SE1550554A1

SE1550554A1 - Method pertaining to an SCR system and an SCR system

Info

Publication number: SE1550554A1
Application number: SE1550554A
Authority: SE
Inventors: Fröberg Magnus; Bökelund Björn; Eliassen Torbjörn; Agrell Fredrik
Original assignee: Scania Cv Ab
Priority date: 2015-05-04
Filing date: 2015-05-04
Publication date: 2016-11-05
Also published as: DE102016005262A1; SE540606C2

Abstract

The invention relates to a method for supplying a reducing agent of an SCR-system comprising an SCR-unit (260) and an ammonia slip catalyst (265) arranged downstream of said SCR-unit (260), a reducing agent dosing unit (250) arranged upstream of said SCR-unit. (260), an exhaust gas ammonia content sensor (222) arranged between said SCR-unit (260) and said ammonia slip catalyst (265) and a first NO-sensor (263) arranged downstream of said ammonia slip catalyst (265), comprising the steps of: - determining (s410) a prevailing first NO-content (NOl) of said exhaust gas by means of said first NO-sensor (263); - determining (s420) a setpoint value (Should) of the exhaust gas ammonia content on the basis of said prevailing first NO-content (NOl) of said exhaust gas; - determining (s430) a current value (Am) of the exhaust gas ammonia content; and - controlling (s440) the reducing agent supply on the basis of a difference (Diff) between said setpoint value (Should) and said current value (Am) of the exhaust gas ammonia content.The invention relates also to a computer program product comprising program code (P) for a computer (200; 210) for implementing a method according to the invention. The invention relates also to a system for improved vehicle stability and a vehicle equipped with the system.Figure 2b for publication

Description

Method pertaining to an SCR system and an SCR system TECHNICAL FIELD The present invention relates to a method for supplying a reducing agent of an SCR-system.The invention relates also to a computer program product comprising program code for a computer for implementing a method according to the invention. lt relates also to a systemfor supplying a reducing agent of an SCR-system and a motor vehicle equipped with the SCR system.

BACKGROUN D Vehicles today use, for example, urea as reducing agent in SCR (selective catalytic reduction)systems which comprise an SCR catalyst in which said reducing agent and NOX gas can reactand be converted to nitrogen gas and water. Various types of reducing agents may be used in SCR systems. AdBlue is an example of a commonly used reducing agent.

One type of SCR system comprises a container which holds a reducing agent. The system hasalso a pump adapted to drawing said reducing agent from the container via a suction hoseand supplying it via a pressure hose to a dosing unit situated adjacent to an exhaust systemofthe vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit isadapted to injecting a necessary amount of reducing agent into the exhaust pipe upstreamofthe SCR catalyst according to operating routines which are stored in a control unit of thevehicle. To make it easier to regulate the pressure when there are small or no dosingamounts, the system comprises also a return hose which runs back to the container from a pressure side ofthe system. ln some SCR-systems an ammonia slip catalyst is arranged downstream of said SCR catalyst.Herein supply of said reducing agent may be controlled on the basis of ammonia content ofsaid exhaust gas, which ammonia is originating from said supplied reducing agent. This controlling may be performed according to a rich-lean dosing procedure. lt is however of various reasons difficult to determine an amount of overdosing according to this procedure,e.g. because of inherent inaccuracy of a provided exhaust gas ammonia content sensor. Saidcontrolling of reducing agent supply is thus inaccurate, which may result in inefficient exhaust gas emission control.

EP2684597 relates to an exhaust gas emission control system comprising an SCR-catalyst andan ammonia content sensor being provided between said SCR-catalyst and an ammonia slip catalyst of an exhaust gas system.

SUMMARY OF THE INVENTION An object of the present invention is to propose a novel and advantageous method for supplying a reducing agent of an SCR-system.

Another object ofthe invention is to propose a novel and advantageous system and a novel and advantageous computer program for supplying a reducing agent of an SCR-system.

Yet another object of the invention is to propose a method, a system and a computerprogram which achieve an efficient and accurate control of supplying a reducing agent of an SCR-system.

Yet another object of the invention is to propose a method, a system and a computerprogram for achieving a robust, reliable and cost-effective supply of a reducing agent of an SCR-system.

Yet another object of the invention is to propose an alternative method, an alternativesystem and an alternative computer program for supplying a reducing agent of an SCR- system.

Some of these objects are achieved with a method for supplying a reducing agent of an SCR-system comprising an SCR-unit and an ammonia slip catalyst arranged downstream of saidSCR-unit according to claim 1. Other objects are achieved with a system according to claim 8.

Advantageous embodiments are depicted in the dependent claims. Substantially the same advantages of method steps ofthe innovative method hold true for corresponding means of the innovative system.

