After-Treatment System and Method for Six-Stroke Combustion Cycle

Info

Publication number: 20140157758
Type: Application
Filed: Dec 12, 2012
Publication Date: Jun 12, 2014
Applicant: CATERPILLAR INC. (Peoria, IL)
Inventors: Ronald Silver (Peoria, IL), Scott B. Fiveland (Metamora, IL), D. Ryan Williams (Edwards, IL)
Application Number: 13/712,182

Abstract

A method and system of operating an internal combustion engine on a six-stroke cycle utilizes an after-treatment system to reduce emissions such as nitrogen oxides. The method and system introduce a first fuel charge to a combustion chamber and combusts the first fuel charge to produce a first stoichiometric lean condition. The method and system next introduce a second fuel charge and combust the second fuel charge to produce a second stoichiometric lean conditions. The exhaust gasses are then directed to a selective catalytic reduction catalyst with a reductant agent to reduce the nitrogen oxides to nitrogen and water.

Description

Description

TECHNICAL FIELD

This patent disclosure relates generally to internal combustion engines and, more particularly, to internal combustion engines that are configured to operate on a six-stroke internal combustion cycle.

BACKGROUND

Internal combustion engines operating on a six-stroke cycle are generally known in the art. In a six-stroke cycle, a piston reciprocally disposed in a cylinder moves through an intake stroke from a top dead center (TDC) position to a bottom dead center (BDC) position to admit air, an air/fuel mixture, and/or an air/exhaust gas mixture into the cylinder. During a compression stroke, the piston moves towards the TDC position to compress the air or the air mixture. During this process, an initial or additional fuel charge may be introduced to the cylinder by an injector. Ignition of the compressed mixture increases the pressure in the cylinder and forces the piston towards the BDC position during a first power stroke. In accordance with the six-stroke cycle, the piston performs a second compression stroke in which it recompresses the combustion products remaining in the cylinder after the first combustion or power stroke. During this recompression, any exhaust valves associated with the cylinder remain generally closed to assist cylinder recompression. Optionally, a second fuel charge may be introduced into the cylinder during recompression to assist igniting the residual combustion products and produce a second power stroke. Following the second power stroke, the cylinder undergoes an exhaust stroke with the exhaust valve or valves open to substantially evacuate combustion products from the cylinder. One example of an internal combustion engine configured to operate on a six-stroke engine can be found in U.S. Pat. No. 7,418,928. This disclosure relates to a method of operating an engine that includes compressing part of the combustion gas after a first combustion stroke of the piston as well as an additional combustion stroke during a six-stroke cycle of the engine.

Some possible advantages of the six-stroke cycle over the more common four-stroke cycle can include reduced emissions and improved fuel efficiency. For example, the second combustion event and second power stroke can provide for a more complete combustion of soot and/or fuel that may remain in the cylinder after the first combustion event. Although the six-stroke method provides some advantages, its implementation with other technologies and its compatibility with other technologies has yet to be fully understood.

SUMMARY

In one aspect, the disclosure describes a method of reducing emissions from an internal combustion engine operating on a six-stroke cycle. The method introduces a first fuel charge to a combustion chamber of the engine to produce a first stoichiometric lean condition and combusts the first fuel charge to produce initial combustion products with hydrocarbons and nitrogen oxides. The method then introduces a second fuel charge to the combustion chamber to produce a second stoichiometric lean condition and combusts the second fuel charge to increase the nitrogen oxides relative to the hydrocarbons. The exhaust gasses are directed to a selective catalytic reduction (SCR) catalyst and a reductant agent is introduced to the SCR catalyst to convert the nitrogen oxides in the exhaust gasses to nitrogen and water.

In another aspect, the disclosure describes an internal combustion engine system operating on a six-stroke cycle. The engine includes a cylinder and a piston reciprocally disposed in the cylinder to move between a top dead center position and a bottom dead center position. To introduce fuel, the engine can include an injector communicating with the cylinder. The engine can also include an exhaust system to direct exhaust gasses from the cylinder. A selective reduction catalyst for selective catalytic reduction (SCR) of nitrogen oxides is disposed in the exhaust system. A reductant agent storage reservoir accommodates a reductant agent and can introduce the reductant agent to the SCR catalyst. The engine system also includes a controller controlling the injector to introduce fuel during a first compression stroke and/or a first power stroke of the piston at a quantity to produce a first stoichiometric lean condition. The controller also introduces fuel during a second compression stroke and/or second power stroke of the piston to produce a second stoichiometric lean condition. The second stoichiometric lean condition may be closer to stoichiometric than the first stoichiometric lean condition.

In yet another aspect, the disclosure describes a method of reducing emissions from an internal combustion engine by introducing a first fuel charge and air to produce a first stoichiometric lean condition and combusting the first fuel charge and air to produce initial combustion products. The method then introduces a second fuel charge to the initial combustion products to produce a second stoichiometric lean condition and combusts the second fuel charge and initial combustion products to produce exhaust gasses. The exhaust gasses are directed to a selective catalytic reduction catalyst (SCR) for selective catalytic reduction of nitrogen oxides. A reductant agent is also introduced to the SCR catalyst to convert a portion of the nitrogen oxides in the exhaust gasses and the reductant agent to nitrogen and water by selective catalytic reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an engine system having an internal combustion engine adapted for operation in accordance with a six-stroke combustion cycle and associated systems and components for assisting the combustion process including a selective catalytic reduction after-treatment system.

