Double-SCR injection system with common rail structure and injection method

Abstract

The invention belongs to the technical field of engine aftertreatment systems. The dual-SCR injection system with the common rail structure comprises a urea box, a urea pump, a nozzle, a filter element, a pressure release valve, the common rail structure, an inverted pump and a control unit, and the common rail structure is connected with the inverted pump through a return pipe; the common rail structure is connected with two urea nozzles, the first urea nozzle is located at the front end of the Lo-SCR, the second urea nozzle is located at the front end of the SCR, the common rail structure is connected with the urea pump through a liquid suction pipe, the common rail structure is connected with the pressure release valve through a backflow pipe, the common rail structure is connected with a pressure sensor, and the inverse pump is connected with the pressure release valve in parallel. The urea box is connected with an NOx sensor, and the control unit is connected with the pressure sensor, the NOx concentration sensor and the urea nozzle. The low emission requirement is met, and meanwhile the urea spraying amount is more accurate.

Description

Technical Field

[0001] The present invention relates to the technical field of engine aftertreatment systems, and in particular to a dual SCR injection system and an injection method with a common rail structure. Background Art

[0002] The statements in this section merely provide background art related to the present invention and do not necessarily constitute prior art.

[0003] Diesel engine emission regulations are becoming increasingly stringent, and nitrogen oxides are the most concerned emission pollutants in the diesel engine field. Currently, selective catalytic reduction technology (SCR) is widely used to purify the NOx concentration in the exhaust. The new restrictions on various emission pollutants mean that more efficient and novel post-treatment technologies need to be developed. The dual SCR design is highly favored under the goal of ultra-low nitrogen oxide emissions.

[0004] However, the current dual SCR system will have obvious pressure fluctuations in the pipeline after the urea nozzle is turned on, resulting in a partial deviation between the actual urea injection amount and the theoretical injection amount. When the actual urea injection amount is greater than the theoretical injection amount, it is easy to cause urea crystallization; when the actual urea injection amount is less than the theoretical injection amount, it is easy to cause insufficient NOx conversion rate; and the current dual SCR system uses two independent injection supply systems, which have a cumbersome structure and a higher failure rate than a shared injection supply system. Summary of the invention

[0005] In order to solve the deficiencies of the prior art, the present invention provides a dual SCR injection system and injection method with a common rail structure, which can make the pressure of the urea solution in the pipeline more stable and the urea injection amount more accurate while meeting the low emission requirements.

[0006] In order to achieve the above object, the present invention adopts the following technical solution: In a first aspect, the present invention provides a dual SCR injection system with a common rail structure.

[0007] A dual SCR injection system with a common rail structure, comprising: a urea tank, a urea pump, a nozzle, a filter element, a pressure relief valve, a common rail structure, a backflow pump and a control unit, wherein the exhaust gas treatment mechanism comprises a Lo-SCR, an ASC, a DOC, a DPF, an SCR and an ASC which are sequentially arranged in an exhaust pipe; The urea tank is used to store urea solution, the urea tank is connected to the urea pump through a suction pipe, the urea tank is respectively connected to the backflow pump and the pressure relief valve through a reflux pipe, and the filter element is located on the suction pipe between the urea tank and the urea pump; The common rail structure is connected to the backflow pump through a reflux pipe, and the common rail structure is connected to two urea nozzles, namely a first urea nozzle and a second urea nozzle, the first urea nozzle is located at the front end of the Lo-SCR, and the second urea nozzle is located at the front end of the SCR; The common rail structure is connected to the urea pump through a liquid suction pipe, and the common rail structure is connected to the pressure relief valve through a return pipe; The common rail structure is connected with a pressure sensor, the backflow pump is connected with the pressure relief valve in parallel, the urea tank is connected with a NOx sensor, and the control unit is respectively connected with the pressure sensor, the NOx concentration sensor and the urea nozzle.

[0008] As a further limitation of the first aspect of the present invention, the return pipe of the common rail structure is connected to one side of the long side of the common rail structure, and the urea nozzle is connected to the other side of the long side of the common rail structure.

