GB2593762A

GB2593762A - Lean-burn gasoline engine aftertreatment system

Info

Publication number: GB2593762A
Application number: GB2004887.2A
Authority: GB
Inventors: Cudmore Stephen; Mcwilliam Lyn
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2020-04-02
Filing date: 2020-04-02
Publication date: 2021-10-06
Also published as: GB202004887D0

Abstract

An exhaust aftertreatment system 100, for a lean burn gasoline engine 200, comprises an inlet 110, a HC-SCR 120, an oxidation catalyst 130 and an outlet 140. An exhaust stream 240 from the engine is received at the inlet and released into an external environment at the outlet. The HC-SCR uses unburned hydrocarbon (HC) from the exhaust stream as a reductant for selective catalyst reduction of nitrogen oxides (NOx) to nitrogen (N2). The oxidation catalyst is downstream the HC-SCR for oxidising carbon monoxide (CO) and hydrocarbon remainders to carbon dioxide (CO2) and water (H2O). The exhaust aftertreatment system does not comprise an oxidation catalyst upstream the HC-SCR. A method of reducing NOx to nitrogen in an exhaust stream from a lean-burn gasoline engine comprises reducing NOx using a reduction catalyst and a hydrocarbon reductant, and subsequently oxidising carbon monoxide and hydrocarbon remainders to carbon dioxide and water using an oxidation catalyst.

Description

Lean-burn gasoline engine aftertreatment system

TECHNICAL FIELD

The present disclosure relates to a lean-burn gasoline engine aftertreatment system. Aspects of the invention relate to an exhaust aftertreatment system, a powertrain and a vehicle using that aftertreatment system and a method of reducing nitrogen oxides (N0) in an exhaust stream.

BACKGROUND

In classic internal combustion engines, gasoline burns best when it is mixed with air in the proportions of 14.7:1 (lambda = 1). Most modern gasoline engines used in vehicles tend to operate at or near the stoichiometric for most of the time. Ideally, when burning fuel in an engine, only carbon dioxide (002) and water (H20) are produced. In practice, the exhaust gas of an internal combustion engine also comprises significant amounts of carbon monoxide (CO), nitrogen oxides (NO) and unburned hydrocarbons.

Increasing environmental concerns and CO2 induced climate change have pushed engine manufacturers to technologies that increase fuel efficiency and reduce unwanted emissions.

One possible route for increasing fuel efficiency is to burn the fuel with an excess of air. Typical lean-burn engines may mix air and fuel in proportions of, e.g., 20:1 (lambda > 1.3) or even 30:1 (lambda > 2).

Advantages of lean-burn engines include, e.g., that they produce lower levels of CO2 and hydrocarbon emissions by better combustion control and more complete fuel burning inside the engine cylinders. The engines designed for lean-burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional gasoline engines. Additionally, lean-burn modes help to reduce throttling losses, which originate from the extra work that is required for pumping air through a partially closed throttle. When using more air to burn the fuel, the throttle can be kept more open when the demand for engine power is reduced.

Lean-burning of fuel does, however, also come with some important disadvantages. For example, if the mixture is too lean, the engine will fail to combust. Especially at low loads and engine speeds, reduced flammability may affect the stability of the combustion process and introduce problems with engine knock. Another downside of lean-burn technology is that an excess of oxygen in the exhaust stream makes it difficult to control NOx emissions using standard aftertreatment technologies. Although NO concentrations in the exhaust stream are considerably lower in a lean-burn engine than in a lambda 1 engine, a lean-burn gasoline engine, at low loads, still emits a lot more NO than a diesel engine. 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 NON, before the exhaust stream leaves the tailpipe of the vehicle.

In modern diesel engines for vehicles, the problem of excess NO emissions is effectively dealt with using Selective Catalytic Reduction (SCR) with the help of a Diesel Exhaust Fluid (DEF).

