Patent Application
20080210575
This application is based on and incorporates herein by reference Japanese Patent Application No. 2007-052000 filed on Mar. 1, 2007.
1. Field of Application
The present invention relates to an improved oxygen sensor for detecting NOx, i.e., nitrous oxide (NO), nitrous dioxide (NO2), etc., in the exhaust gas of an internal combustion engine, with the oxygen sensor being used in a condition of being installed in the engine exhaust system, downstream from a catalytic converter which cleanses exhaust gas. The invention further relates to a method of evaluating the receptivity of the oxygen sensor to NOx.
2. Description of Related Art
In recent years, increasingly severe regulations have been imposed in various countries, concerning the levels of pollutants (in particular, NOx) that are permitted to be emitted in the exhaust gas from motor vehicles. It is therefore becoming necessary to provide oxygen sensors that are capable of detecting very low levels of NOx concentration in engine exhaust gas. Such an oxygen sensor is installed in the exhaust system of a motor vehicle at a location downstream from a catalytic converter, and serves to measure the concentration of any NOx that has not been removed from the exhaust gas by the catalytic converter. The measurement results obtained by the oxygen sensor may be used for example by an engine controller, e.g., engine ECU (electronic control unit), in feedback control for adjusting the air/fuel ratio supplied to the engine such as to maintain the residual NOx concentration close to zero.
Various devices may be installed in the exhaust system of an internal combustion engine, including an air/fuel sensor for measuring the concentration of the exhaust gas produced from the engine, a catalytic converter that is located downstream from the air/fuel sensor in the exhaust system and which removes pollutants including CO gas, CH4 gas and NOx gas, etc., from the exhaust gas, and an oxygen sensor as described above. Adjustment of the engine air/fuel ratio by feedback control based on the detected level of residual NOx, as described above, is necessary due to the fact that there are limitations on the capability of a catalytic converter in removing pollutants from the exhaust gas. By applying such feedback control of the engine air/fuel ratio, the catalytic converter can be utilized as effectively as possible, within its limitations, to ensure that harmful pollutant gases will not be present in the exhaust system at locations downstream from the catalytic converter.
With such a type of oxygen sensor, any small amount of NOx in the exhaust gas that passes into the protective layer 923 is detected by the measured gas side electrode 922. The measurement result (i.e., value of voltage that is developed between the measured gas side electrode and the reference gas side electrode) obtained by the oxygen sensor are supplied to the engine controller. If NOx is detected by the oxygen sensor under a condition in which lean exhaust gas is being cleansed by the catalytic converter, then this indicates that harmful gases including NOx are being passed out from the exhaust system due to the fact that the capability of the catalytic converter to cleanse lean exhaust gas is being exceeded. In that case, the engine controller applies control of the air/fuel ratio supplied to the engine, to modify the air/fuel ratio such as to bring the exhaust gas from the engine into a condition whereby it can be sufficiently cleansed by the catalytic converter.
However since the levels of NOx that flow downstream from the catalytic converter are extremely small, if the protective layer 923 does not have a high value of diffusion resistance, the NOx gas will flow through and out of the protective layer 923 without reacting with the measured gas side electrode 922 to a sufficient extent. Due to this it has been difficult to achieve a sufficiently high performance in detecting minute amounts of NOx in engine exhaust gas using prior art types of oxygen sensor, and there is a requirement for an improved oxygen sensor which would have a capability of detecting such minute amounts of NOx.
It is an objective of the present invention to overcome the above problem, by providing an oxygen sensor capable of detecting minute amounts of NOx that is present in exhaust gas from an engine, at a location downstream from a catalytic converter in the exhaust system of the engine.
It is a further objective of the invention to provide a method of evaluating the receptivity of the oxygen sensor to NOx.
To achieve the above objectives, according to a first aspect, the invention provides an oxygen sensor designed for installation in the exhaust system of an internal combustion engine, downstream from a catalytic converter which cleanses exhaust gas produced from the internal combustion engine, with the oxygen sensor comprising a solid electrolyte which conducts oxygen ions, a measured gas side electrode and a reference gas side electrode respectively disposed at opposing faces of the solid electrolyte, and a protective layer which cover the measured gas side electrode (i.e., covers external areas of that electrode which are not in contact with the solid electrolyte) while allowing the measured gas to permeate to the measured gas side electrode. The oxygen sensor is characterized in that the protective layer is formed to have a thickness that is within a range between 270 μm and 500 μm. The sensor is further characterized in that the protective layer is configured to have a porosity that is within a range between 3% and 7%.
These values are specified since it has been found that if the thickness of the protective layer is made greater than approximately 500 μm or the porosity of the protective layer is made less than approximately 3%, then the diffusion resistance of the protective layer becomes excessively high. As a result, insufficient amounts of NOx permeate through the protective layer to the measured gas side electrode, so that satisfactory detection of small amounts of NOx in the exhaust gas cannot be achieved.
