US20120079813A1

US20120079813A1 - OPERATING METHODS FOR SELECTIVE CATALYTIC REDUCTION OF NOx

Info

Publication number: US20120079813A1
Application number: US12/897,906
Authority: US
Inventors: Thompson M. Sloane; Kevin L. Perry; David L. Hilden; Norman D. Brinkman; Jong H. Lee; Michael B. Viola; Steven J. Schmieg
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2010-10-05
Filing date: 2010-10-05
Publication date: 2012-04-05
Also published as: CN102441327B; DE102011115300A1; DE102011115300B4; CN102441327A

Abstract

Where oxygenated hydrocarbons, such as ethanol, may be considered for use as a reductant to be added to diesel or gasoline engine exhaust for promoting the catalyzed reduction of NOx to N₂, there is a need to continually adjust the amount of the reductant to be added as engine and catalyst operating conditions change. It is found that useful methods, to be practiced using a suitably programmed on-vehicle computer, can be based on a correlation for ethanol, or other specific reductant, with continually measured values of catalyst temperature, the oxygen and NOx contents of the exhaust, and the volumetric flow rate of the exhaust over a reduction catalyst, such as silver supported on alumina, selected for reduction of NOx to nitrogen. Effective amounts of the reductant for substantial reduction of NOx may be reliably determined using at least such parameters.

Description

TECHNICAL FIELD

This invention pertains to methods for managing the selective catalytic reduction of nitrogen oxides (mostly NO and NO₂, collectively, NOx) in the exhaust gas stream from a diesel engine or other lean-burn engine. More specifically, this invention pertains to a method of controlling the addition of a suitable oxygenated hydrocarbon, such as ethanol, to the exhaust gas preparatory to the reduction of NOx in the exhaust stream as it flows into contact with a non-noble metal, selective reduction catalyst.

BACKGROUND OF THE INVENTION

Diesel engines, some gasoline fueled engines, and many hydrocarbon-fueled power plants are operated at higher than stoichiometric air-to-fuel mass ratios for improved fuel economy. Such lean-burning engines and other power sources, however, produce a hot gaseous exhaust with relatively high contents of oxygen, water, and nitrogen oxides (NO_x). In the case of diesel engines, the temperature of the exhaust gas stream from a warmed up engine is typically in the range of about 200 degrees to 400 degrees Celsius, depending on current rate of fuel consumption (loading) of the engine. The exhaust has a representative, exemplary composition, by volume, of about 10% oxygen, 6% carbon dioxide, 0.1% carbon monoxide (CO), 180 ppm hydrocarbons (HC), 235 ppm NO_x, and the balance substantially nitrogen and water. The exhaust gas often contains some very small carbon-rich particles. And to the extent that the hydrocarbon fuel contains sulfur, the exhaust from the combustion source may also contain sulfur dioxide. It is desired to treat such exhaust gas compositions to minimize the discharge of any substance to the atmosphere other than nitrogen, carbon dioxide, and water.
The NO_xgases, typically comprising varying mixtures of nitrogen oxide (NO) and nitrogen dioxide (NO₂), with small amounts of nitrous oxide (N₂O), are difficult to reduce to nitrogen (N₂) because of the relatively high oxygen (O₂) content (and the water content) in the hot exhaust stream. Three-way catalyst systems, used successfully with engines operated at about a stoichiometric air to fuel ratio, have not been effective in reducing NOx to acceptable levels in such an oxygen-rich exhaust.
Lean NOx traps can be efficient at removing NOx from an exhaust stream, but they require expensive noble metals and their long-term durability is uncertain. Selective catalytic reduction methods have been considered where the lean-burn exhaust has been oxidized to complete the oxidation of unburned hydrocarbons to carbon dioxide and water, oxidize carbon monoxide to carbon dioxide, and to oxidize some of the nitrogen oxide (NO) to nitrogen dioxide (NO₂). A reductant material such as NH₃or an NH₃precursor (an aqueous urea solution) is injected into the oxidized exhaust stream and the stream passed over a suitable catalyst for the reduction of much of the NOx to nitrogen and water. Such a practice is referred to as the “selective catalytic reduction” (SCR) of NOx because it is accomplished without affecting other oxidized species in the exhaust stream. But on-board storage of an additional reductant material is required for urea- and ammonia-SCR and, when urea is the reductant, an aqueous solution of urea must be protected from freezing in cold weather.
It would be useful to have a less expensive, more convenient method for achieving NOx reduction on the exhaust flow from a lean-burn engine.

