TECHNICAL FIELD

[0001] The present disclosure relates to skip-fire engine technologies. More specifically, the present disclosure relates to skip-fire fuel-injection engine technologies for engines fueled with two different fuels.

BACKGROUND

[0002] Many related art engine systems utilize skip-firing schemes or fueling schemes. Such related art skip-firing schemes include various skip-firing patterns and various fueling strategies, but they do not provide a solution for increasing the diesel substitution factor (DSF), that is, decreasing the amount of diesel fuel that is consumed and replacing it with another fuel to provide the desired amount of total energy to fulfill the demanded engine load. Because diesel fuel is readily available, and its ignition properties are well known, diesel fuel is known to be used as a pilot fuel for triggering combustion of other fuels that are less readily ignited, such as natural gas or other gaseous fuels. However, those familiar with engines fueled with a main fuel and a pilot fuel to assist with the ignition of the main fuel will understand that other substances, such as dimethyl ether or kerosene, could be substituted as the pilot fuel. Accordingly, references in this disclosure to "diesel" and "DSF" will be understood to make reference to any fuel that can be employed as a pilot fuel to trigger the combustion of a different fuel, which is employed as the main fuel, which on average, constitutes the majority of the fuel that is consumed by the engine. Several references relating to skip-firing schemes are discussed as follows in this Background section.

[0003] For instance, U.S. Patent No. 5,553,575 relates to a "gas-fueled unthrottled internal combustion engine" having an excess air ratio (lambda) that is optimized by selecting automatically and continuously the optimum fraction of combustion chambers firing (OFF) as a function of engine operating parameters. Further lambda adjustment is performed by suitable control of exhaust gas recirculation (EGR), ignition timing, and/or turbo air bypass (TAB). More specifically, the '575 patent discloses a dual- fuel system which can be fueled with port-injected natural gas and an ignition assist system that can comprise a spark plug or a fuel injector for introducing pilot quantities of diesel fuel directly into the combustion chamber. Port-injected natural gas is injected into the intake port, upstream of the engine intake valve so that the natural gas mixes with the intake air during the engine's intake and compression stroke. If the mixture of natural gas and air detonates prematurely during the compression stroke, this premature detonation is commonly referred to as "engine knock" and this can result in significant damage to the engine. To reduce the risk of engine knock, engines such as this normally reduce the compression ratio and/or reduce the amount of natural gas and increase the amount of diesel fuel that is consumed by the engine. Compared to a conventional diesel engine, in which the fuel is injected directly into the combustion chamber late in the compression stroke, forming a stratified charge, fuel that is port-injected has more time to mix with the air, forming a more homogeneous mixture. The '575 patent also discloses that, for port- injected natural gas it is particularly important to maintain lambda within a narrow range for the efficient combustion of the homogeneous mixture, and to avoid misfiring and excessive production of NOx. Accordingly, the '575 patent is directed to a method for controlling lambda and is not directed towards a method of reducing the amount of diesel fuel consumed. [0004] Like the '575 patent, U.S. Patent No. 5,477,830 also relates to an internal combustion engine with natural gas that is injected into the intake air system, upstream of the combustion chamber intake valves. However, the '830 patent is specifically directed to an electronic fuel injection system for the precise distribution of natural gas into each cylinder for engines that use a shared intake port for a pair of cylinders. Duration and timing of the fuel injection pulse and other injection strategies, such as a skip-fire operation, are controlled and enabled, but, like the '575 patent, because the '830 patent is also directed to a dual fuel engine that teaches fumigating the natural gas to form a homogeneous mixture and controlling the air-fuel mixture (lambda), the objective is not increasing the DSF. With any engine that injects a main fuel into the intake air system, and injects a second fuel, such as diesel fuel directly into the combustion chamber, there are times when the DSF is decreased (not increased). For example, when the amount of port injected fuel is limited to prevent engine knock, under these conditions the amount of injected diesel fuel must be increased to satisfy the total amount of energy that needed for the commanded engine load and speed condition. With engines such as those disclosed in the '575 patent and the '830 patent it would not be uncommon under some normal operating conditions for the fuel delivered to the engine to comprise between 50% and 100% diesel fuel.

[0005] Skip-fire techniques are utilized by some conventional gasoline or diesel mono- fueled engines, but for engines that are fueled with only one fuel, DSF is not applicable. Rather, various other purposes for using skip-fire techniques combined with different fuel injection strategies are used in the general field of internal combustion engines, for example, to reduce smoke emissions, to increase boost pressure, and to adjust air to fuel ratio. Some representative skip-fire techniques are discussed in the following paragraphs of this Background section.

