(19)
(11)EP 3 705 880 A1

(12)EUROPEAN PATENT APPLICATION

(43)Date of publication:
09.09.2020 Bulletin 2020/37

(21)Application number: 20170336.0

(22)Date of filing:  08.03.2017
(51)International Patent Classification (IPC): 
G01N 27/16(2006.01)
G01N 27/407(2006.01)
G01N 27/416(2006.01)
F01N 9/00(2006.01)
G01N 33/00(2006.01)
G01N 27/04(2006.01)
G01N 27/409(2006.01)
G01N 27/417(2006.01)
F01N 11/00(2006.01)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 24.03.2016 US 201662312537 P

(62)Application number of the earlier application in accordance with Art. 76 EPC:
17770819.5 / 3411698

(71)Applicant: Kerdea Technologies, Inc.
Greenville, NC 27834 (US)

(72)Inventor:
  • FOSAAEN, Ken Ervin
    Winterville, NC North Carolina 28590 (US)

(74)Representative: Appleyard Lees IP LLP 
15 Clare Road
Halifax HX1 2HY
Halifax HX1 2HY (GB)

 
Remarks:
This application was filed on 20-04-2020 as a divisional application to the application mentioned under INID code 62.
 


(54)RESISTIVE BASED NOX SENSING METHOD AND APPARATUS


(57) Nitrous oxide (NOx) sensors and related methods and systems for use with combustion processes are disclosed. The NOx sensor uses metal oxide semiconductors. The NOx sensor may have two sensing circuits that share a common electrode. The sensing circuits are differentiated by having different porous catalytic filter coatings protecting the metal oxide semiconductors: one sensing circuit has a porous catalytic filter coating that contains a Noxcat material (rhodium, ruthenium, cobalt, palladium, or nickel), while the porous catalytic filter coating of the other sensing circuit is substantially free of Noxcat material. The two sensing circuits are simultaneously exposed to the exhaust gases at a common macro location. The NOx level may be determined based on a difference in resistance between the two sensing circuits and a temperature of the NOx sensor.




Description

BACKGROUND



[0001] This application is related to nitrous oxides sensors, methods of using nitrous oxides sensors, and related systems for use with combustion processes, for example in internal combustion engines and burner systems.

[0002] Nitrous oxides often form in internal combustion engines and burner systems under high temperature conditions. These are primarily NO, NO2 and N2O, and are collectively referred to as NOx. NOx formation is a particular issue in diesel engines, which run leaner and at higher compression ratios than typical spark ignition engines. These nitrous oxides, when released to the atmosphere, may combine with moisture to form nitric acid, leading to adverse health and environmental consequences. To combat this issue, modern engines and burner systems may be equipped with selective catalytic reduction systems which trap the NOx then react them with ammonia (NH3) to form N2 and H2O. This ammonia is typically introduced into the system in the form of urea (CO(NH2)2), which converts to ammonia when heated in the exhaust stream. Nitrous oxide sensors are used to control the performance of such systems. These nitrous oxide sensors are placed after the NOx trap (typically a Selective Catalytic Reduction (SCR) catalyst), and once NOx is detected, the system is triggered to purge the trap using the urea. These nitrous oxide sensors should be durable and be capable of detecting NOx in the 1 to 2000 ppm range, and not experience cross sensitivities to other gasses (H2, CO, sulfur and sulfur compounds, phosphorus and phosphorous compounds, etc.). Many NOx sensors on the market today are electrochemical sensors based on ion conductor technology; such NOx sensors have not proven satisfactory for all situations.

[0003] As such, there remains a need for alternative type of NOx sensors, and related systems and methods.

[0004] In the background art, KR2011-0123558A Wise Control Inc. describes a gas sensor with two sensing pads whose resistance value changes in response to different gases (i.e. one for an oxidising gas and the other for a reducing gas), outputting a sensing voltage corresponding to the change in the resistance values. WO 2011/093975A1 Fosaaen concerns a sub-miniature oxygen sensor packaged for introduction to the exhaust stream of a combustion process. A large step-wise change in the resistance of the element takes place as a result of changes in oxygen content in the exhaust. JP03249555B1 Fuji describes a multicomponent gas sensor. US2016/077044A1 Arkenberg describes an amperometric electrochemical sensor for measuring the concentrations of two or more target gas species in a gas sample or gas stream.

SUMMARY



[0005] The present invention is defined according to the independent claims. Additional features of the invention will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[0006] By way of introduction, this application is related to NOx sensors, methods of using NOx sensors, and related systems for use with combustion processes, for example in internal combustion engines. In particular, the application relates to NOx sensors that that have metal oxide semiconductor-based sensor portions, and related systems/methods. The NOx sensor may be used to sense NOx levels when exposed to exhaust gases of a combustion process.

[0007] In general, the NOx sensor may have two sensing circuits that share a common electrode. The sensing circuits contain the same metal oxide semiconductor material, but are differentiated by having different porous catalytic filter coatings thereon: one sensing circuit has a porous catalytic filter coating that contains a nitrous oxide catalyzing or "Noxcat" material (e.g., rhodium, ruthenium, cobalt, palladium, or nickel), while the porous catalytic filter coating of the other sensing circuit is substantially free of the Noxcat material. The two sensing circuits are simultaneously exposed to the exhaust gases at a common macro location. The NOx level can be determined based on a difference in resistance between the two sensing circuits and the temperature of the NOx sensor. The difference in resistance between the two sensing circuits is a result of the different amount of oxygen that the respective metal oxide semiconductor materials are exposed to due to the catalyzing action of the Noxcat material containing catalytic filter coating. The NOx values may be used to control an operation of the exhaust control system, such as triggering a purging of the NOx trap (e.g., providing urea/ammonia thereto) when the NOx level exceeds a threshold.

