KR102406044B1 - Scr purifying system and method for improving purifying efficiency of scr purifying system - Google Patents

Abstract

The present invention relates to an SCR purification system and a method for improving the SCR purification efficiency that improve purification efficiency and reduce urea solids under low-speed, low-load operating conditions.
The method for improving SCR purification efficiency according to the present invention includes a low-speed/low-load operation determination step (S110) of determining whether the engine 10 of a vehicle is operating at a low speed or a low load, and the engine 10 is operated at a low speed/low load If the load is operating, the cold operation determination step (S120) of determining whether the engine 10 is sufficiently warmed up, and if the engine 10 is sufficiently warmed up, the opening rate of the air quantity control valve 11 is set to the current operating state. A target opening rate setting step (S130) of setting the target opening rate of the air quantity control valve 11 set in advance according to each other, and the target opening rate setting step of setting the opening rate of the air quantity control valve 11 from the initial opening rate (S130) An opening rate reduction step (S140) of reducing the opening rate of the air quantity control valve 11 to the target opening rate set in A pressure reduction step (S150), an SCR temperature increase determination step (S160) of determining whether the temperature of the SCR device 50 has reached a temperature set to start spraying urea water (S160), and the SCR device 50 activates the temperature If it is reached, it includes a urea water injection step (S170) of spraying an aqueous urea solution into the SCR device.

Description

SCR PURIFYING SYSTEM AND METHOD FOR IMPROVING PURIFYING EFFICIENCY OF SCR PURIFYING SYSTEM

The present invention relates to an SCR (Selective Catalytic Reduction) purification system and a method for improving the SCR purification efficiency in which purification efficiency is improved and urea solids are reduced in low-speed, low-load operating conditions.

Exhaust gas of vehicles includes various harmful substances, and emission of these hazardous substances is limited by laws and regulations.

Although the harmful substances are reduced in the combustion process of the engine, the exhaust gas regulation conditions are not satisfied through this, and thus the harmful substances are reduced through various post-treatment devices.

For example, Diesel Oxidation Catalyst (DOC), Diesel 　 Particulate Filter (DPF), etc. for reducing particulate matter (PM) contained in exhaust gas are installed in the exhaust pipe for discharging exhaust gas discharged from the diesel engine, and nitrogen A Selective Catalytic Reduction (SCR) device is installed to reduce oxide (NOx). As the DOC, DPF, and SCR exhaust pipes pass sequentially, harmful substances contained in the exhaust gas are removed so that the exhaust gas satisfying the regulations is finally discharged. In particular, the purification performance of the above purification devices is improved when the temperature of the exhaust gas is sufficiently increased.

However, there is a case where the vehicle repeatedly travels in a low-speed and low-load section of some vehicles. For example, there is a problem in that a bus mainly running in a city center has to operate a short distance and cannot effectively purify foreign substances contained in exhaust gas due to frequent stops.

In particular, if the number of sections in which the vehicle is operated in a low speed/low load section or in cold conditions increases, the purification efficiency of the SCR device for removing nitrogen oxides (NO) does not increase, so a large amount of nitrogen oxides are emitted from the vehicle. There is a problem.

In addition, since the purification efficiency of the SCR device is lowered, the urea solution injected into the SCR device remains, and urea solids are formed in the SCR device.

On the other hand, the following prior art literature discloses a technology related to a 'diesel post-treatment system'.

KR 10-2012-0017018 A

The present invention was invented to solve the above problems, and by controlling the opening degree of the air amount control valve that controls the amount of air flowing into the engine, spraying the urea solution while raising the temperature inside the SCR device, An object of the present invention is to provide an SCR purification system and a method for improving the SCR purification efficiency that increase the purification efficiency of nitrogen oxides (NOx) and prevent urea solids from remaining therein.

The SCR purification system according to the present invention for achieving the above object is an SCR purification system in which a Selective Catalytic Reduction (SCR) device installed in an exhaust pipe discharged from an engine is installed to purify nitrogen oxides discharged from the engine. , an electronic control unit (ECU) controlling the engine, when it is determined that the engine is not sufficiently warmed up while the engine is running at low speed/low load, the amount of air introduced into the engine to increase the temperature of the SCR device Reduces the opening degree of the air amount control valve for controlling the, and when the temperature of the SCR device reaches a predetermined temperature, characterized in that the urea aqueous solution is injected into the SCR device.

