Dual fuel engine operating method and control strategy for optimized heat release

Operating an engine system includes autoigniting a first fuel including a plurality of liquid fuels premixed with air, to trigger ignition of a direct-injected main fuel in a first engine cycle, and receiving data indicative of an undesired heat release of combustion of the first fuel. The undesired heat release may include an undesired heat release rate (HRR) modality such as a multistage combustion. Relative amounts of the plurality of the liquid fuels are varied and admitted to the cylinder in a second engine cycle, and the first fuel having the varied relative amounts autoignited to trigger ignition of the direct-injected main fuel in the second engine cycle. The undesired heat release can be limited in the second engine cycle based on the varied relative amounts of the liquid fuels. The first fuel may include a blend of dimethyl ether (DME), methanol (MeOH), and water. The direct-injected main fuel may include MeOH and water. Related apparatus and control logic is also disclosed.

TECHNICAL FIELD

The present disclosure relates generally to operating an engine system on dual fuels, and more particularly to optimizing a heat release rate (HRR) of an autoignited first fuel pre-mixed with air to trigger ignition of a direct-injected main fuel.

BACKGROUND

Internal combustion engine systems are used the world over for production of electrical power, pressurization and/or transport of liquids and gases, and of course vehicle propulsion. Internal combustion engines employ a well-known process of igniting a fuel and air in an engine cylinder to produce a rapid pressure and temperature rise that drives a piston coupled to a rotatable crankshaft. In recent years, increasingly stringent emissions standards have motivated research and development in alternatives to traditional fossil fuels. So-called alternative fuels theoretically produce reduced levels of certain undesired emissions including oxides of nitrogen, particulate matter, and greenhouse gases at least under certain conditions.

A well-known platform that has experienced considerable technical and commercial success is a dual fuel engine, typically employing a relatively small quantity of a first fuel to ignite a larger quantity of a main fuel. A classic example of a dual fuel engine employs a small pilot injection of diesel fuel directly into a cylinder where the diesel autoignites to trigger ignition of a main charge of a gaseous fuel such as natural gas. In an effort to further improve upon traditional dual fuel strategies, engineering efforts have accelerated in relation to alternative fuels such as alcohol fuels, hydrogen, ether, and still others.

While many engine platforms can theoretically operate on various different fuel types, even seemingly minor changes to fuel composition can radically alter the necessary controls, timing of engine operating events, and even optimal engine hardware that is needed to comply with applicable standards for emissions, efficiency, and certainly cost. International patent application publication WO 2023200588A1 to Schroeder et al. is directed to a compression-ignited dual liquid fuel system and control strategy for flexible fuel operation.

SUMMARY

In one aspect, a method of operating an engine system includes autoigniting a first fuel including a plurality of liquid fuels premixed with air, in a cylinder in an engine, to trigger ignition of a direct-injected main fuel in a first engine cycle. The method further includes receiving data indicative of an undesired heat release of combustion of the first fuel in the first engine cycle, and varying, based on the data, relative amounts of the plurality of liquid fuels premixed with air and admitted to the cylinder in the second engine cycle. The method still further includes autoigniting the first fuel having the varied relative amounts of the plurality of liquid fuels in the cylinder, to trigger ignition of the direct-injected main fuel in the cylinder in the second engine cycle.

In another aspect, an engine system includes an engine having therein a cylinder, and an intake port extending to the cylinder. The engine system further includes a fuel system having a first fuel supply of a first fuel including a plurality of liquid fuels, a first fuel injector fluidly connected to the first fuel supply and arranged to inject the first fuel into a stream of intake air fed through the intake port, a second fuel supply of a main fuel to be ignited via autoignition of the first fuel, a direct fuel injector fluidly connected to the second fuel supply, and a fueling control unit. The fueling control unit is structured to receive data indicative of an undesired heat release of combustion of the first fuel in a first engine cycle, and to vary, based on the data, relative amounts of the plurality of liquid fuels injected into the stream of intake air in a second engine cycle. The fueling control unit is further structured to reduce the undesired heat release of combustion of the first fuel in the second engine cycle based on the varied relative amounts of the plurality of liquid fuels.

In still another aspect, a fuel system includes a first fuel supply for a first fuel including a plurality of liquid fuels, and a first fuel injector fluidly connected to the first fuel supply to inject the first fuel at a first injection location into a stream of intake air fed to a cylinder in an engine. The fuel system further includes a second fuel supply for a main fuel, and a direct fuel injector fluidly connected to the second fuel supply to directly inject the main fuel at a direct injection location into the cylinder. The fuel system still further includes a fueling control unit structured to determine an undesired heat release rate (HRR) of the first fuel autoignited in the cylinder, and to output a fuel composition control command to increase a relative amount of one of the plurality of liquid fuels admitted to the cylinder in a second engine cycle based on the determined undesired HRR.

