U.S. patent application number 10/155289 was filed with the patent office on 2003-10-09 for device and method for denoxing exhaust gas from an internal combustion engine.
Invention is credited to Tost, Rainer.
Application Number | 20030188528 10/155289 |
Document ID | / |
Family ID | 7930148 |
Filed Date | 2003-10-09 |
United States Patent
Application |
20030188528 |
Kind Code |
A1 |
Tost, Rainer |
October 9, 2003 |
DEVICE AND METHOD FOR DENOXING EXHAUST GAS FROM AN INTERNAL
COMBUSTION ENGINE
Abstract
A device and a method for deNOxing exhaust gas from an internal
combustion engine includes taking into account an amount of gaseous
reducing agent (ammonia) that escapes while the internal combustion
engine is at a stand-still as a result of temperature influences in
a calculation of the amount of reducing-agent solution (urea) that
is to be metered when the internal combustion engine is operating.
The gaseous reducing agent is passed to the reduction catalytic
converter through a pressure-relief line that includes a
pressure-control valve, and, in the method, the amount is recorded
by a flowmeter.
Inventors: |
Tost, Rainer; (Nurnberg,
DE) |
Correspondence
Address: |
LERNER AND GREENBERG, P.A.
Post Office Box 2480
Hollywood
FL
33022-2480
US
|
Family ID: |
7930148 |
Appl. No.: |
10/155289 |
Filed: |
May 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10155289 |
May 24, 2002 |
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PCT/DE00/04066 |
Nov 16, 2000 |
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Current U.S.
Class: |
60/286 |
Current CPC
Class: |
B01D 53/9495 20130101;
F01N 3/2066 20130101; F01N 13/0097 20140603; F01N 2610/06 20130101;
F01N 2900/1812 20130101; Y02A 50/20 20180101; B01D 53/90 20130101;
F01N 2900/1808 20130101; Y02A 50/2325 20180101; F01N 2610/02
20130101; B01D 53/9431 20130101; F01N 3/208 20130101; Y02T 10/12
20130101; Y02T 10/24 20130101; F01N 2610/14 20130101 |
Class at
Publication: |
60/286 |
International
Class: |
F01N 003/00 |
Claims
I claim:
1. A device for deNOxing exhaust gas from an internal combustion
engine having an exhaust pipe conveying exhaust gas in an exhaust
direction, comprising: a reduction catalytic converter operating
under an SCR principle, said converter, disposed in the exhaust
pipe; a reducing-agent reservoir for holding a reducing agent; a
metering device for introducing said reducing agent into exhaust
gas flowing to said converter; a reducing-agent pump for delivering
said reducing agent from said reservoir to said metering device;
said pump fluidically connecting said reservoir to said metering
device; said metering device fluidically connecting said pump to
the exhaust pipe upstream of said converter with respect to the
exhaust direction; a pressure-relief line for feeding excess
reducing agent from said reservoir to said converter; said
pressure-relief line fluidically connecting said reservoir to said
converter; and a flow-measuring device for recording an amount of
excess reducing agent passing through said pressure-relief line,
said flow-measuring device disposed in said pressure-relief
line.
2. The device according to claim 1, wherein: said pressure-relief
line has a cross-sectional opening for conveying said excess
reducing agent; a pressure-control valve is disposed in said
pressure-relief line; and said valve opens said cross-sectional
opening when a predetermined pressure exists in said reservoir to
permit said excess reducing agent to pass through said
pressure-relief line.
3. The device according to claim 1, wherein: said pressure-relief
line has a cross-sectional opening for conveying said excess
reducing agent; an electrically controllable valve is disposed in
said pressure-relief line; and said valve opens said
cross-sectional opening when a predetermined pressure exists in
said reservoir to permit said excess reducing agent to pass through
said pressure-relief line.
4. The device according to claim 1, wherein said flow-measuring
device is a flowmeter.
5. The device according to claim 1, wherein said flow-measuring
device is a flowmeter for ammonia.
6. The device according to claim 1, wherein said pressure-relief
line has an exit opening inside said converter.
7. The device according to claim 1, wherein said reducing agent is
a liquid.
