Fluid handling – Self-proportioning or correlating systems – Self-proportioning flow systems
Reexamination Certificate
2000-05-12
2001-08-14
Hepperle, Stephen M. (Department: 3753)
Fluid handling
Self-proportioning or correlating systems
Self-proportioning flow systems
C060S286000
Reexamination Certificate
active
06273120
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a device for introducing a liquid reducing agent into an exhaust gas purification system. It is used particularly, in an internal combustion engine, in the exhaust gas purification system equipped with a regulated or controlled diesel catalyst (“GDK”). It may also be used in an exhaust gas purification system for stationary diesel engines, for example, with a mechanical power of up to a 1000 kW.
Above all other methods, the technique using regulated or controlled diesel catalyst GDK has proved to be advantageous in order to reduce the pollutants, in particular, nitrogen oxides, contained in the exhaust gas of an internal combustion engine operated with air excess, such as, for example, in diesel and lean-burn engines. In the technique, which is based essentially on the method of selective catalytic reduction (“SCR”), the nitrogen oxides are brought into contact with ammonia at a selective catalyst and are converted there into nitrogen and water.
Due to risks associated with using ammonia, specifically, its toxicity, and because of the troublesome smell generated by ammonia, ammonia is not generally carried in a vehicle using an internal combustion engine equipped with a GDK system. The reducing agent necessary for the catalytic conversion of the nitrogen oxides is, therefore, transported in the vehicle in liquid form as an aqueous urea solution. The ammonia is produced from the aqueous urea solution by hydrolysis in a precise quantity required at a particular moment for conversion of the nitrogen oxides. Where stationary smoke gas purification systems are concerned, for example, downstream of power stations, pure ammonia or ammonia water can be used.
German Published, Non-Prosecuted Patent Application DE 44 17 238, provides for directly leading the exhaust line of a truck diesel engine laterally up to a cylindrical inlet chamber, in which is disposed a funnel-shaped perforated plate. At the narrowest point of the funnel, an injection valve is provided, through which an aqueous urea solution is injected into the inner space of the funnel. A homogenous distribution of the urea solution in the exhaust gas over the entire cross section of the inlet chamber is achieved. A hydrolysis catalyst, a DeNOx catalyst and, if appropriate, an oxidation catalyst, follows the inlet chamber.
A solution differing from application DE 44 17 238 is disclosed in International PCT publication WO 96/36797. In order to obtain a sufficient nebulization of the liquid reducing agent to be atomized, i.e., the reducing agent urea, before introduction into the pollutant-laden exhaust gas stream, a mixing device or mixing chamber is provided there. Into the mixing chamber are introduced the liquid reducing agent and the gas, for example, air, for intimate mixing with one another, i.e., to form an emulsion. The mixing chamber is connected through a single mixing line or pipeline to an atomizer nozzle that is disposed in the exhaust gas stream. An adjustable metering valve precedes the mixing chamber. The metering valve aids in the predetermination of the reducing agent quantity necessary per unit time. The reducing agent quantity flowing through the opened metering valve per unit time is directly dependent on the differential pressure across the metering valve. In order to ensure a constant metering rate for the reducing agent in the prior art device, it is necessary to have a constant differential pressure.
The differential pressure prevailing across the metering valve in a prior art device depends both on the pressure in the reducing agent line upstream of the metering valve and on the pressure in the mixing chamber. Therefore, the differential pressure also depends on the gas pressure in the gas line leading into the mixing chamber. A pump preceding the metering valve for conveying the reducing agent generates the pressure. However, both the gas pressure and the pressure in the reducing agent line may experience fluctuations.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a device for introducing a liquid reducing agent into an exhaust gas purification system that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that meters the reducing agent quantity introduced per unit time simply and accurately.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for introducing a liquid reducing agent into an exhaust gas purification system, including a gas line for receiving and carrying a gas, a reducing agent line for receiving and carrying a liquid reducing agent, a mixing chamber for mixing the reducing agent with the gas, the mixing chamber fluidically connected to the gas line and to the reducing agent line, a metering valve disposed in the reducing agent line, and a control device for controlling a pressure in the reducing agent line dependent upon a gas pressure in the gas line.
The device contains a mixing chamber for mixing the reducing agent with a gas, into which mixing chamber open a reducing agent line carrying the reducing agent and a gas line carrying the gas, and a control device for controlling the reducing agent throughput in the reducing agent line in dependence on the gas pressure in the gas line. Accordingly, the reducing agent throughput, i.e., the quantity of reducing agent injected into the mixing chamber per unit time, can be influenced in a precisely directed way, so that it is possible to eliminate a dependence (present in the prior art) on the gas pressure in the gas line, the gas pressure essentially determining the pressure in the mixing chamber. By a precisely directed control of the reducing agent throughput (used as a correcting variable), the gas pressure (disturbance variable) influencing or disturbing the reducing agent throughput, it is possible to keep the reducing agent throughput constant, regardless of the gas pressure in the gas line.
In a particularly preferred refinement of the invention, the control device for controlling the reducing agent throughput is an actuating member for controlling the pressure in the reducing agent line. In particular, the actuating member provided is a pressure control valve that is controlled by the gas pressure and that preferably controls the pressure in the reducing agent line, starting from the inlet of an adjustable metering valve preceding the mixing chamber. Because the reducing agent throughput depends on the pressure drop at the metering valve preceding the mixing chamber, the reducing agent throughput can be influenced by the setting of the pressure in the reducing agent line upstream of the metering valve. In particular, the pressure is controlled such that the pressure difference at the metering valve and, therefore, also the reducing agent throughput, with the metering valve open, are approximately constant.
In one embodiment, the pressure control valve is connected to a branch line opening into the reducing agent line. It is thereby possible to control the pressure in the reducing agent line by the extraction of a reducing agent part stream from the reducing agent line. The branch line is connected to the reducing agent line preferably upstream of the metering valve, as seen in the direction of flow.
In particular, the branch line opens into a reservoir, to which the reducing agent line is also connected for the extraction of the reducing agent. The configuration gives rise to a recirculation of the extracted reducing agent part stream.
In one embodiment, the pressure control valve provided is a three-way valve disposed in the reducing agent line and to which the branch line is connected.
In an alternative embodiment, the pressure control valve is a two-way valve disposed in the branch line.
In accordance with a concomitant feature of the invention, the control device is connected to the gas line through a pressure extraction line, the pressure control valve preferably being controllable pneumatically. The confi
Hofmann Lothar
Mathes Wieland
Greenberg Laurence A.
Hepperle Stephen M.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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