nitrogen oxide purification catalyst 160 or the fuel decomposition catalyst 130 is Judging from deterioration.

The nitrogen oxide purification catalyst 160 or the fuel decomposition catalyst 130 has a high oxidizing capacity at an initial stage, and the temperature difference between the front and rear ends is greater than the set value, and the reactivity with oxygen is also higher than the set value of the oxygen concentration purification catalyst. Big. However, when the catalysts 130 and 160 deteriorate, the oxidation capacity is lowered and the reactivity with oxygen is lowered.

In the fourth step S04, if it is determined that at least one of the catalysts is deteriorated, it is programmed to perform a rapid temperature increase in the fifth step S05, and if it is not deteriorated, in the sixth step S06 It is programmed not to perform rapid temperature increase, and in the seventh step S07, the engine is normally operated.

In the fifth step (S05), the rapid rise in temperature is not immediately performed, the rapid rise in the operating conditions set after restarting.

3 is a control flowchart of an exhaust system according to an embodiment of the present invention.

Referring to FIG. 3, the exhaust system includes the eighth step S08, the ninth system S09, the tenth step S10, the eleventh step S11, the twelfth step S12, and the thirteenth step S13. , And control associated with the fourteenth step S14.

In the fifth step S05, the rapid temperature rise is determined, and after the operation of the engine 100 ends, the restart of the engine 100 is sensed in the eighth step S08.

In the ninth step S09, an operating condition of the engine 100 is detected. In addition, the inlet temperature of the nitrogen oxide purification catalyst 160 is sensed.

The operating conditions of the engine 100 include intake air amount, fuel injection amount, rotation speed, EGR rate, torque, cooling water temperature, engine oil temperature, and external temperature. Here, when the coolant temperature or the engine oil temperature is higher than the set temperature, and the inlet temperature of the nitrogen oxide purification catalyst 160 is lower than the set temperature, rapid temperature rising is performed in the eleventh step S11.

As in the eleventh step S11, after the rapid temperature increase is performed, in the twelfth step S12, when the inlet temperature of the nitrogen oxide purification catalyst 160 falls below a set temperature, the engine ( Fuel is injected more than the value set in 100).

In the thirteenth step S13, when the inlet temperature of the nitrogen oxide purification catalyst 160 becomes higher than a set temperature, in the fourteenth step S14, the rapid heating is stopped. That is, the engine 100 does not inject additional fuel.

As shown in FIGS. 4 and 5, the nitrogen oxide purification catalyst 160 includes first and second catalyst layers 444 and 446 coated on a carrier. The first catalyst layer 444 is disposed close to the exhaust gas, and the second catalyst layer 446 is disposed close to the carrier.

The first catalyst layer 444 oxidizes the nitrogen oxide contained in the exhaust gas, and reduces a portion of the oxidized nitrogen oxide by oxidation / reduction reaction with unburned fuel or hydrocarbon contained in the exhaust gas.

In addition, another portion of the oxidized nitrogen oxide is diffused into the second catalyst layer 446. 4, 5, and 4, the first catalyst layer 444 may include at least one of the zeolite catalyst 412 and the metal catalyst 414 supported on the porous alumina.

The zeolite catalyst 412 is a catalyst in which at least one element of copper, platinum, manganese, iron, cobalt, nickel, zinc, silver, cerium, and gallium is ion exchanged. Chemical reactions occurring in the zeolite catalyst 412 are as follows.

^{Z-Cu 2+ O - + NO} → Z-Cu 2+ (NO 2 -) ads → Z-Cu 2+ + NO 2

^{Z + O - + NO → Z} + (NO 2 -) ads → Z + + NO 2

_{^{Z-Cu 2+ (NO 2 -}} ) ads + NO → Z-Cu 2+ (N 2 O 3) - ads → Z-Cu 2+ O - + N 2 + O 2

ZH ⁺ + C _n H _2n → ZC _n H _{2n + 1} ⁺ → n (ZH) + C _n H _2n ⁺

mNO ₂ + C _n H _2n + → C _n H _2n N _m O _2m → N ₂ + CO ₂ + H ₂ O

Here, Z means zeolite and subscript ads means adsorption.

