EP1979596A1

EP1979596A1 - Method and apparatus for operating a methane-fuelled engine and treating exhaust gas with a methane oxidation catalyst

Info

Publication number: EP1979596A1
Application number: EP07701756A
Authority: EP
Inventors: Richard Ancimer; Mark Dunn; Jonathan Harris; Olivier Lebastard; D. Andrew Lew
Original assignee: Westport Power Inc
Current assignee: Westport Power Inc
Priority date: 2006-02-03
Filing date: 2007-02-01
Publication date: 2008-10-15
Also published as: CN101379279B; WO2007087725A1; CA2534031A1; CN101379279A; CA2534031C; EP1979596A4

Abstract

A method and apparatus is provided for operating a methane-fuelled engine in a lean burn operating mode or a stoichiometric operating mode. When a methane oxidation catalyst is employed to treat the exhaust gas from an engine running in a lean burn operating mode, the catalyst can be inhibited by SOx reducing the catalyst's methane conversion efficiency. When an engine is running in a stoichiometric operating mode, desulphation of the catalyst can occur, thereby restoring the catalyst's methane conversion efficiency. The disclosed method relates to fuelling the engine with a lean fuel mixture when operating the engine at one of a predetermined first set of points on an engine map, and fuelling the engine with a rich fuel mixture when operating the engine at one of a predetermined second set of points on said engine map. The exhaust gas from the engine flows through a methane oxidation catalyst, which is preferred embodiments is adapted to promote the reduction of NOx when the engine is running in the stoichiometric operating mode. The apparatus comprises means for detecting and controlling lambda, and an electronic controller programmed to operate the engine in one of a lean burn mode and a fuel-rich mode at respective predetermined points on an engine map.

Description

METHOD AND APPARATUS FOR OPERATINGA METHANE-FUELLED

ENGINE AND TREATINGEXHAUST GAS WITH A METHANE

OXIDATION CATALYST

Field of the Invention

[0001] The present invention relates to a method and apparatus for operating a methane-fuelled engine and treating exhaust gas with a methane oxidation catalyst. The invention can be applied to vehicle engines or other engines that are operated with variable load cycles to reduce emissions of carbon monoxide, methane and other unburned hydrocarbons.

Background of the Invention

[0002] Natural gas is comprised mostly of methane. Natural gas is burned as a fuel in internal combustion engines because in many markets around the world, natural gas is less expensive on an energy basis compared to diesel or gasoline. In addition, natural gas is cleaner burning compared to diesel or gasoline, which can help to improve air quality, providing another incentive to replace vehicle engines that burn diesel or gasoline with engines that burn natural gas. However, in the exhaust gases expelled from the combustion chambers of an internal combustion engine, there can remain unburned fuel, and the same is true for engines fuelled with natural gas. Because methane is a greenhouse gas, it is desirable to oxidize unburned methane before the exhaust gases leave a vehicle's exhaust pipe. [0003] There are a number of approaches for burning natural gas in an engine. So- called stoichiometric natural gas engines use an actual air to fuel ratio of about 14.6:1, which corresponds to a lambda of 1 , since lambda is calculated by dividing the actual air- fuel ratio by 14.6 (which is the theoretical stoichiometric ideal air- fuel ratio). As defined herein an engine operating in a stoichiometric mode need not necessarily operate with a lambda of exactly 1.0, but with a lambda that is at or near 1.0 whereby there is substantially no excess oxygen in the charge of air and fuel that is combusted in the engine's combustion chamber. U.S. Patent No. 5,131,224 discloses a method of reducing methane exhaust emissions from a natural gas fuelled engine. The '224 patent teaches operating a natural gas engine in a stoichiometric operating mode with an air- fuel mixture that is on average slightly fuel-rich (that is, with lambda being, on average, less than 1.0), and using a platinum or platinum-palladium (non-rhodium) catalytic converter for exhaust gas treatment. As is known in the prior art, emissions of NOx and methane can be reduced by using exhaust gas recirculation and a three- way catalyst, but these techniques add significantly to the overall cost of the system. [0004] So-called lean burn spark ignition ("LBSI") natural gas engines burn a lean mixture of natural gas with ignition triggered by a spark plug. A lean mixture means that there is a surplus of oxygen in the combustion chamber, so lambda is greater than 1. LBSI natural gas engines can be less complicated and less costly, while producing lower emissions of nitrogen oxide (NOx), compared to stoichiometric natural gas engines operating without a three-way catalyst and exhaust gas recirculation, because higher air- fuel ratios result in cooler combustion temperatures. LBSI natural gas engines can also produce lower emissions of carbon dioxide compared to stoichiometric natural gas engines. Accordingly, LBSI engines offer cost and performance advantages over stoichiometric natural gas engines that don't employ exhaust gas recirculation or three-way catalysts. However, a problem with LBSI natural gas engines is that methane oxidation catalysts can be inhibited by exposure to sulfur oxides (SOx), even at very low concentrations, resulting in deteriorating methane oxidation conversion efficiencies. It is believed that the SOx are chemisorbed onto the catalyst wash coat, effectively blocking the conversion site for methane. For example, experimental results have shown that a SOx concentration level of only 1 ppm (w/w) can result in a 25% reduction in methane conversion efficiency in less than 50 hours of operation, and such a concentration can be introduced into the engine exhaust gas from sulfur that is present in the natural gas and engine lubricating oil. For example, a contributor to SOx in the exhaust gas can be the odorant that is normally added to natural gas for olfactory detection. [0005] A report prepared by Engelhard Corporation for the Gas Research Institute, entitled "Catalyst Development for Methane Emissions Abatement From Lean Burn Natural Gas Vehicles" dated November 1997 reports on work done between January 1994 through May 1997, with the objective of developing a lean burn natural gas vehicle catalytic converter that will continue to oxidize methane over the lifetime of the vehicle. A catalyst containing palladium (Pd) such as Pd/alumina (Al₂O₃) was found to be the most active catalyst for methane emissions abatement from natural gas engines. In its final report, Engelhard Corporation reported that it failed to achieve its objective because the presence of SOx in the engine's exhaust gas inhibited catalyst activity. Engelhard Corporation's research focused on developing a sulfur-resistant catalyst and investigating the de-activation mechanism of palladium oxidation catalysts under laboratory conditions that closely resemble actual natural gas vehicle exhaust.

