GB2347219A

GB2347219A - Correcting for water influence the signal of a sensor for reducing gas in exhaust gas

Info

Publication number: GB2347219A
Application number: GB0003734A
Authority: GB
Inventors: Tillmann Braun; Klaus-Jurgen Marquardt; Ralf Moos; Ralf Muller; Carsten Plog
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 1999-02-23
Filing date: 2000-02-17
Publication date: 2000-08-30
Anticipated expiration: 2020-02-17
Also published as: GB2347219B; ITRM20000080A1; IT1315836B1; FR2790088B1; DE19907669C1; ITRM20000080A0; FR2790088A1; GB0003734D0

Abstract

A method of correcting the influence of water on the signal of a sensor for a reducing agent in IC engine exhaust gas, such as ammonia which has been added to reduce NOx. Exhaust-gas moisture is determined taking into consideration moisture originating from combustion and moisture in induction air, the moisture originating from combustion being determined by one or more sensors which characterise the operating state of the IC engine, such air induction air mass flow, fuel mass flow, air temperature or pressure, or from the oxygen content of the exhaust gas. The moisture in the induction air is determined with a moisture sensor. The sensor signal is then corrected in dependance on the determined exhaust-gas moisture. The ammonia injection system described also determines defects in other components of the system.

Description

Method of correcting the signal of a sensor for an exhaust gas of an mternal combustion engme The invention concerns a method of correcting the influence of water on the signal of a sensor for detecting the reducing agent metered into the exhaust gas of an internal combustion engine.

The main emitters of nitrogen oxides (NOx) in the industrialized states are traffic, fossil-fuelled power stations and industrial plants. While power-station and industrial emissions are increasingly being cut back, the proportion accounted for by traffic is becoming more and more significant.

The NOx emissions of petrol-operated spark-ignition engines can be drastically reduced by operation at k=1 and post-engine exhaust emission control by means of a three-way catalytic converter. Owing to its principle, this possibility does not exist in the case of a mixture-controlled diesel engine which is operated with a leanerthan-stoichiometric mixture. On account of the high oxygen content in the exhaust gas, so far no-one has developed a catalytic converter which can reduce raw NOx emissions without the addition of reducing agents, generally hydrocarbons or ammonia-forming compounds.

For removing nitrogen from power-station emissions, SCR methods (selective catalytic reaction methods)-as described for example in DE 245888-are used to convert nitrogen oxides selectively into water and nitrogen by adding the reducing agent ammonia (NH3). Control of this kind has proved to be suitable in the case of the slow changes over time of the exhaust flow rate and the NOx concentration occurring in the power station sector.

In a modified form, this method can also be used for removing nitrogen from diesel-engine exhaust gases. Therefore, numerus methods of reducing nitrogen oxides in exhaust gases by controlled NH3 addition are described for use in a dieseloperated motor vehicle, in particular in a commercial vehicle. See, for example: [1] Lepperhoff G., Schommers J.: Verhalten von SCR-Katalysatoren im dieselmotorischen Abgas [Behaviour of SCR catalysts in diesel-engine exhaust gas].

MTZ 49, (1988), 17-21, and [2] Huthwohl, G., Li Q., Lepperhoff G.: Untersuchung der NOx-Reduzierung im Abgas von Dieselmotoren durch SCR-Katalysatoren [Investigation of the NOx reduction in the exhaust gas of diesel engines by SCR catalysts]. MTZ 54 In all methods, the use of one or more exhaust-gas sensors proves to be advantageous. For instance, a method which requires one or two NOx sensors is described in EP 0 554 766. A method which requires an NH3 sensor is proposed in DE 41 17 143 A1 and a method in which two NH3 sensors prove to be necessary is proposed in DE 42 17 552. For a further method, proposed in DE 195 36 571, an NH3 sensor is likewise indispensable. The exhaust-gas sensors necessary for these methods of nitrogen oxide reduction must not be cross-sensitive to other components occurring in varying concentrations in the exhaust gas, in particular water, carbon dioxide or oxygen. An NOx sensor suitable for such methods also must not be cross-sensitive to NH3 and an NH3 sensor must not be cross-sensitive to NOx.

NH3 sensors suitable for both power-station and diesel-engine applications have been proposed in DE 197 03 796 and in DE 43 34 071, respectively. Such sensors, like other sensors which can detect reducing agents, have cross-sensitivity to water, which occurs in the exhaust gas in variable concentrations. In the detection of the relevant water content in the engine exhaust, this water cross-sensitivity is often reflected by a shift in the zero line of the sensor output signal, but not by a change in the slope of the characteristic, i. e. the sensitivity of the sensor output signal does not change or does so only insignificantly.