[0009] As a further limitation of the first aspect of the present invention, a suction pipe connected to the urea pump is connected to one side of the short side of the common rail structure, and a return pipe connected to the pressure relief valve is connected to the other side of the short side of the common rail structure.

[0010] As a further limitation of the first aspect of the present invention, the backflow pump is used to backflow the remaining urea solution back to the urea tank after the engine is stopped.

[0011] As a further limitation of the first aspect of the present invention, the common rail structure is a common rail pipe. When the urea pump is in operation, the urea solution of set pressure is delivered to the common rail pipe and the urea solution is accumulated in the cavity of the common rail pipe. After the pressure wave of the urea solution is eliminated in the common rail pipe, the urea solution is delivered to each nozzle through the pipeline.

[0012] In a second aspect, the present invention provides a dual SCR injection method with a common rail structure.

[0013] A dual SCR injection method with a common rail structure, using the dual SCR injection system with a common rail structure according to the first aspect of the present invention, comprises: a pre-injection stage and an injection metering stage; During the pre-injection phase, the pressure of the urea solution in the pipeline is established to prepare for the subsequent injection; In the injection metering stage, the urea solution injection amount is calculated according to the real-time NOx concentration, and the control unit controls the urea nozzle to inject the urea solution according to the calculated urea solution injection amount.

[0014] As a further limitation of the second aspect of the present invention, the pre-injection stage includes: standby, pre-pressure building, exhaust test injection, back-pumping and re-pressure building.

[0015] As a further limitation of the second aspect of the present invention, the pre-pressure building includes: when the pressure building condition is met, the urea pump starts to operate to absorb the urea solution in the urea tank, and a pressure of 0.5 MPa is established in the urea pipeline, and the remaining urea flows back to the urea tank through the return liquid pipeline; The exhaust test spray includes: after the pre-pressure is successfully built up, the urea pump stops running, the urea nozzle is opened to exhaust the air in the pipeline and a test spray test is performed, and at the same time, the pressure in the urea pipeline drops below 0.11 MPa; The back-pumping comprises: when the urea injection valve is in an open state, driving the back-pumping pump to back-pump, so that the pressure in the urea pipeline drops to below 0.023 MPa within a set time; The re-pressure building includes: the backflow pump and the urea nozzle are closed, the urea pump starts to operate to absorb the urea solution in the urea tank again to build up pressure, a pressure of 0.5 MPa is established in the urea pipeline, and the remaining urea flows back to the urea tank through the return liquid pipeline.

[0016] As a further limitation of the second aspect of the present invention, when the injection condition is met, the control unit allocates each urea nozzle to inject according to the current exhaust temperature in the exhaust pipe, including: When the exhaust gas temperature at the upstream LO-SCR inlet end is in a low temperature condition below a first temperature threshold, neither the first urea nozzle nor the second urea nozzle sprays urea; When the exhaust temperature at the inlet end of the upstream LO-SCR exceeds the first temperature threshold, the upstream LO-SCR starts to use the first ammonia storage injection, and the downstream SCR does not inject at this time. The first ammonia storage injection is: the upstream nitrogen oxide concentration is measured by a nitrogen oxide sensor arranged at the front end of the upstream LO-SCR, the upstream theoretical required ammonia input amount is calculated, and the ammonia storage injection coefficient is multiplied to obtain the first actual urea injection amount, and the first urea nozzle is controlled to inject at the first actual urea injection amount; When the exhaust temperature at the inlet end of the downstream SCR exceeds the first temperature threshold but is less than the second temperature threshold, the second urea nozzle at the downstream adopts the first efficiency injection, and the first urea nozzle at the upstream continues to adopt the first ammonia storage injection; the first efficiency injection is: the nitrogen oxide concentration at the downstream is measured by the nitrogen oxide sensor arranged at the front end of the downstream SCR, the theoretical required ammonia input at the downstream is calculated, and the injection of the second urea nozzle is controlled according to the theoretical required ammonia input at the downstream; When the exhaust temperature of the downstream SCR exceeds the second temperature threshold, the downstream SCR performs the second ammonia storage injection, and the upstream LO-SCR switches to the second efficiency injection; the second ammonia storage injection includes: measuring the downstream nitrogen oxide concentration through the nitrogen oxide sensor arranged at the front end of the downstream SCR, calculating the downstream theoretical required urea amount according to the downstream exhaust nitrogen oxide concentration, multiplying it by the corresponding ammonia storage injection coefficient to obtain the second actual urea injection amount, and controlling the second urea nozzle to inject with the second actual urea injection amount; the second efficiency injection includes: measuring the upstream nitrogen oxide concentration through the nitrogen oxide sensor arranged at the front end of the upstream LO-SCR, calculating the upstream theoretical required ammonia input amount, and controlling the injection of the first urea nozzle according to the upstream theoretical required ammonia input amount.

