Abstract:
A method and apparatus for controlling fuel injection timing in a compression ignition engine is provided. The method includes monitoring a position of a piston reciprocating in a cylinder between a top dead center (TDC) position and a bottom dead center (BDC) position and injecting a predetermined quantity of fuel into the cylinder when the piston is at least one of reciprocating from said TDC toward BDC during an intake stroke and at BDC reciprocating toward TDC during a compression stroke.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates generally to fuel control systems for compression ignition engines and, more particularly, to a fuel injection system that suppresses emissions generated by compression ignition diesel engines.  
           [0002]    Diesel engine exhaust is a heterogeneous mixture, which contains gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx). Additionally, diesel engine exhaust contains particulate matter (PM), also known as soot. Soot is a solid, dry, solid carbonaceous material that makes up one component in total particulate matter (TPM), and contributes to visible emissions that may exhaust through a diesel exhaust. Because diesel engines operate with an excess of combustion air (lean exhaust), such engines generally have emissions of CO and gas phase HCs that are below EPA limits. However, emissions from diesel engines have been under increasing scrutiny in recent years, and standards, especially for particulate emissions, have become stricter.  
           [0003]    It is known to facilitate reducing emissions of NOx from diesel engines by retarding injection timing. However, retarding injection timing may cause a corresponding increase in particulate emissions, particularly of the dry carbon or soot portion. Emissions of NOx can also be reduced by applying exhaust gas recirculation (EGR) technology or more advanced direct fuel injection systems, modifying the injection timing, increasing the compression ratio, and/or reducing manifold air temperatures. However, implementing such techniques may also cause a corresponding increase in particulate emissions, and/or cause fuel consumption penalties.  
         SUMMARY OF INVENTION  
         [0004]    In one aspect, a method of controlling fuel injection timing in a compression ignition engine is provided. The method includes monitoring a position of a piston reciprocating in a cylinder between a top dead center (TDC) position and a bottom dead center (BDC) position and injecting a pre-determined quantity of fuel into the cylinder when the piston is at least one of reciprocating from the TDC toward BDC during an intake stroke and at BDC reciprocating toward TDC during a compression stroke.  
           [0005]    In another aspect, a compression ignition engine is described. The engine includes an engine block including at least one cylinder, at least one cylinder head covering the at least one cylinder, a piston reciprocating in the each cylinder between a top dead center (TDC) position and a bottom dead center (BDC) position, a combustion air inlet plenum in flow communication with the at least one cylinder, and a fuel injection system including at least one fuel injector, the system configured to inject fuel into the at least one cylinder when each piston is at least one of reciprocating from TDC toward BDC during an intake stroke and at BDC reciprocating toward TDC during a compression stroke.  
           [0006]    In yet another aspect, a railroad locomotive is described. The locomotive includes a compression ignition engine including an engine block including at least ten cylinders, at least one cylinder head covering the cylinders, a piston reciprocating in each cylinder between a top dead center (TDC) position and a bottom dead center (BDC) position, a combustion air inlet plenum in flow communication with each cylinder, and a fuel injection system including at least one fuel injector, the system configured to inject fuel into each cylinder when the piston is at least one of reciprocating from the TDC toward BDC during an intake stroke and at BDC reciprocating toward TDC during a compression stroke.  
           [0007]    In still another aspect, a railroad locomotive is described. The locomotive includes a compression ignition engine including a compression ignition engine including an engine block including at least ten cylinders, at least one cylinder head covering the cylinders, a piston reciprocating in each cylinder between a top dead center (TDC) position and a bottom dead center (BDC) position, a combustion air inlet plenum in flow communication with the cylinder, and a fuel injection system that includes at least one fuel injector mounted in the at least one cylinder head, the fuel injector includes a nozzle that is at least partially within the cylinder, the system configured to inject the fuel at a first pre-determined piston position that corresponds to a crank angle of between about negative three hundred sixty degrees and about zero degrees., and inject a second quantity of fuel into the cylinder at a second pre-determined piston position that corresponds to a crank angle of between about negative forty five degrees and about twenty degrees, such that a fuel/air equivalence ratio of the fuel/air mixture in each cylinder at ignition is between 0.10 and 0.85.  
