Abstract:
An internal combustion engine operated with excess air has an exhaust gas after-treatment device wherein nitrogen oxides in the exhaust gas are reduced by selective catalytic reduction. A pump is connected between a reducing agent container and a metering device for controlled feeding of a reducing agent to the exhaust gas upstream of an SCR catalyst. A pressure accumulator with an associated pressure sensor for intermediately storing the reducing agent is connected between the pump and the metering device. The pump delivers only so much reducing agent into the pressure accumulator as is being used in the exhaust gas after-treatment.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention lies in the automotive technology field. Specifically, the invention relates to an exhaust gas after-treatment process and device for an internal combustion engine equipped with an SCR catalyst. The system is applicable to internal combustion engines operating with excess air, in particular diesel internal combustion engines, using the selective catalytic reduction of nitrogen oxides from the exhaust gas. A reducing agent is delivered from a reducing agent container by a pump and, for specific operating states of the internal combustion engine, is apportioned to the exhaust gas of the internal combustion engine upstream of an SCR catalyst via a metering device. 
     Nitrogen oxide emissions from an internal combustion engine operating with excess air, in particular a diesel internal combustion engine, can be reduced with the aid of selective catalytic reduction (SCR) technology to form atmospheric nitrogen (N 2 ) and water vapor (H 2 O). The reducing agent used is either ammonia gas (NH 3 ), ammonia in aqueous solution, or urea in aqueous solution. The urea is used in this case as an ammonia carrier and is injected with the aid of a metering system upstream of a hydrolysis catalyst into the exhaust system, and converted there by means of hydrolysis into ammonia which then in turn reduces the nitrogen oxides in the actual SCR or DeNO x  catalyst. 
     In prior art metering systems, pumps produce the required injection pressure. The pressure is maintained constant by a pressure regulator. In order to meter the reducing agent, electromagnetically actuated valves, as are known for fuel injection, are used. The quantities of reducing agent to be metered in proportion to the distance traveled correspond to a few percent of the relevant quantity of fuel. In other words, the amounts to be delivered by the pump are relatively small. In known metering systems, the electrically driven pump delivers the reducing agent via a pressure regulator in the loop. Using these pumps for the necessary pressure range, many times the required quantity of reducing agent is delivered, and available pressure regulators require, in order to function according to specification, delivered quantities which are many times the metered quantity. The pressure regulator limits the pressure to a constant value, so that the metered quantity can be controlled through a valve opening time dictated by a control device. A reducing agent circuit of this type has the disadvantage that the continuously running pump consumes an unnecessarily large amount of drive energy and must be designed for long life. 
     Further, in urea SCR systems of this type, it is necessary to monitor the metered quantities of reducing agent since, on the one hand, conventional gas sensors are too inaccurate to reliably record metering errors and, on the other hand, the storage capacity of the catalyst would delay the detection of an error in metering based solely on exhaust gas measurement. In a continuously running pump which provides a constant pressure by virtue of a pressure regulator, the metered quantity cannot be measured, since the quantity flowing through the pressure regulator back to the storage container for the reducing agent is many times the metered quantity. A further problem with such metering devices consists in the fact that the metering accuracy of available cost-effective solenoid valves decreases, as the drive times become small, to such an extent that it is not possible to meter with enough resolution in particular operating states. 
     German utility model DE 297 08 591 U1 discloses a device for feeding ammonia to the exhaust gas flow of a combustion engine. There, a heatable pressure-proof converter, which contains a thermolytically ammonia-releasing substance, or a thermolytically ammonia-releasing mixture of substances, is used as the ammonia source. An ammonia accumulator for intermediate storage of ammonia released from the material by supplying heat is connected upstream of the metering device for the ammonia. The metering device receives control signals from a control unit which processes the data characteristic of the running of the engine, from which it determines the NO x  emission. 