FIGS. 2-8 are cross-sectional views representing an engine cylinder and a piston movably disposed therein at various points during a six-stroke combustion cycle.

FIG. 9 is a chart representing the lift of the intake valve or valves and exhaust valve or valves in millimeters along the Y-axis as measured against crankshaft angle in degrees along the X-axis for a six-stroke combustion cycle.

FIG. 10 is a chart illustrating a comparison of the internal cylinder pressure along the Y-axis in kilopascals (kPa) as measured against crankshaft angle along the X-axis as measured in degrees for a six-stroke combustion cycle.

FIG. 11 is a schematic flow chart representing a possible routine or steps for operating a six-stroke engine in conjunction with a selective catalytic reduction after-treatment system.

DETAILED DESCRIPTION

This disclosure relates in general to an internal combustion engine and, more particularly, to one adapted to perform a six-stroke cycle for reduced emissions and improved efficiencies. Internal combustion engines burn a hydrocarbon-based fuel or another combustible fuel source to convert the potential or chemical energy therein to mechanical power that can be utilized for other work. In one embodiment, the disclosed engine may be a compression ignition engine, such as a diesel engine, in which a mixture of air and fuel are compressed in a cylinder to raise their pressure and temperature to a point at which auto-ignition or spontaneous ignition occurs. Such engines typically lack a sparkplug that is typically associated with gasoline burning engines. However, in alternative embodiments, the utilization of different fuel such as gasoline and different ignition methods, for example, use of diesel as a pilot fuel to ignite gasoline or natural gas, are contemplated and fall within the scope of the disclosure.

Now referring to FIG. 1, wherein like reference numbers refer to like elements, there is illustrated a block diagram representing an internal combustion engine system 100. The engine system 100 includes an internal combustion engine 102 and, in particular, a diesel engine that combusts a mixture of air and diesel fuel. The illustrated internal combustion engine 102 includes an engine block 104 in which a plurality of combustion chambers 106 are disposed. Although six combustion chambers 106 are shown in an inline configuration, in other embodiments fewer or more combustion chambers may be included or another configuration such as a V-configuration may be employed. The engine system 100 can be utilized in any suitable application including mobile applications such as motor vehicles, work machines, locomotives or marine engines, and in stationary applications such as electrical power generators.

To supply the fuel that the engine 102 burns during the combustion process, a fuel system 110 is operatively associated with the engine system 100. The fuel system 110 includes a fuel reservoir 112 that can accommodate a hydrocarbon-based fuel such as liquid diesel fuel. Although only one fuel reservoir is depicted in the illustrated embodiment, it will be appreciated that in other embodiments additional reservoirs may be included that accommodate the same or different types of fuels that the combustion process may also burn. Because the fuel reservoir 112 may be situated in a remote location with respect to the engine 102, a fuel line 114 can be disposed through the engine system 100 to direct fuel from the fuel reservoir 112 to the engine 102. To pressurize the fuel and force it through the fuel line 114, a fuel pump 116 can be disposed in the fuel line. An optional fuel conditioner 118 may also be disposed in the fuel line 114 to filter the fuel or otherwise condition the fuel by, for example, introducing additives to the fuel, heating the fuel, removing water and the like.

To introduce the fuel to the combustion chambers 106, the fuel line 114 may be in fluid communication with one or more fuel injectors 120 that are associated with the combustion chambers. In the illustrated embodiment, one fuel injector 120 is associated with each combustion chamber but in other embodiments a different number of injectors might be used. Additionally, while the illustrated embodiment depicts the fuel line 114 terminating at the fuel injectors 120, the fuel line may establish a fuel loop that continuously circulates fuel through the plurality of injectors and, optionally, delivers unused fuel back to the fuel reservoir 112. Alternatively, the fuel line 114 may include a fuel collector volume or rail (not shown), which supplies pressurized fuel to the fuel injectors 120. The fuel injectors 120 can be electrically actuated devices that selectively introduce a measured or predetermined quantity of fuel to each combustion chamber 106. In other embodiments, introduction methods other than fuel injectors, such as a carburetor or the like, can be utilized.

To supply the air that is combusted with the fuel in the combustion chambers 106, a hollow runner or intake manifold 130 can be formed in or attached to the engine block 104 such that it extends over or proximate to each of the combustion chambers. The intake manifold 130 can communicate with an intake line 132 that directs air to the internal combustion engine 102. Fluid communication between the intake manifold 130 and the combustion chambers 106 can be established by a plurality of intake runners 134 extending from the intake manifold. One or more intake valves 136 can be associated with each combustion chamber 106 and can open and close to selectively introduce the intake air from the intake manifold 130 to the combustion chamber. While the illustrated embodiment depicts the intake valves at the top of the combustion chamber 106, in other embodiments the intake valves may be placed at other locations such as through a sidewall of the combustion chamber. To direct the exhaust gasses produced by combustion of the air/fuel mixture out of the combustion chambers 106, an exhaust manifold 140 communicating with an exhaust line 142 can also be disposed in or proximate to the engine block 104. The exhaust manifold 140 can communicate with the combustion chambers 106 by exhaust runners 144 extending from the exhaust manifold 140. The exhaust manifold 140 can receive exhaust gasses by selective opening and closing of one or more exhaust valves 146 associated with each chamber.