SUMMARY OF THE INVENTION

A method is provided for managing the use and addition of ethanol, or other suitable low molecular weight oxygenated hydrocarbon (OHC), such as methanol, a propanol, a butanol, or an aldehyde of such an alcohol, as an oxygenated hydrocarbon-type reductant in an OHC-SCR process and catalytic reduction reactor for NOx in an oxygen-containing and water-containing lean-burn exhaust. The amount of oxygenated hydrocarbon added to the exhaust stream is continually monitored and adjusted in accordance with certain conditions and properties of the exhaust stream and of the reduction catalyst as identified below in this specification. A goal in continually managing the addition of OHC is to avoid the need for subsequent treatment of the exhaust for ammonia downstream of the OHC-SCR reactor. And a further goal is to use the oxygenated hydrocarbon in amounts that minimize overall fuel consumption by the vehicle engine and its exhaust system.
Most current vehicle engines use the combustion of a hydrocarbon fuel with air for their power source. The engines comprise a plurality of cylinders, each with a reciprocating piston connected to a crankshaft and powertrain system for propulsion of the vehicle. Means are provided for computer-controlled, sequential, timed, introduction of air and fuel into each cylinder. As the pistons reciprocate in their respective cylinders, a controlled air and fuel mixture sequentially enters the respective cylinder where the combustible mixture is compressed and ignited. The combustion in each cylinder drives the piston in a power stroke which is followed by an exhaust stroke in which the combustion by-products are expelled from the cylinder into an exhaust manifold and then into the exhaust conduit of the vehicle. The continual operation of such a lean-burn engine produces a steady exhaust stream in a temperature range and of a composition illustrated above in the Background section of this specification. The fuel may, for example, be diesel fuel, gasoline, natural gas, liquefied petroleum gas (largely propane), dimethyl ether, ethanol, a mixture of ethanol and gasoline, or the like. Hydrogen may also be used as a fuel.
The oxygen and water-containing exhaust may first be directed to a flow-through, catalyst-containing, oxidation converter for the oxidation of carbon monoxide and incompletely burned hydrocarbon fuel constituents. The oxidation converter may also promote the oxidation of some of the nitrogen oxide to nitrogen dioxide in preparation of the exhaust stream for downstream treatment in a selective catalytic reduction reactor for the reduction of NO₂and other NOx constituents to nitrogen before the exhaust stream leaves the vehicle's exhaust system. In accordance with this invention an oxygenated hydrocarbon is added to the exhaust stream at a suitable location upstream of the selective catalytic reduction reactor. An example of a suitable oxygenated hydrocarbon-selective catalytic reduction (OHC-SCR) catalyst for practices of this invention is a material comprising nanometer-size particles of silver (or silver oxide), deposited and supported on micrometer-size particles of alumina. The particles of supported silver catalyst may, for example, be suitably deposited as a thin washcoat layer on the walls of the many small flow-through channels of an extruded, honeycomb-shaped ceramic monolith.
In accordance with embodiments of this invention, ethanol (or other suitable oxygenated hydrocarbon) is injected (or otherwise introduced) into the exhaust, upstream of the OHC-SCR catalyst. In order to more efficiently affect the reduction of NOx constituents as the exhaust flows in intimate contact with the OHC-SCR catalyst, certain continually varying parameters of the OHC-SCR catalyst and the exhaust, as determined by the inventors herein, are continually monitored and the amount of oxygenated hydrocarbon added to the exhaust stream is continually adjusted in compliance with a predetermined strategy based on the values of the determined parameters.
