DE10330913B4 - Method and device for metering reducing agent to the exhaust gas of an internal combustion engine - Google Patents

Abstract

A method of metering NOx reductant (30) to the exhaust of an internal combustion engine (10) via a metering valve (36), wherein the internal combustion engine (10) is operable in at least two mutually different regions (50, 52) of operating parameters at least by a need for reductant (30), wherein an adjustment of a quantity of reductant to be metered to the demand in a first region (50) by varying a continuously metered amount of reducing agent, and in a second region (52) at least partially by a batch intermittent metering Reducing agent is carried out, characterized in that the first region (50) characterized by a relatively high demand for reducing agent (30) and that the second region (52) characterized by a reduced demand for reducing agent.

Description

State of the art

The invention relates to a method for metering NOx reducing agent to the exhaust gas of an internal combustion engine according to the preamble of claim 1.

Moreover, the invention relates to a control device which controls such a method.

Such a method and such a control unit are from the US 2002/0029564 A1 known.

It is well known that the nitrogen oxide content in the exhaust gas of an internal combustion engine can be reduced by a selective catalytic reduction (SCR). For this purpose, a directly reducing substance such as ammonia or a precursor is supplied to the exhaust, which releases only in the exhaust reducing substances. As a precursor, for example, a urea-water solution (HWL) can be used. In a reaction of the urea with the water (combined thermolysis and hydrolysis) produces ammonia (NH3), which is converted in the selective catalytic reduction with nitrogen monoxide (NO) and nitrogen dioxide (NO2) to molecular nitrogen (N2) and water. The selective catalytic reduction takes place in an SCR catalyst. The hydrolysis can be carried out by an upstream hydrolysis catalyst. However, a conversion of the urea-water solution also takes place in the context of the SCR reaction, so that a separate hydrolysis catalyst need not necessarily be present.

The aforementioned document US 2002/0 029 564 A1 differentiates between stationary operating states and operating states with acceleration. In stationary operating states, a "steady state rate" of hydrocarbons is assigned to the exhaust gas (compare, for example, claim 11, feature e). The metering is carried out according to paragraph 90 of this document, characterized in that a fixed amount of hydrocarbons is metered periodically through the metering valve. The metering should be done either continuously or by periodic pulses. In operating states with acceleration, according to paragraph 91 and claim 1, feature e) and ii) of US 2002/0 029 564 A1 a pulsed injection of hydrocarbons, where the pulse width is to correspond to the time span that is being accelerated, resulting in temporarily higher NOx emissions as a result of the acceleration.

The metering of the urea-water solution is often carried out with air support by a so-called metering tube, which projects into the exhaust gas stream. This metering tube consists for example of a bent or alternatively straight steel tube with about 4 mm outer diameter, 2 mm inner diameter and Sprühlochern at the end, through which the urea-water solution is sprayed into the exhaust stream.

In certain engine operating points, the required amount of reducing agent (HWL) is low. At these operating points, some impairments of the HWL dosage have occurred in engine tests which call into question the fulfillment of the respectively valid emission legislation. The observations and investigations of the failed metering devices indicate that deposits of urea or its reaction products accumulate inside and outside the metering tube and lead to clogging of the tube and / or the spray holes after a certain time.

Against this background, the object of the invention is to provide a method for metering NOx reducing agent to the exhaust gas of an internal combustion engine and in the specification of a control device for controlling the process, the accumulation of deposits of urea or its reaction products inside and outside the metering tube avoids and thus maintains the functionality of an exhaust aftertreatment system with Reduktionsmittelzumessung the exhaust even at higher operating hours.

This object is achieved in a method of the type mentioned in that the first area ( 50 ) by a comparatively high demand for reducing agent ( 30 ) and that the second area ( 52 ) characterized by a reduced need for reducing agent.

