Patent Grant
6941747
This invention relates to the treatment of oxides of nitrogen within the exhaust gas emissions of internal combustion engines, and in particular to a method of operating an internal combustion engine to allow such treatment.
The recent and future introduction of increasingly strict internal combustion engine emissions legislation around the world, particularly as this relates to automotive vehicles, has resulted in increasing pressure on engine and vehicle manufacturers to reduce engine emissions, particularly hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) emissions. These emissions are generally treated by a catalytic converter in the exhaust system of the engine, which is intended to convert these potentially harmful gases into preferred substances such as carbon dioxide, nitrogen, oxygen, and water.
NOx emissions present particular challenges for engine and vehicle manufacturers in that typical catalytic converters have been found to be less effective when the engine is operating under lean burn conditions. This is particularly a problem in engines which derive efficiency advantages from lean burn operation, and in particular, stratified charge engines, such as some of those incorporating the Applicant's dual fluid fuel injection system.
Dual fluid fuel injection systems typically utilise compressed gas during each injection event to entrain and atomise a metered quantity of fuel for delivery into the combustion chambers of an internal combustion engine. The Applicant has developed such fuel injection systems and one version thereof is described in the Applicant's U.S. Pat. No. 4,934,329, the details of which are incorporated herein by reference. Generally, a source of compressed gas, for example an air compressor, is required for these fuel injection systems to operate satisfactorily. The term “air” is used herein to refer not only to atmospheric air, but also to other gases including air and exhaust gas or fuel vapour mixtures. In operation, such dual fluid fuel injection systems typically rely on the existence of a differential pressure between the fuel which is metered for subsequent delivery and the compressed gas, typically air, which is used to deliver the fuel to the engine. In this regard, it is normal that the fuel pressure is slightly higher than the air pressure such that the fuel may be metered into a volume of compressed gas in a manner akin to that described in U.S. Pat. No. 4,934,329.
Various methods of engine operation and engine exhaust systems have been proposed to overcome the problem of NOx emissions. One known example, set out in U.S. Pat. No. 5,433,074, proposes the use of a specific NOx adsorbent layer in the catalyst. This layer or coating is intended to absorb NOx emissions under typical low NOx conversion conditions (that is, during lean burn operation of the engine) and release the absorbed NOx under typical high NOx conversion conditions (that is, during richer than stoichiometric operation of the engine). The adsorbent layer is a NOx adsorbent material including Barium (Ba).
However fuel economy in a system utilising such catalysts can be it compromised by the requirement of periodic “flushing” of the system with a rich air-fuel mixture. Further, in order to ensure effective operation of the system, additional sensors may be required to provide feedback to the engine controller for the purpose of determining whether “flushing” is required. The system may also be temperature sensitive, and damage to the adsorbent layer may occur at temperatures above 750-degrees Celsius, whilst effective operation of the storage capacity may be limited to a window of around 300 to 550 degrees Celsius.
It is the aim of this invention to provide an alternative NOx treatment method and device, which overcomes at least some of the disadvantages of the prior art systems.
In accordance with a first aspect of the present invention, there is provided a method of treating NOx emissions in the exhaust gas of an internal combustion engine having catalyst means including at least a first catalyst converter capable of treating NOx, the method including operating the engine in a first mode to promote a first set of conditions and in a second mode to promote a second set of conditions, wherein the first mode of operation includes operating the engine with a lean air-fuel ratio, and the second mode of operation includes operating the engine with a stoichiometric air-fuel ratio, the method further including controlling the operation of the engine during the first mode so as to promote a selective catalyst NOx reduction process at the first catalytic converter.