[0006] U.S. Patent No. 5,826,563 relates to a high horsepower locomotive diesel engine that is operated in a skip-firing mode, wherein the engine includes a plurality of individually controllable, fuel-injected cylinders. The system senses that the engine is operating in a low horsepower mode and has a low fuel demand. The pattern selected for firing the cylinders is arranged such that all cylinders of the engine are fired within a preselected number of crankshaft rotations. The system also senses the engine air-fuel ratio and adjusts the pattern of cylinders being fired so as to maintain exhaust emissions below a preselected level. Additionally, the pattern of fired cylinders may be adjusted to maintain engine operating temperature and as a function of engine speed. Accordingly, the '563 patent relates to skip-fire for the purpose of adjusting the air to fuel ratio and adjusting the total fuel limit value for reducing smoke emissions in locomotive diesel systems.

[0007] U.S. Patent No. 6,405,705 and continuation-in-part U.S. Patent No. 6,823,835 both relate to a diesel engine having a plurality of individually controllable fuel-injected cylinders that is operated in a skip-firing mode to reduce smoke emissions during low power operation. The system senses certain identified engine operating parameters; and, when these parameters exceed predetermined thresholds for a predetermined time, then skip-firing is implemented. Upon implementation of skip-firing, the engine timing angle is reset by a fixed angle; and a multiplication factor is included in the speed loop integrator to ensure that the appropriate fuel volume value is injected into each cylinder immediately upon initiation of skip-firing. However, the '705 patent relates to skip-fire in conjunction with adjusting the air to fuel ratio and adjusting the total fuel limit value for reducing smoke emissions in locomotive engine systems, and the '835 patent relates to adding fuel from skipped cylinders into fueled cylinders for adjusting air to fuel ratio in order to maintain performance parameters.

[0008] U.S. Patent No. 6,408,625 relates to an electric power generation system which includes a back-up electric power generator driven by a four-cycle internal combustion engine. The engine includes a compressor along an intake pathway to deliver pressurized air to the cylinders and a turbine along an exhaust pathway to power the compressor when driven by exhaust discharged from the cylinders. The engine is prepared to accept a generator load by increasing boost pressure provided by the compressor. This increase in boost pressure is accomplished by skip-firing the cylinders in a selected pattern, thereby retarding ignition timing for the cylinders, or by using a combination of these techniques. Accordingly, the '625 patent relates to a skip-firing technique for increasing boost pressure.

[0009] U.S. Patent No. 8,136,497 involves a method for improving starting of an engine that may be repeatedly stopped and started to improve fuel economy. In one embodiment, the method involves using skip-fire when the engine is idling to reduce fuel consumption and prevent the engine speed from overshooting the desired idle speed. Another embodiment is disclosed whereby skip-fire is employed for torque control.

[0010] U.S. Patent Application Publication No. 2011/0253113 relates to an engine that is configured with an exhaust gas recirculation (EGR) system, The EGR system comprises exhaust manifolds from one or more cylinders being connected to an intake system, such cylinders being referred to as donor cylinders. These donor cylinders are the cylinders from which exhaust gas is recirculated to the intake. For an engine which also uses skip- fire, the 2011/0253113 publication relates to various methods and systems for operating an internal combustion engine that has one or more donor cylinders and one or more non- donor cylinders. Accordingly, depending upon the engine operating conditions, the 2011/0253113 publication is directed to methods for choosing whether to skip either donor cylinders or non-donor cylinders when skip-fire is commanded. For example, during an EGR cooler heating mode, the system operates at least one of the donor cylinders at a cylinder load that is sufficient to increase an exhaust temperature for regenerating an EGR cooler and operates at least one of the non-donor cylinders in a low- or no-fuel mode.

[0011] U.S. Patent No. 8,131,447 relates to a variety of methods and arrangements for improving fuel efficiency of internal combustion engines, including selectively skipping combustion events so that other working cycles can operate at a better thermodynamic efficiency. A controller is used to dynamically determine the chamber firings required to provide the engine torque based on the engine's current operational state and conditions. The chamber firings may be sequenced in real time or in near real time in a manner that helps reduce undesirable vibrations of the engine.

[0012] While these background examples may relate to skip-fire techniques in association with a variety of technical problems, they fail to disclose an engine that injects two different fuels directly into the combustion chamber, or any methods for increasing the amount of one fuel that is substituted for the other fuel. More specifically, when diesel fuel is employed as a pilot fuel, none of these background examples discloses increasing DSF and reducing overall diesel pilot fuel consumption in direct- injection compression-ignition engine systems.

SUMMARY

[0013] The present disclosure generally involves a system and method that provide many beneficial features and advantages over the systems and methods referenced in the Background section above, including, significantly reducing overall pilot fuel consumption for engines that use a pilot fuel to trigger the ignition of a different fuel that serves as the main fuel, especially for compression-ignition engine systems. The present system and method also involve a skip-fire fuel-injection strategy that increases the pilot fuel substitution factor, referred to herein as the DSF. Compared to engines that inject fuel into the intake port or elsewhere upstream from the intake valve, for an engine that injects fuel into the combustion chamber either via a pre-chamber or directly, as in the present disclosure, fuel must be injected at higher pressures to overcome the in-cylinder pressure that increases during the compression stroke.

