EMISSIONS

BACKGROUND a. Field of the Invention

The present invention relates to an exhaust system for reducing particulate matter and NO2 levels in exhaust gases from diesel engines, and to a method for using the system. b. Related Art

Historically, NOx and particulate emissions have been of particular concern when developing diesel engines and aftertreatments. More recently, legislation and local air quality improvement schemes are now also targeting NO2 reduction.

SCRT is a technology that can control NOx, particulate, CO and HC (hydrocarbon) emissions. However it is difficult also to control NO2 emissions because significant quantities of NO2 are generated within the system in order to facilitate the passive regeneration of the CRT.

SCR has the potential to control NOx and NO2, but particulate reduction might be difficult to achieve downstream of this because there is limited energy available in the exhaust gas to enable particulate burn (regeneration) over the filter. It is known to provide a system in which the DPF regenerates at low temperatures. The system is connected downstream of a turbocharger or engine and the exhaust gas is split into a main pipe and a bypass. The bypass has a heating element and means for injecting diesel fuel into the exhaust gas stream. A first DOC provides heating through the exothermic heat from catalytic fuel burning. Downstream of the first DOC, the two pipes join together. Gases from the combined pipes are fed to a second DOC and the DPF. The second DOC also serves to burn hydrocarbons, to increase the temperature to a level at which the filter can regenerate. Sensors are used to measure temperatures at various points, and relative gas flows through the pipes are adjusted, together with pre-heating using the electric heater, to ensure that gases reaching the second DOC are above a threshold temperature required for efficient hydrocarbon conversion. The system meters flow through the first DOC between 0% to 100% in order to achieve the ideal flow rate for maximum performance. The use of supplementary heating and the flow rate adjustment system has significant cost and development disadvantages. Moreover, because exhaust gases flow through the DOC, which generates NO2 it is necessary to use a special oxidation catalyst which is selected for low NO2 formation but which is less effective than others for catalysing HC conversion.

US 2010/0037607 describes a system with parallel exhaust gas flows. The system has a NO oxidation catalyst in one stream and a HC oxidation catalyst in the other. The HC catalyst acts as a heater when fuel is injected upstream. This allows selective heating of the DPF without heating of the NO catalyst. This is said to optimise the catalytic activity of the NO oxidation catalyst

US 2003/0089104 describes an exhaust system which has an exhaust line with an oxidation catalyser and a catalytically-coated DPF. During a DPF regeneration phase, post-injection of fuel takes place to provide unburned HC to the oxidation catalyst which oxidises the HC. A bypass circuit and valve arrangement permits exhaust gas with unburned fuel to bypass the oxidation catalyst. When regeneration of the filter is triggered, a proportion of the exhaust gases with injected HC is diverted straight to the DPF without passing through the oxidiser. This arrangement is said to increase the combustion rate of particles trapped in the DPF

Other prior art systems are described in the following documents: JP2010043577, US2006/1 17742, US2010/199634, JP2010209783, EP2014348, US2008/155968, EP2309103, JP2006233947, US201 1/192143, US2009/277159, US201 1/146233, US2010/242438, US2009/178393, WO07/136148, US2009/260346, US2005/031514.

An industry challenge is to develop a DPF system that can regenerate at low temperature, with low tailpipe NO2 and, if necessary, in combination with other aftertreatment systems, such as those for reducing NOx.

SUMMARY OF THE INVENTION

Aspects of the invention are specified in the independent claims. Preferred features are specified in the dependent claims.

The invention provides a system and method for reducing tailpipe NO2 emissions while providing for DPF regeneration.

DEFINITIONS

When used herein, the following definitions define the stated term: "CO" is carbon monoxide.

A "CRT" is a Continuously Regenerating Trap system for the removal of PM from the exhaust gas stream using a wall-flow filter. The system operates passively and is self cleaning. It achieves this by using a catalyst upstream of the filter to produce exhaust gas conditions that enable the carbon fraction of the PM to be burnt off at typical diesel exhaust gas temperatures.

A "DOC" is a Diesel Oxidation Catalyst, which is used to promote burning of diesel hydrocarbons in an exhaust gas flow.

A "DPF" is a Diesel Particulate Filter, which is used to remove PM from exhaust gases.

"HC" means hydrocarbon.

"High HC activity" means a catalyst which when fresh will cause or promote combustion of at least 70% (preferably at least 80%) of HC in an exhaust gas at 300°C.

"High NO2 activity" means a catalyst which when fresh will cause or promote conversion of at least 60% (preferably at least 70%) of NO to NO2 in an exhaust gas at 300°C.

"NO" is nitric oxide.

"NO2" is nitrogen dioxide, a major contributor to photochemical smog and acid rain. It can be removed from an exhaust gas stream by SCR.

"NOx" is a generic term for all oxides of nitrogen.