Conveniently, the catalyst means includes a first catalyst converter arranged in an exhaust system of the engine. Preferably, the first set of conditions include exhaust gases with lean air-fuel ratio and lower relative temperatures. Conveniently, the second set of conditions include exhaust gases with a stoichiometric air fuel ratio. In many cases, the second set of conditions will include higher relative exhaust gas temperatures. Preferably, the exhaust gas temperatures produced by the engine while it operates under the first mode of operation are in the range 200 to 400 degrees Celsius. Preferably, the exhaust gas temperatures produced by the engine while it operates under the second mode of operation are greater than 200 degrees Celsius, and typically the exhaust gas temperature are greater than 400 degrees Celsius. Preferably the relevant exhaust temperature is that of the exhaust gas at the first catalytic converter. Preferably the temperature of the exhaust gas is controlled by way of appropriate operation of the engine to ensure effective operation of the first catalytic converter under the first mode of operation. Preferably the temperature of the exhaust gas in this case is controlled to be within the range 200 to 400 degrees Celsius. Preferably the temperature of the exhaust gas is controlled by way of appropriate operation of the engine to ensure effective operation of the first catalytic converter under the second mode of operation. Preferably the temperature of the exhaust gas In this case is to be greater than approximately 400 degrees Celsius. Conveniently, the operation of the engine is controlled during the first mode so as to generate the exhaust gas emissions having characteristics that can support acceptable levels of Nox conversion within the first catalytic converter.
Preferably the first catalytic converter includes a combination of Pt (or Pd), Rh and Ba elements. Preferably, the first catalytic converter comprises a greater proportion of Pt (ie: it is “Pt rich”) than would be expected in a typical three way catalyst. Preferably the ratio of Pt to Rh in the first catalytic converter is 10:1. Preferably, the proportion of Ba in the first catalyst converter is relatively low as compared to the proportions of Pt and Rh. The operation of the engine during the first mode is controlled so as to promote a selective catalyst reduction process at the first catalytic converter which is normally not supported during lean burn operation. The composition of the first catalytic converter is preferably slightly different to that expected in a typical three way catalyst comprising pt (or Pd) and Rh. Conveniently, the subtle difference in the composition of the first catalyst converter together with the promotion of the first set of conditions during the first mode enable the achievement of higher NOx emission efficiencies than would otherwise be expected form a typical three way catalyst during the said first mode of operation.
Conveniently, the operation of the engine is controlled during the second mode so as to promote high NOx conversion efficiency levels within the first catalytic converter.
Preferably a temperature sensing device is provided in the exhaust system of the internal combustion engine, and the output from the temperature sensing device is used to determine the mode of operation of the internal combustion engine. Preferably a sensed temperature of between 200 and 400 degrees Celsius will result in operation of the engine under the first mode of operation. Preferably a sensed temperature of greater than 400 degrees Celsius will result in operation of the engine under the second mode of operation. This latter mode of operation will typically equate to high engine load operating conditions wherein the temperatures of the exhaust gas are usually higher than during lean burn operation.
Preferably the first catalytic converter is provided in the exhaust system at a position sufficiently downstream of the internal combustion engine that the exhaust gas is allowed to cool somewhat before entering the first catalytic converter.
Preferably a second catalytic converter is provided in a close coupled configuration with the internal combustion engine for the purpose of oxidising hydrocarbon and carbon monoxide emissions in the engine exhaust gases. Preferably the first catalytic converter is a three way catalyst. Conveniently, the engine is direct injected. Preferably, fuel injection to the engine is effected by way of a two fluid fuel injection system,
According to another aspect of the present invention, there is provided an engine exhaust system for treating NOx emissions in the exhaust gas of an internal combustion engine, including catalyst means having at least a first catalyst converter capable of treating NOx, wherein the engine exhaust system is adapted to at least selectively reduce a portion of the NOx emissions when the engine is operated in a first mode and a first set of conditions are promoted, and the first mode of operation includes operating the engine with a lean air-fuel ratio.
According to a further aspect of the present invention, there is provided an electronic control unit for controlling an internal combustion engine having catalyst means including at least a first catalyst converter capable of treating NOx, the electronic control unit operating the engine in a first mode to promote a first set of conditions and in a second mode to promote a second set of conditions, wherein the first mode of operation includes operating the engine with a lean air-fuel ratio, and the second mode of operation includes operating the engine with a stoichiometric air-fuel ratio to thereby treat NOx emissions in the exhaust gas of the engine.