[0014] In this disclosure, high-pressure direct-injection (HPDI) engine systems denote engine systems that introduce at least some of the main fuel and the pilot fuel into a combustion chamber during the compression stroke or near the beginning of the power stroke (this injection timing being referred to herein as "late cycle injection timing"). In an HPDI engine system, the timing for injecting the fuel into the combustion chamber is determined based on the desired timing for ignition of the fuel, with the fuel burning in a stratified combustion mode rather than a pre -mixed combustion mode. When a fuel is injected directly into a combustion chamber there is normally a time delay, referred to herein as the "ignition delay", between the timing for start of injection and the timing for start of ignition. Accordingly, in an HPDI engine system, so that the fuel ignites at the desired time, thereby preventing premature detonation, the timing for start of injection can be determined based upon the timing for start of ignition, minus the ignition delay associated with detected engine operating conditions. That is, for an HPDI engine system, both fuels can be introduced into the combustion chamber after the associated intake valve closes, whether this is accomplished by way of being injected directly into the combustion chamber or indirectly injected via a pre-chamber. As previously noted, engines that use port injection can be "knock limited," meaning that a limit exists on the amount of fuel that can be safely port-injected into the intake air system upstream from the combustion chamber intake valve. Such port-injected engines require an increased amount of directly-injected pilot fuel to constitute the total amount of fuel that is needed on an energy basis. However, for HPDI engine systems, in accordance with the present disclosure, reducing the amount of pilot fuel to only that which is needed for ensuring ignition of the main fuel is possible, because, with late cycle injection timing, less danger of engine knock exists in that the fuel is not introduced into the combustion chamber until the intended time for its ignition. A requisite total fueling is mandated by a requisite engine power for any given engine operating condition.

[0015] In particular, the present method comprises a skip-fire strategy, including a cycle- by-cycle skip-firing pattern, in accordance with the present disclosure. Regardless of whether an engine is a 2-stroke engine or a 4-stroke engine, a power stroke is associated with each cycle. By controlling whether any fuel is introduced into a combustion chamber, the disclosed method involves selectively skipping the firing in each cylinder on a cycle-by-cycle basis. In preferred embodiments, the skipped cylinders are selected in a pattern such that these skipped cylinders reduce the formation of harmonic frequency vibrations in the engine. In addition, the present cycle -by-cycle skip-firing pattern further comprises a switching period during a given cycle which provides sufficient time for determining which cylinders to skip for a subsequent cycle.

[0016] In addition, the present system and method generally involve a skip-fire technique combined with a fuel- injection strategy for modulating the fuel delivery. At low engine loads, as the total fuel requirement decreases, a limit exists as to how much the pilot fuel quantity can be reduced while also ensuring stable combustion. As the pulse width of a pilot fuel injection event decreases, the potential for variability is higher in the amount of pilot fuel that is injected, because many variables exist in addition to the variability in the pulse width, such as the fuel pressure, cylinder pressure, injector-to-injector differences, and cylinder-to-cylinder differences. When the pulse width is very short, the cycle-to- cycle differences in the amount of fuel injected represent a larger fraction of the total fueling compared to the cycle-cycle differences in the amount of fuel injected when the commanded pilot fuel quantity is larger. Accordingly, at low loads, to improve combustion stability, setting a lower limit on the amount of pilot fuel that is commanded is preferable; and, for engines that do set a lower limit on the amount of pilot fuel, by using a skip-fire technique, the overall amount of pilot fuel that is consumed in all of the firing cylinders is lower than in related art engines that use all cylinders without employing a skip-fire technique. It is noteworthy that with the presently disclosed skip fire method, the DSF can be increased in part because the amount of pilot fuel is determined mainly by the amount needed for acting as a pilot fuel and achieving stable combustion. Unlike with the dual fuel engines described in the background, when the engine is under load, the amount of pilot fuel is not determined by the energy needed to satisfy the engine load.

[0017] The present method of skip-firing an engine system, having a plurality of combustion chamber, a fuel-injection system for delivering fuel to each combustion chamber, and said engine system being operable with a main fuel and a pilot fuel, and the method generally comprising: detecting whether the engine is experiencing a predetermined low load condition for applying a skip-fire injection mode; when the predetermined low load condition is detected, determining and selecting at least one combustion chamber of the plurality of combustion chambers designated for skip-firing during a next cycle; skip-firing the selected at least one combustion chamber for a given duration whereby pilot fuel substitution is increased and overall pilot fuel consumption is reduced; determining whether the engine continues experiencing the low load condition during the given duration; and, if so, repeating determining and selecting at least one combustion chamber of the plurality of combustion chambers to skip-fire during the next cycle, and if not, returning the plurality of combustion chambers to a normal injection mode.

