Patent Publication Number: US-2019186391-A1

Title: Dual fuel engine control strategy for limiting cylinder over-pressurization

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a control strategy for a dual fuel engine system, and more particularly to switching the dual fuel engine system to a limited gas-to-liquid substitution mode where cylinder pressure data indicates cylinder over-pressurization. 
     BACKGROUND 
     Internal combustion engines are well known and widely used for propulsive power, electrical power generation, gas compression, liquid and gas transfer, and in various industrial applications. In a conventional four-cycle or two-cycle operating scheme, a mixture of fuel and air is combusted within an engine cycle to produce a rapid pressure rise and induce linear travel of a piston, ultimately rotating a crankshaft to provide torque for various purposes. Spark-ignited engines typically employ a liquid petroleum distillate fuel such as gasoline, or various gaseous fuels in the nature of natural gas, methane, propane, and various mixtures such as biogas, landfill gas, and mine gas. Compression-ignition engines typically utilize fuels such as diesel distillate fuel, biodiesel, and still other liquid fuels. In recent years, there has been significant interest in development of engines and operating strategies that are flexible with regard to fuel utilization. Fuel prices can be fairly dynamic and, moreover, certain fuels that have realized relatively increased abundance in recent years, such as natural gas, can have desirable combustion or emissions properties which are sought to be exploited. 
     One type of engine design that allows for operation with different fuel types combines both a diesel distillate fuel and natural gas. Diesel alone is relatively easy to compression ignite, but can produce undesired emissions. Where natural gas is used as a fuel in a diesel engine, without modification the mixture of natural gas and air can fail to ignite, knock, or have combustion stability problems. Various strategies have been developed that predominantly burn natural gas while using a relatively smaller amount of diesel fuel as a so-called pilot fuel. The diesel pilot fuel can ignite to in turn ignite the natural gas, offering relative predictability and reliability in the timing and manner of ignition, and otherwise combining certain advantages of both fuel types. One example of such an engine is known from U.S. Pat. No. 6,032,617 to Willi et al. 
     The term “substitution” or “substitution ratio” is commonly used to describe the relative contributions of diesel fuel and gaseous fuel in a dual fuel engine at any one time, and can be understood generally as the extent to which gaseous fuel is substituted for what would otherwise be diesel fuel in a single fuel liquid fuel engine. In certain dual fuel engines, particularly at relatively high levels of substitution, combustion events can occur that drive cylinder pressures above a maximum limit, typically based upon the capability or tolerance of the hardware to withstand pressure magnitude and pressure impulses. These relatively extreme pressure events can occur for a variety of reasons, including oil droplets present in the combustion chamber, variation in the composition of the gaseous fuel or gaseous fuel blend, under-delivery or over-delivery of gaseous fuel, temperature variation, or still other factors. 
     SUMMARY OF THE INVENTION 
     In one aspect, a dual fuel engine control system includes a pressure sensor structured for positioning in fluid communication with a combustion cylinder in a dual fuel engine system, and an electronic control unit. The electronic control unit is structured to receive cylinder pressure data from the pressure sensor, and to vary gas-to-liquid fuel substitution in the dual fuel engine system based on the cylinder pressure data. The electronic control unit is further structured to receive cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle during operation of the dual fuel engine system in a normal gas-to-liquid substitution mode. The electronic control unit is further structured to switch the dual fuel engine system to operation in a limited gas-to-liquid substitution mode based on the cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle. The electronic control unit is further structured to return the dual fuel engine system to operation in the normal gas-to-liquid substitution mode, and to receive cylinder pressure data indicative of cylinder over-pressurization in a later engine cycle after returning the dual fuel engine system to operation in the normal gas-to-liquid substitution mode. The electronic control unit is still further structured to output a gas substitution fault signal based on the data indicative of cylinder over-pressurization in a later engine cycle. 