Conversely, if the thickness of the protective layer is made less than approximately 270 μm or the porosity of the protective layer is made greater than approximately 7%, then the diffusion resistance of the protective layer becomes excessively low. In that case, a small amount of NOx that flows into the protective layer will pass through and out of the protective layer excessively rapidly, without reacting with the measured gas side electrode to a sufficient degree. Hence in that case too, satisfactory detection of small amounts of NOx in the exhaust gas cannot be achieved.
However if the protective layer is formed to have a suitable thickness and a suitable porosity, NOx is enabled to penetrate the protective layer but will be hindered from quickly flowing out from the protective layer. Hence, a sufficient amount of NOx can be temporarily retained within the protective layer, ensuring that there will be time for the NOx to react with the measured gas side electrode, and hence sufficient time for the oxygen sensor to detect the level of NOx present in the exhaust gas.
The term “receptivity to NOx”, as applied herein to an oxygen sensor, is to be understood as signifying the extent to which the oxygen sensor achieves an optimum balance between ease of enabling NOx gas to flow into the sensor, to be subjected to detection, and temporary retention (trapping) of NOx to a sufficient degree for enabling effective detection. The effect of improved receptivity of an oxygen sensor to NOx is that improved sensitivity in detecting small amounts of NOx can be achieved.
From a second aspect, the oxygen sensor is preferably configured to have a value of boundary current that is within a range between 0.1 mA and 0.2 mA (with a predetermined fixed voltage applied between the measured gas side electrode and reference gas side electrode) while a gas having a concentration of NO substantially equal to 400 ppm is being supplied to the oxygen sensor.
It has been found that an oxygen sensor having such a value of boundary current has a protective layer with an appropriately high diffusion resistance, thereby ensuring that small amounts of NOx contained in exhaust gas are permitted to flow into the protective layer but will be temporarily retained, i.e., the oxygen sensor can have good receptivity to NOx. If the boundary current is less than 0.10 mA, then the oxygen sensor will have an excessively high diffusion resistance, so that NOx will not readily pass into the protective layer, whereas if the boundary current is greater than 0.20 mA then the diffusion resistance will be excessively low, so that NOx which enters the protective layer will not be retained for a sufficient duration to enable effective detection.
Preferably, the test gas having a concentration of NO substantially equal to 400 ppm is a mixture of N2 and NO gases and is supplied to the sensor at a flow rate substantially equal to 40 liters/minute.
From another aspect, the invention provides a method of evaluating the NOx receptivity of various configurations of oxygen sensor. The method comprises:
supplying a test gas to the oxygen sensor, consisting of a mixture of a combustible gas (at a concentration of equal to or less than 1000 ppm) and NO gas, while successively varying the concentration of NO gas in the test gas,
measuring the respective values of output voltage that are produced by the oxygen sensor, corresponding to successive values of NO concentration, and detecting when the output voltage attains a predetermined voltage, and
evaluating the receptivity of the oxygen sensor to NOx, based on the concentration of NO at which the predetermined voltage is attained.
Considering a plurality of oxygen sensors whose output characteristics are respectively different (e.g., due to differences in porosity of the respective protective layers of the oxygen sensors, etc.), each oxygen sensor can be tested by supplying a test gas while successively varying the concentration of NO in the test gas, and noting the value of NO concentration at which a predetermined value of output voltage is produced from the oxygen sensor. The lower the concentration of NO for which the predetermined output voltage is produced, the better is the receptivity of the oxygen sensor to NOx.
Such evaluation is preferably performed by supplying the test gas to each oxygen sensor at a flow rate of more than 30 L/minute, in order to approximate to the operating conditions of an oxygen sensor that is installed in an actual engine exhaust system.
The NOx receptivity of various configurations of oxygen sensor can thereby be readily compared, enabling an oxygen sensor having improved NOx receptivity (and hence improved sensitivity in detecting low concentrations of NOx in exhaust gas) to be obtained.
A first embodiment will be described referring to
As shown in
The catalytic converter 82 cleanses the exhaust gas of the vehicle engine 80 by removing gaseous pollutants including CH4, CO, NOx, etc.
However if the air/fuel ratio of the vehicle engine 80 is not appropriate, a sufficient degree of cleansing cannot be achieved by the catalytic converter 82, so that some pollutant gases (in particular, NOx) will flow downstream from the catalytic converter 82 and will be emitted to the atmosphere. The oxygen sensor 1 serves to detect whether this condition is occurring, and supplies the detection results to the engine controller 84. The engine controller 84 applies feedback control of the air/fuel ratio supplied to the vehicle engine 80, based on the detection results from the oxygen sensor 1, to reduce or eliminate the emission of residual NOx from the catalytic converter 82.
The measured gas passes through the intake aperture 50 of the measured gas side cover 5 to reach the protective layer 23, via the trap layer 25 and the catalytic layer 24 (as indicated by the arrow G in
When the measured gas changes from a rich to a lean condition, small amounts of NOx will tend to be contained in the measured gas for the reasons described hereinabove, and are detected by the oxygen sensor 1.