According to yet another aspect of the present invention, there is provided an internal combustion engine for use with an exhaust treatment system having reversible NOx adsorbent capability, said engine having a fuel injection system which facilitates operation of said engine with a plurality of air fuel ratios in a range between lean and rich and said engine having an electronic controller for controlling operation of said engine and for selecting between said air fuel ratios, wherein said selection is not directly dependent on the amount of NOx stored or calculated to be stored in said exhaust treatment system.
According to a further aspect of the present invention, there is provided an internal combustion engine and exhaust treatment system for a vehicle, said exhaust treatment system having reversible NOx adsorbent capability, said engine having a fuel injection system which facilitates operation of said engine with a plurality of air fuel ratios in a range between lean and rich and said engine having an electronic controller for controlling operation of said engine and for selecting between said air fuel ratios, wherein the amount of NOx emitted by said engine to said exhaust treatment system over a Euro III drive cycle is no more than four times the Euro III requirement whereby said exhaust treatment system has emissions of NOx, carbon monoxide and hydrocarbons less than said Euro III requirement over said Euro III drive cycle.
According to another aspect of the present invention, there is provided an internal combustion engine for use with an exhaust treatment system having reversible NOx adsorbent capability, said engine having a fuel injection system which facilitates operation of said engine with a plurality of air fuel ratios in a range between lean and substantially stoichiometric and said engine having an electronic controller for controlling operation of said engine and for selecting said substantially stoichiometric air fuel ratio to purge NOx stored in said exhaust go treatment system.
According to another aspect of the present invention, there is provided an internal combustion engine and exhaust treatment system for use in a vehicle, said exhaust treatment system comprising at least one catalyst having three way conversion capability and NOx storage capability, wherein the amount of NOx emitted by said engine to said exhaust treatment system over a Euro III drive art cycle is no more than four times the Euro III requirement whereby said exhaust treatment system has emissions of NOx, carbon monoxide and hydrocarbons less than said Euro III requirement over said Euro III drive cycle, and the volume of the catalyst is less than 150% of the swept volume of said engine.
It will be convenient to further describe the invention with respect to the accompanying drawings that assist in describing various preferred embodiments of the present invention. Other embodiments of the invention are however possible, and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.
In the drawings:
As referred to above, emissions legislation is being introduced around the world that requires engine and vehicle manufactures to reduce the emissions produced by various types of vehicles. An example of such legislation that is applicable to Europe is commonly referred to as the Euro III and Euro IV emissions targets and should be well known to those skilled in the relevant art
The Euro III and Euro IV emissions targets for passenger vehicles powered only by gasoline in respect of HC, CO and NOx emissions are:
To make these measurements of vehicle emissions, a vehicle is typically operated on a dynamometer. The dynamometer is caused to operate with a specific drive cycle that simulates certain real world driving conditions. Euro III and Euro IV have specific drive cycles over which the emissions referred to above are measured, these drive cycles are referred to as the ECE and the EUDC drive cycles.
The emissions that are measured are referred to as tail pipe emissions as they are emitted from the exhaust pipe (often referred to as the “tail pipe”) of the vehicle. In a typical vehicle, emissions from the engine (often referred to as “engine out” emissions) are treated by an exhaust treatment system that typically utilises a catalytic converter which promotes further reduction and oxidation of engine out emissions so that the tail pipe emissions contain a greater proportion of N2, O2, CO2, and H2O than the engine out emissions. Hence the Euro III and Euro IV emissions specify maximum levels of tail out emissions of hydrocarbons, carbon-monoxide and oxides of Nitrogen for various classes of vehicles.
It is preferable that in meeting these emissions targets that the vehicle also have a fuel economy benefit over currently available MPI (Manifold Port Injected) engines and DI (Direct Injection) engines.