Accordingly, this object is achieved in a control device of the type mentioned in that the first area ( 50 ) by a comparatively high demand for reducing agent ( 30 ) and that the second area ( 52 ) characterized by a reduced need for reducing agent.

Advantages of the invention

By these features, the object of the invention is completely solved. The invention is based on the recognition that adjusts a film flow of urea-water solution on the wall in the interior of the metering at all operating points of the internal combustion engine. The transport takes place by shear stresses, which are caused by the passing dosing air in the liquid.

In areas of low levels of NOx in the raw exhaust gas, which usually correlate with relatively low exhaust gas temperatures, small amounts of HWL are required and evidently only a thin liquid film forms on the pipe wall, which badly cools the pipe wall. As a result, poorly wetted zones with elevated wall temperatures form, where the water locally vaporizes and solid urea deposits build up over time. These conditions occur at low to medium engine speeds at low load. Medium and heavy duty trucks typically have low dosages at levels of about 300 to 500 grams of urea-water solution per hour. In comparison, high dosages in this technical environment are typically 2.5 to 8 kg per hour.

Due to the discontinuous intermittent metering, the thickness of the liquid film can be varied over time. As a result, it can be prevented that the thickness of a liquid film decreases more or less uniformly as the reducing agent requirement decreases. By this type of metering periodically a larger film thickness, in a sense, a surge or a wave can be generated. At the larger film thickness, a local dehydration is not expected. On the contrary: With a suitable choice of the repetition frequency and the opening pulse widths of the metering valve, any existing older deposits can be dissolved and washed away. Since conventional SCR catalysts can store urea products and NH3 (ammonia) within certain limits, the process is feasible without affecting the NOx conversion. As a result, the exhaust aftertreatment for NOx reduction can be carried out safely even in commercial vehicles in the lower part load range. The lower part load range is characterized by a comparatively low NOx raw emission and thus by a low reduction agent requirement.

It is preferred that a reversal of the metering of reducing agent takes place at the transition between the first region and the second region without a sudden change in the metered amount of reducing agent.

Due to this configuration, the change between the two types of metering does not affect the time-averaged flow rate. Since the transition from operating points with higher NOx raw emission to operating points with lower raw emissions also without sudden change in the amount of NOx takes place, there is no transition to the lack of reducing agent, which would reduce the NOx conversion, nor to an excess that would result in undesirable emissions of ammonia if breached by the SCR catalyst.

It is also preferred that the metered via the metering valve amount of reducing agent is transported in a line through a gas flowing in the line to the exhaust gas.

By this known type of reducing agent transport, the reducing agent can be sprayed into the exhaust gas. This achieves good metering of the reducing agent metering and good distribution of the reducing agent in the exhaust gas. In addition, the reduction agent transport through a gas opens up further degrees of freedom in influencing the transport of the reducing agent to the exhaust gas. This is exploited in embodiments presented below for avoiding urea deposits in the metering system.

Furthermore, it is preferred that a volume flow of the gas is adjusted controlled in dependence on the reducing agent requirement.

It has been found that a change in the dosing air supply, in addition to the change in the pattern with which a reducing agent metering valve is activated, can further reduce the formation of deposits.

A further preferred embodiment is characterized in that a reduced volume flow is set with reduced reducing agent requirement.

By doing so, the flow rate of the liquid in the film is reduced. With a reduction of the pressure in the metering tube of 1.7 bar to about 1.4 bar and an exhaust gas back pressure of about 1.1 bar, a reduction in the flow rate by about 30 percent resulted. Thus, the wall shear stress in the liquid film decreases sharply and the film thickness increases significantly. At the same time, the transfer of heat by convection, which is responsible for the evaporation of water from the urea-water solution with, is smaller because of the reduced Reynolds number. This reduces the risk of solid deposits forming due to evaporation of water from the film.

Preferably, a time-continuous volumetric flow is set in the first region and a pulsating volumetric flow is set at least temporarily in the second region.

