Reductant addition in exhaust system comprising NOx-absorbent

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

The present invention relates to an exhaust system for a lean-burn internal combustion engine comprising a NO_x-absorbent and, in particular, to a method of controlling reductant addition into the exhaust system for the purpose of regenerating the NO_x-absorbent and reducing NO to N₂.

An exhaust system for a lean-burn internal combustion engine such as a diesel engine or a lean-burn gasoline engine comprising a NO_x-absorbent is known from, for example, EP 0560991.

As used herein, a “NO_x-trap” is a catalyst comprising a NO_x-absorbent and a catalyst for oxidising NO to NO₂. NO_x-traps are also known as “lean NO_x traps” or “LNC”.

A typical NO_x-trap formulation includes a catalytic oxidation component, such as Pt, a NO_x-absorbent, such as compounds of alkali metals e.g. potassium and/or caesium; compounds of alkaline earth metals, such as barium or strontium; or compounds of rare-earth metals, typically lanthanum and/or yttrium; and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NO_x-storage during lean engine operation for this formulation is that, in a first step, the NO reacts with oxygen on active oxidation sites on the Pt to form NO₂. The second step involves adsorption of the NO₂ by the storage material in the form of an inorganic nitrate.

Whilst the inorganic NO_x-storage component is typically present as an oxide, it is understood that in the presence of air or exhaust gas containing CO₂ and H₂O it may also be in the form of the carbonate or possibly the hydroxide.

When the engine runs intermittently under enriched conditions or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO₂. Under richer conditions, these NO_x species are reduced by carbon monoxide, hydrogen and hydrocarbons to N₂, which can take place over the reduction catalyst.

An object of an exhaust system comprising a NO_x-trap is to improve the economy of the engine whilst meeting the relevant emissions standard, e.g. Euro N.

Systems to control reductant addition for the purpose of regenerating a NO_x-trap and reducing desorbed NO are known, but tend to require very complicated control regimes involving multiple sensor inputs and processors to run complex algorithms. As a result, such systems are very expensive.

EP-B-0341832 (incorporated herein by reference) describes a process for combusting particulate matter (PM) in diesel exhaust gas, which method comprising oxidising NO in the exhaust gas to NO₂ on a catalyst, filtering the PM from the exhaust gas and combusting the filtered PM in the NO₂ at up to 400° C. Such a system is available from Johnson Matthey and is marketed as the CRT®.

We have investigated methods of regenerating NO_x-absorbents and we have discovered that it is possible to meet a relevant emission standard, such as Euro IV, with an exhaust system comprising a NO_x-absorbent without the need for complex equipment such as algorithm-programmed processors and a network of sensor inputs. Such a discovery has particular application to the retrofit market.

According to a first aspect of the invention, there is provided an exhaust system for a vehicular lean-burn internal combustion engine comprising a NO_x-absorbent, a reductant injector disposed upstream of the NO_x-absorbent and means, when in use, for controlling reductant addition, wherein the reductant addition control means supplies reductant to the NO_x-absorbent at all vehicle speeds in a duty cycle at a rate which is predetermined to correlate with a desired NO_x conversion at the average duty cycle speed of the vehicle.

The invention of the first aspect has particular application to the retrofit market for vehicles of a limited duty cycle such as buses or refuse trucks. The idea is to determine what rate of reductant injection is required to reduce a chosen quantity of NO_x, e.g. 90%, in a NO_x-absorbent at the average duty cycle speed. For example, when the NO_x-absorbent is a component of a NO_x-trap, the system controller can be arranged, when in use, to generate a continuous tempo and quantity of hydrocarbon (HC) fuel injection e.g. injection at 2 seconds every minute. The system controller can also be arranged to provide occasional relatively long rich HC fuel pulses to ensure that the NO_x-trap is substantially completely regenerated, followed by the more frequent sequence of shorter enrichment pulses to maintain the storing capability of the NO_x-trap. The exact detail of the injection strategy depends on the vehicle and its duty cycle.

At speeds higher than the average duty cycle speed, there would be more NO and a greater mass airflow and so NO_x conversion overall would fall off, because there would be insufficient reductant. However, because higher speed would be less likely e.g. in city centre buses, the system can meet NO emission standards over an entire drive cycle without increasing fuel penalty; equally where the vehicle speed drops below the average duty cycle speed, HC can be emitted, but on average over a duty cycle the system can meet the emission standard for HC. The correlation of the rate of HC injection to average duty cycle speed can be tailored to the particular application, e.g. buses in Manchester (UK) city centre would be expected to encounter different duty cycles to those in London (UK) city centre.

