WO1994027139A1

WO1994027139A1 - Method and device for detection of oxidizable materials in a space

Info

Publication number: WO1994027139A1
Application number: PCT/SE1994/000468
Authority: WO
Inventors: Magnus Lindblad; Edward Jobson; Mikael Larsson; Sören ERIKSSON
Original assignee: Volvo AB
Current assignee: Volvo AB
Priority date: 1993-05-19
Filing date: 1994-05-19
Publication date: 1994-11-24
Anticipated expiration: 1995-11-19
Also published as: JPH09500959A; SE510091C2; SE9301715D0; EP0700518A1; SE9301715L

Abstract

The invention relates to a method and a device for detection of oxidizable materials in the exhaust gas outlet of a combustion engine, by means of a measuring device comprising at least one measuring bridge with at least two sensors (D, R) which are arranged in said exhaust outlet, the measuring bridge being exposed to an electrical operating voltage and one of the sensors forming an active sensor (D) by burning said oxidizable materials on its surface which is heated by the resistance, and thereby increasing its temperature-dependent resistance, the other sensor (R) serving as a reference by changing the resistance similar to the active sensor depending on similar environmental parameters without increasing its temperature by the presence of combustible materials, whereby oxidizable materials are detected by measuring voltage variations in the measuring bridge. According to the invention, a flow of air can be delivered to the exhaust gas outlet upstream of the position of the active sensor (D).

Description

Method and device for detection of oxidizable materials in a space

The present invention relates to a method for detection of oxidizable materials in accordance with the subsequent claim 1.

The present invention also relates to a device for detec¬ tion of oxidizable materials in accordance with the subsequent claim 5.

It is known from GB-A-2 185 579 to detect combustible gaseous hydrocarbons by means of a measuring bridge which comprises pellistors. A pellistor is a resistor with a temperature-dependent resistance of a special kind, which is described below in greater detail. The design of pellistors is described in greater detail in GB 2 044 937, for example.

Previously known solutions in which pellistors have been utilized have in common that they have been utilized within areas where there is a desire to detect the presence of certain harmful substances, due to the risk for explosions, such as hydrocarbons in mines, ships etc., without striving for any indication of a directly readable measured value, such as the percentage or ppm rate of a certain substance.

The object of the present invention is to achieve a method and a device which may be applied within new fields of application.

Said object is obtained by means of the method and the device in accordance with the present invention, the features of which will be apparent from the subsequent claims 1 and 5, respectively.

The invention will now be described in greater detail in connection with a preferred embodiment and with reference to the accompanying drawings, wherein Fig. 1 schematically shows an example of an exhaust gas sensor in the form of a pellistor, and Fig. 2 shows schematically an example of a detection device according to the invention,

Fig. 3 shows schematically a system for diagnosis of a catalytic converter, Fig. 4 shows an exhaust gas sensor according to an alter¬ native embodiment, and Fig. 5 is a side view of the exhaust gas sensor according to Figure 4.

Exhaust gas sensors of the pellistor type are previously known, but here they are described in an advantageous embodiment. The name pellistor is derived from "a pellet- shaped resistor". Thus, the pellistor is an element of electrical resistance in the shape of a platinum wire 2 of small dimensions, for example with a diameter of 50 μ , which is dipped in aluminium oxide and inserted in a sleeve 3 in the form of a catalyst, which for example is based on palladium, platinum, nickel or rhodium. The ends 4, 5 of the platinum wire protrude from the sleeve and form the pellistor's two connections to other components in the measuring device. In order to obtain a high response, a maximum ratio between the area and the volume has been sought. In the present case, this has been achieved by forming the sleeve 3 from a thin film, wherein resistance wires with a surrounding sleeve are formed as a cylinder, which is straight or, for example, bent like a U. With a U- shaped sleeve, one of the sides may be exposed to air and the other side to the gas to be measured. The pellistor is a kind of resistor with a temperature- dependent resistance which also develops heat with a relatively high temperature, for example 500-600°C, when a voltage is applied across the pellistor. When the ignition point has been reached for oxidizable materials coming into contact with the surface of the catalyst sleeve 3, these will burn on the surface. The temperature on the pellistor will then rise further due to the heat of reaction which has developed, whereby the resistance in the platinum wire 2 will rise, which can be observed through current or voltage measurements.