[0018] The disclosed method also comprises at least one of: selecting a different one or more combustion chambers for skip-firing in the following cycle, continuing to skip-fire the same selected combustion chamber(s) for a predetermined number of cycles or for a predetermined time duration, or when a plurality of combustion chambers are selected, continuing to skip-fire a select one or more of the same selected combustion chambers in addition to skip-firing a select one or more of different combustion chambers. After completing the predetermined number of cycles or completing the predetermined time duration, the method further comprises determining whether the engine continues to experience the low load condition; and, if so, repeat determining and selecting at least one combustion chamber of the plurality of combustion chambers to skip-fire during the next cycle, and if not, ending the skip-fire mode and returning the plurality of combustion chambers to a normal injection mode.

[0019] The present skip-fire fuel-injection engine system generally comprises an engine system, such as a compression-ignition engine system, e.g., a diesel engine system, modified and operable for fueling with a main fuel and a pilot fuel. The engine system has a plurality of combustion chambers and a fuel injection system, preferably for separate and independent injection of the pilot fuel and the main fuel. More particularly, in preferred embodiments, the pilot fuel can be diesel fuel and the main fuel can be natural gas, or other suitable gaseous fuels, such as methane, propane, hydrogen, and mixtures thereof. The engine system further comprises a feedback and control system in electronic communication with the fuel injection system. The feedback and control system is adapted to: determine whether the engine is experiencing a low load condition for applying a skip-fire injection mode; determine and select at least one combustion chamber of the plurality of combustion chambers designated for skip-firing during a next cycle. [0020] The present method of fabricating a skip-fire engine system generally comprises providing an engine system that employs a pilot fuel to trigger the ignition of a main fuel, or modifying a diesel engine system for fueling with a main fuel that uses a pilot fuel to trigger ignition of the main fuel. The engine system has a plurality of combustion chambers and a fuel injection system for introducing the pilot fuel and the main fuel into each one of the respective combustion chambers. In preferred embodiments the fuel injection system injects the pilot fuel and the main fuel directly into the combustion chamber. However, the present disclosure also contemplates the method comprising providing a pre-chamber into which one or both fuels are injected. In either case, the common feature is that late cycle injection is enabled after the intake valve is closed. The present fabrication method also comprises providing a feedback and control system in electronic communication with the fuel injection system the diesel engine system. The feedback and control system is adapted to: determine whether the engine is experiencing a low load condition for applying a skip-fire injection mode; determine and select at least one combustion chamber of the plurality of combustion chambers designated for skip- firing during a next cycle; skip-fire the selected at least one combustion chamber for a given duration; determine whether the engine continues experiencing the low load condition during the given duration; and, if so, repeat determining and selecting at least one combustion chamber of the plurality of combustion chambers to skip-fire during the next cycle, and if not, return the plurality of combustion chambers to a normal injection mode. A normal injection mode is defined herein to be a mode in which pilot fuel and main fuel are delivered to each of the engine's respective combustion chambers at a respective timing so that each of the engine's pistons is doing substantially the same amount of work for a given operating condition.

BRIEF DESCRIPTION OF THE DRAWING

[0021] The above aspects and other aspects, features, and advantages of several embodiments of the present disclosure will be more apparent from the following Detailed Description as presented in conjunction with the following several figures of the Drawing.

[0022] FIG. 1 is a schematic diagram illustrating an engine system with a plurality of cylinders, with each a fuel injector for injecting a pilot fuel and a main fuel into each combustion chamber, in accordance with an embodiment of the present disclosure.

[0023] FIG. 2A is a graph illustrating command signals over time for injection fuel into a combustion chamber for a low load operation mode of an engine system, according to the prior art. [0024] FIG. 2B is a graph illustrating command signals over time for injecting fuel into a combustion chamber for a low load operating mode with a skip-fire fuel-injection operation mode of an engine system, in accordance with an embodiment of the present disclosure. [0025] FIG. 3 is a cross-sectional view of one cylinder, illustrating a portion of an HPDI engine system suitable for use in a skip-fire operation mode, in accordance with an embodiment of the present disclosure.

[0026] FIG. 4A is a graph illustrating a ratio of pilot fuel to main fuel as a function of power for an engine system, in accordance with an embodiment of the present disclosure.

[0027] FIG. 4B is a graph illustrating torque as a function of engine speed for an engine system, in accordance with an embodiment of the present disclosure. [0028] FIG. 5 is a flowchart illustrating a skip-fire fuel- injection method, in accordance with an embodiment of the present disclosure. [0030] FIG. 6 is a flowchart illustrating a method of fabricating an engine system, in accordance with an embodiment of the present disclosure.

[0031] Corresponding reference characters indicate corresponding components throughout the several figures of the Drawing. Elements in the several figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be emphasized relative to other elements for facilitating understanding of the various presently disclosed embodiments. Also, common, but well-understood, elements that are useful or necessary in commercially feasible embodiments are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present disclosure.