The Applicant has developed certain engine which utilize a two fluid direct fuel injection system. Simple application of such fuel injection systems to four stroke engines is not, in itself, sufficient to meet these emissions targets and further refinement is required before the above emissions targets can be met. In particular it is necessary to calibrate an engine at various points on the speed load curve (for example the speed load curve detailed in
To fully understand how these emissions targets may be met by use of such a fuel system, the Applicant's two fluid fuel injection system will first be described in some detail with reference to
Referring now to
As best seen in
Referring again to
The opening of each injector valve 34 is cot lied by the ECU 100 via a respective communicating means 101 in timed relation to the engine cycle to effect delivery of fuel from the injection port 32 to a combustion chamber 60 of the engine 20. By virtue of the two fluid nature of the system, fuel is delivered to the cylinder entrained in a gas. The passage 90 is in constant communication with the air duct 13 via the conduit 80 as shown in FIG. 2 and thus, under normal operation, is maintained at a substantially steady air pressure. Upon energization of the solenoid of air injector 12, the valve 34 is displaced downwardly to open the injection port 32 so that a metered quantity of fuel delivered into the air injector 12 by the fuel metering unit 10 is carried by air through the injection port 32 in the combustion chamber 60 of a cylinder of the engine 20.
Typically, the air injector 12 is located within the cylinder head 40 of the engine 20, and is directly in communication with the combustion chamber 60 defined by the reciprocation of a piston 61 within the engine cylinder. As above described, when the injection port 32 is opened and the air supply available via the conduit 80 is above the pressure in the engine cylinder, air will flow from the air duct 13 through the passage 80, passage 90 and, entrained with fuel, injection port 32, into the engine combustion chamber 60.
Turning now to
As is usual in the operation of engine systems of this type, fuel and air are taken in through their respective intakes 202, 204. Combustion then takes place in the engine 200, and exhaust gases pass out of the engine 200. In this Figure, there is illustrated an optimal coupled catalytic converter 206 through which the exhaust gases may pass immediately as they leave the combustion chamber of the engine 200. Exhaust gases then travel along exhaust pipe 212 to the main a catalytic converter 208, and subsequently out the external exhaust outlet 210. The catalytic converter 208 may for example be an underbody catalyst arranged to be a specified distance downstream of an exhaust port (not shown) of the engine.
The engine operation includes two major modes, and two transitional modes (although the engine need not necessarily operate under these modes at all times and other modes of operation are possible). Preferred modal a operation of the engine is best shown in
In lean operation mode (indicated by reference numeral A), the engine Is calibrated to operate in lean burn mode, with a stoichiometric coefficient of preferably greater than 1.3. (ie: The stoichiometric coefficient is 1 for a stoichiometric air-fuel ratio, greater than 1 for a lean air-fuel ratio, and less than 1 for a rich air-fuel ratio.) In the stoichiometric ratio mode (indicated by reference numeral C), the air-fuel ratio is maintained at a substantially stoichiometric level with a stoichiometric coefficient of substantially 1.0. Preferably exhaust gas is re-circulated to the combustion chambers to comprise greater than 25% by mass of the gas in the chamber under lean modes of operation and preferably no greater than 40%. The amount of exhaust gas increasing as the air fuel ratio gets leaner. Exhaust gas may also be re-circulated to the combustion chambers in stoichiometric modes of operation, however dual injection of fuel, as detailed further herein, is preferably employ
Engine operation is preferred in either one of these major modes of operation, however, a first transition mode (indicated by reference numeral B) may be required when transferring between stoichiometric mode C and lean mode A. A transitional peak mode (indicated by reference numeral D) may also be provided, and is used for specific high load operation for generally temporary operation using a fuel rich air-fuel ratio (stoichiometric coefficient less than 1).
