Method for operating an exhaust aftertreatment system of a diesel engine with emission control depending on the stored NH3

Abstract

Method for operating an exhaust aftertreatment system of a diesel engine, which has an exhaust stream (1) in which an SCR catalyst (6) is arranged, and which has a device for injecting an aqueous urea solution into the exhaust stream (1) upstream of the SCR catalyst (6), wherein the raw exhaust emissions and / or the heating of the SCR catalyst (6) during the warm-up phase of the diesel engine after engine start are controlled depending on the ammonia (NH3) stored in the SCR catalyst (6), characterized in that the ammonia (NH3) stored in the SCR catalyst (6) is measured by an antenna sensor (7) and a limit value for the amount of NH3 stored in the SCR catalyst (6) is defined, which should not be undercut before the SCR catalyst (6) reaches a temperature threshold for releasing the dosing of the aqueous urea solution and wherein the required initial time until the NH3 storage quantity reaches the limit value for the NH3 storage quantity is calculated from the actual measured NH3 storage quantity stored in the SCR catalyst (6) and a second time is calculated, which is required until the SCR catalyst (6) has been heated up to the temperature threshold in order to release the dose of aqueous urea solution, where the NO is calculated from the first time and the second time. x -Raw emission is calculated and the exhaust gas temperature is controlled depending on the calculated first time and the calculated second time.

Description

[0001] The present invention relates to a method for operating an exhaust aftertreatment system of a diesel engine which has an exhaust stream in which an SCR catalyst is arranged and which has a device for injecting an aqueous urea solution into the exhaust stream upstream of the SCR catalyst.

[0002] The emission limits for diesel engines are being continually lowered, particularly under real-world driving conditions with varying ambient temperatures. NOₓ plays a key role in this. x Emissions from diesel engines play a major role. Problems arise particularly at cold temperatures during the warm-up phase of the corresponding catalytic converters.

[0003] Modern exhaust aftertreatment systems for diesel engines now feature a so-called SCR catalyst (selective catalytic reduction catalyst). They also have a device for injecting an aqueous urea solution into the exhaust stream upstream of the SCR catalyst. The aqueous urea solution is converted to ammonia (NH3), which is stored in the SCR catalyst and catalytically reduced by the exhaust gas to harmless nitrogen. In this way, the emission of nitrogen oxides (NOx) is reduced. x ) reduced.

[0004] However, if the SCR catalyst is not above a certain temperature threshold, for example 180 °C, and the amount of NH3 stored in the SCR catalyst is very low, the emission reduction described above will only be partially effective. The implementation of a good and reliable NO xReducing emissions during the warm-up phase of an SCR catalyst therefore presents a corresponding challenge.

[0005] It is known that during the warm-up phase of the diesel engine, the NO x -Raw emissions and the SCR catalyst's warm-up phase are to be pre-controlled through appropriate calibration in order to guarantee compliance with emission regulations. However, under real-world driving conditions, it is extremely difficult to provide a suitable calibration that meets all requirements.

[0006] German patent application DE 10 2011 051 476 A1 discloses a method for the exhaust aftertreatment of a diesel engine with an SCR catalyst and a device for injecting a reducing agent at a point upstream of the SCR catalyst. This exhaust aftertreatment method comprises the following steps: determining whether a driving condition requires a rapid heat-up (RHU) logic process to rapidly raise the temperature of an exhaust gas flowing through an exhaust section; detecting an ON / OFF signal that executes or terminates the RHU logic process; determining whether an injection signal is activated to execute the RHU logic process; detecting a fault status of one or more components for executing the RHU logic process; and detecting the exhaust gas temperature during the execution of the RHU logic process and comparing the detected temperature with a modeled value.Accordingly, a malfunction of the RHU logic process is avoided in time, so that the RHU logic process is executed safely and a malfunction is reported to the driver via an error signal, thereby improving the on-board diagnostic function.

[0007] Furthermore, an exhaust gas control device is known from US patent 2015 0 314 239 A1, comprising an SCR catalyst, an ammonia supply device, a temperature detection device for determining the temperature of the SCR catalyst, and an ammonium nitrate quantity detection device. An associated control device is configured to calculate the quantity of ammonia adsorbed onto the SCR catalyst and to reduce the quantity of NO2 flowing into the SCR catalyst when the NOx purification rate is below a threshold value.