"PM" is Particulate Matter, the solid content of exhaust gases, primarily soot (carbon) and ash.

"SCR" is Selective Catalytic Reduction, a process for removing NOx by reducing with a reductant such as ammonia over a catalyst.

"SCRT"® is a combination of SCR and CRT in a single exhaust emissions reduction system. It is capable of removing NOx, PM, HC and CO. SCRT is a registered trade mark of Johnson Matthey PLC. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further illustrated, by way of example only, with reference to the following drawings, in which:

Figure 1 is a schematic representation of an exhaust system in accordance with an aspect of the present invention; Figure 2 is a schematic representation corresponding to Figure 1 , in bypass mode;

Figure 3 is a graph illustrating decrease in NO2 output relative to the engine output level in bypass mode;

Figures 4 and 5 correspond to Figures 2 and 3 for the exhaust system in light-off mode;

Figures 6 and 7 correspond to Figures 2 and 3 for the exhaust system in regeneration mode;

Figure 8 is a graph showing net tailpipe NO2 reduction over an eight hour operation; and Figures 9 to 13 illustrate alternative embodiments of exhaust systems in accordance with the present invention.

DETAILED DESCRIPTION An exhaust system 2 comprises an inlet 6 for receiving exhaust gases from a diesel engine (not shown) and a first conduit 4 connected to a DPF module 10 via an outlet 8. The first conduit 4 is capable of providing a first fluid connection between the inlet 6 and the DPF 10. A second conduit 12 is capable of providing a second fluid connection between the inlet 6 and the DPF 10. The second conduit 12 houses a DOC 14. The DOC catalyst has high HC activity and high NO ₂ activity; when fresh, the DOC converts over 80% HC and over 70% NO ₂ at 300°C. Suitable DOCs with high HC activity and high NO ₂ activity will be well known to those skilled in the art of exhaust emission control.

A fuel injector 16 is arranged to inject fuel into the exhaust gas stream upstream of the DOC. A valve mechanism 18 is adjustable for selectively directing exhaust gases from the inlet 6 to the DPF 10 through the first conduit 4 or the second conduit 12. The fuel injector 16 is between the valve mechanism 18 and the DOC 14. In this example, the fuel injector 16 is upstream of the valve mechanism 18 but could alternatively be in a separate stream within the second conduit 12. Gases which pass through the second conduit 12 encounter the DOC 14 before reaching the DPF 10. Gases which pass through the first conduit 4 reach the DPF 10 without encountering a diesel oxidation catalyst. Filtered exhaust gases exit the DPF 10 via a tailpipe 28.

In this embodiment, the exhaust system 2 is illustrated in combination with an upstream SCR unit 20. The SCR 20 has an injector 22 for introducing a reductant such as ammonia, and an optional inlet module oxidation catalyst 24 and an optional outlet module 26 with slip catalyst. It will be appreciated that the invention is not limited to use with an SCR. It may be used as a standalone active DPF system or may be combined with other aftertreatment or exhaust technologies.

Referring to Figure 2, in bypass (soot filter) operating mode the valve mechanism 18 is open, and exhaust gases entering via the inlet 6 substantially bypass the DOC 14 and reach the DPF 10 without encountering an oxidation catalyst. This limits or eliminates NO2 production over the DOC. A reduction in engine-out NO2 occurs over the DPF 10 by passive reduction of NO2 over accumulated particulate matter. The decrease in NO2 relative to the engine-out level is illustrated in Figure 3; both soot and NO2 levels are reduced by the DPF. Referring now to Figure 4, in light-off mode, the valve mechanism 18 closes off the path through the first conduit 4, diverting exhaust gases through the second conduit 12 and DOC 14. Combustible components of the hot exhaust gases, notably CO and HC, are catalytically oxidised over the DOC 14. We have found that it is during this brief light-off period that the most significant quantity of NO2 is produced, as shown in Figure 5.

When the DOC achieves light-off, the system switches to regeneration mode, and fuel is injected via the fuel injector 16 to oxidise over the DOC (Figure 6). The resulting exotherm increases the temperature of the exhaust gas sufficiently to combust the organic fraction of PM (predominantly carbon/adsorbed HCs). This enables burning of the PM accumulated within the DPF and regeneration of the DPF filtering capacity. Combustion of the organic fraction of PM on the DPF could optionally be catalysed by use of a fuel-borne catalyst so that the temperature required to initiate PM combustion is lowered, requiring less energy to start regeneration. In this embodiment, the fuel injector 16 is arranged and adapted to direct most or substantially all of the injected fuel directly into the second conduit 12. By 'directly' we mean without substantial axial travel in the first conduit 4. As illustrated, the fuel injector 16 is positioned directly opposite the entrance to the second conduit 12 where it branches from the first conduit 4. The injected fuel is directed straight across the first conduit 4 into the second conduit 12. The fuel injector 16 could alternatively be located in a wall of the second conduit 12 upstream of the DOC 14.