DETAILED DESCRIPTION

[0032] The following description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of exemplary embodiments. The scope of the disclosure should be determined with reference to the Claims. Reference throughout this specification to "one embodiment," "an embodiment," or similar language means that a particular feature, structure, or characteristic that is described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0033] Further, the described features, structures, or characteristics of the present disclosure may be combined in any suitable manner in one or more embodiments. In the Detailed Description, numerous specific details are provided for a thorough understanding of embodiments of the disclosure. One skilled in the relevant art will recognize, however, that the embodiments of the present disclosure can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present disclosure.

[0034] Referring to FIG. 1, this schematic diagram illustrates engine system 100, such as a compression-ignition engine system, e.g., a diesel engine system, in accordance with an embodiment of the present disclosure. In this example, the engine has six cylinders 10 that each have two associated fuel injectors mounted to deliver fuel into the combustion chamber defined by each cylinder. Pilot fuel injector 70' is connected to pilot fuel rail 72' and pilot fuel is supplied to pilot fuel rail 72' from pilot fuel supply system 74', which includes pilot fuel pump 76' and pilot fuel storage tank 78'. Main fuel injector 70" is connected to main fuel rail 72" and main fuel is supplied to main fuel rail 72" from main fuel supply system 74", which includes main fuel pump 76" and main fuel storage tank 78". In FIG. 1 pilot fuel injector 70' and main fuel injector 70" are shown in separate bodies, but as shown in FIG. 3, and for example, the applicant's own U.S. Patent Nos. 6,439,192 and 6,761,325, the two injectors can be integrated into a single body. In the illustrated embodiments, because the main fuel is injected directly into the respective combustion chambers, at least some of the main fuel can be injected after the respective intake valves are closed, with the timing being determined to prevent the formation of a combustible mixture that ignites prematurely to cause engine knock. During a normal operating mode, a normal injection mode is employed, with the total fueling delivered to each cylinder 10 comprising a pilot fuel quantity A and a main fuel quantity B with substantially the same amount of fuel and with substantially the same timing for each cylinder for any given operating condition. In preferred embodiments, the pilot fuel comprises a diesel fuel, preferably in a range of approximately 5% of the total fueling, measured on an energy basis. The main fuel can comprise compressed natural gas, preferably in a range of approximately 95% of the total fueling, measured on an energy basis. While the illustrated engine system has six cylinders, it will be appreciated that any engine system with a plurality of combustion chambers can benefit from this method. In the illustrated example, with a six-cylinder engine, a normal injection mode during a normal operating mode comprises modulating a predetermined minimum pilot fuel quantity and modulating a main fuel quantity for each of all six cylinders at a ratio that maintains a requisite engine power. [0035] Referring still to FIG. 1, engine system 100 comprises feedback control system 200 in electronic communication with the fuel injection system. Feedback control system 200 is adapted to: detect whether the engine system 100 is experiencing a predetermined low load condition associated with a skip-fire injection mode; when the predetermined low load is detected, determine and select at least one combustion chamber of the plurality of combustion chambers associated with cylinders 10 that is designated for skip- firing during a next cycle; skip-fire the selected at least one combustion chamber for a given duration; determine whether the engine system 100 continues experiencing the low load condition during the given duration; and, if so, repeat determining and selecting at least one combustion chamber of the plurality of combustion chambers to skip-fire during the next cycle, and if not, return the plurality of combustion chambers to a normal injection mode. Feedback control system 200 can control pilot fuel injector 70' independently from its control of main fuel injector 70" so that the timing for injection of each fuel and the quantity of each fuel injected can be determined to achieve the desired combustion characteristics, and to increase the DSF. Because each fuel injector is individually controlled, feedback control system 200 can be programmed to follow a predetermined cycle-by-cycle skip-firing pattern that reduces vibrations and avoids harmonic frequencies when selecting at least one combustion chamber of the plurality of combustion chambers that is designated for skip-firing. While the skip-fire operation mode can cause irregular structural loading of engine components, for example, the pistons, the crankshaft(s), these effects are minimized at low loads and have less influence with engines that have a greater number of cylinders. In preferred embodiments, the main fuel comprises a gaseous fuel, such as natural gas, that can be stored as liquefied natural gas (LNG) or compressed natural gas (CNG); and the pilot fuel comprises diesel fuel. The given duration for the skip-fire injection mode comprises a switching period having sufficient time for facilitating detecting and selecting the at least one combustion chamber for skip-fire during the next cycle.