[0006] Published SAE technical paper 961971, entitled, "Methane Emissions Abatement from Lean Burn Natural Gas Vehicle Exhaust: Sulfur' s Impact on Catalyst Performance" is authored by employees of Engelhard Corporation. On page 18 of this SAE technical paper the authors disclose a strategy for periodic thermal- reduction regeneration so that the catalyst maintains acceptable activity in the presence of sulfur. Specifically, the disclosed strategy involves operating a natural gas engine to cycle the catalyst between a lean, high space velocity mode lasting 14.5 minutes and a rich, low space velocity mode for 30 seconds. The catalyst temperature increases from 550 degrees Celsius during the lean mode to approximately 650 degrees Celsius during the rich mode. The authors concluded from their experimental results that this strategy only delays the eventual decay in catalyst methane activity. [0007] Published Japanese patent application number JP20002000058777 (publication number JP2003254117A2), entitled "Exhaust Emission Controlling Method" (the '58777 Application), like SAE technical paper 961971, discloses a method of alternately burning the fuel in a lean atmosphere of excessive air and a rich atmosphere of excessive fuel. The exhaust gas from burning the fuel in a rich atmosphere subjects the inhibited catalyst sites to a reducing atmosphere that regenerates the inhibited catalyst sites thereby recovering methane oxidizing catalyst activity and restoring methane oxidization conversion rates. According to the '58777 Application, the timing for regenerating the inhibited catalyst sites can be determined by calculating when the catalyst's methane conversion efficiency has deteriorated by a predetermined amount, with this calculation being based upon a number of parameters including sulfur concentration in the exhaust gas, air-fuel ratio, and exhaust gas temperature. A problem with this approach is that it adds complexity to the control of the engine, since this approach involves calculating the timing for desulphation of the methane oxidation catalyst, and two control strategies for operating the engine in a lean burn or fuel rich mode, since the timing for operating in a fuel rich mode is dependent on a calculated timing which can occur at any point on the engine map. [0008] Accordingly, there is a need for an improved control strategy for operating a methane- fuelled engine to simply and effectively achieve some of lower emissions benefits associated with LBSI methane-fuelled engines, while reducing emissions of unburned methane by managing the performance and desulphation of a methane oxidation catalyst.

Summary of the Invention

[0009] A method is provided for operating a methane- fuelled engine and of treating exhaust gas coming from the engine to reduce emissions of methane and nitrogen oxides. In preferred embodiments of the method is engine is fuelled with natural gas. The disclosed method comprises fuelling the engine with a lean fuel mixture when operating the engine at one of a predetermined first set of points on an engine map; fuelling the engine with a rich fuel mixture when operating the engine at one of a predetermined second set of points on the engine map; and flowing an exhaust gas from the engine through a methane oxidation catalyst. [0010] For example, the predetermined second set of points on the engine map can be associated with when the engine is operating within a predetermined high engine speed range and within a predetermined low engine load range. In a preferred method, the predetermined low engine load range is defined by when the engine is operating at less than 20 percent of maximum engine load and the predetermined high engine speed range is defined by when the engine is operating with a speed that is at least 80 percent of maximum engine speed. Preferably the predetermined second set of points on the engine map are associated with when the engine is operating within a predetermined engine speed range and within a predetermined engine load range in which the engine can be, and the engine is controlled to be, operated in a stoichiometric operating mode with an exhaust gas temperature between the engine and the methane oxidation catalyst of at least 600 degrees Celsius and more preferably a temperature between 650 and 800 degrees Celsius. In one embodiment the predetermined engine load range for operating with a rich fuel mixture corresponds to when inlet manifold pressure is less than about 85 kPa absolute (about 12 psia). [0011] When the engine is operating in a stoichiometric operating mode, the rich fuel mixture can be produced by reducing the air mass flow rate through the intake air manifold and into the engine's combustion chamber. In some embodiments the air mass flow rate can be reduced by throttling air flow through the intake air passage. Methane- fuelled engines equipped with a turbocharger can reduce air mass flow rate by opening a wastegate valve so that some of the exhaust gas is by-passed around the turbocharger' s turbine. If the turbocharger is a variable geometry turbocharger or a variable nozzle turbocharger, the respective geometry or nozzle can be controlled to reduce air mass flow rate.

[0012] In a preferred method, when the engine is operated with a lean fuel mixture, the average lambda of the charge formed in the combustion chamber is at least 1.3, and preferably between 1.3 and 1.7. The method can further comprise fuelling the methane-fuelled engine with a fuel mixture comprising methane and hydrogen, and controlling the lean fuel mixture to have an average lambda between 1.3 and 2.0. When the engine is operated with a rich fuel mixture, the average lambda of the charge formed in the combustion chamber is less than or equal to 1.0, and preferably between 0.95 and 1.0. [0013] The preferred method further comprises activating a spark plug to promote ignition of the lean and rich fuel mixtures and advancing timing for activating the spark plug to an earlier time when fuelling the engine with the lean fuel mixture. [0014] The method preferably comprises oxidizing said methane with in the exhaust gas in the presence of palladium provided by the methane oxidation catalyst. In one embodiment the palladium can be impregnated into a washcoat comprising alumina, and the washcoat can be deposited on a ceramic support comprising silicon carbide or magnesium aluminum silicate (known as "Cordierite"). In other embodiments the method can comprise depositing the washcoat on a metallic support. [0015] hi preferred methods the methane oxidation catalyst promotes the oxidation of methane when the engine is fuelled with a lean fuel mixture and promotes the W

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reduction of nitrogen oxides to nitrogen when the engine is fuelled with a rich fuel mixture.

[0016] The method can further comprise commanding the engine to operate at a point that is one of the predetermined second set of points on the engine map for a predetermined time as a step in the start-up sequence for the engine. If the engine is a prime mover for a vehicle or machine, the controller can be programmed to recognize predefined conditions when the vehicle or machine is stationary, and to command the engine to operate at a point that is one of the predetermined second set of points on the engine map for the lesser of a predetermined time or until the vehicle is no longer stationary.

