KR20070020138A - Reductant addition in exhaust system comprising nox-absorbent - Google Patents

Abstract

An exhaust system for a vehicular lean- burn internal combustion engine comprises a NOX-absorbent, a reductant injector (78) disposed upstream of the NOX-absorbent and means (50), when in use, for controlling reductant addition, wherein the reductant addition control means supplies reductant to the NOX-absorbent at all vehicle speeds in a duty cycle at a rate which is predetermined to correlate with a desired NOX conversion at the average duty cycle speed of the vehicle. ® KIPO & WIPO 2007

Description

REDUCTANT ADDITION IN EXHAUST SYSTEM COMPRISING NOx-ABSORBENT}

The present invention NO _X - is an invention relating to a method of reproducing the absorber and control the reducing agent to the exhaust system for the purpose of reduction of NO _X to N ₂ - lean-burn internal combustion engine exhaust system, in particular NO _X containing the absorbent.

Exhaust systems for lean burn internal combustion engines, such as lean burn gasoline engines and diesel engines containing NO _X -absorbers, are described, for example, in EP 0560991 and the like.

As used herein, "NO _X -trap" is a catalyst comprising a NO _X -absorber and a catalyst for oxidizing NO to NO ₂ . NO _X -traps are also known as "lean NO _X traps" or "LNCs."

Typical NO _X -trap compositions include catalysts such as Pt; Absorber - NO _X, such as potassium and / or a compound of an alkaline earth metal compound such as barium or strontium of the alkali metals such as cesium, or in general compounds of rare earth metals such as lanthanum and / stinging yttrium; And reducing catalysts such as rhodium. The mechanism generally provided for NO _X -storage during lean engine operation for the composition is that in the first step NO reacts with oxygen at the active oxidation site on Pt to form NO ₂ . The second step involves the absorption of NO ₂ by the storage material in the form of inorganic nitrates.

Inorganic NO _x -storage components are generally present as oxides, although it is obvious that in the presence of exhaust gases or air comprising CO ₂ and H ₂ O, they may be in the form of carbonates or in the form of hydroxides.

When the engine is intermittently driven in an enriched or elevated temperature, the nitrate species become thermodynamically unstable and decompose and produce NO or NO ₂ . In concentrated state, such NO _X is reduced to N ₂ by carbon monoxide, hydrogen and hydrocarbons, and this reduction reaction can take place with a reduction catalyst.

The purpose of an exhaust system comprising a NO _X -trap is to increase the economics of the engine while at the same time satisfying the relevant emission standards, for example Euro IV.

Systems for controlling the addition of reducing agents for the purpose of regenerating NO _x -traps and reducing the absorbed NO _x are known, but processors for driving very complex control schemes and complex algorithms, including multiple sensor inputs, are known. need.

EP-B-0341832 describes a process for the combustion of particulate matter (PM) in diesel exhaust. The method comprises oxidizing NO in exhaust gas to NO ₂ on a catalyst, purifying PM from exhaust gas, and burning the filtered particulate matter at NO ₂ up to 400 ° C. This system is supplied by Johnson Matthey and commercialized as CRT ^® .

NO _X - After a study how to play the absorber, the algorithm is programmed the processor and the NO _X which does not require sophisticated equipment such as a sensor input network which - related to the exhaust gas, such as Euro IV with an exhaust system including an absorber It has been found that it is possible to meet the criteria.

According to the first aspect of the invention, NO _X - absorber, NO _X - reducing agent injection, which is disposed upstream of the absorber device, and obtain at the same time providing the vehicle a lean burn internal combustion engine exhaust system comprising means for controlling the addition of the reducing agent there is, the reducing agent addition control means includes: a reducing agent, NO _X in the duty cycle over the whole speed of the vehicle at a predetermined rate related to the NO _X conversion required by the average duty cycle (duty cycle) the speed of the vehicle is supplied to the absorber.

The present invention in the first aspect has particular use in the retrofit retrofit market for vehicles of limited duty cycles, such as buses and waste trucks. This is to determine how much reducing agent injection rate is needed to reduce the amount of NO _x selected, for example to 90%, in the NO _X -absorbent at the average duty cycle rate. For example, where the NO _X -absorber is a component of the NO _X -trap, upon operation, the system controller may be configured to generate a continuous tempo and amount of hydrocarbon (HC) fuel injection, eg at 2 seconds per minute. Can be. In addition, the system controller can be configured to provide a relatively long, rich HC fuel pulse from time to time to ensure that the NO _X -trap is substantially completely regenerated, after which it maintains the storage capacity of the NO _X -trap. In order to follow a more frequent series of shorter enrichment pulses. The precise and detailed injection method depends on the vehicle and the duty cycle of the vehicle.