     In another aspect, a method of operating a dual fuel engine system includes receiving data indicative of cylinder over-pressurization in an earlier engine cycle during operation of the dual fuel engine system in a normal gas-to-liquid substitution mode, and switching the dual fuel engine system to operation in a limited gas-to-liquid substitution mode in response to the data indicative of cylinder over-pressurization in an earlier engine cycle. The method further includes returning the dual fuel engine system to operation in the normal gas-to-liquid substitution mode, and receiving data indicative of cylinder over-pressurization in a later engine cycle after returning the dual fuel engine system to operation in the normal gas-to-liquid substitution mode. The method still further includes outputting a gas substitution fault signal in response to the data indicative of cylinder over-pressurization in a later engine cycle. 
     In still another aspect, a dual fuel engine system includes an engine having an engine housing with a plurality of combustion cylinders formed therein, and a dual fuel system coupled with the engine. The dual fuel system includes a plurality of liquid fuel admission valves for conveying a liquid fuel into the plurality of combustion cylinders, and at least one gaseous fuel admission valve for conveying a gaseous fuel into the plurality of combustion cylinders. The dual fuel engine system further includes a dual fuel control system having a plurality of pressure sensors in fluid communication with the plurality of combustion cylinders, and an electronic control unit. The electronic control unit is structured to receive cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle during operation of the dual fuel engine system in a normal gas-to-liquid substitution mode. The electronic control unit is further structured to switch the dual fuel engine system to operation in a limited gas-to-liquid substitution mode based on the cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle. The electronic control unit is further structured to return the dual fuel engine system to operation in the normal gas-to-liquid substitution mode, and to receive cylinder pressure data indicative of cylinder over-pressurization in a later engine cycle after returning the dual fuel engine system to operation in the normal gas-to-liquid substitution mode. The electronic control unit is still further structured to output a gas substitution fault signal in response to the cylinder pressure data indicative of cylinder over-pressurization in a later engine cycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially sectioned side diagrammatic view of a dual fuel engine system, according to one embodiment; 
         FIG. 2  is a block diagram of a control unit, according to one embodiment; 
         FIG. 3  is a concept diagram of cylinder pressure states, according to one embodiment; 
         FIG. 4  is a graph of dual fuel engine system operating states over time, according to one embodiment; and 
         FIG. 5  is a flowchart illustrating example control logic flow, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a dual fuel engine system  10  according to one embodiment, and including an internal combustion dual fuel engine  11  having an engine housing  12  with a plurality of pistons  14  positioned at least partially within a plurality of combustion cylinders  18  formed in engine housing  12 . Dual fuel engine system  10  (hereinafter “engine system  10 ”) also includes a crankshaft  16  coupled with pistons  14  in a generally conventional manner. In a typical embodiment, internal combustion engine  11  (hereinafter “engine  11 ”) is a multi-cylinder engine with cylinders  18  in an in-line configuration, a V-configuration, or any other suitable configuration. Pistons  14  are movable within engine housing  12  to compress a mixture of air and fuel in corresponding combustion cylinders  18  to an autoignition threshold, with a liquid fuel such as diesel distillate liquid fuel serving as a pilot fuel igniting a main charge of a gaseous fuel such as natural gas, as further discussed herein. As will be further apparent from the following description, engine system  10  is uniquely configured to vary and/or suspend substitution of gaseous fuel for liquid fuel (hereinafter “gas-to-liquid substitution”) in a manner that protects engine system  10  from over-pressurization and consequent hardware damage or performance degradation. 
     Engine system  10  further includes an air system  20  having an air inlet  22  structured to feed intake air for combustion to a compressor  24  in a turbocharger  32 . Air compressed by way of compressor  24  can be fed through an aftercooler  26 , and thenceforth to an intake manifold  28 . A plurality of intake runners  30  feed air from intake manifold  28  to individual cylinders  18 . In one implementation, a gaseous fuel for combustion is delivered into the flow of incoming intake air downstream of aftercooler  26  and into intake manifold  28  or intake runners  30 . In another implementation, gaseous fuel could be delivered into the flow of incoming intake air upstream of compressor  24 , for instance, and typically by way of a gaseous fuel delivery valve (not shown) positioned fluidly between air inlet  22  and compressor  24 . In still another embodiment, gaseous fuel is directly injected by way of gaseous fuel injectors into combustion cylinders  18 . An exhaust manifold  34 , that includes or is fluidly connected with exhaust runners  36  is in fluid communication with combustion cylinders  18  and feeds exhaust gases to a turbine  38  of turbocharger  32 , with the exhaust subsequently conveyed to an exhaust outlet or tailpipe  40 , possibly first passing through one or more exhaust aftertreatment devices (not shown) or potentially even a second turbine stage of a second turbocharger. 
