Abstract:
An engine includes one or more cylinders configured to receive gaseous fuel for use in combustion. Two gaseous fuel injectors per cylinder, include: a first injector directed at the corresponding cylinder and having an injection capacity of a first amount of gaseous fuel per injection, and a second injector directed at the corresponding cylinder and having an injection capacity of a second amount of gaseous fuel per injection, the second amount being greater than the first amount. The engine is operable in a first mode in which a per-cylinder fuel demand is at or below the first amount, and only the first injector is operable for each cylinder. The engine is operable in a second mode in which the per-cylinder fuel demand is greater than the first amount, and only the second injector is operable for each cylinder.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to gaseous fuel engines with fuel injectors for introducing gaseous fuel (i.e., fuel that naturally exists in a gaseous state, rather than a liquid state) into intake ports of an internal combustion engine, for example in a passenger vehicle. Gaseous fuels include natural gas (primarily methane) and derivatives thereof, such as butane and propane, but do not include gasoline. Natural gas can be used to power internal combustion engines. Compared to conventional engines, vehicles run on natural gas are environmentally friendly while outputting less engine noise than traditional diesel-powered engines. 
     SUMMARY OF THE INVENTION 
     The invention provides, in one aspect, an engine having one or more cylinders configured to receive gaseous fuel for use in combustion. Two gaseous fuel injectors, per cylinder, are directed at each of the plurality of cylinder. A first injector has an injection capacity of a first amount of gaseous fuel per injection. A second injector has an injection capacity of a second amount of gaseous fuel per injection, the second amount being greater than the first amount. The engine is operable in a first mode in which a per-cylinder fuel demand is at or below the first amount, and only the first injector is operable for each cylinder. The engine is operable in a second mode in which the per-cylinder fuel demand is greater than the first amount, and only the second injector is operable for each cylinder. 
     The invention provides, in another aspect, a method of operating a gaseous fuel engine having one or more cylinders. Two injectors are provided per cylinder: a first injector and a second injector. An engine load is analyzed. In a first mode of operation, gaseous fuel is injected into each cylinder via only the corresponding first injector when the engine load requires an amount of gaseous fuel that is less than or equal to a first amount. In a second mode gaseous fuel is injected into each cylinder via only the corresponding second injector when the engine load requires a second amount of gaseous fuel, greater than the first amount. 
     The invention provides, in yet another aspect, a method of operating a gaseous fuel engine having one or more cylinders. Two gaseous fuel injectors are provided per cylinder: a first injector and a second injector. The second injector has a higher injection capacity than an injection capacity of the first injector. An intake valve per cylinder is configured to transition between a closed position and an open position. An intake valve opening duration, in which the intake valve is in the open position, is analyzed. Gaseous fuel is injected into each cylinder only within the corresponding intake valve opening duration. The injection of gaseous fuel includes injecting gaseous fuel into each cylinder via only the corresponding first injector in a first mode of engine operation when the intake valve opening duration is greater than a predetermined duration. The injection of gaseous fuel further includes injecting gaseous fuel into each cylinder via only the corresponding second injector in a second mode of operation when the intake valve opening duration is less than the predetermined valve opening duration. 
     Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a conventional engine having fuel injectors positioned at the downstream ends of individual intake runners. 
         FIG. 2  is a schematic representation of an engine utilizing a first injector according to one embodiment of the present invention. 
         FIG. 3  is a schematic representation of an engine utilizing a second injector according to one embodiment of the present invention. 