     2.Summary of the Invention 
     It is accordingly an object of the invention to provide an exhaust gas after-treatment process and device for an internal combustion engine equipped with an SCR catalyst, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows metering small quantities of reducing agent and, at the same time, presents an opportunity to check the metering system in terms of its functional integrity. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method of after-treating exhaust gas of an internal combustion engine operating with excess air, in particular a diesel engine, wherein nitrogen oxides in the exhaust gas are reduced by selective catalytic reduction. The method comprises the following steps: 
     pumping a reducing agent from a reducing agent container into a pressure accumulator; 
     delivering, during specific operating states of an internal combustion engine, quantities of the reducing agent to an exhaust gas of the internal combustion engine upstream of an SCR catalyst, by opening a metering device and taking a quantity of reducing agent currently required for exhaust gas after-treatment from the pressure accumulator; and 
     wherein the pumping step comprises delivering only a quantity of the reducing agent required by the metering to the pressure accumulator. 
     In accordance with an added feature of the invention, a pressure in the pressure accumulator is determined with a pressure sensor and a metered amount of reducing agent is deduced from a pressure difference before and after a respective metering phase. 
     In accordance with an additional feature of the invention, the metered amount of reducing agent is evaluated from a characteristic curve in dependence on the measured pressure values. In a preferred embodiment, the characteristic curve is stored in a memory of a control unit for exhaust gas after-treatment. 
     In accordance with another feature of the invention, the metering system is checked for leaktightness during pauses between metering in which no reducing agent is added. 
     In accordance with a further feature of the invention, after an end of a metering pulse, a pressure is measured in the pressure accumulator, and, after a predetermined period of time has elapsed, it is determined whether the pressure in the pressure accumulator has fallen below a predetermined limit value, and if the pressure has fallen below the limit value, the metering system is classified as not being leaktight. 
     In accordance with again an added feature of the invention, a pressure in the pressure accumulator is sensed with a pressure sensor, and, when the pressure falls below a lower pressure threshold value, the pump is turned on, and when the pressure exceeds an upper pressure threshold value, the pump is turned off. 
     In accordance with again an additional feature of the invention, a drive time of the pump is measured during which the pressure lies between the lower and upper pressure threshold values and the drive time is used as a criterion for a functionality of the pump. 
     In accordance with again another feature of the invention, a charging and discharging time of the pressure accumulator is measured and the charging and discharging time is used as a criterion for the functionality of the pressure accumulator. 
     In accordance with a preferred embodiment of the invention, the pressure accumulator used in the method is a flexible line connecting the pump to the metering device. 
     With the above and other objects in view there is provided, in accordance with the invention, an exhaust gas after-treatment device for an internal combustion engine operating with excess air, in particular a diesel engine, wherein nitrogen oxides in the exhaust gas are reduced by selective catalytic reduction. The novel device comprises: 
     a reducing agent container; 
     a metering device for controlled feeding of a reducing agent to an exhaust gas of an internal combustion engine upstream of an SCR catalyst in an exhaust gas flow direction; 
     a pump communicating with the reducing agent container for pumping reducing agent from the reducing agent container; and 
     a pressure accumulator in form of a flexible line connected between the pump and the metering device for intermediately storing the reducing agent, and a pressure sensor operatively associated with the pressure accumulator. 
     In other words, the novel system includes the following main components: control unit, pump, pressure accumulator, pressure sensor and metering valve. The pump is turned on by the control computer and delivers reducing agent into a pressure accumulator. By opening the metering valve, it is possible to inject the reducing agent into the exhaust of the internal combustion engine. According to the invention, the pump delivers in this case only as much reducing agent solution as is used up for the exhaust gas after-treatment. This results, on the one hand, in very short pump drive times, which considerably simplifies the design of the pump motor in terms of heating and, on the other hand, in very short total operating times, that is to say reduced requirements in terms of service life. In addition to the savings in the pump design, the weight of the system is also reduced by the omission of the pressure regulator, and the electrical power consumption is reduced. 