As shown in Figure 7, NO2 production is suppressed when fuel injection takes place. The catalyst has preferred selectivity towards HC oxidation rather than NO oxidation. Tailpipe NO2 output is relatively high during the light-off mode, as illustrated in Figure 5, but this mode is operated for only a small proportion of the duty cycle. This offsets some, but not all, of the NO2 reduction that occurs during the rest of the duty cycle in bypass (soot filter) mode. Results for an eight hour shift at steady state operation are shown in Figure 8. Cumulative tailpipe NO2 is substantially reduced with respect to cumulative engine-out NO2. The invention provides a regenerative DPF system with low tailpipe NO2. The system may regenerate at low temperatures, particularly if a fuel-borne catalyst is used, and it may be used in combination with other aftertreatment systems, such as those for reducing NOx.

In the embodiment illustrated in Figure 1 , the valve mechanism 18 is a simple butterfly valve located in the first conduit. When the valve 18 is open, most exhaust gases pass through the first conduit to the DPF.

The relatively low gas flow through the second conduit 12 can be eliminated completely by use of a second valve 19 in the second conduit, as illustrated in the embodiment shown in Figure 9. The first valve 18 and the second valve 19 are independently controllable to allow exhaust gases to flow through only the first conduit 4 or the second conduit 12 according to the operating mode.

Referring now to Figure 10, another embodiment is illustrated, which uses a three- way valve 18 at the junction of the conduits 4,12 for selectively directing exhaust gases through either or both conduits.

The first conduit 4 and the second conduit 12 may be arranged in any convenient manner. In the embodiment illustrated in Figure 1 1 , the conduits 4,12 are coaxial. A valve 18 in the central second conduit 12 selectively permits exhaust gas flow through the DOC 14 during light-off and regeneration modes. Because the second conduit 12 is aligned with the exhaust gas inlet 6, gases quickly reach the DOC 14 when the valve is opened. Accordingly, this embodiment provides fast light-off when light-off mode is selected. It will be understood that the DOC 14 could alternatively be located in the outer annular passage, which would function as the second conduit. It will be further understood that the valve mechanism 18 could alternatively be provided in the outer annular passage. The embodiment illustrated in Figure 12 is a similar fast light-off arrangement to that of Figure 1 1 , but with an alternative valve mechanism 18 that allows all exhaust gases to be diverted through either the inner conduit or the outer annular conduit. The valve mechanism 18 comprises a first valve disc 18a and a second valve disc 18b, each of which is provided with at least one outer aperture 30 and at least one inner aperture 32 which correspond respectively with the first conduit 4 and the second conduit 12. Relative rotation of the valve discs selectively brings the outer apertures 30 into alignment and the inner apertures 32 out of alignment. Further relative rotation of the valve discs selectively brings the outer apertures 30 out of alignment and the inner apertures 32 into alignment. When the inner apertures 32 are aligned, the central (second) conduit 12 is opened, and when the outer apertures 30 are aligned, the annular (first) conduit 4 is opened. At intermediate alignments the valve mechanism 18 optionally prevents gas flow through either conduit.

As illustrated, the DOC 14 is located in the inner conduit which functions as the second conduit 12, while the annular conduit functions as the first conduit 4. It is appreciated that the DOC could alternatively be provided in the annular conduit. It will be appreciated that various sensors and control systems may be employed to optimise performance of the exhaust system. For example, the temperature of exhaust gases may be monitored at or downstream of the DOC to determine when light-off occurs, and to trigger fuel injection at or after light-off. Accurate control of the fuel injection rate is preferred in order to limit unburned fuel passing through the DOC. The pressure difference over the DPF may be monitored, and

regeneration mode triggered when a threshold Δρ value is reached, corresponding to a threshold level of channel blockage within the DPF.

The system may be open loop, closed loop, feedforward, feedback or use other suitable monitoring and control methods.

The system is simple in terms of hardware and control, providing cost and operating advantages. For example, whereas the prior art system requires control of flow through the first catalyst anywhere between 0% and 100% in order to achieve desired flow rates through the DOC, this is not necessary in the present system. The valve mechanism can simply be switched between zero and maximum flow depending on the operating mode required.

The system uses fewer components than prior art systems, and operates differently to achieve both DPF regeneration and low NO2 levels at the tailpipe. The described prior art system cannot achieve low tailpipe NO2 because there will always be exhaust gas flowing through a catalyst that will generate NO2.

The prior art system requires a special oxidation catalyst selected for good HC conversion and low NO2 formation. An advantage of the present system is that it can use a very active catalyst that typically has both high HC and NO2 conversion, lighting off at lower temperatures and having higher efficiency. The layout and operation of the system is such that low tailpipe NO2 is achieved relative to NO2 going into the system.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.