SE536173C2 - Calibration system for a virtual NOx sensor in an exhaust after-treatment system for an internal combustion engine - Google Patents

Description

If the SCRw catalyst increases, the NH3 level must be reduced to avoid NHg emissions (ie, excess NH; released from the SCR catalyst), which may reduce the conversion efficiency for the SCR catalyst.

In summary, in order to meet stricter environmental requirements, more and more vehicle manufacturers use SCR catalytic converter systems to purify diesel exhaust gases kv than nitrogen oxides (NOX). This is done by injecting ammonia solution into an SCR catalyst that helps convert NOX particles into nitrogen and water. The exhaust gas purification strategy should take into account that enough NOX is converted while not wanting to inject too much ammonia, for both the driving economy and the environment.

The US patent application with number US-2008/0250778 and the Swedish patent with number SE-530435 describe known devices in the field of technology.

US-2008/0250778 relates to a method for regulating the amount of NH; stored in a catalyst for an exhaust gas treatment system comprising determining the amount of NH; into the catalyst based on a dosing frequency of a dosing agent injected into the exhaust stream upstream of the catalyst and determining the amount of NH; out of the catalyst.

The accumulated mass of NH; in the catalyst is calculated by the amount in, and out, respectively, from the catalyst, and the dosing frequency is then calculated using the accumulated mass.

SE-530435 relates to a method for monitoring the function of an exhaust after-treatment system of a motor vehicle. According to this known system, a frequency analysis is performed of a parameter related to exhaust gases flowing out of, for example, a catalyst. Based on the results of the frequency analysis, information about the system's function can be obtained.

Thus, in order to minimize NOX emissions, careful control of the dosage of the reducing agent, Lex, is required. of urea, to the SCR catalyst. The control aims to provide optimal storage of ammonia at different temperatures. However, the storage cannot be measured directly, which together with the fact that the SCR catalyst works very differently at different temperatures makes the control very difficult.

There is a legal requirement that emissions of nitrogen oxides (NOX) must be measured with a sensor located after the SCR catalyst. In this way, the driver can be made aware that the vehicle emits unauthorized amounts of nitrogen oxides, this may, for example, be due to the urea being refueled with too low a concentration.

The sensors used to measure the content of nitrogen oxides in the exhaust gases are often very expensive components. NOx sensors are made of ceramic metal oxides, usually yttrium-stabilized zirconium (YSZ). YSZ is compressed into a solid ceramic that conducts oxygen ions at high temperatures, from approx. 400 ° C. To obtain a measuring signal, a pair of precious metal electrodes are placed on the surface and the voltage or current variations of an electrical signal can then be measured as a function of the NOx concentration.

High demands are placed on the sensor to achieve the sensitivity and robustness required when measuring exhaust fumes. Therefore, the cost of NOX sensors is relatively high.

Furthermore, it is very difficult to use the downstream NOx sensor to also control the SCR system in feedback, among the arm due to the SCR catalyst having a strongly varying dynamic at different temperatures and the condition of the SCR catalyst is not observable. Therefore, in order to achieve optimal control, an additional NOx sensor is often provided which is placed upstream of the SCR catalyst to control the amount of added reducing agent, which entails an additional cost.

JP-3124907 describes an exhaust after-treatment system which comprises a bypass duct with an oxygen sensor and based on the output signal from the oxygen sensor one can get an indication of the function of the SCR catalyst.

The object of the present invention is to provide a system for determining and calibrating a virtual NOx sensor upstream of the SCR without having to use a NOX sensor located upstream of the SCR catalyst. Summary of the Invention The above object is achieved with the invention defined by the independent claim.

Preferred embodiments are defined by the dependent claims.

According to the invention, the problem is solved by constructing the SCR catalyst so that a small fl of exhaust gases, i.e. a small part of the total exhaust gas fate, for a short period of time is allowed to pass an alternative path past the SCR catalyst. The passage is designed for a short time delay for the fate passing through the bypass so that the measured value downstream of the SCR catalyst corresponds as well as possible to a calculated virtual NOx value upstream of the SCR catalyst. An additional requirement is that fl the fate of the bypass is substantially less than fl the fate of the SCR catalyst so as not to have an excessive increase in emissions.

The contribution of the smaller fl fate to the measured NOX content at the NOx sensor located downstream of the SCR catalyst can be detected by increasing the NOx content during the time when the bypass line is connected, and the measured NOX levels will then be a function of upstream NOX. holder.

The present invention is based on periodically passing an amount of exhaust gas through the bypass so that it is possible to calibrate a modeled, virtual value for the NOX content upstream of the SCR catalyst based on measured NOK content downstream of the SCR catalyst. It needs to be done so often that calibration can take into account disturbances that occur, for example, when another fuel is used or when the humidity changes.