According to an aspect of the invention there is provided a method for supplying a reducingagent of an SCR-system arranged for a combustion engine comprising an SCR-unit and anammonia slip catalyst arranged downstream of said SCR-unit, a reducing agent dosing unitarranged upstream of said SCR-unit, an exhaust gas ammonia content sensor arrangedbetween said SCR-unit and said ammonia slip catalyst and a first NOX-sensor arranged downstream of said ammonia slip catalyst, comprising the steps of: - determining a prevailing first NOX-content of said exhaust gas by means of said first NOX-sensor;- determining a setpoint value of the exhaust gas ammonia content on the basis of said prevailing first NOX-content of said exhaust gas;- determining a current value of the exhaust gas ammonia content; and - controlling the reducing agent supply on the basis of a difference between said setpointvalue and said current value of the exhaust gas ammonia content. By determining saidsetpoint value of the exhaust gas ammonia content of said exhaust gas from saidcombustion engine on the basis of said prevailing first NOX-content a reliable, accurate androbust controlling the reducing agent supply is achieved. Further, it is advantageous todetermine said exhaust gas ammonia content between said SCR-catalyst and said ammonia slip catalyst.The method may comprise the steps of:- determining a prevailing exhaust gas flow; - determining said setpoint value of the exhaust gas ammonia content on the basis of saiddetermined prevailing exhaust gas flow. Said prevailing exhaust gas flow may be measuredby means of an exhaust gas mass flow sensor and/or by means of a model forestimating/calculating/modelling said prevailing exhaust gas flow. Said exhaust gas massflow may be determined regarding a position downstream said combustion engine and upstream said SCR-catalyst. Hereby a versatile method is achieved. By determining said setpoint value of the exhaust gas ammonia content of said exhaust gas from saidcombustion engine on the basis of said prevailing exhaust gas flow a reliable, accurate and robust contro||ing the reducing agent supply is achieved.The method may comprise the steps of:- determining a prevailing second NOX-content of said exhaust gas upstream of said SCR-unit; - determining said setpoint value of the exhaust gas ammonia content on the basis of saiddetermined prevailing second NOX-content. By determining said setpoint value of theexhaust gas ammonia content of said exhaust gas from said combustion engine on the basisof said prevailing second NOX-content of said exhaust gas upstream of said SCR-unit a reliable, accurate and robust contro||ing the reducing agent supply is achieved.

By determining said setpoint value of the exhaust gas ammonia content of said exhaust gasfrom said combustion engine on the basis of said prevailing first NOX-content, second NOX-content and said prevailing exhaust gas flow a highly accurate method for contro||ing said supply of reducing agent is achieved.The method may comprise the steps of:- determining a prevailing temperature of said SCR-unit; - determining said setpoint value of the exhaust gas ammonia content on the basis of saiddetermined prevailing temperature of said SCR-unit. Said prevailing temperature of said SCR-unit can be determined directly by means of a temperature sensor. Said prevailingtemperature of said SCR-unit can be determined indirectly by means of a suitable model, e.g.on the basis of information about a prevailing exhaust gas flow and/or a prevailingtemperature of said exhaust gas. This advantageously provides versatility. By determiningsaid setpoint value of the exhaust gas ammonia content of said exhaust gas from saidcombustion engine on the basis of said prevailing temperature of said SCR-unit a reliable, accurate and robust contro||ing the reducing agent supply is achieved.

By determining said setpoint value of the exhaust gas ammonia content of said exhaust gas from said combustion engine on the basis of said prevailing first NOX-content, second NOX- content, said prevailing exhaust gas flow and said prevailing temperature of said SCR-unit a highly accurate method for controlling said supply of reducing agent is achieved.

The method may comprise the step of: - controlling the reducing agent supply by alternating the supply in a specified intervalaround a nominal supply value. Hereby a robust and accurate function of controlling isachieved which also is related to relatively small computational burden of a control unit of the SCR-system.

The method may comprise the step of: - adapting said nominal supply value to said SCR-system by changing said nominal supplyvalue. Hereby a more optimal dosing of said reduction agent is achieved, which in turnprovides numerous advantageous, not the least from an environmental perspective. Herebycontrolling of reduction agent supply advantageously is adapted to the particular SCR-system of relevance.

The method may comprise the step of: - adapting said specified interval for adaption to said difference between said setpoint valueand said current value of the exhaust gas ammonia content if an exhaust gas temperature ofthe SCR-unit exceeds a predetermined temperature level. Hereby a more adequate rich-leandosing procedure is achieved, in particular during operation where said exhaust gas temperature is relative high, such as e.g. more than 400 degrees Celsius.

According to an aspect of the invention there is provided a system for supplying a reducingagent of an SCR-system comprising an SCR-unit and an ammonia slip catalyst arrangeddownstream of said SCR-unit, a reducing agent dosing unit arranged upstream of said SCR-unit, an exhaust gas ammonia content sensor arranged between said SCR-unit and saidammonia slip catalyst and a first NOX-sensor arranged downstream of said ammonia slip catalyst, comprising: - means for determining a prevailing first NOX-content of said exhaust gas; - means for determining a setpoint value of the exhaust gas ammonia content on the basis of said prevailing first NOX-content of said exhaust gas;- means for determining a current value of the exhaust gas ammonia content; and - means for contro||ing the reducing agent supply on the basis of a difference between said setpoint value and said current value of the exhaust gas ammonia content.The system may comprise:- means for determining a prevailing exhaust gas flow; - means for determining said setpoint value ofthe exhaust gas ammonia content on the basis of said determined prevailing exhaust gas flow.The system may comprise: - means for determining a prevailing second NOX-content of said exhaust gas upstream of said SCR-unit; - means for determining said setpoint value ofthe exhaust gas ammonia content on the basis of said determined prevailing second NOX-content.The system may comprise:- means for determining a prevailing temperature of said SCR-unit; - means for determining said setpoint value ofthe exhaust gas ammonia content on the basis of said determined prevailing temperature of said SCR-unit.The system may comprise: - means for contro||ing the reducing agent supply by alternating the supply in a specified interval around a nominal supply value.The system may comprise: - means for adapting said nominal supply value to said SCR-system by changing said nominal supply value.

The system may comprise: - means for adapting said specified interval for adaption to said difference between saidsetpoint value and said current value of the exhaust gas ammonia content if an exhaust gas temperature of the SCR-unit exceeds a predetermined temperature level.

According to an aspect of the invention there is provided a vehicle comprising a systemaccording to what is presented herein. Said vehicle may be a motor vehicle. Said vehicle may be any from among a truck, bus or passenger car.

According to an aspect of the invention there is provided a computer program for controllingsupply a reducing agent of an SCR-system comprising an SCR-unit and an ammonia slipcatalyst arranged downstream of said SCR-unit, wherein said computer program comprisesprogram code for causing an electronic control unit or a computer connected to saidelectronic control unit to perform the steps according to anyone ofthe claims 1-7, when run on said electronic control unit or said computer.