The ECU determines that the vehicle is operated at low speed/low load when any one of the engine rotation speed, the vehicle speed, the accelerator opening degree, the fuel consumption, and the engine load information is less than or equal to a preset value characterized in that

The ECU, when at least one of the temperature of the engine coolant, the inlet temperature of Diesel Oxidation Catalyst (DOC) installed in the exhaust pipe of the engine, and the temperature of the air flowing into the engine is lower than a preset value, the engine of the vehicle It is characterized in that it is determined that the cold operation is in progress.

The ECU may reduce an opening rate of the air amount control valve from a maximum open state to a target opening rate determined according to an operating state of the engine.

The ECU sets the target opening rate of the air quantity control valve from mapping data in which the target opening rate is stored according to a fuel injection timing, an injection amount of fuel, and an air amount required for combustion.

On the other hand, the method for improving SCR purification efficiency according to the present invention includes a low speed/low load operation determination step of determining whether the engine of a vehicle is operating at low speed or low load, and if the engine is operating at low speed/low load, the engine a cold operation determination step of determining whether the engine has been sufficiently warmed up, and a target opening rate setting step of setting the air amount control valve opening rate to the target opening rate of the air amount control valve preset according to the current operating state, if the engine is sufficiently warmed up; , an opening rate reduction step of reducing the opening rate of the air quantity control valve from the initial opening rate to the target opening rate set in the target opening rate setting step; and the opening rate of the air quantity control valve is reduced A supercharging boost pressure reduction step in which the supercharging booster pressure is reduced according to the If so, it includes a urea water injection step of injecting an aqueous urea water solution into the SCR device.

In the low-speed/low-load driving determination step, when at least one of the engine rotation speed, the engine speed, the throttle valve opening degree, fuel consumption, and engine load information is less than or equal to a preset value, the vehicle is set at low speed/low load. It is characterized in that it is judged to be driven by a load.

In the cold driving determination step, when at least one of the temperature of the engine coolant, the inlet temperature of Diesel Oxidation Catalyst (DOC) installed in the exhaust pipe of the engine, and the temperature of the air flowing into the engine is lower than a preset value, the It is characterized in that it is determined that the engine is running cold.

In the step of reducing the opening rate, the opening rate of the air quantity control valve is reduced from a state in which the air quantity control valve is maximally opened to the target opening rate.

In the step of determining the completion of the SCR temperature increase, the activation temperature of the SCR device is set to 240 to 260.

The target opening rate setting step is characterized in that it is set from mapping data stored in advance in a direction in which fuel efficiency increases and exhaust gas decreases.

In the target opening rate setting step, the mapping data is set according to an injection timing of fuel, an injection amount of fuel, and an amount of air required for combustion.

In the low speed/low load driving determination step, when the vehicle is driven in a state other than low speed/low load, the urea water injection step is performed.

When it is determined that the engine is warmed up in the cold driving determination step, the urea water injection step is performed.

According to the SCR purification system and the method for improving SCR purification efficiency of the present invention having the above configuration, when it is determined that the vehicle is operating at a low speed and a low load state, the opening degree of the air amount control valve is reduced to reduce the turbo booster pressure. By doing so, the temperature of the SCR device is increased.

In the low-speed, low-load state, the temperature of the SCR device is maintained at a temperature capable of increasing the nitrogen oxide purification efficiency, thereby improving the nitrogen oxide purification efficiency in the SCR device.

In addition, it is possible to prevent the occurrence of urea solids in the SCR device.

1 is a block diagram showing an SCR purification system according to the present invention;
2 is a flowchart illustrating a method for improving SCR purification efficiency according to the present invention.

Hereinafter, an SCR purification system and a method for improving SCR purification efficiency according to the present invention will be described in detail with reference to the accompanying drawings.

The SCR purification system according to the present invention is installed in the exhaust system of a vehicle equipped with an engine 10 as shown in FIG.

An air control valve (ACV; Air Control Valve, 11 ) for controlling the amount of air introduced into the engine 10 is installed on one side of the engine 10 . The air amount control valve 11 is normally open to the maximum, but the opening rate is reduced according to the control of an electronic control unit (ECU) 60 to control the amount of air flowing into the engine 10 .