In one embodiment of the first aspect, an oxidation catalyst is disposed between the reductant injector and the NO_x-absorbent for increasing the temperature of the NO_x-trap for regeneration and/or to remove oxygen from the exhaust gas to ensure a rich exhaust gas for regeneration of the NO_x-absorbent.

In a particular arrangement, the NO₁-trap and systems for delivering reductant described herein are disposed downstream of the arrangement described in EP-B-0341832, mentioned hereinabove. That is, a catalyst for oxidising NO to NO₂ is followed by an optionally catalysed filter then a reductant injector followed by the NO_x-absorbent.

In one embodiment, the NO_x-absorbent for use in the invention is a component of a NO_x-trap.

Unless otherwise described, the catalysts for use in the present invention are coated on high surface area substrate monoliths made from metal or ceramic or silicon carbide, e.g. cordierite, materials. A common arrangement is a honeycomb, flow-through monolith structure of from 100-600 cells per square inch (cpsi) such as 300-400 cpsi (15.5-93.0 cells cm², e.g. 46.5-62.0 cells cm²).

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

A method of reducing NO_x in the exhaust gas of a vehicular lean-burn internal combustion engine according to a second aspect of the invention comprises absorbing NO_x from the exhaust gas in a NO_x absorbent, contacting the NO_x absorbent with a reductant to regenerate the NO_x-absorbent at all vehicle speeds in a duty cycle, and reducing NO_x to N₂, wherein a rate of reductant injection correlates with a desired NO_x conversion at the average duty cycle speed.

In order that the present invention may be more fully understood, embodiments thereof will now be described with reference to the accompanying drawings, in which:

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

FIG. 2 is a schematic graph plotting quantity of fuel against time showing a fuel injection strategy for use in the system of FIG. 1;

FIG. 3 is a schematic of a working embodiment of the invention;

FIG. 4 is a graph showing the upstream Air/Fuel Ratio (AFR) as a function of road speed in the embodiment of FIG. 3;

FIG. 5 is a graph showing NO_x measurements at the idle condition for the embodiment of FIG. 3;

FIG. 6 is a graph showing the corresponding system temperatures at the idle condition for the trace shown in FIG. 5;

FIG. 7 is a graph showing NO_x measurements at 40 mph for the embodiment of FIG. 3;

FIG. 8 is a graph showing the corresponding temperature measurements at 40 mph for the trace shown in FIG. 7; and

FIG. 9 is a graph showing the NO_x conversion as a function of road speed for the system of FIG. 3.

In the system 50 depicted in FIG. 1, 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 a switch providing power to the ICU 62 if the master power switch 54 is on, the engine is running as determined by an AC ripple from the alternator 56 present after a DC blocking capacitor 58 and the output of a suitably placed thermocouple 60 to detect the exhaust system is above a minimum pre-determined temperature for reduction of NO_x on a suitable NO_x-trap. The master switch 54 need not be connected to the key-on position.

The CSC 52 is designed to generate a continuous tempo and quantity of HC injection when all three features (master switch position, detection of alternator ripple and exhaust gas temperature above a pre-determined minimum) coincide. When the CSC 52 is on, power is supplied to the injection pump 64 and the ICU 62 that operates a solenoid valve 66 to produce a series of pulses to enrich the exhaust gas before it passes over an oxidation catalyst upstream of the NO_x absorbing components. Typically the injection controller will provide occasional relatively very long rich pulses to ensure that the NO_x-trap is substantially completely empty and this is followed by a more frequent sequence of shorter enrichment pulses, e.g. injection at 2 seconds every minute, to maintain the storing capability of the NO_x-trap (see FIG. 2).

This fuel injection rate is correlated to a chosen NO_x conversion e.g. 90% at the average duty cycle speed. The exact detail of the injection strategy depends on the vehicle and its duty cycle.