8. In an internal combustion engine having an exhaust pipe
conveying exhaust gas in an exhaust direction, a device for
deNOxing exhaust gas in the exhaust pipe comprising: a reduction
catalytic converter operating under an SCR principle, said
converter, disposed in the exhaust pipe; a reducing-agent reservoir
for holding a reducing agent; a metering device for introducing
said reducing agent into the exhaust gas flowing to said converter;
a reducing-agent pump for delivering said reducing agent from said
reservoir to said metering device; said pump fluidically connecting
said reservoir to said metering device; said metering device
fluidically connecting said pump to the exhaust pipe upstream of
said converter with respect to the exhaust direction; a
pressure-relief line for feeding excess reducing agent from said
reservoir to said converter; said pressure-relief line fluidically
connecting said reservoir to said converter; and a flow-measuring
device for recording an amount of excess reducing agent passing
through said pressure-relief line, said flow-measuring device
disposed in said pressure-relief line.
9. A method for deNOxing exhaust gas from an internal combustion
engine, which comprises: determining, as a function of operating
parameters of at least one of the internal combustion engine and a
reduction catalytic converter operating under an SCR principle, an
amount of reducing-agent solution to be metered and introducing the
amount of the reducing agent into an exhaust pipe upstream of the
converter while the internal combustion engine is operating; when
the internal combustion engine is at a stand-still, feeding gaseous
reducing agent formed as a result of temperature effects to the
converter; and recording and taking into account an amount of the
gaseous reducing agent during a determination of an amount of
reducing-agent solution to be metered during operation of the
internal combustion engine.
10. The method according to claim 9, which further comprises
supplying the gaseous reducing agent at a location inside the
converter through a pressure-relief line connecting a
reducing-agent reservoir and the converter.
11. The method according to claim 10, which further comprises
feeding the gaseous reducing agent to the converter when a pressure
in the reservoir exceeds a predetermined pressure level.
12. The method according to claim 11, which further comprises
opening the pressure-relief line with a valve device disposed in
the pressure-relief line when the predetermined pressure level is
reached.
13. The method according to claim 11, which further comprises
determining an amount of the gaseous reducing agent with a
flow-measuring device disposed in the pressure-relief line.
14. The method according to claim 12, which further comprises
determining an amount of the gaseous reducing agent from a value
for the pressure in the reservoir and a duration of an opening of
the valve device.
15. The method according to claim 13, wherein: the reducing agent
is aqueous urea solution; and the flow-measuring device is a
flowmeter for ammonia.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application PCT/DE00/04066, filed Nov. 16, 2000,
which designated the United States and which was not published in
English.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0002] The invention relates to a device and a method for deNOxing
exhaust gas from an internal combustion engine.
[0003] The reduction of the nitrogen oxide emissions from an
internal combustion engine that operates with excess air, in
particular, a diesel internal combustion engine, can be effected
with the aid of selective catalytic reduction (SCR), to form
atmospheric nitrogen (N.sub.2) and water vapor (H.sub.2O). The
reducing agents used are either gaseous ammonia (NH.sub.3), ammonia
in aqueous solution, or urea in aqueous solution. The urea serves
as an ammonia carrier and is injected into the exhaust system with
the aid of a metering system upstream of a hydrolysis catalytic
converter, where it is converted into ammonia by hydrolysis, and
the ammonia then reduces the nitrogen oxides in the actual SCR or
deNOx catalytic converter.
[0004] The important components of such a metering system are a
reducing-agent vessel, a pump, a pressure sensor, and a metering
valve. The pump conveys the reducing agent stored in the
reducing-agent vessel to the metering valve, by which the reducing
agent is injected into the exhaust-gas stream upstream of the
hydrolysis catalytic converter. The metering valve is actuated
through signals from a control device such that a defined,
currently required amount of reducing agent is supplied as a
function of operating parameters of the internal combustion engine
(German Patent DE 197 43 337 C1, corresponding to U.S. Pat. No.
6,082,102 to Wissler et al.).
[0005] An advantage of the ammonia-releasing substances that are
present in aqueous solutions, such as, for example, urea, is that
the storage, handling, delivery, and metering are, in technical
terms, relatively simple to implement. A drawback of these aqueous
solutions is that, in the event of heating above a defined
temperature limit, which in turn is dependent, inter alia, on the
concentration of the dissolved substance, thermal decomposition of
the solution starts to occur in the reducing-agent tank.
