MXPA06014739A - Reductant addition in exhaust system comprising nox. - 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.

Description

ADDITION OF REDUCER IN AN EXHAUST SYSTEM THAT COMPRISES AN ABSORBENT NOx SUBSTANCE DESCRIPTION OF THE INVENTION The present invention relates to an exhaust system for a poor combustion internal combustion engine comprising a NOx absorbing substance and in particular, with a method for controlling the addition of reducing agent in the exhaust system with the purpose of regenerating the NOx absorbing substance and reducing the proportion of NOx with respect to N2. An exhaust system for a poor combustion internal combustion engine such as a diesel engine or a poor combustion gasoline engine comprising an N0X absorbent substance is known, for example from EP 0560991. As used herein, A "NOx trap" is a catalyst comprising a NOx absorbing substance and a catalyst for oxidizing NO to N02. NOx traps are also known as "poor NOx traps" or "LNC." A typical NOx trap formulation includes a catalytic oxidation component such as Pt, an N0X-absorbing substance such as alkali metal compounds, eg potassium. or cesium, alkaline earth metal compounds such as barium or strontium, or compounds of rare earth metals, typically lanthanum or yttrium, and a reduction catalyst, for example rhodium, a mechanism commonly used for storage of N0X during poor motor operation. For these formulations, in a first stage, the NO reacts with oxygen at sites of active oxidation on Pt to form N02 The second stage involves the adsorption of N02 by the storage material in the form of an inorganic nitrate. Inorganic NOx storage is typically present as an oxide, it is understood that in the presence of air or an exhaust gas that It has C02 and H20, it can also be in the form of a carbonate or possibly hydroxide. When the engine runs intermittently under enriched conditions or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or N02. Under richer conditions, these N0X species are reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which is carried out on the reduction catalyst. An objective of an exhaust system comprising an N0X trap is to improve engine economy while meeting the relevant emission standards, for example Euro IV. Systems for controlling the addition of reductant for the purpose of regenerating an N0X trap and reducing the desorbed N0X are known, but tend to require very complicated control regimes involving multiple sensor inputs and processors to perform complex algorithms. As a result, such systems are very expensive. EP-B-0341832 (incorporated herein by reference) discloses a process for burning particulate matter (PM) in a diesel exhaust gas, which method comprises oxidizing NO in the exhaust gas to N02 in a catalyst, filtering the PM of the exhaust gas and burn the PM filtered in the N02 up to 400 ° C. This system is available from Johnson Matthey and is marketed as the CRTMR. We have investigated methods for regenerating NOx absorbers and have discovered that it is possible to satisfy a relevant emission standard, such as Euro IV, with an exhaust system comprising N0X absorbent without the need for complex equipment such as processors programmed by algorithms and a network of sensor input. Such a discovery has particular application in the market of the update. According to a first aspect of the invention, there is provided an exhaust system for a lean combustion internal combustion engine, comprising an N0X absorbent, a reducer injector placed upstream of the N0X absorbent and a medium, when in use, to control the addition of reducer, wherein, the reducer addition control means supplies reducer to the N0X absorbent at full vehicle speed in a duty cycle at a speed which is predetermined to relate to a desired conversion of N0X at an average work cycle speed of the vehicle. The invention of the first aspect has particular application in the market of the update for vehicles of a limited work cycle such as buses or trucks of rubbish. The idea is to determine what rate of reducer injection is required to reduce a selected amount of NOx, for example 90% in an N0X absorbent substance at an average duty cycle speed. For example, when the N0X absorbing substance is a component of a NOx trap, the system controller may be distributed, when in use, to generate a continuous synchronization and a hydrocarbon (HC) fuel injection amount, for example injection of 2 seconds every minute. The system controller can also be arranged to provide occasional relatively long, rich HC fuel pulses to ensure that the N0X trap has been substantially completely regenerated, followed by a more frequent sequence of shorter enrichment pulses to maintain the capacity of the N0X trap. storage of the N0X trap. The exact detail of the injection strategy depends on the vehicle and its work cycle. At speeds greater than the average duty cycle speed, there will be more N0X and a greater mass of air flow and thus the conversion of N0X will generally decrease, because there is insufficient reducer. However, because a higher speed is less likely, for example in buses in the city center, the system can meet the N02 emission standards over a complete driving cycle without increasing the fuel penalty.; Also when the vehicle speed falls below the average work cycle speed, HC can be issued but on average over a work cycle the system can satisfy the emission standard for HC. The ratio of the injection rate of HC to the average working cycle speed can be adapted to the particular application, for example buses in the city center of Manchester (United Kingdom) can be expected to find work cycles different from those found in the center of the city of London (United Kingdom). In an embodiment of the first aspect, an oxidation catalyst is placed between the reducer injector and the N0X absorbent substance to increase the temperature of the N0X trap for regeneration or to extract oxygen from the exhaust gas to ensure a rich exhaust gas for the generation of the absorbent substance of N0X. In a particular embodiment, the N0X trap and the systems for reducing supply described herein are placed downstream of the distribution described in EP-B-0341832 mentioned in the above. That is, a catalyst for oxidizing NO to N02 is followed by an optionally catalysed filter and then a reducing injector followed by N0X sorbent. In one embodiment, the N0X absorbent for use in the invention is a component of a N0X trap. Unless otherwise described, the catalysts for use in the present invention are coated on high surface area substrate monoliths made of metal or ceramic materials or silicon carbide, for example cordiarite. A common distribution is a honeycomb, of a monolithic flow through structure with 15.5-93.0 cells / cm2, for example 46.5-62.0 cells / cm2 (100-600 cells per square inch (cpsi) such as 300-400 cpsi). The internal combustion engine can be a diesel or poor combustion gasoline engine, such as a direct fuel injection engine. The diesel engine can be a light duty motor or a heavy duty motor, as defined by the relevant legislation. A method for reducing NOx in the exhaust gas of an internal combustion engine with poor vehicular combustion, according to one aspect of the invention, comprises absorbing N0X from the exhaust gas into an N0X absorbing substance, contacting the absorbent substance of N0X with a reducer to regenerate the N0X absorbent at full vehicle speed in a duty cycle and reduce N0X to N2, where the reducer injection speed is related to the desired conversion of NOx to the working cycle speed average. In order that the present invention be better understood, the embodiments thereof will now be described with reference to the accompanying drawings, in which: Figure 1 shows a schematic system according to a first aspect of the invention; Figure 2 is a schematic graph showing the amount of fuel versus time shown by the fuel injection strategy for use in the system of Figure 1; Figure 3 is a schematic of a working mode of the invention; Figure 4 is a graph showing the upstream air / fuel ratio (AFR) as a function of the road speed in the embodiment of Figure 3; Figure 5 is a graph showing the measurements of N0X in a free condition for the embodiment of Figure 3; Figure 6 is a graph showing the corresponding system temperatures in the free condition for the trace shown in Figure 5; Figure 7 is a graph showing the N0X measurements at 64 km / h (40 mph) for the embodiment of Figure 3; Figure 8 is a graph showing the temperature measurements corresponding to 64 km / h (40 mph) for the trace shown in Figure 7; and Figure 9 is a graph showing the conversion of NOx as a function of the road speed for the system of Figure 3. In the system 50 shown in Figure 1, the number 52 is a conditional system controller.