[0036] FIGS. 2A and 2B illustrate how the disclosed skip-fire technique increases the pilot fuel substitution factor, resulting in reduced overall pilot fuel consumption. FIG. 2A shows the injector command signals for a six cylinder engine system operated without skip-firing. The cylinder numbers along the left hand side of the figure represent cylinder firing order. Over the same time scale, indicated by crank angle degrees, for each cylinder there is a respective command signal for the pilot fuel, Ai, A ₂ , A3, A4, A5 and A6, and a respective command signal for the main fuel, Bi, B2, B3, B ₄ , B5 and Be. Because there is a minimum quantity of pilot fuel to ensure stable combustion, at lower load conditions there is a minimum amount of pilot fuel that is delivered to the pilot fuel injectors when all six cylinders are firing in a normal operating mode. FIG. 2B shows the injector command signals the same six cylinder engine operated with skip-firing. In this example, no fuel is delivered to half of the cylinders (Α ₂ ', Α ₄ ' and A^) so while the same minimum amount of pilot fuel is injected into each cylinder, pilot fuel consumption is reduced by half, and the main fuel B2', B ₄ ' and Be' can be increased to the cylinders that are firing to make up the energy requirement to satisfy the demanded engine load. In this way, the engine system is operable using a lower ratio of the pilot fuel to a total fuel quantity measured on an energy basis at predetermined low load conditions compared to the same engine system when operated without employing a skip-fire operating mode. Like the pilot fuel injectors, with the main fuel injectors there is also a lower limit on the amount of fuel that can be consistently injected so increasing the amount of main fuel that is injected into each combustion chamber using the skip-fire technique also helps to improve combustion stability. Using the disclosed skip-fire technique, and increasing DSF at low load helps to improve the overall DSF. During operation over a range of engine loads for a typical operating cycle for engines used to power a vehicle, with the skip-fire technique, the pilot fuel comprises approximately 5% or less of a total fueling measured on an energy basis. The main fuel comprises approximately 95% or more of a total fueling measured on an energy basis. Under certain predetermined load conditions, the engine system can be operable in a mode when the pilot fuel is the only fuel consumed by the engine system. During skip-firing operating mode different patterns for cylinder firing can be employed such that over time all cylinders are fired. It is desirable to fire each cylinder during skip-firing operating mode, eventually, such that diesel accumulation in the injector is reduced. Due to the pressure differential between gas and diesel, diesel accumulation in the injector causes the diesel to flow into the gas rail. For example, during one engine cycle cylinders 1, 3 and 5 can be fired, and during the next engine cycle cylinders 2, 4 and 6 can be fired. It is not a requirement that the cylinders to be fired are changed from engine cycle to engine cycle.

[0037] Referring to FIG. 3, this cross-sectional view illustrates one cylinder of skip-fire fuel-injection engine system 100 (as shown in FIG. 1) that comprises a plurality of cylinders, suitable for use in a skip-fire operation mode, in accordance with an embodiment of the present disclosure. By example only, the engine system 100 generally comprises: a cylinder 10 formed by cylinder walls 24 of an engine block 25; a piston 20 mechanically and rotatably coupled with a piston rod (not shown) by way of a pin (not shown) disposed through opening 30; intake manifold 40 for delivering air in direction I into combustion chamber 50 by way of operation of intake valve 41; exhaust manifold 60 for delivering exhaust in direction E away from combustion chamber 50 by way of operation of exhaust valve 61; and fuel injector 70, being one component of a fuel injection system, for delivering fuel to the combustion chamber. In this illustrated embodiment, fuel injector 70 is designed to have two injector assemblies in one body for injecting a pilot fuel, such as a diesel pilot, and a main fuel, such as natural gas. In preferred embodiments fuel injector 70 can comprise concentric needles, one needle for controlling the injection of the pilot fuel and another needle for controlling the injection of the main fuel. In another embodiment, the fuel injector can inject the pilot fuel and the main fuel together or in yet other embodiments (not shown) for engines with more space to accommodate other arrangements, there can be two separate injection valve assemblies in the same body (for example, side by side and parallel to each other), or (as shown in FIG. 1) two separate fuel injectors each with its own body mounted separately.

[0038] Referring to FIG. 4A, this graph illustrates a ratio R of pilot fuel to main fuel (%) as a function of power P (hp) for an engine system, in accordance with an embodiment of the present disclosure, compared to the same data for an engine system operating under the same conditions but without using skip-fire. When engine system 100 is operating near idle at low power the quantity of pilot fuel consumed relative to main fuel increases, since a minimum quantity of pilot fuel is required for ignition. Line 200 represents the ratio R as a function of power for engine system 100 not operating in skip fire mode, and line 210 represents the same relationship when engine system 100 switches to skip fire mode at powers below PI . As can be seen by line 210, skip-firing operating mode reduces the ratio of pilot fuel to main fuel, thereby improving DSF.

