Method and apparatus for posttreatment of an exhaust gas from an internal combustion engine

A method of and an apparatus for posttreatment of an exhaust gas from an internal combustion engine, in which a substance to be mixed with the exhaust gas is sprayed in metered quantities into an exhaust gas line through which the exhaust gas flows. The substance is sprayed onto an impact plate disposed inside the exhaust gas line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 2004/002556 filed on Nov. 19, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an improved method of and apparatus for posttreatment of an exhaust gas from an internal combustion engine.

2. Prior Art

Apparatuses, with which a substance to be mixed with the exhaust gas from an internal combustion engine is sprayed in metered quantities into an exhaust gas line through which exhaust gas flows are known for two different purposes.

First, a urea-water solution (UWS) is sprayed into the exhaust gas from diesel engines, to reduce the proportion of nitrogen oxides NOxin the exhaust gas. The urea in the sprayed-in solution is converted here in the exhaust gas line itself, or in a hydrolysis catalytic converter optionally located in the exhaust gas line, into ammonia NH3, which in a downstream SCR catalytic converter reduces the nitrogen oxides NOx, forming nitrogen N2and water H2O. The delivery of the urea-water solution is done either with air reinforcement, through a metering pipe protruding into the exhaust gas flow, or by means of a cooled injection valve. In both cases, particularly at low exhaust gas speeds, it cannot be avoided that some of the urea-water solution will meet the wall of the exhaust gas line, which particularly at low exhaust gas or wall temperatures leads to the development of deposits of urea or urea reaction products. In these deposits, urea or urea reaction products are stored, and at a later point they can be undesirably released again, and particularly in a transient mode of operation of the engine, they can be a hindrance to an unproblematic, optimized denitrification process. Moreover, the deposits can increase the flow resistance of the exhaust gas line and thus impair the exhaust gas flow.

Second, for heating exhaust gas from internal combustion engines downstream of the engine, fuel is injected into the exhaust gas, in order to attain the exhaust gas temperatures required for regenerating a downstream NOx, catalytic converter or diesel particle filter (DPF). The fuel here is usually sprayed purposefully onto a portion of the wall of the exhaust gas line, in order to vaporize the fuel faster, because of the contact with the heated wall of the line, than would take place solely by delivery into the hot exhaust gas. However, since the exhaust gas line is more or less severely cooled, among other factors by the relative wind of the motor vehicle, it cannot always be assured that the wall temperature of the exhaust gas line will be above the boiling temperatures of the higher-molecular ingredients of the commercial fuels used.

SUMMARY AND ADVANTAGES OF THE INVENTION

The method and apparatus of the invention, offer the advantage over the prior art that the temperature of an impact plate, mounted inside the exhaust gas line, is generally higher, because of lower heat losses, than that of an adjacent wall of the exhaust gas line. This also brings about faster heating up of the substance sprayed into the exhaust gas flow as it strikes the impact plate, located in the spraying direction of the spray device, or on striking its impact face diametrically opposite the spray device.

When the method and the apparatus are used for injecting a liquid or solid reducing agent into the exhaust gas flow, it has been found that this accelerating heating up of the reducing agent and, in the case of an SCR process, the resultant faster conversion of the urea into its subsequent products, leads to a reduction in deposits in the exhaust gas line, and as a result an unwanted storage of reducing agent in such deposits is avoided, and in exhaust gas tests, in which especially in the USA and Japan many operating points are in the range of relatively low exhaust gas temperatures of <250° C., an improvement in the dynamics and conversion rates can be attained.

When the method and the apparatus are used for post-engine combustion by fuel injection into the exhaust gas, the accelerated heating up of the fuel causes faster evaporation and thus an acceleration of the exothermal oxidation of the fuel, as a result of which the exhaust gas is heated for regenerating the catalytic converter or for burning off the Diesel particle filter. For further speeding up the evaporation, it may be advantageous to apply a thin coating of a porous ceramic material, metal wool, or some other suitable material onto the impact plate, so that the surface area of the impact face can be increased.

In general terms, by means of the impact plate of the invention, the speed of conversion of material, or of a change in the aggregate state of the substance delivered into the exhaust gas flow, can be increased, which is often advantageous in the ensuing exhaust gas treatment.

In a preferred feature of the invention, it is provided that the impact plate has a low thermal capacity, so that because of its low thermal inertia, it heats up very quickly to the temperature of the exhaust gas, and that for being secured in the exhaust gas line, the impact plate is joined to the wall of that line by connecting elements with poor thermal conductivity, so that at relatively high travel speeds associated with greater cooling of the wall, because of the relative wind, only little heat is dissipated from the impact plate to the wall.

To improve the mixing of the substance, evaporating from the impact plate, with the exhaust gas flowing past the impact plate, a further advantageous feature of the invention provides that the exhaust gas is made turbulent downstream of the impact plate in terms of the flow direction, preferably by a static mixer disposed downstream of the impact face in terms of the flow direction, which increases the turbulence of the exhaust gas flow downstream of the impact face and thus assures improved mixing of the exhaust gas flow there.