[0006] At high temperatures, for example, when the vehicle equipped
with an exhaust-gas aftertreatment installation of this type is
parked at locations with high insolation, or even while the vehicle
is operating in hot regions, the reducing agent, which can be at
least partially converted into ammonia, may be overheated. The
decomposition vapor pressure, which increases as the temperature
rises, for, for example, an aqueous urea solution, leads to the
formation of ammonia and, therefore, to an increase in pressure in
the reservoir.
[0007] In order, on one hand, to prevent the reservoir from being
destroyed by an unacceptably high pressure and, on the other hand,
to prevent slippage of ammonia, in particular, when the filler neck
of the reservoir is opened, European Patent Application EP 0 577
853 B1 discloses, in an exhaust-gas aftertreatment installation for
an internal combustion engine of the type described in the
introduction, connecting a pressure-relief line, which feeds excess
reducing agent to the deNOx catalytic converter, to the reservoir
for the reducing agent. The pressure-relief line is connected to
the inlet of the deNOx catalytic converter, i.e., to the side that
faces the internal combustion engine. A pressure-control valve is
incorporated in the pressure-relief line. As a result, the amount
of excess ammonia that is to be received by the deNOx catalytic
converter can be limited within the scope of the compressive
strength of the reservoir.
[0008] In the prior art pressure relief method, although it is
possible to avoid an unacceptably high build-up of pressure in the
reservoir, the amount of reducing agent that is fed to the
catalytic converter through the pressure-relief line can only be
taken into account to an insufficient extent during the metering
strategy.
SUMMARY OF THE INVENTION
[0009] It is accordingly an object of the invention to provide a
device and method for deNOxing exhaust gas from an internal
combustion engine that overcomes the hereinafore-mentioned
disadvantages of the heretofore-known devices and methods of this
general type and that reliably prevents an unacceptably high
pressure in a reducing-agent reservoir of an exhaust-gas
aftertreatment device of the type described in the introduction
without impairing the metering accuracy.
[0010] With the foregoing and other objects in view, there is
provided, in accordance with the invention, a device for deNOxing
exhaust gas from an internal combustion engine having an exhaust
pipe conveying exhaust gas in an exhaust direction includes a
reduction catalytic converter operating under an SCR principle, the
converter, disposed in the exhaust pipe, a reducing-agent reservoir
for holding a reducing agent, a metering device for introducing the
reducing agent into exhaust gas flowing to the converter, a
reducing-agent pump for delivering the reducing agent from the
reservoir to the metering device, the pump fluidically connecting
the reservoir to the metering device, the metering device
fluidically connecting the pump to the exhaust pipe upstream of the
converter with respect to the exhaust direction, a pressure-relief
line for feeding excess reducing agent from the reservoir to the
converter, the pressure-relief line fluidically connecting the
reservoir to the converter, and a flow-measuring device for
recording an amount of excess reducing agent passing through the
pressure-relief line, the flow-measuring device disposed in the
pressure-relief line.
[0011] As a result of the amount of gaseous reducing agent that
escapes while the internal combustion engine is at a stand still,
due to temperature influences being taken into account during the
determination of the amount of reducing-agent solution that is to
be metered when the internal combustion engine is operating, not
only is the operational reliability increased, even in the range of
critical ambient conditions, for example, in summertime operation,
but also a high metering accuracy is achieved.
[0012] The targeted utilization of the gaseous reducing agent that
is released by the heating, i.e., ammonia, when an aqueous urea
solution is used as reducing agent, prevents slippage of reducing
agent because, when a predetermined pressure level is reached in
the reducing-agent reservoir, the gaseous reducing agent is passed
into the reduction catalytic converter through a pressure-relief
line. The amount of gaseous reducing agent that flows in is
advantageously recorded by a flowmeter in the pressure-relief line
and is taken into account during the calculation of the amount of
reducing agent. For example, when the internal combustion engine is
operating, liquid reducing agent is only injected again in a
controlled manner into the exhaust pipe of the internal combustion
engine when the gaseous reducing agent in the reduction catalytic
converter has been consumed.