(CSC), number 54 is a master switch, number 56 is an alternator, number 58 is a blocking capacitor, number 60 is a thermocouple, number 62 is an injection controller (ICU), number 64 It is a fuel pump, the number 66 is a valve, the number 68 is a fuel injector and the number 70 is a positive energy line. The CSC 52 is a switch that provides power to the ICU 62 if the master power switch 54 is turned on, the motor is running as determined by an AC rectifier from the alternator 56 present in a DC block capacitor 58 and the output of the a thermocouple 60 suitably positioned to detect the exhaust system that is above a minimum predetermined temperature for reduction of N0X in a suitable NOx trap. The master switch 54 does not need to be connected in the on position. The CSC 52 is designed to generate continuous synchronization and HC injection quantity when all three characteristics (master switch position, alternator rectifier detection and exhaust gas temperature above a predetermined minimum) coincide. When the CSC 52 is turned on, power is supplied to the injection pump 64 and the ICU 62 operating a solenoid valve 66 produces a series of pulses to enrich the exhaust gas before it passes over an oxidation catalyst upstream of the reactor. the absorbent components of N0X. Typically, the injection controller will provide occasional relatively long very rich pulses to ensure that the NOx trap is substantially completely empty and this is followed by a more frequent sequence of shorter enrichment pulses, for example injection at two seconds, each minute to maintain the storage capacity of the N0X trap (see Figure 2). This fuel injection rate is related to a selected conversion of NOx, for example 90% at the average work cycle speed. The exact detail of the injection strategy depends on the vehicle and its work cycle. Although very generally the systems using NOx traps described herein have been developed to provide simple control mechanisms for predicting when the NOx trap regeneration should take place, with particular application to the update, many vehicles already include a range of sensors to enter data in the ECU to control other aspects of the operation of the vehicle. By appropriate reprogramming of the ECU, it is possible to adapt one or more of the existing sensor inputs for the purpose of predicting the remaining NOx trap capacity. These include, but are not limited to, the elapsed time, predetermined or predicted, from the ignition or the previous regeneration, upon detecting the state of a suitable clock means; the air flow over the T C or vacuum of the manifold; ignition timing; the speed of the engine; the position of the choke, the composition of the exhaust gas, for example using a sensor?, preferably a sensor? linear; the amount of fuel injected into the engine; when the vehicle includes an exhaust gas recirculation circuit (EGR), the position of the EGR valve and therefore the detected amount of EGR; the engine cooling temperature and when the exhaust system includes a N0X sensor, the amount of N0X detected upstream or downstream of the NOx trap. When the clock mode is used, the predicted time can be adjusted later in response to input data. The following specific example is provided by way of illustration only.