[0017] A methane-fuelled engine is disclosed that comprises an intake air manifold defining a passage through which air can flow into a combustion chamber of the engine; a fuel metering valve operable to regulate the mass flow rate of a fuel comprising methane that is introduced into the combustion chamber through a fuel supply pipe; a throttle disposed inside the intake air manifold for regulating the mass flow rate of air that is introduced into the combustion chamber; an exhaust manifold defining a passage that is in communication with the combustion chamber for receiving combustion products from the combustion chamber and directing the combustion products to an exhaust pipe; a methane oxidation catalyst disposed in the exhaust pipe; at least one sensor associated with the methane-fuelled engine for calculating or measuring lambda in the exhaust manifold or in the intake air manifold; and an electronic controller programmed to operate the engine in one of a lean burn mode and a fuel-rich mode at respective predetermined points on an engine map. [0018] In a preferred embodiment the at least one sensor is a lambda sensor with a sensing probe disposed in the exhaust manifold or the exhaust pipe upstream from the methane oxidation catalyst, and the lambda sensor is operable to send signals representative of the measured lambda value to the electronic controller. [0019] In another preferred embodiment the at least one sensor comprises a first mass flow sensor associated with the intake air manifold and a second mass flow sensor associated with the fuel supply pipe, and the first and second mass flow sensors are operable to send signals representative of the respective air and fuel mass flow rates to the electronic controller, and the electronic controller is programmable to calculate lambda of a charge formed in the combustion chamber.

[0020] In yet another preferred embodiment, the at least one sensor comprises a first temperature sensor associated with the intake air manifold; a first pressure sensor associated with the intake air manifold; a second temperature sensor associated with the fuel supply pipe; a second pressure sensor associated with the fuel supply pipe; and the electronic controller is programmable to process data collected from the first and second temperature sensor and the first and second pressure sensor to calculate lambda of a charge formed in the combustion chamber. [0021] In the preferred apparatus, the methane oxidation catalyst comprises palladium. The palladium can be impregnated in a washcoat comprising alumina and the washcoat can be deposited on a ceramic support comprising silicon carbide or magnesium aluminum silicate. In another embodiment the washcoat can be deposited on a metallic support. [0022] The methane oxidation catalyst consists of a number of components.

Generally, it is preferred that the methane oxidation catalyst comprises at least one catalytically active component, namely a noble metal selected from the group consisting of palladium, platinum and rhodium. The methane oxidation catalyst preferably further comprises at least one oxygen storage component selected from the group consisting of cerium oxide (known as "Ceria"), and a combination of cerium and zirconium. In addition, the methane oxidation catalyst preferably further comprises a scavenger for hydrogen sulfide. The methane oxidation catalyst can be a three-way catalyst of the type that has been developed for automotive applications, though such catalyst formulations have not been developed for use with lean burn methane-fuelled engines.

[0023] In a preferred embodiment, the methane oxidation catalyst comprises palladium for oxidizing methane when the engine is operating in a lean burn mode, and also rhodium for reducing nitrogen oxides to nitrogen when the engine is operating in a stoichiometric mode. The methane oxidation catalyst can further comprise cerium oxide to serve as an oxygen storage component, and a scavenger for hydrogen sulfide. [0024] The fuel can be introduced into the intake air manifold of the engine in a number of ways. For example, the engine can comprise a port fuel injection valve for introducing the fuel into an intake port between the intake air manifold and the combustion chamber. In another embodiment, a sparger can be employed for introducing the fuel from the fuel supply pipe into the intake air manifold upstream from a throttle. Instead of a sparger, the engine can comprise a plurality of ports provided in a wall of the intake air manifold upstream from the throttle, wherein the fuel from the fuel supply pipe is flowable from a plenum through the plurality of ports into the intake air manifold. An advantage of this arrangement over a sparger is that it does not block any of the flow area through the intake air manifold.

[0025] In yet another embodiment, the engine can comprise a fuel injection valve with a nozzle disposed in the combustion chamber for introducing the fuel directly into the combustion chamber. The fuel can be introduced into the combustion chamber during the intake stroke or early in the compression stroke while the combustion chamber pressure is still relatively low.

[0026] The disclosed invention has advantages over the prior art. Compared to a stoichiometric natural gas fuelled engine or an engine that operates with an even richer fuel mixture as taught by the '244 patent, the presently disclosed engine and method can operate most of the time in a lean burn mode, to achieve lower emissions of NOx and carbon dioxide, while operating only with a rich fuel mixture when the engine is operating in a predetermined region of the engine map. The authors of SAE technical paper 961971 described a method of operating in a lean burn mode for 14.5 minutes and a fuel rich mode for 30 seconds, but their conclusion was that this approach only delayed the eventual decay in catalyst performance. Unlike the approach taught in SAE technical paper 981971, with the presently disclosed method and apparatus experimental results show that methane oxidation catalyst performance can be restored to methane conversion rates of 85% to 90% at maximum fuelling. The presently disclosed method is also simpler to implement than a timed periodic catalyst desulphation cycle because two parallel engine control strategies are not required for lean burn and fuel rich modes. With a timed desulphation cycle, the engine could be operating under conditions anywhere on the engine map when it is time for a desulphation cycle, so parallel control strategies are needed for every operating point on the engine map. With the presently disclosed method and apparatus, the engine only operates in a fuel rich mode when the engine is operating in a predetermined region of the engine map. Japanese patent application number JP20002000058777 also disclosed a method of periodically switching between lean burn and fuel rich operating modes, but this method has the same disadvantages as the method taught by SAE technical paper 981971.

[0027] Accordingly, the presently disclosed method and apparatus provides an approach for operating a methane-fuelled engine that can reduce emissions of NOx and carbon dioxide compared to a stoichiometric methane- fuelled engine.

Experimental results have shown that the disclosed method and apparatus can allow a methane- fuelled engine to operate most of the time in a lean burn mode while maintaining the conversion efficiency of a methane oxidation catalyst by operating in a fuel rich mode when operating in a predetermined region of the engine map.