To actuate the intake valves 136 and the exhaust valves 146, the illustrated embodiment depicts an overhead camshaft 148 that is disposed over the engine block 104 and operatively engages the valves. As will be familiar to those of skill in the art, the camshaft 148 can include a plurality of eccentric lobes disposed along its length that, as the camshaft rotates, cause the intake and exhaust valves 136, 146 to displace or move up and down in an alternating manner with respect to the combustion chambers 106. Movement of the valves can seal and unseal ports leading into the combustion chamber. The placement or configuration of the lobes along the camshaft 148 controls or determines the gas flow through the internal combustion engine 102. As is known in the art, other methods exist for implementing valve timing such as actuators acting on the individual valve stems and the like. Furthermore, in other embodiments, a variable valve timing method can be employed that adjusts the timing and duration of actuating the intake and exhaust valves during the combustion process to simultaneously adjust the combustion process.

To assist in directing the intake air into the internal combustion engine 102, the engine system 100 can include a turbocharger 150. The turbocharger 150 includes a compressor 152 disposed in the intake line 132 that compresses intake air drawn from the atmosphere and directs the compressed air to the intake manifold 130. Although a single turbocharger 150 is shown, more than one such device connected in series and/or in parallel with another can be used. To power the compressor 152, a turbine 156 can be disposed in the exhaust line 142 and can receive pressurized exhaust gasses from the exhaust manifold 140. The pressurized exhaust gasses directed through the turbine 156 can rotate a turbine wheel having a series of blades thereon, which powers a shaft that causes a compressor wheel to rotate within the compressor housing.

To filter debris from intake air drawn from the atmosphere, an intake air filter 160 can be disposed upstream of the compressor 152. In some embodiments, the engine system 100 may be open-throttled wherein the compressor 152 draws air directly from the atmosphere with no intervening controls or adjustability. In other embodiments, to assist in controlling or governing the amount of air drawn into the engine system 100, an adjustable governor or intake throttle 162 can be disposed in the intake line 132 between the intake air filter 160 and the compressor 152. Because the intake air may become heated during compression, an intercooler 166 such as an air-to-air heat exchanger can be disposed in the intake line 132 between the compressor 152 and the intake manifold 130 to cool the compressed air.

To reduce emissions and assist adjusted control over the combustion process, the engine system 100 can mix the intake air with a portion of the exhaust gasses drawn from the exhaust system of the engine through a system or process called exhaust gas recirculation (EGR). The EGR system forms an intake air/exhaust gas mixture that is introduced to the combustion chambers along with the intake air. In one aspect, addition of exhaust gasses to the intake air displaces the relative amount of oxygen in the combustion chamber during combustion that results in a lower combustion temperature and reduces the generation of nitrogen oxides. Two exemplary EGR systems are shown associated with the engine system 100 in FIG. 1, but it should be appreciated that these illustrations are exemplary and that either one, both, or neither can be used on the engine. It is contemplated that selection of an EGR system of a particular type may depend on the particular requirements of each engine application.

In the first embodiment, a high-pressure EGR system 170 operates to direct high-pressure exhaust gasses to the intake manifold 130. The high-pressure EGR system 170 includes a high-pressure EGR line 172 that communicates with the exhaust line 142 downstream of the exhaust manifold 140 and upstream of the turbine 156 to receive the high-pressure exhaust gasses being expelled from the combustion chambers 106. The system is thus referred to as a high-pressure EGR system 170 because the exhaust gasses received have yet to depressurize through the turbine 156. The high-pressure EGR line 172 is also in fluid communication with the intake manifold 130. To control the amount or quantity of the exhaust gasses combined with the intake air, the high-pressure EGR system 170 can include an adjustable EGR valve 174 disposed along the high-pressure EGR line 172. Hence, the ratio of exhaust gasses mixed with intake air can be varied during operation by adjustment of the adjustable EGR valve 174. Because the exhaust gasses may be at a sufficiently high temperature that may affect the combustion process, the high-pressure EGR system can also include an EGR cooler 176 disposed along the high-pressure EGR line 172 to cool the exhaust gasses.

In the second embodiment, a low-pressure EGR system 180 directs low-pressure exhaust gasses to the intake line 132 before it reaches the intake manifold 130. The low-pressure EGR system 180 includes a low-pressure EGR line 182 that communicates with the exhaust line 142 downstream of the turbine 156 so that it receives low-pressure exhaust gasses that have depressurized through the turbine, and delivers the exhaust gasses upstream of the compressor 152 so it can mix and be compressed with the incoming air. The system is thus referred to as a low-pressure EGR system because it operates using depressurized exhaust gasses. To control the quantity of exhaust gasses re-circulated, the low-pressure EGR line 182 can also include an adjustable EGR valve 184.