[0017] As a further limitation of the second aspect of the present invention, after the vehicle is turned off and the power is turned off, the control unit controls the urea pump to stop working, the urea nozzle is opened, the backflow pump starts working, the urea solution passes through the reflux pipe from the nozzle, and the urea solution in the urea pump is returned to the urea tank. After the backflow pump works for a set time, the backflow process is completed.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. In the present invention, an innovative common rail structure is used to stabilize the pressure of the urea pipeline, making the injection pressure more accurate, thereby making the urea injection amount more accurate, achieving the best conversion effect, and preventing the phenomenon of insufficient injection amount leading to low conversion rate or excessive injection amount leading to urea crystallization.

[0019] 2. The common rail structure of the present invention can stabilize and adjust the pressure of the urea solution in the pipe in a timely manner as needed, and is both a pressure retainer and a storage of high-pressure urea solution. When used as a storage of high-pressure urea solution, the common rail structure stores the urea solution provided by the urea pump in the common rail structure, and provides the required urea solution to the urea nozzle in a timely manner, using the pressure and time control form, so that the process of spraying urea and the process of supplying urea become two relatively independent processes; when used as a pressure retainer, firstly, the volume of the common rail structure is large, which can absorb and buffer the pressure wave to a certain extent. During the propagation of the pressure wave, its energy will gradually be absorbed and attenuated by the common rail structure, thereby reducing the impact of the pressure wave on the system; secondly, the common rail system is equipped with a pressure sensor, which can monitor the urea pressure in the common rail structure in real time. The control unit accurately adjusts the urea supply of the urea pump and the injection time of the urea nozzle according to the feedback of the pressure sensor, thereby further balancing and stabilizing the pressure in the common rail structure.

[0020] 3. In the present invention, a backflow pump is used to backflow the urea solution in the pipe into the urea tank after the engine stops running, so as to prevent urea from crystallizing in the pipe and causing blockage.

[0021] 4. In the present invention, the common rail structure is applied to simplify the two sets of independent urea pipelines in the original dual SCR urea injection system, and a common set of urea supply structure is used, so that the structure is simplified, the problem of the original redundant structure of the dual SCR system is optimized, the failure rate of the urea injection system is reduced, and its reliability and durability are improved.