Also as described above, the protective layer 23 of this embodiment has a thickness in the range 270˜500 μm and a porosity in the range 3˜7%, since these have been found to be optimum ranges for achieving a high degree of sensitivity in detecting NOx in the measured gas. Specifically, with such values of thickness and porosity of the protective layer 23, once NOx gas has penetrated into the protective layer 23, it will be held trapped within the protective layer 23 (as indicated by the curved arrows g in
It has thus been found that by setting appropriate values of thickness and porosity for the protective layer of an oxygen sensor as described above, it becomes possible for the sensor to reliably detect even very small amounts of NOx contained in a measured gas that is supplied to the oxygen sensor.
The first embodiment has been described above for the case of utilizing a flat layer-configuration gas sensor element 2 for the oxygen sensor. However the principles described are equally applicable to a cup-configuration gas sensor element 2 having the form shown in partial cross-sectional view in
The invention further provides a method of evaluating the receptivity of the oxygen sensor to NOx, as will be described based on the following example, referring to
This evaluation method is described in greater detail in the following, for the case in which a number of oxygen sensor specimens are to be respectively evaluated for the purpose of comparing their receptivities to NOx. Firstly a plurality of oxygen sensor specimens are prepared, having respectively different operating characteristics. It will be assumed that three oxygen sensor specimens are utilized in this example, respectively designated as specimen 1, specimen 2 and specimen 3.
The rate of flow of the test gas is set as 40 L/minute, with the rate of flow of the H2O gas of the test gas being set as 5 L/minute and the temperature of the test gas fixed at 500° C. The respective concentrations of combustible gaseous constituents of the test gas (i.e., CO and CH4) are fixed at 200 ppm and 50 ppm. When the concentration of NO in the test gas is altered, the concentration of N2 in the test gas is altered correspondingly, such as to maintain the rate of flow of the test gas at the predetermined value. The respective concentrations of the test gas CO and CH4 are thereby held fixed, as the NO concentration is varied.
The relationship between values of NO concentration and corresponding output voltage from the oxygen sensor is then measured, for each of the specimens 1 to 3. These relationships are exemplified by the characteristics designated as L1, L2 and L3 in
The lower the NO concentration at which the sensor output voltage becomes 0.6 V, the better is the receptivity of the oxygen sensor to NOx. It can thus be understood that with this example, the predetermined value of 0.6 V is used as an index for evaluating the receptivity of an oxygen sensor with respect to NOx. In the following, the NO concentration at which the output voltage produced by an oxygen sensor attains the predetermined value (with this example, 0.6 V) will be referred to as the NOx sensitivity of that sensor. For example if the predetermined output voltage is attained when the NO concentration is 400 ppm, then the NOx sensitivity will be designated as 400 ppm. Hence, the better is the receptivity of the oxygen sensor to NOx, the smaller will be the value of NOx sensitivity of the sensor (i.e., a smaller NOx sensitivity value signifies better detection sensitivity).
With the example of
It can thus be understood that the invention provides a simple and effective method of evaluating the receptivity of oxygen sensors to NOx.
The range of oxygen sensor output voltage values from 0.5 V to 0.65 V, designated by the letter A in
Another example of evaluating the receptivity of an oxygen sensor to NOx will be described referring to the
A gas mixture having a similar constitution to that described for the second embodiment above, and having an NO concentration that was varied within the range 100 ppm to 600 ppm was supplied to each of the specimens. The receptivity of each oxygen sensor to NOx was evaluated using a predetermined sensor output voltage value of 0.6 V, as described for the second embodiment above.
The evaluation results are shown in
A total of 20 oxygen sensor specimens were prepared, each having a different combination of values of thickness and porosity of the protective layer, with the thickness values being selected from among 100 μm, 200 μm, 300 μm, 400 μm and 500 μm, and with the porosity values being selected from among 8%, 7%, 5%, and 3%. The NOx sensor value at which an output voltage of 0.6 V was produced was then measured for each of the specimens, as described for the second embodiment above.
In
As can be understood from
With respect to the value of 400 ppm which is used as the standard value for evaluating NOx receptivity with this example, it should be noted that under current regulations concerning emission control, the emission control apparatus of a vehicle must be capable of detecting a concentration of NOx in exhaust gas that is as low as 400 ppm.
It can thus be understood from this example that if the porosity is equal to or less than 7% and the protective layer of an oxygen sensor has a thickness of equal to or greater than 270 μm, a sufficient degree of NOx sensitivity can be achieved.
An example of investigating the relationship between NOx sensitivity (as defined hereinabove) of an oxygen sensor and boundary current values will be described in the following. Eight oxygen sensor specimens were prepared, with the NOx sensitivity of each these having been measured beforehand by using the method of the second embodiment hereinabove. For each oxygen sensor, the value of boundary current (which flows between the measured gas side electrode and reference gas side electrode when a predetermined fixed voltage is applied across these electrodes) was measured under a condition of supplying a test gas to the measurement electrode through the protective layer, with the NO concentration of the test gas being fixed at 400 ppm and with the test gas being supplied to the sensor (i.e., to the protective layer) at a flow rate of 40 L/minute and a temperature of 500° C.
The measurement results are shown in
Hence these results show that an oxygen sensor having good receptivity to NOx can be obtained if the protective layer of the oxygen sensor is configured such that the boundary current is less than 0.2 mA.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-052000 | Mar 2007 | JP | national |