Brief Description of the Drawings

[0028] Figure 1 is a schematic diagram of an apparatus for practicing the disclosed method. The apparatus comprises a fuel injection system, an engine combustion chamber, a catalytic converter, and an electronic controller. In this embodiment a port injector is employed to introduce fuel into the intake port and a lambda sensor disposed in the exhaust pipe upstream from the methane oxidation catalyst is employed to control the air/fuel ratio of the charge that is delivered to the combustion chamber. [0029] Figure 2 is a schematic diagram of a second embodiment of an apparatus for practicing the disclosed method. The apparatus of Figure 2 is like the apparatus of Figure 1 except that a sparger is employed to introduce fuel into the intake air manifold and mass flow sensors for the intake air and the fuel are employed instead of a lambda sensor to control the air/fuel ratio of the charge that is delivered to the combustion chamber. [0030] Figure 3 is a schematic diagram of a third embodiment of an apparatus for practicing the disclosed method. The apparatus of Figure 3 is like the apparatuses of Figures 1 and 2 except that fUel is introducible into the intake air manifold through a plurality of ports and temperature and pressure sensors are employed measure the pressure and temperature of the intake air and the fuel, so that the electronic controller can calculate the mass flow of air and fuel and thereby control the air/fuel ratio of the charge that is delivered to the combustion chamber. Figure 3 also shows an embodiment with a turbocharging system that comprises a turbocharger and a wastegate. Engines that have a turbocharging system can use it to control the air/fuel ratio of the charge that is delivered to the combustion chamber. [0031] Figure 4 is an engine map showing a region on the map where the engine can be operated with a lean fuel mixture and a region where the engine can be operated with a rich fuel mixture.

[0032] Figure 5 is a plot of lambda against engine load for different engine speeds. A plurality of lines are plotted with each line corresponding to a different engine speed. This plot shows by example an application of the method showing how an engine can be run under lean burn conditions at most engine speeds and engine loads and that a target lambda can be determined as a function of engine speed and load. [0033] Figure 6 is a plot of methane concentration against time, showing the regenerative effect on the methane oxidation catalyst by application of the disclosed method. The plotted data is from experimental results that show the effectiveness of the disclosed method in regenerating a methane oxidation catalyst.

Detailed Description of Preferred Embodiment(s)

[0034] With reference to the figures, like-named components with like reference numbers separated by multiples of one hundred refer to like components in different embodiments.

[0035] Figure 1 is a schematic view of apparatus 100 for practicing the disclosed method. The apparatus comprises fuel injection system 110, engine combustion chamber 120, catalytic converter 140, and electronic controller 150. In this embodiment fuel injection system 110 comprises port injector 112, which introduces fuel from fuel supply pipe 114 into intake port 116. Air flows into combustion chamber 120 through intake air manifold 118, and throttle 119 regulates the flow of air through intake air manifold 118. Combustion chamber 120 is defined by cylinder block 122, piston 124, and cylinder head 126. The engine can comprise spark plug 128 for triggering ignition of the charge inside combustion chamber 120. As is well- known for internal combustion engines, intake valve 130 is operable to control the flow of the air and fuel mixture into combustion chamber 120 from intake air manifold 118, and exhaust valve 132 is operable to control the flow of combustion products from combustion chamber 120 to exhaust manifold 134. Exhaust gas flows from exhaust manifold 134 to a methane oxidation catalyst provided inside catalytic converter 140. From catalytic converter 140, the exhaust gas flows to engine exhaust pipe 142.

[0036] In the embodiment illustrated by Figure 1, lambda sensor 152 is disposed in exhaust manifold 134 upstream from the methane oxidation catalyst. Lambda measurements from sensor 152 are employed to control the air/fuel ratio of the charge that is delivered to the combustion chamber. According to the disclosed invention, with data received from lambda sensor 152, electronic controller 150 is programmable to operate the engine in a lean burn mode under most operating conditions with the charge delivered to combustion chamber 120 having an average lambda greater than 1.1. Under certain predetermined operating conditions that are predefined on an engine map, such as a predefined range of engine speed and a predefined range of engine torque, the engine is controlled to operate with a richer air-fuel mixture, with the charge delivered to combustion chamber 120 having a lambda less than or equal to about 1.0 and preferably between 0.95 and 1.0. In the embodiment of Figure 1 , electronic controller 150 processes the measurement of lambda from lambda sensor 152 and determines from an engine map what the target lambda is given the current values for engine speed and engine torque. In this embodiment, the lambda sensor is located in exhaust manifold 134 because it is difficult to measure lambda inside the combustion chamber, and with a fuel injection system that uses port injectors it is difficult to measure lambda upstream from the combustion chamber. However, it is possible for electronic controller 150 to determine lambda inside combustion chamber 120 from measurements of lambda taken in exhaust manifold 134 or at any point in an exhaust pipe upstream from catalytic converter 140. Accordingly, using the measurement of lambda taken by sensor 152, electronic controller 150 can use throttle 1 19 and port injector 112 to control lambda according to an engine map so that the engine is operated in a lean burn mode most of the time (with an average lambda between 1.3 and 1.7), and in a stoichiometric mode (with a lambda less than or equal to about 1.0 and preferably between 0.95 and 1.0) in predetermined areas of the engine map, and by this method, the methane oxidation catalyst is periodically regenerated to drive sulfur from the catalyst's active sites and to restore methane conversion efficiency. [0037] Figure 2 is a schematic diagram of a second embodiment of an apparatus that can be used to practice the disclosed method. Apparatus 200 is like apparatus 100 except that sparger 212 is employed to introduce fuel into the intake air manifold and mass flow sensors 254 and 256 for respectively measuring the mass flow of intake air and fuel are employed instead of a lambda sensor to control the air/fuel ratio of the charge that is delivered to the combustion chamber. [0038] Apparatus 200 comprises fuel injection system 210, engine combustion chamber 220, catalytic converter 240, and electronic controller 250. Sparger 212 introduces fuel from fuel supply pipe 214 into intake air manifold 218 upstream from throttle 219. The fuel and air are mixed inside intake manifold 218 prior to being introduced into combustion chamber 220. Throttle 219 regulates the flow of the air- fuel mixture into combustion chamber 220. Combustion chamber 220 is defined by cylinder block 222, piston 224, and cylinder head 226. The engine can comprise spark plug 228 for triggering ignition of the charge inside combustion chamber 220. Apparatus 200 further comprises intake valve 230, which is operable to admit the air and fuel mixture into combustion chamber 120, and exhaust valve 232, which is operable to allow combustion products to flow from combustion chamber 220 to exhaust manifold 234. Exhaust gas flows from exhaust manifold 234 to a methane oxidation catalyst provided inside catalytic converter 240. From catalytic converter 240, the exhaust gas flows to engine exhaust pipe 242. [0039] Apparatus 200 can be employed to practice the subject method in the same manner as apparatus 100 except that the manner of determining lambda in the combustion chamber is different. Instead of using lambda measurements, controller 250 uses mass flow measurements from sensors 254 and 256 to determine lambda. Then electronic controller can operate fuel metering valve 215 and throttle 219 to make lambda greater than 1.1 to operate in a lean combustion mode under most conditions, and to make lambda less than or equal to about 1.0 and preferably between 0.95 and 1.0 in a predetermined area on the engine map.