During the lean mode operation A, the temperature of the exhaust gas at the entry to the main catalyst 208 is preferably in the range of 200 to 400 degrees Celsius. In stoichiometric operation C, the temperature of the exhaust gas at the entry to the main catalyst 208 is typically above 400 degrees Celsius. Conveniently, in this latter mode of operation, the engine can be controlled by way of a dual injection strategy such as that disclosed in the Applicants' International Patent Application No. PCT/AU98/01004, the contents of which are included herein by reference.
Control of the system can be performed in two different ways. Firstly, the mode of the engine can be controlled on the basis of the known or estimated temperature of the exhaust gas. In this case, a sensor 214 can provide information to the engine management system for the purposes of controlling the engine operation appropriately. Secondly, the temperature of the exhaust gas can be controlled to fit the mode of operation under which the engine is currently operating or is desired to operate. Exhaust gas temperature may be controlled, for example, by varying ignition timings from cycle to cycle (corresponding variations of fuelling level may also be required). Of course, a combination of these two methods of control can also be used.
The main catalytic converter 208 is a three way converter which catalytically treats hydrocarbons, carbon monoxide gases and nitrous oxides. The Applicant has found that a Pt—Rh—Ba catalytic converter is particularly useful, and specifically has found that the characteristics of a Johnson-Matthey development version D2681/JM370 provides especially good results. This catalytic converter has a ratio of Pt:Rh of 10:1 in the catalytically active part of the converter. The catalytic converter also has a small proportion of Ba therein.
It is believed that the operation of the engine 200 in mode A so as to promote exhaust gases with a lean air fuel ratio and relatively lower gas temperatures supports a selective NOx reduction process that is not typically supported by a normal 3 way catalyst. It is further believed that this selective NOx reduction process is further supported by the presence of a Pt rich catalytic converter, and perhaps still further by the presence of some Ba on the converter. This selective NOx reduction process promotes the reduction of NOx emissions down to the less harmiful components such as N2O, N2 and O2. Alternate theory suggests that the Ba may, at least in part, provide NOx adsorption capabilities, and may even act as a catalyst commonly referred to as a Lean NOx Trap (LNT) or Lean NOx Catalyst (LNC). This allows some of the NOx to be stored for conversion into less harmful emissions when the engine operates in mode C as described in greater detail herein.
In mode C, the engine 200 is controlled in such a way to take advantage of the high conversion efficiencies that the catalyst converter 208 can provide under stoichiometric operating conditions, these conditions being synonymous with higher exhaust gas temperatures and higher load operating by points.
The use of the close coupled catalytic converter 206 as illustrated in
In an alternate embodiment, a three way catalyst may be re-located from a close coupled position to an underbody position. An underbody position is a position remote from the engine bay and associated fire wall, and is typically between the ground and the underside of the floor of the vehicle. In this instance the three way catalyst is preferable located in a position adjacent a catalyst having NOx adsorbent properties, such as a catalyst having Ba as a constituent. Preferably, the catalyst having NOx adsorbent properties operates additionally as a three way catalyst. The three way catalyst that has been re-located to an underbody position is preferably located in a single canister together with the catalyst having NOx adsorbent properties. Preferably the three way catalyst is located at the inlet of the canister and the catalyst with NOx adsorbent properties is located at the outlet of the canister. Locating the three way catalyst adjacent the inlet of the canister allows the three way catalyst to be heated by the exhaust gasses emitted from the engine. This transfer of heat to the three way catalyst also serves to cool the exhaust before it flows through the catalyst with NOx adsorbent properties. In this way both the three way catalyst and the catalyst with NOx adsorbent properties are generally maintained within their respective windows of operational temperatures. Some control of the engine may be required to achieve this. Specifically control of variables such as fuel per cycle and ignition timing may also be implemented to maintain exhaust gas temperatures in a range sufficient to keep the catalysts in their operational temperature windows. As the three way catalyst is now located in an underbody position it is preferable that it is rapidly heated at starting of the engine. Such heating being commonly referred to as a light off strategy and may be achieved through use of a heating element such as a resistive heating element or by use of exhaust gases as detailed in the Applicants U.S. Pat. No. 5,655,365 or any other suitable means. It has been found that optimum results may be achieved by location of the underbody catalyst a distance of between 1.0 m and 1.5 m along the exhaust system from the engine.