[0008] In one example, a nitrous oxide sensor has a substrate; a first oxygen sensing circuit affixed on a first side of the substrate; and a second oxygen sensing circuit affixed on the first side of the substrate. The first oxygen sensing circuit comprises a first sensing electrode; a common electrode; and a first metal oxide semiconductor portion electrically bridging a first physical gap between the first sensing electrode and the common electrode. The second oxygen sensing circuit comprises a second sensing electrode; the common electrode; and a second metal oxide semiconductor portion electrically bridging a second physical gap between the second sensing electrode and the common electrode. A first catalytic filter layer is disposed over the first metal oxide semiconductor portion, with the first catalytic filter layer comprising a Noxcat material. The Noxcat material is one of rhodium, ruthenium, cobalt, palladium, or nickel. A second catalytic filter layer is disposed over the second metal oxide semiconductor portion, with the second catalytic filter layer being substantially free of the Noxcat material. The first and second metal oxide semiconductor portions are disposed in spaced relation to each other such that there is a gap between the first and second metal oxide semiconductor portions. The first and second catalytic filter layers are disposed in spaced relation to each other. The first and second oxygen sensing circuits are configured to provide similar resistances when subjected to an environment free of nitrous oxide, but to provide substantially different resistances when subjected to an environment containing nitrous oxide(s).

[0009] In a second example, a nitrous oxide sensor includes a substrate; an electrically conductive common electrode mounted on a first side of the substrate; an electrically conductive first sensing electrode mounted on the first side of the substrate in spaced relation to the common electrode; and an electrically conductive second sensing electrode mounted on the first side of the substrate in spaced relation to the common electrode. The common electrode is disposed between the first and second sensing electrodes. The first sensing electrode and the common electrode are configured to form a first comb structure, and the second sensing electrode and the common electrode are configured to form a second comb structure. A first metal oxide semiconductor layer is disposed over the first comb structure and forms a first semiconductor bridge between the first sensing electrode and the common electrode. A second metal oxide semiconductor layer is disposed over the second comb structure and forms a second semiconductor bridge interconnecting the second sensing electrode and the common electrode. The first and second metal oxide semiconductor layers are spaced from each other. A first catalytic filter layer is disposed over the first metal semiconductor oxide layer, and a second catalytic filter layer is disposed over the second metal oxide semiconductor layer. The first catalytic filter layer comprises a Noxcat material and the second catalytic filter layer is substantially free of Noxcat material.

[0010] In a third example, a nitrous oxide sensor assembly includes the nitrous oxide sensor of the first example and/or the second example. In addition, the nitrous oxide sensor assembly includes a first voltage divider operatively connected to the first oxygen sensing circuit, and a second voltage divider operatively connected to the second oxygen sensing circuit, and processing circuitry. The processing circuitry is operative to: a) determine a first voltage drop associated with the first oxygen sensing circuit, the first voltage drop proportional to a resistance of the first oxygen sensing circuit; b) determine a second voltage drop associated with the second oxygen sensing circuit, the second voltage drop proportional to a resistance of the second oxygen sensing circuit; c) determine a temperature of the sensor; and d) determine if a purge of a NOx trap associated with a combustion process is needed by determining if a difference between the first voltage drop and the second voltage drop is greater than a first threshold. The first threshold may advantageously be based on the determined temperature of the nitrous oxide sensor. If the nitrous oxide sensor includes a heater portion, such as disposed on a second side of the substrate, the processing circuitry may be operative to determine the temperature of the nitrous oxide sensor by determining a voltage drop across a resistor disposed in series with the heater portion. In some sub-aspects, the processing circuitry is further operative to trigger a purge of the NOx trap in response to determining that the difference between the first voltage drop and the second voltage drop is greater than the first threshold.

[0011] In a fourth example, any NOx sensor disclosed herein may be used to detect nitrous oxide in exhaust from a combustion process. Such a method may include exposing the NOx sensor to the exhaust from the combustion process. The NOx sensor may have first and second oxygen sensing circuits that share a common electrode; wherein the first oxygen sensing circuit comprises a first metal oxide semiconductor portion in contact with the common electrode; wherein the second oxygen sensing circuit comprises a second metal oxide semiconductor portion in contact with the common electrode. The method includes passing some of the exhaust through a first catalytic filter layer to reach the first metal oxide semiconductor portion. The method also includes simultaneously passing some of the exhaust through a second catalytic filter layer to reach the second metal oxide semiconductor portion. The method includes at least partially converting nitrous oxide in the exhaust to oxygen while passing through the first catalytic filter layer prior to reaching the first metal oxide semiconductor portion, but in parallel nitrous oxide in the exhaust is not converted to oxygen while passing through the second catalytic filter layer to reach the second metal oxide semiconductor portion. The method also includes determining a temperature of the NOx sensor. The method also includes determining a nitrous oxide level in the exhaust based on a resistance differential between the first oxygen sensing circuit and the second oxygen sensing circuit, and the determined temperature. The first and second oxygen sensing circuits are configured to provide similar resistances when subjected to an environment free of nitrous oxide, but to provide substantially different resistances when subjected to an environment containing nitrous oxide(s). The process may continue by comparing the resistance differential to a threshold, and triggering a purging of the NOx trap (e.g., providing urea/ammonia thereto) when the determined NOx level exceeds the threshold.

[0012] The various examples discussed above may be used alone or in any combination. The various apparatus disclosed herein may operate according to any combination of various methods disclosed herein, and vice versa. Further, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS



[0013] 

Fig. 1 shows a schematic representation of an engine, having an NOx sensor of the present invention and/or where the method of the one or more embodiments of the present invention may be implemented.

Fig. 2 shows a schematic representation of the electrode configurations of one embodiment of an NOx sensor.

Fig. 3 shows the apparatus of Fig. 2 with the addition of the MOS layer and the differing porous protective (catalytic filter) layers.

Fig. 4 shows a side view of the apparatus of Fig. 3.

Fig. 5 shows a simplified schematic of the NOx sensor of Fig. 3 connected to a controller via a connecting circuit.

Fig. 6 shows one multiple-cylinder configuration with a common NOx sensor for all cylinders.

Fig. 7 shows an exemplary process flow for one or more aspects.


DETAILED DESCRIPTION



[0014] In one or more aspects, the present application is directed to a method of sensing NOx levels using a resistive based NOx sensor exposed to exhaust gases of a combustion process, and related devices and systems. In exemplary embodiments, the NOx sensor may have two sensing circuits that share a common electrode. The sensing circuits contain the same metal oxide semiconductor material, but are differentiated by having different porous catalytic filter coatings thereon: one sensing circuit has a porous catalytic filter coating that contains a nitrous oxide catalyzing or "Noxcat" material, while the porous catalytic filter coating of the other sensing circuit is substantially free of the Noxcat material. The term Noxcat material, as used herein, means any one of the following materials: rhodium, ruthenium, cobalt, palladium, or nickel, which are all class VIII elements in the periodic table of elements, and which act as catalysts to convert NOx to N2 and O2 at exhaust gas temperatures. The two sensing circuits are simultaneously exposed to the exhaust gases at a common macro location. The NOx level is determined based on a difference in resistance between the two sensing circuits. The sensed NOx level may be used to control an operation of the exhaust control system, such as triggering a purging of the NOx trap (e.g., providing urea/ammonia thereto) when the NOx level exceeds a threshold.