[0013] When the vehicle is parked, the values for pressure and
opening time of a valve device disposed in the pressure-relief line
can be stored by an intelligent sensor configuration and, after the
internal combustion engine has been started these values are
interrogated by a control unit that controls the metering of the
reducing agent, are transmitted and the stored current reduction
catalytic converter level can be corrected accordingly.
[0014] In accordance with another feature of the invention, the
pressure-relief line has a cross-sectional opening for conveying
the excess reducing agent, a pressure-control valve is disposed in
the pressure-relief line, and the valve opens the cross-sectional
opening when a predetermined pressure exists in the reservoir to
permit the excess reducing agent to pass through the
pressure-relief line.
[0015] In accordance with a further feature of the invention, the
pressure-relief line has a cross-sectional opening for conveying
the excess reducing agent, an electrically controllable valve) is
disposed in the pressure-relief line, and the valve opens the
cross-sectional opening when a predetermined pressure exists in the
reservoir to permit the excess reducing agent to pass through the
pressure-relief line.
[0016] In accordance with an added feature of the invention, the
flow-measuring device is a flowmeter, preferably, for ammonia.
[0017] In accordance with an additional feature of the invention,
the pressure-relief line has an exit opening inside the
converter.
[0018] In accordance with yet another feature of the invention, the
reducing agent is a liquid.
[0019] With the objects of the invention in view, there is also
provided a method for deNOxing exhaust gas from an internal
combustion engine including the steps of determining, as a function
of operating parameters of at least one of the internal combustion
engine and a reduction catalytic converter operating under an SCR
principle, an amount of reducing-agent solution to be metered and
introducing the amount of the reducing agent into an exhaust pipe
upstream of the converter while the internal combustion engine is
operating, when the internal combustion engine is at a stand-still,
feeding gaseous reducing agent formed as a result of temperature
effects to the converter, and recording and taking into account an
amount of the gaseous reducing agent during a determination of an
amount of reducing-agent solution to be metered during operation of
the internal combustion engine.
[0020] In accordance with yet a further feature of the invention,
the gaseous reducing agent is supplied at a location inside the
converter through a pressure-relief line connecting a
reducing-agent reservoir and the converter.
[0021] In accordance with yet an added feature of the invention,
the gaseous reducing agent is fed to the converter when a pressure
in the reservoir exceeds a predetermined pressure level.
[0022] In accordance with yet an additional feature of the
invention, the pressure-relief line is opened with a valve device
disposed in the pressure-relief line when the predetermined
pressure level is reached.
[0023] In accordance with again another feature of the invention,
an amount of the gaseous reducing agent is determined with a
flow-measuring device disposed in the pressure-relief line.
[0024] In accordance with again a further feature of the invention,
an amount of the gaseous reducing agent is determined from a value
for the pressure in the reservoir and a duration of an opening of
the valve device.
[0025] In accordance with a concomitant feature of the invention,
the reducing agent is aqueous urea solution and the flow-measuring
device is a flowmeter for ammonia.
[0026] Other features that are considered as characteristic for the
invention are set forth in the appended claims.
[0027] Although the invention is illustrated and described herein
as embodied in a device and method for deNOxing exhaust gas from an
internal combustion engine, it is, nevertheless, not intended to be
limited to the details shown because various modifications and
structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of
equivalents of the claims.
[0028] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof,
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a block circuit diagram of an active tank-pressure
control configuration and controlled pressure relief into the
exhaust-gas catalytic converter according to the invention; and
[0030] FIG. 2 is a block circuit diagram of a passive tank-pressure
control configuration and controlled pressure relief into the
exhaust-gas catalytic converter according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] A feature that is common to both FIGS. 1 and 2 is that only
those components of the internal combustion engine and the
associated device for deNOxing exhaust gas that are required to
gain an understanding of the invention are illustrated. In
particular, the fuel circuit has not been illustrated. In the
exemplary embodiments, the internal combustion engine shown is a
diesel internal combustion engine, and aqueous urea solution is
used as reducing agent for the aftertreatment of the exhaust gas.
Identical components are provided with identical reference symbols
throughout the figures and are only explained once, with reference
to the description relating to FIG. 1.