In further accordance with an illustration of an example of a method of this invention, a measure or determined value of each of the following four parameters (at least) are continually obtained at timely intervals and used in managing the addition of the oxygenated hydrocarbon: (i) the temperature of the OHC-SCR catalyst, representations of the amounts of NOx (ii) and of oxygen (iii) in the exhaust just upstream of the OHC-SCR catalyst, and (iv) a measure of the flow rate of the exhaust passing over or through the catalyst. In the case of the exhaust flow rate, a measure such as the gas hourly space velocity (SV, h⁻¹) or the equivalent may be measured or determined. In determining space velocity at a particular moment, the ratio of the current unit volume per hour of exhaust gas at standard conditions of temperature and pressure and a predetermined fixed volume of the OHC-SCR catalyst may, for example, be used. In practices of the invention it is important to obtain accurate and reliable representations of oxygen content in the exhaust, upstream of the OHC-SCR catalyst, for each determination of OHC addition to the exhaust.
Suitable sensors for measuring the current temperature of the selective reduction catalyst and for determining a useful value of the flow rate of the exhaust are available. Similarly, sensors are available for continually providing or determining useful instantaneous concentrations of oxygen (e.g., as a percentage of the total exhaust gas) and NOx (e.g., in parts per million of exhaust gas composition). A suitably programmed, computer-based control system may be used for receiving such data from the respective sensors and for using such available data for continually and rapidly analyzing these parameters for use in specifying the amounts of oxidized hydrocarbon reductant to be added to the exhaust stream throughout each operation of the vehicle engine. Such a sensor and computer-based system may function as part of or in cooperation with an engine control system used, for example, in measuring, determining, and controlling air and fuel additions to the engine, and other engine operations.
In accordance with preferred embodiments of the invention, and by way of illustrative example, a suitable addition control system for the OHC-SCR reductant may consider the current values of the above specified exhaust/OHC-SCR parameters and determine a reductant amount (mass or volume) to be added to the exhaust stream. For example, the control system may be programmed to add a molar value of a reductant (such as ethanol) per mole of NOx constituents then determined or estimated to be in the exhaust based on a combination of the current values of the stated four parameters. As will be described in more detail below in this specification, a table of parameter combinations and corresponding reductant addition values may be predetermined for any (or each) reductant contemplated for use in the specific OHC-SCR system for a particular engine and vehicle. The table may be stored in the memory of a vehicle computer control system serving to manage on-going reductant additions to the exhaust stream. It is recognized that like engines in different vehicles may use different reductants, different mechanisms or means for adding a reductant to an exhaust stream, and different tables of parameter combinations and corresponding reductant addition values. A principal goal of the management of OHC addition is to achieve effective reduction of NOx to nitrogen, while minimizing the formation of ammonia in the reduction reactor and minimizing the effect of the reductant addition on the over-all fuel economy of the engine-vehicle system.
In other embodiments of the invention, the current values of the four parameters may be used in a vehicle computer system to calculate a current reductant amount based on a use of stored data or computational programs.
The oxygenated hydrocarbon reductant material will be carried in a suitable reservoir on the vehicle and delivered through a suitable conduit for addition to the engine or exhaust stream. Preferably, the reductant material is ethanol and, of course, ethanol may be the fuel for the vehicle, or a constituent of the fuel. Where, for example, the fuel is a mixture of ethanol and diesel fuel or of ethanol and gasoline, such an oxygenated hydrocarbon/hydrocarbon mixture may be used as the OHC-SCR reductant additive.
Other objects and advantages will be apparent from a review of detailed illustrations of specific embodiments of practices of the invention which are presented (as non-limiting illustrations) below in this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the flow of an exhaust stream from the exhaust manifold of a diesel engine (or other lean-burn engine or power plant) through an exhaust conduit system including an oxidation catalyst reactor and an OHC-SCR catalyst reactor. Illustrations of sensor locations and reductant addition locations are shown in this figure.

