TECHNICAL FIELD

The present disclosure relates to improvements in hydrocarbon selective catalytic reduction. Aspects of the invention relate to a hydrocarbon selective catalytic reduction unit, an exhaust aftertreatment system, a powertrain and a vehicle using the hydrocarbon selective catalytic reduction unit. The present disclosure further relates to a method of reducing nitrogen oxides (NO _x ) in an exhaust stream.

BACKGROUND

Internal combustion engines burn hydrocarbon fuel (CxHy) with oxygen (02), resulting in carbon dioxide (C02) and water (H20). The oxygen used for this process typically come from air, taken from the immediate environment of the engine and contains nitrogen (N ₂ ). In the combustion process, some of this nitrogen combines with the oxygen to form nitrogen oxides (NO _x ), such as nitric oxide (NO) and nitrogen dioxide (N0 ₂ ).

NOx emissions from vehicles are an important environmental concern, since they lead to smog and acid rain. Increasingly strict regulations are designed to reduce the total amount of NOx emitted through transport and industrial activity. A variety of technical solutions is available for removing the NO _x from the exhaust stream before the exhaust stream is released into the environment.

For example, in modern diesel engines for vehicles, the problem of excess NO _x emissions is often dealt with using Selective Catalytic Reduction (SCR) with the help of a Diesel Exhaust Fluid (DEF). The DEF is an aqueous urea ((NFh^CO) solution that is injected into the hot exhaust stream, which makes the water evaporate and the urea decompose into ammonia and carbon dioxide. In the presence of oxygen and a catalyst, the ammonia reduces the NO _x into nitrogen and water.

WO 2008/026002 A1 describes a hydrocarbon SCR (HC-SCR), wherein hydrocarbons present in the exhaust stream or injected by a special hydrocarbon injector are used as a reductant to convert nitrogen oxides to nitrogen, in the presence of a catalyst. HC-SCR brings the advantage that the injected hydrocarbon can be sourced from the fuel tank and no additional DEF tank is needed to provide for the necessary reductant. A problem with HC- SCR is that it typically works best at temperatures around and above about 350°C and is less effective at lower temperatures. In WO 2008/026002 A1 this problem is allegedly tackled by using an oxidation catalyst upstream the HC-SCR in order to significantly increase the N0 ₂ :N0 _X ratio, which leads to a more effective NO _x reduction in the HC-SCR. Also a diesel engine aftertreatment system currently sold by Cataler, uses an oxidation catalyst upstream a HC-SCR. However, the use of an oxidation catalyst upstream the HC-SCR comes with a few disadvantages, one of them being the it may oxidise some of the hydrocarbons from the exhaust stream, which then can’t be used for reducing NO _x in the HC-SCR anymore. Consequently, more fuel has to be injected into the aftertreatment system, which reduces the fuel efficiency of the vehicle.

A proven approach for increasing the effectiveness of a HC-SCR at lower temperatures is to dope the catalyst with platinum (Pt) particles. However over 350°C this results in promoting competitive reactions that consume the available hydrocarbons, but do not lead to NO _x reduction. This degrades the performance of the HC-SCR.

In view of current and future environmental legislation (EUR06, China Stage 6b, SULEV30, etc.), it is important that some effective measures are taken to get rid of most of that NOx, before the exhaust stream leaves the tailpipe of the vehicle. It is therefore an aim of the present invention to address one or more of the disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide a hydrocarbon selective catalytic reduction unit, an exhaust aftertreatment system, a powertrain, a vehicle and a method as claimed in the appended claims

According to an aspect of the present invention there is provided a HC-SCR unit for selective catalyst reduction of nitrogen oxides (NOx) to nitrogen (N2) using hydrocarbon (HC) as a reductant. The HC-SCR unit comprises an inlet for receiving an exhaust stream, a catalyst bed for activating the reduction of the nitrogen oxides, and an outlet for emitting the exhaust stream. The catalyst bed comprises a first catalyst zone with a first catalyst coating and a second catalyst zone with a second catalyst coating, the second catalyst coating being different from the first catalyst coating.

By using two or more zones with different catalyst coatings, it is possible for the HC-SCR to perform optimally under varying circumstances. The different catalyst coatings may, e.g., be particularly effective at different temperatures, different predetermined HC concentrations, NO _x concentrations, N0 ₂ :N0 _X ratios or HC:NO _x ratios. It is to be noted that the word ‘unit’ is herein to be interpreted as in referring to a ‘functional unit’ and that the first and second catalyst zone are not necessarily housed in one and the same housing. The HC-SCR unit may, e.g., be formed by two consecutive housings, an inlet of the second housing being coupled to an outlet of the first housing. It is, however, preferred to provide the first and second catalyst zone on one continuous catalyst bed and in a single housing, since this provides for a more compact solution.

