BACKGROUND

The present disclosure relates to a device to convey a chemical reactant into the exhaust stream of a combustion engine, comprisinga mixer housing having an inlet opening through which the exhaust flow enters the mixer housing,a metering pipe passing through the mixer housing, towards which the exhaust flow flowing into the mixer housing flows in a transverse direction, and having a first end and a second end,a metering unit which is arranged at the first end of the metering pipe and can be connected to a reactant supply for dispensing reactant into the metering pipe, andmeans for generating a swirl flow of the exhaust flow.
Furthermore, an exhaust gas purification system is described for reducing the NOx content of the exhaust gas of an internal combustion engine using such a device.

Modern internal combustion engines, for example if they are diesel engines which are mainly used as dynamically loaded diesel engines, are equipped with an exhaust gas purification system to meet emission requirements. These requirements relate, among other things, to a limit on NOx emissions. In such exhaust gas purification systems, NOx emissions are in many cases brought about by a selective catalytic reaction of the nitrogen oxides (NOx-) contained in the exhaust gas. So-called SCR catalysts (SCR: selective catalytic reduction) are used for this purpose. A reducing agent is required as the reaction medium for the desired catalytic reduction of nitrogen oxides to take place at such an SCR catalyst. Ammonia is typically used for this purpose, which is introduced into the SCR catalyst in the form of liquid urea upstream of the catalyst. A specific temperature is required to release the reducing agent (ammonia) contained in the precursor. Therefore, it is necessary that the exhaust gas temperature is sufficiently high for effective denitrification of the exhaust gases. The reactions to release the reducing agent from the precursor do not occur spontaneously, especially if the exhaust gas temperatures are not sufficiently high, but require a certain amount of time. For this reason, a certain flow path is required between the metering unit with which the precursor is fed and the SCR catalyst. Once the reducing agent (ammonia) is released, the desired catalytic reaction to reduce or eliminate the nitrogen oxides carried in the exhaust gas takes place at the SCR catalyst. It is not desirable if precursor which has not yet been converted flows against the SCR catalyst, since the liquid droplets would then be deposited on the upstream surface of the SCR catalyst. In addition, when flowing into the SCR catalyst, the best possible uniform distribution of the reducing agent released in the exhaust gas flow and also a uniform velocity distribution over its cross-sectional area is desired. Precipitation of liquid precursor in the exhaust gas tract should also be avoided as far as possible.

In order to meet these requirements for the supply of a reducing agent metered as a liquid precursor, WO 2015/018971 A1 as well as WO 2016/207484 A1 propose to arrange a metering pipe inside a mixer housing, at one end of which a metering unit for injecting the precursor is arranged. The metering pipe is designed as a hole or screen plate and has a conical shape, with its cross-sectional area decreasing in the direction away from the metering unit. The metering pipe is hit by a flow at right angles to its longitudinal extension. The end section of the metering pipe engages in a jacket tube opposite the metering unit, leaving an annular gap. In the device according to this prior art, a swirl flow is generated on the outer side of the metering pipe. This flow is accelerated within the annular gap between the metering pipe and the jacket tube, which gap serves as a nozzle. Due to the speed difference between the external swirl flow and the exhaust gas flow inside the metering pipe, the precursor injected into the metering pipe in the form of liquid droplets is intermixed with the exhaust gas flow. A baffle plate is arranged on the outlet side of the metering pipe, through which the exhaust gas flow is deflected into the opposite direction. The reducing agent is sprayed onto this guide element, which is distributed in the exhaust gas flow deflected by the element. The reducing agent is released over a short distance, such that this previously known device can also be used with limited available installation space. Due to the multiple deflection of the exhaust gas flow in respectively opposite flow directions, a respective exhaust gas back pressure must be accepted.

WO 2011/163395 A describes another metering and mixing arrangement for use in an exhaust gas aftertreatment system. In this prior art, the metering pipe is also configured as a perforated plate tube, but has a cylindrical shape. The metering pipe is arranged transversely with respect to the exhaust gas flow to be cleaned, located in a housing. Upstream of the incoming exhaust gas in the direction of flow, there is a guide element in the housing which is adapted to guide the incoming exhaust gas past the metering pipe in order to generate a swirl flow on the outside of the metering pipe.