It is also known that, both in the case of resistive exhaust-gas sensors and in the case of capacitive exhaust-gas sensors, effects of ageing occur after prolonged use, these effects being reflected by a shift in the zero line of the sensor output signal, while the slope of the characteristic remains constant.

DE 43 32 512 C2 discloses an optochemical sensor for detecting ammonia which has a moisture cross-sensitivity with respect to water. To compensate for the moisture cross-sensitivity, the moisture is measured with an additional moisture sensor.

In this case, the same type of sensor as the ammonia sensor may be used as the moisture sensor.

An ammonia sensor for use under agricultural conditions is described in the Caplus Databank on STN; AN: 1999: 8649, AB; Rechenbach, T. et al. in Eurosensors XII, Prox. 12th Eur. Conf. Solid-States Transducers 9th UK Conf. Sens. Their Appl.

(1998), Vol. 1,556-559. In addition to an NH3-sensitive layer, it comprises an H20 sensitive layer to compensate for the moisture cross-sensitivity.

The present invention seeks to provide a low-cost and reliable method by which it is possible to correct the influence of water on the signal of an exhaust-gas sensor which detects the concentration of reducing agent in the exhaust gas of an internal combustion engine.

According to the present invention there is provided a method of correcting the influence of water on the signal of an exhaust-gas sensor adapted to detect the concentration of reducing agent in the exhaust gas of an internal combustion engine, wherein -the exhaust-gas moisture present in the exhaust gas is determined, taking into consideration the moisture originating from the combustion and the moisture contained in the induction air, the moisture originating from the combustion being determined by means of one or more sensors which characterize the operating state of the internal combustion engine or the oxygen content of the exhaust gas, and correcting the sensor signal supplied by the exhaust-gas sensor in dependence upon at least some of the data so determined.

Advantageous developments of the method according to the invention are the subject of further claims.

According to the method of the invention of correcting the influence of water on the signal of an exhaust-gas sensor which detects the concentration of reducing agent in the exhaust gas of an internal combustion engine, the exhaust-gas moisture present in the exhaust gas is determined and consequently the sensor signal supplied by the exhaust-gas sensor is corrected.

The reducing agent may be, in particular, ammonia or a compound which releases ammonia or is converted into ammonia.

The concentration of water in the exhaust gas cl20, tut is made up of two components: one component CHp, air, which originates from the atmospheric humidity in the induction air and depends on the ambient conditions, and one component cmo. r, which originates from the combustion of the fuel. The following applies: CH20, tot = CH20, air + CH20, (1) The component CH2O, F, which originates from the combustion of the fuel and changes constantly with the driving state, can be determined from other data known to the engine electronics, such as for example the air mass flow mA or fuel mass flow mF.

In addition to the direct measurement of the air mass flow mA by means of a suitable sensor, this variable may also be obtained from the measurement of the engine speed, charge-air temperature and charge-air pressure. mF is also referred to as fuel consumption. The relationship between mA, mF and CH2O, F is as follows : =' m (2) mX + mL (2) The factor a is about 1.2 and indicates how much water is produced from the fuel during combustion. The following applies for diesel fuel: 1 kg of diesel fuel burns to produce about 1.2 kg of water. If the concept of the moisture equivalent ME is introduced, the following applies for this: FA = ##### (3) giving: CH20. F = a x ME (4) The following applies for the only slowly changing component cH20, air contained in the induction air: mL CH2OLoft = f x = f x (1-FA) (5) mK + mL Here, f indicates the amount of water contained in the air; by contrast with the popular definition of absolute humidity, however, in water mass related to air mass.

The water component originating from the induction air CH2o. air in the exhaust gas can, according to one embodiment of the invention, be measured directly by means of a moisture sensor. Alternatively, the influence of this component can also be derived from the knowledge of other variables, so that it does not have to be measured directly in every case. An embodiment of this is described in detail below.

Alternatively or additionally, however, the oxygen content of the exhaust gas may also be used for the evaluation. This has the advantage that no data available only in central engine electronics are required for determining the NH3 concentration.