At speeds above the average duty cycle rate, there will be more NO _x and greater mass airflow, so the NO _x conversion will decrease. This is because there is not enough reducing agent. However, for example in city buses, the speed is less likely to be high, so the system will meet the NO _X emission criteria for the entire operating cycle without increasing fuel consumption. Equivalently, HC may be emitted when the vehicle speed drops below the average duty cycle speed, but on average over the duty cycle the system may meet the HC emission criteria. The correlation between HC injection rate and average duty cycle speed can be tailored for a particular purpose (eg, a bus running in Manchester City, UK, must meet a different duty cycle than in London).

In one embodiment of the first aspect, to increase the temperature of the NO _x -trap for regeneration and / or to remove oxygen from the exhaust so that the exhaust can be concentrated for regeneration of the NO _x -absorbent, An oxidation catalyst is disposed between the reducing agent injector and the NO _x -absorbent.

In certain configurations, the system for delivering NO _x -traps and reducing agents described herein is disposed downstream of the configurations described in EP-B-0341832. That is, the catalyst for oxidizing NO to NO ₂ is optionally followed by a filter on which the catalyst is acted, and the reducing agent injector is followed by a NO _X -absorbent.

In one embodiment, the NO _X -absorbent used in the present invention is a component of the NO _X -trap.

Unless otherwise stated, the catalysts used in the present invention are coated on the high surface area of the carrier monolith made of metal or ceramic or silicon carbide material such as cordierite. A common configuration is a honeycomb monolithic structure through which fluids pass, ranging from 100-600 cpsi in cells per square inch (cpsi), for example 300-400 cpsi (15.5-93.0 cells cm ^-2 , for example 46.5-62.0 cells). cm ^-2 ).

The internal combustion engine may be a diesel or lean burn gasoline engine, such as a gasoline direct injection engine. The diesel engine may be a light engine or a heavy engine as defined by the relevant legislation.

A method of reducing NO _x in the exhaust gas of a vehicle lean burn internal combustion engine according to a second aspect of the present invention comprises the steps of: absorbing NO _X from the exhaust gas in a NO _X -absorbent, in a duty cycle over the entire speed of the vehicle, NO _{X-NO X} in order to reproduce the absorbent-step of the absorbent in contact with the reducing agent, and NO _X steps, and the reducing agent injection rate include the reduction to N ₂ is associated with the NO _X conversion required by the average duty cycle speed .

1 shows a schematic system according to a first aspect of the invention.

FIG. 2 is a schematic graph showing the amount of fuel over time, illustrating a fuel injection method when driven in the system of FIG.

3 is a schematic diagram showing an operational embodiment of the present invention.

4 is a graph showing the upstream air / fuel ratio (AFR) as a function of road speed in the embodiment of FIG.

FIG. 5 is a graph illustrating NO _x measurement in an idle state with respect to the embodiment of FIG. 3.

FIG. 6 is a graph showing the corresponding system temperature in the idle state for the trace shown in FIG.

FIG. 7 is a graph showing NO _x measurements at 40 mph for the example of FIG. 3.

FIG. 8 is a graph showing the corresponding temperature measurement at 40 mph for the trace shown in FIG.

FIG. 9 is a graph showing NO _x conversion as a function of road speed for the system of FIG.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings for a deep understanding of the present invention.

In the system 50 shown in FIG. 1, reference numeral 52 is a conditional system controller (CSC), 54 is a master switch, 56 is an alternator, 58 is a blocking capacitor, 60 is a thermocouple, 62 is an injection controller (ICU), 64 is a fuel pump, 66 is a valve, 68 is a fuel injector, and 70 is a positive power line. The CSC 52 is driven as detected by the AC ripple from the alternator whose master power switch 54 is on, the engine is located behind the DC blocking capacitor 58, and the exhaust is exhausted. Switch to power the ICU 62 when the output of the thermocouple 60 properly positioned to detect the system is above the minimum predetermined temperature for the reduction of NO _X on the appropriate NO _X trap. to be. The master switch 54 need not be connected to the key-on position.

CSC 52 is designed to generate a continuous HC injection tempo and quantity when all three characteristics (position of master switch, detection of alternator ripple, and exhaust gas temperature above a predetermined minimum value) coincide. When the CSC 52 is on, it drives the solenoid valve 66 to generate a series of pulses to concentrate the exhaust gas before it passes through the oxidation catalyst upstream of the NO _x -absorbing component. Power is supplied to the ICU 62 and the injection pump 64. In general, the injection controller will provide a relatively very long rich pulse from time to time to make the NO _X -trap substantially completely empty, and thereafter further to maintain the capacity of the stored NO _X -trap. There is a frequent series of shorter enrichment pulses, such as injection at 2 seconds per minute (see FIG. 2).