Whilst, very generally, the systems employing NO_x-traps described herein have been developed to provide simple control mechanisms to predict when NO_x-trap regeneration should be done, with particular application to retrofit, many vehicles already include a range of sensors to input data to the ECU for controlling other aspects of vehicular operation. By suitable re-programming of the ECU it is possible to adopt one or more of such existing sensor inputs for the purposes of predicting remaining NO_x-trap capacity. These include, but are not limited to, predetermined or predicted time elapsed from key-on or previous regeneration, by sensing the status of a suitable clock means; airflow over the TWC or manifold vacuum; ignition timing, engine speed; throttle position; exhaust gas redox composition, for example using a lambda sensor, preferably a linear lambda sensor, quantity of fuel injected in the engine; where the vehicle includes an exhaust gas recirculation (EGR) circuit, the position of the EGR valve and thereby the detected amount of EGR; engine coolant temperature; and where the exhaust system includes a NO_x sensor, the amount of NO_x detected upstream and/or downstream of the NO_x-trap. Where the clock embodiment is used, the predicted time can be subsequently adjusted in response to data input.

The following specific Example is provided by way of illustration only.

EXAMPLE

The exhaust system (10) (shown in FIG. 3) of a single deck bus fitted with a 6 litre turbocharged engine and comprising engine turbo (12), type approved to European Stage 1 emission limits, was modified to incorporate a three-way splitter (14) for diverting the exhaust gas into one of three parallel legs (16), the exhaust gas flow in each leg being of equal velocity flow. Each leg (16) comprised a chamber (18) containing an oxidation catalyst (20) followed by a NO_x-trap (22). The gas flows were then combined downstream of the NO_x-traps and the total exhaust gas flow was passed through a “clean up” oxidation catalyst (24) to remove any unburned hydrocarbons (HC) exiting the NO_x-traps before the exhaust gas was passed directly to the atmosphere. A fuel injector (26) comprising a fuel solenoid (28) was sited in front of each oxidation catalyst (20), a NO_x sensor (29) in front of the exhaust splitter (14), combined NO_x/air fuel ratio sensors (30) behind the NO_x-traps and thermocouples (T1, T2, T3, T4) measuring temperatures in front of and behind the oxidation catalysts (20) and at the exit of the reactors. The oxidation catalysts (20) and the NO traps (22) were each coated on ceramic flow-through monoliths at 400 cells in (62 cells cm⁻²) and 0.06 in (0.15 mm) wall thickness. The oxidation catalysts (20) were 5.66 in (144 mm) diameter×3 in (76 mm) and volume 75.5 in³ (1.24 litre), the NO_x-traps (22) were the same diameter but 6 in (152 mm) long and the “clean up” catalyst (24) 10.5 in (267 mm) diameter×3 in (76 mm) long and volume 260 in³ (4.26 litres).

The experiments described here were conducted using one leg of the split exhaust only. The vehicle was operated using diesel fuel containing 50 ppm sulphur and run at steady speeds of idle, 10, 20, 30 and 40 mph for periods of time; fuel was injected at each of these points and the air fuel ratio during injection determined as shown in FIG. 4. The combination of time and duration (2 seconds injection, one per minute per leg) was selected empirically as it gave the best combination of exhaust gas temperatures (to maintain the NO_x-trap within an active temperature window) and NO_x conversion. Simultaneously the NO emissions pre- and post- the system together with the temperature profiles were measured.

FIG. 5 shows the NO emissions (ppm) from the engine and after the NO_x-trap for the idle condition together with the air fuel ratio measured after the NO_x-trap. FIG. 6 shows the temperature traces for the same period. From FIG. 5 it is seen that when fuel is injected at the start of the idle period, the air fuel ratio drops from lean to rich as expected from the predictions in FIG. 3 and, after the initial NO_x breakthrough, good NO_x conversion is seen. With time, the air fuel ratio remains lean throughout the injection event but good NO_x conversion is still maintained. The exotherm (T2) generated over the oxidation catalyst helps maintain the temperature of the NO_x-trap within its operating window of 220-550° C. An exotherm (T3) is also registered across the NO_x-trap, some of which is caused by combustion of unreacted gaseous reductant from the oxidation catalyst. We interpret this result to mean that some of this exotherm is from the combustion of unburned fuel droplets reacting on the surface of the NO_x-trap as time increases at this engine idle condition. This is because the system inlet temperature falls so as to be insufficient to vaporise the incoming fuel and the rear sensor measured air/fuel ratio spikes become less pronounced and more rounded, suggesting a sequence of the deposition, vaporisation and then the subsequent oxidation of the fuel droplets. The local richness caused by this event also serves to maintain the observed NO_x-trap operation efficiency.