The DEF is an aqueous urea ((NH2)200) solution that is injected into the hot exhaust gas stream, which makes the water evaporate and the urea decompose into ammonia and carbon dioxide (002). In the presence of oxygen (02) and a catalyst, the ammonia reduces the NOx into nitrogen (N2) and water (H20). Although this technology is very effective for reducing NO emissions in diesel engines, it comes with some disadvantages that do not make it a preferred solution for dealing with NOx emissions in lean-burn gasoline engines. The tank and injection system required for providing the DEF to the exhaust stream take up space in and add weight to the vehicle. The filling of the tank may be an annoyance for some users and structurally adds maintenance costs to the ownership of the vehicle. Also, DEF is not easily available everywhere, which may make it difficult for consumers to ensure that the DEF tank remains properly filled.

A known alternative for reducing the NO content of the exhaust stream of a lean-burning engine is the use of a Lean NO Trap (LNT). In an LNT, an adsorbent, such as a zeolite or a chemical storage including Barium or Cerium, traps the NO molecules. Once the trap is full, it cannot adsorb more NO and needs to be regenerated. During the regeneration, the trapped NO is converted to nitrogen and water. An important disadvantage of an LNT is that the periodic regeneration requires the engine to operate under rich conditions. The increased CO2 emissions resulting from such regeneration thus partly negate the main positive effect of the use of a lean-burn engine.

It is 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 an exhaust aftertreatment system, a powertrain and a vehicle using that aftertreatment system and a method of reducing nitrogen oxides (NO) in an exhaust stream as claimed in the appended claims According to an aspect of the present invention there is provided an exhaust aftertreatment system for a lean-burn gasoline engine, the aftertreatment system comprising an inlet, a HC-SCR, an oxidation catalyst and an outlet. An exhaust stream from the engine is received at the inlet. The HC-SCR is provided for selective catalyst reduction of nitrogen oxides (NO4 to nitrogen (N2) using unburned hydrocarbon (HC) from the exhaust stream as a reductant. The oxidation catalyst is provided downstream the HC-SCR for oxidising carbon monoxide (CO) and hydrocarbon (HC) remainders to carbon dioxide (002) and water (H20). At the outlet, the exhaust stream is released into an external environment of the exhaust aftertreatment system. According to this aspect of the invention, the exhaust aftertreatment system does not comprise an oxidation catalyst upstream the HC-SCR.

Hydrocarbon selective catalyst reduction as such has been used before. However, up till now, HC-SCR has mainly been used for diesel engines. The inventors have found that the typical temperatures and hydrocarbon and NO2 output of a lean-burn gasoline engine could make HC-SCR a surprisingly suitable technique for NC x reduction. It is noted that known diesel engine HC-SCR systems, such as sold by Cataler or as disclosed in the international patent application WO 2008/026002 Al, also include an oxidation catalyst. However, in such diesel engine HC-SCR systems, it is essential that the oxidation catalyst is provided upstream the HC-SCR in order to provide the NO2 that is necessary to allow for the primary and desirable reactions to actually occur. The NO2:NOx ratio in the diesel engine exhaust stream is generally about 10-20%, which is far too low for allowing the HC-SCR to have any success in the reduction of NOx. The oxidation catalyst therefore oxidises not only the CO, but also the NO from the diesel engine exhaust stream. Even in WO 2008/026002 Al, where a small DOG (Diesel Oxidation Catalyst) is provided, downstream the HC-SCR, a larger main DOC is still needed upstream the HC-SCR, to oxidise the CO and provide the NO2 that is needed for the subsequent NOx reduction.