[0022] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0024] Figure 1 A schematic diagram of a dual SCR injection system with a common rail structure provided by the present invention; Figure 2 A schematic flow chart of a dual SCR injection method with a common rail structure provided by the present invention; Figure 3 A schematic diagram of a flow chart of the control unit provided by the present invention adjusting the injection of each urea nozzle according to the current exhaust working condition; Figure 4 A schematic diagram of the urea back extraction process provided by the present invention; Among them, 1. urea tank; 2. urea pump; 3. first urea nozzle; 4. second urea nozzle; 5. pressure relief valve; 6. common rail structure; 7. backflow pump; 8. engine; 9. exhaust pipe; 10. control unit. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0026] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0027] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0028] Embodiment 1: In this implementation, if Figure 1As shown, a dual SCR injection system with a common rail structure 6 is proposed, comprising: a urea tank 1, a urea pump 2, a nozzle, a filter element, a pressure relief valve 5, a common rail structure 6, a backflow pump 7 and a control unit 10, wherein the exhaust gas treatment mechanism comprises Lo-SCR (low concentration selective catalytic reducer), ASC (ammonia oxidation catalyst), DOC (oxidation catalyst), DPF (wall flow particulate filter), SCR (selective catalytic reducer) and ASC (ammonia oxidation catalyst) which are sequentially arranged in an exhaust pipe 9; The urea tank 1 is used to store urea solution. The urea tank 1 is connected to the urea pump 2 through a suction pipe. The urea tank 1 is connected to the backflow pump 7 and the pressure relief valve 5 through a reflux pipe. The filter element is located on the suction pipe between the urea tank 1 and the urea pump 2. The common rail structure 6 is connected to the backflow pump 7 through a return pipe (a throttling hole is provided in the return pipe between the common rail structure 6 and the backflow pump 7. After the throttling hole is added, the pressure drop at the return outlet is faster than that inside the common rail. Due to the throttling effect, the pressure drop inside the common rail structure 6 is buffered, and the rail pressure fluctuation is attenuated, so that the rail pressure fluctuation inside the common rail structure 6 is reduced); The common rail structure 6 is connected to two urea nozzles, namely a first urea nozzle 3 and a second urea nozzle 4. The first urea nozzle 3 is located at the front end of the Lo-SCR, and the second urea nozzle 4 is located at the front end of the SCR. The common rail structure 6 is connected to the urea pump 2 through a liquid suction pipe, and the common rail structure 6 is connected to the pressure relief valve 5 through a return pipe; The common rail structure 6 is connected to a pressure sensor, the backflow pump 7 is connected in parallel with the pressure relief valve 5, the urea tank 1 is connected to a NOx sensor, and the control unit 10 is connected to the pressure sensor, the NOx concentration sensor and the urea nozzle respectively.

[0029] In this implementation, the return pipe of the common rail structure 6 is connected to one side of the long side of the common rail structure 6 , and the urea nozzle is connected to the other side of the long side of the common rail structure 6 .

[0030] In this implementation, the suction pipe connected to the urea pump 2 is connected to one side of the short side of the common rail structure 6 , and the return pipe connected to the pressure relief valve 5 is connected to the other side of the short side of the common rail structure 6 .

[0031] In this implementation, the urea tank 1 is used to store urea solution and adopts a fully enclosed structure; In this implementation, the urea pump 2 is used to establish pressure in the pipeline and provide a urea solution of a certain pressure to the nozzle; In this implementation, the nozzle is used to spray a certain amount of urea solution into the exhaust pipe 9; In this implementation, the filter element is used to absorb impurities in the urea solution to prevent clogging; In this implementation, the pressure relief valve 5 is used to limit the pressure of the urea solution in the pipeline, and releases the pressure when the pressure is too high to prevent the pipeline pressure from being too high, and returns the excess urea solution to the urea tank 1; In this implementation, the backflow pump 7 is used to backflow the remaining urea solution in the pipeline back to the urea tank 1 after the engine 8 is stopped; In this implementation, the control unit 10 is connected to the pressure sensor, the NOx concentration sensor and the valve of the urea nozzle, respectively, and is used to calculate the required urea injection amount and control the switch of the valve of the urea nozzle to perform injection according to the signals of the pressure sensor and the NOx concentration sensor; In the present implementation, the common rail structure 6 adopts a common rail pipe, which includes a larger internal cavity. When the urea pump 2 is in operation, the urea solution with a certain pressure is delivered to the common rail pipe and accumulated in the internal cavity. After the pressure wave of the urea solution is eliminated in the common rail pipe, it is delivered to each nozzle (i.e., the first urea nozzle 3 and the second urea nozzle 4) through the pipeline.

[0032] In this embodiment, if Figure 2 As shown, the system is powered on, and the fault code, temperature signal and speed signal are obtained to determine whether the pressure building conditions are met. If not, the system returns to continue to obtain the fault code, temperature signal and speed signal. If so, pre-pressure building, exhaust, test emptying and rapid backflow are performed to build pressure. After the pressure is built, the fault code, temperature signal and speed signal are obtained again to determine whether the injection conditions are met. If not, the system continues to obtain the fault code, temperature signal and speed signal. If so, the nozzle is controlled to spray.