[0040] Figure 3 is a schematic diagram of a third embodiment of an apparatus that can be used for practicing the disclosed method. Apparatus 300 uses a ring of ports 312 that open onto intake air manifold 318 from fuel supply pipe 314, and temperature and pressure sensors are employed to measure the pressure and temperature of the intake air and the fuel, so that the electronic controller can calculate the mass flow of air and fuel and thereby control the air/fuel ratio of the charge that is delivered to the combustion chamber.

[0041] Apparatus 300 comprises fuel injection system 310, engine combustion chamber 320, catalytic converter 340, and electronic controller 350. An annular plenum supplies fuel to ports 212 and fuel flows into intake air manifold 318 upstream from throttle 319. Like apparatus 200, some mixing of the fuel and air can occur in intake air manifold 318 prior to the air-fuel mixture being introduced into combustion chamber 320 to form a charge therein. Throttle 319 can be used to regulate the flow of the air- fuel mixture into combustion chamber 320. [0042] Combustion chamber 320 is defined by cylinder block 322, piston 324, and cylinder head 326. The engine can comprise spark plug 328 for triggering ignition of the charge inside combustion chamber 320. Apparatus 300 further comprises intake valve 330, which is operable to admit the air and fuel mixture into combustion chamber 320, and exhaust valve 332, which is operable to allow combustion products to flow from combustion chamber 320 to exhaust manifold 334.

[0043] In this embodiment, exhaust gas flowing from exhaust manifold 334 can be directed to a turbine of turbocharger 338 or can be directed to by-pass the turbine by operation of wastegate valve 336. Air intake 316 directs the intake air to turbocharger 338 and then to intake air manifold 318 through air passage 317. As is known to persons skilled in engine technology, turbocharger 338 is driven by exhaust gas and can be employed to boost the pressure of the intake air. After the exhaust gas exits the turbine, exhaust pipe 339 directs the exhaust gas to catalytic converter 340. When wastegate valve 336 is open, exhaust gas flows through exhaust pipe 337 to catalytic converter 340. The turbine of turbocharger 338 can be a variable geometry turbine or a variable nozzle turbine. By controlling the wastegate to control how much exhaust gas flows through the turbine and/or by controlling the variable geometry or nozzle of the turbine the mass airflow through intake air manifold 118 is controlled, whereby lambda can be controlled in cooperation with fuel metering valve 315. That is, one can control lambda by controlling fuel metering valve 315 and throttle 119, in combination with one of, wastegate 336, or the variable geometry or the variable nozzle turbine of turbocharger 338, or wastegate 336 and the variable geometry or the variable nozzle turbine of turbocharger 338.

[0044] Like the other embodiments, apparatus 300 can be employed to practice the subject method. Electronic controller 350 can calculate air mass flow rate from measurements taken by air temperature sensor 358 and air pressure sensor 360. Similarly, electronic controller 350 can calculate fuel mass flow rate from measurements taken by fuel temperature sensor 362 and fuel pressure sensor 364. Then electronic controller 350 can make reference to an engine map to determined the desired operating mode (lean burn or stoichiometric) based upon current engine operating conditions. When the engine is operating in a predetermined area of the engine map, electronic controller 350 controls fuel metering valve 315 and throttle 319 to deliver a richer fuel mixture to the combustion chamber, preferably with a lambda between 0.95 and 1.0. When the engine is operating at a point outside the predetermined area for operating with a richer fuel mixture, electronic controller 350 controls fuel metering valve 315 and throttle 319 to deliver a lean fuel mixture to the combustion chamber, preferably with an average lambda between 1.1 and 1.7.

[0045] Figures 1 through 3 are illustrative of different embodiments of an apparatus for practicing the disclosed method. Persons skilled in the technology will understand that variations can be made to the illustrated embodiments without departing from the spirit and scope of the disclosed apparatus. For example, the ring of ports 312 shown in Figure 3 for introducing the fuel into the intake manifold can be substituted for sparger 212 in Figure 2 with the same effect. In other variations, the lambda sensor of Figure 1 can be substituted for the sensors of the illustrated embodiments of Figures 2 and 3, or the fuel mass flow sensor 256 of Figure 2 can be substituted for pressure and temperature sensors 362 and 364 in the embodiment of Figure 3. An important feature of the disclosed apparatus is at least one sensor associated with the methane- fuelled engine that measures a parameter that can be used by the electronic engine controller to calculate or measure lambda directly in the exhaust manifold or in the intake air manifold, and the combination of such one or more sensors with an electronic engine controller that can command and control the engine to run in a lean burn mode under most operating conditions, and with a richer fuel mixture, closer to stoichiometric, with lambda less than or equal to about 1.0, under predetermined conditions defined by an engine map.