The results of the experiment with the bus held at a steady speed of 40 mph are shown in FIGS. 7 and 8. Here the exhaust flow rate was much higher but the same injection flow rate was used as at idle and the exhaust was expected to remain lean throughout the injection periods (FIG. 3). However, apart from the breakthrough spikes when the fuel is first injected, NO_x is reduced over the remaining operating time, although not as efficiently as at idle. The exotherm (T3) over (T2) was sometimes lower than at idle, but because of the heat capacity of the increased flow rate of exhaust gases, it is very significant. Therefore an exothermic reaction is still taking place and again we believe that this is because some unburned fuel droplets are being carried through the oxidation catalyst and being combusted on the NO_x-trap. The persistence of fuel droplets, despite the higher inlet temperature of the oxidation catalyst, is expected to occur because the greater exhaust flow rate that is likely to carry the droplets through the oxidation catalyst as is shown by the significant exotherm measured across the NO_x-trap and the trap regeneration observed in apparently lean conditions.

FIG. 9 presents the trend in calculated average NO_x conversion efficiency, as a function of speed, for the system. FIG. 3 indicates rich exhaust gas conditions do not occur above about 6 mph but good NO_x conversions were obtained under lean conditions across a wider speed range. This is especially relevant in the range from idle to 30 mph which is the most common operating range for an urban city bus.

Claims

1. An exhaust system for a vehicular lean-burn internal combustion engine comprising: a NOx-absorbent; a reductant injector disposed upstream of the NOx absorbent; and means, 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 NO conversion at the average duty cycle speed of the vehicle.

2. An exhaust system according to claim 1, wherein the NOx-absorbent 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. An exhaust system according to claim 2, wherein the alkaline earth metal is selected from the group consisting of barium, magnesium, strontium and calcium.

4. An exhaust system according to claim 2, wherein the alkali metal is selected from the group consisting of potassium and caesium.

5. An exhaust system according to claim 2, wherein the rare earth metal is selected from the group consisting of cerium, yttrium, lanthanum and praseodymium.

6. An exhaust system according to claim 2, wherein the alkaline earth metal compound, the alkali metal compound or the rare earth metal compound is supported on a support material.

7. An exhaust system according to claim 6, wherein the support is selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixtures or a composite oxide of any two or more thereof.

8. An exhaust system according to claim 6, wherein the NOx-absorbent comprises the support.

9. An exhaust system according to claim 1, wherein the NOx-absorbent is a component of a NOx-trap comprising a catalyst for oxidising NO.

10. An exhaust system according to claim 9, wherein the NO oxidation catalyst comprises a platinum group metal.

11. An exhaust system according to claim 10, wherein the NOx-trap comprises a NO reduction catalyst.

12. An exhaust system according to claim 1, comprising an oxidation catalyst disposed between the reductant injector and the NOx-absorbent.

13. An exhaust system according to claim 1, comprising a catalyst for oxidising NO to NO2 disposed upstream of the reductant injector.

14. An exhaust system according to claim 13, wherein the NO oxidation catalyst is platinum on an alumina support.

15. An exhaust system according to claim 13, comprising an optionally catalysed particulate filter disposed between the oxidation catalyst and the reductant injector.

16. An exhaust system according to claim 9, wherein the NOx-trap comprises a particulate filter.

17. An exhaust system according to claim 1, comprising control means when in use, intermittently to enrich the exhaust gas composition for regenerating the NOx-absorbent.

18. An exhaust system according to claim 24, wherein the control means, when in use, supplies reductant to the NOx-trap only when the catalyst is active for NOx reduction.

19. A diesel engine comprising an exhaust system according to claim 1.

20. A light-duty diesel engine according to claim 19.

21. A method of reducing NO in the exhaust gas of a vehicular lean-burn internal combustion engine, which method comprising absorbing NOx from the exhaust gas in a NOx-absorbent, contacting the NOx-absorbent with a reductant to regenerate the NOx-absorbent, at all vehicle speeds in a duty cycle, and reducing NOx to N2, wherein a rate of reductant injection correlates with a desired NOx conversion at the average duty cycle speed.

22. An exhaust system according to claim 10, wherein the platinum group metal is selected from the group consisting of platinum, palladium and a combination of platinum and palladium.

23. An exhaust system according to claim 11, wherein the NOx reduction catalyst comprises rhodium.

24. An exhaust system according to claim 9, comprising control means when in use, to intermittently enrich the exhaust gas composition for regenerating the NOx-absorbent.