In an embodiment of the HC-SCR unit of the invention, the first catalyst coating is selected for activating the reduction at temperatures above a first activation temperature and the second catalyst is selected for activating the reduction at temperatures below the first activation temperature. This allows the second catalyst zone to reduce the NO _x concentration in the exhaust stream when the temperature drops below the first activation temperature. The first activation temperature may, e.g., be about 350°C.

Preferably, the second catalyst zone is provided downstream the first catalyst zone. This is for example useful in situations wherein the second catalyst zone may be able to deplete the exhaust stream of the hydrocarbons that the first catalyst zone needs for reducing the NO _x content of the exhaust stream.

Optional coating materials for both catalyst coatings may comprise silver-alumina (AG-AI203), Iron (Fe), Copper (Cu), or Cobalt (Co), all possibly in combination with a zeolite. The second catalyst coating may comprise a platinum group metal, in order to make it particularly suitable for reducing NO _x concentrations at lower temperatures than in the first catalyst zone.

According to another aspect of the invention an exhaust aftertreatment system for an internal combustion engine is provided. The aftertreatment system comprises an inlet for receiving an exhaust stream from the engine, a HC-SCR unit as described above, and an outlet for releasing the exhaust stream into an external environment of the exhaust aftertreatment system. Further, a powertrain and a vehicle are provided, comprising an internal combustion engine and such an exhaust aftertreatment system. The inlet of the exhaust aftertreatment system is coupled to an exhaust outlet of the engine.

The internal combustion engine may be a lean-burn gasoline engine. A known disadvantage of lean-burn gasoline engines is that the temperature of the exhaust stream is relatively low, especially at low engine loads, which makes it difficult to reduce its NO _x content with conventional aftertreatment systems. With the HC-SCR unit according to the invention, NO _x reduction is made possible at lower temperatures, without having to compromise on the HC- SCR performance at higher temperatures.

According to a further aspect of the invention, a method is provided for reducing nitrogen oxides (NOx) in an exhaust stream from an internal combustion engine to nitrogen (N2), using hydrocarbon (HC) as a reductant. The method comprises a step of passing the exhaust stream over a first catalyst zone with a first catalyst coating for activating the reduction of the nitrogen oxides, and a step of passing the exhaust stream over a second catalyst zone with a second catalyst coating for activating the reduction of the nitrogen oxides. The second catalyst coating being different from the first catalyst coating. This method brings all the benefits already described above for the other aspects of the invention.

In an embodiment, the first catalyst coating may be selected for activating the reduction at temperatures above a first activation temperature and the second catalyst may be selected for activating the reduction at temperatures below the first activation temperature.

In an embodiment, the first activation temperature may be about 350°C.

In an embodiment, the passing the second catalyst zone may follow the passing the first catalyst zone.

In an embodiment, the first catalyst coating and/or the second catalyst coating may comprise silver-alumina (AG-AI203), Iron (Fe), Copper (Cu), or Cobalt (Co).

In an embodiment, the first catalyst coating and/or the second catalyst coating may further comprise a zeolite.

In an embodiment, the second catalyst coating may comprise a platinum group metal, such as platinum (Pt).

In an embodiment, all the hydrocarbon (HC) used as the reductant may be unburned hydrocarbon (HC) derived from combustion of gasoline in the lean-burn gasoline engine.

In an embodiment, the hydrocarbon (HC) used as the reductant may comprise hydrocarbons (HC) from fuel injected into the exhaust stream, downstream the engine. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic representation of an engine and an exhaust aftertreatment system according to an embodiment of the invention;

Figure 2 shows a schematic representation of a cross section of a hydrocarbon selective catalyst reduction unit according to an embodiment of the invention;

Figure 3 shows a vehicle according to an embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic representation of an internal combustion engine 200 and an exhaust aftertreatment system 100 according to an embodiment of the invention. The engine 200 is a gasoline engine 200 that is configured to burn gasoline from the fuel tank 210, using intake air 230. The engine 200 shown has four cylinders for combustion of the fuel, but the invention is equally applicable to engines having 3, 6, 8, 12 or any other number of cylinders. The engine 200 is further configured to burn the gasoline in a lean environment, i.e. with an excess of air present in the air-fuel mixture in the cylinders. The engine 200 may exclusively operate lean or vary between leaner and richer burning modes. While the engine 200 shown is an internal combustion engine 200, the engine may be a hybrid engine that further includes an electric motor. It is noted that the invention described herein is not limited to use in combination with a lean operating gasoline engine 200 as shown in Figure 1 , but is similarly useful for reducing the NO _x output of, e.g., rich and stoichiometric burning gasoline engines or diesel engines. Further, the usefulness of this invention is not limited to vehicles, but extends to other products and machines, powered by an internal combustion engine.