The exhaust gas discharged from the engine 10 is discharged to the outside through the exhaust pipe 12 connected to the exhaust manifold of the engine 10 . The exhaust pipe 12 may be provided with various post-treatment devices for reducing harmful substances contained in the exhaust gas. For example, as shown in FIG. 1 , the exhaust pipe 12 has a Diesel Oxidation Catalyst (DOC) 30 ), Diesel 　 Particulate 　 Filter (40), SCR (Selective Catalytic Reduction) device 50 may be installed. The DOC 30 and the DPF 40 are used to reduce particulate matter (PM) included in the exhaust gas, and the SCR device 50 removes nitrogen oxides (NOx) included in the exhaust gas. used to reduce

The DOC 30 burns particulate matter (PM) collected in the DOC 30 by injecting boil-off gas from the fuel tank 21 . In the fuel tank 21 , the fuel pressurized through the fuel pump 22 is injected through the injector installed in the engine 10 , and the fuel evaporation gas evaporated in the fuel tank 21 or the fuel pump 22 . (HC; hydrocarbon) is collected in the HC dosing module 24 through the filter 23 and then, when air is injected from the air tank 26 connected to the HC dosing module 24, the fuel boil-off gas together with the air is periodically sprayed to the front end of the DOC 30 through the HC injection nozzle 25 to burn the particulate matter collected in the DOC 30 . Temperature sensors 31 and 32 are installed at the front and rear ends of the DOC 30, respectively.

In addition, the differential pressure between the pressure at the front end and the rear end of the DPF 40 is measured through the pressure sensor 41 , and when the differential pressure is equal to or greater than a predetermined differential pressure, the fuel boil-off gas is burned or the exhaust of the engine 10 . By increasing the temperature of the gas, the particulate matter collected in the DPF 40 is burned. The concentration of particulate matter included in the exhaust gas passing through the DOC 30 and the DPF 40 is measured through the PM sensor 42 installed at the rear end of the DPF 40 .

On the other hand, the SCR device 50 for purifying nitrogen oxides (NOx) uses the urea water stored in the urea water tank 51 through the urea water supply module 52, urea water installed at the front end of the SCR device 50 It is sprayed with the injection nozzle 53 to purify the nitrogen oxides in the SCR device 50 . A front NOx sensor 56 and a rear NOx sensor 57 for measuring the concentration of nitrogen oxide contained in the exhaust gas are installed at the front and rear ends of the SCR device 50, respectively, and the SCR device 50 Temperature sensors 54 and 55 for measuring the temperature of are installed at the front and rear ends of the SCR device 50 .

The SCR device 50 is controlled by a dosing control unit (DCU, 70) for controlling the injection of urea water. That is, the DCU 70 receives the nitrogen oxide concentration from the front NOx sensor 56 and the rear NOx sensor 57 , and controls the SCR device 50 through the temperature sensors 54 and 55 . The temperature of the inlet and outlet is input, and the operation of the urea water injection nozzle 53 is controlled, so that the urea water is injected into the SCR device 50 . The DCU 70 is controlled by the ECU 60 .

When it is determined that the engine 10 is not sufficiently warmed up while the engine 10 is operated at low speed/low load, the ECU 60 returns to the engine 10 so that the temperature of the SCR device 50 rises. Reduce the opening degree of the air amount control valve 11 for controlling the amount of air introduced, and when the temperature of the SCR device 50 reaches a predetermined temperature, control to spray the aqueous urea solution into the SCR device 50 do.

The ECU 60 sets the vehicle to low speed/low load when any one of the rotation speed of the engine 10, the speed of the vehicle, the accelerator opening degree, fuel consumption, and engine load information is less than or equal to a preset value. It is determined that the operation is performed, and the temperature of the SCR device 50 is controlled so that the temperature of the SCR device 50 is increased by reducing the opening degree of the air amount control valve 11 .

In addition, the ECU 60 determines at least one of a temperature of engine coolant, an inlet temperature of Diesel Oxidation Catalyst (DOC) installed in the exhaust pipe 12 of the engine 10 , and a temperature of air flowing into the engine 10 . When is lower than a preset value, it is determined that the engine of the vehicle is running in a cold state, and the opening degree of the air amount control valve 11 is reduced to control the temperature of the SCR device 50 to rise.

When the ECU 60 controls the air amount control valve 11 , the target opening rate depends on the fuel injection timing, the amount of fuel injected, and the amount of air required for combustion from a state in which the air amount control valve 11 is maximally opened. Based on the stored mapping data, a target opening rate of the air amount control valve 11 is set, and the air amount control valve 11 is controlled.