[0032] Referring now to the figures of the drawings in detail and
first, particularly to FIG. 1 thereof, there is shown a block
circuit diagram that represents a highly simplified form of a
diesel internal combustion engine 1 that is equipped with a device
2 for deNOxing exhaust gas and to which the air required for
combustion is supplied through an induction duct 3, which is only
partially illustrated. On the outlet side, the internal combustion
engine 1 is connected to an exhaust pipe 4, further along the
exhaust pipe 4 is disposed an SCR storage reduction catalytic
converter 5, referred to below simply as a reduction catalytic
converter.
[0033] To control the internal combustion engine 1, a conventional
engine management system (EMS) 6 is connected to the internal
combustion engine 1 through a data and control line 7, which is
only diagrammatically illustrated in the figure. Signals from
sensors (e.g., temperature sensors for intake air, charge air,
coolant, load sensor, speed sensor) and signals for actuators
(e.g., injection valves, final control members) are transmitted
between the internal combustion engine 1 and the engine management
system 6 through the data and control line 7.
[0034] The device 2 for deNOxing exhaust gas has, in addition to
the reduction catalytic converter 5, which includes, by way of
example, a plurality of catalytic converter units that are
connected in series and are not described in more detail, a
metering control unit (MCU) 8, a reducing-agent reservoir 9 with an
electrically actuable reducing-agent pump 10 for delivering the
reducing agent, and a metering device, in the form of a metering
valve 11. The reducing-agent pump 10 is connected by a suction line
21 to the reducing-agent reservoir 9 and by a feed line 12 to the
metering valve 11. In addition, a non-illustrated oxidation
catalytic converter may be disposed upstream and/or downstream of
the reduction catalytic converter 5.
[0035] In this exemplary embodiment, the reducing agent used is
aqueous urea solution that is stored in the reducing-agent
reservoir 9. On the top of the reducing-agent reservoir 9 there is
a pressure sensor 13, which transmits a signal that corresponds to
the pressure in the reducing-agent reservoir 9 to the metering
control unit 8. The reducing-agent reservoir 9 is also associated
with further non-illustrated sensors that record the temperature of
the aqueous urea solution and the filling level in the
reducing-agent reservoir 10.
[0036] Moreover, the signals from a non-illustrated temperature
sensor disposed upstream of the reduction catalytic converter 5 and
from a non-illustrated exhaust-measuring pick-up, e.g., a NOx
sensor, disposed downstream of the reduction catalytic converter
are transmitted to the metering control unit 8.
[0037] When required, the metering control unit 8 actuates the
electromagnetic metering valve 11, to which urea solution is
supplied from the reducing-agent reservoir 9 through the feed line
12 and with the aid of the reducing-agent pump 10. The urea
solution is injected into the exhaust pipe 4 upstream of the
reduction catalytic converter 5 by the metering valve 11.
[0038] For reciprocal exchange of data, the metering control unit 8
is electrically connected to the engine management system 6, for
example, through a CAN bus 14. The operating parameters that are of
relevance for calculation of the amount of urea solution to be
metered, such as the engine speed, the air mass, the fuel mass, the
control distance of an injection pump, the exhaust-gas mass flow,
the operating temperature, the charge-air temperature, the start of
injection, etc., are transmitted to the metering control unit 8
through the bus 14.
[0039] It is also possible for the functions of the metering
control unit 8 for the reducing-agent metering system to be
integrated into the engine management system 6 of the internal
combustion engine.
[0040] Working on the basis of these parameters and the measured
values for the exhaust gas temperature and the NOx content in the
exhaust gas, the metering control unit 8 calculates the quantity of
urea solution that is to be injected and transmits a corresponding
electrical signal, through an electrical connection line that is
not shown in more detail, to the metering valve 11. The urea is
hydrolyzed and thoroughly mixed as a result of its injection into
the exhaust pipe 4. The catalytic reduction of the NO.sub.x, in the
exhaust gas to form N.sub.2 and H.sub.2O takes place in the
catalytic converter units of the reduction catalytic converter.