The present invention provides a robust control of the addition of reducing agents and eliminates the need for a NOX sensor upstream of the SCR catalyst, which results in a simplified and thus cheaper construction.

According to the invention, the variation in measured NOX content downstream of the SCR catalyst is thus utilized as a part of the fate which is passed past the SCR catalyst gives rise to. Thus, the present invention provides a periodic calibration of the NOx virtual sensor located upstream of the SCR catalyst.

Brief Description of the Drawings Figure 1 shows a schematic block diagram illustrating the present invention.

Figure 2 shows graphs illustrating the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION With reference to the accompanying drawings, the invention will now be described in detail.

An internal combustion engine 2 with an associated exhaust gas treatment system 4 is shown schematically in Figure 1. The exhaust gases leaving the internal combustion engine 2 are passed through an exhaust line 6 and discharged to the environment via an exhaust outlet 8. A selective catalytic reduction catalyst (SCR catalyst) is provided. the exhaust gases from the internal combustion engine 2 pass through the SCR catalyst 10 before being discharged to the environment via the exhaust outlet 8. Preferably, the internal combustion engine is arranged in a motor vehicle, but of course, within the scope of the present invention, it is possible to apply the invention to an internal combustion engine. in industry or on a ship.

Reducing agents, e.g. urea, is injected by means of an injector 12 into the exhaust gases in the exhaust line 6 upstream of the SCR catalyst 10. The injection device 12 comprises one or more injection means 14 arranged in the exhaust line 6 in the form of injection nozzles or the like, and a storage container 16 for reducing agents connected thereto. The injection device also comprises a control means 20 connected to the control means 18. The control means 18 is controlled by the control means 20, which determines the amount of reducing agent to be injected into the exhaust gases based on the prevailing drive conditions of the combustion engine 2 and the SCR catalyst 10 and depending on a control signal 21 from a processing agent 42.

The reducing agent may be urea (CO (NH 2) 2), ammonia (NH 3) or hydrocarbon (fuel). 536 '173 6 The exhaust gas treatment system often also includes a diesel oxidation catalyst (DOC), (not shown in the figure), which is located upstream of the SCR catalyst 10. In the DOC, reactions take place which reduce the emission levels of the exhaust gases. Furthermore, a diesel particle filter (DPF), (not shown in the figure), can be arranged upstream or downstream of the SCR catalyst to further reduce emission levels.

A NOx sensor 22 is provided in the exhaust line 6 downstream of the SCR catalyst 10.

The sensor 22 is arranged to generate a measurement signal (NOXUT) representing the NOx content in the exhaust gases flowing out of the SCR catalyst 10, i.e. The NOX content in the exhaust gases at the outlet of the SCR catalyst. The measurement signal may be a continuous signal representing the continuous changes of the measured parameter, i.e. forms a continuous stream of measured values regarding the size of the parameter, but is usually recorded as a discrete time signal which forms a series of successive and discrete measured values regarding the size of the parameter.

The processing means 42 comprises a memory 44, and is arranged to receive the measurement signal NOxgT from the sensor 22 and to generate the control signal 21 for the injection device 12.

Furthermore, there is a mass flow meter 24 arranged to measure the mass flow (FLTOT) of the exhaust gases upstream of the SCR catalyst 10 and to emit a mass flow signal (FLTOT), which is applied to the processing means 42, depending on the measured mass flow.

The exhaust after-treatment system 4 comprises, according to the invention, a bypass line 30 adapted to direct exhaust gases from a position upstream of the SCR catalyst 10 past the SCR catalyst 10 for a predetermined time interval, and that these exhaust gases are returned to the exhaust gases passing the SCR catalyst 10 in a position upstream NOX sensor 22. The processing means 42 is then adapted to store NOX values (NOxi) from the NOX sensor 22 which is measured when the bypass line 30 is connected to NOx values (NOxz) which are measured when it is not switched on, and which are measured in time in near the connection to the connection. In terms of time in connection with when the connection took place, the values measured in the order of magnitude mean at most one number of seconds before and after the connection, respectively. 536 173 7 The processing means 42 is adapted to calculate a virtual value (NOXUS) for the NOX content in the exhaust gases upstream of the SCR catalyst based on N0x | , NOxg, mass F LTOT and a desert FLBp through the dry connection line when this is connected.

According to one embodiment, the system 4 comprises a valve device 32 arranged in the bypass line 30, and that the monitoring device 40 is adapted to emit a connection signal 31 for controlling the connection of the bypass line 30 by opening and closing the valve device 32. In the figure the valve device is illustrated arranged in the mouth of the bypass line . However, the valve device can be arranged anywhere in the bypass line.