This may take place either by an oxygen sensor being integrated into the NH3 sensor housing, or by an oxygen sensor suitable for oxygen-rich exhaust gases being fitted as an additional exhaust-gas sensor. The increased cost is offset by the advantage that the determination of the water component originating from the combustion of the fuel cnzo. F in the exhaust gas can take place independently of data on the input side of the engine, such as the air mass flow mA and fuel consumption mF. Consequently, the NH3 sensor operates independently of the engine control, and can also be used, for example, directly as a threshold switch. Such an arrangement provides the additional advantage that it is possible to dispense with the fuel consumption measurement and/or the measurement of the induction air mass flow.

The oxygen sensor supplies an output signal, which indicates the oxygen partial pressure PO2 of the exhaust gas. The air ratio), can be concluded from the PO2 :

The factor v is fuel-dependent and, with pure octane, is about 0.36.

The relationship between the air ratio A, the air mass flow mA and the fuel consumption mF is as follows: # mL x 1 FA x1 (7) mx Lmin (1-FA)Lmin Ljnin indicates the air requirement in the case of complete combustion.

Depending on the fuel, Lmin is approximately 14.8 kg of air/kg of fuel. It is accordingly evident that, with an oxygen sensor in the exhaust gas, it is possible to dispense with the air mass flow sensor and/or the fuel consumption measurement.

In a further embodiment, the summary concentration of water vapour concentration cH20, tot = cH20, air + cH2o, F can also be measured by a moisture sensor.

To sum up, there are the following preferred methods of measuring atmospheric humidity: la Determination of the concentration of water vapour in the exhaust gas produced during combustion by determination of the moisture equivalent ME from the fuel mass flow (mF) and air mass flow (mA). lb Determination of the concentration of water vapour in the exhaust gas produced during combustion by determination of the moisture equivalent ME from the oxygen partial pressure of the exhaust gas by means of a X probe.

2a Determination of the summary concentration of water vapour from the concentration of water vapour contained in the induction air and from the concentration of water vapour CH2o. tot produced in the exhaust gas during combustion by an atmospheric humidity sensor in combination with the determination of the moisture equivalent ME according to method la, i. e. by determination of the fuel mass flow (mF) and air mass flow (mA).

2b Determination of the summary concentration of water vapour from the concentration of water vapour contained in the induction air and from the concentration of water vapour CH20, tot produced in the exhaust gas during combustion by an atmospheric humidity sensor in combination with the determination of the moisture equivalent ME according to method lb, i. e. by measurement of the oxygen partial pressure p02 of the exhaust gas by means of a x probe The invention is explained in more detail on the basis of a preferred embodiment with reference to the drawings, in which: Figure 1 shows the signal change of an NH3 sensor in the range of 0-100 ppm of NH3 for concentrations of water vapour of 2.5% by volume, 5% by volume and 10% by volume, based on 0 ppm of NH3 and 2.5% by volume of H20 in the NH3. The measurements reveal that only the zero point, but not the NH3 sensitivity of the sensor, is changed by the concentration of water vapour.

Figure 2 shows by the example of three different ambient (- 20 C, 60% r. h.; 22 C, 60% r. h.; 30 C, 100% r. h.; r. h.: relative humidity) the variation in the zero point of the sensor from Figure 1, dependent on the operating state of an engine which is characterized by the moisture equivalent ME. According to Equations 3 and 4, ME is a measure of the concentration of water vapour produced in the exhaust gas during combustion. With constant ambient conditions, the zero point of the sensor changes as a result of the operating state of the engine according to a function. This function is shifted virtually parallel when there is a change in ambient humidity.

Figures 3,4 show various stages of an exemplary embodiment of the method according to the invention on the basis of flow diagrams.

A particularly advantageous embodiment of the sensor correction according to the invention is described below on the basis of an SCR catalyst system. In this case, nitrogen oxides are converted at the SCR catalyst with NH3 being added. The metering of the NH3 is to be controlled in such a way that, as far as possible, the entire metered-in NH3 is consumed during the catalytic reaction, so that no NH3 gets into the environment via the exhaust gas flow. This undesired NH3 concentration in the exhaust gas is referred to as NH3 leakage. For monitoring the NH3 concentration, in the exhaust gas there is arranged an NH3 sensor, the water cross-sensitivity of which must be corrected.

The method comprises two stages: -Determination of the zero-value function (Figure 3), -Functional monitoring of the sensor system and exhaust emission control system used (Figure 4).