For the lean-burn gasoline engine aftertreatment system used in accordance with the invention, the engine-out exhaust stream has a much better NO2:NOx ratio that improves even further for increasing lambda. A more compact and efficient aftertreatment system is thus obtained by not using an upstream oxidation catalyst, but only using a downstream oxidation catalyst. The CO oxidation is performed downstream the HC-SCR with the added bonus of cleaning up any remaining hydrocarbons that slipped through the HC-SCR (typically about 20%) in the same catalyst unit. A further advantage of not using an oxidation catalyst, upstream the HC-SCR is that an upstream oxidation catalyst would deplete the exhaust stream of hydrocarbons which are needed for the NO reduction performed in the HC-SCR.

In an embodiment of the exhaust aftertreatment system according to the invention, the exhaust aftertreatment system does not comprise a fuel injector for injecting additional hydrocarbon into the exhaust stream downstream the engine.

The activity of the HC-SCR is highly dependent on the availability of hydrocarbons and the ratio of hydrocarbon to NOR. While, up till now, HC-SCR has mainly been used for diesel engines, the inventors have found that the typical hydrocarbon output of a lean-burn gasoline engine could make HC-SCR a surprisingly suitable technique for NO reduction. In a lean-burn gasoline engine, the hydrocarbon emissions are significantly higher than for diesel engines operating at similar loads (up to 70% more for most of the engine map). As a result, HC-SCR can be performed in lean-burn gasoline engines, without requiring the injection of additional hydrocarbons into the exhaust stream. This significantly reduces the cost and complexity of the aftertreatment system and increases the overall fuel efficiency of the engine. It has further been found that a further contributing factor to the efficiency of the HC-SCR is that the exhaust stream hydrocarbons of a lean-burn gasoline engine comprise significantly shorter chain hydrocarbons than the exhaust stream of a diesel engine or injected fuel directly obtained from the fuel tank.

In an embodiment of the exhaust aftertreatment system according to the invention, the exhaust aftertreatment system does comprises a fuel injector for injecting additional hydrocarbon into the exhaust stream downstream the inlet and upstream the HC-SCR.

Although this may add cost and complexity to the system, it also provides the opportunity to supplement the engine-out HC with a specified amount of additional HC, if and when the circumstances require this for achieving optimal effectiveness of the HC-SCR.

The HC-SCR may comprise a silver-alumina (AG-A1203) catalyst, with or without the addition of platinum (Pt).

According to another aspect of the invention, a powertrain for a vehicle is provided comprising a lean-burn gasoline engine and an exhaust aftertreatment system as described above.

According to a further aspect of the invention, a vehicle comprising such a powertrain is provided.

According to a further aspect of the invention, a method is provided for reducing nitrogen oxides (NO) in an exhaust stream from a lean-burn gasoline engine to nitrogen (N2). The method comprises reducing nitrogen oxides (NO.) using a reduction catalyst and a hydrocarbon (HC) reductant, and subsequently oxidising carbon monoxide (CO) and hydrocarbon (HC) remainders to carbon dioxide (CO2) and water (H20) using an oxidation catalyst.

The hydrocarbon (HC) used as the reductant may be unburned hydrocarbon (HC) derived from combustion of gasoline in the lean-burn gasoline engine and/or hydrocarbons (HC) from gasoline 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 an engine and an exhaust aftertreatment system 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 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. Even though the engine 200 shown is an internal combustion engine 200, the engine may be a hybrid engine that further includes an electric motor.

Burning gasoline in the cylinders of the engine 200 results in an exhaust gas, which includes carbon monoxide (CO), carbon dioxide (002), water vapour (H20), 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 NOx (NO and NO2) and the hydrocarbons. To achieve this aim, the aftertreatment system 100 comprises at least two stages, a hydrocarbon selective catalyst reductor (HC-SCR) 120 and an oxidation catalyst 130.

The HC-SCR 120 is provided and configured for the reduction of NO to nitrogen (N2) using unburned hydrocarbon from the exhaust stream 240 as a reductant. The primary and desirable reactions are: NO + -> NO2 0" + 02 -> CxHyOz Cx1-1yOz + NO2 -> N2 + CO2 + H20 The primary reactions are promoted by the presence of a catalyst coating. A suitable catalyst for the HC-SCR 120 may, e.g., comprise silver-alumina (AG-A1203). Further improved catalyst properties may be achieved by further including platinum (Pt) or another platinum group metal, such as ruthenium, rhodium, palladium, osmium or iridium.