To further reduce emissions generated by the combustion process, the engine system 100 can include one or more after-treatment devices disposed along the exhaust line 142 that treat the exhaust gasses before they are discharged to the atmosphere. One example of an after-treatment device is a selective catalytic reduction (SCR) system 190 for reducing nitrogen oxides such as NO and NO₂, commonly referred to as NO_X. In an SCR system 190, the exhaust gasses are combined with a reductant agent such as ammonia or an ammonia precursor such as urea and are directed through a catalyst 192 that chemically converts or reduces the nitrogen oxides in the exhaust gasses to nitrogen and water. For example, the reaction and reduction of nitrogen oxides can occur according to the following representative equation:

NH₃+NO_X=N₂+[H₂O] (1)

In the disclosed embodiment, the SCR catalyst can be a vanadium-based catalyst in which vanadium is the active material that causes the reaction. One advantage of a vanadium-based SCR catalyst is its high tolerance to sulfur and sulfates that may be present in the exhaust gasses. In other types of SCR catalysts, sulfur can gather on the active sites of the catalyst material reducing the effectiveness of the catalyst. In other embodiments, other types of SCR catalysts such as copper zeolite or iron zeolite may be used. To provide the reductant agent used in the process, a separate storage tank 194 may be associated with the SCR system 190. An electrically-operated SCR injector 196 in fluid communication with the storage tank 194 can be disposed either in the exhaust line 142 upstream of the vanadium-based SCR catalyst 192 or directly into the vanadium-based SCR catalyst to introduce the reductant agent to the exhaust gasses. Optionally, to mix the reductant agent and exhaust gasses, various mixers or pre-mixers can be disposed in the exhaust line 142. Another after-treatment system that may be included in an embodiment is a diesel oxidation catalyst (DOC) 198 made from metals such as palladium and platinum that can convert hydrocarbons and carbon monoxide in the exhaust gasses to carbon dioxide. Representative equations for this reaction are:

CO+½O₂→CO₂ (2)

[HC]+O₂→CO₂+H₂O (3)

In contrast to the SCR reaction, the DOC 198, by reacting components that may already be present in the exhaust gasses, does not require a reductant agent. In various embodiments, the DOC 198 can be placed either upstream or downstream of the SCR catalyst.

To coordinate and control the various systems and components associated with the engine system 100, the system can include an electronic or computerized control unit, module or controller 200. The controller 200 is adapted to monitor various operating parameters and to responsively regulate various variables and functions affecting engine operation. The controller 200 can include a microprocessor, an application specific integrated circuit (ASIC), or other appropriate circuitry and can have memory or other data storage capabilities. The controller can include functions, steps, routines, data tables, data maps, charts and the like saved in and executable from read only memory or another electronically accessible storage medium to control the engine system. Although in FIG. 1, the controller 200 is illustrated as a single, discrete unit, in other embodiments, the controller and its functions may be distributed among a plurality of distinct and separate components. To receive operating parameters and send control commands or instructions, the controller can be operatively associated with and can communicate with various sensors and controls on the engine system 100. Communication between the controller and the sensors can be established by sending and receiving digital or analog signals across electronic communication lines or communication busses. The various communication and command channels are indicated in dashed lines for illustration purposes.

For example, to monitor the pressure and/or temperature in the combustion chambers 106, the controller 200 may communicate with chamber sensors 210 such as a transducer or the like, one of which may be associated with each combustion chamber in the engine block 104. The chamber sensors 210 can monitor the combustion chamber conditions directly or indirectly. For example, by measuring the backpressure exerted against the intake or exhaust valves, the controller 200 can indirectly measure the pressure in the combustion chamber 106. The controller can also communicate with an intake manifold sensor 212 disposed in the intake manifold 130 and that can sense or measure the conditions therein. To monitor the conditions such as pressure and/or temperature in the exhaust manifold 140, the controller 200 can similarly communicate with an exhaust manifold sensor 214 disposed in the exhaust manifold. From the temperature of the exhaust gasses in the exhaust manifold 140, the controller 200 may be able to infer the temperature at which combustion in the combustion chambers 106 is occurring.

To measure the flow rate, pressure and/or temperature of the air entering the engine, the controller 200 can communicate with an intake air sensor 220. The intake air sensor 220 may be associated with, as shown, the intake air filter 160 or another intake system component such as the intake manifold. The intake air sensor 220 may also determined or sense the barometric pressure or other environmental conditions in which the engine system is operating.

To further control the combustion process, the controller 200 can communicate with injector controls 230 that can control the fuel injectors 120 operatively associated with the combustion chambers 106. The injector controls 230 can selectively activate or deactivate the fuel injectors 120 to determine the timing of introduction and the quantity of fuel introduced by each fuel injector. To further control the timing of the combustion operation, the controller 200 in the illustrated embodiment can also communicate with a camshaft control 232 that is operatively associated with the camshaft 148. Alternatively, the controller 200 may communicate with and control any other device used to monitor and/or control valve timing.