[0039] Referring to FIG. 4B, this graph illustrates torque T as a function of engine speed ES (rpm) for an engine system, in accordance with an embodiment of the present disclosure, compared to the same data for an engine system operating under the same conditions but without using skip-fire. Engine system 100 operates in skip-fire mode in region 230, representing a low load region of engine operation below line 220 representing torque T as a function of engine speed ES. [0040] Referring to FIG. 5, this flowchart illustrates a method M for skip-firing an engine system such as the one shown in FIG. 1 , the engine system having a plurality of cylinders and a fuel-injection system, and the engine system being operable with a main fuel and a pilot fuel, in accordance with an embodiment of the present disclosure. In particular, the present skip-fire strategy comprises a cycle-by-cycle skip-firing pattern (combustion chambers are selectively skipped per cycle) which reduces the formation of harmonic frequency vibrations in the engine system; and, when used in combination with direct fuel injection or direct injection (both pilot fuel and main fuel being delivered directly into the combustion chamber or a pre-chamber after the intake valve closes by way of a direct injector), injection timing is selected to trigger ignition at the desired time, preventing premature ignition and engine knock. In addition, the present cycle-by-cycle skip-firing pattern further comprises a switching duration during a given cycle which provides a sufficient time period for determining which combustion chambers to skip for the subsequent cycle.

[0041] Still referring to FIG. 5, the skip-fire method M for skip-firing an engine system comprises: detecting engine load, as indicated by block 5001; determining when the predetermined low load condition is detected, as indicated by block 5002; when the predetermined low load condition is detected, determining and selecting at least one combustion chamber of the plurality of combustion chambers designated for skip-firing during a next cycle, as indicated by block 5003; skip-firing the selected at least one combustion chamber for a given duration, as indicated by block 5004; determining whether the engine system continues experiencing the low load condition during the given duration, as indicated by block 5005; and, if so, repeating determining and selecting at least one combustion chamber of the plurality of combustion chambers to skip-fire during the next cycle, as indicated by block 5006, and if not, returning the plurality of combustion chambers to a normal injection mode, as indicated by block 5007, thereby increasing a pilot fuel substitution factor, and thereby reducing an overall pilot fuel consumption. [0042] Still referring to FIG. 5, in preferred embodiments the method M for skip-firing an engine system further comprises delivering at least some of the main fuel into a combustion chamber after closing of an associated intake valve, whereby a timing for delivery of at least some of the main fuel is selected to prevent premature ignition; delivering the at least some of the main fuel and the pilot fuel into the combustion chamber through a fuel injection system; controlling timing for delivery of the pilot fuel independently from timing for delivery of at least some of the main fuel; and injecting the pilot fuel into the combustion chamber separately from the main fuel. [0043] Still referring to FIG. 5, in the method M for skip-firing an engine system, selecting at least one combustion chamber of the plurality of combustion chambers designated for skip-firing further comprises following a predetermined cycle-by-cycle skip-firing pattern that reduces formation of harmonic frequency vibrations in the engine system. The main fuel comprises a gaseous fuel, such as natural gas. The pilot fuel comprises diesel fuel. The given duration comprises a switching period having sufficient time for facilitating detecting and selecting at least one combustion chamber for skip-fire during the next cycle. The engine system is operable using a lower ratio of the pilot fuel to a total fuel quantity measured on an energy basis at the predetermined low load condition compared to the same engine system when operated without skip-firing. With some engines that use a pilot fuel to ignite a main fuel, when the engine load decreases below a predetermined level, at which the minimum pilot fuel amount provides all of the requisite energy to deliver the commanded load, the amount of main fuel is reduced to zero. By using the disclosed skip-fire operating mode, the amount of time that an engine is fuelled only with pilot fuel is reduced because the skip-fire operating mode extends the range of operation at low operating loads where the main fuel provides at least some of the energy required to deliver the demanded engine load. In preferred embodiments, by using a skip-fire operating mode, overall pilot fuel consumption can be reduced, so that the pilot fuel comprises approximately 5% or less of a total fueling measured on an energy basis. The main fuel comprises approximately 95% or more of a total overall fueling measured on an energy basis. Under certain predetermined load conditions, the engine system can still be operable in a mode when the pilot fuel is the only fuel consumed by the engine system, but the range of these conditions is reduced compared to the same engine that does not use a skip-fire operating mode.

[0048] Referring to FIG. 6, this flowchart illustrates a method M _fab of fabricating a skip- fire fuel-injection engine system S, in accordance with an embodiment of the present disclosure. The method M _fab comprises: providing an engine system 100 having a plurality of combustion chambers 50 and a fuel injection system, the engine system being operable with a main fuel and a pilot fuel, as indicated by block 8001; and providing a feedback and control system in electronic communication with the fuel injection system, as indicated by block 8002. The feedback and control system 200 is adapted to: detect whether the engine system 100 is experiencing a predetermined low load condition associated with a skip-fire injection mode; when the predetermined low load is detected, determine and select at least one combustion chamber 50 of the plurality of combustion chambers 50 designated for skip-firing during a next cycle; skip-fire the selected at least one combustion chamber 50 for a given duration; determine whether the engine system 100 continues experiencing the low load condition during the given duration; and, if so, repeat determining and selecting at least one combustion chamber 50 of the plurality of combustion chambers 50 to skip-fire during the next cycle, and if not, return the plurality of combustion chambers 50 to a normal injection mode, whereby a pilot fuel substitution factor is increased, and whereby an overall pilot fuel consumption is reduced.