     Engine system  10  further includes a dual fuel system  41  having a first fuel supply  42  that is a liquid fuel supply, and one or more liquid fuel pumps  44  structured to convey and pressurize liquid fuel for delivery to a plurality of liquid fuel admission valves  46 . Liquid fuel admission valves  46  can include liquid fuel injectors each positioned partially within one of combustion cylinders  18  and structured to directly inject liquid fuel therein. Alternatives such as port-injected designs are also contemplated. Moreover, pressurization of fuel to injection pressures could occur within liquid fuel admission valves  46  by way of fuel pressurization plungers driven by a cam, for instance. 
     Fuel system  41  also includes a second fuel supply  48  that is a gaseous fuel supply structured to contain a gaseous fuel in a compressed gaseous form or a cryogenically stored liquid form. A gaseous fuel pump  50  can pump the gaseous fuel in the subject liquid form or gaseous form to vaporization and pressurization equipment  52  of generally known design. From vaporization and pressurization equipment  52  the gaseous fuel can be fed to at least one gaseous fuel admission valve  54 . As noted above, a plurality of gaseous fuel delivery points by way of a plurality of gaseous fuel admission valves  54  is contemplated, however, the present disclosure is not thereby limited and delivery of gaseous fuel at a single point such as to intake manifold  28  or upstream of intake manifold  28  could be employed. 
     Engine system  10  further includes a dual fuel engine control system  56  (hereinafter “control system  56 ”) having one or more pressure sensors  58  structured for positioning in fluid communication with combustion cylinders  18  in engine system  10 . In the illustrated embodiment, each pressure sensor  58  is an in-cylinder pressure sensor which will be understood to include a movable or deformable element (not shown) that is responsive to fluid pressure in the corresponding combustion cylinder  18 . Pressure sensors  58 , hereinafter described in but not limited to the singular, produces an electrical output such as a voltage or a current that provides data indicative of cylinder pressure in real time. Control system  56  also includes an electronic control unit  60  structured to receive cylinder pressure data from pressure sensor  58 , and to vary gas-to-liquid fuel substitution in engine system  10  based on the cylinder pressure data, the significance of which will be further apparent from the following description. 
     Referring also now to  FIG. 2 , there is shown a block diagram of electronic control unit  60  illustrating example elements thereof. Electronic control unit  60  can include an input/output or I/O interface  62 , and a processor  64 . Processor  64  can include any suitable computerized data processing device such as a microprocessor, or a microcontroller. Processor  64  may be coupled with an electronic timer  65 , resident on electronic control unit  60  for example, and with a computer readable memory  66 . Computer readable memory  66  can include computer executable instructions recorded thereon for monitoring and/or controlling various aspects of operation of engine system  10 , including fueling control software  68 , a normal gas-to-liquid substitution map  70 , and a limited gas-to-liquid substitution map  72 . Normal gas-to-liquid substitution map  70  (hereinafter “fueling map  70 ”) might store fueling control commands, fueling amounts, or other fueling information according to a first map coordinate, engine speed information according to a second coordinate, and indicated mean effective pressure (IMEP) according to a third coordinate. Limited gas-to-liquid substitution map  72  (hereinafter “fueling map  72 ”) may be analogously configured. Fueling control software  68  can read fueling control values, such as for commanding a fueling amount of at least one of gaseous fuel and liquid fuel, from fueling map  70  when gas-to-liquid substitution is being employed in normal operation, and can read fueling control values, from fueling map  72  from a second mode, as further discussed herein. 