In embodiments having an intake throttle 162, the controller 200 can communicate with a throttle sensor 240 associated with the throttle and that can control the amount of air drawn into the engine system 100. The controller 200 can also be operatively associated with either or both of the high-pressure EGR system 170 and the low-pressure EGR system 180. For example, the controller 200 can be communicatively linked to a high-pressure EGR control 242 associated with the adjustable EGR valve 174 disposed in the high-pressure EGR line 172. Similarly, the controller 220 can also be communicatively linked to a low-pressure EGR control 244 associated with the adjustable EGR valve 184 in the low-pressure EGR line 182. The controller 220 can thereby adjust the amount of exhaust gasses and the ratio of intake air/exhaust gasses introduced to the combustion process. In addition to controlling the EGR system, the controller can also be communicatively linked to a SCR injector control 246 associated with the SCR injector 196 to adjustably control the timing and amount of reductant agent introduced to the exhaust gasses.

The engine system 100 can operate in accordance with a six-stroke combustion cycle in which the reciprocal piston disposed in the combustion chamber makes six or more strokes between the top dead center (TDC) position and bottom dead center (BDC) position during each cycle. A representative series of six strokes and the accompanying operations of the engine components associated with the combustion chamber 106 are illustrated in FIGS. 2-8. FIG. 9 is a chart showing the valve lift in millimeters along the Y-axis compared to the crank angle in degrees along the X-axis. Lift of the intake valve is indicated in solid lines and lift of the exhaust valve in dashed lines. FIG. 10 depicts the cylinder pressure in kilopascals (kPa) along the Y-axis compared to crank angle in degrees along the X-axis. Additional strokes, for example, 8-stroke or 10-stroke operation and the like, which would include one or more successive recompressions, are not discussed in detail herein as they would be similar to the recompression and re-combustion that is discussed, but are contemplated to be within the scope of the disclosure. The actual strokes are performed by a reciprocal piston 250 that is slidably disposed in an elongated cylinder 252. The cylinder 252 may be bored into the engine block, or may alternatively be defined within a cylindrical sleeve installed into a cylinder block. One end of the cylinder 252 is closed off by a flame deck surface 254 so that the combustion chamber 106 defines an enclosed space between the piston 250, the flame deck surface and the inner wall of the cylinder. The reciprocal piston 250 moves between the TDC position where the piston is closest to the flame deck surface 254 and the BDC position where the piston is furthest from the flame deck surface. The motion of the piston 250 with respect to the flame deck surface 254 thereby defines a variable volume 258 that expands and contracts.

Referring to FIG. 2, the six-stroke cycle includes an intake stroke during which the piston 250 moves from the TDC position to the BDC position causing the variable volume 258 to expand. During this stroke, the intake valve 136 is opened so that air or an air mixture may enter the combustion chamber 106, as represented by the positive bell-shaped intake curve 270 indicating intake valve lift in FIG. 9. Referring to FIG. 3, once the piston 250 reaches the BDC position, the intake valve 136 closes and the piston can perform a first compression stroke moving back toward the TDC position and compressing the variable volume 258 that has been filled with air during the intake stroke. It is contemplated that additional features, such as simulating a variable displacement cylinder volume by closing the intake valve before the piston reaches BDC or maintaining the intake valve open past BDC, which can be used to control the amount of air that enters the cylinder, may be used but are not shown or discussed in the present discussion for simplicity. As indicated by the upward slope of the first compression curve 280 in FIG. 10, this motion increases the pressure and temperature of fluids in the combustion chamber. In diesel engines, the compression ratio can be on the order of 15:1 although other compression ratios are common.

As illustrated in FIG. 4, in those embodiments in which only air is initially drawn into the combustion chamber 106, the fuel injector 120 can introduce a first fuel charge 260 into the variable volume 258 to create an air/fuel mixture as the piston 250 approaches the TDC position. The quantity of the first fuel charge 260 can be such that the resulting air/fuel mixture is lean of stoichiometric, meaning there is an excess amount of oxygen from what is theoretically required to fully combust the quantity of fuel that was provided to the combustion chamber. Under stoichiometric conditions, the proportion of air and fuel is such that all air and fuel will react together with little or no remaining excess of either component. The air/fuel ratio can be expressed as a lambda valve (λ) calculated by the following representative equation:

λ=(Air/Fuel_actual)/(Air/Fuel_stoich) (4)

Accordingly, the lambda valve at stoichiometric conditions equals 1.0 with larger values indicating lean conditions and smaller values indicating fuel rich conditions. For diesel fuel, the air/fuel ratio at stoichiometric conditions is about 14.5:1 to about 14.7:1. In the disclosed six-stroke embodiment, the air/fuel ratio created by the first fuel charge can be about 30:1 to produce a first stoichiometric lean condition. At an instance when the piston 250 is at or close to the TDC position and the pressure and temperature are at or near a first maximum pressure, as indicated by point 282 in FIG. 10, the air/fuel mixture may ignite. In embodiments where the fuel is less reactive, i.e., the fuel has a lower propensity to auto-ignite when pressurized and heated, such as in gasoline burning engines, ignition may be induced by a sparkplug, by ignition of a pilot fuel or the like. During a first power stroke, the combusting air/fuel mixture expands forcing the piston 250 back to the BDC position as indicated in FIGS. 4 to 5. The piston 250 can be linked or connected to a crankshaft 256 so that its linear motion is converted to rotational motion that can be used to power an application or machine. The expansion of the variable volume 258 during the first power stroke also reduces the pressure in the combustion chamber 106 as indicated by the downward sloping first expansion curve 284 in FIG. 10. At this stage, the variable volume contains the resulting combustion products 262 that may include unburned fuel, even though the air/fuel mixture in the chamber was lean (due to incomplete combustion), in the form of hydrocarbons. The variable volume may further include carbon monoxide, nitrogen oxides such as NO and NO₂commonly referred to as NO_X, and excess oxygen from the intake air due to the lean conditions.