FIG. 2 is a schematic illustration of an OHC-SCR reactor. In this embodiment particles of a silver-alumina catalyst are applied as a washcoat material to the flow-through channels of a cordierite monolith honeycomb body. The silver-alumina catalyst-bearing monolith is enclosed in a suitable metal housing with an inlet and outlet for exhaust gas flow.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides an operating strategy for the use of ethanol and other suitable oxygenated hydrocarbons as an OHC-SCR reducing agent in the exhaust of a lean-burn engine. Ethanol is a suitable and preferred reductant for NOx over a silver-alumina catalyst, yielding NOx conversion efficiencies of 70-99% over a wide range of diesel and gasoline engine operating conditions. The oxygenated hydrocarbon reacts with NOx and oxygen at catalyst sites to form nitrogen, carbon dioxide and water. A significant portion of the NOx and the reductant are thus converted to substances tolerated by the environment to which the exhaust is released.
In particular, an average NOx conversion efficiency of 81% was achieved on a Heavy-Duty Federal Test Procedure schedule run on a 6.6 liter diesel engine with ethanol injection into the exhaust stream. Ethanol is a particularly effective OHC-SCR reductant, allowing reduction of NOx at catalyst temperatures as low as 250° C. Partial oxidation products of ethanol, such as acetaldehyde, as well as other alcohols are also comparably effective OHC-SCR reductants. Through a combination of experiments and mathematical modeling, an operating strategy has been developed for highly efficient removal of NOx from the exhaust during engine operation using ethanol as the OHC-SCR reductant. Other oxygenated hydrocarbons (or mixtures of oxygenated hydrocarbons with hydrocarbons) that are suitable for OHC-SCR could also be used, with only small modifications to the operating strategy for reductant additions as disclosed in this specification.
Practices of the method of controlling reductant additions may be better understood following an illustration of the operation of a diesel engine (or other lean-burn engine) and its exhaust system, including a catalyst-containing reactor in the exhaust path for the reduction of NOx constituents.
FIG. 1 is a schematic, functional block diagram of an exemplary diesel engine and exhaust system for OHC-SCR treatment of the NOx content of the exhaust.
Referring now to FIG. 1, a schematic, functional block diagram of an exemplary engine-exhaust system 10 is presented. The engine 12 may be, for example, a gasoline-type internal combustion engine, a diesel-type internal combustion engine, a hybrid-type engine, and/or another suitable type of engine. The engine 12 generates torque by combusting an air/fuel mixture within cylinders 14 of the engine 12. The engine 12 may include any suitable number of cylinders, such as the cylinder 14. For example, the engine 12 may include 2, 3, 4, 5, 6, 8, 10, or 12 cylinders.
Air is drawn sequentially into each cylinder 14 of engine 12 through intake manifold 13′. A throttle valve 13″ actuated by electronic throttle controller (ETC) 13′″ controls the flow of air into each cylinder 14 of the engine 12. The air mixes with fuel from one or more fuel injectors 15 to form an air and fuel mixture, which in the case of diesel engines and other lean-burn engines has an air-to-fuel mass ratio greatly in excess of the stoichiometric ratio of the fuel, which for diesel fuel and gasoline is about 14.7. The air/fuel mixture is sequentially ignited and combusted in each cylinder 14 to produce torque to propel a vehicle. And combustion of the air/fuel mixture produces exhaust which is expelled from each cylinder 14 into an exhaust manifold 17. The exhaust stream exits exhaust manifold 17 as a continuous stream during engine operation and flows into exhaust system 16.
The exhaust system 16 includes treatment catalysts, sensors, computer control modules, and the like that cooperate to reduce the amount of nitrogen oxides (NOx) in the exhaust. The exhaust system 16 typically includes an oxidation catalyst 20, a reductant material injector 22, and an OHC-SCR catalyst 24. The OHC-SCR catalyst 24 is usually carried in an OHC-SCR reactor container, an example of which is illustrated in FIG. 2. In FIG. 1 the exhaust gas flows from exhaust manifold 17 through a first exhaust conduit section 21 to oxidation catalyst 20. The oxidized exhaust flows through a second exhaust conduit section 23 to the OHC-SCR catalyst 24 and then through a third conduit section 25. The exhaust gas may exit the exhaust system through third conduit section 25. Exhaust system 16 may also include other devices such as a filter or trap (not shown) for removing carbonaceous particulate matter from the exhaust stream.
In this example the reductant material injector 22 injects an oxygenated hydrocarbon reductant material (e.g., ethanol) into the second exhaust conduit section 23, upstream of the OHC-SCR catalyst 24. The OHC-SCR catalyst 24 may adsorb the ethanol or other oxygenated hydrocarbon reductant and promote reaction of the reductant with NOx constituents and oxygen in the flowing exhaust stream to reduce NOx constituents to nitrogen. The OHC-SCR catalyst 24 is preferably a catalyst of silver particles supported on alumina particles. The OHC-SCR catalyst 24 may be used in combination with a diesel particulate filter or in any other suitable configuration.