     From the pressure drop during and after a metering phase, it is possible to evaluate the quantity of reducing agent metered. Inaccuracies in the valve drive or in the valve throughput do not therefore affect the metering accuracy. If the pressure falls below a predetermined minimum pressure, the pump is restarted and the pressure is built up again. The quantity of reducing agent metered may advantageously, if appropriate sensors are present, also be evaluated from the displacement of a pressure accumulator diaphragm or the number of displacements with a predetermined volume. 
     Because of the capacity for storing the reducing agent in the SCR catalyst, metering pulses may be dispensed with during the pressure build-up phase in order not to impair the accuracy of the quantitative calculation for the reducing agent. Through this pumping and metering strategy, sufficiently accurate metering of the reducing agent can be achieved without additional sensors. 
     Since the quantity of reducing agent metered is determined through the pressure accumulator, the metering valve can be operated with very short drive pulses and at a high frequency, without entailing disadvantages in the accuracy of the metering. 
     The system according to the invention also offers extensive self-diagnosis features. By measuring the charging time of the pressure accumulator or the rate at which the pressure rises, changes in the pump characteristics can be detected. In the metering phase, by comparing the cumulative valve drive times with the pressure decrease or the pressure accumulator displacements, changes in the functioning of the valve can be detected. By checking the charging and discharging times, errors in the functioning of the pressure accumulator can be ascertained. These types of errors include, on the one hand, changes in the spring constant due to the spring breaking in sprung diaphragms or pistons or, on the other hand, loss of gas if a diaphragm supported by a gas volume is used. Further, changes in the storage capacity due to deformation of the diaphragm or impeded pistons can be detected. If neither the pump nor the metering valve are being driven, the pressure in the system must remain constant. A drop in pressure indicates a leak in the system. The errors picked up may be stored in an error memory and/or communicated to the driver acoustically or optically. 
     Without loss of metering accuracy, the system allows choice of the injection pressure level. For optimum distribution of the reducing agent in the exhaust gas, the injection profile of the metering valve can be matched to the engine operating states (speed, load, exhaust gas back pressure) by varying the pressure. 
     The quantity that is metered in is defined no longer through the valve opening time but through the pressure profile or the number or magnitude of the pressure accumulator displacements. In this case, use is made of the fact that, in the case of a urea SCR system, it is not the quantity per metering pulse but only the integral envelope or profile which needs to be precise. The metering pulses do not, in contrast to fuel metering, need to be synchronized with the crankshaft, i.e. the accumulator charging phase in which metering should not be carried out, for example in order not to lose accuracy, does not place any restriction on the functionality of the system. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an exhaust gas after-treatment process and device for an internal combustion engine equipped with an SCR catalyst, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a first exemplary embodiment of an NO x  reduction system with a pressure accumulator and a separate pump; and 
     FIG. 2 is a block diagram of a second exemplary embodiment in which the pressure accumulator and the pump are integrated to form a single component. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be understood by those of skill in the pertinent art that the two block diagrams of the NO x  reduction systems in the drawing include only those components which are necessary for an understanding of the invention. In particular, in both cases, the heating device with associated temperature sensors, as well as the device for measuring the filling level, have been omitted in relation to the reducing agent container. 
     Referring now FIG. 1 in detail there is seen a reducing agent container  10  from which reducing agent, for example urea dissolved in water, is delivered by means of an electrically driven reducing agent pump, referred to below as pump  11  for the sake of simplicity. The intake side of the pump  11  is connected via a delivery line  12  to the reducing agent container  10 . From the output side of the pump  11 , a feedline  13  leads to a metering valve  14 . The metering valve  14  is arranged in an exhaust pipe  15 , upstream of an SCR catalyst  16  in the exhaust gas flow direction (indicated by the arrows). The nozzle opening of the metering valve  14  protrudes into the exhaust pipe  15 . 
     The feedline  13  contains a pressure accumulator  17  which, for example, has a sprung diaphragm  171 . Further, a pressure sensor  18  is arranged in the feedline  13 , downstream of the pressure accumulator  17  as viewed in the direction in which the reducing agent is delivered. The pressure sensor  18 , the pump  11  and the metering valve  14  are respectively connected, via electrical lines (not indicated in further detail) to a control unit  19  which controls the addition of the reducing agent to the exhaust  15 . For this purpose, the control unit  19  has an electronic computer unit  22  or processor  22  which also performs error detection and diagnostic routines. 