The valve device can, for example, consist of a simple damper which can assume two states, a closed state when no. Fate is allowed to pass and an open state when a fl fate, FLBP, is allowed to pass through the line. The time interval when the bypass is switched on is preferably longer than one second but preferably shorter than 30 seconds and even more preferably shorter than 10 seconds. The time interval must be long enough for a stable measurement signal to be obtained, but must not be too long because the emissions then increase too much.

According to a further embodiment, the system comprises a second fate meter 33 adapted to measure said fate through the dry connection FLBP.

An alternative to arranging a second speedometer is instead to calculate the flow through the dry bypass FLBp based on the FLTOT and the dimensions of the bypass line. This calculation is performed, for example, in the processing means 42.

The control signal 21 used to control the injection device 12 is determined and generated by the processing means 42, inter alia in dependence on NOXUS.

According to the embodiment, the bypass line 30 is connected periodically at a certain interval between the connections which is in the range 0.5-3 hours. 535 173 8 In order to obtain stable values, it is important that the flow FLTOT is relatively constant when the bypass line 30 is connected, for example FLTOT must have been substantially constant for at least a predetermined time which may be in the order of 10-15 seconds.

The calculation of NOXUS is preferably performed only if the values for NOX; are relatively constant in connection with the connection of the bypass line. This means that the calculation of NOx-us is performed only if the value of NOX; measured in time before connection is substantially equal to the value of NOX; which is measured in time after the bypass line is not connected again after being connected.

During the period of time that the bypass is switched on and a fl fate passes through it, the NOx sensor downstream will thus measure a NOX content which is the sum of the NOX content in the bypassed fl fate, which then has the same NOX content as before the SCR catalyst, and The NOX content that has passed through the SCR catalyst where the NOX content has been reduced due to of the reduction.

The processing means is adapted to calculate a virtual value for the NOX content in the exhaust gases upstream of the SCR catalyst based on measured values from the sensor and on values for the mass flow taken during said time interval when the bypass line was connected.

This calculated value is used to calibrate the NOX virtual sensor that outputs a value-representing value upstream of the SCR catalyst.

More specifically, the calculation of the virtual value of the NOX content upstream of the SCR catalyst takes place as follows: Assume, for example, that the flow through the bypass line Fßp is 10% of the total fl Fmï. Then the measured NOX content downstream of the SCR catalyst measured during the connection will be NOX; = 0, 9> virtual, calculated NOX content upstream of the SCR catalyst can be expressed as: 536 '173 9 NOX_U5 = (NOxl - 0.9> <NOX | - 9> If instead, which is more realistic, fl the fate through the bypass line is 1% of the total fl fate, the expression of the NOX content upstream of the SCR catalyst will be: NOX.US = (NOxj - 0,99> <NOX; - 99> The calculation of NOXUS takes place at regular intervals where the time between consecutive calibrations should preferably be as long as possible to reduce emissions of exhaust gases that do not pass the SCR catalyst, but where the calibration must take place often enough so that the calculations are as reliable as possible. According to one embodiment, the bypass line is connected periodically at a certain interval between the connections which may be in the order of a number of seconds and upwards.

The time that the feed connection is connected, ie. the length of the measuring interval, according to one embodiment comprises a predetermined number of dosing periods for the reducing agent, for example -15, which corresponds to 5-15 seconds if the dosing period is 1 second long. However, the engagement need not be synchronized with the timing of injection of the reducing agent.

Of course, the control signal for the injection device also includes control instructions related to the normal function of the injection device, i.e. when the feed coupling is not connected.

The reducing agent is added by the injector 12 by injecting the agent at a dosing frequency F, with a dosing period time L (L = 1 / F) defined as the time between the beginning of two consecutive injections, the injection taking place during an adjustable dosing time t of said dosing period time, where 0 reducing agents are varied by varying the dosing frequency and / or dosing time for the injection of the reducing agent. 536 173 10 It is further possible to vary the amount of reducing agent added by changing the dosing pressure, i.e. the pressure applied to the agent during injection. This can of course be done in combination with a variation of the dosing frequency and / or the dosing time.

The proportion of untreated exhaust gases that reach the NOx sensor 22, when the feed clutch is engaged, will vary with the mass. A larger flow will give a reduced share through the smaller connection, ie. bypass line, e ersom as the back pressure increases more in the smaller passage at larger fl fates. According to one embodiment, when no fate meter is arranged in the bypass line 30, values for FLBP are taken, for example, from a table which is preferably stored in the memory 44, where associated values for FLTOT and the exhaust mass F FBB are stored.