           [0008]    In yet another aspect, a railroad locomotive is described. The locomotive includes a compression ignition engine including an engine block including at least ten cylinders, at least one cylinder head covering the cylinders, a piston reciprocating in each cylinder between a top dead center (TDC) position and a bottom dead center (BDC) position, a combustion air inlet plenum in flow communication with each cylinder, and a fuel injection system including at least one fuel injector mounted in the combustion air inlet plenum, the fuel injector including a nozzle, the nozzle at least partially within the combustion air inlet plenum, the system configured to inject fuel into the cylinders at a crank angle of between about negative three hundred sixty degrees and about three hundred sixty degrees, such that a fuel/air equivalence ratio of a fuel/air mixture in the cylinder at ignition is between 0.10 and 0.85. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 is a front-side isometric view of a compression ignition diesel engine.  
         [0010]    [0010]FIG. 2 is a simplified cross sectional view of a portion of a four-stroke cycle diesel engine with manifold fumigation.  
         [0011]    [0011]FIG. 3 is a cross sectional view of a portion of an alternative embodiment of a four-stroke cycle, medium speed diesel engine with in-cylinder premixing.  
         [0012]    [0012]FIG. 4 is a cross sectional view of a portion of the engine shown in FIG. 3 at the end of a compression stroke wherein a premixed charge is ignited by a pilot spray.  
         [0013]    [0013]FIG. 5 is a graph illustrating exemplary emissions levels as a function of air-fuel ratio in the exemplary internal combustion engine. 
     
    
     DETAILED DESCRIPTION  
       [0014]    The basic combustion process for diesel engines involves a diffusion-type combustion of liquid fuel. More specifically, as liquid fuel is injected into compressed hot cylinder air, the fuel evaporates and mixes with the surrounding air to form a flammable mixture. This is a continuing process that happens over time as the fuel is injected into the cylinder. The mixture formed initially will combust and raise the local temperature before the later evaporated fuel has time to fully mix with air. As a result, the later burned fuel is subjected to high temperatures with insufficient air and under such conditions, high temperature pyrolysis of fuel may occur, thus forming soot. As the combustion proceeds in the cylinders, a substantial portion of the soot may be burned-up as a result of exposure to air in the cylinder. The soot will continue to be burned up in the engine until the power stroke volume expansion sufficiently lowers the cylinder temperature, at which time the chemical reaction is stopped, and any non-combusted soot remaining in the cylinder is discharged from the engine as smoke or particulate emission when the exhaust valve is opened.  
         [0015]    [0015]FIG. 1 is a front-side isometric view of a compression ignition diesel engine  10  and includes a turbo charger  12  and a plurality of power cylinders  14 . For example, a twelve-cylinder engine  10  has twelve power cylinders  14  while a sixteen-cylinder engine  10  has sixteen power cylinders  14 . Engine  10  also includes an air intake manifold  16 , a fuel supply line  18  for supplying fuel to each power cylinder  14 , a water inlet manifold  20  used in cooling engine  10 , a lube oil pump  22  and a water pump  24 . An intercooler  26  connected to turbo charger  12  facilitates cooling turbo-charged air before it enters respective power cylinder  14 . In an alternative embodiment, engine  10  is a Vee-type engine, wherein power cylinders  14  are arranged in an offset angle from adjacent power cylinders  14 .  
         [0016]    [0016]FIG. 2 is a cross sectional view of a portion of a four-stroke cycle, medium speed diesel engine  10  with manifold fumigation. In one embodiment, engine  10  is a locomotive engine. Engine  10  includes an engine block  112  that defines a cylinder  114  including a cylinder head  116  and a circumferential wall surface or liner  118 . A combustion air intake port  120  and an exhaust gas port  122  communicate through cylinder head  116  with cylinder  114 . Air intake port  120  is in flow communication with cylinder  114  through an intake valve (not shown) and exhaust gas port  122  is in flow communication with cylinder  114  through an exhaust valve (not shown). Air intake port  120  includes at least one fuel injection port  128  communicating with a fuel injector  130  including an injector nozzle  131 . In an alternative embodiment, additional fuel injectors  130  are provided to facilitate achieving a homogeneous gas-phase mixture of combustion air and fuel. Fuel injector  130  is in communication with a fuel supply system  132  that includes a subsystem configured to regulate a temperature of the fuel to facilitate achieving an optimal vaporization. Air intake port  120  is in communication with an air supply system  133  that includes a sub-system configured to regulate a temperature of the combustion air to facilitate achieving an optimal gas-phase mixing.  