[0049] Still referring to FIG. 6, in the method M _f a _b , the feedback and control system 200 is further adapted to: deliver at least some of the main fuel into a combustion chamber 50 after closing of an intake valve 41, such as an associated intake valve, whereby a timing for delivery of the at least some of the main fuel is selected to prevent premature ignition; deliver at least some of the main fuel and the pilot fuel into combustion chamber 50through a fuel injection system, such as an integrated pilot and main fuel injector 70; control timing for delivery of the pilot fuel independently from the timing for delivery of at least some of the main fuel; inject the pilot fuel into the combustion chamber 50 separately from the main fuel; and follow a predetermined cycle-by-cycle skip-firing pattern that reduces formation of harmonic frequency vibrations in the engine system 100 when selecting at least one combustion chamber 50 of the plurality of combustion chambers 50 designated for skip-firing.

[0050] Still referring to FIG. 6, in the method M _fab , the main fuel can comprise a gaseous fuel, such as natural gas. The pilot fuel can comprise diesel fuel. The given duration comprises a switching period having sufficient time for facilitating detecting and selecting at least one combustion chamber for skip-fire during the next cycle. The engine system is operable using a lower ratio of the pilot fuel to a total fuel quantity measured on an energy basis at the predetermined low load condition compared to the same engine system when operated without skip-firing. The pilot fuel can comprise approximately 5% or less of a total fueling measured on an energy basis. The main fuel can comprise approximately 95% or more of a total fueling measured on an energy basis. Under certain predetermined load conditions, the engine system is operable in a mode when the pilot fuel is the only fuel consumed by the engine system with the range of these conditions being reduced compared to an otherwise equivalent engine that does not have a skip-fire operating mode.

[0051] In all embodiments, the fuel injection system comprises a fuel injection assembly for injecting a pilot fuel and a main fuel. In some embodiments, this fuel injection assembly has one body, comprising a nozzle for injecting the two fuels directly into the engine's combustion chamber. In preferred embodiments the fuel injection assembly comprises two separate and independently operable fuel injection valves, one for the pilot fuel and one for the main fuel. These two fuel injection valves can be concentric or parallel (side by side) in the same body of the fuel injection assembly (as depicted in FIG. 3). When there are two separate fuel injection valves, because of the different mass densities of the two fuels, preferably, there are two sets of orifices so the flow area through the orifices can be made to accommodate the desired flow rate of each fuel. In other embodiments, if the engine has sufficient space to mount two separate fuel injection valves, the fuel injection assembly can comprise two separate fuel injection valves, each housed in its own body (as shown in FIG. 1). In yet another embodiment, the fuel injection assembly can comprise at least one fuel injection valve that injects one of the two fuels into a pre-chamber. In all embodiments, unlike in a dual fuel engine where one of the fuels is injected into the intake air and then enters the combustion chamber with the intake air through the engine intake valve, with the subject fuel injection assembly, after the engine intake valve is closed, both the pilot fuel and the main fuel can be injected through the fuel injection assembly into the combustion chamber either directly into the combustion chamber or indirectly through a pre-chamber. In this disclosure, injection of the two fuels through such a fuel injection assembly requires the fuels to be raised to injection pressures sufficient to overcome the late-cycle in-cylinder pressure, which is higher than the air pressure in the intake air manifold and intake ports. The advantages of such high-pressure direct-injection engine systems include reduced tendency for engine knock, enabling higher compression ratios, and no displacement of intake air by fuel. Accordingly, high-pressure direct- injection is defined to refer to systems that use a fuel injection assembly such as the embodiments described herein. [0052] Information as herein shown and described in detail is fully capable of attaining the above-described object of the present disclosure, the presently preferred embodiment of the present disclosure, and is, thus, representative of the subject matter which is broadly contemplated by the present disclosure. The scope of the present disclosure fully encompasses other embodiments which may become obvious to those skilled in the art, and is to be limited, accordingly, by nothing other than the appended claims, wherein any reference to an element being made in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural and functional equivalents to the elements of the above-described preferred embodiment and additional embodiments as regarded by those of ordinary skill in the art are hereby expressly incorporated by reference and are intended to be encompassed by the present claims.

[0053] Moreover, no requirement exists for a system or method to address each and every problem sought to be resolved by the present disclosure, for such to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. However, that various changes and modifications in form, material, work-piece, and fabrication material detail may be made, without departing from the spirit and scope of the present disclosure, as set forth in the appended claims, as may be apparent to those of ordinary skill in the art, are also encompassed by the present disclosure.