     Electronic control unit  60  is further structured to receive cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle during operation of engine system  10  in a normal gas-to-liquid substitution mode. As noted above, fueling control values in the normal gas-to-liquid substitution mode may be determined according to fueling map  70 . Electronic control unit  60  is also structured to switch engine system  10  to operation in a limited gas-to-liquid substitution mode based on the cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle. The term earlier is used herein in a relative sense in comparison to engine cycles further discussed and occurring later in time than an earlier engine cycle. An earlier engine cycle could precede a later engine cycle by a few minutes, several hours, or potentially several days. A middle engine cycle discussed in the present disclosure occurs in time between an earlier engine cycle and a later engine cycle, and an initial engine cycle precedes an earlier engine cycle. No particular separation in time amongst engine cycles is intended herein. 
     Electronic control unit  60  may further be structured to return engine system  10  to operation in the normal gas-to-liquid substitution mode, and to receive cylinder pressure data indicative of cylinder over-pressurization in a later engine cycle after returning engine system  10  to operation in the normal gas-to-liquid substitution mode. In an implementation, electronic control unit  60  can be further structured to start timer  65  based on the cylinder pressure data indicative of cylinder over-pressurization in an earlier engine cycle, and may further be structured to return engine system  10  to operation in the normal gas-to-liquid substitution mode based on expiration of timer  65 . These procedures can be thought of as receiving an indication of detected or likely cylinder over-pressurization, during gaseous fuel substitution at a relatively higher level, and responsively lowering gaseous fuel substitution for a period of time determined by timer  65 , if circumstances in the interim do not justify disabling gaseous fuel substitution or taking some other action. It should be appreciated that during operation in the normal gas-to-liquid substitution mode the relative extent of gaseous fuel substitution may be based on a first criterion, such as an ignitability criterion, for example, meaning that gaseous fuel could be supplied without limitation, subject to usual engine speed and engine load demands, so long as a sufficient amount of diesel fuel is delivered to provide robust and reliable pilot ignition. The specific amount of diesel fuel needed in such circumstances could be empirically determined. Additionally or alternatively, the relative extent to which gaseous fuel is substituted for diesel fuel could be user-specified based on factors such as gaseous fuel availability, price, or engine performance parameters that are desired. In the limited gas-to-liquid substitution mode the relative extent of gaseous fuel substitution could be based on a second criterion, such as a criterion that is a minimum deliverable amount of gaseous fuel, for example. Providing a minimum deliverable amount of gaseous fuel might be desirable to maintain some minimum level of operation of the gaseous fuel side of fuel system  41 . Alternatively, in the limited gas-to-liquid substitution mode, the relative extent of gaseous fuel substitution might be 0. In other words, the limited gas-to-liquid substitution mode could include a diesel-only mode in some embodiments. Moreover, it should also be appreciated that while electronic control unit  60  is structured to determine fueling commands by way of a stored first map such as fueling map  70  in the normal gas-to-liquid substitution mode, and by way of a stored second map such as fueling map  72  in the limited gas-to-liquid substitution mode, in other instances a single multi-coordinate map might be used, or a number of fueling maps greater than two might be used, or another fueling control strategy altogether. 