According to an aspect of the invention there is provided a computer program for controllingsupply a reducing agent of an SCR-system comprising an SCR-unit and an ammonia slipcatalyst arranged downstream of said SCR-unit, wherein said computer program comprisesprogram code stored on a computer-readable medium for causing an electronic control unitor a computer connected to said electronic control unit to perform the steps according to anyone of the claims 1-7.

According to an aspect of the invention there is provided a computer program for controllingsupply a reducing agent of an SCR-system comprising an SCR-unit and an ammonia slipcatalyst arranged downstream of said SCR-unit, wherein said computer program comprisesprogram code stored on a computer-readable medium for causing an electronic control unitor a computer connected to said electronic control unit to perform the steps according to anyone of the claims 1-7, when run on said electronic control unit or said computer.

According to an aspect of the invention there is provided a computer program productcontaining a program code stored on a computer-readable medium for performing methodsteps according to anyone of c|aims 1-7, when said computer program is run on an electronic control unit or a computer connected to said electronic control unit.

According to an aspect of the invention there is provided a computer program productcontaining a program code stored non-volatile on a computer-readable medium forperforming method steps according to anyone of c|aims 1-7, when said computer program is run on an electronic control unit or a computer connected to said electronic control unit.

Further objects, advantages and novel features of the present invention will becomeapparent to one skilled in the art from the following details, and also by putting theinvention into practice. Whereas the invention is described below, it should be noted that itis not confined to the specific details described. One skilled in the art having access to theteachings herein will recognise further applications, modifications and incorporations in other fields, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the present invention and its further objects and advantages, thedetailed description set out below should be read in conjunction with the accompanyingdrawings, in which the same reference notations denote similar items in the various diagrams, and in which: Figure 1 schematically illustrates a vehicle according to an embodiment ofthe invention;Figure 2a schematically illustrates a subsystem for the vehicle depicted in Figure 1, accordingto an embodiment of the invention; Figure 2b schematically illustrates a subsystem for the vehicle depicted in Figure 1, accordingto an embodiment of the invention; Figure 3 schematically illustrates a diagram presenting an ammonia setpoint value as a function of tailpipe NOX of a subsystem, according to an embodiment of the invention; Figure 4a is a schematic flowchart of a method according to an embodiment of theinvention; Figure 4b is a more detailed schematic flowchart of a method according to an embodimentofthe invention; and Figure 5 schematically illustrates a computer according to an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 depicts a side view of a vehicle 100. The exemplified vehicle 100 comprises a tractorunit 110 and a trailer 112. The vehicle may be a heavy vehicle, e.g. a truck or a bus. lt may alternatively be a car. lt should be noted that the invention is suitable for application in any SCR system and istherefore not confined to SCR systems of motor vehicles. The innovative method and theinnovative SCR system in one aspect of the invention are well suited to other platformswhich comprise an SCR system than motor vehicles, e.g. watercraft. The watercraft may be of any kind, e.g. motor boats, steamers, ferries or ships.

The innovative method and the innovative SCR system according to one aspect of theinvention are also well suited to, for example, systems which comprise industrial engines and/or engine-powered industrial robots.

The innovative method and the innovative SCR system according to one aspect of theinvention are also well suited to various kinds of power plants, e.g. an electric power plant which comprises an engine-powered generator.

The innovative method and the innovative SCR system are also well suited to any enginesystem which comprises an engine and an SCR system, e.g. on a locomotive or some other platform.

The innovative method and the innovative SCR system are also well suited to any system which comprises a NOX generator.

The term ”link” refers herein to a communication link which may be a physical connectionsuch as an opto-electronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link.

The term ”line” refers herein to a passage for holding and conveying a fluid, e.g. a reductantin liquid form. The line may be a pipe of any desired size and be made of any suitable material, e.g. plastic, rubber or metal.

The term ”reductant” or ”reducing agent” refers herein to an agent used for reacting withcertain emissions in an SCR system. These emissions may for example comprise NOX gas. Theterms ”reductant” and ”reducing agent” are herein used synonymously. ln one version, saidreductant is so-called AdBlue. Other kinds of reductants may of course be used. AdBlue isherein cited as an example of a reductant, but one skilled in the art will appreciate that theinnovative method and the innovative SCR system are feasible with other types ofreductants, subject to necessary adaptations in control algorithms for executing program code in accordance with the innovative method.

Herein said setpoint value may be denoted desired value. Herein said setpoint value may alternatively be denoted target value.

Figure 2a depicts a subsystem 299 of the vehicle 100. The subsystem 299 is situated in thetractor unit 110. lt may be part of an SCR system. lt comprises in this example a container205 arranged to hold a reductant. The container 205 is adapted to contain a suitable amount of reductant and also to being replenishable as necessary.

A first line 271 is adapted to leading the reductant to a pump 230 from the container 205.The pump 230 may be any suitable pump. lt may be adapted to being driven by an electricmotor. lt is adapted to drawing the reductant from the container 205 via the first line 271and supplying it via a second line 272 to a dosing unit 250. The dosing unit 250 may comprisean electrically operated dosing valve by means of which a flow of reductant added to the exhaust system can be controlled. The pump 230 is adapted to pressurising the reductant in 11 the second line 272. Said dosing unit 250 is provided with a throttle unit against which said pressure of the reductant is built up in the subsystem 299.

The dosing unit 250 is arranged to supply said reducing agent to an exhaust gas system (seeFigure 2a) of the vehicle 100. ln particular the dosing unit 250 is arranged to in a controlledway supply a suitable amount of reducing agent to an exhaust gas system of the vehicle 100.According to an example said dosing is performed according to a rich-lean dosing process,involving a determined difference between a setpoint value Should and a current value ofsaid exhaust gas ammonia content. According to this embodiment an SCR-unit 260 (seeFigure 2b) is arranged downstream of a position of the exhaust gas system where injection of reducing agent is achieved by means of the dosing unit 250.