The fuel injection rate is associated with a conversion which is a selected NO _X conversion, for example 90%, at an average duty cycle speed. The details of the injection method depend on the vehicle and the duty cycle of the vehicle.

Very generally, the systems using the NO _x -traps described herein have been developed to provide special use in the older equipment retrofit market with simple control mechanisms that predict when NO _x -trap regeneration should occur. Many vehicles already include a series of sensors that input data into the ECU to control other elements of the vehicle drive. By properly reprogramming the ECU, it is possible to employ one or more of these sensors to predict residual NO _x -trap capacity. This includes (but is not limited to) a predetermined or predicted time elapsed from key-on or previous playback, which includes the appropriate state of recording means; State of flow through the TWC or manifold vacuum; State of ignition timing; State of engine speed; State of throttle position; The state of the exhaust gas redox compound, for example using a lambda sensor, preferably a linear lambda sensor; The amount of fuel injected from the engine; Where the vehicle comprises an exhaust gas recirculation (EGR) circuit, the position of the EGR valve and the amount of EGR measured accordingly; Engine coolant temperature; And when the exhaust system includes a NO _X sensor, the amount of NO _X detected upstream and / or downstream of the NO _X -trap. In the case where recording means are used, the estimated time can be adjusted as a result of the data input.

(Example)

The following examples are merely illustrative.

A single-floor bus exhaust system 10 (see FIG. 3) comprising an engine turbo 12 of the type suitable for a 6 liter turbocharged engine and meeting the European Stage 1 emission limits is provided with three parallel legs (16). It is modified to include a three-way splitter 14 for switching to one of (). In each leg the exhaust flows at the same speed. Each leg 56 includes a chamber 18 with an NO _x -trap 22 followed by an oxidation catalyst 20. Gas flow is NO _X - are combined downstream of the trap, before the exhaust gas is directly passed into the atmosphere NO _X - the total exhaust gas flow of "clean-up (clean up) in order to remove unburned hydrocarbons (HC) coming out of the trap Pass through an oxidation catalyst 24. A fuel injector 66 comprising a fuel solenoid 28 is disposed in front of each oxidation catalyst 20, and a NO _X sensor 29 is disposed in front of the exhaust splitter 54, combined NO _x / air. The fuel ratio sensor 30 is disposed behind the NO _x -trap, and thermocouples T1, T2, T3, and T4 for measuring temperature are disposed before and after the oxidation catalyst 20 and at the reactor outlet. Oxidation catalyst 20 and NO _x -trap 22 are coated on a ceramic fluid passage and monolith with a 400 cell in ^-2 (62 cell cm ^-2 ) and 0.06 in (0.15 mm) wall thickness, respectively. The oxidation catalyst 20 is 5.66 inches (144 mm) in diameter, 3 inches (76 mm) in length, and has a volume of 75.5 inches ³ (1.24 liters). The diameter of the NO _x -trap 22 is the same as the diameter of the oxidation catalyst 20 but is 6 inches (152 mm) in length. The "clean up" catalyst 24 has a diameter of 10.5 inches (267 mm), a length of 3 inches (76 mm), and a volume of 260 inches ³ (4.26 liters).

The experiment described in this embodiment is performed using only one leg of the exhaust split. The vehicle is driven using diesel fuel containing 50 ppm of sulfur and is driven at constant idling speeds of 10, 20, 30 and 40 mph during operation. Fuel is injected at each point with the air-fuel ratio during the injection determined as shown in FIG. The combination of time and duration (two-second injection, once per minute per leg) determines the exhaust gas temperature (temperature to maintain the NO _X -trap within the dynamic temperature window) and NO _X. Empirically selected to achieve the best combination between transforms. At the same time, the NO _x -emissions before and after the system together with the temperature profile are measured.

5 is NO _X from the engine. Exhaust (ppm), and NO _X - During the idle state with the air-fuel ratio measured after the NO _X trap - trap after the NO _X Exhaust (ppm). 6 shows temperature traces during the same time. In FIG. 5, if fuel is injected at the start of idle time, the air-fuel ratio can be observed to fall from lean to excess, as predicted from the expectation in FIG. 3, with good NO _X conversion after initial NO _X breakthrough. This is observed. Over time, the air-fuel ratio remains sparse throughout the injection operation, but good NO _X conversion remains. The exotherm T2 generated over the oxidation catalyst allows the temperature of the NO _X -trap to be maintained between operating windows of 220-550 ° C. In addition, the exotherm T3 is recorded across the NO _x -trap, some of which are generated as the unreacted gas reducing agent is burned from the oxidation catalyst. This result is interpreted to mean that some of the heat generation occurs from the combustion of unburned fuel droplets reacting on the surface of the NO _X -trap over time in the engine idle state. This is because the system inlet temperature is insufficient to vaporize the incoming fuel, and also the air / fuel ratio peak measured by the rear sensor, indicating a series of settling, vaporization and thus sequential oxidation of the fuel droplets. ) Is not displayed correctly and becomes rounder. Some of the excess thus caused also serves to maintain the observed NO _x -trap operating efficiency.