[0015] In one or more aspects, the sensor element is composed of a dielectric substrate onto which a platinum (or other suitable material) resistor is applied. This resistor can either be used as a means of measuring the temperature of the sensor element in a passive state due to the strong linear relationship between the resistance and the temperature, or (preferably) it can be used to control the temperature of the sensor element to a narrow value by adjusting the applied voltage; in this case it would function as a heater.

[0016] In one or more aspects, also applied to the ceramic substrate, advantageously on the opposite side, is a refractory (ex: platinum, palladium, or other suitable material) conductor consisting of at least three discrete electrodes. One of these electrodes is a common electrode, which forms two intermeshing combs with the other electrodes. One or more metal oxide semiconducting (MOS) materials are applied over these combs to form respective semiconductor bridges. The MOS materials may be an n-type MOS (ex: TiO2) and/or a p-type MOS (ex: Cr2O3). Both materials require thermal energy (heat) to convert them from essentially dielectric materials to semiconducting materials with the following distinction: a) the resistivity of n-type semiconductors increases with increased oxygen exposure, and b) the resistivity of p-type semiconductors decreases with increased oxygen exposure.

[0017] Porous coatings overlay these MOS materials. The porous coatings contain various catalytic materials to enhance responsiveness and/or protect the sensitive MOS material from contaminants and harmful deposits.

[0018] In one or more aspects, the MOS material and geometry used on each comb is advantageously the same with respect to material and geometry. The intention is that the combs would be held at essentially the same temperature due to their proximity to each other, and they would otherwise respond to the exhaust gasses to which they are exposed to in the same way with respect to their resistance if not for a difference in their respective porous catalytic coatings. The porous coating over one of MOS materials contains a Noxcat material (e.g., rhodium), whereas the porous coating over the other MOS material does not contain the Noxcat material. The purpose of the Noxcat material is to convert NOx to N2 and O2. As a result, the oxygen to which the MOS materials is exposed is therefore different, resulting in a different resistance, with the amount of the resistance difference depending upon the temperature, geometry of the sensing materials, electrodes, etc. Under these conditions, the resistance measurement difference is indicative of the presence of NOx, since the extra oxygen came from the conversion of NOx to N2 and O2. With an n-type MOS material the resistance of the Noxcat material catalyzed portion will be higher than the non-Noxcat material bearing portion, while for p-type semiconductors, the Noxcat material bearing portion will have a lower resistance than the non-Noxcat material bearing portion.

[0019] In one or more aspects, measurement of the resistance difference can be carried out in various ways including the following: Each leg of the sensor is placed in series with two resistors of known value (a voltage divider) and a fixed DC voltage (example 5VDC) is applied across the circuit as shown in Figure 5. The voltage drop across one of the two resistors in each leg is indicative of the resistance of the sensor element portion, which, in turn, is indicative of the state of the exhaust gas for that portion of the sensor, and the difference in the resistance is indicative of the presence of NOx in the exhaust gas.

[0020] By heating the NOx sensor and exposing it to the exhaust stream, the NOx sensor's resistance ranges will change according to the element temperature as well as the oxygen and NOx content of the gas to which it is exposed. The element temperature can be controlled by adjusting the voltage to the heater to hit a target heater resistance (calculated from applied voltage and measured current).

[0021] In one or more aspects, the more complete system is comprised of a sensor containing the sensor element described above (the three electrodes, the MOS material bridges, the catalytic layers, and the optional heater), which is packaged in a housing that is durable enough for the exhaust system environment, circuitry housed in the electronic control unit (or intermediate box) connected to the NOx sensor's heater and oxygen-sensitive MOS portions of the element, and a logic system to convert the range and differences in resistance along with temperature information to a NOx value.

[0022] The NOx sensor and accompanying circuitry described herein has applicability for single and multiple combustion chamber (piston, rotary, etc.) spark ignition (SI) as well as compression ignition (diesel and HCCI) engines and burner systems (boilers, heaters, etc.) where detection of NOx is important to the emissions management control system. However, for simplicity, the discussion herein may generally be in the context of an NOx sensor for a small displacement gasoline powered internal combustion engine, but it should be understood that the NOx sensor(s) disclosed herein may be used in other internal combustion engine applications, such as hydrogen powered engines, other hydrocarbon powered engines, diesel engines, Homogeneous Charge Compression Ignition (HCCI) engines, and Reactivity Controlled Compression Ignition (RCCI) engines. Further, the disclosed method(s) may be used with other combustion processes, such as, for example, those found in furnaces, boilers, and water heaters.

[0023] Figure 1 shows a schematic of an exemplary internal combustion engine 10, which may be of any type (e.g., piston, rotary, nutating disk, etc.). The engine 10 includes at least one combustion chamber 12 with associated piston, valves, etc. (not shown), an intake plenum 18, an exhaust plenum 19, and an engine management system 30. The intake plenum 18 supplies air to the combustion chamber 12. A mass airflow (MAF) sensor or manifold air pressure (MAP) sensor 22, advantageously with associated temperature sensor, is disposed in the intake plenum 18 so that the incoming air conditions may be monitored as the airflow changes due to the opening or closing of the throttle body 15, which is used to control the load/speed of the engine, typically based on operator demand. A controllable fuel metering system, such as a fuel injector 16, supplies fuel to the combustion chamber under control of the engine management system 30. For diesel engines, the fuel injector 16 typically sprays fuel directly into the combustion chamber 12, and ignition takes place as a result of reaching a particular compression level. For spark ignition engines, the fuel injector 16 may spray fuel into the intake plenum 18 or directly into the combustion chamber 12, and a spark ignition device 14, e.g., spark plug, operates under the control of the engine management system 30 to ignite the air and fuel mixture in the combustion chamber 12 at the desired time in the cycle for proper combustion. A NOx sensor 50 is disposed in the exhaust plenum 19, downstream of a NOx trap 21 to sense the amount of NOx in the exhaust gases. Note that the NOx trap 21 may be placed in any appropriate location in the exhaust plenum. The engine management system 30 includes one or more processing circuits 32 (or, "processing circuitry" 32, collectively "controller") that control the fuel supply amount and timing, ignition timing, and other engine parameters based on the input from various sensors and the programming of the processing circuits 32. For example, the engine management system 30 uses the NOx sensor 50, as described below, to help control the urea injector 17 so that the engine 10 operates with desired exhaust conditions. As can be appreciated, the controller 32 may be a centralized microprocessor, or may be a distributed architecture, where different portions of the controller 32, which may be physically distributed, handle different engine functions, such as the computations and signaling associated with the NOx sensor 50, with suitable hardware, firmware, and/or software programming to carry out the functions described herein. Other than the particulars of the NOx sensor 50 and the operation of the processing circuit(s) 32 related to the NOx sensor 50, described in greater detail below, the configuration and operations of the engine 10 are well known to those of skill in the art, and are not discussed further herein in the interests of brevity.