Burning gasoline in the cylinders of the engine 200 results in an exhaust gas, which includes carbon monoxide (CO), carbon dioxide (CO2), water vapour (H2O), nitrogen oxides (NO, NO2), unburned hydrocarbons (HC) and, possibly, additional substances. The exhaust gas from the different cylinders is combined into an exhaust stream 240, which is led into an inlet 110 of the exhaust aftertreatment system 100.

The aftertreatment system 100 is configured to remove environmentally harmful emissions from the exhaust stream 240, before it leaves the outlet 140 of the aftertreatment system 100 and the tailpipe of the vehicle. More specifically, it is important that the aftertreatment system 100 removes as much of possible of the CO, the NO _x (NO and NO2) and the hydrocarbons. To achieve this aim, the aftertreatment system 100 comprises two stages, a hydrocarbon selective catalyst reductor (HC-SCR) 120 and an oxidation catalyst 130. In Figure 1 , the oxidation catalyst 130 is placed downstream the HC-SCR 120, but as will be explained in more detail below, an oxidation catalyst may alternatively (or additionally) be provided upstream the HC-SCR 120.

The HC-SCR 120 is provided and configured for the reduction NO _x to nitrogen (N2) using unburned hydrocarbon from the exhaust stream 240 as a reductant. The primary and desirable reactions are:

NO + 0 ₂ NO2

C _X Hy + O2 CxHyOz

C _x HyOz + NO2 N ₂ + C0 ₂ + H2O

The primary reactions are promoted by the presence of a catalyst coating. A suitable catalyst coating for the HC-SCR 120 may, e.g., comprise silver-alumina (AG-AI ₂ O ₃ ).

The activity of the HC-SCR 120 is highly dependent on the availability of hydrocarbons and the ratio of hydrocarbon to NO _x . To ensure the presence of enough hydrocarbons in the HC- SCR 120, a fuel injector 115 may be provided. The fuel injector 115 uses fuel from the fuel tank 210 and injects it directly into the exhaust stream 240, upstream the HC-SCR 120. The amount of fuel that is injected may vary and may, e.g., be controlled based on engine parameters (engine RPM, load estimates, etc.) and/or sensor readings indicating the instantaneous composition of the exhaust stream 240 and/or the tailpipe emissions. For example, a hydrocarbons sensor 116 may be provided upstream the HR-SCR 120 to measure the concentration of hydrocarbons in the exhaust stream 240. When it is determined that the HC concentration is too low, hydrocarbons can be added by injecting the right amount of fuel into the exhaust stream. For determining the optimal amount of hydrocarbons, also the NO _x concentration in and the temperature of the exhaust stream 240 may be taken into account and appropriate sensors for measuring these and other parameters may be provided. It is noted that for lean-burn gasoline engines, the HC concentration of the exhaust stream 240 is typically significantly higher than for a diesel engine, and no fuel injector 115 may be provided at all.

The main competitive reaction in the HC-SCR 120 breaks up the hydrocarbon chains and produces CO2 without reducing any NO _x in the process:

C _x H _y + O2 CO2 + H2O

As it is not always possible to create the circumstances wherein the primary reactions are promoted, and the competitive reaction is reduced, also here a higher hydrocarbon content of the exhaust stream could help to ensure that, despite of the competitive reaction, sufficient hydrocarbons are available for NO _x reduction.

In addition to the HC-SCR 120, the aftertreatment system 100 comprises an oxidation catalyst 130, which in this embodiment is provided downstream the HC-SCR 120. The oxidation catalyst is provided for oxidising carbon monoxide (CO) from the exhaust stream 240 and any hydrocarbon remainders that slip through the HC-SCR 120. In the oxidation catalyst 130, the CO and hydrocarbons are combined with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). From the oxidation catalyst 130, the cleaned-up exhaust stream can leave the aftertreatment system 100 and enter the environment of the vehicle in the form of tailpipe emissions 250.

When the engine 200 is a diesel engine, an oxidation catalyst is likely to be provided upstream the HC-SCR 120 in order to provide the NO2 that is necessary to allow for the primary and desirable reactions to actually occur. The NC>2:NO _x ratio in the diesel engine exhaust stream is generally about 10-20%, which is too low for allowing the HC-SCR 120 to successfully reduce the NO _x concentration of the exhaust stream. The upstream oxidation catalyst oxidises not only the CO, but also the NO from the diesel engine exhaust stream. An additional, optionally smaller, oxidation catalyst may be provided, downstream the HC-SCR 120 to deal with hydrocarbons that slipped through the HC-SCR 120. For the lean-burn gasoline engine aftertreatment system 100 of Figure 1 , the engine-out exhaust stream 240 has a much better NC>2:NO _X ratio that improves even further for increasing lambda, and only one downstream oxidation catalyst is used.