[0008] From publication EP 3 081 776 A1, another exhaust aftertreatment system with an SCR catalyst and an NH3 feed is known, wherein a high-frequency RF probe is arranged and configured within the SCR catalyst to measure ammonia (NH3) in the SCR catalyst. A controller is provided for detecting measured concentrations of nitrogen oxides (NOx) or NH3 upstream and downstream of the SCR catalyst and for measuring NH3 storage. The controller is further configured to use both the estimated NH3 storage and the measured NH3 storage to issue a control signal for the injection system.

[0009] Document US 2011 O 265 457 A1 discloses a further exhaust gas purification system with an SCR catalyst and an ammonia feed device for an internal combustion engine. A control device for controlling the internal combustion engine and the ammonia feed device such that a calculated uptake of excess ammonia by the catalytic reduction is at least equal to or greater than a predetermined value.

[0010] Finally, publication EP 3 128 142 A1 also shows an exhaust gas purification system for an internal combustion engine, with a urea-water injection mechanism for the exhaust gas and a catalyst designed to adsorb ammonia and remove NOx by using the adsorbed ammonia. Control electronics are designed to set a target amount of ammonia adsorbed onto the catalyst and to regulate the corresponding amount of urea-water injection. A first parameter is determined, which correlates with the period during which the catalyst temperature is greater than or equal to a threshold value, and the amount of ammonia adsorbed onto the catalyst is reduced when the determined parameter is greater than or equal to a predefined value.

[0011] The present invention is based on the objective of providing a method of the type mentioned above with which the emissions of the diesel engine can be controlled in a particularly favorable and simple manner under real driving conditions.

[0012] According to the invention, this problem is solved in a method of the specified type by controlling the raw exhaust emissions and / or the heating of the SCR catalyst during the warm-up phase of the diesel engine after engine start depending on the ammonia (NH3) stored in the SCR catalyst.

[0013] The method according to the invention has the advantage that, in particular, the NO xEmissions under real driving conditions can be controlled in a cost-effective and simple manner. This is achieved based on the NH3 stored in the SCR catalyst. The corresponding amount of ammonia stored in the SCR catalyst is measured and used as the basis for controlling emissions.

[0014] The measurement of the ammonia stored in the SCR catalyst is carried out using an antenna sensor that is appropriately positioned on the SCR catalyst.

[0015] In the process according to the invention, a limit value (N_NH3_lim) for the amount of NH3 stored in the SCR catalyst is defined, which should not be undercut before the SCR catalyst reaches a temperature threshold (T_SCR_thr) for releasing the dose of aqueous urea solution, so that emissions can be kept below a limit value. This threshold value can, for example, be 180 °C.

[0016] From the actual measured amount of NH3 stored in the SCR catalyst (N_NH3_real), the required initial time (delta_t_NH3) until the NH3 storage quantity reaches the limit (M_NH3_lim) is calculated. The calculation can be performed as follows: delta_t_NH3=(M_NH3_real−M_NH3_lim) / (NOx_raw*MAF / mol_mas) (1) which mean: NO x _raw = NO x -Concentration as raw emission measured by NO x -Sensor upstream SCR MAF exhaust mass airflow Mole mass, molar mass of the exhaust gas

[0017] The second time (delta_t_SCR_heat) required until the SCR catalyst has been heated to the temperature threshold (T_SCR_thr, for example 180 °C) in order to release the dose of aqueous urea solution is also calculated, as follows: delta_t_SCRT_heat=(T_SCR_thr−T_SCR_real)*C_p_SCR / (T_exh−T_SCR_real)*c_p*MAF)) which mean: T_SCR_real current SCR temperature C_p_SCR thermal capacity of the SCR T_exh Exhaust gas temperature upstream of the SCR catalyst c_p specific thermal capacity of the exhaust gas

[0018] From the first time, which is calculated from the actual measured amount of NH3 stored in the SCR catalyst, and the second time, which is needed until the SCR catalyst has been heated to the temperature threshold, the NO is then calculated. x -Raw emission calculated. Specifically, the NO can be calculated from equations (1) and (2) given above. x -Raw emissions are calculated as follows: NOx_raw<(M_NH3_real−M_NH3_lim)*(T_exh−T_SCR_real) *c_p*mol_mas / (T_SCR_thr−T_SCR_real)*C_p_SCR)

[0019] In any case, during the warm-up phase, the time delta_t_NH3 should be longer than the time delta_t_SCR_heat.

[0020] The NO x-Raw emission should be below the value calculated according to equation (3). If the NO x -Sensor measured NO x If the raw emission is higher than the raw emission calculated in the manner described above (according to equation (3)), the EGR rate can be increased.