The logic for the ECU 60 to control the opening degree of the air amount control valve 11 to reduce nitrogen oxides in the SCR device 50 is specifically disclosed in the SCR purification efficiency improvement method to be described later.

On the other hand, Figure 2 discloses a method for improving SCR purification efficiency according to the present invention.

The method for improving SCR purification efficiency according to the present invention includes a low-speed/low-load operation determination step (S110) of determining whether the engine 10 of a vehicle is operating at a low speed or a low load, and the engine 10 is operated at a low speed/low load If the load is operating, the cold operation determination step (S120) of determining whether the engine 10 is sufficiently warmed up, and if the engine 10 is sufficiently warmed up, the opening rate of the air amount control valve 11 is set to the current operating state. A target opening rate setting step (S130) of setting the target opening rate of the air quantity control valve 11 set in advance according to each other, and the target opening rate setting step of setting the opening rate of the air quantity control valve 11 from the initial opening rate (S130) An opening rate reduction step (S140) of reducing the opening rate of the air quantity control valve 11 to the target opening rate set in The pressure reduction step (S150), the SCR temperature increase completion determination step (S160) of determining whether the temperature of the SCR device 50 has reached the activation temperature, and the SCR device 50. If the activation temperature has been reached, the SCR device It includes a urea water injection step (S170) of spraying an aqueous urea water solution into the inside.

In the low-speed/low-load operation determination step S110, it is determined whether the engine 10 mounted on the vehicle is being operated at a low speed or a low load. If the engine 10 is operating at a high speed or a high load, the temperature of the exhaust gas discharged from the engine 10 is high so that the SCR device 50 can be maintained at a high temperature, so that the nitrogen in the SCR device 50 is Oxide (NOx) can be purified normally. However, due to road congestion and frequent stops, buses running downtown have a long section operated at low speed or low load, and at this time, there is a problem in that nitrogen oxides contained in exhaust gas are not sufficiently purified. The present invention is intended to solve this problem, and it is determined whether the engine 10 is being operated at a low speed or a low load.

At this time, in the low-speed/low-load operation determination step (S110), at least one of the rotation speed of the engine 10, the speed of the engine 10, the opening degree of the throttle valve, fuel consumption, and engine load information is preset If it is less than the set value, it is determined that the engine 10 is operated at low speed/low load.

In the cold driving determination step ( S120 ), if the engine 10 is operating at a low speed/low load, it is determined whether the engine 10 is sufficiently warmed up. Since the engine 10 can operate normally only when it is warmed up, even if the engine 10 is operating at a low speed/low load, it is determined whether the engine 10 has been warmed up and the temperature of the SCR device 50 after the warming up is completed. In order to increase the , each step described later is allowed to proceed.

Here, in the cold driving determination step ( S120 ), at least one of the temperature of the engine coolant, the inlet temperature of Diesel Oxidation Catalyst (DOC) installed in the exhaust pipe 12 of the engine, and the temperature of the air flowing into the engine 10 . When is lower than a preset value, it is determined that the engine of the vehicle is cold driving.

In the target opening rate setting step ( S130 ), if the engine 10 is sufficiently warmed up, the opening rate of the air quantity control valve 11 is set as the target opening rate of the air quantity control valve 11 set in advance according to the current operating state.

The air amount control valve 11 is for controlling the amount of air flowing into the engine 10, and is initially in the maximum open state, and in order to increase the temperature of the SCR device 50, the supercharging boost pressure must be lowered. , to decrease the degree of opening of the air amount control valve 11 .

Accordingly, in the target opening rate setting step ( S130 ), the target opening rate of the air amount control valve 11 is set.

In this case, in the target opening rate setting step S130 , the target opening rate of the air amount control valve 11 is set in a direction in which fuel efficiency increases and exhaust gas decreases, which may be set from previously stored mapped data.

In particular, in the mapped data, the opening rate of the air amount control valve 11 is mapped according to the fuel injection timing, the fuel injection amount, and the amount of air required for combustion, and this mapping data is stored in the ECU 60 and is Accordingly, the target opening rate of the air amount control valve 11 is set according to the fuel injection timing, the fuel injection amount, and the amount of air required for combustion.