         [0017]    While the present invention is described in the context of a locomotive, it is recognized that the benefits of the invention accrue to other applications of diesel engines. Therefore, this embodiment of the invention is intended solely for illustrative and exemplary purposes and is in no way intended to limit the scope of application of the invention.  
         [0018]    A piston  134  is slidingly disposed in cylinder  114  and includes a face  136  that is adjacent cylinder head  116 , and a circumferential sidewall surface  138  that is spaced from cylinder  114  by a predetermined clearance gap  140 . Piston  134  includes a plurality of closely spaced, annular grooves  141 , each of which is configured to receive an annular, split, compression ring seal  142  for establishing a compression seal between piston sidewall surface  138  and cylinder liner  118 . Piston  134  is shown in a bottom-dead-center (BDC) stroke position, in which piston face  136  and cylinder head  116  are at their furthest relative distance. Piston  134  reciprocates inside cylinder  114  between BDC and a top-dead-center (TDC) stroke position in which piston face  136  and cylinder head  116  are at their closest relative distance. Thus, cylinder  114  has a maximum working volume above piston face  136  when piston  134  is at BDC, and a minimum working volume above piston face  136  when piston is at TDC. The ratio of the BDC volume to the TDC volume is known as the compression ratio of cylinder  114 .  
         [0019]    In operation, piston  134  reciprocates between TDC and BDC positions. More specifically, the movement of piston  134  from TDC to BDC is referred to as a downstroke and the movement of piston  134  from BDC to TDC is referred to as an upstroke. Starting from a position wherein piston  134  is at TDC, during or after a firing of fuel in cylinder  114  from a previous cycle, a first downstroke or power stroke occurs after combustion when piston  134  is driven away from cylinder head  116  by a force of rapidly expanding combustion gases. The force acting on piston  134  is transmitted to connecting parts (not shown) to deliver power to drive an engine shaft (not shown). For reference, a piston position at TDC during firing is known as a crank angle of zero degrees. After piston  134  reaches BDC, or a crank angle of one-hundred eighty degrees, the next stroke of the cycle begins. A first upstroke or exhaust stroke expels depleted exhaust gases from cylinder  114 . As piston  134  moves toward cylinder head  116 , the volume of cylinder  114  decreases, causing the exhaust gas pressure in cylinder  114  in increase. On the exhaust stroke, the exhaust valve opens to allow the increasingly pressurized exhaust gas to escape cylinder  114 . After piston  134  reaches TDC, or a crank angle of three hundred sixty degrees, a second down stroke or, intake stroke occurs, and the air inlet valve is open and injector  130  is pressurized by fuel supply system  132 . Because of the cyclic nature of the strokes referred to, a crank angle of three hundred sixty degrees and negative three hundred sixty degrees are equivalent. Combustion air at a regulated predetermined temperature and at a regulated predetermined pressure passes injector nozzle  131  as it is forced into cylinder  114 . Injector  130  releases a pressurized stream  148  of fuel through nozzle  131  into the combustion air stream in inlet  120 . In one embodiment, stream  148  is released at a crank angle of between about negative three hundred sixty degrees and three hundred sixty degrees. Nozzle  131  is configured to atomize the fuel passing therethrough. The warmed and atomized fuel vaporizes in inlet  120  and mixes homogeneously with the combustion air prior to entering cylinder  114 . By the time piston  134  reaches BDC, cylinder  114  is substantially filled with a homogeneous fuel/air mixture.  
         [0020]    At BDC or a crank angle of negative one hundred eighty degrees, piston  134  reverses travel and begins a first upstroke or compression stroke. As piston  134  moves closer to cylinder head  116 , the volume of cylinder  114  decreases, causing the temperature and pressure of the homogeneous fuel/air mixture to increase to an ignition point wherein combustion takes place. Combustion takes place near TDC or a crank angle of zero degrees, and is controlled by varying a fuel/air mixture and engine operating parameters to occur at an optimum point in the stroke. In one embodiment, the fuel/air mixture and engine operating parameters are controlled by, for example, exhaust gas recirculation (EGR), water injection directly into the cylinder, water injection into the intake manifold, variable valve timing, variable compression ratio, and/or variable geometry turbomachinery to optimize the cylinder pre-compression conditions. This is in contrast to at least some known combustion processes wherein liquid fuel is injected into the cylinder near the top of the compression stroke. Injecting fuel into inlet  120  and modulating the fuel and air to achieve a homogeneous mixture at the end of the intake stroke changes the combustion mode from a diffusion flame to a lean-mixed combustion event.  