The percentage of NOx that is removed from the exhaust via the NOx reaction with the oxygenated hydrocarbon reductant is referred to as conversion efficiency or NOx conversion rate. In accordance with this invention, the NOx conversion rate is importantly related to the continual determination of how much reductant is added to the exhaust gas stream upstream of the OHC-SCR catalyst 24.
Practices herein for determining the amount of reductant to be added to the OHC-SCR catalyst 24 make use of a group of sensors that may be considered as components of an exhaust system 16. The exhaust system 16 may comprise NOx sensors 28 and 30 and temperature sensors (often thermocouples) 32, 34, and 36. The exhaust system 16 also includes an oxygen sensor 38 which also plays an important role in the practice of this invention. One NOx sensor 28 is located upstream of the oxidation catalyst 20, and another NOx sensor 30 is located downstream of the OHC-SCR catalyst 24. In other embodiments, the NOx sensor 28 is located between the oxidation catalyst 20 and the OHC-SCR catalyst 24. A temperature sensor may also be located inside or near the OHC-SCR catalyst 24. In some practices of the invention it may also be useful to sense the presence of other constituents, such as ammonia, in the third conduit section 25, downstream of the OHC-SCR catalyst 24.
NOx sensors and oxygen sensors come in different forms and may be used in different forms. Often, they are electrochemical devices that compare an exhaust stream with air or other reference material. Their signals are received and used by an engine control module or other computer-based module. And the signals from a NOx sensor or an oxygen sensor may be used by a control module in combination with other sensor signals, such as a mass air flow sensor, in obtaining a useful value for current NOx concentrations or oxygen concentration in the exhaust stream.
The NOx sensors 28 and 30 are thus used in obtaining a measure of NOx concentration upstream and downstream of the OHC-SCR catalyst 24, respectively. In other words, the NOx sensors 28 and 30 are used to obtain a measure or value of NOx flowing into and out of the OHC-SCR catalyst 24. The NOx sensors 28 and 30 generate signals corresponding to the values of concentration of NOx (ppm) at their respective locations, which are referred to as NOx_inand NOx_out, respectively.
The temperature sensors 32, 34, and 36 are located in various places throughout the exhaust system 16. For example, as shown in FIG. 1, the temperature sensor 34 is located downstream of the oxidation catalyst 20 and upstream of the OHC-SCR catalyst 24, and the temperature sensor 36 is located downstream of the OHC-SCR catalyst 24. The temperature sensor 32 is located upstream of the oxidation catalyst 20. The temperature sensors 32, 34, and 36 each measure temperature of the exhaust at their respective locations and output a signal that corresponds to that measured temperature. The signals output by the temperature sensors 32, 34, and 36 are referred to as T_A, T_B, and T_c, respectively, in FIG. 1.
An engine control module (ECM) 40 controls the torque output of the engine 12. In this function, the ECM 40 may receive signals from one or more sensors including an intake manifold absolute pressure sensor (MAP), a mass air flow sensor (MAF), a throttle position sensor (TPS), an intake air temperature sensor (IAS), and sensors of other engine operating parameters for control of engine operation. In the practice of this invention, the ECM 40 also includes a reductant addition control module 42 that controls the mass flow rate of the oxygenated hydrocarbon reductant injected by the reductant material injector 22. In this manner, the reductant addition control module 42 controls the amount of reductant material supplied to the OHC-SCR catalyst 24. The reductant addition control module 42 is an element of a computer, pre-programmed according to the principles of the present disclosure to manage the addition of reductant material for the reduction of NOx constituents in an exhaust stream flowing in contact with OHC-SCR catalyst 24. The reductant material may be stored, for example, in the fuel tank or other suitable reservoir and delivered through a conduit, not shown, to reductant material injector 22.
As indicated schematically in FIG. 1, reductant addition control module 42 receives signals from temperature sensors, one or more NOx composition sensors and at least one oxygen sensor. In addition to the temperature sensors and exhaust gas constituent sensors, a sensor detecting exhaust mass flow rate is provided upstream of the OHC-SCR catalyst 24 for continual determination by module 42 of exhaust gas volumetric flow rate for the calculation of the space velocity of the exhaust (or a like exhaust gas volume) in contact with the OHC-SCR catalyst 24. In some embodiments of the invention, the reductant addition control module may use data, for example, for mass air flow and fuel flow to the engine to determine or estimate the current exhaust gas flow rate.