Additional sensors (HC, NO _x , CO, etc.) may be provided downstream the HC-SCR 120 and/or the oxidation catalyst 130 for monitoring the effectiveness of the HC-SCR 120, the oxidation catalyst 130, or the aftertreatment system 101 as a whole. Based on signals received from such sensors, the timing and dosage of the fuel injections may be adapted for optimal results in terms of NO _x reduction and fuel efficiency.

Figure 2 shows a schematic representation of a cross section of a hydrocarbon selective catalyst reduction unit (HC-SCR) 120 according to an embodiment of the invention. The HC- SCR 120 comprises an inlet 241 for receiving the exhaust stream 240 from the engine 200, a first SCR zone 301 and a second SCR zone 302 for reducing the NO _x content of the exhaust stream 240 and an outlet 242 for releasing the exhaust stream 240 for further processing by other parts of the aftertreatment system 100 or for release into the environment of the vehicle in the form of tailpipe emissions 250.

The NO _x reducing reactions indicated above take place inside the HC-SCR 120. While the exhaust stream 240 flows through the HC-SCR 120, contact with the catalyst coatings 310, 320 promotes the primary and desirable reactions. In the HC-SCR 120 of Figure 2, the two separate zones 301 , 302 comprise different catalyst coatings 310, 320. In Figure 2, which is only a schematic representation of an embodiment of a HC-SCR 120 according to the invention, the two separate zones 301, 302 are of equal size. In other embodiments, the different zones may be differently sized. Also, three or more different zones, using three or more different catalyst coatings may be provided for optimally enjoying the benefits of the invention.

A typical and effective catalyst coating for the HC-SCR 120 may, e.g., comprise silver-alumina (AG-AI2O3) and could be used as a basis for the catalyst coatings 310, 320 used in the HC- SCR of Figure 2. Other catalyst coatings that can be used may comprise Iron (Fe), Copper (Cu) or Cobalt (Co), which all have been seen to work at various efficiencies, some with the inclusion of a zeolite. According to the invention, the first catalyst coating 310 is different from the second catalyst coating 320, which makes the first SCR zone 301 optimally effective in different circumstances than the second SCR zone 302. For example, the first catalyst coating 310 is selected for activating the NO _x reduction at temperatures above a first activation temperature and the second catalyst coating 320 is selected for activating the reduction at temperatures below that first activation temperature. The first catalyst coating 310 may thus be a high temperature coating 310 and the second catalyst coating 320 a low temperature coating 320. Alternatively, the different catalyst coatings 310, 320 may be optimised for, e.g., different predetermined HC concentrations, NO _x concentrations, N0 ₂ :NO _x ratios or HC:NO _x ratios.

A typical way to improve low temperature activation of HC-SCR catalyst coatings has been to dope the catalyst coating with platinum (Pt) particles. For example, in the embodiment of Figure 2, the first catalyst coating 310 may be a silver-alumina coating and the second catalyst coating 320 may be a silver-alumina coating with added platinum particles. In the first SCR zone 301 , the primary and desirable reactions are then promoted at temperatures above the first catalyst’s activation temperature of about 350°C. In the second SCR zone 302, the platinum doped catalyst coating 320 enables efficient NO _x reduction at temperatures well below 350°C. Multiple zones with different amounts of added platinum may be provided for optimal NO _x reduction in different temperature ranges.

At lower temperatures (e.g. < 350°C), the platinum doping effectively promotes the primary and desirable reactions. Over 350°C, however, the platinum doping may promote the competitive reactions too. This could lead to depletion of the hydrocarbon reductant and therewith degrade the performance of the HC-SCR 120. To avoid the depletion of hydrocarbons at these higher temperatures, the high temperature catalyst coating 310 is provided in the first, i.e. upstream, SCR zone 301 of the HC-SCR and the lower temperature catalyst coating 320 in the rear SCR zone 302. This ensures that the platinum particles in the low temperature catalyst coating 320 will not have a negative effect on the NO _x reduction in the front, higher temperature SCR zone 301. It is, however, noted that the invention is not limited to this specific setup.

It is noted that suitable catalyst coatings with lower activation temperatures (< 350°C) may also be obtained by, e.g., adding particles of other platinum group metals, such as ruthenium, rhodium, palladium, osmium, or iridium. Also, the higher temperature base coating to which the platinum group metals are added may not be silver-alumina and the base coating used in the second zone 302 may not be the same material as used for the high temperature coating 310 in the first zone 301.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.