         [0021]    The traditional direct-injection system referred to above generates a mixing-controlled burn during the heat release process in the diesel engine cycle. The fuel and air burn at a stoichiometric ratio of approximately one, in localized areas at a flame front, although the overall mixture in cylinder  114  is lean. This results in high temperatures at the flame front of the combustion event, which causes high levels of NOx emissions. Also due to the heterogeneous nature of the diffusion flame, there are fuel rich regions that may burn with insufficient oxygen, thus producing large quantities of soot and particulate matter. In contrast, the fuel and air are uniformly mixed within the present invention such that the entire mixture is at an overall lean equivalence ratio. This process facilitates eliminating the formation of soot and also results in low NOx emissions due to the low flame temperatures and because there is no locally rich zone of combustion and rather, ignition occurs substantially spontaneously and concurrently at many points in cylinder  114 .  
         [0022]    [0022]FIG. 3 is a cross sectional view of a portion of an alternative embodiment of a four-stroke cycle, medium speed diesel engine  149  with in-cylinder premixing. FIG. 4 is a cross sectional view of a portion of the engine shown in FIG. 3 at the end of a compression stroke wherein a premixed charge is ignited by a pilot spray. Engine  149  is substantially similar to Engine  10  shown in FIGS. 1 and 2 and components in engine  149  that are identical to components of engine  10  are identified in FIG. 3 using the same reference numerals used in FIG. 2. Accordingly, engine  149  includes an engine block  112  that defines a cylinder  114  including a cylinder head  116  and a circumferential wall surface or liner  118 . A combustion air intake port  120  and an exhaust gas port  122  communicate through cylinder head  116  with cylinder  114 . Air intake port  120  is in flow communication with cylinder  114  through an intake valve (not shown) and exhaust gas port  122  is in flow communication with cylinder  114  through an exhaust valve (not shown). Cylinder head  116  includes at least one fuel injection port  128  communicating with a fuel injector  130  including an injector nozzle  131 .  
         [0023]    In operation, piston  134  reciprocates between TDC and BDC positions. Starting from a position wherein piston  134  is at TDC at a crank angle of negative three hundred sixty degrees, an intake stroke occurs and the air inlet valve is open. Combustion air at a regulated predetermined temperature and at a regulated predetermined pressure passes inlet  120  as it is forced into cylinder  114 . When piston  134  reaches BDC or a crank angle of negative one hundred eighty degrees, cylinder  114  is substantially filled with combustion air. At BDC, piston  134  reverses travel and begins a compression stroke and the air inlet valve is closed. Injector  130  releases a first, main pressurized stream  150  of fuel through nozzle  131  into cylinder  114 . In one embodiment, stream  150  is released at a crank angle of between approximately negative three hundred sixty degrees and approximately zero degrees. First pressurized stream  150  contains all or a portion of the fuel that will be injected during that cycle. Nozzle  131  is configured to atomize the fuel passing through it. The warmed and atomized fuel vaporizes in cylinder  114  and mixes homogeneously with the combustion air in cylinder  114 . During the compression stroke, as piston  134  moves closer to cylinder head  116 , the volume of cylinder  14  decreases, causing the temperature and pressure of the combustion air/fuel mixture to increase. Injector  130  releases a second pressurized stream  152  (see FIG. 4) of fuel through nozzle  131  into cylinder  114 . In one embodiment, stream  150  is released at a crank angle between approximately negative forty five degrees and approximately twenty degrees. The second stream  152  of fuel contains the remaining fuel that will be injected during that stroke. The injection of the second, pilot stream  152  of fuel ignites the homogenous air/fuel mixture in cylinder  114 . Combustion takes place near TDC and is controlled to occur at an optimum point in the stroke. The combustion process is controlled by regulating the temperature of the fuel, the temperature of the combustion air, the timing and duration of the main injection stream and the timing and duration of the pilot injection stream.  
         [0024]    With a dual injection strategy, a portion of, or all of, the fuel is injected early in the engine cycle, during the intake stroke and at the beginning of the compression stroke. This allows enough time for the fuel and the in-cylinder gas to mix before ignition. A homogeneous mixture is created in this process and this mixture is ignited by injecting a portion of the fuel near TDC. The pilot injection will trigger combustion throughout the homogeneous fuel-air mixture. In an alternative embodiment, the homogeneous mixture auto-ignites without the use of a pilot stream. In the exemplary embodiment, the early fuel injection is achieved by a cam-driven fuel injector system. In an alternative embodiment, the fuel injection system uses an advanced injection technology such as, a common-rail fuel system or advanced unit pump and unit injectors. Additionally, combustion is controlled using supplemental injection of inert media such as, for example, exhaust gas, water or additional air.  