A third line 273 runs between the dosing unit 250 and the container 205. The third line 273is arranged to lead back a certain amount of reducing agent which been feed to the dosingunit 250 to the container 205. With this configuration there is advantageously achievedcooling of the dosing unit 250. ln this way the dosing unit 250 is cooled by means of a flow ofthe reducing agent as this is fed through the dosing unit 250 from the pump 230 to the container 205.

The first control unit 200 is arranged for communication with the pump 230 via a link L292and is adapted to control the operation of the pump 230 in order to for example regulateflows of reductant within the subsystem 299. The first control unit 200 is adapted tocontrolling an operating power of the pump 230 by regulating the electric motor associated with said pump 230.

The first control unit 200 is arranged for communication with the dosing unit 250 via a linkL250 and is adapted to controlling the operation of the dosing unit 250 in order for exampleto regulate the supply of reductant to the exhaust gas system of the vehicle 100. The firstcontrol unit 200 is adapted to controlling the operation of the dosing unit 250 in order for example to regulate the supply of reductant back to the container 205. 12 A second control unit 210 is arranged for communication with the first control unit 200 via alink L210 and may be detachably connected to it. lt may be a control unit external to thevehicle 100. lt may be adapted to conducting the innovative method steps according to theinvention. The second control unit 210 may be arranged to perform the inventive methodsteps according to the invention. lt may be used to cross-load software to the first controlunit 200, particularly software for conducting the innovative method. lt may alternatively bearranged for communication with the first control unit 200 via an internal network on boardthe vehicle. lt may be adapted to performing substantially the same functions as the firstcontrol unit 200, such as controlling the reducing agent supply on the basis of a differencebetween said setpoint value Should and said current value Am ofthe exhaust gas ammoniacontent, which setpoint value Should is determined on the basis of a prevailing tail pipe NOX-content of said exhaust gas. This is depicted in greater detail below. The innovative methodmay be conducted by the first control unit 200 or the second control unit 210, or by both of them.

Figure 2b schematically illustrates a subsystem 289 of the vehicle 100 shown in Figure 1,according to an embodiment of the invention. The subsystem 289 may comprise a part of the inventive SCR system.

An engine 231 is during operation generating an exhaust gas flow which is lead via a firstpassage 235 to an SCR-unit 260. Said engine 231 may be a combustion engine. A secondpassage 245 is arranged to lead exhaust gas from said SCR-unit 260 to an ammonia slip catalyst 265.

Said SCR-system may also comprise a DPF-unit (Diesel Particulate Filter) and/or a DOC-unit (Diesel Oxidation Unit). These units may be arranged at the first passage 235.

The first control unit 200 is arranged for communication with said engine 231 via a link L231.The first control unit 200 is arranged to control operation of the engine 231 according tostored operational routines. The first control unit 200 is arranged to control operation of the dosing unit 250 for injecting reducing agent into the first passage 235. 13 An exhaust gas ammonia content sensor 222 is arranged in the second passage 245. Saidexhaust gas ammonia content sensor 222 is arranged for communication with the firstcontrol unit 200 via a link L222. The exhaust gas ammonia content sensor 222 is arranged tocontinuously or intermittently determine a prevailing exhaust gas ammonia content Am inthe second passage 245. The exhaust gas ammonia content sensor 222 is arranged tocontinuously or intermittently send signals S3 comprising information about the prevailing exhaust gas ammonia content Am to the first control unit 200 via said link L222.

A first NOX sensor 263 is arranged downstream of said ammonia slip catalyst 265 at a thirdpassage 255, which third passage 255 is arranged to lead purified exhaust gas to asurrounding of the subsystem 289. Said first NOX sensor 263 is arranged for communicationwith the first control unit 200 via a link L263. The first NOX sensor 263 is arranged tocontinuously or intermittently determine a prevailing content of NOX in the third passage255. The first NOX sensor 263 is arranged to continuously or intermittently send signals S1comprising information about the prevailing first NOX content NOxl to the first control unit 200 via said link L263.

A second NOX sensor 233 is arranged upstream of said SCR-unit 260 at said first passage 235.Said second NOX sensor 233 is arranged for communication with the first control unit 200 viaa link L233. The second NOX sensor 233 is arranged to continuously or intermittentlydetermine a prevailing content NOX2 in the first passage 235. The second NOX sensor 233 isarranged to continuously or intermittently send signals S2 comprising information about a prevailing content of NOX to the first control unit 200 via the link L233.

The first control unit 200 may according to one embodiment be arranged to by means of astored model calculate a prevailing content of NOX in the first passage 235. The first controlunit 200 is arranged to, on the basis of information about for example into said combustionengine 231 injected amount of fuel, calculate a prevailing content of NOX in said first passage 235.

A sensor (not shown) for measuring a prevailing air mass flow on an inlet side of the engine 231 may be provided. Said air mass flow sensor is arranged to continuously or intermittently 14 determine a prevailing air mass flow MF and continuously or intermittently send signalscomprising information thereof to the first control unit 200 via a suitable link (not shown).Hereby said first control unit 200 is arranged to determine a prevailing exhaust gas flow on the basis of said signals and information about prevailing fuel supply to the engine 231.

The first control unit 200 may according to one embodiment be arranged to by means of astored model calculate a prevailing exhaust gas mass flow MF in the first passage 235. Thefirst control unit 200 is arranged to, on the basis of information about for example operationstate of said combustion engine 231, calculate a prevailing exhaust gas mass flow MF in said first passage 235.