EXAMPLE The exhaust system (10) (shown in Figure 3) of a single bus with a turbocharged 6-liter engine and comprising a turbo engine (12), of the type approved by the emission limits. of the European Stage 1, and modified to incorporate a three-way divider (14) to divert the exhaust gas to one of the three parallel columns (16), the exhaust gas flow in each column is of the same velocity of flow. Each column (16) is constituted by a chamber (18) containing an oxidation catalyst (20) followed by a NOx trap (22). The gas flows are then combined downstream of the N0X traps and the total exhaust gas flow is passed through a "cleaning" oxidation catalyst (24) to remove any unburned hydrocarbon (HC ) that leaves the N0X traps before the exhaust gas passes directly into the atmosphere. A fuel injector (26) comprising a fuel solenoid (28) is placed in front of each oxidation catalyst (20), a N0X sensor (29) in front of the exhaust divider (14), sensors (30) of the NOx / fuel air ratio combined behind the N0X traps and thermocouples (TI, T2, T3, T4) that measure the temperatures in the front and behind the oxidation catalysts (20) and in the output of the reactors The oxidation catalyst (20) and the NOx traps (22) are each covered in ceramic through-flow monoliths of 62 cells / cm 2) 400 cells / inch2) and with a wall thickness of 0.15 mm (0.06 inches). The oxidation catalysts (20) have a diameter of 144 mm (5.66 inches) x 76 mm (3 inches) and a volume of 1.24 liters (75.5 inches3), the traps of N0X (22) are the same diameter but 152 mm (6 inches) long and a "cleaning" catalyst (24) of 267 mm (10.5 inches) in diameter x 76 mm (3 inches) in length and a volume of 4.26 liters (260 inches3). The experiments described here are carried out using a split column only. The vehicle is operated using a diesel fuel containing 50 ppm sulfur and operating at free or idle stable speeds, and at 16 km / h (10 mph), 32 km / h (20 mph), 48 km / h (30 mph) and 64 km / h (40 mph) for periods of time; the fuel is injected at each of these points and the proportion of air and fuel during injection is determined, as shown in Figure 4. The combination of time and duration (2 injection seconds, one per minute per column) is empirically selects data that provides the best combination of exhaust gas temperatures (to maintain the NOx trap within an active temperature range) and N0X conversion. Simultaneously, the N0X emissions before and after the system along with the temperature profiles were also measured. Figure 5 shows the emissions of NOx (in ppm) of the engine and after the NOx trap for the free condition together with the proportion of air fuel measured after the NOx trap. Figure 6 shows the temperature traces for the same period. From figure 5 it is observed that when fuel is injected at the beginning of the free period, the proportion of air and fuel descends from a poor to a rich condition, as expected from the predictions in figure 3 and, after the initial start of N0X, a good conversion of N0X is observed. Over time, the proportion of air and fuel remains poor during the injection event but a good conversion of N0X is still maintained. The exotherm (T2) generated on the oxidation catalyst helps maintain the temperature of the NOx trap within its operating range of 220-550 ° C. An exotherm (T3) is also registered through the N0X trap, part of which is caused by combustion of the unreacted gaseous reductant of the oxidation catalyst. We interpret this result which means that part of this exotherm is from the combustion of unburned fuel droplets that react on the surface of the NOx trap as time increases in this free motor condition. This is because the intake temperature of the system drops so that it is insufficient to vaporize the incoming fuel and the peaks in the air / fuel ratio measured by the rear sensor become less pronounced and more rounded, suggesting a sequence of deposition, vaporization and then subsequent oxidation of the fuel droplets. The local wealth caused by this event also serves to maintain the operating efficiency of the observed N0X trap. The results of the experiment with the bus that is maintained at a steady speed of 64 km / h (40 mph) are shown in figures 7 and 8. Here, the exhaust flow rate is much higher but the same rate is used. injection flow that when free and the exhaust is expected to remain free during the injection periods (figure 3). However, in addition to starting peaks when the fuel is injected for the first time, it is reduced N0X during the remaining operating time, although not as efficiently as when it is free. Sometimes the exotherm (T3) on (T2) is lower than when it is free, but due to the thermal capacity of the increased flow rate of the exhaust gases, it is very significant. Therefore, an exothermic reaction is still carried out and again we consider that this is because some unburned fuel droplets are transported through the oxidation catalyst and burned in the N0X trap. The persistence of fuel droplets, despite the higher intake temperature of the oxidation catalyst, is expected to occur because the higher exhaust flow rate is likely to entrain the droplets through the oxidation catalyst as shown by the significant exotherm measured through the N0X trap and the regeneration of the trap observed in apparently poor conditions. Figure 9 shows the trend in average NOx conversion efficiency calculated, as a speed function, for the system. Figure 3 indicates rich exhaust gas conditions that do not occur above about 10 km / h (6 mph) but good NOx conversions are not obtained under poor conditions over a wider speed range. This is especially relevant in the range from free to 48 km / h (30 mph) which is the most common operating range for a city bus.