DETAILED DESCRIPTION

Referring toFIG.1, there is shown a dual fuel internal combustion engine system10, according to one embodiment. Engine system10includes an engine12having an engine housing14with a plurality of combustion cylinders16formed therein. A plurality of pistons18are positioned in cylinders16and each is movable between a top-dead-center position and a bottom-dead-center position in a generally conventional manner. Engine12is typically operated in a 4-stroke engine cycle although the present disclosure is not thereby limited. Pistons18are coupled to a crankshaft20that is rotated to operate a load such as an electrical generator, a compressor, a pump, or a driveline in a vehicle to name a few examples. Cylinders16may be of any number in any suitable arrangement such as a V-pattern, an in-line pattern, or still another. An engine head22is attached to engine housing14.

Engine system10also includes an intake conduit24supplying intake air to a compressor28in a turbocharger26. The intake air is pressurized by way of compressor28and fed through an aftercooler32to an intake manifold34. A plurality of intake runners36extend from intake manifold34to engine head22. Exhaust from combustion of fuel in cylinders16is fed by way of an exhaust manifold46to an exhaust conduit48and through a turbine30of turbocharger26, again in a generally conventional manner. A plurality of intake valves42are supported in engine head22and each movable to open and close fluid communication between an intake port38extending through engine head22and each respective combustion cylinder16. A plurality of exhaust valves40are also supported in engine head22and each movable to open and close fluid communication between one combustion cylinder16and exhaust manifold36by way of exhaust ports44. Although the present disclosure is not thereby limited, each combustion cylinder16may be associated with a total of two intake valves40and a total of two exhaust valves42.

Engine system10also includes a fuel system50. Fuel system50includes a first fuel supply52for a first fuel, and a plurality of first fuel injectors54each positioned at a first fuel injection location of engine system10and fluidly connected to first fuel supply52. Each first fuel injector54, referred to herein, at times, in the singular, may be arranged as a port fuel injector, such that the first injection location is a port fuel injection location. First fuel injector54may be positioned to inject the first fuel directly into, or close to and just upstream of, the respective intake port38. Fuel system50also includes a second fuel supply56for a second or main fuel, and a plurality of second fuel injectors58each positioned at a second fuel injection location of engine system10and fluidly connected to second fuel supply56. Each second fuel injector58, also referred to herein, at times, in the singular, may be arranged as a direct fuel injector. The first fuel may be injected into a stream of pressurized intake air fed through intake port38. “Fed through” intake port38means to be fed or having been partially fed. The second fuel injection location includes a direct injection location, such that the second fuel is directly injected into a mixture of the first fuel and pressurized intake air in the respective combustion cylinder16.

In the illustrated embodiment, first fuel supply52is illustrated as a fuel conduit. Thus, the fuel conduit supplies the first fuel. In various implementations, first fuel supply52may include not only a fuel conduit but also an accumulator or other storage volume of the first fuel that is not explicitly shown inFIG.1. Fuel system50also includes a fuel pump60structured to transfer the second fuel to a high-pressure pump66that pressurizes the second fuel to an injection pressure and feeds the same to second fuel injectors58. Fuel pump60also feeds the second fuel to a reactor64wherein the second fuel is transformed into the first fuel. Reactor64may include appropriate hardware such as a heating element85for the transformation of the second fuel into the first fuel. In an embodiment, the first fuel includes a fuel blend including a plurality of liquid fuels, for example an ether, including for example dimethyl ether or DME, and methanol or MeOH, as well as water. The second fuel may include an alcohol fuel, such as or MeOH. Reactor64can transform a blend of MeOH and water from second fuel supply56by way of a well-known alcohol dehydration process to provide a feed of a fuel blend of DME, MeOH, and water to another fuel pump62that pressurizes the fuel blend to an injection pressure for supplying to first fuel injectors54from first fuel supply52. Thus, the first fuel may include a fuel blend of DME, MeOH, and water that is premixed with air before and/or during admitting the first fuel to cylinder16. The second fuel may include a blend of MeOH and water. Other fuel blends and compositions are nevertheless within the scope of the present disclosure. Moreover, in some implementations the first fuel is not derived from the second fuel at least on-board engine system10, and the two fuels are independently stored and supplied. The present disclosure is applicable without limitation to the plumbing arrangement, number of fuel storage volumes, number of pumps, valves, or other components in fuel system50, or processes for making or blending fuels, so long as a supply of two suitable fuels can be provided.