In FIG. 1 delivery of the reducing agent to the exhaust stream was accomplished by injection of the reductant into the exhaust stream, upstream from the OHC-SCR catalyst. Other methods of delivery of the reducing agent include; 2) injection into a fired engine cylinder, late in the expansion stroke; 3) injection into a non-fired, or deactivated, cylinder into which no combustion fuel has been added; 4) injection into a cylinder just before deactivation by the closing of intake and exhaust valves, followed by opening of the exhaust valve on a subsequent cycle to allow the reductant and any reaction products of the reductant to enter the exhaust system. In practices in which oxygenated hydrocarbon is added through the vehicle engine it may be preferred to minimize or eliminate the use of an oxidation reactor in the exhaust stream.
Before discussing methods for continually determining a suitable quantity of reductant to be added to the exhaust stream for effective reduction of NOx, it may be helpful to illustrate an example of a reactor (or container) for through-passage of an NOx-containing and reductant-containing exhaust and efficient contact of the gas with an OHC-SCR reduction catalyst 24 placed in the catalytic reactor. FIG. 2 illustrates an OHC-SCR reactor 60.
Referring to FIG. 2, OHC-SCR reactor 60 may comprise a round tubular stainless steel body 62 for tightly enclosing, for example, an extruded, round cylindrical, honey-comb shaped, cordierite catalyst support body 64 which is seen in the broken-out window in the side of body 62. Support body 64 may be formed of other known and suitable ceramic or metallic materials. In this embodiment, cordierite support body 64 is formed with many exhaust gas flow-through channels that extend from an upstream, exhaust gas inlet face 65 of the catalyst support body to a downstream, exhaust gas outlet face (not visible in FIG. 2) of the catalyst support body. These small flow-through channels are represented as crossing lines in the illustration of exhaust gas inlet face 65. For example, 400 flow-through channels per square inch of inlet face are typically formed during extrusion of the ceramic body. A silver-on-alumina particle catalyst in the form of a washcoat is coated on the walls of each of the channels of the honeycomb structure. The diameter of steel body 62 and enclosed silver-based reduction catalyst support body 64 is enlarged with respect to the upstream and downstream exhaust conduits so as to reduce drag on the exhaust stream as it engages the inlet face 65 of the catalyst support body 64 and flows through the washcoated channels. Support body 64 is sealed into steel body 62 so that exhaust flow is directed into contact with the supported OHC-SRC catalyst on the channel walls of support body 64.
As seen in FIG. 2, the upstream end of steel enclosure body 62 (as indicated by exhaust flow direction arrow 66) is enclosed by an expanding stainless steel exhaust inlet section 68. Exhaust inlet 70 of inlet section 68 is sized and adapted to receive exhaust flow from exhaust conduit 23 (FIG. 1). Exhaust inlet section 68 is welded to steel enclosure body 62. In a like manner, the downstream end (exhaust flow direction arrow 72) of steel enclosure body 62 is enclosed by a flow narrowing, steel exhaust section 74 with an exhaust outlet 76 adapted to be welded to a downstream exhaust conduit piece.
A temperature sensor (not illustrated in FIG. 2) may be located within steel enclosure body 62. Such a sensor may be located, for example, at the upstream 65 and/or the downstream end of catalyst support body 64.
In accordance with practices of this invention it is necessary to obtain a measure of the volumetric flow rate of an NOx-containing exhaust stream over a silver-on-alumina selective reduction catalyst. In the illustration of FIG. 2, the exhaust stream is divided at inlet face 65 of catalyst support body 64 and caused to flow through many very small channels to which the silver catalyst has been applied in the form of washcoated particles. In this embodiment, it is often preferred to use the outer, superficial, volume of the support body 64 as the volume of the catalyst. Thus, a useful measure of a space velocity of an NOx-containing exhaust stream with respect to a washcoated, multichannel support body may be determined by dividing an hourly volumetric flow rate of the exhaust by the outer volume of the cordierite body containing the exhaust flow channels. The volumetric flow rate of may, for example, be determined at its actual temperature and pressure or at a standardized temperature and pressure.
An important control variable in these operating strategies for oxygenated hydrocarbon reductant addition to an exhaust stream is the ratio of the reductant concentration to the NOx concentration just upstream of the OHC-SCR catalyst. As illustrated in FIG. 1, sensors are used to measure the NOx concentration at a given operating condition, and the appropriate amount of reductant is introduced into the exhaust. The reductant concentration is expressed as the concentration of carbon atoms contained in the reductant relative to the concentration of oxides of nitrogen, and will be designated as the Cl(reductant)/NOx ratio. Accordingly, when ethanol is used as the reductant material, one mole of ethanol provides two moles of Cl reductant in a Cl/NOx ratio.

