Patent Application
20220082042
This invention relates to a structures and methods of operation of catalytic converters, particulate filters (PFs), selective catalytic reduction (SCR) units, and like structures for treating exhaust gases to reduce harmful pollution and has particular but not exclusive application to reducing pollution from internal combustion engines at start-up and when idling.
Catalytic converters, PFs and SCR units are used in internal combustion engines to reduce noxious exhaust emissions arising when fuel is burned as part of the combustion cycle. Significant among such emissions are carbon monoxide and nitric oxide. These gases are dangerous to health but can be converted to less noxious gases by oxidation respectively to carbon dioxide and nitrogen/oxygen. Other noxious gaseous emission products, including unburned hydrocarbons, can also be converted either by oxidation or reduction to less noxious forms. The conversion processes can be effected or accelerated if they are performed at high temperature and in the presence of a suitable catalyst being matched to the particular noxious emission gas that is to be processed and converted to a benign gaseous form. For example, typical catalysts for the conversion of carbon monoxide to carbon dioxide are finely divided platinum and palladium, while a typical catalyst for the conversion of nitric oxide to nitrogen and oxygen is finely divided rhodium.
Catalytic converters, PFs and SCR units have low efficiency when cold, i.e. the running temperature from ambient air start-up temperature to a temperature of the order of 300° C. or “light-off” temperature, being the temperature where the metal catalyst starts to accelerate the pollutant conversion processes previously described. Light-off is often characterized as the temperature at which a 50% reduction in toxic emissions occurs and for gasoline is approximately 300° C. Below light-off temperature, little to no catalytic action takes place. This is therefore the period during a vehicle's daily use during which most of the vehicle's polluting emissions are produced. Getting the catalytic converter, PF or SCR unit hot as quickly as possible is important to reducing cold start emissions.
For simplicity and clarity of illustration, elements illustrated in the accompanying figure are not drawn to common scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Advantages, features and characteristics of the present invention, as well as methods, operation and functions of related elements of structure, and the combinations of parts and economies of manufacture, will become apparent upon consideration of the following description and claims with reference to the accompanying drawings, all of which form a part of the specification, wherein like reference numerals designate corresponding parts in the various figures, and wherein:
A gaseous emissions treatment assembly may take any of a number of forms. Typical of these is a known catalytic converter having a cylindrical substrate body 10 usually made of ceramic material and often called a brick, an example of which is shown in
In the catalytic converter, interiors of the tubular passages 12 are wash-coated with a layer containing a particular catalyst material. A wash-coat typically contains a base material, suitable for ensuring adherence to the cured ceramic material of the substrate body, and entrained particulate catalyst material for promoting specific pollution-reducing chemical reactions. Examples of such catalyst materials are platinum and palladium which are catalysts effective in converting carbon monoxide and oxygen to carbon dioxide, and rhodium which is a catalyst suitable for converting nitric oxide to nitrogen and oxygen. Other catalysts are known which promote high temperature oxidation or reduction of other gaseous materials. The wash-coating is prepared by generating a suspension of the finely divided catalyst in a ceramic paste or slurry, the ceramic slurry serving to cause the wash-coat layer to adhere to the walls of the ceramic substrate body. As an alternative to wash-coating to place catalyst materials on the substrate body surfaces, the substrate body material itself may contain a catalyst so that brick walls themselves present catalyst material at the internal surfaces bounding the cells.