[0024] Referring now to Figs. 2-4, the NOx sensor 50 may be a resistive-based NOx sensor, with an exhaust/NOx sensing portion 60 mounted to a dielectric substrate 52. The dielectric substrate 52 may be a fired ceramic dielectric substrate of any suitable type known in the semiconductor art. The NOx sensor 50 may or may not include a resistance-based heater portion 54 (see Fig. 4), also mounted to the dielectric substrate 52, but in spaced relation to the sensing portion 60.

[0025] In some embodiments, the sensing portion 60 includes a common electrode 62, a first sensing electrode 70, and a second sensing electrode 80, all disposed on a first or top side 52a of the dielectric substrate 52. The common electrode 62 includes a central area 61 and two sets of fingers 64, each set including a plurality of fingers that form respective combs 66,68. The combs 66,68 may advantageously extend in opposite lateral directions from the central area 61, although this is not required. For the configuration shown in Figs. 2-4, the first sensing electrode 70 is disposed on one lateral side of the common electrode 62 in spaced relation thereto. The first sensing electrode 70 includes a set of a plurality of fingers 72 that form a comb 74, and a terminal area 76. The comb 74 of the first sensing electrode 70 is advantageously intermeshed with comb 66 of the common electrode 62 to form a comb structure, with the respective fingers 64,72 being spaced from each other by a gap so that there is not direct contact between the two electrodes 62,70. Likewise, the second sensing electrode 80 is disposed on the opposite lateral side of the common electrode 62 from the first sensing electrode 70, and in spaced relation to the common electrode 62. The second sensing electrode 80 includes a plurality of fingers 82 that form a comb 84. The comb 84 of the second sensing electrode 80 is advantageously intermeshed with comb 68 of the common electrode 62 to form another comb structure, with the respective fingers 64,82 being spaced from each other by a gap so that there is not direct contact between the two electrodes 62,80. Further, there is no direct contact between the first and second sensing electrodes 70,80. Note that the electrodes 70,80 are electrically conductive, and may be formed of any suitable material, such as platinum or palladium or other suitable refractory conductor. Note that the length and spacing of the fingers 64,72,82 of combs 66,68,74,84, and their particular materials, may be adjusted as desired for the particular operating conditions for the NOx sensor 50.

[0026] An n-type (e.g., TiO2) or p-type (e.g., Cr2O3) metal oxide semiconductor (MOS) layer 56 is placed over combs 66,74 so that the layer 56 forms a semiconductor bridge between the first sensing electrode 70 and the common electrode 62. Likewise, an n-type or p-type metal oxide semiconductor layer 58 is placed over combs 68,84 so that the layer 58 forms a semiconductor bridge between the second sensing electrode 80 and the common electrode 62. Note that MOS layer 56 and MOS layer 58 should have identical material composition and optionally geometries (other than being potential mirror images of each other), and therefore be of the same type (i.e., p-type or n-type), and isolated from each other (no contact, e.g., with a gap between them). The layers 56,58 are advantageously sintered to their respective electrodes 62,70,80 to ensure good physical and electrical contact.

[0027] Relevant to the discussion below, it should be understood that an n-type semiconductor has a resistance that is significantly lower and has a positive relationship with oxygen content when exposed to exhaust gases from a combustion process operating in the rich region, while the resistance is orders of magnitude higher and may be relatively uncorrelated to the oxygen content in the lean region. Conversely, a p-type semiconductor has a resistance that is significantly lower and has a negative relationship with oxygen content when exposed to exhaust gases from a combustion process operating in the lean region, while the resistance is orders of magnitude higher and may be relatively uncorrelated to the oxygen content in the rich region.

[0028] Respective porous dielectric protective coating layers 59a, 59b are placed over the semiconductor layers 56,58, and neighboring portions of the electrodes 62,70,80. These porous dielectric protecting coating layers 59a, 59b may sometimes be referred to herein as catalytic filter layers 59a, 59b. These catalytic filter layers 59a, 59b are distinct from each other and are therefore separated by a corresponding gap 57. Note that the "gaps" described herein, including gap 57 may be empty voids, the gaps can alternatively be entirely/partially filled with suitable material, such as a dielectric frit or glaze material, as is desired. Both catalytic filter layers 59a, 59b may include catalytic precious metal(s), such as platinum, and/or palladium, as well as oxygen storage components such as cerium oxide or other suitable material as may be necessary to achieve the desired functional characteristics of the sensing portion 60. These catalytic materials may be an initial part of the composition of the catalytic filter layers 59a, 59b, or added as to impregnate the catalytic filter layers 59a, 59b in a subsequent operation. In addition, one of the catalytic filter layers, for example coating layer 59a, contains Noxcat material (at a defined level, such as .25% to 2.5%), while the other catalytic filter layers (in this example 59b) is substantially free of the Noxcat material. Substantially free, in this context, means that the concentration of the Noxcat material is approximately zero, and at least low enough not to convert any substantial amounts of NOx to N2 and O2, so as to not substantially change the resistance of the associated MOS layer due to the presence of NOx in the exhaust gas (as compared to an identical exhaust gas without the NOx).