As is apparent from Fig. 2, the detection device according to the invention is designed as a measuring bridge which, in the shown example, is a Wheatstone bridge with a first branch in which a first exhaust gas sensor in the form of the pellistor described above is connected. In the follo¬ wing, this is referred to as the active pellistor D. In a second bridge, a second exhaust gas sensor in the form of a second pellistor R is connected, which lacks the above described catalytic sleeve and thus forms a non-active reference pellistor which is intended to compensate for certain environmental factors such as temperature, pressu¬ re, flow, humidity etc. It should be noted that the resistance elements of the exhaust gas sensors R, D do not necessarily have to be pellet-shaped, but other geometric shapes are also possible.

In the two other branches of the bridge there are connected two identical resistors 6, 7 with relatively low resi¬ stance, in the shown example 270 ohm. Parallel to the reference pellistor 1 there is connected a resistor 8, the size of which is specific for each pair of pellistors. Over the two connection points 9, 10 of the pellistors D, R, which points are not common, a voltage source 11 is connected for voltage measurements. Parallel to this there is connected a potentiometer 12 , the moveable terminal 13 of which is connected to the common connection point 14 of both the fixed resistors 6, 7. Over this connection point 14 and the common connection point 15 of the pellistors, there is connected a voltage measuring device 16 which, for example, comprises a signal amplifier for amplification of the voltage signal from the Wheatstone bridge by, for example, a magnitude of 10.

When the bridge is balanced, i.e. when the pellistors' resistance values are mutually equal with any mutal deviations being compensated for by the resistor 8, no voltage difference will arise across the points 14 and 15. Tolerances of the components, non-balance in the bridge may also be compensated for by means of the potentiometer 12, so that a zero value of the voltage measuring device 16 may be adjusted in an environment when oxidizable materials are not present.

When the detection device, and in particular the two pellistors, are placed in a space with oxidizable materials at the same time as the voltage source 11 is connected so that the temperature in the sleeve 3 of the sensor pellis¬ tor rises to a value which causes oxidazable materials to burn on the surface, the resistance of the sensor pellistor will increase. This will result in a non-balance in the measuring bridge so that a voltage arises between the point 14 and 15, which may be read directly from the voltage measuring device 16 in the form of a measured value, or may after signal treatment indicate the presence and also the content of oxidizable materials.

According to the invention, the detection device is especially intended for measuring the operating conditions of combustion engines. One example of such a detection is the detection of the combustion of the fuel, which can be utilized for laboratory tests as well as for a continous supervision of the operation. In the latter case, the detection device may be comprised in an automatic control¬ ling system for adjustment of parameters which influence the combustion conditions so that an optimal combustion may be obtained, for example controlling the fuel injection or the timing of the ignition. In this regard, the detection device is arranged with at least one pair of pellistors, i.e. both the active pellistor and the reference pellistor, in the outlet of the combustion chambers, immediately after these or further away in the exhaust system.

The detection device may also detect the function of a catalytic converter in the exhaust outlet of a combustion engine. In this regard, the detection device may be placed after the catalytic converter in order to analyze the exhaust gases and more precisely the presence or the content of non-burnt oxidizable materials, or for determi¬ ning the conversion ratio of the catalytic converter, i.e. the coefficient of efficiency of the catalytic converter. In the latter case, a twin system with two pairs of pellistors is used, one pair being arranged before the catalytic converter and one after the catalytic converter. By using double measuring bridges, double output signals are obtained, which are compared and signal-treated in a signal treatment system. However, for stable engines it is sufficient with one pair of pellistors after the catalytic converter for detection of the coefficient of efficiency.

In Figure 3 there is schematically shown a system for diagnosis of a catalytic converter 17, which comprises an exhaust gas sensor 18 which comprises a first, active pellistor D and a second pellistor R (see also Figure 2) . The exhaust gas sensor 18, which is arranged upstream in relation to the catalytic converter 17, is part of a measuring bridge in accordance with the above-mentioned. The measuring bridge comprises a voltage measuring device 16 which is connected to a computer-based control unit 19, which is adapted to register the values of the voltage in the measuring bridge being measured by the voltage easu- ring device 16. Furthermore, there is provided an additio¬ nal exhaust gas sensor 20 in the form of a second pair of pellistors (which comprises two pellistors, analogous to what has been mentioned above) arranged downstream of the catalytic converter 17. The second exhaust gas sensor 20 is connected to the control unit 19 via a second voltage measuring device 21.