     The control unit  19  is connected via a serial interface  20  (for example a CAN BUS) to a control unit  21  of the internal combustion engine and/or to other control units. Via this interface, data such as, for example, engine speed, control movement and boost air temperature are transmitted to the control unit  19 . Further, the control unit  19  contains various functional units for driving the actuators (reducing agent heater, pump) and for evaluating the signals delivered by the sensors (filling level sensor, temperature of the reducing agent, pressure in the pressure accumulator or in the feedline). For this purpose, the control unit  19  has, amongst other things, a memory  23  which stores a plurality of characteristic curves or characteristics, as well as threshold values, whose significance will be explained in more detail. 
     In the first illustrative embodiment, the pressure accumulator  17  and the pump  11  form separate functional units, although these may also be combined with the pressure sensor  18  to form a single module in order to decrease the costs for the housing parts and lines and to reduce the assembly outlay. 
     It is also possible to integrate the functions of the control unit  19  for the reducing agent metering system into the control unit  21  of the internal combustion engine. 
     The functional operation of the system is as follows: After the pump  11  has been triggered using appropriate drive signals from the control unit  19 , reducing agent is delivered from the reducing agent container  10  and pumped into the pressure accumulator  17  and into the feedline  13 . The sprung (spring-loaded) diaphragm  171  of the pressure accumulator  17  is deflected in the direction denoted by an arrow symbol, against the force of a spring  26 . Using the pressure sensor  18 , the pressure is measured and compared with an upper threshold value stored in the memory  23 . If the currently measured pressure value exceeds the predetermined threshold value (for example 2-5 bar), then the pump  11  is turned off. If, because of a plurality of brief injection pulses for the metering valve  14 , the pressure falls below a predetermined lower threshold value, then the pump  11  is turned on again. The required drive time of the pump  11  is measured in order to detect errors (for example inadequate pump power) in the pressure system. 
     Both the lower and the upper threshold values for the pressure can be freely set by the computer unit  22  of the control unit  19 , that is to say optimally adjusted for the respective state of the internal combustion engine or the instantaneous exhaust gas temperature. 
     In the first illustrative embodiment, the quantity of reducing agent metered may advantageously be determined using a characteristic curve for the pressure accumulator volume as a function of the pressure. For this purpose, the pressure is measured by means of the pressure sensor  18  before and after a series of metering pulses. 
     Through simple table access, the volume difference, that is to say the metered quantity of reducing agent, can then be determined. For this purpose, the memory  23  stores a characteristic curve which represents the relationship between pressure and pressure accumulator volume. The input variable is in this case the pressure recorded by the pressure sensor  18 . If the memory  23  of the control unit  19  also stores a characteristic curve for the throughput of the metering valve as a function of pressure, the metering valve can be checked for the correct throughput. In pauses between metering, for example if the exhaust gas temperature is too low and consequently no reducing agent is being added, the system can be checked for leaktightness. To do this, the pressure directly after the end of a metering pulse is recorded and, after a particular time has elapsed, a check is made as to whether the pressure has fallen below a predetermined limit value. If this is the case, it is concluded that there is a sealing fault in the metering system. This may be due to a leak in the reducing agent container, the lines or the pressure accumulator, or because the metering valve no longer closes fully, so that there is a residual flow. The result of the diagnosis may be communicated to the driver of the motor vehicle equipped with the internal combustion engine acoustically and/or optically by means of a display device  24  which is activated by corresponding signals from the control unit  19 . A further option is to enter the error in an error memory and/or make changes to the control routines of the internal combustion engine, so that the emissions remain below a limit value even in the event of a fault. 
     In a simplified embodiment, the pressure accumulator  17 , represented as a separate component in FIG. 1, may be produced as a flexible line taking the place of feedline  13 , in particular in the form of a hose (flexibly expandable hose), which connects the output side of the pump  11  to the metering valve  14 . 