Referring to FIG. 6, in the six-stroke cycle, the piston 250 can perform another compression stroke in which it compresses the combustion products 262 in the variable volume 258 by moving back to the TDC position. During the second compression stroke, both the intake valve 136 and exhaust valve 146 are typically closed so that pressure increases in the variable volume as indicated by the second compression curve 286 in FIG. 10. However, given that the gasses in the variable volume are at a relatively elevated temperature following the first combustion event, in some embodiments, to prevent too large a pressure spike, the exhaust valve 146 may be briefly opened to discharge some of the contents in a process referred to as blowdown, as indicated by the small blowdown curve 272 in FIG. 9. When the piston 250 reaches the TDC position shown in FIG. 6, the fuel injector 120 can introduce a second fuel charge 264 into the combustion chamber 106 that can intermix with the combustion products 262 from the previous combustion event. Referring to FIG. 10, at this instance, the pressure in the compressed variable volume 258 will be at a second maximum pressure 288. The second maximum pressure 288 may be greater than the first maximum pressure 282 or may be otherwise controlled to be about the same or lower than the first maximum pressure.

The quantity of the second fuel charge 264 provided to the cylinder, in conjunction with oxygen that may remain within the cylinder, can be selected to approach the stoichiometric condition for combustion but still be slightly lean of stoichiometric. For example, the air/fuel ratio resulting from the second fuel charge 264 can between about 14.7:1 and about 22:1 and, more precisely, between about 17:1 and about 20:1 so as to produce a second stoichiometric lean condition that, although lean, is still closer to stoichiometric than the first stoichiometric condition. When the piston 250 is at or near the TDC position and the combustion chamber 106 reaches the second maximum pressure 288, the second fuel charge 264 and the previous combustion products 262 may spontaneously ignite. Referring to FIGS. 6 to 7, the second ignition and resulting second combustion expands the contents of the variable volume 258 forcing the piston 250 toward the BDC position resulting in a second power stroke driving the crankshaft 256. The second power stroke also reduces the pressure in the cylinder 252 as indicated by the downward slopping second expansion curve 290 in FIG. 10.

The second combustion event can further incinerate the unburned combustion products from the initial combustion event such as particulate matter, unburned fuel and soot. The quantity or amount of hydrocarbons in the resulting second combustion products 266 remaining in the cylinder 252 may also be reduced. However, because the second combustion event occurs near the stoichiometric point and thus at a higher temperature, it may produce excess nitrogen oxides relative to the hydrocarbons. Referring to FIG. 8, an exhaust stroke can be performed during which the momentum of the crankshaft 256 moves the piston 250 back to the TDC position with the exhaust valve 146 opened to discharge the second combustion products to the exhaust system. Alternatively, additional recompression and re-combustion strokes can be performed in accordance with an 8-stroke, 10-stroke or like operating mode of the engine. With the exhaust valve opened as indicated by the bell-shaped exhaust curve 274 in FIG. 9, the pressure in the cylinder can return to its initial pressure as indicated by the low, flat exhaust curve 292 in FIG. 10. The exhaust gasses can be directed through the after-treatment system including the vanadium-based SCR catalyst 192 illustrated in FIG. 1.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to internal combustion engines operating on a six-stroke cycle, as in the disclosed embodiment, or to engines operating with a greater number of strokes. In general, the disclosure is applicable to any engine operating mode that includes recompression and re-combustion of byproducts remaining in a cylinder from a previous combustion. Referring to FIG. 11, there is illustrated a control strategy 300 that can be implemented by the engine system 100 to operate the engine system according to a six-stroke cycle. The controller 200 illustrated in FIG. 1 can conduct or carry out many of the indicated steps. According to the control strategy 300, the controller 200 in a first measuring step 302 can determine the mass airflow into the engine system 100 by monitoring the throttle sensor 240 or optionally indirectly by measuring the pressure in the intake manifold 130 using a reading from the intake manifold sensor 212 in a volumetric efficiency or other similar calculation. In a fuel determination step 304, the controller 200 can utilized the determined mass airflow to select the amount or quantity of fuel for the combustion process. This can include determining the relative amount of fuel for the first and second fuel charges to bring about the desired air/fuel ratios. In a first introduction step 310, the controller 200 from FIG. 1 directs the fuel injector 120 to introduce a first fuel charge into the combustion chamber 106. The quantity of the first fuel charge can be such that a first stoichiometric lean condition is created in the combustion chamber. During a first combustion event 312, the first fuel charge is combusted with a portion of the oxygen in the combustion chamber to produce the first power stroke. The resulting first or initial combustion products may include excess hydrocarbons, nitrogen oxides and oxygen. These first or initial combustion products can be recompressed in the combustion chamber 106 by the piston returning to the TDC position.