Determination of the zero-value function: in the warming-up phase of the SCR catalyst, i. e. as long as no NH3 is being metered in, the dependence of the sensor zero point on the operating state of the engine is measured (zero-value function, also referred to below as zero line of the sensor signal). In addition to the advantage of automatic correction of the cross-sensitivity to water vapour according to Figure 1, and consequently the influence of atmospheric humidity, this also offers the possibility of automatic correction of a possibly occurring long-term drift of the sensor zero point.

Functional monitoring: after determination of the zero line of the sensor signal and after the beginning of the NH3 metering, continuous functional monitoring of the entire system can be carried out. In the course of this, the actual value of the NH3 sensor is compared with the setpoint value obtained from the zero-value function (which here corresponds to NH3=0) and plausibility aspects are considered. This offers the advantage of being able to distinguish on the basis of threshold values whether a malfunction that has occurred was caused by a fault in the SCR system or in the sensor system.

In detail, the method proceeds as follows: For determining the zero-value function according to Figure 3, the signal of the NH3 sensor is measured for a number of engine operating phases after starting the engine and before beginning NH3 metering. At the same time, the concentration of water vapour produced during combustion is determined, for example according to method la or lb.

After the elapse of a fixed number of measurements, the standard deviation of the values obtained for the moisture equivalent ME is calculated. It must reach at least a fixed value for it to be possible subsequently to determine the slope a and the axis intercept b of the zero-value function. If appropriate, further measurements are carried out. Once this function has been determined, the slope a must be plausible and a fixed correlation coefficient R2 must at least have been reached before NH3 metering is begun.

Otherwise, a fixed number of repetitions (counting variable z) of the entire algorithm are carried out until all the criteria are satisfied. If the determination of the zero-value function is unsuccessful, this means there is a defect in at least one of the sensors used and a corresponding diagnostic message is issued.

The functional monitoring according to Figure 4 proceeds as follows.

Once NH3 metering has started, the currently measured NH3 concentration (NH3 actual) and the exhaust-gas moisture originating from combustion are determined according to method la or lb. With the aid of the zero-value function, a setpoint signal (NH3se,) of the NH3 sensor is calculated on the basis of the determined exhaust-gas moisture originating from combustion and is compared with the actual value (NH3aCtual). If the deviation exceeds a fixed value, the NH3 metering is reduced until NH3 leakage is impossible.

Subsequently, the zero-value function is newly determined in the way already described above. There follows a comparison both of the slope a and of the axis intercept b of the previous zero-value function with the newly determined zero-value function.

If these deviations lie within a fixed small tolerance, for example 3%, this means that the zero-value function has not changed. Consequently, the established deviation of the setpoint value and actual value of the NH3 sensor signal is attributable to NH3 leakage. The NH3 metering is resumed at a reduced rate. If a fixed number (counting variable n) of instances of NH3 exceeding the described value are established, the NH3 metering is stopped and a diagnostic signal is issued to the effect that the SCR catalyst or the NH3 metering device is defective.

If the deviations of the slope a and/or the axis intercept b lie between the previous zero-value function and the newly determined zero-value function within a tolerance range of, for example, 3-10%, this means that the zero-value function has changed as a result of the drift of at least one of the sensors being used (for example the . probe, air mass flow sensor etc.). A further possible cause is a change in the humidity of the ambient air. The NH3 metering is subsequently resumed and the setpoint value calculation of the NH3 sensor signal is performed on the basis of the new zero-value function.

If the deviation of the slope and/or the axis intercept between the previous zero-value function and the newly determined zero-value function is greater than the fixed greater tolerance, for example 10%, this means that the zero-value function has changed as a result of a serious change in at least one of the sensors being used.

Nevertheless, the NH3 metering can be resumed and the setpoint value calculation of the NH3 sensor signal is performed on the basis of the new zero-value function. If, however, this case occurs a fixed number of times (counting variable m), the NH3 metering is stopped and a diagnostic signal is issued to the effect that there is a defect in the sensor system.

In a further embodiment, instead of the moisture originating from combustion, the total moisture as a sum of the moisture originating from combustion and the moisture contained in the induction air may be determined for the determination of the zero-value function. In the zero-value function, the sensor signal of the exhaust-gas sensor is then plotted against the total moisture. The moisture measurement may be performed, for example, by one of the methods 2a, 2b Otherwise, the method proceeds in precisely the same way as described on the basis of Figures 3 and 4. The advantage of this embodiment is that a change in the moisture of the ambient air is ruled out in the functional monitoring as a cause of a deviation between the setpoint value and actual value of the NH3 sensor signal and therefore a reduction in the NH3 metering occurs less frequently.