Exhaust gases from diesel (compression combustion) engines contain more nitrogen oxides than gasoline (spark combustion) engines. Long-term exposure to nitrogen oxides even at low levels can cause temporary or permanent respiratory problems. In selective catalytic reduction (SCR) units, a liquid reductant is injected into a diesel engine exhaust flow to combine with nitrogen dioxide and nitric oxide (referred to collectively as NOx) in the exhaust gas. A preferred reductant is aqueous urea (2(NH2)2CO which is often referred to as diesel exhaust fluid (DEF). In the presence of a catalyst, ammonia resulting from thermal decomposition of the urea combines with the nitrogen oxides to produce less harmful products, chiefly nitrogen and water. Other reductants such as anhydrous ammonia and aqueous ammonia may also be used as an alternative to urea although especially for automotive application, on-board storage presents greater difficulty. Suitable catalysts may be any of certain metals oxides (such as those of molybdenum, vanadium, and tungsten), certain precious metals and zeolites. The typical temperature range for a SCR reaction is from 360° C. to 450° C. with a catalyst such as activated carbon being used to stimulate lower temperature reactions. As in gasoline (spark combustion engines), diesel (pressure combustion) engines may experience a period after a start-up where the exhaust temperature is too cool for effective SCR NOx reduction processes to take place. Other catalytic converters in which the present invention finds application for preheating or supplementary heating are lean NOX catalyst systems, lean NOX trap systems and non-SCR systems. The present invention is applicable also to each of these nitrogen oxide emissions treatment assemblies.
A gaseous emissions treatment assembly may have a series of the substrate bodies or bricks 10, each having a particular catalyst layer or emissions treatment mode depending on the noxious emission to be reduced or neutralized. Gaseous emissions treatment bricks may be made of materials other than fired ceramic, such as stainless steel. Also, they may have different forms of honeycombed cells or passages than those described above. For example, cells can be round, square, hexagonal, triangular or other convenient cross-sectional shape. In addition, if desired for optimizing strength and low thermal capacity or for other purposes, some of the extruded honeycomb walls can be formed so as to be thicker than other of the walls or formed so that there is some variety in the shape and size of cells. Junctions between adjacent interior walls of a cell can be sharp angled or can present curved profiles.
Typically, as shown in
Brick shape, profile and cell densities vary among different manufacturers. For example, some bricks are round and some are oval. Some assemblies have single stage bricks that are generally heavily wash-coated with the catalyst metals, while others may have two or three bricks with different wash-coatings on each brick. Some exhausts have 900, 600 and 400 cpsi cell densities used in the full exhaust assembly, while others use only 400 cpsi bricks throughout. A close-coupled converter may be mounted up close to the exhaust manifold with a view to reducing the period between start-up and light-off temperature. An underfloor converter can be located further from the engine where it takes relatively longer to heat up, but can be relatively larger and used to treat the majority of gases once the exhaust assembly is up to temperature. In another configuration, a unit for reducing the period to light-off temperature and a unit to deal with high gas flow after light-off are mounted together in a common casing.
At one or more locations in the assembly, sensors mounted in the exhaust gas flow including within or adjacent the substrate body provide feedback to the engine control system for emission checking and tuning purposes. Aside from start-up, control of fuel and air input has the object typically of maintaining a 14.6:1 air: fuel ratio for an optimal combination of power and cleanliness. A ratio higher than this produces a lean condition—not enough fuel. A lower ratio produces a rich condition—too much fuel. The start-up procedure on some vehicles runs rich for an initial few seconds to get heat into the engine and ultimately the catalytic converter. The structures and operating methods described below for indirectly heating the catalyst layers and the exhaust gases can be used with each of a close-coupled catalytic converter, an underfloor converter, and a combination of the two. Outputs from the temperature sensors are taken to a controller at which the monitored temperature or temperatures are used to control when induction heating is switched on and off. Using an appropriate algorithm implemented at the controller, the monitored temperatures may also be used to control specific effects of the applied heating processes to achieve a particular heating pattern.
In use, the encased brick (or bricks) is mounted in the vehicle exhaust line to receive exhaust gases from the engine and to pass them to the vehicle tail pipe. The passage of exhaust gases through the gaseous emissions treatment unit heats the ceramic brick 10 to promote catalyst activated processes where the flowing gases contact the catalyst layer. Especially when the vehicle engine is being run at optimal operating temperature and when there is substantial throughput of exhaust gases, such treatment units operate substantially to reduce the presence of noxious gaseous emissions entering the atmosphere. Such units have shortcomings however at start-up when the interior of the brick is at low temperature, during idling during city driving or when waiting for a coffee at a Tim Hortons drive-through, and between electric driving periods for hybrid vehicles.