A temperature sensor 221 is arranged upstream of said SCR-unit 260 at said first passage235. Said temperature sensor 221 is arranged for communication with the first control unit200 via a link L221. The temperature sensor 221 is arranged to continuously or intermittentlydetermine a prevailing temperature of the exhaust gas in the first passage 235. Thistemperature corresponds to a prevailing temperature T of said SCR-unit 260. Thetemperature sensor 221 is arranged to continuously or intermittently send signalscomprising information about a prevailing temperature of the exhaust gas to the first controlunit 200 via the link L221. Said first control unit 200 is according to an example arranged todetermine said prevailing temperature T of said SCR-unit 260 on the basis of said prevailingtemperature of the exhaust gas in the first passage 235 and said prevailing exhaust gas flow MF according to a model stored in a memory of said first control unit 200.

According to an example there is provided a temperature sensor (not shown) for measuringa prevailing temperature T of said SCR-unit 260 which sensor is arranged at said SCR-unit260. Said temperature sensor is arranged to continuously or intermittently determine aprevailing prevailing temperature T of said SCR-unit 260 and continuously or intermittentlysend signals comprising information thereof to the first control unit 200 via a suitable link (not shown).

The first control unit 200 may according to an embodiment be arranged to by means of a stored model calculate a prevailing temperature of the exhaust gas in the first passage 235.

The first control unit 200 may be arranged to on the basis of information about for exampleinto said engine 231 injected amount of fuel and exhaust gas mass flow calculate a prevailing temperature of the exhaust gas in the first passage 235.

Said dosing unit 250 is arranged at said first passage 235 for providing said reducing agent into the exhaust gas in the first passage 235.

The first control unit 200 is arranged to determine a setpoint value Should of the exhaust gas ammonia content on the basis of said prevailing first NOX-content NOxl of said exhaust gas.

The first control unit 200 is arranged to control the reducing agent supply on the basis of adifference Diff between said setpoint value Should and said current value Am ofthe exhaust gaS ammOﬂla COﬂteHt.

Hereby said first control unit 200 is arranged to continuously or intermittently determinesaid difference Diff between said setpoint value Should and said current value Am of theexhaust gas ammonia content and control the reducing agent supply to said second passage 245 on the basis thereof. lf said current value Am is larger than said setpoint value Should so called lean dosing bymeans of said dosing unit 250 is performed. This is controlled by means of said first control unit 200. lf said current value Am is smaller than said setpoint value Should so called rich dosing bymeans of said dosing unit 250 is performed. This is controlled by means of said first control unit 200. ln this way a rich-lean dosing process if performed during operation of said engine 231 and said SCR-system.

Hereby a nominal value ANR (Ammonia NOX Ratio) is provided about which dosing is performed. According to an example said ANR value is 1.0. A dosing constant Const is hereby 16 also provided. Said constant Const may be a predetermined value. According to one example said constant Const is 0.15.

This means that when performing rich dosing said first control unit 200 is performing dosingfor achieving an ANR value of 1.15. This means that when performing lean dosing said first control unit 200 is performing dosing for achieving an ANR value of 0.85.

According to an embodiment said nominal value may be adapted according to routinesstored in a memory of said first control unit 200. This means that said nominal value may beset to any suitable value being more relevant for a unique SCR-system and thereto associated engine 231. A nominal value NV may thus for example be set to be 0.95 or 1.2.

According to an embodiment said constant Const may be subject for adaption. This is inparticular relevant when the SCR-unit is relatively warm, e.g. experiencing temperaturesabove e.g. 400 degrees Celsius. Hereby said constant Const may be adapted on the basis ofsaid difference Diff. According to an example said constant Const may be defined as a valuerepresenting said difference Diff multiplied by a suitable constant K wherein said value Diff relates to a difference between said setpoint value Should and said current value Am.

According to one example said ANR is defined by: AN R= NV+/- Diff*K Where NV is a nominal value, e.g. 1.0, and an interval relating to the term Diff*K never can exceed an interval defined by +/-Const.

Figure 3 schematically illustrates a diagram wherein a setpoint value Should of the exhaustgas ammonia content is presented as a function of said first NOX-content NOX1 of saidexhaust gas. Said setpoint value Should is given in ppm (parts per million). Said first NOX- content NOX1 is given in ppm. 17 According to this example four graphs A, B, C and D are presented. Said four graphs are predetermined graphs.

Said graph A is associated with a relatively low exhaust gas mass flow MF and a relatively lowsecond NOX-content NOXZ of said exhaust gas. Hereby said graph A is associated with relatively low levels of said setpoint value Should of the exhaust gas ammonia content.

Said graph B is associated with a relatively low exhaust gas mass flow MF and a relativelyhigh second NOX-content NOXZ of said exhaust gas. Hereby said graph B is associated withrelatively low levels of said setpoint value Should of the exhaust gas ammonia content, however higher levels than said graph A.

Said graph C is associated with a high exhaust gas mass flow MF and a relatively low secondNOX-content NOXZ of said exhaust gas. Hereby said graph C is associated with relatively highlevels of said setpoint value Should of the exhaust gas ammonia content, and higher levels than said graphs A and B.

Said graph D is associated with a high exhaust gas mass flow MF and a relatively high secondNOX-content NOXZ of said exhaust gas. Hereby said graph D is associated with relatively highlevels of said setpoint value Should of the exhaust gas ammonia content, and higher levels than said graph C.

The diagram presented with reference to Figure 3 hereby schematically depicts thecorrelation between said setpoint value Should of the exhaust gas ammonia content and the parameters exhaust gas mass flow MF and second NOX-content NOXZ.

Said diagram may comprise a number of relevant graphs relating to various values of saidexhaust gas mass flow MF and second NOX-content NOXZ, respectively, as functions of aprevailing first NOX-content NOxl of said exhaust gas. Also graphs relating to also theprevailing temperature T ofthe SCR-unit 260 may be provided according to an embodiment.

Herein a number of graphs may be presented depending upon prevailing values of said 18 exhaust gas mass flow I\/IF, second NOX-content NOX2 and temperature T ofthe SCR-unit 260 as a function of a prevailing first NOX-content NOxl of said exhaust gas.