During a second introduction step 320, the controller 200 directs the fuel injector 120 to introduce a second fuel charge into the combustion chamber 106. The controller 200 can adjust the quantity of the second fuel charge to create a second stoichiometric lean condition in the combustion chamber. The second stoichiometric lean condition can be closer to stoichiometric than the first stoichiometric lean condition resulting from the first fuel charge, but still includes an excess of oxygen. The second fuel charge and the first or initial combustion products from the first combustion event are combusted in a second combustion event 322 that produces a second power stroke. Because the second combustion event 322 occurs closer to the stoichiometric point, meaning that most of the hydrocarbons in the fuel will combust with the oxygen, the disclosed engine system 100 can have a better fuel efficiency than a typical four-stroke diesel engine. Additionally, because of the excess oxygen in the second stoichiometric lean condition, the second combustion event 322 can combust or burn a significant amount of particulate matter in the combustion chamber. For example, the quantity of the resulting particulate matter may be approximately less than 0.04 grams/kilowatt-hour, which is sufficient to meet the Environmental Protection Agency's Tier IV regulations for large diesel engines. However, because the second combustion event 322 occurs closer to the stoichiometric point and possibly in the presence of excess oxygen, the combustion process can occur at higher temperatures increasing the generation of nitrogen oxides. For example, the operating temperature of the engine may be between about 380° to about 450° C., which can be measured by monitoring exhaust temperature. Alternatively, the heat rejection of the engine may be monitored by measuring an engine coolant or lubrication oil temperatures using a thermocouple or other sensing device. Therefore, the resulting exhaust gasses will generally contain a larger ratio of nitrogen oxides compared to hydrocarbons.

To reduce emissions, the engine system 100 can direct the exhaust gasses to the after-treatment devices disposed in the exhaust system. Because the second combustion event 322 burns off a significant portion of the particulate matter, a diesel particulate filter (DPF) which is often required to trap such particulate matter is unnecessary. However, because the exhaust gasses resulting from the second combustion event still contains more oxidants (O₂and NO_X) than reductants (H₂and CO), traditional three-way catalysts used in spark-ignition gasoline engines may not function properly because those catalysts typically require oxidants to be generally equal to reductants.

Accordingly, the engine system 100 can include the vanadium-based SCR catalyst 192 to reduce the amount of the nitrogen oxides in the exhaust gasses. To determine the actual amount of nitrogen oxides in the exhaust gasses, the controller 200 in a nitrogen oxide sensing step 330 can communicate with the exhaust sensor 214 disposed in the exhaust manifold 140 upstream of the SCR catalyst 192. The amount of nitrogen oxides can be determined indirectly by, for example, sensing the temperature of the exhaust gasses and extrapolating from other known variables to estimate the quantity of nitrogen oxides that the first and second combustion events 312, 322 would likely produce. The controller 200 then determines the amount of reductant agent necessary to convert the nitrogen oxides in a second reductant agent determination step 332 and, in a subsequent SCR instruction step 334, communicates an appropriate command to the SCR injector 196 to introduce the necessary quantity of reductant agent. In a resulting conversion step 336, the vanadium-based SCR catalyst converts at least a portion of the nitrogen oxides to nitrogen and water pursuant to equation (1) above. In fact, vanadium-based SCR catalyst can have an efficiency of greater than 90% if correctly sized to the associated engine system. Because it can convert nitrogen oxides in the presence of varying amounts of excess oxygen in the exhaust gasses, inclusion of the vanadium-based SCR catalyst enables the disclosed six-stroke engine system to operate with the second fuel charge being slightly lean of stoichiometric and to operate at higher temperatures that are responsible for generating the nitrogen oxides. For example, nitrogen oxides can be generated in an amount of about 8 grams/kilowatt-hour, greater than the 0.5-5.0 grams/kilowatt-hours amount associated with four-stroke diesel engines. Because the disclosed after-treatment method uses a reductant agent different than hydrocarbons from unburned fuel and can lack a DPF that may require regeneration by burning additional fuel, the method can conserve fuel and raise fuel efficiencies.

In a contemplated variation, the control strategy 300 can include functionality to ensure that the appropriate constituents are present in the exhaust gasses to optimize the disclosed emission control methodology. For example, because the second combustion event is intended to combust substantially most of the particulate matter but generates significant nitrogen oxides, the vanadium-based SCR catalyst can perform as the primary after-treatment device reducing the nitrogen oxides. Further, the engine system 100 can be configured to produce nitrogen oxides in a relative amount greater than hydrocarbons that the vanadium-based SCR catalyst might not treat. To determine if the first and second combustion events are producing nitrogen oxides and hydrocarbons in the correct proportions, the controller 200 in a hydrocarbon sensing step 340 can measure the quantity of the hydrocarbons in the exhausts. In an adjustment decision step 342, the controller can decide if the amount of hydrocarbons is too great, either as an absolute value or in comparison to the nitrogen oxides measured in the nitrogen oxide sensing step 330. The controller 200 can feed this information back to fuel determination step 304 to adjust the amount of fuel introduced for the first and second combustion events 312, 322.