In one aspect of the present invention, a gaseous emissions treatment assembly such as that shown in
The resistive heating wires 20 are fixed in place by a friction fit at least partially achieved by closely matching wire exterior area dimensions to the cell area dimensions so that surface roughness of the wire surface and the cell walls 14 locks the resistive heating wires 20 in place. Wires can also be formed with a non-linear element such as a bow or crimp so that the bow or crimp is straightened somewhat as the wire is inserted into a cell and so is held by the cell walls as inherent resilience of the wire tends to return it to its original bow or crimped shape. This is caused at least a part by the resistive heating wire bearing against a part of the cell walls 14 and so enhancing the friction fit to retain the resistive heating wire in place. The overall friction fit can be such as to resist gravity, vibration, temperature cycling, and pressure on the wires as exhaust gases pass through the substrate body. Wires may alternatively, or in addition, be fixed into the cells by bonding outer surface parts of the wires to interior surfaces of respective cells. A suitable composite adhesive may be a blend of materials chosen to reduce temperature cycling stress effects in which there may be significant metal wire expansion/contraction, but vanishingly small expansion/contraction of the ceramic substrate. This differential could otherwise produce stresses at the adhesive interface between the two materials. By using such a composite adhesive, movement of a bonded wire relative to the surrounding cell walls may be reduced while maintaining high heat transfer.
In one embodiment, the resistive heating wires are identical to each other in terms of composition and cross-sectional size and they extend the full length of each occupied cell. One of more of the wires can alternatively extend only part way along a cell; either located near the front, near the back, or at an intermediate location along the length of the cell. One or more of the wires can alternatively have a different composition and/or resistivity from other wires. One or more of the wires can alternatively have a different cross-sectional area and therefore resistance per unit length than other wires.
The looped and woven wire configurations shown in
In one manufacturing method for the
Heating is preferably controlled using a pulse width modulated (PWM) electrical power source in which heating control is determined by changing the frequency and pulse width of current pulsing. Control of heating is dependent on a number of factors. Applied heating current can be tuned to the resistive wire load to achieve high efficiency in terms of generating heat and speed to light-off. A heating pattern in the ceramic substrate is determined by appropriate location and configuration of the metal wires. Heating effects can be modified by appropriate adjustment of electrical power applied to individual resistive hearing wires. In particular, the applied current can be changed with time so that there is interdependence between the level of heating and the particular operational phase from pre-start-up to highway driving.
Of some importance is whether the resistive heating wire is in stationary air or is in a gas flow such as exhaust gas from vehicle engine operation or another induced flow. For example, in a pre-ignition phase, resistive heating wires are normally in stationary air while in a post-ignition phase, resistive heating wires are normally in a moving exhaust gas flow. If the heating wire is in stationary air, there is no gas flow to transfer heat away from the wire and so there is a heightened risk of the resistive heating wire overheating and burning out. Consequently, periods of applying heating current are reduced in the absence of airflow. Such a stepped current supply is for example adopted for a hybrid vehicle having an internal combustion engine operating during periods of driving time and an electric motor operating during intervening periods of driving time.
Whether from the viewpoint of performance or to prevent damage to the resistive heating wires, one or more temperature sensors can be positioned within or adjacent to the assembly. The temperature of a resistive heating wire may also be monitored so that specific and directed reduction in heating can be implemented if, for example, there is a localised risk of resistive heating wire damage or burnout.
In an alternative method of monitoring and controlling temperature, the rate of change of resistance with temperature for a heating wire is calibrated in prior testing. Such testing can be either or both of testing stock wire or testing specific catalytic converter units before their deployment. In operation of the catalytic converter, voltage applied to the heating elements is fixed and the current drawn by the heating elements is measured. From this measurement, the resistance of the heating elements is deduced and using a lookup table in the catalytic converter microcontroller, a temperature value is derived at any given time representing the average temperature of the heating elements inside the catalytic converter, PF or SCR unit (including any heating element parts located outside the ceramic substrate). The microcontroller uses PWM (pulse width modulation) to vary the current output to generate a desired temperature indicative of a steady state resistance). The temperature is that of the heating elements and region of the ceramic substrate in direct contact with the heating elements. However, the temperature may not be uniform throughout the ceramic substrate. Temperature variations will depend on the mass of the ceramic substrate and where the heating elements are placed in the substrate.