It has been shown that precisely the generation of a pulsating volume flow of dosing air in the second region, ie where the amount of the reducing agent to be metered by a Variation of the repetition frequency of the opening pulse widths is adjusted to the needs, leading to a further reduction of the formation of deposits.

It is preferred that the level of the volume flow pulsating in the second area is smaller than the level of the volume flow in the first area.

It is also preferred that the level of the volumetric flow in the second region is reversed in a ramp-shaped manner to the higher level for the first region during a transition into the first region.

It is also particularly preferred that the pulsating volume flow is adjusted when the metering valve is closed.

These measures further enhance the advantage of reduced deposit formation. In practice, it has been found in particular that these features in combination with the above features have a particularly great positive influence on the desired reduction in the formation of deposits.

In a preferred embodiment, the repetition frequency in the first range is between 2 and 15 Hz, and it is set in the second range so that the metering valve is closed in each case for time intervals of 0.5 s to 10 s.

These intervals have proven to be a good compromise for realizing comparatively large reductant streams at high NOx raw emissions of the engine and comparatively small reductant streams in the region of lower NOx raw emissions of the engine in conjunction with a reduction in the formation of deposits in the dosing system.

With regard to the control unit, it is preferred that it controls at least one of the above-mentioned methods and configurations.

Further advantages will become apparent from the description and the accompanying figures.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

drawings

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Show it:

1 schematically the technical environment in which the invention unfolds its effect;

2 a representation of typical areas of operating points of an internal combustion engine, which differ by their need for reducing agent, in qualitative form;

3 a first drive pattern of a metering valve, as in a first area after 2 is used; and

4 a second control pattern of a metering valve, as in a second area after 2 is used.

Description of the embodiments

1 shows an internal combustion engine 10 with an exhaust system 12 a reductant dosing system 14 and a metered air system 16 , The reducing agent metering system 14 has a controller 18 on. At the control unit 18 it can be the control unit with which the internal combustion engine 10 is controlled. Alternatively, a separate dosing control device may be provided. For controlling a supply of reducing agent to the exhaust system 12 processes the controller 18 Signals of a sensor 20 about operating parameters of the internal combustion engine 10 , The sensors 20 includes in particular a speed sensor. Next controls the controller 18 a power actuator 22 of the internal combustion engine 10 , The power actuator can be, for example, an arrangement of injection valves, via which the internal combustion engine 10 a torque-determining amount of fuel is supplied.

In the internal combustion engine 10 it is preferably a diesel engine used to drive medium or heavy trucks. The exhaust gases of the internal combustion engine 10 be over an exhaust pipe 24 and an oxidation catalyst 25 into a catalyst 26 guided. In the exhaust pipe 24 is upstream of the catalyst in the flow direction of the exhaust gases 26 a metering tube 28 arranged. About this dosing tube 28 becomes reducing agent 30 from a reducing agent reservoir 32 in the exhaust gas in front of the catalyst 26 sprayed. The catalyst 26 is a Denox catalyst used in conjunction with the sprayed reductant 30 reduces the nitrogen oxide content in the exhaust gas. As a Denox catalyst 26 Therefore, in particular, an SCR catalyst in question, as mentioned in the introduction. An SCR catalyst 26 is preferably preceded by an oxidation catalyst, which is not explicitly shown in the figures for reasons of clarity. In the sprayed reducing agent 30 it is preferably a urea-water solution from which by a thermolysis and Hydrolysis reaction ammonia is formed with the nitrogen oxides from the exhaust gas in the SCR catalyst 26 reacts to molecular nitrogen and water.