The activity of the HC-SCR 120 is highly dependent on the availability of hydrocarbons and the ratio of hydrocarbon to NOx. Although HC-SCR, up till now, has mainly been used for diesel engines, the inventors have found that the typical hydrocarbon output of a lean-burn gasoline engine could make HC-SCR a surprisingly suitable technique for NOx reduction. In a lean-burn gasoline engine, the hydrocarbon emissions are significantly higher than for diesel engines operating at similar loads (up to 70% more for most of the engine map). As a result, HC-SCR can be performed in lean-burn gasoline engines, without requiring the injection of additional hydrocarbons into the exhaust stream. This significantly reduces the cost and complexity of the aftertreatment system and increases the overall fuel efficiency of the engine.

It has further been found that a further contributing factor to the efficiency of the HC-SCR 120 is that the exhaust stream hydrocarbons of a lean-burn gasoline engine 200 comprise significantly shorter chain hydrocarbons than the exhaust stream of a diesel engine or injected fuel directly obtained from the fuel tank 210.

The main competitive reaction in the HC-SCR 120 breaks up the hydrocarbon chains and produces CO2 without reducing any NCI in the process: CH y + 02 -> CO2 + H20 As it is not always possible to create the circumstances wherein the primary reactions are promoted, and the competitive reaction is reduced, also here the higher hydrocarbon content of the exhaust stream could help to ensure that, despite the competitive reaction, sufficient hydrocarbons are available for NOx reduction.

In addition to the HC-SCR 120, the aftertreatment system 100 comprises an oxidation catalyst 130, which 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 (02) to produce carbon dioxide (CO2) and water (H20). From the oxidation catalyst 130, the cleaned-up exhaust stream can leave the aftertreatment system 100 and enter the environment of the car in the form of tailpipe emissions 250.

It is noted that known diesel engine HC-SCR systems, such as sold by Cataler or as disclosed in the international patent application WO 2008/026002 Al, also include an oxidation catalyst. However, in such diesel engine HC-SCR systems, it is essential that the oxidation catalyst is provided upstream the HC-SCR in order to provide the NO2 that is necessary to allow for the primary and desirable reactions to actually occur. The NO2:N0" ratio in the diesel engine exhaust stream is generally about 10-20%, which is far too low for allowing the HC-SCR to have any success in the reduction of NOR. The oxidation catalyst therefore oxidises not only the CO, but also the NO from the diesel engine exhaust stream. Even in WO 2008/026002 Al, where a small DOC (Diesel Oxidation Catalyst) is provided, downstream the HC-SCR, a larger main DOC is still needed upstream the HC-SCR, to oxidise the CO and provide the NO2 that is needed for the subsequent NO reduction.

For the lean-burn gasoline engine aftertreatment system 100 of Figure 1, the engine-out exhaust stream 240 has a much better NO2:NOx ratio that improves even further for increasing lambda. A more compact and efficient aftertreatment system 100 is thus obtained by not using an upstream oxidation catalyst, but only using a downstream oxidation catalyst 130. The CO oxidation is thus performed downstream the HC-SCR 120 with the added bonus of cleaning up any remaining hydrocarbons that slipped through the HC-SCR 120 (typically about 20%) in the same catalyst unit. A further advantage of not using an oxidation catalyst, upstream the HC-SCR 120 is that an upstream oxidation catalyst would deplete the exhaust stream 240 of hydrocarbons and would make it necessary to inject fresh hydrocarbons between the oxidation catalyst and the HC-SCR 120. With the setup of Figure 1, such injection is not necessary.