The values being measured in the voltage measuring devices 16 and 21 of the voltage non-balance in the two respective measuring bridges in which the exhaust gas sensors 18, 20 are a part are delivered to the control unit 19. There¬ after, in the control unit 19 a conversion of the measured voltage values to measures of the concentration of CO and HC compounds in the exhaust gases occurs. This conversion takes place through the fact that CO and HC compounds react with oxygen on the catalytic surfaces of the exhaust gas sensors 18, 20. These reactions generate heat which increases the temperature of said catalytic surfaces. The higher the concentration of combustible material, the greater the heat generation will take place, provided that there is a sufficient amount of oxygen. The increase of temperature provides an increased resistance in the measuring device, which results in a voltage difference being measured in the voltage measuring devices 16, 21, respectively.

In practice, the voltage may be measured for known gas mixtures, after which a relationship between this increase of the temperature and the concentration of CO of HC compounds is calculated. This relationship, which has proven to be substantially linear over a very large temperature range, may then be employed when determining the concentration of CO and HC compounds in a certain exhaust gas mixture.

The efficiency, or rather the coefficient of efficiency, of the catalytic converter 17, may be determined through the relationship η = (1 - X_after/X_before) , where X^ denotes the concentration (in g/s) of CO and HC compounds in the exhaust gas downstream of the catalytic converter 17, and ^x _before denotes the concentration of CO and HC compounds upstream of the catalytic converter 17. The value of the coefficient of efficiency 77 is then used for indicating whether the catalytic converter is not operating or has a reduced capacity for purifying the exhaust gases. On the other hand, it is also important to be able to determine that the catalytic converter 17 is not defective if the vehicle for some other reason has too high emissions.

According to an alternative embodiment, only one exhaust gas sensor 20 (in the form of just one pair of pellistors) may be utilized, arranged downstream of the catalytic con¬ verter 17. In order to determine the coefficient of efficiency of the catalyst 17 it is necessary to know the emissions before and after the catalytic converter 17. With only one exhaust gas sensor 20, the emissions and thus also the coefficient of efficiency of the catalyst may be determined for every given condition of operation (i.e. the relationship between load and engine speed) by comparing a measured value and a stored value corresponding to the emissions from a normal engine.

Furthermore, the values of the CO and HC compounds, which have been calculated according to that mentioned above, may be utilized in a feedback loop in order to minimize the emissions after the catalytic converter. In this regard, the measured values which are delivered from the exhaust gas sensor may, for example, be used to increase or decrease the amount of injected fuel, adjust the ignition point or change the amount of EGR (Exhaust gas recir- culation) .

The values which have been measured by the voltage measu¬ ring devices 16, 21 may also be used in order to determine a value of the temperature of the exhaust gases. A value of the exhaust gas temperature is derived from the resistance of one of the resistor elements of the exhaust gas sensor. The value of the exhaust gas temperature is utilized for protecting the exhaust system (in particular the catalytic converter) from being overheated. By measuring the exhaust gas temperature, the delivery of fuel and the ignition may be controlled so that one gets closer to the limit of the allowable temperature but without risking that the tempera¬ ture becomes too high. This will lead to a decreased fuel comsumption (for certain driving conditions) . In addition to this, the exhaust gas temperature may be used for an auxiliary control of the efficiency of the catalytic converter. Finally, the exhaust gas temperature may be used for determining the light-off temperature of a specific catalytic converter.

In accordance with an additional embodiment, the signal from the exhaust gas sensor may be analyzed during a starting procedure for detecting at what point of time the catalytic converter will ignite (the so called "light-off time") .

Besides detection of the exhaust gases, the detection device may also be used for detecting and analyzing the vapour pressure of the fuel directly in the fuel tank, in which a pair of pellistors detects the quantity of hydro- carbons above the liquid surface of a liquid fuel in the tank. Due to the fact that the fuel in the gaseous phase in the tank has an excess of combustible materials at "normal" temperatures, the amount of oxygen in the tank is in reality detected. The vapour pressure may then be cal¬ culated from this. Preferably, the measurement takes place immediately before a cold start, possibly with a warm start when stability has commenced, i.e. a stability in the tank due to a communication with the atmosphere. With stable conditions, the outside temperature is also measured in order to create a reference and a correction factor.

When arranging the pellistor pair of the detection device in the exhaust gas outlet of a combustion engine, the device may be used for detecting incorrect ignition by analyzing the combustion heat. Furthermore, transient emissions may be characterized by examining the output signal of the detection device. In this regard, the amount of non-burnt materials in the emissions may be analyzed at varying engine speeds, particularly during acceleration and retardation.