In the step of reducing the opening rate ( S140 ), the opening rate of the air quantity control valve 11 is adjusted to the target opening rate set in the step of setting the target opening rate ( S130 ). The initial opening rate of the air amount control valve 11, that is, the target opening rate set in the target opening rate setting step S130 at the maximum opening is adjusted.

The supercharging boost pressure reduction step S150 reduces the pressure in front of the compressor of the turbocharger installed in the intake pipe, that is, the supercharging boost pressure. The supercharging boost pressure may be achieved by lowering the opening rate of the air amount control valve 11 in the opening rate reducing step ( S140 ).

When the supercharging boost pressure is lowered, the air-fuel ratio in the engine 10 is lowered, the temperature of the exhaust gas discharged from the engine 10 is increased, and the exhaust gas having the increased temperature is sent to the SCR device (50). By being introduced, the temperature of the SCR device 50 is increased.

In the SCR temperature increase determination step (S160), the temperature of the SCR device 50 rises and the urea solution is sprayed into the SCR device 50 to determine whether the temperature is raised to a temperature that can prevent the formation of urea solids. Check it. In the SCR device 50, when the urea solution is injected using the urea water injection nozzle 53 into the SCR device 50 only when the temperature is raised to an appropriate temperature, the urea solution is Since it is possible to purify nitrogen oxides contained in exhaust gas by reacting with the catalyst of the device 50 , the temperature of the SCR device 50 is raised to an activation temperature capable of purifying nitrogen oxides in the SCR device 50 . determine whether

It is determined whether the temperature of the SCR device 50 is raised to an activation temperature using the temperature sensor 54 installed at the front end of the SCR device 50 .

Here, the activation temperature of the SCR device in the SCR temperature rising completion determination step (S160) is preferably set to 240 °C to 260 °C, particularly preferably set to 250 °C. This is because, when the temperature reaches 250° C., when urea water is injected into the SCR device 50, urea solids do not exist in the SCR device 50 and nitrogen oxides are purified.

On the other hand, when it is determined that the temperature of the SCR device 50 has risen to an activation temperature set to start spraying of urea water in the SCR temperature increase completion determination step (S160), the following urea water injection step (S170) is performed. However, if it is determined that the temperature of the SCR device 50 has not risen to the activation temperature set to start injecting urea water, the air amount control valve ( While controlling the engine 10 while maintaining the opening degree of 11) at the target opening rate, the SCR temperature increase determination step S160 is repeatedly performed.

In the urea water injection step (S170), when the SCR device 50 has reached the activation temperature, the urea water solution is injected into the SCR device. An SCR catalyst capable of purifying nitrogen oxide is coated inside the SCR device 50. By injecting urea water into the SCR device 50 using the urea water injection nozzle 53, It is possible to purify nitrogen oxides (NOx) contained in the exhaust gas discharged from the engine 10 .

On the other hand, if it is determined that the engine 10 is not operating at a low speed or a low load in the low speed/low load operation determination step S110, the urea water injection step S170 is performed immediately.

In addition, even when it is determined that the engine 10 is in a warmed-up state in the cold driving determination step S120 , the urea water injection step S170 is performed immediately.

If it is not the low speed/low load of the engine 10 or the warming-up of the engine 10 is completed, the temperature of the exhaust gas discharged from the engine 10 can sufficiently increase the temperature of the SCR device 50 Since there is a state, even if the urea solution is sprayed to the SCR device 50, the purification efficiency is lowered or urea solids are not formed.

As each step as described above is continuously performed until the engine of the vehicle is stopped, even in a vehicle with many sections running at low speed/low load, such as a bus running downtown, the temperature of the SCR device 50 is By spraying the urea solution in a state in which the temperature of the SCR device 50 is sufficiently increased by increasing the temperature of the exhaust gas to increase, the purification efficiency of the nitrogen oxides (NOx) can be increased.

10: engine 11: air amount control valve
12: exhaust pipe 21: fuel tank
22: fuel pump 23: filter
24: HC dosing module 25: HC injection nozzle
26: air tank 30: DOC
31: temperature sensor 32: temperature sensor
40: DPF 41: pressure sensor
42: PM sensor 50: SCR device
51: urea water tank 52: urea water supply module
53: urea water injection nozzle 54: temperature sensor
55: temperature sensor 56: front NOx sensor
57: rear NOx sensor 60: ECU
70: DCU
S110: Low speed/low load operation judgment stage
S120: cold operation judgment step
S130: Target opening rate setting step
S140: Reduction of the opening rate
S150: Supercharge boost pressure reduction stage
S160: SCR temperature increase completion judgment step
S170: urea water injection step

