Measuring and testing – Gas analysis – Gas of combustion
Reexamination Certificate
2002-08-29
2003-06-03
Noori, Max (Department: 2855)
Measuring and testing
Gas analysis
Gas of combustion
Reexamination Certificate
active
06571601
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on and claims the priority date of German Application No. 101 42 236.9, filed on Aug. 29, 2001, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The invention relates to an improved method for determining the reducing agent concentration (NH
3
) in the exhaust-gas flow of an internal combustion engine.
Nitrogen can be removed from diesel engine exhaust gases by using the selective catalytic reduction (SCR), with ammonia (NH
3
) as reducing agent. For this purpose, ammonia is carried along in the vehicle either directly or in the form of a compound from which the ammonia is obtained. For the catalytic reduction reaction, ammonia is metered into the engine exhaust gas at a specific ratio to the NO
x
content that is momentarily present in the exhaust gas. A stoichiometric NH
3
—NO
x
ratio must exist in order to achieve the maximum possible NO
x
conversion. A lower ratio leads to a lower conversion while a higher ratio leads to a so-called “NH
3
breakthrough.” The optimum use of the nitrogen-removing catalytic converter is achieved with the aid of a zeolitic NH
3
gas sensor, which is installed as a control element or as an NH
3
breakthrough sensor in the exhaust-gas flow behind the catalytic converter. The NH
3
—NO
x
ratio can thus always be adjusted to the maximum possible NO
x
conversion.
The use of a zeolitic NH
3
gas sensor requires that a connection be found between the measuring variable and the NH
3
concentration to be determined at the sensor. Besides the scaling, this connection is used to compute the compensation of undesirable lateral effects, in particular those to water (H2
O
) in the form of water vapor in the exhaust gas.
A method for correcting the influence of H2
O
on the signal of an NH
3
gas sensor is known from German Patent No. DE 199 07 669 C1. The reference suggests determining a so-called zero-value function of the gas sensor, which is then used to correct the measuring values during the continued operation. In order to determine the zero-value function, the gas sensor signal is measured during several different operating phases of the engine while the engine is running, without feeding NH
3
to the catalytic converter system. The H
2
O concentration in the exhaust gas, the so-called moisture equivalent FÄ (ME), is determined at the same time. The moisture equivalent characterizes the operating state of the engine and is determined from the fuel-mass flow together with the air-mass flow, or from the oxygen partial pressure of the exhaust gas—respectively also in combination with a moisture sensor for determining the H2
O
concentration in air that is suctioned in—or with a moisture sensor in the exhaust-gas flow. In all cases, the ascending gradient and the axial section of the zero-value function, which is approximated as a straight line, is determined from the measured values. In order to correct the measured values of the NH
3
sensor during operation, the respective zero-value function is determined and the measured sensor value is then corrected accordingly.
However, the known method has a number of disadvantages.
The delayed response (response threshold) of the NH
3
sensor for low NH
3
concentrations is not taken into account. In the same way, the influence of the NO
x
gas components on the response threshold and the NH
3
sensitivity are not taken into account. When determining the zero-value function, a linear approximation of the dependence between measured value (Cp) and NH
3
concentration is made, which provides acceptable results only for NH
3
concentrations up to approximately 50 ppm. The adsorption and conversion behavior of the NH
3
sensor, which leads to a distortion of the gas composition arriving at the sensor, is also not taken into consideration.
SUMMARY OF THE INVENTION
Thus, it is the object of the invention to provide a method for correcting the signal from a NH
3
sensor, which method provides an exact value for the measured NH
3
concentration, even with low NH
3
concentrations. At the same time, the method takes into account the adsorption behavior and the conversion behavior of the NH
3
sensor.
The above object generally is achieved according to the invention by a method for determining the reducing agent concentration (NH
3
) in the exhaust-gas flow of an internal combustion engine with a (zeolitic) NH
3
gas sensor that supplies a base measuring value, wherein the base measuring value is initially corrected by an offset value and by a correction value that depends on the H
2
O concentration of the exhaust gas to form an intermediate value. Following this, the intermediate value is corrected by an additional value that depends on the NO
x
concentration of the exhaust gas to obtain a corrected NH
3
measuring value.
The values for the dependence of the measured values on the NO
x
concentration and the NH
3
concentration are recorded in a 2-dimensional table.
The two adjacent columns in the table, which best match the given NO
x
concentration, are selected to determine the NH
3
concentration. Following that, the line that best matches the first intermediate value is determined in the first selected column and the final measured value is then determined through interpolation, initially between the columns and then between the lines.
REFERENCES:
patent: 5540047 (1996-07-01), Dahlheim et al.
patent: 6119448 (2000-09-01), Emmerling et al.
patent: 6247303 (2001-06-01), Broer et al.
patent: 6367320 (2002-04-01), Kueper et al.
patent: 199 07 699 (2000-11-01), None
Irion Eckard
Knezevic Aleksandar
Leye Holger
Smuk Mirko
Conti Temic Microelectronic GmbH
Harrison Monica D.
Kunitz Norman N.
Noori Max
Venable LLP
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