To provide for secure fastening and a defined position of the impact plate in the tail pipe and relative to the spraying direction, the impact plate is preferably bent in tubular form such that it can mounted at a slight spacing from the wall of the exhaust gas line, coaxially in the exhaust gas line, and has a beveled face end, through which the substance can be sprayed from a spray nozzle, oriented at an acute angle to the flow direction of the exhaust gas, of the spray device onto an impact face located on the inside of the impact plate and diametrically opposite from the spray nozzle.

To enable economical manufacture of the apparatus, the impact plate is preferably shaped together with the connecting elements serving to secure it in the exhaust gas line and together with the static mixer, if present, that serves to make the exhaust gas turbulent, in the form of a stamped and bent part made of thin sheet metal, which to prevent corrosion may for instance comprise stainless steel.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1shows a short portion of an exhaust manifold or tail pipe2of a motor vehicle with a diesel engine, between the engine (not shown) and a catalytic converter4, which is also partly shown on the right-hand side ofFIG. 1, and in which nitrogen oxides NOxcontained in the tail pipe exhaust gas are reduced by reacting with ammonia NH3, forming nitrogen N2and water H2O.

For generating the ammonia that is needed for reducing the nitrogen oxides in the catalytic converter4, a urea-water solution is sprayed, upstream in terms of the flow direction from a catalytic converter4in a manner known per se, through a spray nozzle6of a cooled injection valve or metering valve8into the exhaust gas flowing through the tail pipe2. The spray nozzle6is retained by an adapter10, which is inserted into the cylindrical wall of the tail pipe2, and the nozzle is inclined to the longitudinal axis12of the tail pipe2so that a convergent spray stream or mist14emerging from the spray nozzle6enters the tail pipe2at an acute angle α to the flow direction (arrow S) of the exhaust gas.

To prevent the spray stream or mist14from striking a portion, diametrically opposite the spray nozzle6, of the wall16of the tail pipe2that is cooled by the relative wind during travel of the motor vehicle, a thin impact plate18is mounted inside the tail pipe2directly downstream of the spray nozzle6; with a part of its inside20pointing in the direction of the spray nozzle6, it forms an impact face22for the droplets of the spray stream or mist14that reach as far as the impact plate18.

The impact plate18comprises a metal sheet bent into a tube, which is inserted coaxially into the tail pipe2of circular cross section and which has a somewhat smaller diameter than the tail pipe2, so that between the wall16of the tail pipe2and the outside of the impact plate18, an annular gap24, with a gap width of a few millimeters, remains open. For securing the impact plate18, a plurality of spacer feet26of a material with poor thermal conductivity are used, spanning the annular gap24, in order to minimize the heat dissipation from the impact plate18, bathed by the hot exhaust gas, to the wall16of the tail pipe2that is cooled by the relative wind.

The face end28of the tubular impact plate18toward the front in terms of the flow direction, and adjacent to the spray nozzle6, is beveled, and the longer wall portion having the impact face22is diametrically opposite the spray nozzle6, so that the spray stream or mist14emerging from the spray nozzle6and passing through the beveled face end28of the tubular impact plate18strikes the impact face22and moistens it with the urea-water solution.

The thickness of the material comprising the impact plate18is relatively slight, or in other words less than the thickness of the wall16of the tail pipe2, so that because of its low thermal capacity and the poor thermal conductivity of the spacer feet26, it very quickly heats up to the temperature of the hot tail pipe gases, and unlike the externally cooled wall of the tail pipe2, it also maintains that temperature. Thus the urea-water solution striking the impact face22also heats up quickly and evaporates from the impact face22, so that even at low exhaust gas speeds and/or low exhaust gas temperatures downstream of the impact face22, no deposits of urea or reaction products of the urea form in the tail pipe2or on the impact plate18.

Because of the slight thickness of the material of the impact plate18, incorporating the impact plate18does not substantially increase the flow resistance in the tail pipe2.

To assure better mixing of the reducing agent, evaporated from the impact face22, with the tail pipe exhaust gases, a static mixer30, shown schematically as a guide vane inFIG. 1, can optionally be disposed downstream of the impact face22and assures intensive turbulence of the reducing agent together with the tail pipe exhaust gas.

FIGS. 3 through 5show a similar impact plate18, but it is used in post-engine heating of a tail pipe exhaust gas for the sake of regenerating an NOxstorage catalytic converter or a Diesel particle filter (not shown); instead of the urea-water solution, a metered quantity of fuel is injected through the spray nozzle6into the tail pipe2.

Here, the impact plate18, on a part of its inside20facing toward the spray nozzle6, is provided with a coating of metal wool31, by which the surface area of this impact face22, moistened by the spray stream or mist14, is increased.

The static mixer30is also embodied here integrally with the impact plate18and comprises a part32of the metal sheet which protrudes, in the form of a sector32, past the rear edge of a sheet-metal blank34of the impact plate18and is joined to the blank34by a thin bridge of material36, as shown inFIG. 5, and which [part32], after the bending of the blank34into a tube, is bent inward into the tube, as best seen fromFIGS. 3 and 4. The part32of the sheet, bent over and fixed with its tip38on the inside20of the tubular impact plate18, assures that the tail pipe exhaust gas as is made quite turbulent as it emerges from the impact plate18and thus assures good mixing of this exhaust gas with the fuel evaporated in it and on the hot impact face22. The spacer feet26here are also formed by parts of the impact plate18, which protrude in the form of narrow tongues past the two opposed end edges40,42of the blank34.

The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.