Claims

Claims 1. A method of correcting the influence of water on the signal of an exhaustgas sensor adapted to detect the concentration of reducing agent in the exhaust gas of an internal combustion engine, wherein -the exhaust-gas moisture present in the exhaust gas is determined, taking into consideration the moisture originating from the combustion and the moisture contained in the induction air, the moisture originating from the combustion being determined by means of one or more sensors which characterize the operating state of the internal combustion engine or the oxygen content of the exhaust gas, and correcting the sensor signal supplied by the exhaust-gas sensor in dependence upon at least some of the data so determined.
2. A method according to Claim 1, wherein the sensors characterizing the operating state of the internal combustion engine are induction-air mass flow meters and/or fuel mass flow meters and/or revolution counters in conjunction with sensors for determining the charge-air temperature and charge-air pressure and/or an exhaust-gas oxygen sensor.
3. A method according to Claim 1, wherein the moisture contained in the induction air is determined by a moisture sensor.
4. A method according to any one of the preceding claims, wherein -during a measuring phase in which no reducing agent is present at the exhaust-gas sensor, the moisture originating from the combustion is determined for a plurality of engine operating states and the associated signal of the exhaust-gas sensor is determined, and from this a measure of the moisture contained in the induction air is established, -during a measuring phase in which the reducing agent may be present at the exhaust-gas sensor, the moisture originating from the combustion and the associated signal of the exhaust-gas sensor are determined, the value for the moisture originating from the combustion and the constant value for the moisture of the induction air established in the previous measuring phase being used for correcting the sensor signal.
5. A method according to any one of the preceding claims, wherein -during a measuring phase in which no reducing agent is present at the exhaust-gas sensor, the moisture originating from the combustion is determined for a plurality of engine operating states and the associated signal of the exhaust-gas sensor is determined and consequently the zero line of the sensor signal is defined ; during a measuring phase in which the reducing agent may be present at the exhaust-gas sensor, the moisture originating from the combustion and the associated signal of the exhaust-gas sensor are determined, the sensor signal thus established being corrected by recourse to the zero line of the sensor signal established without exposure to reducing agent.
6. A method according to any one of the preceding Claims 1 to 3, wherein -during a measuring phase in which no reducing agent is present at the exhaust-gas sensor, the sum of the moisture originating from the combustion and the moisture contained in the induction air is determined for a plurality of engine operating states and the associated signal of the exhaust-gas sensor is determined and consequently the zero line of the sensor signal is defined; during a measuring phase in which the reducing agent may be present at the exhaust-gas sensor, the sum of the moisture originating from the combustion and the moisture contained in the induction air as well as the associated signal of the exhaust-gas sensor are determined, the sensor signal thus established being corrected by recourse to the zero line of the sensor signal established without exposure to reducing agent.
7. A method according to any one of Claims 5 or 6, wherein, from the comparison of the signal of the exhaust-gas sensor measured during a measuring phase in which the reducing agent may be present at the exhaust-gas sensor with the signal of the exhaust-gas sensor measured without exposure to reducing agent, a fault diagnosis with respect to the function of the system components concerned is carried out.
8. A method according to Claim 7, wherein, if the difference between the signal of the exhaust-gas sensor measured during a measuring phase in which the reducing agent may be present at the exhaust-gas sensor and the signal of the exhaust-gas sensor measured without exposure to reducing agent exceeds a predetermined maximum value, the addition of reducing agent is reduced to such an extent that no reducing agent is present any longer in the exhaust gas and, following this, a renewed determination of the sensor zero line is carried out, the comparison of the old sensor zero line with the new sensor zero line being used either to obtain a fault diagnosis or to change the control parameters of the reducing agent metering system.
9. A method according to Claim 8, wherein formation of the difference between the signal of the exhaust-gas sensor measured during a measuring phase in which the reducing agent may be present at the exhaust-gas sensor and the signal of the exhaustgas sensor measured without exposure to reducing agent takes into account a plurality of measurements of these variables.
10. A method according to any one of the preceding claims, wherein the reducing agent is ammonia or a compound which releases ammonia or is converted into ammonia.
11. A method of correcting the influence of water on the signal of an exhaustgas sensor adapted to detect the concentration of reducing agent in the exhaust gas of an internal combustion engine, substantially as described herein, with reference to and as illustrated in, the accompanying drawings..