In addition, at least one metal of platinum, palladium, rhodium, iridium, ruthenium, tungsten, chromium, manganese, iron, cobalt, copper, zinc, and silver may be used as the metal catalyst 414 supported on the porous alumina. The chemical reaction occurring in the metal catalyst 414 supported on the porous alumina is as follows.

NO + O ₂ → (NO _x ) _ads

THC + (NO _x ) _ads → THC-ONO or THC-NO ₂

THC-NO ₂ → THC-NCO

THC-NCO + NO + O ₂ → N ₂ + CO ₂ + H ₂ O

Here, THC means hydrocarbon. As mentioned above, hydrocarbons are intended to represent compounds composed of carbon and hydrogen contained in exhaust gases and fuels.

The second catalyst layer 446 stores a portion of the nitrogen oxide oxidized in the first catalyst layer 444, and desorbs the stored nitrogen oxide by fuel injected additionally according to a set condition to the first catalyst layer 444. To be reduced at.

As mentioned above, the set condition is a case where the nitrogen oxide stored in the second catalyst layer 446 is greater than or equal to the value set in the map data so that the reduction reaction of the nitrogen oxide in the first catalyst layer 444 is activated.

The second catalyst layer 446 includes a noble metal 408 and a nitrogen oxide storage material 406. Barium oxide (BaO) may be used as the nitrogen oxide storage material 406. The precious metal 408 facilitates the storage of nitrogen oxides by the nitrogen oxide storage material 406. As the precious metal 408, various metal materials such as platinum and palladium may be used.

Hereinafter, the operating principle of the present invention will be described in detail.

NOx storage mode

When the nitrogen oxide stored in the second catalyst layer 446 is smaller than the set value, the nitrogen oxide included in the exhaust gas is oxidized in the first catalyst layer 444, and a part of the oxidized nitrogen oxide is hydrocarbon contained in the exhaust gas. And an oxidation / reduction reaction to reduce nitrogen gas, and the other part is stored in the second catalyst layer 446. In this process, the hydrocarbons contained in the exhaust gas are oxidized to carbon dioxide. The reaction occurring in the first catalyst layer 444 is briefly expressed as in the following equation.

NO + 1 / 2O ₂ → NO ₂

NO + HC → 1 / 2N ₂ + CO ₂

In addition, when another portion of the oxidized nitrogen oxide and nitrogen oxide included in the exhaust gas are diffused and stored in the second catalyst layer 446, the noble metal 408 of the second catalyst layer 446 may be a nitrogen oxide storage material ( 406 facilitates storage of nitrogen oxides. The reaction occurring in the second catalyst layer 446 is briefly expressed as in the following equation.

BaO + 2NO ₂ + 1 / 2O ₂ → Ba (NO ₃ ) ₂

NOx Regeneration Mode

When the nitrogen oxide stored in the second catalyst layer 446 is equal to or greater than a set value, the controller 110 causes the second injector 120 to further inject fuel. The additional injected fuel passes through the fuel decomposition catalyst 130 (DFC), and in this process, the fuel is split into low molecular hydrocarbons and converted. In addition, a portion of the low molecular weight hydrocarbon is converted into a hydrocarbon combined with oxygen passes through the nitrogen oxide purification catalyst 160.

At this time, in the second catalyst layer 446, the nitrogen oxide is desorbed through the substitution reaction with the hydrocarbon, which is briefly expressed as follows.

Ba (NO ₃ ) ₂ + 3CO → BaCO ₃ + 2NO + 2CO ₂

In addition, in the first catalyst layer 444, the nitrogen oxide is reduced to nitrogen gas and the hydrocarbon / oxygen combined hydrocarbon by the oxidation / reduction reaction between the nitrogen oxide desorbed from the second catalyst layer 446 and the hydrocarbon / hydrocarbon combined hydrocarbon. Is oxidized to carbon dioxide. This is briefly expressed as follows.

NO + HC / Oxygenated HC = 1 / 2N ₂ + CO ₂

As described above, nitrogen oxides and hydrocarbons contained in the exhaust gas are purified.