[0041] A pressure-relief line 16 branches off in the upper part of
the reducing-agent reservoir 9, in particular, at a filler neck 15
of the reducing-agent reservoir 9. The branching at the filler neck
15, in combination with a non-illustrated float valve, ensures that
it is impossible for any liquid reducing agent to enter the
pressure-relief line 16 even when the reducing-agent vessel 9 is
completely full. The pressure-relief line 16 ends at a location 17
inside the reduction catalytic converter 5. Selecting the feed
point in such a way reliably prevents ammonia from being able to
flow toward the internal combustion engine as a result of a stack
effect forming through the residual heat of the exhaust system when
the internal combustion engine 1 is at a stand-still. Such a
configuration prevents possible corrosion damage to parts of the
internal combustion engine 1, in particular, to bearings, housing
parts, valve seats, and piston heads as a result of the chemically
aggressive nature of ammonia.
[0042] A valve 18, which can be controlled by electrical control
signals from the metering control unit 6, and a flow-measuring
device 19, e.g., a flowmeter for ammonia (NH.sub.3), which when the
valve 18 is open transmits a signal corresponding to the throughput
of the gaseous reducing agent escaping to the metering control unit
6, are disposed along the relief line 16.
[0043] The operation of this device, as diagrammatically
illustrated in FIG. 1, is explained below.
[0044] The pressure in the reducing-agent reservoir 9 is
continuously monitored with the aid of the pressure sensor 13. An
increase in the temperature causes gaseous ammonia to evolve, which
leads to an increase in the pressure in the reducing-agent
reservoir 9. If the pressure in the reducing-agent reservoir
exceeds a limit value, which is determined according to
application, inter alia, as a function of the geometry and
configuration of the reducing-agent reservoir, and also of the
ambient pressure, and that is stored in a memory 22 of the metering
control unit 8, the electric valve 18 is opened by signals from the
metering,control unit 8. Ammonia flows through the flow-measuring
device 19 into the reduction catalytic converter 5. When the
pressure in the reducingagent vessel 9 has been completely reduced,
in which connection the signal from the pressure sensor 13 is
reevaluated, the valve 18 is closed.
[0045] Because the ammonia storage capacity of the SCR catalytic
converter is limited by its volume and its temperature, and the
efficiency of the reduction catalytic converter is also determined
by the quantity of ammonia stored, the quantity of ammonia that
flows into the reduction catalytic converter 5 as a result of the
evolution of gas is recorded by the flow-measuring device 19, and
the value is stored in the memory 22 of the metering control unit
6. While the internal combustion engine 1 is operating, the
metering control unit 6 cyclically determines the efficiency of the
reduction catalytic converter and a desired value for the quantity
of reducing agent that is to be metered. The quantity of reducing
agent is calculated from operating parameters of the internal
combustion engine, such as the air mass, operating temperature,
catalytic converter temperature, and/or load. The quantity of
reducing agent so calculated is then corrected base upon the
additional quantity of ammonia that has already been supplied to
the reduction catalytic converter during the evolution of gas. For
such a purpose, the signal from the flowmeter 19 is evaluated. The
evaluation can be achieved, for example, by storing a relationship
between the quantity of gaseous ammonia that has escaped and the
associated quantity of reducing agent (aqueous urea solution) in a
characteristic diagram or a table. The metered quantity of aqueous
urea, which is calculated as a function of the operating point, is
then reduced by such an amount. It is, therefore, possible to
reliably avoid both an unacceptably high pressure in the
reducing-agent reservoir and slippage of ammonia.
[0046] FIG. 2 shows a tank-pressure control configuration that,
unlike the exemplary embodiments described above, is not active.
Rather, it is passive, and, therefore, an easier and less expensive
way of reducing the pressure in the reducingagent vessel without
influencing the accuracy of metering.
[0047] The device fundamentally corresponds to the structure that
has been explained with reference to FIG. 1. The difference is that
there is no need for a pressure sensor 13, and it is not an
electrically actuable valve 11, but rather a mechanically acting
pressure-control valve 20 that is disposed in the relief line 16.
The pressure-control valve 20 opens automatically when a
predetermined pressure is reached in the reducing-agent vessel 9.
The quantity of gaseous ammonia that escapes is in such a case,
too, recorded by the flow-measuring device 19 and is taken into
account in the metering strategy in the same manner as that
described above.
[0048] As an alternative to the flow-measuring device 19 that
records the quantity of ammonia, it is also possible to determine
the quantity of gaseous ammonia that has escaped based upon the
signals from the pressure sensor 13 and the opening time of the
relief valve 18, 20, for example, by a characteristic diagram or a
table that is stored in the memory 22.
* * * * *