         [0025]    The dual injection strategy allows engine  149  to operate in a different combustion mode compared to a direct injection engine. The combustion strategy is changed from a diffusion flame to a lean-premixed or partially pre-mixed combustion event. In this embodiment, a portion of, or all of, the fuel used in the cycle is uniformly mixed with the in-cylinder air so that the majority of the mixture is at a lean equivalence ratio at the time of combustion. This process facilitates eliminating the formation of soot and also results in low NOx emissions due to the low flame temperatures.  
         [0026]    [0026]FIG. 5 is a graph illustrating exemplary emissions levels as a function of air-fuel ratio in an exemplary internal combustion engine  10 . A horizontal axis of graph  200  represents a fuel/air equivalence ratio scale  202  with a corresponding air/fuel ratio scale  204 . The fuel/air equivalence ratio is defined as the actual fuel-to-air mass ratio divided by the stoichiometric fuel-to-air mass ratio. A fuel/air equivalence ratio that is stoichiometric if the fuel/air equivalence ratio is greater in value than 0.9 and less in value than 1.1. A lean fuel/air mixture has a fuel/air equivalence ratio of less than 0.9. A rich fuel/air mixture has a fuel/air equivalence ratio of greater than 1.1.  
         [0027]    A vertical axis  206  of graph  200  represents concentrations of constituents of internal combustion engine exhaust. A band  208  shows the range of a concentration of hydrocarbon emissions that is emitted by an internal combustion engine operating at fuel/air equivalence ratios shown on axis  202 . Likewise, a band  210  shows the range of a concentration of NOx emissions that is emitted by an internal combustion engine operating at fuel/air equivalence ratios shown on axis  202  and band  212  shows the range of a concentration of carbon monoxide emissions that is emitted by an internal combustion engine operating at fuel/air equivalence ratios shown on axis  202 .  
         [0028]    As discussed above, the basic combustion process for direct injection diesel engines involves a diffusion-type combustion of liquid fuel. The mixture formed initially after the fuel is injected into the cylinder will combust and raise the local temperature before the later evaporated fuel has time to fully mix with air. The result is areas of rich mixture combustion, stoichiometric mixture combustion, and lean mixture combustion occurring in the cylinder at the same time. Even though the overall mixture is held to a lean fuel/air equivalence ratio, localized areas of rich mixture combustion and stoichiometric mixture combustion raise outlet emissions levels of NOx, HC and CO unacceptably. By comparison, operation with a lean homogeneous mixture produces less emissions of NOx, HC and CO. Engine  10  and engine  149  may operate in area  214  with a fuel/air equivalence ratio of less than 0.85 homogeneous throughout cylinder  114  at the time of ignition. A fuel/air equivalence ratio of less than approximately 0.85 that is homogeneous throughout cylinder  114  at the time of ignition ensures lower NOx, HC and CO generation and subsequent emissions. Operation of engines  10  and  149  at a fuel/air equivalence ratio of less than approximately 0.75 is governed by fuel economy and combustion stability considerations. In the exemplary embodiment, engines  10  and  149  operate at a fuel/air equivalence ratio of between about 0.10 to about 1.00. In an alternate embodiment, engines  10  and  149  operate at a fuel/air equivalence ratio of between about 0.20 to about 0.60. In an another alternate embodiment, engines  10  and  149  operate at a fuel/air equivalence ratio of between about 0.75 to about 0.85.  
         [0029]    The above-described diesel engine fuel injection systems are cost-effective and highly reliable. Each system includes an injector that injects fuel into a diesel engine combustion air volume such that a homogeneous fuel/air mixture results early in the engine cycle. Such injection facilitates complete burning of the fuel at lower temperatures resulting in less particulate emissions being formed and less NOx being generated. As a result, the fuel injection system facilitates reducing engine emissions in a cost-effective and reliable manner.  
         [0030]    Exemplary embodiments of diesel engine fuel injection systems are described above in detail. The systems are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. Each diesel engine fuel injection systems component can also be used in combination with other diesel engine fuel injection systems components.  
         [0031]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.