Reducing Agent Metering System with Evacuation of the Reducing Agent Line after Completion of Metering

The invention relates to a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line (1) and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, a Venturi tube (2) being integrated into the delivery line (1) and having a branch (3) which discharges into the nozzle, at the outlet (4) of the Venturi tube (2) there being an openable shutoff element so that with the shutoff element opened the reducing agent which is located in the nozzle and the branch (3) is intaken as a suction medium into the Venturi tube (2) and is carried away via the outlet (4).

Identical parts, components and modules are identified in the figures with identical reference numbers.

FIGS. 1 to 4each show an extract from the delivery line1of a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction. The metering system is connected to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump which is not shown via the delivery line1.

A Venturi tube2which in the exemplary embodiments shown inFIGS. 1 to 3has a narrowing5and furthermore in the region of the narrowing5a branch3is integrated into the delivery line1. The branch3widens to a line diameter corresponding to the delivery line1and discharges into the nozzle which is not shown and via which the reducing agent is injected into the exhaust gas line of the internal combustion engine.

As can be taken fromFIGS. 1 to 3, downstream of the branch3a cross sectional widening6of the Venturi tube follows. At the outlet4of the Venturi tube there is a switchable shutoff element by means of which the outlet4of the Venturi tube can be closed off.

During metering operation of the metering system the shutoff element at the outlet4of the Venturi tube2is closed so that the reducing agent which has been delivered via the delivery line1is delivered to the nozzle via the branch3and is injected into the exhaust gas line of the internal combustion engine.

When metering is completed the shutoff element at the outlet4of the Venturi tube2is opened so that the reducing agent which has been delivered by the delivery line1discharges via the outlet4of the Venturi tube2. As a result of the narrowing5within the Venturi tube2the delivered liquid is accelerated, i.e. the dynamic pressure rises dramatically. As a result of this, the static pressure drops dramatically since the total pressure is constant. As a result of the Venturi effect which arises within the Venturi tube2, in this way the fluid which is located in the nozzle and in the branch line3toward the nozzle is intaken and entrained by the main flow of the narrowing5of the Venturi tube2and carried away via the outlet4.

In the embodiment as shown inFIG. 1the branch3in the region of the narrowing5is spaced apart from a cross sectional widening6of the Venturi tube2, which widening follows downstream. Here the branch3is located vertically on the Venturi tube2.

In the version shown inFIG. 2the branch3is likewise located vertically on the Venturi tube2and is likewise located in the region of the narrowing5of the Venturi tube2. In the exemplary embodiment according toFIG. 2the branch3is however not spaced upstream in front of a cross sectional widening6of the Venturi tube2, but is located half in the cross sectional widening6of the Venturi tube2, as can be taken fromFIG. 2.

In the embodiment of the invention shown inFIG. 3the branch3is located diagonally on the Venturi tube2so that an acute angle α is included between the branch3and the Venturi tube2. This means that with the shutoff element closed, at the outlet4of the Venturi tube2the flow in this case is deflected by 135° into the branch3toward the nozzle which is not shown. It has been shown that this arrangement with an acute angle α between the branch3and the Venturi tube2is efficient especially for evacuation of the branch line3and the nozzle when the blocking element at the outlet4of the Venturi tube2is opened when the metering is completed.

In the embodiment according toFIG. 3the branch3in the region of the narrowing5of the Venturi tube2is located half in a cross sectional widening6.

In the exemplary embodiments according toFIGS. 1 to 3the narrowing5of the Venturi tube2has an inside diameter of 1 mm. The branch line can likewise have an inside diameter of 1 mm or even a smaller diameter than the narrowing5of the Venturi tube2, the branch line3then widening in the direction to the nozzle to a line diameter corresponding to the delivery line1. But it has been shown that the branch line3and the nozzle can be especially efficiently evacuated when the branch line3in the connection region to the Venturi tube2has a diameter which is somewhat smaller than the narrowing5of the Venturi tube2.

FIG. 4shows another embodiment of the invention. Here a Venturi tube2is integrated into the delivery line1, the Venturi tube in this case having the classic structure of a jet pump. From the delivery line1a propellant nozzle discharges into the mixing chamber7which has a cross sectional narrowing and is followed downstream by the diffusor8. The branch3toward the nozzle is located in the region of the mixing chamber. Downstream, the Venturi tube at the outlet4has a switchable shutoff element which is closed during metering operation so that the delivered fluid discharges via the branch line3toward the nozzle and is injected into the exhaust gas line of the internal combustion engine. When metering is completed the shutoff element at the outlet4of the Venturi tube2is opened so that the overall arrangement according toFIG. 4works like a jet pump and the reducing agent which has been delivered via the delivery line1is used as the propellant medium and the fluid which is located in the branch line is intaken as a suction medium into the mixing chamber7and is carried away via the outlet4.

The embodiment according toFIG. 4has an extremely complex structure, but it has been shown in tests that this embodiment also forms the most effective version for prompt and reliable evacuation of the branch line3and the nozzle.

The invention thus makes available a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, in which by means of one especially advantageous configuration of the delivery line1, by integration of a Venturi tube2and a change of the flow direction by switching the switchable shutoff element at the outlet4of the Venturi tube2by a change of the flow direction a Venturi effect is produced which is used according to the principle of a suction jet pump in order to evacuate the branch line3and the nozzle and to intake the reducing agent which is located in the branch line3into the Venturi tube2and to carry it away via the outlet4. The reducing agent solution which is to be metered is used at the same time as a propellant medium of a jet pump.

It is especially advantageous here that an extra line with compressed air or the like is not necessary for evacuating the system. Furthermore the reducing agent which is located in the branch line3can be returned again to the tank without its being injected unused into the exhaust gas line. Altogether the construction is distinctly simpler and the entire system is distinctly simplified compared to the prior art. When the outlet4of the Venturi tube2is connected to the suction line of the pump, after evacuating the branch line3toward the nozzle and the sensors, the pump is also partially ventilated; this is used to protect the entire system against freezing.