[0033] More specifically, it is mainly divided into two stages, the pre-injection stage and the injection metering stage. First, in the pre-injection stage, the pressure of the urea solution in the pipeline is established to prepare for the subsequent injection. Then, the urea injection amount is calculated according to the real-time NOx concentration in the exhaust pipe 9, and the control unit 10 controls the urea nozzle to spray the urea solution.

[0034] The pre-injection stage is the entire process from the standby state of the urea pump 2 to the pressure building of the urea pump 2 in preparation for injection, which is mainly divided into the processes of standby, pre-pressure building, exhaust test injection, rapid backflow, and re-pressure building.

[0035] Certain conditions must be met during the pre-pressure buildup phase, including: (1) there are no related fault codes that affect the pressure buildup of the urea pump 2; (2) the exhaust temperature exceeds the preset value; (3) the engine 8 speed is greater than 500 rpm; (4) the system is successfully thawed (urea temperature is higher than -8°C).

[0036] The pre-pressure building process specifically includes: when the pressure building condition is reached, the urea pump 2 starts to operate to absorb the urea solution in the urea tank 1, and a pressure of 0.5 MPa is established in the urea pipeline, and the remaining urea flows back to the urea tank 1 through the reflux pipe.

[0037] The exhaust test injection process specifically includes: after the pre-pressure is successfully built up, the urea pump 2 stops running, the urea nozzle is opened to exhaust the air in the pipeline and a test injection test is performed, and at the same time, the pressure in the urea pipeline drops below 0.11 MPa.

[0038] The rapid back-pumping stage specifically includes: when the urea injection valve is in an open state, the back-pumping pump 7 is driven to rapidly back-pump, so that the pressure in the pipeline drops to below 0.023 MPa in a short time.

[0039] The re-pressure building process specifically includes: the backflow pump 7 and the urea injection valve are closed, the urea pump 2 starts to operate to absorb the urea solution in the urea tank 1 again to build up pressure, and a pressure of about 0.5 MPa is established in the urea pipeline, and the remaining urea flows back to the urea tank 1 through the return liquid pipeline.

[0040] Determine whether the injection conditions are met, specifically, including: (1) SCR inlet temperature exceeds 180°C; (2) there is no SCR-related fault code; (3) pre-injection is successful (urea pre-injection pressure is 5 bar); (4) system thawing is successful (urea temperature is higher than -8°C).

[0041] like Figure 3 The control unit 10 adjusts the injection of each urea nozzle according to the current exhaust working condition, specifically including: When the exhaust temperature at the upstream LO-SCR inlet end (similar to the exhaust temperature after the turbine of the engine 8) is below the first temperature threshold (preferably 180°C), the first urea nozzle 3 and the second urea nozzle 4 do not spray urea; When the exhaust temperature at the inlet end of the upstream LO-SCR exceeds the first temperature threshold, the upstream LO-SCR starts to use the first ammonia storage injection, and the downstream SCR does not inject at this time. The first ammonia storage injection is: the upstream nitrogen oxide concentration is measured by the nitrogen oxide sensor arranged at the front end of the upstream LO-SCR, the upstream theoretical required ammonia input amount is calculated, and the ammonia storage injection coefficient is multiplied to obtain the first actual urea injection amount, and the first urea nozzle 3 is controlled to inject with the first actual urea injection amount; When the exhaust temperature at the inlet end of the downstream SCR exceeds the first temperature threshold but is less than the second temperature threshold (preferably 250°C here), the downstream second urea nozzle 4 adopts the first efficiency injection, and the upstream first urea nozzle 3 continues to adopt the first ammonia storage injection; the first efficiency injection is: the downstream nitrogen oxide concentration is measured by the nitrogen oxide sensor arranged at the front end of the downstream SCR, the downstream theoretical required ammonia input amount is calculated, and the injection of the second urea nozzle 4 is controlled according to the downstream theoretical required ammonia input amount; When the exhaust temperature of the downstream SCR exceeds the second temperature threshold, the downstream SCR performs the second ammonia storage injection, and the upstream LO-SCR switches to the second efficiency injection; the second ammonia storage injection includes: measuring the downstream nitrogen oxide concentration through the nitrogen oxide sensor arranged at the front end of the downstream SCR, calculating the downstream theoretical required urea amount according to the downstream exhaust nitrogen oxide concentration, multiplying it by the corresponding ammonia storage injection coefficient to obtain the second actual urea injection amount, and controlling the second urea nozzle 4 to inject with the second actual urea injection amount; the second efficiency injection includes: measuring the upstream nitrogen oxide concentration through the nitrogen oxide sensor arranged at the front end of the upstream LO-SCR, calculating the upstream theoretical required ammonia input amount, and controlling the injection of the first urea nozzle 3 according to the upstream theoretical required ammonia input amount.