Fuel system50may further include a fuel composition valve92coupled with an electrical actuator94and fluidly connected to second fuel supply56. Fuel composition valve92is movable between an open position and a closed position based on a fuel composition control command further discussed herein. Fuel system50also includes a fuel mixing conduit90extending from fuel composition valve92to a fuel outlet96. Fuel outlet96defines a fuel-fuel mixing location for mixing the main fuel into a feed of a first fuel to first fuel injector54.

It will be appreciated in view of the example structure illustrated inFIG.1that the second fuel can be selectively added to the first fuel output from reactor64to obtain a desired composition of the first fuel. In some instances, reactor output can vary such that a blend ratio of the first fuel output from reactor64itself can vary. In some instances, a blend ratio of, for example, DME, MeOH, and water, that is produced by reactor64can be suboptimal. Reactor output can be manipulated to some extent by adjusting an output of heater85, for example, or by other means. In some instances, however, reactor output cannot be controllably varied, or cannot be controllable varied fast enough to respond to system needs. Thus, selectively adding MeOH at the fuel-fuel mixing location defined by fuel outlet96can enable the blend ratio to be adjusted relatively rapidly on-the-fly. In still other instances, an engine system according to the present disclosure might not utilize a reactor to produce the first fuel at all, and a plurality of liquid fuels forming a fuel blend, or a plurality of liquid fuels stored separately and blended on-board, might be utilized. In such instances, an analogous strategy of selectively supplying a constituent of a fuel blend to achieve a desired blend ratio might be applied. The blending location(s) and still other factors might also vary from those illustrated inFIG.1. Still other embodiments contemplated herein contemplate obtaining a desired first fuel composition entirely by way of controlling reactor64or feedstocks to reactor64without supplementation of the main fuel or other fuel constituent to the first fuel at all.

Turning now toFIG.2, there is shown an engine system110including an engine112having a cylinder116formed therein, and an intake port138extending to cylinder116. Engine system110may be configured in certain respects analogously to engine system10discussed above and will be understood to include certain features and functionality analogous to those of engine system10discussed above but not specifically illustrated and/or described. Engine112includes a fuel system150having a first fuel supply152, and a second fuel supply156. First fuel supply152may contain and convey a first fuel generally analogous to the embodiment described above, and feed the same via a pump162from a reactor164to a first fuel injector154. Second fuel supply156may contain a second fuel or a main fuel, again generally analogous to the foregoing description of engine system10shown inFIG.1. A pump166pressurizes the main fuel and supplies the same to a direct fuel injector158.

Fuel system150also includes a pump160fluidly connected to second fuel supply156and structured to convey the main fuel to reactor164, functionally analogous to reactor64discussed above, and to a pump166. Pump166pressurizes the main fuel to be supplied to direct fuel injector158. In the illustrated embodiment, a fuel mixing conduit190extends between pump166and a fuel composition valve192, and from fuel composition valve192to a fuel outlet196defining a fuel-air mixing location. Fuel outlet196may be formed by a third fuel injector155arranged as a port injector. Third fuel injector155is thus fluidly connected to second fuel supply156and arranged to inject the main fuel into a stream of intake air fed through intake port138.

Whereas fuel system50can be operated to vary relative amounts of a plurality of liquid fuels by varying a blend ratio of the plurality of liquid fuels, in fuel system150varying relative amounts of a plurality of liquid fuels may include varying an injection amount of one of the plurality of liquid fuels injected separately of a liquid fuel blend, into a feed of intake air through intake port138to cylinder116. For example, first fuel injector154can be operated to inject a blend of DME, MeOH, and water into a stream of intake air fed through intake port138, and third fuel injector155can be selectively operated to inject MeOH and water into the feed of intake air fed through intake port138to provide a desired mixture of DME, MeOH, and water in the first fuel premixed with air and to be autoignited in cylinder116, as further discussed herein.

Returning now toFIG.1, engine system10further includes a control system68. It will be understood that the description herein of control system68in engine system10applies generally analogously to engine system110and other engine systems contemplated herein. Control system68includes an electronic control unit or fueling control unit70having a processor72and a computer readable memory74. Processor72can include any suitable programmable logic unit, such as a microprocessor or a microcontroller, that is, or includes, a central processing unit (CPU). Memory74includes any suitable volatile or non-volatile computer readable memory such as RAM, ROM, FLASH, a hard drive, or still another. Memory74stores computer executable instructions to be executed by processor72, as well as maps, tables, or other data structures, to carry out the features and functions of the present disclosure. Fueling control unit70could include one processor and one memory, and/or multiple processors and/or multiple memories without limitation. Control system68may also include an engine timing sensor76coupled to fueling control unit70, such as an engine crank angle timing sensor structured to output an engine timing signal. Control system68also includes at least one intake manifold sensor78structured to monitor intake manifold pressure or IMAP and intake manifold temperature or IMAT and output corresponding signals to fueling control unit70. A cylinder pressure sensor is shown at numeral80. Control system68may also include at least one reactor sensor84structured to monitor a reactor temperature and a reactor pressure of reactor64. Control system68may also include a plurality of variable valve actuators82each coupled to one of intake valves40to vary an intake valve closing timing.