For dosing the reducing agent 30 indicates the reductant dosing system 14 first a pump 34 on, the reducing agents 30 from the reservoir 32 via a metering valve 36 to a mixing chamber 38 promotes. In addition to the reducing agent 30 becomes the mixing chamber 38 Dosing air from the dosing air system 16 fed. The air from the dosing air system 16 flows over the mixing chamber 38 and a line 39 as well as via the dosing tube 28 in the exhaust pipe 24 , This forms an aerosol. The aerosol is introduced into the line by the dosing air 39 torn, in which a liquid film separates, the inner walls of the pipe 39 covered. The administration 39 typically has an inside diameter of 4 mm and the dosing tube 28 typically has an inner diameter of 2 mm. Under the influence of the flowing metering air is formed in the line 39 and in the dosing tube 28 a film of reducing agent, from the metering air to spray holes at the exhaust side of the metering tube 28 is transported and sprayed into the exhaust via said spray holes.

The dosing air system 16 can be identical to the pressure supply that is already present in lorries for supplying compressed air brake systems. Such compressed air supplies are known to have a reservoir 40 on top of a compressor 42 is supplied with compressed air. The compressor 42 is usually the internal combustion engine 10 driven, for example via a toothed belt 44 , For the reducing agent metering system 14 Air volumes of the order of a few liters per minute are needed. Such a need can be easily met by existing compressed air supplies in trucks.

The of the dosing air system 16 to the reductant dosing system 14 Delivered dosing air is typically through a supercritical throttle 48 limited to, for example, 20 l / min. Here, a supercritical throttle is understood as meaning a throttle which, in the case of pressure differences above a critical threshold, delivers a flow which no longer increases as the pressure difference increases. The pressure difference is the difference between the absolute pressures on the input side and the output side of the throttle. For an embodiment of a method in which the Dosierluftmenge is controlled, has the reducing agent metering system 14 optionally a controllable valve 46 on. Both the controllable valve 46 as well as the metering valve 36 can be designed as two-position switching 2-way valve (2/2-valve) and each of the control unit 18 to be controlled.

In the 2 are possible operating ranges of an internal combustion engine 10 represented in a plane which is spanned by axes for speed values n and torque values M. The curve 49 includes the set of all possible (M, n) value pairs that are present in the operation of the internal combustion engine 10 can result. This set of all possible value pairs (M, n) is divided into a first range 50 and a second area 52 on. The first area 50 is characterized by a comparatively high demand for reducing agents 30 out. In contrast, the second area stands out 52 by a reduced need for reducing agent 30 out.

To the reducing agent 30 To dose as needed, controls the controller 18 the metering valve 36 alternately in the open and closed state. In addition, the Dosierluftmenge via the controllable valve 46 from the controller 18 to be controlled. In the first area 50 the control of the metering valve takes place 36 and the controllable valve 46 according to the patterns of 3 , 3 shows the mass flow dm / dt of the reducing agent 30 and the dosing air over time. There is the pulse sequence 54 the opening state of the metering valve 36 and thus the flow through the metering valve 36 at. A high level of the signal of the pulse train 54 corresponds to an open metering valve 36 and a low level of the pulse train 54 corresponds to a closed metering valve 36 ,

The pulse sequence 54 consists of single pulses 56 together, the one pulse width or opening duration 58 own and with a period duration 60 follow one another. In the example of 3 is a period of half a second recognizable, which corresponds to a drive frequency of 2 Hz. It is understood, however, that the metering valve 36 can also be controlled with other frequencies, with frequencies between 2 Hz and 10 Hz, in particular 4 Hz, have been found to be particularly suitable. The amount of reductant added 30 will be in the first operating area 50 continuously adapted to actual needs. The metering takes place continuously, because at the frequencies mentioned behind metering valve 36 a continuous stream of reducing agent 30 established. The variation of the metered amount can be achieved by varying the opening duration 58 at fixed period 60 respectively.

The Dosierluftmenge is in the example of 3 kept constant over time. This is in the 3 through the line 62 shown, the constant volume flow of the dosing via the permanently open controllable valve 46 represents. The constant metered air volume flow drives an average reducing agent flow from the mixing chamber 38 to the dosing tube 28 as he goes through the line 64 in the 3 is represented. With an extension of the opening duration 58 would be the average reductant flow 64 climb. Accordingly, he would at a reduction of the opening duration 58 decline.