[0021] The exhaust gas temperature is also controlled by the method according to the invention, specifically as a function of equation (1) and equation (2). If the NO x -Raw emission is set as the target value through calibration, or if, under given conditions, a further NO x Since further raw emission reduction is no longer possible, the required exhaust gas temperature is calculated as follows: T_exh>T_SCR_real+(T_SCR_thr−T_SCR_real)*C_p_SCR*NOx_raw / (c_p*mol_mas*(M_NH3_real−M_NH3_lim))

[0022] According to an advantageous embodiment, the exhaust gas temperature is set so that it is higher than the exhaust gas temperature calculated according to equation (4). This is achieved by means of exhaust gas recirculation and / or electric catalyst heating or post-injection.

[0023] Using the methods described above, based on NH3 loading in the SCR catalyst during the heating phase, the NO x Emissions under real driving conditions can be controlled cheaply and easily. The NH3 loading in the SCR catalyst can be monitored using antenna sensors and / or NO. x -Sensors are determined.

[0024] The invention is explained in detail below with reference to two exemplary embodiments in conjunction with the drawing. The drawing shows: Fig. 1 a schematic representation of a first embodiment of an exhaust aftertreatment system for a diesel engine; and Fig. 2 a second embodiment of a corresponding exhaust aftertreatment system.

[0025] In the Fig. In the schematically depicted exhaust aftertreatment system of a diesel engine, the exhaust gas from the diesel engine is routed from left to right via an exhaust stream 1. The exhaust gas passes successively through an oxidation catalyst 2, a diesel particulate filter 3, and an SCR catalyst 6. Upstream of the SCR catalyst 6, an aqueous urea solution is fed into the exhaust stream at point 5 from a container 4. According to the invention, an antenna sensor 7 is provided on the SCR catalyst 6 to measure the amount of ammonia (NH3) stored there. This sensor transmits its signal to a control / evaluation device (not shown), which calculates the amount of NH3 stored in the SCR catalyst 6.

[0026] The in Fig. The embodiment shown in section 2 differs from that of the Fig.1 only by using a NO instead of an antenna sensor 7 x -Sensor 8 upstream and a NO x Sensor 9 is provided downstream of the SCR catalyst 6. Both sensors can also be used to determine the amount of ammonia stored in the SCR catalyst 6, whereby in this case the SCR efficiency is calculated from the measured values ​​supplied by both sensors and the amount of ammonia stored in the SCR catalyst is estimated on this basis.

[0027] In both cases, the amount of NH3 measured in the manner described above and stored in the SCR catalyst 6 is used to calculate the raw exhaust emissions and / or the heating of the SCR catalyst during the warm-up phase of the diesel engine after engine start. In this way, the NOₓ emissions, in particular, can be determined. x Emissions can be controlled in a simple and cost-effective way under real driving conditions.

Claims

[1] Method for operating an exhaust gas aftertreatment system of a diesel engine having an exhaust line (1) in which an SCR catalyst (6) is arranged, and having a device for injecting an aqueous urea solution into the exhaust line (1) upstream of the SCR catalyst (6), wherein the raw exhaust gas emissions and / or the heating of the SCR catalyst (6) are controlled during the warm-up phase of the diesel engine after the engine start as a function of the ammonia (NH3) stored in the SCR catalyst (6), characterized by , that the ammonia (NH3) stored in the SCR catalyst (6) is measured by an antenna sensor (7) and a limit value is defined for the amount of NH3 stored in the SCR catalyst (6), which should not be undercut before the SCR catalyst (6) reaches a temperature threshold value for releasing the dosage of the aqueous urea solution and wherein a required first time is calculated from the measured NH3 quantity actually stored in the SCR catalyst (6) until the NH3 storage quantity has reached the limit value for the NH3 storage quantity and a second time is calculated which is required until the SCR catalyst (6) has been heated up to the temperature threshold in order to deliver the dosage of the aqueous urea solution, where the calculated first time and the calculated second time are the NO x -raw emission is calculated and the exhaust gas temperature is controlled depending on the calculated first time and the calculated second time. [2] Method according to claim 1, characterized by that when the NO x -Sensor measured NO x -Raw emission is higher than the calculated raw emission, the EGR rate of the exhaust gas aftertreatment system is increased. [3] Method according to claim 1, characterized bythat the required exhaust gas temperature is determined mathematically. [4] Method according to claim 3, characterized by that the exhaust gas temperature is adjusted by means of exhaust gas recirculation and / or electrical catalyst heating so that it is higher than the calculated exhaust gas temperature.

Images