In one embodiment of the invention, the resistive wire section is very close in cross sectional area and/or shape to the cross-sectional area and shape of the cell within which it is accommodated. Substantially matching cross-sections so that the cell is almost fully blocked is advantageous in many respects. Close matching is generally the most efficient for heating which is predominantly conduction at touching points and radiation across any small gaps between the wire and the cell. The cross-sectional shape of the resistive heating wire sections and/or the passages can be any of round, square, hexagonal, triangular, octagonal or other convenient shape. Although matching cell and wire profiles are desirable, it is not essential that they match.
In a preferred embodiment, wires are inserted or threaded into selected passages before the unit is wash coated with catalyst-containing wash coat slurry. The wash coat is deposited as a layer on the open cells and, in the same operation, fills, at least to some extent, gaps between the walls of the resistive heating wires and the walls of the passages in which they are accommodated. The wash coat in the occupied cells locks in the inserted or threaded resistive heating wires and can supplement or replace the previously described bonding and resilient element structures for retaining resistive wires in place. In use, with the resistive heaters either directly contacting the passage walls or joined by wash coat and/or bonding material, just about all heat transfer within the heater-filled passages is by conduction. In the wire loop embodiments of
The resistive heating wires or pins require wiring connections to a current source, this being indicated in the illustrated figures by the term ‘manifold’. Connections between supply wiring and resistively heated wires can be by any of resistance welded, laser welded, diffusion welded, brazed or the like.
Leads to and from the resistive elements are made of high conductivity metals or alloys capable of surviving the exhaust gas temperatures that are generated at the vehicle catalytic converter, PF or SCR unit. Examples of such metals are copper, aluminum and Inconel™ alloys.
For the resistive heating wire sections, nichrome, a nickel chromium alloy which may be alloyed with amounts of other materials such as iron, is particularly valuable.
Stainless steel falls somewhere in between high and low resistivity metals. With suitable alloying metals and appropriate wire cross-sectional area, stainless steel can function as a resistive heating wire section element. With other alloying metals and different wire cross-sectional area, it can function as the wire conductor sections. Inconel™ can also be suitably alloyed and gauged to function as either a high resistivity heating section or a low resistivity lead section. In one particular example, copper comprises the inlet and outlet manifold metals and nichrome constitutes the resistance heated sections.
As shown in several of
Manufacturing using completed three- or multi-part wire assemblies is achieved by reel feeding both the conductor lead and resistive wire into a cutting and welding station. The feeder measures out the required length of the particular wire, cuts it to length and then this is held in the welding fixture. Then, the next wire of the other material is fed out, cut to length and then held in the welding fixture adjacent to the first wire. Joint are preferably either butt joints or overlap joints. Resistive or laser welding are preferred methods for fusing dissimilar materials together because no additional bonding material is needed at the weld site. The methods also produce a defined compositional transition between the two materials and thus a defined heating transition between them as well. A weld fixture is used to maintain the placement of wires when welding a resistive section to conducting section joint. For a three-section wire, a second joint is effected in the same manner and either the completed wire is ejected before a next wire is formed or a composite wire is continuously formed, collected on a reel and stored until later when it is cut to length.
The pigtail embodiment of
In the embodiment of
Leads to the resistive heating elements may have parts located within the casing 18 and parts located outside the casing. In one embodiment, the leads are separate from each other within the casing and are combined outside the casing. Respective insulated through-wall connectors mechanically consolidate the input leads and the output leads into a main input cable at or near one end of the casing and a main output cable at or near the other end of the casing. The ceramic substrate is separated from the inner wall of the casing by a layer of insulation acting to provide both heat and electrical insulation.