As mentioned above, have in the dosage of small amounts of reducing agent with a drive pattern for the metering 36 as is the pulse sequence 54 corresponds, deposits formed in the dosing system that affect the functioning of the system. In order to avoid the formation of these deposits, the control unit switches 18 then to another driving pattern, when the operating point of the internal combustion engine 10 in the second area 52 located.

An example of a second drive pattern is in 4 shown. In the 4 can the pulses 66 and 68 in a first embodiment in each case as openings of the metering valve 36 be understood. To generate a surge, the metering valve 36 during a period of time 70 triggered with a duty cycle opening. The amount of reductant added 30 is then at least partially varied by the distance 72 between two such periods 72 is varied. The greater the distance 72 is selected, the smaller is the over several opening periods (time periods 72 ) average of the metered amount of reducing agent.

The time periods 70 and the distance 72 two pulse trains 66 and 68 is, for example, just chosen so that averaged over time a mean reducing agent flow 74 gives as he the average reductant flow 64 to 3 equivalent. This shows that it is possible to switch between the two patterns without a sudden change in the reducing agent dosage. It has been shown that by the longer distances 72 Successful opening of the metering valve 36 a somewhat wavy transport of reducing agent 30 from the mixing chamber 38 to the dosing tube 28 takes place. It has also been shown that this modified transport mechanism prevents the formation of deposits in the metering system between the mixing chamber 38 and the spray holes of the metering tube 28 Effectively prevented or at least reduced.

This desired effect can be further enhanced by the fact that the volume flow 76 the metering air is also set pulsating. It has been found to be particularly advantageous, the dosing air volume flow 76 to vary so that it is parallel in time with openings of Reduktionsmittelzumessventils 36 a constant value 78 has. Furthermore, it has proven to be advantageous, the value of the volume flow of the dosing in the phases 72 reduce in which the metering valve 36 that the flow of the reducing agent 30 controls, is closed. In this case, the volume flow at a controllable valve 46 be reduced with variable opening cross section to a mean, non-zero value, as indicated by the reference numeral 86 in the 4 is represented. Preferably, however, the stream of metered air is detected with a digitally switchable between a closed position and an open position 2-way valve (2/2 valve) 3 digitally controlled as indicated by the sequence of levels 82 . 88 and 84 in the 4 is shown. About the working as air pressure control valve 46 will be in front of the throttle 48 the pressure varies. As a result of the resulting varying pressure drop across the throttle 48 turns in the line 39 a pulsating air flow, as exemplified schematically by the dashed curve 76 in the 4 is shown. Preferably, the flow rate dm / dt is already initially below the Dosierluftmenge 62 to 3 , In other words: For small quantities of dosing agent 30 is preferred also the Dosierluftmenge from the level 62 to the level 78 lowered. In the later transition to larger quantities of reducing agent 30 Preferably, a ramp-shaped increase of the dosing air quantity to the level 62 as it is in the 4 through the curve 91 is shown. The ramp-shaped raising preferably takes place after a dead time has elapsed 89 in which the level of the metering air initially at a lower level, for example, the level 78 , remains.

The by changing the Dosierluftmengen levels 78 . 86 . 82 . 88 . 84 and 80 in the 4 Pulsation of the dosing air shown is preferred in the periods 72 generated in which the metering valve 36 remains closed.

Alternatively or additionally, a blockage of the metering tube 28 be counteracted by a sufficiently large increase in the temperature of the metering tube. The heat input into the dosing tube 28 can be increased, for example, via ribs on the outside of the metering tube, which absorb heat from the exhaust gas. In this case, an increase by a factor which is greater than two is preferred. The resulting increase in the temperatures of the wall of the metering tube to values above about 135 ° C then leads to a melting of any urea deposits.