[0042] like Figure 4 This implementation also provides a back-pumping process. When the vehicle is turned off and the power is turned off, the control unit 10 controls the urea pump 2 to stop working, the urea nozzle is turned on, the back-pumping pump 7 is turned on, and the urea solution flows back to the urea tank 1. It is determined whether it is emptied. If so, the back-pumping is completed; if not, it is recorded as "incomplete emptying" and the back-pumping is completed. The back-pumping action of this implementation lasts for 60s (i.e., the set time, which can be set according to the specific situation and will not be repeated here).

[0043] It is understandable that if the switch power supply of the control unit 10 (such as ECU) is disconnected due to battery disconnection or other reasons before the back-pulling is completed, the control unit 10 will record a fault code, which can be read as "incomplete back-pulling count" in the subsequent diagnostic software.

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A dual SCR injection system with a common rail structure, characterized in that: include: A urea tank, a urea pump, a nozzle, a filter element, a pressure relief valve, a common rail structure, a backflow pump and a control unit, wherein the exhaust gas treatment mechanism includes a Lo-SCR, an ASC, a DOC, a DPF, an SCR and an ASC which are sequentially arranged in the exhaust pipe; The urea tank is used to store urea solution, the urea tank is connected to the urea pump through a suction pipe, the urea tank is respectively connected to the backflow pump and the pressure relief valve through a reflux pipe, and the filter element is located on the suction pipe between the urea tank and the urea pump; The common rail structure is connected to the backflow pump through a reflux pipe, and the common rail structure is connected to two urea nozzles, namely a first urea nozzle and a second urea nozzle, the first urea nozzle is located at the front end of the Lo-SCR, and the second urea nozzle is located at the front end of the SCR; The common rail structure is connected to the urea pump through a liquid suction pipe, and the common rail structure is connected to the pressure relief valve through a return pipe; The common rail structure is connected with a pressure sensor, the backflow pump is connected with the pressure relief valve in parallel, the urea tank is connected with a NOx sensor, and the control unit is respectively connected with the pressure sensor, the NOx concentration sensor and the urea nozzle.

2. The dual SCR injection system with a common rail structure according to claim 1, characterized in that: The return pipe of the common rail structure is connected to one side of the long side of the common rail structure, and the urea nozzle is connected to the other side of the long side of the common rail structure.

3. The dual SCR injection system with a common rail structure according to claim 1 or 2, characterized in that: A liquid suction pipe for connecting with the urea pump is connected to one side of the short side of the common rail structure, and a return pipe for connecting with the pressure relief valve is connected to the other side of the short side of the common rail structure.

4. The dual SCR injection system with a common rail structure according to claim 1 or 2, characterized in that: The backflow pump is used to backflow the remaining urea solution back to the urea tank after the engine is stopped.

5. The dual SCR injection system with a common rail structure according to claim 1 or 2, characterized in that: The common rail structure is a common rail pipe. When the urea pump is in operation, the urea solution of set pressure is delivered to the common rail pipe and the urea solution is accumulated in the cavity of the common rail pipe. After the pressure wave of the urea solution is eliminated in the common rail pipe, the urea solution is delivered to each nozzle through the pipeline.

6. A dual SCR injection method with a common rail structure, characterized in that: A dual SCR injection system with a common rail structure according to any one of claims 1 to 5, comprising: a pre-injection stage and an injection metering stage; During the pre-injection phase, the pressure of the urea solution in the pipeline is established to prepare for the subsequent injection; In the injection metering stage, the urea solution injection amount is calculated according to the real-time NOx concentration, and the control unit controls the urea nozzle to inject the urea solution according to the calculated urea solution injection amount.