Thus by measuring said prevailing first NOX-content NOxl of said exhaust gas and payingregard to at least one of said parameters exhaust gas mass flow I\/IF, second NOX-contentNOX2 and temperature T of the SCR-unit 260 said setpoint value Should of the exhaust gas ammonia content may be determined according to an aspect of the invention.

Figure 4a schematically illustrates a flow chart of a method for supplying a reducing agent ofan SCR-system comprising an SCR-unit 260 and an ammonia slip catalyst 265 arrangeddownstream of said SCR-unit 260, a reducing agent dosing unit 250 arranged upstream ofsaid SCR-unit 260, an exhaust gas ammonia content sensor 222 arranged between said SCR-unit 260 and said ammonia slip catalyst 265 and a first NOX-sensor 263 arrangeddownstream of said ammonia slip catalyst 265. The method comprises a method step s401.

Said method step s401 comprises the steps of: - determining a prevailing first NOX-content NOxl of said exhaust gas by means of said firstNOX-sensor 263;- determining a setpoint value Should of the exhaust gas ammonia content on the basis of said prevailing first NOX-content NOxl of said exhaust gas; - determining a current value Am of the exhaust gas ammonia content; and - controlling the reducing agent supply on the basis of a difference Diff between saidsetpoint value Should and said current value Am of the exhaust gas ammonia content. After the method step s401 the method ends.

Figure 4b schematically illustrates a flow chart of a method for supplying a reducing agent ofan SCR-system comprising an SCR-unit 260 and an ammonia slip catalyst 265 arrangeddownstream of said SCR-unit 260, a reducing agent dosing unit 250 arranged upstream of said SCR-unit 260, an exhaust gas ammonia content sensor 222 arranged between said SCR- 19 unit 260 and said ammonia slip catalyst 265 and a first NOX-sensor 263 arranged downstream of said ammonia slip catalyst 265. The method comprises a method step s410.

The method step s410 comprises the method step of determining a prevailing first NOX-content NOxl of said exhaust gas. Said first NOX-content NOxl is a prevailing NOX-content ofthe exhaust gas in the third passage 255. Said first NOX-content NOxl may also be referred toas a tail pipe NOX-value. This may be performed by means of said first NOX-sensor 263. Saidfirst NOX-content NOxl of said exhaust gas may be determined intermittently or continuously. After the method step s410 a subsequent method step s420 is performed.

The method step s420 comprises the step of determining a setpoint value Should of theexhaust gas ammonia content on the basis of said prevailing first NOX-content NOxl of saidexhaust gas. This may be performed by means of said exhaust gas ammonia content sensor 222.The method step s420 may comprise the steps of:- determining a prevailing exhaust gas flow I\/IF; - determining said setpoint value Should of the exhaust gas ammonia content on the basis ofsaid determined prevailing exhaust gas flow I\/IF. Said prevailing exhaust gas flow MF may bedetermined by measurement by means of an air mass flow sensor. Said prevailing exhaustgas flow MF may be determined by means of a model stored in a memory ofthe first controlunit 200. Said setpoint value Should ofthe exhaust gas ammonia content may according toan embodiment be determined on the basis of said prevailing first NOX-content NOxl of said exhaust gas and said determined prevailing exhaust gas flow I\/IF.

The method step s420 may comprise the steps of: - determining a prevailing second NOX-content NOX2 of said exhaust gas upstream of said SCR-unit; - determining said setpoint value Should of the exhaust gas ammonia content on the basis ofsaid determined prevailing second NOX-content NOX2. Said prevailing second NOX-contentNOX2 may be determined by measurement by means of said second NOX-sensor 233 beingarranged at the first passage 235. Said prevailing second NOX-content NOX2 may bedetermined by means of a model stored in a memory of the first control unit 200. Saidsetpoint value Should of the exhaust gas ammonia content may according to anembodiment be determined on the basis of said prevailing first NOX-content NOxl of saidexhaust gas, said determined prevailing exhaust gas flow I\/IF and said second NOX-contentNOX2. Said setpoint value Should of the exhaust gas ammonia content may according to anembodiment be determined on the basis of said prevailing first NOX-content NOxl of saidexhaust gas and said second NOX-content NOX2. Said prevailing first NOX-content NOxl of said exhaust gas may be determined continuously or intermittently.

The method step s420 may comprise the steps of: - determining a prevailing temperature T of said SCR-unit 260; - determining said setpoint value Should of the exhaust gas ammonia content on the basis ofsaid determined prevailing temperature T of said SCR-unit 260. Said prevailing temperatureT of said SCR-unit 260 may be determined on the basis of measurement by means of saidtemperature sensor 221 being arranged at the first passage 235. Said prevailing temperatureT of said SCR-unit 260 may be determined by means of a temperature sensor being arrangedat said SCR-unit 260. Said prevailing temperature T of said SCR-unit 260 may be determinedby means of a model stored in a memory ofthe first control unit 200. Said setpoint valueShould of the exhaust gas ammonia content may according to an embodiment bedetermined on the basis of said prevailing first NOX-content NOxl of said exhaust gas, saiddetermined prevailing exhaust gas flow I\/IF, said second NOX-content NOX2 and saidprevailing temperature T of said SCR-unit 260. Said setpoint value Should of the exhaust gasammonia content may according to an embodiment be determined on the basis of saidprevailing first NOX-content NOxl of said exhaust gas and said prevailing temperature T ofsaid SCR-unit 260. Said prevailing temperature T of said SCR-unit 260 may be determined continuously or intermittently. 21 After the method step s420 a subsequent method step s430 is performed.

The method step s430 comprises the step of determining a current value Am of the exhaustgas ammonia content. This may be performed by means of said exhaust gas ammoniacontent sensor 222 being arranged in said second passage 245. Said current value Am of the exhaust gas ammonia content may be determined continuously or intermittently.