[0029] Referring to Figs. 4-5, a first oxygen sensing circuit 42 is formed by the common electrode 62, the MOS layer 56, the first sensing electrode 70. The first oxygen sensing circuit 42 includes the MOS layer 56 having the catalytic filter layer 59a of a first type, and is therefore associated with the catalytic filter layer 59a of the first type. A second oxygen sensing circuit 44 is formed by the common electrode 62, the MOS layer 58, the second sensing electrode 80. The second oxygen sensing circuit 44 includes the MOS layer 58 having the catalytic filter layer 59b of a second type, and is therefore associated with the catalytic filter layer 59b of the second type. In an illustrative example, the catalytic filter layer of the first type comprises Noxcat material, while the catalytic filter layer of the second type is substantially Noxcat material free. In another illustrative example, the catalytic filter layer of the first type is substantially Noxcat material free, and the catalytic filter layer of the second type comprises Noxcat material.

[0030] When Noxcat material is present in the catalytic filter layer (e.g. 59a), and that catalytic filter layer is exposed to the exhaust stream having NOx present, the Noxcat material interacts with NOx to produce N2 and O2. Thus, in an example where the MOS layer 56 is covered by the porous catalytic filter layer with Noxcat material (59a), while MOS layer 58 is covered by a porous catalytic filter layer that is substantially Noxcat material free (59b), MOS layer 56 responds, when NOx is present in the exhaust plenum 19 at the location of the NOx sensor 50, in a way so as to indicate a higher oxygen level than the MOS layer 58 covered by the porous layer without Noxcat material (59b). For example, when MOS layers 56,58 are both n-type semiconductors, then MOS layer 56 has a significantly higher resistance than MOS layer 58, when, in the absence of NOx, the two layers 56/58 would have substantially similar resistances. Therefore, despite being exposed to a common exhaust having a certain (and shared) actual oxygen level, the two sensor circuits 42,44 provide different resistances, with the difference in resistance proportional to the amount of NOx present in the exhaust plenum 19 after the NOx trap 21.

[0031] In some, but not all, embodiments, an optional heater portion 54 is advantageously disposed on the bottom side 52b of the dielectric substrate 52, generally opposite from the exhaust sensing portion 60. See Fig. 4. In other embodiments (not shown), the heater portion 54 may be disposed on the same side 52a of the dielectric substrate 52 as the exhaust sensing portion 60, provided the two portions 54, 60 are electrically isolated (not electrically connected other than through ground). This can be achieved by applying a dielectric layer (not shown) over the heater portion 54 via screen printing, vapor deposition, or other techniques known in the art. In still other embodiments, the heater portion 54 is omitted. The heater portion 54 may take any suitable form and may be formed of any suitable material such as tungsten, platinum, palladium, other precious metals, ceramic (such as silicon nitride Si3N4), or other materials known in the art. Referencing Figs. 4-5, the heater portion 54 may be advantageously connected to ground and to a voltage source, such as a twelve volt DC voltage source VSi. The heater portion 54 may also have a suitable protective and/or electrically insulating layer 59, as desired.

[0032] The NOx sensor 50 may have suitable connections for power and other signals. For example, in some embodiments, the NOx sensor 50 has four contacts or leads 55, 69, 79, 89 for making suitable connections. Lead 55 is electrically connected to the heater portion 54, and functions as a power (+) lead for the heater portion 54. Lead 69 is electrically connected to the common electrode 62, and acts as a ground (-) lead for the sensing portion 60. Lead 79 is electrically connected to the first sensing electrode 70, and acts as the output from first sensing circuit 42. Lead 89 is electrically connected to the second sensing electrode 80, and acts as the output from second sensing circuit 44. Lead 69 may also function as a ground lead for heater portion 54, or there may be an additional lead (not shown) for a separate ground lead for heater portion 54.

[0033] Together, the NOx sensor 50 and the controller 32 form a nitrous oxide sensing assembly or system 5. In some embodiments, the NOx sensor assembly 5 is such that the NOx sensor 50 is connected directly to the controller 32 so that the sensed oxygen level data from the NOx sensor 50 is supplied to the controller 32. In other embodiments, the NOx sensor assembly 5 includes a connecting circuit 90 that interconnects the NOx sensor 5 to the computational portions of controller 32. See Fig. 5. Note that the connecting circuit 90, if present, may be a sub-portion of the controller 32, be a separate component or components between the NOx sensor 50 and the controller 32, or dispersed in any suitable manner. In one or more illustrative embodiments, the connecting circuit 90 includes voltage source VSi which may be, as illustrated, a twelve volt DC voltage source. Voltage source VSi connects to one side of heater portion 54 via shunt resistor RHs and lead 55. Suitable leads disposed on either side of shunt resistor RHs allow the voltage drop VHs across shunt resistor RHs to be measured. Note that the line LHS for VHS, as with other voltage drop lines herein, is illustrated and discussed as being a single line merely for the sake of simplicity, but are typically two lines each. The opposing end of heater portion 54 connects to ground via lead 69.

[0034] The connecting circuit 90 also includes a constant voltage source CV, which will be assumed to be powered using a nominal voltage of five volts or any other suitable stable power source that is available. The constant voltage source CV connects to the first sensing circuit 42 via resistors R1 and R2 and lead 79. As can be appreciated, resistors R1 and R2 form a first voltage divider, with the voltage drop V2 across resistor R2 measured and provided by line L2. The sensing circuit 92 is completed to ground via the first sensing circuit 42 (e.g., first sensing electrode 70, MOS layer 56, and common electrode 62). The constant voltage source CV connects to the second sensing circuit 44 via resistors R3 and R4 and lead 89. As can be appreciated, resistors R3 and R4 form a second voltage divider, with the voltage drop V4 across resistor R4 measured and provided by line L4. The sensing circuit 94 is completed to ground via second sensing circuit 44 (e.g., sensing electrode 80, semiconductor layer 58, and common electrode 62). Note that in alternate embodiments, resistor Ri and/or resistor R3 may be omitted from their respective circuits, or additional resistors may be added to their respective circuits for scaling or calibration purposes.

[0035] The connecting circuit 90 provides voltage drop V2 to the controller 32 via line L2, voltage drop VHs to controller 32 via line L HS, the actual voltage Vs of voltage source VS-\ to controller 32 via line Ls, and voltage drop V4 to the controller 32 via line L4.