The electrical leads and the resistive heating elements are made of materials that develop oxidation resistance when exposed to air and to hot emissions gas. Consequently, they resist environmental degradation without the need for directly contacting insulation. The leads are installed with minimal possible slack and even some slight tension to limit vibration during use. Vibration could otherwise jeopardize lead integrity in the long term or may result in undesirable metal contact with other elements of the assembly. When operating in a catalytic converter, PF or SCR unit, the leads, by virtue of their low area cross-section and the occupation density of the cells, do not add substantially to back pressure which might otherwise adversely affect vehicle engine performance.
Wiring the heating element, whether individual, looped or woven, is complex as there are both inlet and outlet connections to each of the resistance heating wires. With several hundred resistive heating wires for one ceramic substrate requires a large number of conducting leads.
Heating from a wire section predominantly exposed to air or flowing gaseous emissions mainly derives from radiation and convection. Heating from a wire section retained closely within a cell of the ceramic substrate is mainly conductive. At a transition between two wire sections, there is some combination of all three heat transfer mechanisms.
One example, for a 4 kW 48 V catalytic converter application with a 500° C. light-off temperature, used a 400 cpsi ceramic substrate 4 inches in diameter and 4 inches long. Wire gauge was 19 or 0.0359 inch diameter to substantially fill the cross-sectional area of the cells. A nichrome C resistive heating element was used having a composition of 60% nickel, 16% chromium and 24% iron so giving a service temperature up to 1000° C. This example had a 1:15 array pattern of occupied: unoccupied cells. 10 resistive wire heating elements were woven back and forth 17 times (34 channels blocked per wire). The 10 woven wires were connected in parallel to meet required power and voltage ratings.
In another example, a 400 cpsi ceramic substrate with 0.0045-inch wall thickness had 0.0455-inch effective diameter cells accommodating respectively 0.0359-inch effective diameter wires. This left a gap between the wire surface adjacent wall about 0.005 inches which enabled relatively easy wire insertion or weaving at the time of manufacture. A smaller gap is possible if manufacturing processes result in wire and cell walls that are particularly accurately shaped and dimensioned. Lowering the gap size with a view to better heat transfer during use is a compromise with maintaining ease of wire insertion into a cell. Ceramic glue/mastic or catalyst metal wash coat slurry was used to fill at least part of the gap and so increase heat transfer during use. In one example, the resistive heating wire was heated to a 500° C. maximum in order to heat the surroundings.
Other variations and modifications will be apparent to those skilled in the art and the embodiments of the invention described and illustrated are not intended to be limiting. The principles of the invention contemplate many alternatives having advantages and properties evident in the exemplary embodiments.
Referring to
In a resistive heating system or an inductive heating system, if high power heating is required, this may result in greater size and/or cost of electrical components due to the high current draw. Splitting the power/current between two heating techniques—one resistive and the other inductive—can reduce the size and cost of such components. While more electrical components may be required overall, the size of these components is reduced such that system packaging is smaller compared with a single high power heating system. Electrical components can scale exponentially in size and cost with power/current requirements.
In a further alternative combination system, for fixed packaging requirements, the combination of resistive and inductive heating is structured and operated to boost the attainable power level beyond what is reasonably attainable for a single resistive or inductive system. Thus, for a specific design of resistive or inductive heating system, there is a highest efficiency operating regime for converting input energy to usable heat. Operating at non-optimal power levels reduces the efficiency of a particular heating system, whether for a resistive or inductive system. The efficiency level typically might range from a minimum of 50% up to maximum of 90% efficiency. Taking a total power requirement and splitting it between two heating systems allows overall efficiency to improve. Here, operating parameters are set so that neither the resistive nor inductive system is allowed to drop below a middle efficiency (say, 70%) level, with each heating technology taking a portion of the total power requirement. Starting at a low power level, the single-system would have the lowest efficiency, but the two-system approach would have a middle efficiency. As the power level is increased, the efficiency improves in both the single- and two-system approaches until the first heating system in the two-system approach is maxed out. This would also be a first system maximum efficiency while the single-system is at mid-efficiency. Increasing power further increases the efficiency of the single-system whereas the two-system approach would see a slight drop from the first maximum efficiency as the second heating technology is now working with the first. When both the heating technologies work together in the two-system approach, they can both run below their max powers at efficiencies slightly lower than the maximum, or one can run at maximum power and efficiency and other heating technology runs at its middle efficiency, with any combination between the two or an inverse of the example presented. At a maximum power requirement, the single-system and two-system arrangements are each heating at their maximum efficiency. The advantage of the two-system heating approach is that the total efficiency is always well above the minimum of the single-system.