     Referring now to FIG. 2, there is shown a further illustrative embodiment of the system according to the invention, in which the pressure accumulator and the pump are integrated to a single unit. 
     The reference numeral  10  again denotes a reducing agent container, from which the reducing agent, for example urea dissolved in water, is delivered by means of an electrically driven diaphragm pump  111 . For this purpose, the diaphragm pump  111  is connected via a delivery line  12  to the reducing agent container  10 . Where the diaphragm pump  111  joins the delivery line  12 , a nonreturn valve  25  is provided which lets the reducing agent through only in the direction indicated by an arrow symbol to a pressure accumulator directly connected to the diaphragm pump. The pressure sensor  18  is also integrated in the diaphragm pump  111  and thus measures the pressure in the pressure accumulator  17 . From the pressure accumulator  17 , a feedline  13  leads to a metering valve  14  which is arranged in an exhaust pipe  15 , upstream of an SCR catalyst  16  in the exhaust gas flow direction (arrow symbol), in such a way that its nozzle opening projects into the exhaust pipe  15 . 
     The pressure sensor  18 , the system driving the diaphragm pump  111  and the metering valve  14  are respectively connected via electrical lines (not indicated in further detail) to a control unit  19  which controls the addition of the reducing agent to the exhaust  15 . For this purpose, the control unit  19  has an electronic computer unit  22  which also carries out error detection and diagnostic routines. 
     The control unit  19  is connected via a serial interface  20  (for example a CAN BUS) to a control unit  21  of the internal combustion engine and/or to other control units. Via this interface, data such as, for example, engine speed, control movement and boost air temperature are transmitted to the control unit  19 . Further, the control unit  19  contains various functional units for driving the actuators (reducing agent heater, pump) and for evaluating the signals delivered by the sensors (fill level sensor, temperature of the reducing agent, pressure in the pressure accumulator or in the feedline). For this purpose, the control unit  19  has, amongst other things, a memory  23  which stores a plurality of characteristic curves or characteristics, as well as threshold values, whose significance will be explained in more detail. 
     In the second illustrative embodiment, using a diaphragm pump, there is the possibility of utilizing the diaphragm  172  both for the pressure accumulator function and for the pump function. The diaphragm  172  is acted on by a spring  26  in order to produce the system pressure. For the pumping process, the diaphragm is deflected by the system driving the diaphragm pump (electric motor) against the force of the spring  26 . The deflection of the diaphragm  172  may, for example, be recorded with a mechanical displacement sensor or a switch, and transmitted as information to the control unit. The deflection is compared with a predetermined value, which is stored in the memory  23 , and as soon as this value of the deflection is reached, a pressure is built up, by the spring force of the membrane  172 , which is essentially determined by the spring characteristics, the deflection and the diaphragm area. 
     If the metering valve  14  is then driven using corresponding drive signals from the control unit  19 , the pressure in the system decreases. The value for the pressure in the system is measured continuously using the pressure sensor  18  and compared with a minimum pressure value stored in the memory  23  of the control unit. It is preferable to use short pulses as drive signals in order to avoid cooling out the catalyst. If, after a plurality of such metering pulses, the pressure reaches the predetermined minimum value, the pump drive system is restarted and the diaphragm  172  is deflected against the spring  26 . 
     It is also possible to start this process after a predetermined minimum deflection of the diaphragm  172  is reached, corresponding to a maximum relaxation of the spring  26 , for example using the signal from a position switch, and this is suitable especially in the case of purely mechanically controlled pumps. It is possible to determine the metered quantity of reducing agent and detect errors in terms of sealing faults in the system in the same way as described with reference to the illustrative embodiment according to FIG.  1 . 
     If the diaphragm pump has a position switch or another way of ascertaining the displacement, the displacement volume may be provided directly through the deflection of the diaphragm, by means of which either redundancy in determining the metered quantity is obtained, which can help with the self-diagnosis capacity and the metering accuracy, or else it is possible to do without the pressure sensor.