Claims (14)

In the SCR purification system, the SCR (Selective Catalytic Reduction) device installed in the exhaust pipe discharged from the engine is installed to purify the nitrogen oxides discharged from the engine,
ECU (electronic control unit) for controlling the engine,
When it is determined that the engine is not sufficiently warmed up while the engine is running at a low speed/low load, the opening degree of the air amount control valve controlling the amount of air flowing into the engine is reduced to increase the temperature of the SCR device, and the SCR When the temperature of the device reaches a predetermined temperature, the SCR purification system, characterized in that for injecting the aqueous urea solution into the SCR device. According to claim 1,
The ECU determines that the vehicle is operated at low speed/low load when any one of the engine rotation speed, vehicle speed, accelerator opening degree, fuel consumption, and engine load information is less than or equal to a preset value SCR purification system characterized. According to claim 1,
When at least one of a temperature of engine coolant, an inlet temperature of Diesel Oxidation Catalyst (DOC) installed in an exhaust pipe of the engine, and a temperature of air flowing into the engine is lower than a preset value, the engine of the vehicle is cooled SCR purification system, characterized in that it is determined that it is in operation. According to claim 1,
and the ECU reduces an opening rate of the air amount control valve from a maximum open state to a target opening rate determined according to an operating state of the engine. According to claim 1,
The ECU sets the target opening rate of the air quantity control valve from mapping data in which the target opening rate is stored according to a fuel injection timing, an injection amount of fuel, and an air quantity required for combustion. A low-speed/low-load operation determination step of determining whether the engine of the vehicle is being operated at a low speed or a low load;
If the engine is operating at a low speed/low load, a cold operation determination step of determining whether the engine is sufficiently warmed up;
a target opening rate setting step of setting the opening rate of the air quantity control valve to the target opening rate of the air quantity control valve preset according to the current operating state when the engine is sufficiently warmed up;
an opening rate reducing step of reducing the opening rate of the air quantity control valve from the initial opening rate to the target opening rate set in the target opening rate setting step;
a supercharging boost pressure reduction step in which the supercharging booster pressure is reduced as the opening rate of the air amount control valve is reduced;
SCR temperature increase completion determination step of determining whether the temperature of the SCR device has reached the activation temperature set to start spraying urea water;
If the SCR device has reached the activation temperature, SCR purification efficiency improvement method comprising a urea water injection step of spraying an aqueous urea solution into the SCR device. 7. The method of claim 6,
In the low-speed/low-load driving determination step, when at least one of the engine rotation speed, the engine speed, the throttle valve opening degree, fuel consumption, and engine load information is less than or equal to a preset value, the vehicle is set at low speed/low load. A method for improving SCR purification efficiency, characterized in that it is determined that the vehicle is operated with a load. 7. The method of claim 6,
In the cold driving determination step, when at least one of the temperature of the engine coolant, the inlet temperature of Diesel Oxidation Catalyst (DOC) installed in the exhaust pipe of the engine, and the temperature of the air flowing into the engine is lower than a preset value, the A method for improving SCR purification efficiency, characterized in that it is determined that the engine is in cold operation. 7. The method of claim 6,
In the step of reducing the opening rate, the SCR purification efficiency improvement method, characterized in that the opening rate of the air quantity control valve is reduced from a state in which the air quantity control valve is maximally opened to the target opening rate. 7. The method of claim 6,
SCR purification efficiency improvement method, characterized in that the activation temperature of the SCR device in the step of determining the completion of the SCR temperature increase is set to 240 °C to 260 °C. 7. The method of claim 6,
The target opening rate setting step is a method for improving SCR purification efficiency, characterized in that it is set from mapping data stored in advance in a direction in which fuel efficiency is increased and exhaust gas is decreased. 12. The method of claim 11,
In the target opening rate setting step, the method for improving SCR purification efficiency, characterized in that the mapping data is set according to an injection timing of fuel, an injection amount of fuel, and an amount of air required for combustion. 7. The method of claim 6,
In the low-speed/low-load operation determination step,
SCR purification efficiency improvement method, characterized in that the urea water injection step is performed when the vehicle is driven in a state other than low speed/low load. 7. The method of claim 6,
When it is determined that the engine is warmed up in the cold driving determination step, the urea water injection step is performed.

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