Stochastic Process Modeling of Reductant Addition for OHC-SCR

The inventors herein have made use of Stochastic Process Modeling (SPM) to provide numerical relationships between exhaust gas parameters and silver catalyst temperatures for managing oxidized hydrocarbon additions to an NOx-containing exhaust stream. Stochastic Process Modeling is an advanced empirical modeling method that interpolates between data points after adjusting for the noise that is present in the data used to construct the model. The SPM tools that were used were based on Matlab, and were obtained from Ricardo through the Design of Experiments for Powertrain Engineering (DEPE) Consortium.
In one example of the use of this modeling, stochastic process models were developed based on operating engine data for NOx conversion taken during two different known diesel engine operating cycles: the Heavy Duty Federal Test Procedure (HDFTP) and the Supplemental Emissions Test (SET), also a U.S. federal test procedure. It was determined that the required input data from the experiments would be the catalyst inlet temperature, the concentrations of each of NOx, Cl hydrocarbon (ethanol), and oxygen at the catalyst inlet, and the gas hourly space velocity over the catalyst. The output variable was either the NOx conversion efficiency or the related post OHC-SCR NOx concentration.
The experiments were carried out with a 6.6 liter turbocharged diesel engine. The exhaust system consisted of a diesel oxidation catalyst downstream of the exhaust manifold, followed by a ten-liter Ag/Al₂O₃OHC-SCR catalyst. Ethanol was injected into the exhaust system between the oxidation catalyst and the silver catalyst bed when the measured exhaust temperature at the point of injection was greater than 220° C. Measurements of exhaust species were obtained using a fast Fourier transform infrared (FTIR) spectrometer and were made every second during each of the engine cycles.
The testing of the engine as it was operated over the two federal test cycles produced several thousand data points. A satisfactory model for the HDFTP was constructed using 164 data points, which were successfully analyzed by Stochastic Process Modeling for implementation in an engine control module for control of ethanol additions to the specific diesel engine as it is operated in cycles like or overlapping the test cycles. However, in order to provide a summary of illustrative operating parameters in this text, ranges of selected operating variables were arbitrarily grouped into high and low temperature categories.
For purposes of illustration, values of these operating variables are grouped into low and high ranges. The low range of these variables are the following:

Temperature<300 C

NOx concentration<200 ppm
O₂concentration<11%
space velocity<50000 hr⁻¹.
The high range of these variables are for values exceeding these upper limits of the low range. Such a grouping results in sixteen illustrative operating conditions, consisting of the sixteen different combinations of low and high values of the four operating variables. Since each illustrative operating condition represents a range of variable values, the Cl(reductant)/NOx ratio required to achieve maximum NOx conversion may also be a range. The range of the Cl(reductant)/NOx ratio required to achieve maximum NOx conversion within each of the sixteen illustrative operating conditions is presented. This range has been determined from stochastic process modeling of engine data acquired on the HDFTP and the Supplemental Emissions Test (SET). For ethanol as the reductant, this is shown in Table I:

TABLE I

Operating strategy for ethanol as the HC-OHC-SCR reductant.

T	NOx	SV	O₂	C1(Ethanol)/NOx

low	low	low	low	2
low	low	low	high	2-40
low	low	high	low	2-3
low	low	high	high	2
low	high	low	low	2-37
low	high	low	high	1-2
low	high	high	low	1-2
low	high	high	high	1-14
high	low	low	low	3-32
high	low	low	high	1-40
high	low	high	low	2-31
high	low	high	high	6-72
high	high	low	low	9-12
high	high	low	high	7-23
high	high	high	low	4-26
high	high	high	high	6-10

The stochastic process modeling of engine data can provide the desired Cl/NOx ratio at any desired engine operating condition, if the engine data is comprehensive enough. The above table was constructed using stochastic modeling prediction (by interpolating the engine data) of the desired Cl/NOx ratio at 3 to 5 specific engine conditions that meet each of the high-low criteria, which is the reason for the ranges given. In actual practice, the number of specific engine conditions to be used for a particular test procedure will have to be determined using a stochastic process model that is based on engine data for that particular test procedure.
In other words, engine calibration will use many more than 16 points to define controllable engine conditions over a federal test procedure, for example, or other selected, desired engine operating cycle(s). But once the stochastic process model is developed from approximately 100-200 appropriately-chosen engine test operating conditions for a particular test procedure, the model can be used to predict the desired Cl/NOx ratio at any operating condition during the test procedure. The number of required predicted operating conditions needed for a particular test procedure will be determined based on the amount of NOx reduction required while avoiding the need to store large quantities of reductant on-board and to not exceed an acceptable effective fuel economy penalty due to the consumption of ethanol.
Thus, for actual engine calibration purposes, the preferred Cl(ethanol)/NOx ratio will be determined at each of a large and suitable number of different operating conditions. Factors to consider in determining the preferred Cl(ethanol)/NOx ratio, in addition to the four variables defining the operating condition, are the mass flow rate of NOx that exits the OHC-SCR catalyst, the mass flow rates of other undesirable exhaust components, such as ammonia, that may be a by-product of the OHC-SCR catalyst operation, and the equivalent fuel economy penalty incurred by adding ethanol to the exhaust. In this embodiment, the ethanol is stored on-board the vehicle in a container which is separate from the normal fuel tank, and used as needed in the exhaust system. Other means of supplying the required reductant could also be used, as described below. The engine fuel used could be diesel fuel, gasoline, hydrogen, natural gas, LPG, dimethylether, or other suitable lean-burn engine fuel.
Stochastic process modeling has also been applied to OHC-SCR of NOx in the exhaust from a four-cylinder gasoline fueled, lean-burn operated, spark ignition, direct injection (SIDI) engine. Ethanol and mixtures of 85% ethanol, 15% gasoline (by volume, a fuel known as E85) were used as the OHC reductant additive. The engine was operated on the New European Driving Cycles when the engine was in a warmed-up operating mode in which the exhaust was at a temperature suitable for effective silver catalyst function. The testing and data acquisition were conducted as described above with respect to the diesel engine. Several thousand data points were obtained for the engine operating cycles. Again, satisfactory models for driving cycles were developed using a few hundred data points which were analyzed by Stochastic Process Modeling for implementation in an engine control module for control of ethanol additions to the specific gasoline engine as it is operating in cycles like or overlapping the test cycles.
The advantages of ethanol or other effective oxygenate as a reductant for OHC-SCR can be realized in the exhaust of an engine that is designed to run on any fuel-ethanol mixture, or any fuel-oxygenate blend. In this latter case the reductant could conveniently use the contents of the fuel tank as the source for the OHC-SCR reductant. The larger the concentration of the effective oxygenate in the fuel, the larger the improvement in the NOx conversion efficiency that can be achieved. If desired, the oxygenate could be separated from the fuel before injection into the exhaust in order to concentrate it and make the injected reductant more effective. Such a separation could be achieved with a membrane, an extraction with water or other suitable chemical, or with another suitable method. A procedure similar to that described above could be used to develop an operating strategy for any engine fuel-oxygenate mixture.