         FIG. 4  is a graph illustrating an exemplary group of operational ranges of the engine of  FIGS. 2-3 , according to fuel injection quantity and engine speed. The graph illustrates distinct regions of operation for first and second gaseous fuel injectors, according to an exemplary method of the present invention. 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
     Gaseous fuel engines may begin with an engine designed for more common fuels such as diesel fuel or gasoline. Components of these engines are retrofitted to allow the engine to run on a gaseous fuel.  FIG. 1  shows an engine  16  including an intake manifold  20  and a cylinder head  24  of a known configuration. Air enters the intake manifold  20  via the throttle body  28 . A throttle valve  32  located within the throttle body  28  selectively opens and closes to limit or prevent the passage of air through the throttle body  28 . The air within the throttle body is diverted through numerous intake runners  36  (i.e., one intake runner  36  per piston cylinder  56 ). The intake runners  36  align with intake ports  44  within the cylinder head  24 . Fuel is provided from a single gaseous fuel tank or common fuel supply  46 , through a fuel line  50 , to multiple fuel injectors  48 . A fuel injector  48  is outfitted to inject fuel  52  into each of the respective intake ports  44 . The injected fuel  52  mixes with the air to create an air-fuel mixture. An intake valve  40 , located within each intake port  44  selectively prohibits the air-fuel mixture from reaching a piston cylinder  56  when the intake valve  40  is in a closed position. When the intake valve  40  is in an open position (i.e., intake stroke, suction stroke), the air-fuel mixture enters the piston cylinder  56  where combustion occurs. An exhaust valve  42  is located downstream of each cylinder  56  and is configured to open after combustion has occurred. 
     Certain engines, such as some diesel engines, do not have individual intake runners, but rather include a common intake plenum  160 , such as the engine  116  as shown in  FIGS. 2-3 . The common intake plenum  160  is located directly downstream of the throttle body  128  and throttle valve  132 , upstream of the multiple intake ports  144 , and provides a fluid communication path between the multiple piston cylinders  156 . As shown in  FIGS. 2 and 3 , the engine  116  can include six intake ports  144  for six piston cylinders  156 , however, the present invention equally applies to any number of piston cylinders  156  of an engine  116  with a number of cylinders (i.e., at least one piston cylinder  156 ), and any number of intake ports  144  per piston cylinder  156 . 
     A gaseous fuel engine utilizes gaseous fuel injectors  148 A,  148 B. For example, an engine  116  may be retrofitted to run on the gaseous fuel (i.e., natural gas). Fuel is provided from a single gaseous fuel tank or common fuel supply  146 , through a fuel line  150 , to multiple fuel injectors  148 A,  148 B. Fuel injectors  148 A,  148 B, are positioned to inject fuel into the common intake plenum  160 , and are aligned with the intake ports  144 . Therefore, in use, the fuel injectors direct the injected fuel  152  towards the corresponding intake port  144 . Alternatively, the injectors  148 A,  148 B may inject downstream of the common intake plenum yet upstream of a corresponding intake valve  140 . However, with a gaseous fuel  152  and a common intake plenum  160 , it is possible that injected fuel  152  can travel through the common intake plenum  160  to additional intake ports  144 . This can increase the amount of injected gaseous fuel  152  in some intake ports  144  and decrease the amount in others. This inconsistency can lead to poor combustion within the piston cylinders  156 . Therefore, in certain embodiments, open valve injection is implemented. 
     With open valve injection, fuel  152  is injected towards the intake ports  144  of the cylinder head  124  only when the corresponding intake valve  140  is open. This prevents or at least limits the amount of injected fuel  152  which bounces off a closed intake valve  140  and spreads through the common intake plenum  160 . The fuel  152  mixes with a flow of intake air which is provided through the throttle body  128 , is selectively throttled via the throttle valve  132 , and mixes with the injected fuel  152  in the common intake plenum  160 . However, in order to supply the largest fuel demand of the engine  116  within an intake valve opening duration (i.e., the time that the intake valve  140  is in an open position), a second fuel injector  148 B, distinct from the first injector  148 A, with a sufficiently large flow capacity is provided for each cylinder  156 . The second fuel injectors  148 B may not be suitable for injecting very small amounts of fuel  152  when the engine  116  is running at idle or with a low load. Therefore, an additional injector, a first gaseous fuel injector  148 A is provided. 