In a further embodiment, the three way catalyst and catalyst with NOx adsorbent properties form separate parts of the same three way catalyst brick. The catalyst with NOx adsorbent properties forming that part of the brick to which Ba is added.
With these arrangements, the catalyst with the NOx adsorbent properties may be regenerated by operating the engine with a stoichiometric air fuel ratio (note: regeneration of a NOx adsorbent catalyst is often referred to as “purging” the catalyst).
It is preferable that when operating the engine in mode A, ie lean mode, that the combustion chamber gas comprise 25% or more EGR by mass. EGR being an acronym for Exhaust Gas Re-circulation. EGR means re-circulation of some of the exhaust gasses into the inlet manifold of the engine and hence into the combustion chambers of the engine.
Preferably the combustion chamber gases comprise between 25% and 40% EGR by mass with the percentage of EGR increasing as the air fuel ratio increases (ie as the air fuel ratio gets more lean).
By maintaining the engine out NOx to a level of approximately twice the Euro III tail pipe emissions, the applicant has found the above referenced three way catalyst with NOx adsorbent properties to be particularly effective. It is believed that with PGM (precious group metals—ie Pt, Pd, Rh etc) loadings that are relatively standard for manifold port injected vehicles, engine out NOx emissions of between three and four times Euro III may be emitted whilst the catalyst will still be effective for meeting Euro III requirements. Such a catalyst having an engine swept volume (ESV) of less than 150% and preferably less than 110%. It is believed that engine out CO emissions should at the same time be in the order of three times or less Euro III emissions in order to meet Euro III emissions requirements. Further it is believed that the engine out HC emissions should be in the order of ten times or less Euro III emissions in order to meet Euro III emissions. Preferably the engine is calibrated across its speed load range so that its emissions do not to exceed these limits over a particular drive cycle. This may require that when the engine is operated in a lean mode that the air fuel ratio correspond with a lambda value no less than 1.3. More over as the lambda value increases, the ECR percentage should also generally increase to a limit of approximately 40%. In some circumstances, an air fuel ratio corresponding to a lambda of between 1.0 and 1.3 may be selected when transitioning between a lean air fuel ratio operating point and a stoichiometric air fuel ratio operating point.
Selection of whether a load point should be lean or stoichiometric, and if lean, the limit to which it can be lean is generally determined for an engine during calibration. A trade off between lean operation, power requirements, NOx levels and levels of other emissions will be required. However, to meet Euro III and Euro IV requirements at least, it is believed that lean operating points should be calibrated to have ISNOx (Indicated Specific NOx) emissions levels in the range between 0.7 and 2.0 grams per kilowatt hour in order for a three way catalyst with some NOx adsorbent properties to be utilised. It is believed that by calibrating the engine so that the emissions are maintained in the above bounds that PGM loadings similar to current MPI vehicles may be utilised. Optimally, the catalyst may have a size of less than 150% ESV (engine swept volume) and preferably less than 110% ESV. This range of calibration points is believed to provide optimum operation of an engine capable of generating engine out NOx of approximately one and one half times Euro III levels, three times Euro III CO levels and ten times Euro III HC levels. Calibration with lower NOx levels may be possible, however a larger three way catalyst may be required and fuel consumption may also deteriorate. Hence it is believed that the above range of ISNOx in combination with an exhaust treatment system having a three way catalyst and a catalyst having some NOx adsorbent properties provides an optimum configuration for meeting Euro III and/or Euro IV emissions targets.