The method step s430 may comprise the step of determining a difference between saidsetpoint value Should and said current value Am of the exhaust gas ammonia content. Thismay be performed continuously or intermittently. This may be performed by means of said first control unit 200.

After the method step s430 a subsequent method step s440 is performed.

The method step s440 comprises the step of contro||ing the reducing agent supply on thebasis of a difference between said setpoint value and said current value of the exhaust gasammonia content. This may be performed by means of said first control unit 200. Saidcontro||ing of the reducing agent supply to the first passage 235 is according to anembodiment based on a lean-rich dosing principle. Herein said contro||ing of the reducingagent supply is performed by alternating the supply in a specified interval around a nominalsupply value. Said nominal supply value may be a predetermined value, also referred to asANR. Said predetermined nominal value may be 1.0. Said specified interval may be a predetermined interval.

According to an example said nominal specified value may be adapted to operation of saidengine 231 and SCR-system. Also, during operation of said SCR-system also said specified interval around said predetermined nominal value may be adapted.

After the method step s440 the method ends. 22 Figure 5 is a diagram of one version of a device 500. The control units 200 and 210 describedwith reference to Figure 2b may in one version comprise the device 500. The device 500comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory550. The non-volatile memory 520 has a first memory element 530 in which a computerprogram, e.g. an operating system, is stored for contro||ing the function of the device 500.The device 500 further comprises a bus controller, a serial communication port, I/O means,an A/D converter, a time and date input and transfer unit, an event counter and aninterruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

The computer program comprises routines for contro||ing a supply process of a reducingagent of an SCR-system comprising an SCR-unit 260 and an ammonia slip catalyst 265arranged downstream of said SCR-unit 260, a reducing agent dosing unit 250 arrangedupstream of said SCR-unit 260, an exhaust gas ammonia content sensor 222 arrangedbetween said SCR-unit 260 and said ammonia slip catalyst 265 and a first NOX-sensor 263 arranged downstream of said ammonia slip catalyst 265.

The computer program P comprises routines for determining a prevailing first NOX-contentNOxl of said exhaust gas by means of said first NOX-sensor 263. The computer program Pcomprises routines for determining a setpoint value Should of the exhaust gas ammoniacontent on the basis of said prevailing first NOX-content NOxl of said exhaust gas. Thecomputer program P comprises routines for determining a current value Am of the exhaustgas ammonia content. The computer program P comprises routines for contro||ing thereducing agent supply on the basis of a difference Diff between said setpoint value Should and said current value Am of the exhaust gas ammonia content.

The computer program P may comprise routines for determining a prevailing exhaust gasflow I\/IF and determining said setpoint value Should of the exhaust gas ammonia content on the basis of said determined prevailing exhaust gas flow I\/IF.

The computer program P may comprise routines for determining a prevailing second NOX- content NOX2 of said exhaust gas upstream of said SCR-unit 260 and determining said 23 setpoint value Should of the exhaust gas ammonia content on the basis of said determined prevailing second NOX-content NOXZ.

The computer program P may comprise routines for determining a prevailing temperature Tof said SCR-unit 260 and determining said setpoint value Should of the exhaust gas ammonia content on the basis of said determined prevailing temperature T of said SCR-unit.

The computer program P may comprise routines for contro||ing the reducing agent supply by alternating the supply in a specified interval around a nominal supply value NV.

The computer program P may comprise routines for adapting said nominal supply value to said SCR-system by changing said nominal supply value NV.

The computer program P may comprise routines for adapting said specified interval foradaption to said difference between said setpoint value and said current value of theexhaust gas ammonia content if an exhaust gas temperature in the SCR-system exceeds a predetermined temperature level, e.g. 350 or 400 degrees Celsius.

The program P may be stored in an executable form or in compressed form in a memory 560 and/or in a read/write memory 550.

Where it is stated that the data processing unit 510 performs a certain function, it meansthat it conducts a certain part of the program which is stored in the memory 560 or a certain part of the program which is stored in the read/write memory 550.

The data processing device 510 can communicate with a data port 599 via a data bus 515.The non-volatile memory 520 is intended for communication with the data processing unit510 via a data bus 512. The separate memory 560 is intended to communicate with the dataprocessing unit via a data bus 511. The read/write memory 550 is arranged to communicatewith the data processing unit 510 via a data bus 514. The links L210, L220, L221, L222, L231,L233, L250, L263 and L292, for example, may be connected to the data port 599 (see Figure2a and Figure 2b). 24 When data are received on the data port 599, they are stored temporarily in the secondmemory element 540. When input data received have been temporarily stored, the dataprocessing unit 510 will be prepared to conduct code execution as described above.According to one embodiment signals received on the data port 599 comprise informationabout a prevailing first NOX-content NOxl, a prevailing second NOX-content NOXZ, aprevailing exhaust gas ammonia content Am, a prevailing air mass flow into said engine 231,a prevailing temperature of said exhaust gas and/or a prevailing temperature T of said SCR- unit 260.The signals received on the data port 299 may be used by the device 500 for: - determining a setpoint value Should of the exhaust gas ammonia content on the basis of said prevailing first NOX-content NOxl of said exhaust gas;- determining a current value Am of the exhaust gas ammonia content; and - controlling the reducing agent supply on the basis of a difference Diff between said setpoint value Should and said current value Am of the exhaust gas ammonia content.

Parts of the methods herein described may be conducted by the device 500 by means of thedata processing unit 510 which runs the program stored in the memory 560 or theread/write memory 550. When the device 500 runs the program, methods herein described are executed.