The control strategy 300 can performed additional treatment steps to help reduce emissions. In an embodiment, to ensure that little if any hydrocarbons remain, the engine system 100 in a diesel oxidation step 344 can direct the exhaust gasses through the diesel oxidation catalyst 198 where remaining hydrocarbons can react with the excess oxygen remaining from the lean second combustion event. In another embodiment, to reduce the production of nitrogen oxides by the combustion process, the engine system 100 in an EGR step 346 can activate either the high pressure EGR system 170 or low pressure EGR system 180 to re-circulate a portion of the exhaust gasses with the intake air.

It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

1. A method of reducing emissions from an internal combustion engine operating a six-stroke cycle, the method comprising:

introducing a first fuel charge to a combustion chamber of the internal combustion engine to produce a first stoichiometric lean condition in the combustion chamber and combusting the first fuel charge to produce initial combustion products including hydrocarbons and nitrogen oxides;

introducing a second fuel charge to the combustion chamber to produce a second stoichiometric lean condition and combusting the second fuel charge and the initial combustion products to produce exhaust gasses and increase the nitrogen oxides relative to the hydrocarbons;

directing the exhaust gasses to a selective catalytic reduction (SCR) catalyst;

introducing a reductant agent to the SCR catalyst; and

converting nitrogen oxides in the exhaust gasses to nitrogen and water.

2. The method of claim 1, wherein the second stoichiometric lean condition is closer to stoichiometric than the first stoichiometric lean condition.

3. The method of claim 1, wherein the fuel is diesel having a stoichiometric air/fuel ratio of about 14.5:1 to about 14.7:1.

4. The method of claim 3, wherein the first stoichiometric lean condition has an air/fuel ratio of approximately 30:1 and the second stoichiometric lean condition has an air/fuel ratio of between about 14.7:1 to about 20:1.

5. The method of claim 1, wherein combusting of the second fuel charge occurs at an engine operating temperate of about 380° to about 450° C.

6. The method of claim 1, wherein the SCR catalyst is a vanadium-based SCR catalyst.

7. The method of claim 6, wherein the reductant agent is selected from the group consisting of ammonia and urea.

8. The method of claim 1, wherein the exhaust gasses include particulate matter in an amount less that 0.04 grams/kilowatt-hour so as to not require a diesel particulate filter.

9. The method of claim 1, further comprising oxidizing hydrocarbons remaining in the exhaust gasses with a diesel oxidation catalyst.

10. An internal combustion engine system operating on a six-stroke cycle comprising:

an internal combustion engine including a cylinder, a piston reciprocally disposed in the cylinder to move between a top dead center position and a bottom dead center position, an injector communicating with the cylinder to introduce fuel, and an exhaust system communicating with the engine to direct exhaust gasses from the cylinder;

a selective reduction catalyst for selective catalytic reduction (SCR) of nitrogen oxides disposed in the exhaust system;

a reductant agent storage reservoir accommodating a reductant agent and communicating with the exhaust system to introduce the reductant agent to the SCR catalyst; and

a controller controlling the injector to introduce fuel during a first compression stroke and/or a first power stroke of the piston at a quantity to produce a first stoichiometric lean condition in the cylinder, and to introduce fuel during a second compression stroke and/or a second power stroke of the piston to produce a second stoichiometric lean condition in the cylinder.

11. The system of claim 10, wherein the second stoichiometric lean condition is closer to stoichiometric than the first stoichiometric lean condition.

12. The system of claim 11, wherein the fuel is diesel having a stoichiometric point of between about 14.5:1 to about 14.7:1.

13. The system of claim 12, wherein the first stoichiometric lean condition has an air/fuel ratio of approximately 30:1 and the second stoichiometric lean condition has an air/fuel ratio of a about 14.7:1 to about 22:1.

14. The system of claim 10, wherein the SCR catalyst is a vanadium-based SCR catalyst.

15. The system of claim 10, wherein the system lacks a diesel particulate filter.

16. A method of reducing emissions from an internal combustion engine comprising introducing a first fuel charge and air to a combustion chamber of the internal combustion engine to produce a first stoichiometric lean condition and combusting the first fuel charge and air to produce initial combustion products;

introducing a second fuel charge to the combustion chamber with the initial combustion products to produce a second stoichiometric lean condition and combusting the second fuel charge and initial combustion products to produce exhaust gasses;

directing the exhaust gasses from the combustion chamber to a selective catalytic reduction catalyst (SCR) for selective catalytic reduction of nitrogen oxides;

introducing a reductant agent to the SCR catalyst;

converting nitrogen oxides in the exhaust gasses and the reductant agent to nitrogen and water by selective catalytic reduction.

17. The method of claim 16, wherein the second stoichiometric lean condition is closer to stoichiometric than the first stoichiometric lean condition.

18. The method of claim 16, wherein combusting of the second fuel charge and the initial combustion products occurs at temperature between about 380° C. to about 450° C.

19. The method of claim 16, wherein nitrogen oxides are present in the exhaust gasses in a greater relative amount than hydrocarbons.

20. The method of claim 16, wherein the exhaust gasses have less than 0.04 grams/kilowatt-hour of particulate matter.