In such a combination system, a ‘contained’ wire or other metal may be both a resistive heater and an inductive heater, a suitable exemplary material being Kanthal™. Such an insert can be used for resistive heating and inductive heating at the same or different times. If heated inductively and resistively at the same time, inductive heating has current more concentrated in a skin region of the metal while in resistive heating, current is relatively evenly spread throughout the cross-section of the metal content. In a variation, some metal inserts are dedicated to, and specifically adapted for, resistive heating, while other metal inserts are dedicated to, and specifically adapted for, inductive heating.
The above-mentioned US patents introduce various modifications and variations of a basic inductive heating system for aftertreatment of gaseous emissions such as those described in U.S. Pat. No. 10,352,214. Many of the structures and methods described in such patents can be adapted to resistive heating as now contemplated, whether or not combined with inductive heating. Such structures and methods include, for example, substitution of the metal wires 20 by a hollow wire, metal powder, molten metal, and deposited metal film. Such patents describe also exemplary methods for insertion and retention of these materials in such passages. It will be understood by those of ordinary skill in the art that structures and methods described in such patents can be adopted and applied, with or without some adaptation, in building and operating a resistive or resistive/inductive heating system as described above.
While the embodiments described above describe the use of metal wires within selected cells of a ceramic substrate, metal within the selected cells can be other than a wire, and include such forms as metal powder, molten metal, metal in matrix, metal deposited in a selected cell and metal in the material of the ceramic structure. The use of the word ‘wire’ throughout the specification and when applied to the wire interior of a selected cell is intended to cover such alternative embodiments of metal having desired resistivity characteristics. The use of the word ‘insert’ in the specification connotes more than just placement and is intended to cover pouring, flowing, deposition and like processes for introducing metal into a selected call.
In a combined resistive/inductive heating system ‘contained’ metal temperature affects the form of heating differently. Resistance heating efficiency improves with rising temperature because the resistance to current flow generally increases as temperature increases. Induction heating efficiency increases with temperature up to the Curie point, but the efficiency then drops off rapidly as magnetic properties that support induction heating are lost. While temperature control versus efficiency is challenging to balance, this can be mitigated by limiting the temperature and operating at the most efficient balance of the two heating systems. Alternatively, in a more general operating practice, induction is used at lower temperatures and resistance heating is used at higher temperatures.
To effectively combine the different heating schemes, real estate and other structural compromises may be necessary. Firstly, 3 or 4 electrical connections are necessary compared to only 1 or 2 for a single heating system. Secondly, resistance heating requires a positive voltage and a ground for current flow. For vehicular systems, ground may be to the exhaust system and thus to the chassis. Grounding to the exhaust system can be effected as a connection to the sheet metal inside the aftertreatment system. Induction heating requires 2 leads for the AC signal to and from the coil. Two controllers/power supplies are necessary because resistance heating requires a DC supply and the induction requires an AC supply. However, as noted previously, the physical size of the two-system design need not be more than that of a single—induction or resistive—heating system on its own.
The present application claims priority pursuant to 35 U.S.C. 119(e) to pending U.S. Provisional Application Ser. No. 63/105,727 entitled “Apparatus and method for gaseous emissions treatment with resistive heating” filed Sep. 17, 2020, the disclosure of which application is incorporated herein by reference in its entirety and made part of the present application for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 63079983 | Sep 2020 | US |