     As noted above, engine system  10  can be operated in the limited gas-to-liquid substitution mode for a period of time such as a period of time determined by way of timer  65 . Reasons for cylinder over-pressurization could include fuel droplets in a cylinder, variation in a composition of gaseous fuel or a gaseous fuel blend, or variations in fuel delivery, to name a few examples. These and other factors causing cylinder over-pressurization could be resolved by enabling engine system  10  to operate in the limited gas-to-liquid substitution mode whilst simultaneously or eliminating the risk of cylinder over-pressurization that could lead to hardware damage or performance degradation. Accordingly, once switched to operation in the limited gas-to-liquid substitution mode, engine system  10  may be thought of as having an opportunity for self-correction. As noted, if, during this period of time for potential self-correction, another cylinder over-pressurization event is detected, it might be determined that engine system  10  should be shut down or perhaps operated in a so-called limp home mode, for instance. Occurrence of cylinder over-pressurization in a limited gas-to-liquid substitution mode could be indicative of a hardware problem or a controls problem, such as excessive intake manifold temperature, improper valve timing, or various other problems. In any event, once the opportunity for self-correction has ended, an attempt may be made to return engine system  10  to operation in the normal gas-to-liquid substitution mode. Cylinder pressure data indicating cylinder over-pressurization in the later engine cycle as discussed herein may indicate that self-correction has not and/or is not likely to occur. Based on the cylinder pressure data indicative of cylinder over-pressurization in a later engine cycle, electronic control unit  60  can output a gas substitution fault signal. The fault signal could be logged as a designated error code in memory  66 , for instance, and control system  56  can take further action in response to the fault detection to ensure continued availability of operation of engine system  10 . For instance, electronic control unit  60  may be further structured to switch engine system  10  to operation in a liquid-only mode based on the gas substitution fault signal. A liquid-only mode could include the limited gas-to-liquid substitution enabled by fueling map  72 , where the substitution limit is 0, or another fueling map that is a liquid-fueling-only map could be used, for example. It is further contemplated that rather than merely an earlier engine cycle and a later engine cycle where cylinder over-pressurization is detected, in some instances a third occurrence of cylinder over-pressurization and two opportunities for self-correction might be required before a fault state is triggered. To this end, electronic control unit  60  may be further structured to receive cylinder pressure data indicative of cylinder over-pressurization in an initial engine cycle preceding the earlier engine cycle during operation of engine system  10  in the normal gas-to-liquid substitution mode. 
     Referring now also to  FIG. 3 , it will be recalled that pressure sensor  58  produces cylinder pressure data potentially continuously, but typically at least over the course of a majority of a compression stroke and an ignition stroke in a given engine cycle. Cylinder pressure data indicative of over-pressurization in the later engine cycle, and potentially also in the earlier engine cycle and where considered the initial engine cycle, can include cylinder pressure magnitude data and also cylinder pressure timing data. In this way, electronic control unit  60  can determine not only a peak cylinder pressure but also a time duration of occurrence of peak cylinder pressure, or cylinder pressure above some threshold. A specific threshold in this regard could be determined empirically for each engine, and will typically be based upon a hardware limit. In  FIG. 3 , a fault-triggered or over-pressurization zone  76  is shown, and a no-fault or normal zone  78 , in a concept diagram  74  where pressure is shown on the X-axis and time is shown on the Y-axis. In diagram  74  a threshold  80  separates zone  76  from zone  78 , with cylinder pressure events shown at  82  and  86  within zone  78 . Cylinder pressure events  84  and  88  are shown in zone  76 . It can be appreciated that a relatively high pressure magnitude but a relatively short time duration is associated with pressure event  82 , whereas a relatively long time duration and relatively low pressure magnitude is associated with pressure event  84 . Event  82  is within zone  78 , where electronic control unit  60  would likely not output a gas substitution fault signal, whereas event  84  is within zone  76  where electronic control unit  60  would likely output a gas substitution fault signal. Event  86  and event  88  can both be seen to be associated with a medium cylinder pressure magnitude and a medium cylinder pressure duration, but are nevertheless upon opposite sides of threshold  80 . From  FIG. 3  and the accompanying description it will be understood that electronic control unit  60  can be structured to compare cylinder pressure magnitude and cylinder pressure timing to a threshold determined according to a magnitude-duration sliding scale. Another way to understand the principles set forth in  FIG. 3  is that a relatively high pressure and relatively short duration need not trigger a fault, whereas a relatively lower pressure but a relatively longer duration might trigger a fault. Individual dual fuel engine systems or classes of dual fuel engine systems can be calibrated to determine cylinder pressure timings, and cylinder pressure magnitudes, that define the magnitude-duration sliding scale according to sensitivity to over-pressurization, operational characteristics, and other factors. 