[0036] The controller 32 may determine the NOx level based on the voltage levels supplied to it by the sensing circuits 42,44. In particular, the controller 32 may determine the voltage difference between V2 and V4, and then determine the NOx level based thereon, advantageously taking into account the temperature of the NOx sensor 50.

[0037] The temperature of the NOx sensor 50 may be determined by controller 32 based on the resistance of the heater portion 54. For example, the current IH in the heater portion 54 may be calculated as the voltage drop VHs across the shunt resistor RHs, divided by the resistance of the shunt resistor RHs, or IHS = VHS/RHS. Then, the resistance RH of the heater portion 54 may be calculated based on the voltage drop across the heater portion 54 divided by the current IH through the heater portion 54. Thus, RH may be calculated as RH = (VS-VHS)/IHS. Then, using RH, temperature T may be calculated using a suitable formula, for example T = (M x RH) + B, where the slope M and the constant B are dependent on the heater portion 54 design. As can be appreciated, M and B can be determined in a calibration process, and the relevant values stored in memory of the engine management system 30 for use by the controller 32.

[0038] The controller 32 receives the inputs derived from the NOx sensor 50 and other sensors, and advantageously controls the operation of the urea injector 17, and optionally other functions such a fuel metering, ignition timing, and other engine functions.

[0039] The discussion above has generally been in the context of the temperature of the NOx sensor 50 being derived from the resistance of the heater portion 54 that is part of the NOx sensor 50. Thus, the heater portion 54 fills two roles: heating the NOx sensor 50 and sensing temperature thereof. However, in some embodiments, a temperature sensor distinct from the heater portion 54 may alternatively employed. Thus, the NOx sensor 50 may include a thermocouple or other suitable temperature sensing device 51, in addition to the exhaust sensing portion 60 and the optional heater portion 54. Such a temperature sensor 51 is shown in Fig. 5 in dashed lines to indicate its optional presence, with its connection line to controller 32 not shown for simplicity. And, the temperature of the NOx sensor 50 may be determined by controller 32 based on data from the temperature sensor 51 in a conventional fashion, either as an alternative or in addition to determining the temperature based on the resistance of the heater portion 54.

[0040] The discussion above has generally been in the context of controlling an engine 10 having a single cylinder/combustion chamber. However, a similar approach may be used with engines having multiple cylinders, such as that shown in Fig. 6 with cylinders J, K, L, and P. In Fig. 6, a single common NOx sensor 50 is used for multiple cylinders.

[0041] The discussion above has generally been in the context of an internal combustion engine; however, the present invention is not limited in application to internal combustion engines. Indeed, the NOx sensing method described above can be used to control NOx reduction processes generally. Thus, for example, the method(s) described herein may be used for NOx measurement and NOx reduction in combustion processes in a furnace or a water heater. As with the engine-based discussion above, the NOx sensor 50 is disposed so as to sense exhaust gases in the exhaust plenum 19 from the combustion process.

[0042] From the above, it can be seen that the NOx sensor 50 may be used to detect nitrous oxide in exhaust from a combustion process. For example, as shown in Fig. 7, one method for detecting nitrous oxide in exhaust from a combustion process is to expose the NOx sensor 50 to the exhaust from the combustion process (step 1 10). As discussed above, the NOx sensor 50 has first and second oxygen sensing circuits 42,44 that share a common electrode 62; wherein the first oxygen sensing circuit 42 comprises a first metal oxide semiconductor portion 56 in contact with the common electrode 62; wherein the second oxygen sensing circuit 44 comprises a second metal oxide semiconductor portion 58 in contact with the common electrode 62. Some of the exhaust passes through a first catalytic filter layer 59a (step 120) to reach the first metal oxide semiconductor portion 56. Simultaneously, some of the exhaust passes through a second catalytic filter layer 59b (step 130) to reach the second metal oxide semiconductor portion 58. At step 120, nitrous oxide in the exhaust is converted at least partially to oxygen while passing through the first catalytic filter layer 59a prior to reaching the first metal oxide semiconductor portion 56; but during parallel step 130, nitrous oxide in the exhaust is not converted partially to oxygen while passing through the second catalytic filter layer 59b to reach the second metal oxide semiconductor portion 58. The process also includes, at step 135, determining the temperature of the NOx sensor, which may be accomplished in any suitable way, such as calculated from the heater current IH or the voltage drop across a resistor RHs disposed in series with the heater portion 54, or be based on data from a separate temperature sensor 51. Note that step 135 may occur before or after or concurrent with step 120 and 130. The process continues, at step 140, with determining a nitrous oxide level in the exhaust based on a resistance differential between the first oxygen sensing circuit 42 and the second oxygen sensing circuit 44 and the determined temperature of the NOx sensor. As pointed out above, the first and second oxygen sensing circuits 42,44 are configured to provide similar resistances when subjected to an environment free of nitrous oxide, but to provide substantially different resistances when subjected to an environment containing nitrous oxide(s). The process may continue to the optional step of comparing the resistance differential to a threshold (step 150), and triggering a purging of the NOx trap 21 (e.g., providing urea/ammonia thereto) (step 160) when the determined NOx level exceeds the threshold. The threshold may advantageously vary based on the determined temperature of the sensor, so as to compensate for the temperature dependent response of the MOS materials of layers 56,58 to the presence of oxygen.

[0043] The discussion above has generally been in the context of catalytic filer layer 59a having a particular Noxcat material, while the other catalytic filter layer 59b is substantially free of that particular Noxcat material. Thus, catalytic filer layer 59a may comprise rhodium, while catalytic filter layer 59b is substantially free of rhodium. This is an example using a single Noxcat material, and is believed to be an advantageous configuration. However, in other aspects or embodiments, catalytic filer layer 59a may comprise a combination of multiple Noxcat materials (e.g., rhodium and nickel), while catalytic filter layer 59b is substantially free of those Noxcat materials. Claim language of "wherein the first catalytic filter layer comprises a Noxcat material... wherein the second catalytic filter layer is substantially free of the Noxcat material," and the like, is intended to cover both situations (single Noxcat material and combination of two Noxcat materials), and situations where more Noxcat materials are involved.

[0044] The methods and engine control systems discussed above provide the opportunity for enhanced combustion and/or engine control so that greater fuel economy and/or reduced emissions may be achieved.