Figure 2 shows graphs showing both measured NOX content or the SCR catalyst (top graph) where the levels also measured NOX values; and NOX; has been marked, partly the dosage of the reducing agent (middle graph), and partly the control signal to the valve device for connection of the bypass line where "O" indicates open valve device and "C" indicates closed valve device (bottom graph).

As mentioned above, the reducing agent is added to the injector 12 by injecting the agent at a dosing frequency F, with a dosing period time L (L = 1 / F) defined as the time between the beginning of two consecutive injections, the injection taking place during an adjustable dosing time t of said dosing period time , where 0 to vary the dosing sequence and / or the dosing time before the injection of the reducing agent. In the example shown in Figure 2, the connection line is connected for three injection periods numbered 1, 2 and 3.

In the graphs, the connection of the bypass line has been synchronized with the injection of reducing agent; however, this is not at all necessary for applying the application.

Thus, by measuring the amplitudes for the NO x content during the switch-on period, and before and after the switch-on period, and taking into account the measured fl fate, 536 173 ll a calculated value for the NOX content upstream of the SCR catalyst can be determined which value i.a. can be used to control the injection device.

During the time period when the bypass connection is switched on, a fl fate will pass the feed bypass where the NOx content is unchanged, which is used to calculate a virtual value for the NOX content in the exhaust gases upstream of the SCR catalyst (NOWS) based on measured values from NOx sensor 22 and on values for the mass taken up during said time interval when the bypass line 30 was connected, and for values taken up in temporal connection with the connection of the bypass line.

The present invention is not limited to the above-described preferred embodiments.

Various alternatives, modifications and equivalents can be used. The above embodiments are therefore not to be construed as limiting the scope of the invention, as defined by the appended claims.

Claims (9)

10 15 20 25 30 536 173 12 Patent claims An exhaust gas aftertreatment system (4) for an internal combustion engine (2), comprising an SCR catalyst (10) (selective catalytic reduction catalyst); a NOX sensor (22) adapted to determine the content of nitrogen oxides (NOX) in exhaust gases flowing out of said SCR catalyst (10) and to emit a measurement signal (NOXUT) depending on the NOx content; an injector (12) adapted to add a reducing agent to the exhaust gases flowing into the SCR catalyst (10), a processing means (42) comprising a memory (44), the processing means (42) being arranged to receive the measurement signal from the NOx sensor (22) and generating a control signal (21) for the injection device (12), and a mass fate meter (24) arranged to measure the mass fate (F LTOT) for the exhaust gases upstream of the SCR catalyst and to emit a mass fate signal (FLTOT) depending on the measured mass fate. , characterized in that the degassing treatment system (4) comprises a bypass line (30) adapted to direct a portion of the total amount of exhaust gases from a position upstream of the SCR catalyst (10) past the SCR catalyst (10) for a predetermined time interval, and that these exhaust gases are returned to the exhaust gases which have passed the SCR catalyst (10) in a position upstream of said NOX sensor (22), the processing means (42) being adapted to store NOX values (NOxi) from NO the x-sensor (22) which is measured when the bypass line (30) is connected to the NOx value (NOxg) which is measured when it is not connected, and which is measured in time in close proximity to the connection, the processing means (42) being adapted to calculate a virtual value (NOXUS) passed the NOx content in the exhaust gases upstream of the SCR catalyst based on NOX1, NOx1, mass flow FLTOT and a desert F LBP through the bypass line when it is switched on. The exhaust gas treatment system according to claim 1, wherein the system comprises a valve device (32) arranged in the bypass line (30), and that the processing means (42) is adapted to emit a switching signal (31) for controlling the connection of the dry bypass line (30) by opening and closing the valve assembly (32). 10 15 20 25 535 '173 13 The exhaust gas treatment system according to claim 1 or 2, wherein said time interval when the bypass is engaged is less than 30 seconds. The exhaust gas treatment system according to any one of claims 1-3, wherein said time interval is shorter than 10 seconds. The exhaust gas treatment system according to any one of claims 1-4, wherein the system comprises a second fate meter (33) adapted to measure said fate through the pre-coupling FLBP. The exhaust gas treatment system according to any one of claims 1-4, wherein said fate through the dry connection FLBP is calculated based on FLTOT and the dimensions of the bypass line. Exhaust gas treatment system according to any one of the preceding claims, wherein said processing means (42) is adapted to generate said control signal in dependence on NOX-U5. Exhaust after-treatment system according to one of Claims 1 to 7, in which the bypass line (30) is connected when the FLTOT has been constant for at least a predetermined time. Exhaust gas treatment system according to any one of claims 1-8, wherein the calculation of NOxm is performed only if the values for NOX; in time before and after the connection of the bypass line are essentially the same.