     INDUSTRIAL APPLICABILITY 
     Referring to the drawings generally, but in particular now to  FIG. 4 , there is shown a graph  90  illustrating engine system operation according to one embodiment of the present disclosure, with time (potentially in hours) shown on the X-axis and gas-to-liquid substitution percent shown on the Y-axis. A trace is shown at  91  and illustrates the general patterns of substitution variation that might be observed where engine system  10  is transitioned through different modes with attempted substitution of gaseous fuel for liquid fuel, and eventual reversion to liquid-only mode. In a period  92  gaseous fuel substitution is increased up to a level shown, for example, that is at about 80 percent in gas-to-liquid substitution, and where engine system  10  is then operated at that substitution for a period  94 . A cylinder over-pressurization event is detected at  96 , and thereafter substitution is reduced in a period  98  as engine system  10  is switched to the limited gas-to-liquid substitution mode, in which it is operated for a period  100 . Period  100  represents an opportunity for self-correction as discussed herein. In a period  102  gas substitution is again attempted and the substitution percent is increased from about 30 percent in period  100  to about 80 percent in a period  104 . Another over-pressurization event is shown at  106 , and substitution is again reduced in a period  108 , with engine system  10  then operated in the limited gas-to-liquid substitution mode in a period  110 . In a period  112  substitution is again increased and engine system  10  operated in the normal gas-to-liquid substitution mode in a period  114 . At  116  another cylinder over-pressurization event is detected. Cylinder over-pressurization event  116  can include a third cylinder over-pressurization event, for example, that triggers outputting the gas substitution fault signal, and subsequent operation for a period  118  in liquid-only mode. It will be appreciated that over-pressurization event  96  could be detected in an initial engine cycle as contemplated herein, over-pressurization event  106  detected in an earlier engine cycle or a middle engine cycle, and over-pressurization event  116  detected in a later engine cycle. As also noted, the periods of time such as period  100  and period  110  where engine system  10  is allowed opportunity for self-correction could be determined individually for a dual fuel engine, or a class of dual fuel engines. 
     Turning now to  FIG. 5 , there is shown a flowchart  200  illustrating example methodology and control logic flow. At a block  210  electronic control unit  60  receives data from pressure sensor  58  indicative of cylinder over-pressurization in an earlier engine cycle. From block  210 , the logic can advance to block  220  to switch engine system  10  to limited gas-to-liquid substitution mode. From block  220 , the logic can advance to a block  230  to return engine system  10  to the normal gas-to-liquid substitution mode. From block  230 , the logic can advance to block  240  to receive data indicative of cylinder over-pressurization in a later engine cycle. From block  240  the logic can advance to block  260  to output the gas substitution fault signal, and to block  270  to disable gaseous fuel delivery or otherwise operate engine system  10  in the liquid-only mode. It will be appreciated that the illustration of  FIG. 5  is exemplary only. Embodiments are contemplated where 2, 3, 4, 5, or still another number of cylinder over-pressurization events will occur prior to a fault condition being determined. It should also be appreciated that electronic control unit  60  might also receive data indicative of cylinder over-pressurization in a middle engine cycle, during operation of engine system  10  in the limited gas-to-liquid substitution mode. This would correspond, for example, to data indicative of cylinder over-pressurization being received during period  100  or period  110  in the  FIG. 4  illustration. Electronic control unit  60  could output a hardware-faulted signal based on the data indicative of cylinder over-pressurization in the middle engine cycle to enable logging an error code, shutting down engine system  10 , initiating a limp-home mode, or taking still another action. 
     The present description is for illustrative purposes only, and should not be construed to narrow the breadth of the present disclosure in any way. Thus, those skilled in the art will appreciate that various modifications might be made to the presently disclosed embodiments without departing from the full and fair scope and spirit of the present disclosure. Other aspects, features and advantages will be apparent upon an examination of the attached drawings and appended claims. As used herein, the articles “a” and “an” are intended to include one or more items, and may be used interchangeably with “one or more.” Where only one item is intended, the term “one” or similar language is used. Also, as used herein, the terms “has,” “have,” “having,” or the like are intended to be open-ended terms. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.