[0045] For more information about forming the electrodes 62,70,80 and the MOS layers 56,58, and the like, and/or temperature compensation of the sensed oxygen levels, see U.S. Patent Application Publication 20140130779 and/or U.S. Patent Nos. 8586394 and 8959987.

[0046] The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope of the invention. The present embodiments are, therefore, to be considered as illustrative and not restrictive.

[0047] In one example there has been described a nitrous oxide sensor, comprising: a substrate; a first oxygen sensing circuit affixed on a first side of the substrate; a second oxygen sensing circuit affixed on the first side of the substrate; wherein the first oxygen sensing circuit comprises: a first sensing electrode; a common electrode; a first metal oxide semiconductor portion electrically bridging a first physical gap between the first sensing electrode and the common electrode; wherein the second oxygen sensing circuit comprises: a second sensing electrode; the common electrode; a second metal oxide semiconductor portion electrically bridging a second physical gap between the second sensing electrode and the common electrode; wherein a first catalytic filter layer is disposed over the first metal oxide semiconductor portion; wherein the first catalytic filter layer comprises a Noxcat material; wherein the Noxcat material is one of rhodium, ruthenium, cobalt, palladium, or nickel; wherein a second catalytic filter layer is disposed over the second metal oxide semiconductor portion; wherein the second catalytic filter layer is substantially free of the Noxcat material; wherein the first and second metal oxide semiconductor portions are disposed in spaced relation to each other such that there is a third physical gap between the first and second metal oxide semiconductor portions; wherein the first and second catalytic filter layers are disposed in spaced relation to each other; wherein the first and second oxygen sensing circuits are configured to provide similar resistances when subjected to an environment free of nitrous oxide, but to provide substantially different resistances when subjected to an environment containing nitrous oxide(s).

[0048] In one example, the Noxcat material is rhodium. In one example the nitrous oxide sensor further comprises a heater portion disposed on a second side of the substrate, the second side opposite the first side. In one example, the first and second metal oxide semiconductor portions are both n-type semiconductors, or are both p-type semiconductors. In one example, the first and second sensing electrodes are mirror images of each other. 6. The nitrous oxide sensor of claim 1 , wherein the gap is an empty void.

[0049] In one example there has been described a nitrous oxide sensor assembly, comprising: a nitrous oxide sensor as noted above; a first voltage divider operatively connected to the first oxygen sensing circuit; a second voltage divider operatively connected to the second oxygen sensing circuit; processing circuitry operative to: determine a first voltage drop associated with the first oxygen sensing circuit, the first voltage drop proportional to a resistance of the first oxygen sensing circuit; determine a second voltage drop associated with the second oxygen sensing circuit, the second voltage drop proportional to a resistance of the second oxygen sensing circuit; determine a temperature of the nitrous oxide sensor; determine if a purge of a NOx trap associated with a combustion process is needed by determining if a difference between the first voltage drop and the second voltage drop is greater than a first threshold.

[0050] In one example, the first threshold is based on the determined temperature of the nitrous oxide sensor. In one example, the nitrous oxide sensor further comprises a heater portion disposed on a second side of the substrate; wherein the processing circuitry is operative to determine the temperature of the nitrous oxide sensor by determining a voltage drop across a resistor disposed in series with the heater portion. In one example, the processing circuitry is further operative to trigger a purge of the NOx trap in response to determining that the difference between the first voltage drop and the second voltage drop is greater than the first threshold.

[0051] In one example there has been described a nitrous oxide sensor, comprising: a substrate; an electrically conductive common electrode mounted on a first side of the substrate; an electrically conductive first sensing electrode mounted on the first side of the substrate in spaced relation to the common electrode; an electrically conductive second sensing electrode mounted on the first side of the substrate in spaced relation to the common electrode; wherein the common electrode is disposed between the first and second sensing electrodes; wherein the first sensing electrode and the common electrode are configured to form a first comb structure; wherein the second sensing electrode and the common electrode are configured to form a second comb structure; a first metal oxide layer semiconductor layer disposed over the first comb structure and forming a first semiconductor bridge between the first sensing electrode and the common electrode; a second metal oxide semiconductor layer disposed over the second comb structure and forming a second semiconductor bridge interconnecting the second sensing electrode and the common electrode; wherein the first and second metal oxide semiconductor layers are spaced from each other; a first catalytic filter layer disposed over the first metal semiconductor oxide layer; a second catalytic filter layer disposed over the second metal oxide semiconductor layer; wherein the first catalytic filter layer comprises rhodium and the second catalytic filter layer is substantially rhodium free.

[0052] In one example the nitrous oxide sensor includes a heater portion disposed on a second side of the substrate, the second side opposite the first side. In one example, the first and second metal oxide layers are n-type semiconductors. In one example, the first and second metal oxide layers are p-type semiconductors. In one example, the first and second sensing electrodes are mirror images of each other.

[0053] In one example there has been described a method of detecting nitrous oxide in exhaust from a combustion process, the method comprising: exposing a sensor to the exhaust from the combustion process; wherein the sensor has first and second oxygen sensing circuits that share a common electrode; wherein the first oxygen sensing circuit comprises a first metal oxide semiconductor portion in contact with the common electrode; wherein the second oxygen sensing circuit comprises a second metal oxide semiconductor portion in contact with the common electrode; passing some of the exhaust through a first catalytic filter layer to reach the first metal oxide semiconductor portion, and simultaneously passing some of the exhaust through a second catalytic filter layer to reach the second metal oxide semiconductor portion; wherein nitrous oxide in the exhaust is at least partially converted to oxygen while passing through the first catalytic filter layer prior to reaching the first metal oxide semiconductor portion, but nitrous oxide in the exhaust is not converted to oxygen while passing through the second catalytic filter layer to reach the second metal oxide semiconductor portion; and determining a temperature of the sensor; determining a nitrous oxide level in the exhaust based on a resistance differential between the first oxygen sensing circuit and the second oxygen sensing circuit and the determined temperature; wherein the first and second oxygen sensing circuits are configured to provide similar resistances when subjected to an environment free of nitrous oxide, but to provide substantially different resistances when subjected to an environment containing nitrous oxide(s).

[0054] In one example the method includes exposing the sensor to the exhaust at a point downstream from a NOx trap. In one example the method includes comparing the resistance differential to a threshold, the threshold varying based on the determined temperature of the sensor; and in response to the resistance differential exceeding the threshold, triggering a purging of a NOx trap disposed along an exhaust associated with the combustion process.