[0046] Figure 4 is an illustration of an engine map with engine torque on the y-axis and engine speed on the x-axis. Line 400 defines the upper torque limit for the engine so that the space between line 400 and the x-axis is the operating range of the engine. The axes in Figure 4 do not show units because this figure illustrates the approach taught by the presently disclosed method, which can be applied to the engine map of any engine. According to the method, the engine is controlled to operate in a lean burn mode under most operating conditions, and in a stoichiometric mode under certain predetermined operating conditions. In figure 4, area 401 is the space on the illustrated engine map where the engine is controlled to operate in a lean burn mode. That is, when the engine torque and the engine speed defines a point on the engine map in area 401 , an engine controller controls the air to fuel ratio in the intake air manifold to deliver a charge to the engine's combustion chambers with an average lambda greater than 1.1. Line 402 defines the boundary of area 403 , which is a predetermined region on the engine map where the engine is controlled to operate with a richer fuel mixture that is closer to stoichiometric. That is, when the engine torque and the engine speed defines a point on the engine map in area 403, an engine controller controls the air to fuel ratio to deliver a charge to the engine's combustion chambers with an average lambda between about 0.95 and 1.0. [0047] In Figure 4, the predetermined region for burning a richer fuel mixture that is defined by line 402 is somewhat arbitrary. This predetermined region could be anywhere on the engine map, but it is preferably in a region that is utilized frequently while representing a minor proportion of the anticipated normal operating cycle, so that the engine operates mostly in a lean burn mode. In addition, as is described in more detail below, with respect to Figure 6, for reducing the time for oxidizing the catalyst at the end of the desulphation cycle, it is preferred to choose an area on the engine map for stoichiometric operation where the exhaust gas temperature is| normally higher than 600 degrees Celsius, and more preferably above 650 degrees Celsius when the engine operates in the stoichiometric operating mode. [0048] Figure 5 shows a plot of lambda against engine load for different engine speeds that could be used to implement the engine map illustrated by Figure 4. The legend on the right hand side of Figure 5 sets out the unique symbols that are employed to plot lines corresponding to different engine operating speeds in rpm. In accordance with the disclosed method, for most points on the engine operating map, the engine is operated in a lean burn mode with an average lambda greater than 1.1. The desired lambda programmed into the engine map changes with engine speed, increasing gradually as engine load increases, with the desired lambda being generally higher for higher engine speeds. The exception to this pattern is when the engine is operating in the predefined engine speed and load range when the engine operates in a "stoichiometric" operating mode which in this example is when lambda is less than or equal to 1.0, or when the engine is transitioning from the stoichiometric operating mode to the normal lean burn operating mode. In the example of Figure 5, for the stoichiometric operating mode the predefined engine speed range is between 2500 and 2800 rpm, and the predefined engine load range is from zero to about 20 percent of the maximum load. When the engine speed is 3000 rpm and engine load is less than 50% of maximum engine load, lambda is commanded to values that help with the transition from the stoichiometric operating mode to the lean burn operating mode, and vice versa.

[0049] Following a set of predetermined values for lambda, determined as a function of engine load and engine speed, the disclosed method can be employed to operate an engine under lean burn conditions most of the time to reduce engine emissions, with periodic desulphation of the methane oxidation catalyst occurring automatically when under predefined conditions the engine runs in a stoichiometric operating mode. [0050] Figure 6 is a plot of experimental data, which shows the effectiveness of the disclosed method in regenerating a methane oxidation catalyst that has been inhibited by the adsorption of SOx. The data was collected from a Cummins™ 5.9 liter engine fuelled with natural gas. The methane oxidation catalyst comprised a catalytic washcoat of alumina impregnated with palladium deposited on a support comprising magnesium aluminum silicate. In other embodiments the support could be metallic, or silicon carbide. Figure 6 plots methane concentration in the exhaust gas downstream from the methane oxidation catalyst with the concentration measured in units of parts per million (ppm). The data plotted in Figure 6 illustrates the effect on methane concentration of switching from a lean burn operating mode to a stoichiometric operating mode. On the left hand side, the engine is running in a lean burn operating mode and methane concentration is about 675 ppm. In the illustrated data, the conversion efficiency of the methane oxidation catalyst has been allowed to decline significantly to better demonstrate the regenerative capabilities of the disclosed method. That is, on the left hand side of the graph, the methane conversion efficiency of the catalyst is approximately 65% and catalytic activity is severely inhibited by adsorption of SOx to the active sites of the methane oxidation catalyst. After desulphation of the methane oxidation catalyst by the disclosed method, the methane conversion efficiency was restored to 85-90%. By practicing the disclosed method, desulphation occurs whenever the engine is operated in the part of the engine map where the engine controller is programmed to operate the engine in a stoichiometric operating mode and depending upon the size of the stoichiometric region on the engine map and the normal operating cycle of the engine, desulphation can be made to occur with enough frequency that the methane oxidation catalyst is prevented from being inhibited to the degree shown in Figure 6, and less time is required to restore methane conversion efficiency to 85-90%. [0051] In Figure 6, as indicated by the legend, three sets of data are plotted against the same time scale. Methane concentration measured in parts per million (ppm) and engine torque measured in Nm share the same scale on the left-hand y-axis with units from zero to one thousand. Engine speed measured in revolutions per minute (rpm) use the scale on the right-hand y-axis with units from zero to three thousand. At about 1220 seconds on the depicted time scale the engine operating mode was switched from a lean burn mode with a lambda between 1.4 and 1.5, to a stoichiometric mode with an average lambda of around 1. In the tests performed to collect this data, the engine speed was kept constant until the end of the desulphation cycle but the engine load was reduced, as shown by the plotted engine speed and engine torque. This simulates switching to a stoichiometric operating mode at a predetermined region of the engine map where engine speed is kept at about 2800 rpm, and engine load is well less than 20 percent of maximum load. There are some transient effects associated with the change in engine load and the warming of the methane oxidation catalyst to a higher temperature that helps with desulphation. In the plotted data for methane concentration because of the location of the methane sensor downstream from the methane oxidation catalyst there can be a lag between the measured methane concentration and the methane concentration in the exhaust stream that is leaving the catalytic converter that houses the methane oxidation catalyst. [0052] Figure 6 shows that with the disclosed method, regeneration of the methane oxidation catalyst by desulphation can occur in a very short period of time. Desulphation occurs when the excess methane in the exhaust gas is converted to carbon monoxide and hydrogen, and the hydrogen strips the SOx from the methane oxidation catalyst by reacting with the sulfur to produce H₂S and H₂SO₄. In the conducted experiments the time period for operating in the stoichiometric operating mode is about 60 seconds but the methane oxidation catalyst desulphation is completed after about 40 seconds. In the experiments, the engine is operated in the stoichiometric operating mode longer than was needed for the desulphation to occur because the switching was done manually, and by operating the engine beyond the time needed for desulphation, it was confirmed that desulphation was completed to the fullest extent possible. That is, the increase in methane concentration at around the 1260 second mark indicates that desulphation is complete. When the engine was switched back to the lean burn operating mode and the transient effects had subsided, the measured methane concentration confirmed that the methane conversion efficiency was restored to 85-90%.