     The injectors  148 A,  148 B may be operated at a variable energizing time which varies the quantity of injected fuel  152  per injection, up to a maximum capacity. For given operating conditions of the engine  116 , the first gaseous fuel injectors  148 A have an injection capacity of a first amount of gaseous fuel per injection and are limited to injecting no more than the first amount.  FIG. 2  shows a first operational mode of the engine  116  in which only the first injector  148 A is in operation for each of the cylinders  156 . Under the same operating conditions of the engine  116 , the second gaseous fuel injectors  148 B have an injection capacity of a second amount of gaseous fuel  152  per injection. The second gaseous fuel injectors  148 B are limited to injecting no more than the second amount.  FIG. 3  shows a second operational mode of the engine  116  in which only the second injector  148 B is in operation for each of the cylinders  156 . The second amount is greater than the first amount. However, the first and second fuel injectors  148 A,  148 B are not configured to simultaneously inject, rather, a control unit (not shown) determines which fuel injector  148 A,  148 B to use. 
     As shown in  FIG. 4 , injection quantity is plotted against engine speed, for an exemplary set of operating conditions (e.g., 7 bar inlet pressure; 120 crank angle degrees; maximum injection duration). At these conditions, the engine  116  may operate within a specific range  264 . A first mode  248 A, indicated by a diagonal cross-hatch, specifies engine loads at which the first injector  148 A injects a quantity of gaseous fuel  152 . The second injector  148 B is not capable of accurately injecting the amount of gaseous fuel  152  desired in the first area  248 A. A second mode  248 B, indicated by a stippling, specifies engine loads at which the second injector  148 B injects a quantity of gaseous fuel  152 . The first injector  148 A is not capable of injecting the amount of gaseous fuel  152  desired in the second area  248 B. A third mode  248 C, indicated by a cross-hatch, specifies engine loads at which both the first injector  148 A and the second injector  148 B are capable of injecting the requested quantity of gaseous fuel  152 . The control unit determines which fuel injector  148 A,  148 B to use, and the gaseous fuel  152  is injected with one of the two injectors  148 A,  148 B. If the fuel demand falls within the third mode, the control unit (not shown) may default to continue injecting with the injector used in the previous injection. 
     The line  268  represents a first amount, which is the upper limit of the first injector  148 A, wherein, at the specified conditions, the first injector  148 A is unable to inject more fuel per injection. The line  272  represents a second amount, which is the upper limit of the second injector  148 B, wherein, at the specified conditions, the second injector  148 B is unable to inject more fuel per injection. The line  276  represents a third amount, which is a non-zero lower limit of the second injector  148 B. The third amount is defined as the smallest injection target amount at which the second injector  148 B can meet the target within a predetermined acceptable range (e.g., less than 5 percent deviation from the specified target amount, less than 1 percent deviation from the specified target amount, etc.). 
     The intake valve  140  of each cylinder  156  transitions between an open position and a closed position in controlled relation to the crankshaft rotation and piston stroke. An intake valve opening duration is a length of time in which each of the intake valves  140  is in the open position. The intake valve opening duration will generally decrease as the operating speed of the engine  116  is increased. This assumes that each intake valve  140  is held open for a consistent number of crank angle degrees, however, this parameter may be variable (e.g., corresponding to an engine equipped with variable valve control). The intake valve opening duration provides a restriction, which limits the amount of fuel  152  which can be provided to the cylinder  140 . As an alternative to, or in combination with engine-load dependent injection, as shown in  FIG. 4 , the control unit can determine which fuel injector  148 A,  148 B is appropriate for injecting the gaseous fuel  152  based on the intake valve opening duration. 
     The first injector  148 A injects gaseous fuel  152  into the respective piston cylinder  156  when the engine  116  is operating with a first intake valve opening duration. The second injector  148 B injects gaseous fuel  152  into the respective piston cylinder  156  when the engine  116  is operating with a second intake valve opening duration, less than the first intake valve opening duration. Therefore, when the intake valve  140  is open for a short duration, in which the first injector  148 A is unable to inject a requested amount of fuel, the second injector  148 B, with a higher injection capacity than the first injector  148 A, injects the gaseous fuel  152 . 
     When the engine  116  is operating with a third intake valve opening duration, in which either of the first and the second fuel injectors  148 A,  148 B are configured to inject, the control unit determines which fuel injector  148 A,  148 B to use, and the gaseous fuel  152  is injected with one of the two injectors  148 A,  148 B. If the engine  116  is operating with the third intake valve opening duration, the control unit (not shown) may continue injection with the injector used in the previous injection.