Selection between air fuel ratio and modes A, B, C and D is demonstrated with reference to the dual mode strategy detailed in
Returning to step 510, if the vehicle is in a high gear then the process moves to step 535 and the engine load threshold variable “E1” is set to F_High, being a high load value. The process then moves to step 540 where it is determined whether or not the engine Is currently operating with an air led mode or a fuel led mode. If it is operating with an air led mode then the process moves to step 530, otherwise the process moves to step 550 where the engine load threshold value is reduced by the high gear hysteresis number which defines a hysteresis band for transitioning from an air led mode to a fuel led mode (ie, a hysteresis for engine loads when moving from Mode C to Mode A) under high gear operating conditions, after which the process moves to step 530.
At step 530 the process determines whether or not the current engine load is greater than the current engine load threshold E1. If it is not, then the process moves to step 555 and a fuel led (or lean air fuel ratio) is identified and the engine operates in mode A.
If at step 530 the current engine load is greater than the current engine threshold value E1 then the process moves to step 565 and operation an air led mode is identified. The process then moves to step 570 where if the engine load is greater than engine threshold value E2 then the engine operates in Mode D, which is a mode with rich air fuel ratios. If however at step 570 the current engine load is identified as being less than E2 ten the process moves to step 580 which corresponds with Mode C, ie a stoichiometric air fuel ratio.
In preferred embodiments, an additional step 585 may be introduced intermediate step 570 and step 580. This step may determine whether or not the exhaust gas is within a predetermined range, such as range suitable for efficient operation of a catalyst with NOx adsorbent characteristics. If it is within this range, then the process may then operate at additional step 590 in Mode B.
In a further embodiment, the catalyst with NOx adsorbent properties may be regenerated at a sufficient rate when operating the engine with a stoichiometric air fuel ratio (ie lambda=1.0) that saturation of the catalyst can be avoided. This allows the engine to operate under typical driving conditions such that a NOx sensor may not be required. As such the air fuel ratio for engine load conditions may be selected independently of NOx stored on the catalyst or calculated as stored on the catalyst. This is because the engine load will typically dictate stoichiometric or rich operating conditions from time to time. As such, this intermittent operation at these lower air fuel ratios, as occurs under typical vehicle operating conditions, will often be sufficient to maintain the catalyst in a non-saturated state.
Alternately, the catalyst may be monitored, either directly by a NOx sensor or indirectly by some other means, such as an exhaust gas temperature sensor. Where it is monitored directly, the engine can be operated by selecting a stoichiometric air fuel ratio from time to time so as to ensure that the catalyst does not saturate. Such an arrangement having an advantage that the fuel economy is not greatly penalised as may be the case where the engine is operate with a rich air fuel ratio.
Indirect monitoring of the NOx stored on the catalyst may be achieved by a cumulative measure of NOx emitted from the engine. This may be achieved by monitoring the engine operating conditions over a period of time. For example the period of time that the engine has spent at various operating points. If it is known the amount of NOx that is likely to be emitted at these operating points then the amount of NOx can be estimated. These operating points may be identified as either one of engine speed or engine load or both. In these circumstances, the engine may be deliberately operated with a stoichiometric air fuel ratio, even though a lean air fuel ratio may be sufficient for current engine operating conditions, so as to regenerate the NOx adsorbent catalyst.
Alternate methods of estimating when to have stoichiometric excursion from a lean mode of operation so as to regenerate the catalyst may be employed. For example, the amount of time since a stoichiometric excusion last occurred or the amount of time since the engine last operated with a stoichiometric operating condition for a period of time to purge the catalyst of a significant proportion of the NOx adsorbed thereto.
The method according to the present invention is applicable to both two stroke and four stroke engines incorporating direct injection systems and particularly those operation with a dual fluid fuel injection system. Modifications and variations as would be deemed obvious to the person skilled in the art are included within the ambit of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| PQ2722 | Sep 1999 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU00/01064 | 9/8/2000 | WO | 00 | 6/13/2002 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO01/18374 | 3/15/2001 | WO | A |
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| Number | Date | Country |
|---|---|---|
| 19807203 | Aug 1999 | DE |
| 2000-87788 | Mar 2000 | JP |
| WO 9901648 | Jan 1999 | WO |