[0053] Though not shown on Figure 6, the temperature of the exhaust gas directed to the methane oxidation catalyst increases when the engine is switched to a stoichiometric operating mode. In the testing that produced the data shown in Figure 6, in the lean burn operating mode preceding the 1220 second mark, even with a higher engine torque relative to the desulphation time period, the temperature of the exhaust gas entering the catalytic converter is about 645 degrees Celsius. When the engine switches to a stoichiometric operating mode, the engine torque is much lower but the temperature of the exhaust gas entering the catalytic converter increases to about 700 degrees Celsius. Exhaust gas temperature is preferably kept below 800 degrees Celsius because temperatures higher than that can damage the methane oxidation catalyst. When the engine switches back to a lean burn operating mode after the 1275 second mark, the temperature of the exhaust gas entering the catalytic converter declines back to about 645 degrees Celsius. It is believed that desulphation of the methane oxidation catalyst is worse when the temperature of the methane oxidation catalyst is below 600 degrees Celsius and with the pre-determined region of the engine map for operating in a stoichiometric operating mode that was selected for the experimental data plotted in Figure 6, when operating in a stoichiometric operating mode with a high engine speed and low engine torque, the exhaust gas is at a desirable temperature for desulphation of the methane oxidation catalyst. [0054] It is believed that the methane oxidation catalyst is more effective in oxidizing methane when the catalyst is in an oxidized state, and that after the sulfur is driven from the active sites of the catalyst by the disclosed desulphation cycle, the methane oxidation catalyst is not oxidized. Accordingly, after running an engine is a stoichiometric operating mode and completing a desulphation cycle, when the engine switches back to a lean burn operating mode the methane conversion efficiency of the catalyst is not restored until the methane catalyst is oxidized, which explains some of the transient effects observed in the measured methane concentration after switching from a desulphation mode to a normal lean burn operating mode. [0055] Under normal operating conditions, in practicing the disclosed method, an engine may run under stoichiometric conditions at random intervals. For example when a vehicle is downshifting or coasting down a hill the engine may operate briefly in the region on the engine map that is designated for stoichiometric operation. The size of the predetermined engine speed and load ranges on the engine map can be selected based on the engine's anticipated operating cycle, to provide adequate time periods for automatically regenerating the methane oxidation catalyst. Whenever the engine operates in the predefined conditions associated with stoichiometric operating mode, the methane oxidation catalyst can be fully regenerated or partially regenerated, depending upon the length of time that the engine is operating in the stoichiometric operating mode.

[0056] Because the methane oxidation catalyst can be fully regenerated in a short period of time (about 40 seconds in the experimental example when the catalyst is severely inhibited), the start up sequence for the engine can optionally include an engine warm up sequence that includes desulphation of the methane oxidation catalyst so that the engine can begin each operating cycle by restoring the methane conversion efficiency to a higher level by regenerating the methane oxidation catalyst. That is, as part of the start-up sequence, the engine controller can be programmed to command the engine to operate for a preset time within the area of the engine map where the engine is operated in the stoichiometric operating mode. In another embodiment, the engine controller can be programmed to run in the stoichiometric operating mode at other predetermined times when a vehicle is stationary. For example, if the disclosed methane-fuelled engine is the prime mover for a garbage truck, when the truck is stationary and the engine controller recognizes that the operator has stopped the vehicle to pick up or dump out a load, the engine controller can command the engine to operate in the stoichiometric operating mode for a preset time or until the vehicle is commanded to move from its stationary position. In these examples (start-up and predetermined times when a vehicle is stationary), the same method is employed for desulphation of the methane oxidation catalyst. That is, the engine is operated in a stoichiometric operating mode within a predetermined area of the engine map. The only difference is that the engine controller can be programmed to operate the engine in that part of the map when the engine controller recognizes predetermined operating conditions, instead of when the engine operates in that part of the engine map during the normal course of operating the engine.

[0057] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

Claims

What is claimed is:

1. A method of operating a methane- fuelled engine and of treating exhaust gas coming from said engine to reduce emissions of methane and nitrogen oxides, said method comprising: fuelling said engine with a lean fuel mixture when operating said engine at one of a predetermined first set of points on an engine map; fuelling said engine with a rich fuel mixture when operating said engine at one of a predetermined second set of points on said engine map; and flowing an exhaust gas from said engine through a methane oxidation catalyst.

2. The method of claim 1 wherein said predetermined second set of points on said engine map are associated with when said engine is operating within a predetermined high engine speed range and within a predetermined low engine load range.

3. The method of claim 2 wherein said predetermined low engine load range is defined by when said engine is operating at less than 20 percent of maximum engine load.

4. The method of claim 2 wherein said predetermined high engine speed range is defined by when said engine is operating with a speed that is at least 80 percent of maximum engine speed.

5. The method of claim 1 wherein said predetermined second set of points on said engine map are associated with when said engine is operating within a predetermined engine speed range and within a predetermined engine load range.

6. The method of claim 5 wherein said predetermined engine speed range is between 2500 rpm and 2800 rpm.

7. The method of claim 5 wherein said predetermined engine load range corresponds to when inlet manifold pressure is less than about 85 kPa absolute (about 12 psia).

8. The method of claim 5 wherein said predetermined engine load range corresponds to when the temperature of exhaust gas exiting said combustion chamber is at least 600 degrees Celsius.

9. The method of claim 5 wherein said predetermined engine load range corresponds to when the temperature of exhaust gas exiting said combustion chamber is at least 650 degrees Celsius and less than 800 degrees Celsius.

10. The method of claim 1 wherein said rich fuel mixture is produced by reducing air mass flow rate to said engine.

11. The method of claim 10 wherein said air mass flow rate is reduced by throttling air flow through an intake air passage.

12. The method of claim 10 wherein said air mass flow rate is reduced by opening a wastegate valve when said methane-fuelled engine is equipped with a turbocharger.

13. The method of claim 10 wherein said air mass flow rate is reduced by controlling the geometry of a variable geometry turbocharger.