Figure 2 shows a schematic representation of an engine 200 and an exhaust aftertreatment system 101 according to an embodiment of the invention. The aftertreatment system 101 shown here is largely similar to the aftertreatment system 100 shown in Figure 1. The main difference is that it adds a fuel injector 115 for selectively injecting additional hydrocarbons into the exhaust stream 240, upstream of the HC-SCR 120. Although the lean-burn gasoline engine 200 will generally provide sufficient hydrocarbons for effective NO reduction in the HC-SCR 120, there may be situations (e.g. under higher loads and at higher engine speeds) wherein additional hydrocarbons may be needed for a more effective operation of the HC-SCR 120. The control of the amount of fuel injected into the exhaust stream may, e.g., be 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 concentration in and the temperature of the exhaust stream may be taken into account and appropriate sensors for measuring these and other parameters may be provided.

Additional sensors (HC, NO", 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 NOx reduction and fuel efficiency.

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.

Claims

CLAIMS1. An exhaust aftertreatment system (100, 101) for a lean-burn gasoline engine (200), the aftertreatment system (100, 101) comprising: - an inlet (110) for receiving an exhaust stream (240) from the engine (200), a HC-SCR (120) for selective catalyst reduction of nitrogen oxides (N0x) to nitrogen (N2) using unburned hydrocarbon (HC) from the exhaust stream (240) as a reductant, an oxidation catalyst (130), provided downstream the HC-SCR (120), for oxidising carbon monoxide (CO) and hydrocarbon (HC) remainders to carbon dioxide (002) and water (H20), and an outlet (140) for releasing the exhaust stream (240) into an external environment of the exhaust aftertreatment system (100, 101), wherein the exhaust aftertreatment system (100, 101) does not comprise an oxidation catalyst upstream the HC-SCR (120).
2. An exhaust aftertreatment system (100, 101) according to claim 1, wherein the exhaust aftertreatment system (100) does not comprise a fuel injector for injecting additional hydrocarbon into the exhaust stream (240) downstream the engine (200).
3. An exhaust aftertreatment system (100, 101) according to claim 1, wherein the exhaust aftertreatment system (101) further comprises a fuel injector (115) for injecting additional hydrocarbon into the exhaust stream (240) downstream the inlet (110) and upstream the HC-SCR (120).
4. An exhaust aftertreatment system (100, 101) according to any of the preceding claims, wherein the HC-SCR (120) comprises a silver-alumina (AG-A1203) catalyst.
5. An exhaust aftertreatment system (100, 101) according to any of the preceding claims, wherein the catalyst comprises platinum (Pt).
6. A powertrain for a vehicle comprising a lean-burn gasoline engine (200) and an exhaust aftertreatment system (100, 101) according to any one of the preceding claims, the inlet (110) of the exhaust aftertreatment system (100, 101) being coupled to an exhaust outlet of the engine (200).
7. A vehicle (10) comprising a powertrain as claimed in claim 6.
8. A method of reducing nitrogen oxides (NO.) in an exhaust stream (240) from a lean-burn gasoline engine (200) to nitrogen (N2), the method comprising reducing nitrogen oxides (NO.) using a reduction catalyst and a hydrocarbon (HC) reductant, and subsequently oxidising carbon monoxide (CO) and hydrocarbon (HC) remainders to carbon dioxide (002) and water (H20) using an oxidation catalyst (130).
9. A method according to claim 8, wherein all the hydrocarbon (HC) used as the reductant is unburned hydrocarbon (HC) derived from combustion of gasoline in the lean-burn gasoline engine (200).
10. A method according to claim 8, wherein the hydrocarbon (HC) used as the reductant comprises hydrocarbons (HC) from gasoline injected into the exhaust stream (240), downstream the engine (200).
11. A method according to any of claims 8-10, wherein the reduction catalyst comprises silver-alumina (AG-A1203).
12. A method according to any of claims 8-11, wherein the reduction catalyst comprises platinum (Pt).