As with the prior art previously appreciated, this prior art also follows the concept of generating a swirl flow outside a metering pipe configured in the manner of a perforated plate, which flow continues into the pipe interior as a result of the openings arranged in the manner of a grid. The disadvantage of such a design of such an exhaust gas purification system is that the reducing agent supplied in liquid form can be deposited in an uncontrolled manner in the perforations of the metering pipe on the respective flow shadow side, such that it cannot be guaranteed that the reducing agent quantity currently required for sufficient denitrification is actually supplied to the downstream SCR catalytic converter. NOx slip can therefore not be ruled out.

A mix box for an exhaust system of an internal combustion engine for mixing additives into an exhaust gas flow is known from WO 2016/142292 A1 In this device, too, there is a transverse flow against the metering pipe, namely through an inlet tube arranged parallel to the metering pipe, which is designed as a strainer tube in its section parallel to the outlet tube, such that the exhaust gas emerges from the inlet tube in the radial direction. A flow volume is located between these two pipes. This is bounded on the edges by flow guide elements which influence the deflection of the exhaust gas flow leaving the inlet pipe in the circumferential direction or in the radial direction relative to the outlet pipe serving as the metering pipe. Thus, in this prior art as well, a swirl flow is generated outside the metering pipe and enters it through openings made in the outlet tube. In a further development, the inflow openings of the metering pipe (outlet tube) are equipped with vanes projecting from the outer shell surface. The spray cone with which the liquid reducing agent precursor is injected into the metering pipe is designed such that it impinges on the inner surface of the metering pipe in the area of the inflow opening of the metering pipe.

Even if the required flow path for releasing the reducing agent contained in a liquid precursor as a chemical reactant is possible on a shorter flow distance compared to other designs due to the effect described above, and this device can therefore also be used in confined installation space conditions, it would be desirable if the required installation space could be further reduced and if it were nevertheless possible to convert the precursor to release the reactant contained therein even at lower exhaust gas volume flows, without having to accept an unacceptable deposition of liquid precursor droplets.

SUMMARY

Proceeding from this background, the present disclosure is therefore based on further developing a device of the type mentioned at the outset in order to meet these requirements.

This is provided, according to the present disclosure, by a device of the type mentioned at the outset, in which the means for generating the swirl flow are designed to generate it within the metering pipe, and for this purpose, the metering pipe has at least one inflow opening extending over a circumferential surface segment of no less than 45° and over at least a portion of the length of the metering pipe located inside the mixer housing, with a shovel-like hood arranged on the metering pipe and directing the exhaust gas flow eccentrically into the inflow opening.

In this device for feeding a chemical reactant into the exhaust tract of an internal combustion engine, particularly for feeding ammonia to an SCR catalytic converter, wherein this chemical reactant is injected into the exhaust tract in the form of a liquid precursor divided into droplets, a swirl flow is first generated inside the metering pipe as a result of the deflection of the exhaust gas flow flowing transversely against the metering pipe via the at least one shovel-like hood into the metering pipe. This allows the formation of a particularly high-energy swirl flow by simple means and without necessarily having to accept an exhaust back pressure that is sometimes too high. A higher-energy swirl flow with a correspondingly higher flow rate favors droplet transport of liquid precursor droplets introduced into the swirl flow.

The swirl flow of the exhaust gas is generated inside the metering pipe by utilizing the lateral flow of the exhaust gas into the metering pipe. The metering pipe has at least one inflow opening through which the exhaust gas flow enters the metering pipe. This at least one inflow opening extends in the circumferential direction of the metering pipe over a shell surface segment of no less than 45°. In the longitudinal extension of the metering pipe, this at least one inflow opening extends over at least 40 to 60% of the length of the section of the metering pipe located in the mixer housing in the metering pipe flow space. The metering pipe flow space may be bounded by the mixer housing itself or by a component located inside the mixer housing. In the metering pipe flow space, incoming exhaust gas can flow around the metering pipe. Nevertheless, the desired swirl flow in the metering pipe is only provided within it by deflecting the exhaust gas flow into the metering pipe. According to one embodiment, the inflow opening extends in the direction of the longitudinal extension of the metering pipe over at least 70 to 75% of its length located inside the mixer housing. Due to the extension of the at least one inflow opening in the circumferential direction of the metering pipe outlined above, the number of possible inflow openings is limited. Moreover, the cross-sectional area of the at least one inflow opening with its shovel-like hood for receiving the waste gas flow is correspondingly large, such that for this reason alone deposits of reactants, such as liquid urea droplets as ammonia precursors, are virtually ruled out. The cross-sectional area of the at least one inflow opening is adapted to the velocity of the swirl flow to be generated and to the expected exhaust gas volume flow. Due to the design of this feed device, the cross-sectional area against which the exhaust gas flows against the metering pipe is significantly larger than the cross-sectional area of the metering pipe.