For a good correlation, it is also desired that the exhaust gas flow does not have too rich a mixture, which is a result of too rich a fuel/air mixture. In applications involving rich mixtures, the pellistors may be arranged in a by-pass flow where there is added air in order to make the mixture leaner. Alternatively, a voltage may be applied over a ceramic carrier (ion pump) which on one of its sides has access to the atmosphere and on its other side has access to the gas environment being analyzed. In Figure 4 there is shown an embodiment of an exhaust gas sensor 22 which delivers oxygen to the exhaust gases. The exhaust gas sensor 22 comprises an essentially box-shaped sensor body 23 which is formed by an ion-conducting ceramic substance, preferably Zr0₂. On the cover 24 of the sensor body 23 there is arranged a first active electrode 25, which is preferably formed by a platinum conductor pattern. The sensor 22 also comprises a reference electrode 26, which likewise comprises a conducting platinum pattern. The electrodes 25 and 26, respectively, correspond to the pellistor pair D and R, respectively, which have been described above with reference to Figure 2.

According to Figure 5, the sensor 22 also comprises a third electrode 27 which is arranged on the underside of the cover 24 and which is composed of a metal which allows dissociation of oxygen, e.g. nickel or platinum. The third electrode 27 is arranged so that it is in contact with the surrounding air. A current is directed from the first, active electrode 25 on the exhaust gas side to the third electrode 27. In this way, there is a transport of oxygen ions from the surrounding air to the first, active elec¬ trode 25. Here, the oxygen will react with excess hydrocar¬ bons and carbon monoxide. In this manner, a proper measu¬ ring signal is also obtained from the exhaust gas sensor also "rich" exhaust gases, i.e. exhaust gases having an oxygen deficit. By adding oxygen, the reading of the exhaust gas sensor 22 thus becomes independent of the oxygen content of the exhaust gases. In order to simplify the -process, there is arranged, preferably in connection with the exhaust gas sensor, means 28 for giving the third electrode a certain negative potential (of about a few volts) in relation to the first and the second electrode 25, 26.

It should be noted that the exhaust gas sensor 22 does not have to be arranged in a by-pass flow, but may also be arranged in connection with the exhaust gas outlet itself.

Practical trials have shown that the amount of added oxygen may vary within a relatively large interval. There is therefore no need for a precise control of the amount of oxygen excess, the important aspect being that there is a sufficiently large excess of oxygen.

It should be noted that the excess of oxygen may be provided by adding oxygen only, or alternatively some oxygen-containing gas such as for example air.

From different experiments, it has been verified that the reading of the detector device over a large measuring field is largely proportional, which has been observed by measuring concentrations of hydrocarbons. A good correla¬ tion has also been observed when measuring carbon monoxide.

Non-burned remains in the exhaust system of a combustion engine may be formed by a plurality of substances. By means of the detection device in accordance with the invention, materials with different ignition temperature may be selected by giving the sensor pellistor different operating voltages, and thereby different temperatures created by the resistance. This may be solved either by providing twin detection devices with different operating voltages, or by operating one single detection device in different modes or intervals with separate voltage levels for the different time intervals, with an analysis which in a corresponding way is time-divided in connection with the signal treat¬ ment.

The invention is not limited to the embodiment which has been described above and in the drawings, but may be varied within the scope of the appended claims. For example, the measuring bridge may be designed with another circuit solution. With ideal components the fixed resistor 8 as well as the potentiometer 12, for example, may be omitted. Furthermore, there may be different voltage and resistance values.

Claims

1. Method for detection of oxidizable materials in the exhaust gas outlet of a combustion engine by means of a measuring device comprising at least one measuring bridge with at least two sensors (D, R) which are arranged in said exhaust gas outlet, the measuring bridge being exposed to an electrical operating voltage and one of the sensors forming an active sensor (D) by burning said oxidizable materials on its surface which is heated by the resistance, and thereby increasing its temperature-dependent resis- tance, and the other sensor (R) serving as a reference by changing the resistance, similar to the active sensor, depending on similar environmental parameters without increasing its temperature through the presence of com¬ bustible materials, whereby oxidizable materials are being detected by measuring voltage variations in the measuring bridge, c h a r a c t e r i z e d i n that a flow of air is delivered to the exhaust gas outlet upstream of the position of the active pellistor (D) .

2. Method according to claim 1, c h a r a c t e r i ¬ z e d i n that an electrical voltage is applied over a ceramic carrier, which on its one side has access to the atmosphere and on its other side has access to the exhaust gases in the outlet.