The foregoing description of the preferred embodiments of the present invention isprovided for illustrative and descriptive purposes. lt is not intended to be exhaustive, nor tolimit the invention to the variants described. Many modifications and variations willobviously suggest themselves to one skilled in the art. The embodiments have been chosenand described in order to best explain the principles of the invention and their practicalapplications and thereby make it possible for one skilled in the art to understand theinvention for different embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for supplying a reducing agent of an SCR-system comprising an SCR-unit (260)and an ammonia slip catalyst (265) arranged downstream of said SCR-unit (260), a reducingagent dosing unit (250) arranged upstream of said SCR-unit (260), an exhaust gas ammoniacontent sensor (222) arranged between said SCR-unit (260) and said ammonia slip catalyst(265) and a first NOX-sensor (263) arranged downstream of said ammonia slip catalyst (265), comprising the step of: - determining (s410) a prevailing first NOX-content (NOxl) of said exhaust gas by means ofsaid first NOX-sensor (263), characterized by the steps of: - determining (s420) a setpoint value (Should) ofthe exhaust gas ammonia content on the basis of said prevailing first NOX-content (NOxl) of said exhaust gas;- determining (s430) a current value (Am) of the exhaust gas ammonia content; and - contro||ing (s440) the reducing agent supply on the basis of a difference (Diff) between said setpoint value (Should) and said current value (Am) of the exhaust gas ammonia content.

2. The method according to claim 1, comprising the steps of:- determining (s420) a prevailing exhaust gas flow (I\/|F); - determining (s420) said setpoint value (Should) of the exhaust gas ammonia content on the basis of said determined prevailing exhaust gas flow (I\/IF).

3. The method according to claim 1 or 2, comprising the steps of: - determining (s420) a prevailing second NOX-content (NOX2) of said exhaust gas upstream of said scR-unit (26o); 26 - determining (s420) said setpoint value (Should) of the exhaust gas ammonia content on the basis of said determined prevailing second NOX-content (NOX2).

4. The method according to anyone of claims 1-3, comprising the steps of: - determining (s420) a prevailing temperature (T) of said SCR-unit (260); - determining (s420) said setpoint value (Should) of the exhaust gas ammonia content on the basis of said determined prevailing temperature (T) of said SCR-unit (260).

5. The method according to anyone of claims 1-4, comprising the step of: - contro||ing (s440) the reducing agent supply by alternating the supply in a specified interval around a nominal supply value.

6. The method according to claim 5, comprising the step of: - adapting said nominal supply value to said SCR-system by changing said nominal supply value.

7. The method according to claim 5 or 6, comprising the step of: - adapting said specified interval for adaption to said difference (Diff) between said setpointvalue (Should) and said current value (Am) of the exhaust gas ammonia content if an exhaust gas temperature (T) of the SCR-unit (260) exceeds a predetermined temperature level.

8. A system for supplying a reducing agent of an SCR-system comprising an SCR-unit (260)and an ammonia slip catalyst (265) arranged downstream of said SCR-unit (260), a reducingagent dosing unit (250) arranged upstream of said SCR-unit (260), an exhaust gas ammonia content sensor (222) arranged between said SCR-unit (260) and said ammonia slip catalyst 27 (265) and a first NOX-sensor (263) arranged downstream of said ammonia slip catalyst (265), comprising: - means (200; 210; 500; 263) for determining a prevailing first NOX-content (NOX1) of saidexhaust gas, characterized by: - means (200; 210; 500) for determining a setpoint value (Should) ofthe exhaust gas ammonia content on the basis of said prevailing first NOX-content (NOX1) of said exhaust gas; - means (200; 210; 500; 222) for determining a current value (Am) ofthe exhaust gas ammonia content; and - means (200; 210; 500; 250) for contro||ing the reducing agent supply on the basis of adifference (Diff) between said setpoint value (Should) and said current value (Am) of the exhaust gas ammonia content.

9. The system according to claim 8, comprising: - means (200; 210; 500) for determining a prevailing exhaust gas flow (I\/|F); - means (200; 210; 500) for determining said setpoint value (Should) ofthe exhaust gas ammonia content on the basis of said determined prevailing exhaust gas flow (I\/IF).

10. The system according to claim 8 or 9, comprising: - means (200; 210; 500; 233) for determining a prevailing second NOX-content (NOX2) of said exhaust gas upstream of said SCR-unit (260); - means (200; 210; 500) for determining said setpoint value (Should) ofthe exhaust gas ammonia content on the basis of said determined prevailing second NOX-content (NOX2).

11. The system according to anyone of claims 8-10, comprising: 28 - means (200; 210; 500; 221)for determining a prevailing temperature (T) of said SCR-unit(260); - means (200; 210; 500) for determining said setpoint value (Should) ofthe exhaust gasammonia content on the basis of said determined prevailing temperature (T) of said SCR- unn(z6o)

12. The system according to anyone of claims 8-11, comprising: - means (200; 210; 500; 250) for controlling the reducing agent supply by alternating the supply in a specified interval around a nominal supply value.

13. The system according to claim 12, comprising: - means (200; 210; 500) for adapting said nominal supply value to said SCR-system by changing said nominal supply value.

14. The system according to claim 12 or 13, comprising: - means (200; 210; 500) for adapting said specified interval for adaption to said difference(Diff) between said setpoint value (Should) and said current value (Am) of the exhaust gasammonia content if an exhaust gas temperature (T) of the SCR-unit (260) exceeds a predetermined temperature level.

15. I\/|otor vehicle (100; 110) comprising a system according to any of claims 8-14.

16. I\/|otor vehicle (100; 110) according to claim 15, wherein said motor vehicle is any of a truck, bus or passenger car. 29

17. A computer program (P) pertaining to a for supplying a reducing agent of an SCR-system,wherein said computer program (P) comprises a program code for causing an electroniccontrol unit (200; 500) or a computer (210; 500) connected to said electronic control unit (200; 500) to perform the steps according to anyone of claims 1-7.

18. A computer program product comprising a program code stored on a computer readablemedium for performing method steps according to anyone of claims 1-7, when said programcode is run on an electronic control unit (210; 500) or a computer (210; 500) connected to said electronic control unit (200; 500).