A special feature of this device is that each inflow opening is equipped with a shovel-like hood arranged thereon. Due to the shovel-like design of the hood, the inflow opening is enclosed at the sides and rear and at least largely covered in the radial direction. It is entirely possible for the mouth of the hood to project over the front inflow-side edge of the inflow opening covered by it. Such a hood serves to direct the exhaust gas flow flowing transversely towards the metering pipe into the inflow opening. For this purpose, at least one hood arranged on an inflow opening points with its mouth in the direction of the inlet opening of the exhaust gas into the mixer housing, so that the exhaust gas entering the mixer housing flows at least partially directly into the hood and thus into the inflow opening of the metering pipe. The tangential deflection of the exhaust gas flow into the metering pipe, which typically has a circular cross-sectional geometry, leads to the desired exhaust gas swirl flow on the inner wall of the metering pipe. Since the swirl flow is generated inside the metering pipe, only lower flow losses have to be accepted, in contrast to previously known designs of such equipment. If multiple inflow openings are provided and consequently also multiple hoods are arranged on the outer surface of the metering pipe, their hood mouths are oriented in the same direction viewed in the circumferential direction of the metering pipe. Since the metering pipe is located in the mixer housing, crossing it, the lateral inflow area of the exhaust gas flow is significantly larger than the cross-sectional area of the typically cylindrical metering pipe. As a result, when the exhaust gas flow enters such a hood and thus the interior of the metering pipe, it experiences significant acceleration, both in the direction of rotation of the swirl flow and in the longitudinal extension of the metering pipe. Since the metering pipe is closed on its first end by the metering unit, the swirl flow within the metering pipe propagates from this end to the second end of the metering pipe.

In this device for feeding a chemical reactant into the exhaust tract of an internal combustion engine, the slope of the swirl flow generated in the metering pipe can be adjusted via the exhaust gas back pressure. The lower the gradient at which the swirl flow moves towards the second end of the metering pipe, the longer the flow path, but without having to extend the actual length of the metering pipe for this purpose. For this reason, the overall length of conventional equipment for the preparation required to release the desired ammonia, for example, can be reduced by more than 50% if aqueous urea solution is fed in.

In a preferred embodiment, the spray angle at which the liquid reactant is injected into the metering pipe is set up in such a way that the precursor droplets only come into contact with the inner wall of the metering pipe downstream of the at least one inflow opening, at least to the extent that they have not previously been picked up by the exhaust gas swirl flow. The section of the metering pipe in the flow direction of the exhaust gas downstream of the inflow openings is the section in which the highest flow rates of the exhaust gas swirl flow are observed. In addition, the exhaust gas flowing into the device generally flows against the outside of this section of the metering pipe adjacent to the at least one inflow opening and is heated by this in a particular way. This is utilized in a special design of an operation of this feed device. In this mode of operation, contrary to the prevailing doctrine, liquid precursor is injected into the metering pipe at a volume such that it impinges on this heated inner wall section of the metering pipe. As a result, larger precursor droplets are broken up into smaller droplets by the impact and, in addition, this heated metering pipe section is used to release the reactant contained in the precursor, such as ammonia. Through this process, the required flow path for the release of the reactant and for the desired uniform distribution of the same within the exhaust gas flow can already be achieved in a shorter pipe section. The liquid precursor is preferably metered in such a way that, after a first metering cycle, the subsequent metering cycle is not started until precursor sprayed onto the inner wall of the metering pipe has evaporated. Preferably, the precursor is dispensed under pressure via the metering unit, i.e., it is injected into the interior of the metering pipe in the axial direction at a predetermined pressure, for example, a pressure between 6 and 10 bar.