Claims (15)

1. An exhaust system for a poor combustion internal combustion engine, in a vehicle, comprising an N0X absorbing substance, a reducer injector placed upstream of the N0X absorbent substance and a medium which, when in use, controls the addition of reducing agent, wherein the reducing agent addition control means supplies the NOx absorbent substance at full vehicle speed in a duty cycle at a rate which is predetermined to be related to the desired conversion of NOx in the speed of work cycle by means of the vehicle.

2. Exhaust system as described in claim 1, wherein the NOx absorbing substance is selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds and mixtures of any two or more thereof .

3. Exhaust system as described in claim 2, wherein the alkaline earth metal or each of them is selected from the group consisting of barium, magnesium, strontium and calcium, the alkali metal or each of them are selected from the group which consists of potassium and cesium and the rare earth metal or each of them is selected from the group consisting of cerium, yttrium, lanthanum and praseodymium.

4. Exhaust system as described in claim 2 or 3, wherein the alkaline earth metal compound or each of them, the alkali metal compound or each of them or the rare earth metal compound or each of they are supported on a support material.

5. Exhaust system as described in claim 4, wherein the NOx-absorbing substance comprises the support.

6. Exhaust system as described in any preceding claim, wherein the NOx absorbent is a component of a N0X trap comprising a catalyst for oxidizing NO, optionally platinum or palladium.

7. Exhaust system as described in claim 6, wherein the NOx trap comprises a N0x reduction catalyst, optionally rhodium.

8. Exhaust system as described in any preceding claim, comprising an oxidation catalyst placed between the reducer injector and the NOx absorbing substance.

9. Exhaust system as described in any preceding claim, comprising a catalyst for oxidizing NO to N02 placed upstream of the reducing injector.

10. Exhaust system as described in claim 9, comprising an optionally catalyzed particulate filter positioned between the oxidation catalyst and the reducer injector.

11. Exhaust system as described in claim 6 or 7, wherein the NOx trap comprises a particulate filter. An exhaust system as described in any preceding claim, comprising a control means which, when in use, intermittently enriches the exhaust gas composition to regenerate the NOx absorbing substance. An exhaust system as described in claim 12, when dependent on claim 6 or 7, wherein the control means, when in use, supplies reducer to the NOx trap only when the catalyst is active for reducing the NOx. 14. Diesel engine comprising an exhaust system as described in any preceding claim, optionally a light duty diesel engine. 15. Method for reducing NOx in an exhaust gas of a lean combustion internal combustion engine in a vehicle, which method comprises absorbing NOx from the exhaust gas into a NOx absorbing substance, contacting the NOx absorbing substance with a reducer for regenerating the NOx absorbing substance, at all vehicle speeds in a duty cycle, and reducing NOx to N02, wherein the rate of reducer injection is related to a desired NOx conversion at the cycle speed of average work.