7. The dual SCR injection method with a common rail structure according to claim 6, characterized in that: The pre-injection stage includes: standby, pre-pressure building, exhaust test spray, back pumping and re-pressure building.

8. The dual SCR injection method with a common rail structure according to claim 7, characterized in that: The pre-pressure building includes: when the pressure building condition is met, the urea pump starts to operate to absorb the urea solution in the urea tank, and a pressure of 0.5 MPa is established in the urea pipeline, and the remaining urea flows back to the urea tank through the return pipeline; The exhaust test spray includes: after the pre-pressure is successfully built up, the urea pump stops running, the urea nozzle is opened to exhaust the air in the pipeline and a test spray test is performed, and at the same time, the pressure in the urea pipeline drops below 0.11 MPa; The back pumping includes: when the urea injection valve is in the open state, driving the back pumping pump to back pump, so that the pressure in the urea pipeline drops to below 0.023MPa within a set time; The re-pressure building includes: the backflow pump and the urea nozzle are closed, the urea pump starts to operate to absorb the urea solution in the urea tank again to build up pressure, a pressure of 0.5 MPa is established in the urea pipeline, and the remaining urea flows back to the urea tank through the return liquid pipeline.

9. The dual SCR injection method with a common rail structure according to claim 7 or 8, characterized in that: When the injection conditions are met, the control unit allocates each urea nozzle injection according to the current exhaust temperature in the exhaust pipe, including: When the exhaust gas temperature at the upstream LO-SCR inlet end is in a low temperature condition below a first temperature threshold, neither the first urea nozzle nor the second urea nozzle sprays urea; When the exhaust temperature at the inlet end of the upstream LO-SCR exceeds the first temperature threshold, the upstream LO-SCR starts to use the first ammonia storage injection, and the downstream SCR does not inject at this time. The first ammonia storage injection is: the upstream nitrogen oxide concentration is measured by a nitrogen oxide sensor arranged at the front end of the upstream LO-SCR, the upstream theoretical required ammonia input amount is calculated, and the ammonia storage injection coefficient is multiplied to obtain the first actual urea injection amount, and the first urea nozzle is controlled to inject at the first actual urea injection amount; When the exhaust temperature at the inlet end of the downstream SCR exceeds the first temperature threshold but is less than the second temperature threshold, the second urea nozzle at the downstream adopts the first efficiency injection, and the first urea nozzle at the upstream continues to adopt the first ammonia storage injection; the first efficiency injection is: the nitrogen oxide concentration at the downstream is measured by the nitrogen oxide sensor arranged at the front end of the downstream SCR, the theoretical required ammonia input at the downstream is calculated, and the injection of the second urea nozzle is controlled according to the theoretical required ammonia input at the downstream; When the exhaust temperature of the downstream SCR exceeds the second temperature threshold, the downstream SCR performs the second ammonia storage injection, and the upstream LO-SCR switches to the second efficiency injection; the second ammonia storage injection includes: measuring the downstream nitrogen oxide concentration through the nitrogen oxide sensor arranged at the front end of the downstream SCR, calculating the downstream theoretical required urea amount according to the downstream exhaust nitrogen oxide concentration, multiplying it by the corresponding ammonia storage injection coefficient to obtain the second actual urea injection amount, and controlling the second urea nozzle to inject with the second actual urea injection amount; the second efficiency injection includes: measuring the upstream nitrogen oxide concentration through the nitrogen oxide sensor arranged at the front end of the upstream LO-SCR, calculating the upstream theoretical required ammonia input amount, and controlling the injection of the first urea nozzle according to the upstream theoretical required ammonia input amount.

10. The dual SCR injection method with a common rail structure according to claim 7 or 8, characterized in that: After the vehicle is turned off, the control unit controls the urea pump to stop working, the urea nozzle is opened, and the backflow pump starts working. The urea solution passes through the reflux pipe from the nozzle and sends the urea solution in the urea pump back to the urea tank. After the backflow pump works for the set time, the backflow process is completed.

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