In one embodiment of such a device, the at least one inflow opening is arranged off-center with respect to the longitudinal extent of the metering pipe within the metering pipe flow space, that is, offset in the direction toward the metering unit. This means that the distance of the hood of the at least one inflow opening from the wall section defining the metering pipe flow space in the direction towards the second end of the metering pipe is greater than the distance of this hood from the opposite wall. This ensures that a good proportion of the exhaust gas flows against the outside of the metering pipe section located downstream of the at least one inflow opening in the direction of flow of the exhaust gas through the metering pipe, and typically also flows around that pipe. This section of the metering pipe is directly exposed to the exhaust gas flowing through the at least one inflow opening, with the result that this section of the metering pipe is particularly well heated by the incoming exhaust gas. This section of the outer surface of the metering pipe heated by the incoming exhaust gas can be about four to six times as long as the section of the metering pipe from the hood to the opposite wall defining the metering pipe flow space. The length of the one or multiple inflow openings in the longitudinal direction of the metering pipe also influences the different spacing.

The direction of transport in the longitudinal direction of the metering pipe ensures that the injected precursor droplets are transported in the longitudinal direction of the metering pipe away from the location of injection and typically do not accumulate, for example, in the vicinity of the injection, are partially deposited and can build up there.

In one embodiment, flush openings are provided in the metering pipe in the metering pipe section adjacent to the metering unit, which openings may well be arranged at a direct radial distance from the injectors, in order to obtain an exhaust gas flow also directly in the area of the precursor droplet delivery within the metering pipe in order to avoid possible precursor droplet deposits in the area of the injector nozzle(s). This exhaust gas entering the interior of the metering pipe generates a partial exhaust gas flow directly at the injector orifice(s) and thus at the metering flange carrying the metering unit. This keeps the injector(s) free of precursor droplet deposits. The swirl flow generated within the metering pipe does not or only to a minor extent affect the centrally arranged injector(s), which is why flush openings of this type are a very simple but effective means of keeping the injectors free of precursor deposits. This is supported by turbulences which occur between the jacket flow flowing along the inner wall of the metering pipe and the partial exhaust gas flow entering through the flush openings, Such flush openings can be holes arranged in the manner of a grid, certainly in the form of a circumferential ring. The openings may have a circular or oblong cross-sectional area. The cross-sectional area of the sum of the flush openings can be used to adjust the exhaust gas back pressure and thus also the gradient of the swirl flow in the direction towards the second end of the metering pipe.

In such a device, it is expedient for a hood to have its mouth pointing toward the inlet opening of the mixer housing. Then the exhaust gas enters directly into the hood mouth and is eccentrically directed into the metering pipe. As a result of the associated reduction in cross-section, the exhaust gas flow experiences the desired acceleration. The leading edge of the inflow opening of the metering pipe in the direction of flow of the exhaust gas is typically located in the area of the central longitudinal plane of the metering pipe running transversely to the direction of flow of the exhaust gas, or is located a few angular degrees in front of this plane, According to one embodiment of such a device, this inflow opening extends over about 90°, or over a few angular degrees more than 90°, e.g. about 95 to 100°. According to one embodiment of such a device, the hood is a separately manufactured component which is placed on the outer shell surface of the metering pipe and welded to it on three sides. This simplifies the degrees of freedom in the manufacture of the hood with respect to its geometry. Welding the hood to the outer surface of the metering pipe is advantageous, since there are no elements protruding from the inner wall of the metering pipe for fastening the hood to or on the metering pipe, which could influence and thus impair the swirl flow flowing along said inner wall. In such a hood, the rear or baffle wall of the hood, which serves to direct the flow, is arranged on the rear edge of the inflow opening with respect to the direction of flow of the exhaust gas.

The shape of such a hood can be used to influence the formation of the swirl flow, According to a first example embodiment, the hood is trapezoidal in a developed view, wherein its wall opposite the inflow opening is adjusted to the contour of the inner wall of the mixer housing. The trapezoidal shape reduces the cross-sectional area enclosed by the hood in the direction of the swirl tube interior, causing the exhaust gas flow to accelerate.

In another embodiment, the at least one hood is tapered from its mouth to its rear end. This allows the hoods to be designed with a lower height, such that the inflow openings can be moved closer to the metering flange if an approximately hemispherical mixer housing is provided. As a result, the mixer housing can be designed more compactly.