14. The method of claim 10 wherein said air mass flow rate is reduced by controlling a nozzle of a variable nozzle turbocharger.

15. The method of claim 1 wherein said lean fuel mixture has an average lambda of at least 1.3.

16. The method of claim 1 wherein said lean fuel mixture has an average lambda between 1.3 and 1.7.

17. The method of claim 1 further comprising fuelling said methane- fueled engine with a fuel mixture comprising methane and hydrogen, and controlling said lean fuel mixture to have an average lambda between 1.3 and 2.0.

18. The method of claim 1 wherein said rich fuel mixture has an average lambda less than or equal to 1.0.

19. The method of claim 1 wherein said rich fuel mixture has an average lambda between 0.90 and 1.0.

20. The method of claim 1 further comprising activating a spark plug to promote ignition of said lean and rich fuel mixtures and advancing timing for activating said spark plug to an earlier time when fuelling said engine with said lean fuel mixture.

21. The method of claim 1 further comprising controlling said engine to deliver said exhaust gas to said methane oxidation catalyst with a temperature that is greater than 600 degrees Celsius when operating said engine with said rich fuel mixture.

22. The method of claim 1 further comprising controlling said engine to deliver said exhaust gas to said methane oxidation catalyst with a temperature that is greater than 650 degrees Celsius and less than 800 degrees Celsius when operating said engine with said rich fuel mixture.

23. The method of claim 1 further comprising fuelling said engine with natural gas.

24. The method of claim 1 further comprising oxidizing methane in said exhaust gas in the presence of palladium provided by said methane oxidation catalyst.

25. The method of claim 24 further comprising impregnating said palladium into a washcoat comprising alumina.

26. The method of claim 25 further comprising depositing said washcoat on a ceramic support comprising silicon carbide or magnesium aluminum silicate.

27. The method of claim 25 further comprising depositing said washcoat on a metallic support.

28. The method of claim 1 wherein said methane oxidation catalyst promotes the oxidation of methane when said engine is fuelled with a lean fuel mixture and promotes the reduction of nitrogen oxides to nitrogen when said engine is fuelled with a rich fuel mixture.

29. The method of claim 1 wherein said engine is a prime mover for a vehicle or machine and a controller is programmed to recognize predefined conditions when said vehicle or machine is stationary and to command said engine to operate at a point that is one of said predetermined second set of points on said engine map for the lesser of a predetermined time or until said vehicle is no longer stationary.

30. The method of claim 1 further comprising commanding said engine to operate at a point that is one of said predetermined second set of points on said engine map for a predetermined time as a step in a start up sequence for said engine.

31. A methane- fuelled engine comprises: an intake air manifold defining a passage through which air can flow into a combustion chamber of said engine; a fuel metering valve operable to regulate mass flow rate of a fuel comprising methane that is introduced into said combustion chamber through a fuel supply pipe; a throttle disposed inside said intake air manifold for regulating mass flow rate of air that is introduced into said combustion chamber; an exhaust manifold defining a passage that is in communication with said combustion chamber for receiving combustion products from said combustion chamber and directing said combustion products to an exhaust pipe; a methane oxidation catalyst disposed in said exhaust pipe; at least one sensor associated with said methane-fuelled engine for calculating or measuring lambda in said exhaust manifold or in said intake air manifold; and an electronic controller programmed to operate said engine in one of a lean burn mode and a fuel-rich mode at respective predetermined points on an engine map.

32. The engine of claim 31 wherein said at least one sensor is a lambda sensor with a sensing probe disposed in said exhaust manifold or said exhaust pipe upstream from said methane oxidation catalyst, and said lambda sensor is operable to send signals representative of the measured lambda value to said electronic controller.

33. The engine of claim 31 wherein said at least one sensor comprises a first mass flow sensor associated with said intake air manifold and a second mass flow sensor associated with said fuel supply pipe, and said first and second mass flow sensors are operable to send signals representative of the respective air and fuel mass flow rates to said electronic controller, and said electronic controller is programmable to calculate lambda of a charge formed in said combustion chamber.

34. The engine of claim 31 wherein said at least one sensor comprises: a first temperature sensor associated with said intake air manifold; a first pressure sensor associated with said intake air manifold; a second temperature sensor associated with said fuel supply pipe; a second pressure sensor associated with said fuel supply pipe; and said electronic controller is programmable to process data collected from said first and second temperature sensor and said first and second pressure sensor to calculate lambda of a charge formed in said combustion chamber.

35. The engine of claim 31 wherein said methane oxidation catalyst comprises palladium.

36. The engine of claim 35 wherein said palladium is impregnated in a washcoat comprising alumina.

37. The engine of claim 36 wherein said washcoat is deposited on a ceramic support comprising silicon carbide or magnesium aluminum silicate.

38. The engine of claim 36 wherein said washcoat is deposited on a metallic support.

39. The engine of claim 35 wherein said methane oxidation catalyst further comprises rhodium.

40. The engine of claim 39 wherein said methane oxidation catalyst further comprises cerium oxide.

41. The engine of claim 40 wherein said methane oxidation catalyst further comprises a scavenger for hydrogen sulfide.

42. The engine of claim 31 where said methane oxidation catalyst is a three- way catalyst.

43. The engine of claim 31 wherein said methane oxidation catalyst comprises at least one noble metal selected from the group consisting of palladium, platinum, and rhodium.

44. The engine of claim 43 wherein said methane oxidation catalyst further comprises at least one oxygen storage component selected from the group consisting of cerium oxide and a combination of cerium and zirconium.

45. The engine of claim 44 wherein said methane oxidation catalyst further comprises a scavenger for hydrogen sulfide.

46. The engine of claim 31 further comprising a port fuel injection valve for introducing said fuel into an intake port between said intake air manifold and said combustion chamber.

47. The engine of claim 31 further comprising a sparger for introducing said fuel from said fuel supply pipe into said intake air manifold upstream from a throttle.

48. The engine of claim 31 further comprising a plurality of ports provided in a wall of said intake air manifold upstream from said throttle, wherein said fuel from said fuel supply pipe is flowable through said plurality of ports into said intake air manifold.

49. The engine of claim 31 further comprising a fuel injection valve with a nozzle disposed in said combustion chamber for introducing said fuel directly into said combustion chamber.