3. Method according to either claim 1 or claim 2, c h a ¬ r a c t e r i z e d i n that misfires in the combustion engine are detected.

4. Method according to any of the preceding claims, c h a r a c t e r i z e d i n that the delivered voltage is varied in order to detect different materials with different ignition temperatures.

5. Device for detection of oxidizable materials in the exhaust gas outlet of a combustion engine, in which device there is comprised at least one measuring bridge with at least two sensors (D, R) which are adapted to be arranged in said exhaust gas outlet, and an electrical voltage source for applying a voltage to the measuring bridge, by means of which one of the sensors (D) forms an active sensor adapted to burn said oxidizable materials on its surface which is heated by the resistance and thereby in¬ creasing its temperature-dependent resistance, and the other sensor (R) is a reference sensor adapted to change the resistance similar to the active sensor pellistor depending on similar environmental parameters, whereby an output signal from the bridge in the form of an electrical voltage constitutes an indication of the presence of oxidizable materials in the exhaust gas outlet, c h a - r a c t e r i z e d i n that the device comprises means for delivering a flow of air upstream of the position of the sensor pellistor (D) .

6. Device according to claim 5, c h a r a c t e r i - z e d i n that said means comprises a ceramic carrier over which an electrical voltage is applied, which carrier on its one side has access to the atmosphere and on its other side has access to the exhaust gases in the outlet.

7. Device according to either claim 5 or 6, c h a r a c ¬ t e r i z e d i n that the sensors (D, R) are encased in a protecting cover.

8. Method for detection of oxidizable materials in the exhaust gas outlet of a combustion engine, by means of a measuring device comprising at least one measuring bridge with at least two sensors (D, R) which are arranged in said exhaust gas outlet, the measuring bridge being exposed to an electrical operating voltage and one of the sensors forming an active sensor (D) by burning said oxidizable materials on its surface which is heated by the resistance, and thereby increasing its temperature-dependent resistan¬ ce, and the other sensor (R) serving as a reference sensor by changing the resistance similar to the active sensor depending on similar environmental parameters without increasing its temperature by the presence of combustible materials, whereby oxidizable materials are detected by measuring variations of the voltage in the measuring bridge, c h a r a c t e r i z e d i n that the coeffici¬ ent of efficiency of a catalytic converter (17) for purification of the exhaust gases is analyzed by arranging an active sensor (R) downstream of the catalyst (17) .

9. Method according to claim 8, c h a r a c t e r i ¬ z e d i n that the coefficient of efficiency of the catalytic converter is analyzed by arranging an additional active sensor upstream of the catalytic converter (17) .

10. Method according to either claim 8 or claim 9, c h a ¬ r a c t e r i z e d i n that the delivered voltage is varied for detection of different materials with different ignition temperatures.

11. Method according to any one of the preceding claims, c h a r a c t e r i z e d i n that both of the sensors (D, R) are arranged above the liquid level in a fuel tank for liquid fuel for detection of the vapour pressure.

12. Method according to any one of claims 8 to 11, c h a ¬ r a c t e r i z e d i n that a flow of air is delivered to the exhaust gas outlet upstream of the position of the active sensor (25) .

13. Method according to claim 12, c h a r a c t e r i ¬ z e d i n that an electrical voltage is applied across a ceramic carrier which on its one side has access to the atmosphere and on its other side has access the exhaust gases in the outlet.

14. Device for detection of oxidizable materials in the exhaust gas outlet of a combustion engine, which device comprises at least one measuring bridge with at least two sensors (D, R) which are adapted to be arranged in said exhaust gas outlet, and an electrical voltage source for applying a voltage to the measuring bridge by means of which one of the sensors (D) forms an active sensor adapted to burn said oxidizable materials on its surface which is heated by the resistance, and thereby increasing its temperature-dependent resistance, and the other sensor (R) is a reference sensor adapted to change the resistance similar to the active sensor depending on similar environ¬ mental parameters, an output signal from the bridge in the form of an electrical voltage constituting an indication of the presence of oxidizable materials in the exhaust gas outlet, c h a r a c t e r i z e d i n that at least one sensor pellistor (D) is arranged downstream of the cataly¬ tic converter (17) , whereby the device is adapted for analysis of the efficiency of the catalytic converter (17) .

15. Device according to claim 14, c h a r a c t e r i ¬ z e d i n that an additional sensor is arranged upstream of the catalytic converter (17) .

16. Device according to either claim 14 or claim 15, c h a r a c t e r i z e d i n that the sensors (D, R) are encased in a protecting cover.