In one embodiment, the mixer housing is hemispherical in shape, which means that the mixer housing requires very little installation space. In addition, this allows the arrangement of the device in different positions in the space. Such a design is possible because the swirl flow is generated inside the metering pipe. The hemispherical inner shell surface of the mixer housing avoids fluidic dead spaces, which would increase flow resistance due to the turbulence generated therein. In addition, due to this design of the mixer housing, in which the opening of the hemispherical mixer housing is the inlet opening, the exhaust gas flow experiences a certain reduction in cross-sectional area and thus already a certain acceleration before it is directed into the at least one inflow opening of the metering pipe. The metering pipe crossing the mixer housing in its center divides the exhaust gas flow, so to speak. If the exhaust gas flow against the metering pipe, the heat carried in the exhaust gas flow is effectively transferred to the metering pipe, which in turn counteracts a precipitation of precursor droplets, also by condensation on the inner side of the metering pipe. For this reason, the deflector housing part typically present at the outlet of a first exhaust gas purification unit, for example a particulate filter, possibly with an upstream oxidation catalyst, can be used to accommodate the metering pipe therein and thus to use this housing part for a feed device according to the present disclosure. As a result, this component, which is already available in terms of its installation space, can be expediently used for liquid precursor metering, without requiring significantly more installation space.

Further acceleration of the exhaust gas flow for generating the swirl flow is achieved according to one embodiment in that the at least one inflow opening has a trapezoidal outline geometry in a developed view of the outer surface of the metering pipe, wherein the shorter side of this outline geometry with respect to the flow direction of the exhaust gas is the rear edge following the longitudinal extension of the metering pipe. In the case of at least one inflow opening, the hood associated with this inflow opening is located inside the mixer housing. Typically, particularly if the mixer housing has a hemispherical inner surface, the contour of the hood follows the inner contour of the mixer housing. A gap may be left between the hood and the inner shell surface of the mixer housing. In this gap, the incoming exhaust gas flow, insofar as it is not directed by the hood through the inflow opening into the metering pipe, is also accelerated somewhat and flows around the outside of the hood and the metering pipe. This also creates a specific swirl flow on the outside with respect to the metering pipe. This supports the all-round heating of the metering pipe, above all, in its section between the inflow openings covered by the hoods and the wall defining the metering pipe flow space in the direction of the second end of the metering pipe. In addition, this allows the design of a metering pipe with, for example, two inflow openings, which are typically arranged diametrically opposite to each other with respect to the longitudinal axis of the metering pipe. The exhaust gas flow that bypasses the metering pipe is then captured and fed to the swirl flow inside the metering pipe via the hood of this second inflow opening. Likewise, such a device may be equipped with more than two inflow openings, for example, four inflow openings. The number of inflow openings should not exceed six to eight. If the number of inflow openings is larger, the energy input for forming the exhaust swirl flow is low. An embodiment with two to four inflow openings is preferred.

According to one embodiment, the mixer housing is made of two parts. According to one embodiment, the longitudinal axis of the metering pipe is located in the area of the seam of the two mixer housing parts. This allows easy integration of the metering pipe into the mixer housing. Typically, the mixer housing is equipped with thermal insulation on the outside for heat preservation. With such a configuration of the mixer housing or the feed device, the at least one hood can be mounted outside the mixer housing on the metering pipe.

In another embodiment, the mixer housing is made in one piece, and the metering pipe is inserted therein. In such a configuration, the at least one hood is mounted on the metering pipe when the latter is already inserted into the mixer housing.

In the design of the mixer housing with an inlet opening whose cross-sectional area is significantly larger than the cross-sectional area of the metering pipe, this larger cross-sectional area can be used to connect the mixer housing directly to the outlet of an exhaust gas purification unit upstream of the mixer in the direction of flow of the exhaust gas. Such an exhaust gas purification unit may be a particulate filter or an oxidation catalyst. Against this background, it is particularly advantageous to design the mixer housing with a hemispherical geometry, since the inlet opening is then circular, which is the typical outline geometry of the housing of an exhaust gas purification unit. The diameters of the mixer housing and the exhaust gas purification unit upstream thereof are matched to each other. For this reason, conical housing parts and exhaust line sections between the units that are otherwise used can be avoided, which in turn contributes to a reduction in required installation space.

According to a preferred embodiment, the device described is part of an exhaust gas purification system with which the nitrogen oxides carried in the exhaust gas flow are reduced. An SCR catalyst is then installed downstream of the device in the direction of flow of the exhaust gas.

The device described above can be used for all internal combustion engines operating with excess air, including, for example, gas engines or hydrogen engines.