Claims

1. A nitrous oxide sensor (50), comprising:

a substrate (52);

a first oxygen sensing circuit (42) affixed on a first side (52a) of the substrate (52); and

a second oxygen sensing circuit (44) affixed on the first side (52a) of the substrate (52);

wherein the first oxygen sensing circuit (42) comprises:

a first sensing electrode (70);

a common electrode (62) acting as electrical ground;

a first metal oxide semiconductor portion (56) electrically bridging a first physical gap between the first sensing electrode (70) and the common electrode (62);

a first catalytic filter layer (59a) disposed over the first metal oxide semiconductor portion (56), wherein the first catalytic filter layer (59a) comprises a Noxcat material comprising any of rhodium, ruthenium, cobalt, palladium, or nickel; and

a first output lead (79) electrically connected to the first sensing electrode (70) and acting as an output from the first sensing circuit (42);

wherein the second oxygen sensing circuit (44) comprises:

a second sensing electrode (80);

the common electrode (62) acting as ground potential;

a second metal oxide semiconductor portion (58) electrically bridging a second physical gap between the second sensing electrode (80) and the common electrode (62), wherein the second metal oxide semiconductor portion (58) is disposed in spaced relation to the first metal oxide semiconductor portion (56) such that there is a third physical gap (57) between the first and second metal oxide semiconductor portions (56, 58), and wherein the first and second metal oxide semiconductor portions (56, 58) are both n-type semiconductors, or are both p-type semiconductors;

a second catalytic filter layer (59b) disposed over the second metal oxide semiconductor portion (58) and in spaced relation to the first catalytic filter layer (59a), wherein the second catalytic filter layer (59b) is substantially free of the Noxcat material; and

a second output lead (89) electrically connected to the second sensing electrode (80) and acting as an output from the second sensing circuit (44);

wherein the first and second oxygen sensing circuits (42, 44) are configured to provide similar resistances at the first and second output leads (79, 89) when subjected to an environment free of nitrous oxide, and to provide substantially different resistances when subjected to an environment containing nitrous oxide(s).


 
2. The nitrous oxide sensor (50) of claim 1, further comprising a heater portion (54) disposed on a second side (52b) of the substrate (52), the second side (52b) opposite the first side (52a).
 
3. The nitrous oxide sensor (50) of claim 1 or 2, wherein the first and second metal oxide semiconductor portions (56, 58) both have the same material composition and geometries.
 
4. The nitrous oxide sensor (50) of any of claims 1 to 3, wherein the first and second sensing electrodes (70, 80) are mirror images of each other.
 
5. The nitrous oxide sensor (50) of any of claims 1 to 4, wherein the Noxcat material is present in the first catalytic filter layer (59a) in the range 0.25% to 2.5%.
 
6. The nitrous oxide sensor (50) of any of claims 1 to 5, wherein the Noxcat material is rhodium.
 
7. The nitrous oxide sensor (50) of any of claims 1 to 6, wherein the third physical gap (57) is an empty void.
 
8. The nitrous oxide sensor (50) of any of claims 1 to 7:

wherein the first sensing electrode (70) and the common electrode (62) are configured to form a first comb structure;

wherein the second sensing electrode (80) and the common electrode (62) are configured to form a second comb structure; and

wherein the first comb structure and the second comb structure extend in opposite lateral directions from a central area of the common electrode (62).


 
9. A nitrous oxide sensor assembly (5), comprising:

the nitrous oxide sensor (50) of any of claims 1 to 8; and

a connecting circuit (90) comprising:

a first voltage divider (R1, R2) operatively connected to the first oxygen sensing circuit (42) to provide a first voltage output (LV2); and

a second voltage divider (R3, R4) operatively connected to the second oxygen sensing circuit (44) to provide a second voltage output (LV4).


 
10. A method of detecting nitrous oxide in exhaust from a combustion process, the method comprising:

exposing (110) the sensor of any of claims 1 to 8, or the assembly of claim 9, to the exhaust from the combustion process;

passing (120) some of the exhaust through the first catalytic filter layer (59a) to reach the first metal oxide semiconductor portion (56), and simultaneously passing (130) some of the exhaust through the second catalytic filter layer (59b) to reach the second metal oxide semiconductor portion (58);

wherein nitrous oxide in the exhaust is at least partially converted to oxygen while passing through the first catalytic filter layer (59a) prior to reaching the first metal oxide semiconductor portion (56), whereas nitrous oxide in the exhaust is not converted to oxygen while passing through the second catalytic filter layer (59b) to reach the second metal oxide semiconductor portion (58), whereby the first and second oxygen sensing circuits (42, 44) are configured to provide similar resistances when subjected to an environment free of nitrous oxide, but to provide substantially different resistances when subjected to an environment containing nitrous oxide or nitrous oxides; and

determining (140) a nitrous oxide level in the exhaust based on a resistance differential between the first oxygen sensing circuit (42) and the second oxygen sensing circuit (44).


 
11. The method of claim 10, wherein the determining (14) comprises determining the resistance differential based on a first voltage drop (V2) from a voltage divider (R1, R2) associated with the first oxygen sensing circuit (42), the first voltage drop (V2) being proportional to the resistance of the first oxygen sensing circuit (42), and a second voltage drop (V4) from a second voltage divider (R3, R4) associated with the second oxygen sensing circuit (44), the second voltage drop (V4) being proportional to the resistance of the second oxygen sensing circuit (44).
 
12. The method of claim 10 or 11, wherein the exposing the sensor (50) to the exhaust comprises exposing the sensor (50) to the exhaust at a point downstream from a NOx trap (21).
 
13. The method of any of claims 10 to 12, further comprising:

determining (135) a temperature of the sensor (50);

comparing (150) the resistance differential to a threshold, the threshold varying based on the determined temperature of the sensor (50); and

in response to the resistance differential exceeding the threshold, triggering (160) a purging of a NOx trap (21) disposed along an exhaust plenum (19) associated with the combustion process.


 
14. The method of any of claims 10 to 13, wherein the first metal oxide semiconductor portion (56) and the second metal oxide semiconductor portion (58) have the same material composition and geometries.
 
15. The method of any of claims 10 to 14, wherein the combustion process occurs in an internal combustion engine (10).
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description