Also alternatively or additionally, a blockage of the metering tube 28 be counteracted by a sufficiently large reduction in the temperature of the metering tube. A reduction of the temperature can be achieved by lowering the heat input into the metering tube 28 As a result of a consistent thermal insulation of the metering tube from the exhaust pipe and the exhaust stream can be achieved. At low wall temperatures below 100 ° C, the boiling of the water can be prevented. As a result, deposits of solid substances are avoided.

With consistent application, these measures are also in isolation to achieve the object of the invention in a position.

Claims (13)

Method for metering NOx reducing agent ( 30 ) to the exhaust gas of an internal combustion engine ( 10 ) via a metering valve ( 36 ), wherein the internal combustion engine ( 10 ) in at least two distinct areas ( 50 . 52 ) can be operated by operating parameters that at least by a need for reducing agent ( 30 ), wherein an adaptation of an amount of reductant to be metered to the demand in a first range ( 50 ) by varying a continuously metered amount of reducing agent, and in a second range ( 52 ) Takes place at least partially by a discontinuously intermittent metering of the reducing agent, characterized in that the first region ( 50 ) by a comparatively high demand for reducing agent ( 30 ) and that the second area ( 52 ) characterized by a reduced need for reducing agent. A method according to claim 1, characterized in that a reversal of the metering of reducing agent in the transition between the first area ( 50 ) and the second area ( 52 ) without a sudden change in the amount of reducing agent ( 30 ) he follows. A method according to claim 1 or 2, characterized in that via the metering valve ( 36 ) metered amount of reducing agent ( 30 ) in a line ( 39 ) is transported to the exhaust gas by a gas flowing in the conduit. Method according to claim 3, characterized in that a volume flow ( 62 ) of the gas as a function of the need for reducing agent ( 30 ) is controlled. A method according to claim 4, characterized in that with reduced need for reducing agent ( 30 ) a reduced volume flow ( 86 . 88 ) is set. Method according to claim 4 or 5, characterized in that in the first area ( 50 ) a temporally continuous volume flow ( 62 ) is set. Method according to one of claims 4 to 6, characterized in that in the second area ( 52 ) at least temporarily a pulsating volume flow ( 76 ) is set. Method according to claim 7, characterized in that the level ( 78 ) of the pulsating volume flow 76 smaller than the level of the volumetric flow 62 in the first area ( 50 ). Method according to claim 8, characterized in that the level ( 78 . 80 ) of the volume flow 76 in the second area ( 52 ) when moving to the first area ( 50 ) ramp up to the higher level ( 62 ) for the first area ( 50 ) is reversed. Method according to one of claims 7 to 9, characterized in that the pulsating volume flow ( 76 ) with closed metering valve ( 36 ) is set. Method according to one of the preceding claims, characterized in that the metering valve ( 36 ) in the first area ( 50 ) is clocked at a repetition frequency between 2 and 15 Hz and in the second range ( 52 ) is closed in each case for time intervals of 0.5 s to 10 s. Control unit ( 18 ) for controlling methods for metering NOx reducing agent ( 30 ) to the exhaust gas of an internal combustion engine ( 10 ) via a metering valve ( 36 ), wherein the internal combustion engine ( 10 ) in at least two distinct areas ( 50 . 52 ) can be operated by operating parameters that at least by a need for reducing agent ( 30 ), wherein the control unit ( 18 ) the amount of reducing agent to be metered ( 30 ) in a first area ( 50 ) of operating parameters by varying a continuously metered amount of reducing agent, and in a second range of operating parameters ( 52 ) At least partially controlled by an intermittent discontinuous metering of a reducing agent, characterized in that the first region ( 50 ) by a comparatively high demand for reducing agent ( 30 ) and that the second area ( 52 ) characterized by a reduced need for reducing agent. Control unit ( 18 ) according to claim 12, characterized in that it controls at least one of the methods according to claims 2 to 11.

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