In the embodiment of the present invention, instead of continuously performing further injection of fuel, the control unit 110 performs further injection of fuel in the second injector 120 when the ratio of HC / NOx in exhaust gas is equal to or less than a preset value. Is performed. Therefore, since fuel is additionally injected under conditions optimized according to the operating conditions, slip of hydrocarbon is prevented and fuel economy is improved.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, And all changes to the scope that are deemed to be valid.

1 is a schematic structural diagram of an exhaust system according to an embodiment of the present invention.

2 is a control flowchart of an exhaust system according to an embodiment of the present invention.

3 is a control flowchart of an exhaust system according to an embodiment of the present invention.

4 is a schematic diagram illustrating a case where nitrogen oxide is stored in an exhaust system according to an exemplary embodiment of the present invention.

5 is a schematic diagram illustrating a case where nitrogen oxide is desorbed in an exhaust system according to an embodiment of the present invention.

<Description of the symbols for the main parts of the drawings>

100: engine

110: control unit

120: injector

130: diesel fuel cracking (DFC)

140: diesel particulate filter (DPF)

150: exhaust line

160: nitrogen oxide purification catalyst (NOx trap)

170a, 170b: first and second oxygen sensors

180a, 180b, 180c, 180d: first, second, third and fourth temperature sensors

190: differential pressure sensor

Claims (10)

An exhaust line through which exhaust gas discharged from the engine passes; A nitrogen oxide purification catalyst installed in the exhaust line to reduce nitrogen oxide contained in the exhaust gas; An injector installed at a front end portion of the nitrogen oxide purification catalyst to further inject fuel for generating a reducing agent in order to desorb, reduce and remove the nitrogen oxide stored in the nitrogen oxide purification catalyst; And If it is determined that the nitrogen oxide purification catalyst is deteriorated, a control unit for rapidly increasing the temperature of the nitrogen oxide purification catalyst by further injecting fuel from the engine under the set operating conditions; Including, The control unit, And after the fuel is further injected from the injector, determining that the nitrogen oxide purification catalyst is deteriorated when a temperature difference between the front and rear ends of the nitrogen oxide purification catalyst is less than or equal to a set value. delete delete The method according to claim 1, The set operating condition is, And a cooling water temperature or an oil temperature when the engine is restarted is higher than or equal to a predetermined temperature, and the nitrogen oxide purification catalyst inlet temperature is lower than or equal to a predetermined temperature. The method according to claim 1, And when the temperature of the front end of the nitrogen oxide purification catalyst reaches a set temperature, the engine further stops injecting fuel. The method according to claim 1, If the temperature of the front end portion of the nitrogen oxide purification catalyst during the operation falls below the set temperature, the control unit is characterized in that the engine further injects fuel to rapidly increase the temperature. The method according to claim 1, A fuel decomposition catalyst is further installed between the injector and the nitrogen oxide purification catalyst to convert fuel further injected from the injector into a reducing agent and to raise a temperature of a subsequent stage by an oxidation reaction. The nitrogen oxide purification catalyst, A portion of the nitrogen oxide contained in the exhaust gas is reduced by using unburned fuel or hydrocarbon, and another portion of the nitrogen oxide is stored by diffusing the inside into the nitrogen oxide purification catalyst by using a reducing agent formed in the fuel decomposition catalyst. An exhaust system that desorbs and reduces stored nitrogen oxides. 8. The method of claim 7, A catalyst filtration filter installed between the fuel decomposition catalyst and the nitrogen oxide purification catalyst to collect and remove particulate matter in exhaust gas; And A differential pressure sensor for detecting an amount of particulate matter collected in the catalyst filtration filter by detecting a pressure difference between front and rear ends of the catalyst filtration filter; Exhaust system further comprising. 8. The method of claim 7, The control unit, And the fuel decomposition catalyst is deteriorated, and when the set operating condition is satisfied, the engine further rapidly injects fuel to rapidly increase the temperature of the nitrogen oxide purification catalyst. 8. The method of claim 7, The control unit, And after the fuel has been injected in the injector, when the temperature difference between the front and rear ends of the fuel decomposition catalyst is less than or equal to a set value, the exhaust system determines that the fuel decomposition catalyst is degraded.

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