Claims

1. A method of determining amounts of additions of an oxidized hydrocarbon reductant material for the catalyst-enhanced reduction of NOx constituents in the exhaust from a vehicle powered by a lean-burn engine; the vehicle engine comprising a plurality of combustion cylinders in which a fuel and air are mixed and ignited for powering reciprocating pistons in the cylinders and an exhaust manifold, connected to the cylinders, in which manifold the combustion products from the cylinders are combined as sequential portions of an exhaust gas stream comprising oxygen, water, nitrogen, and NOx; and the vehicle comprising an exhaust conduit for conducting the exhaust gas stream to a reactor into contact with a silver/alumina catalyst, contained within the reactor, for the chemical reduction of NOx constituents by reaction with the added oxidized hydrocarbon reductant; the method comprising, as the vehicle is being operated and the exhaust stream produced:

continually measuring (a) the oxygen content of a portion of the exhaust stream and (b) the NOx content of a portion the exhaust stream before the stream enters the reduction catalyst-containing reactor;

continually measuring (c) the temperature of the reduction catalyst in the reduction catalyst-containing reactor;

continually measuring (d) the gas hourly space velocity of the exhaust gas stream flowing in contact with the reduction catalyst; and, using at least the four measured values (a-d),

continually determining an amount of the reductant material, to maximize NOx reduction by the silver catalyst, to be added to a current portion of the exhaust gas stream before it enters the reduction reactor, such determination being made using a pre-programmed, on-vehicle computer and using at least the current four measured values; and

adding the currently determined amount of reductant to the exhaust stream before it flows into contact with the reduction catalyst.

2. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which the reductant material comprises an alcohol selected from the group consisting of methanol, ethanol, a propanol, a butanol, or an aldehyde of such an alcohol.

3. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which the reductant material comprises ethanol.

4. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which the reduction catalyst comprises particles of silver supported on particles of alumina.

5. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which the additions of reductant material are made in the exhaust conduit upstream of the reduction catalyst.

6. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which additions of reductant material are injected into combustion products in an engine cylinder and then enter the exhaust manifold and exhaust conduit.

7. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which additions of reductant material are made into an engine cylinder and exhausted from the cylinder into the exhaust manifold without combustion of the reductant material.

8. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which the fuel of the vehicle engine is one or more of the materials selected from the group consisting of diesel fuel, gasoline, hydrogen, natural gas, liquefied petroleum gas, ethanol, methanol, and dimethyl ether.

9. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 1 in which the continual determination of the amount of reductant material to be added to the exhaust stream comprises reference by the on-vehicle computer to a predetermined look-up table of addition amounts of a selected reductant material for selected ranges of the four measured values.

10. A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in claim 9 in which the reductant material is ethanol, and the continual determination of the amount of ethanol to be added to the exhaust stream comprises reference by the on-vehicle computer to a predetermined look-up table of addition amounts of ethanol for selected amounts of the four measured values.