Results of testing with a bus maintained at a constant speed of 40 mph are shown in FIGS. 7 and 8. The exhaust flow rate is higher here, but the same injection flow rate is used in the idle state, and the exhaust is required to be in a lean state throughout the injection time (see FIG. 3). However, apart from the breakthrough spike when the fuel is first injected, NO _X is reduced over the remaining operating time, although not as efficient as in the idle state. The exotherm T3 above the exothermic T2 is sometimes lower than in the idle state, but because of the increased heat capacity of the exhaust gas, this is very important. Therefore, an exothermic reaction is still taking place, and it is also believed that this phenomenon is because some of the unburned fuel droplets are carried through the oxidation catalyst and burned on the NO _X -trap. It is expected that fuel droplets will persist despite the high inlet temperature of the oxidation catalyst, as more exhaust flow rates result from the marked exotherm measured through the NO _X -trap and the trap regeneration observed in the apparently sparse state. It will carry the droplet through it.

9 shows the trend of the average NO _x conversion efficiency calculated for the system as a function of speed. 3 shows that no excess exhaust gas condition occurs above about 6 mph, and a good NO _X conversion can be obtained in lean conditions over a wide speed range. This is particularly relevant in the range of 30 mph from idle (which is the most common operating range for urban buses).

Claims (21)

In the vehicle exhaust system for lean burn internal combustion engine, NO _X - absorber, NO _X - reducing agent injector 68 is disposed upstream of the absorbent, and obtain at the same time includes means (50) for controlling the addition of the reducing agent, The reducing agent addition control means, NO _X in the duty cycle over the whole speed of the vehicle at a predetermined rate related to a reducing agent and the NO _X conversion required by the average duty cycle speed of the vehicle exhaust system, characterized in that to be supplied to the absorber. The method of claim 1, The NO _x -absorber is selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, compounds of rare earth metals, and mixtures of two or more thereof. The method of claim 2, The alkaline earth metal is exhaust gas, characterized in that selected from the group consisting of barium, magnesium, strontium and calcium. The method of claim 2, The alkali metal is exhaust gas, characterized in that selected from the group consisting of potassium and cesium. The method of claim 2, And said rare earth metal is selected from the group consisting of cerium, yttrium, lanthanum and praseodymium. The method according to any one of claims 2 to 5, And said alkaline earth metal compound, said alkali metal compound or said rare earth metal compound is supported on a support material. The method of claim 6, And said support material is selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixtures or compounds of two or more thereof. The method of claim 6, NO _x -Absorbent comprising said support material. The method according to any one of claims 1 to 8, An NO _x -absorbent is a component of a NO _X -trap comprising a catalyst for oxidizing NO. The method of claim 9, The NO oxidation catalyst comprises a platinum group metal, optionally platinum and / or palladium. The method according to claim 9 or 10, The NO _x -trap comprises an NO _x reduction catalyst, optionally rhodium. The method according to any one of claims 1 to 11, And an oxidation catalyst disposed between the reducing agent injector (68) and the NO _x -absorbent. The method according to any one of claims 1 to 12, And a catalyst for oxidizing NO to NO ₂ disposed upstream of the reducing agent injector. The method of claim 13, And the NO oxidation catalyst is platinum on an alumina support material. The method according to claim 13 or 14, An exhaust system, characterized in that it comprises an optionally catalyzed particulate filter disposed between the oxidation catalyst and the reducing agent injector. The method according to any one of claims 9 to 11, The NO _x -trap comprises an particulate filter. The method according to any one of claims 1 to 16, And, in operation, control means for intermittently concentrating the exhaust gas composition to regenerate the NO _X -absorbent. The method according to claim 17, which cites any one of claims 9 to 11. Wherein said control means supplies, upon operation, a reducing agent to the NO _x -trap only when the catalyst is activated for NO _x reduction. A diesel engine comprising an exhaust system according to any of the preceding claims. The method of claim 19, The diesel engine is characterized in that the diesel engine for light weight. In the method for reducing NO _x in the exhaust gas of a vehicle lean burn internal combustion engine, NO _X - step to absorb NO _X in the exhaust gas from the absorber, In duty cycle over the whole speed of the vehicle, NO _X - the step of contacting the sorbent with a reducing agent, and - NO _X absorbent to regenerate the Reducing NO _X to N ₂ , Reductant injection rate is related to the NO _x conversion required at the average duty cycle rate.

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