Fueling control unit70is in control communication with first fuel injector54and second or direct fuel injector58, with first fuel injector54and second fuel injector58being electrically actuated so as to open and close to controllably inject fuel in response to electrical control signals in a manner that will be familiar to those skilled in the art. In some embodiments, a hydraulically actuated, pneumatically actuated, or mechanically actuated strategy could be used to operate one or both of first fuel injector54and second fuel injector58. Fueling control unit70is also in control communication with actuator94and potentially heater85. During at least one mode of operation of engine system10a relatively small amount of the first fuel, a fuel blend containing DME, MeOH, and water, for example, is port injected by way of first fuel injector54into a stream of pressurized intake air fed through intake port38. Fueling control unit70may be structured to output a first fueling command to first fuel injector54to inject the first fuel into the stream of pressurized intake air. Fueling control unit70may be further structured to output a second fueling command to direct fuel injector58to inject the second fuel directly into the respective combustion cylinder16and into a mixture of the first fuel and pressurized intake air therein.

Fueling control unit70may be further structured to receive data indicative of an undesired heat release rate (HRR) of combustion of the first fuel in a first engine cycle, and to vary, based on the data, relative amounts of the plurality of liquid fuels injected into the stream of intake air fed to the cylinder in a second engine cycle. In an embodiment, the data indicative of undesired HRR may include cylinder pressure data produced by cylinder pressure sensor80. Monitoring cylinder pressure in conjunction with engine timing can indicate that heat release is earlier than desired, later than desired, proceeds faster than desired, proceeds slower than desired, more intense than desired, or defines other features of a HRR curve that are undesired. In one example, the undesired HRR can include a multistage or multimodal HRR such as a bimodal HRR, as further discussed herein.

Fueling control unit70may be further structured to reduce the undesired HRR of combustion of the first fuel in the second engine cycle based on the varied relative amounts of the plurality of liquid fuels. Put differently, fueling control unit70may operate to limit or eliminate features or modalities of HRR that are undesirable and can lead to reduced efficiency of engine system10or other undesired operating characteristics. By way of example, a bimodal HRR curve can include an early peak or hump and a later peak or hump, such that the early combustion of the first fuel undesirably opposes a piston moving in a cylinder to compress the fluids in the cylinder. As further discussed herein, varying the relative amounts of the plurality of liquid fuels in the cylinder is observed to drive HRR toward a target HRR in at least some instances. As discussed herein, the varying of the relative amounts may be by way of varying a composition of a fuel blend injected via first fuel injector54in engine system10, or in the case of engine system110by way of separately injecting one of the plurality of fuels via third fuel injector155. In still other instances the varying a composition of a fuel blend could include combinations of these strategies, or separately or in combination including control of reactor64.

Referring also now toFIG.3, there is shown a graph200illustrating HRR curves in an engine cycle for a first blend ratio of the first fuel at210and for a second blend ratio of the first fuel at220. In the illustrated example,210shows HRR for a blend of MeOH:DME:H2O=0.40:0.50:0.10 by mass, and220shows HRR for a blend of MeOH:DME:H2O=0.20:0.50:0.30 by mass. It can be noted that 210 includes a first peak or hump beginning around −30 degrees crank angle, and a second peak or hump beginning around −20 degrees crank angle. Trace220includes only a single early peak or hump beginning after −20 degrees crank angle. Trace210shows an undesired heat release in the form of a bimodal heat release rate curve as well as earlier heat release than what might be considered optimal. Trace220shows a desired or target HRR curve where autoignition of the first fuel occurs just prior to ignition of a main fuel triggered by the autoignition of the first fuel.

Thus, for a similar mass fraction of DME in each of the two fuel blends, the fuel blend210with a lesser relative proportion of MeOH tends to result in heat release that is earlier in an engine cycle and less smooth than fuel blend220with a greater relative proportion of MeOH. A more normalized, single-stage or unimodal, HRR curve with later heat release as in the case of the fuel blend220may be more efficient and better controllable. In at least some instances, it may be expected that a MeOH amount of about 30% by mass in a MeOH, DME, and water fuel blend, or less, may be associated with an early two-stage or bimodal ignition as in the case of trace210. A MeOH amount of about 40% by mass in a MeOH, DME, and water fuel blend, or greater, may be associated with a desirable single-stage ignition as in trace220. As used herein the term “about” and similar relative terms can be understood to mean generally or approximately as would be understood by a person of ordinary skill in the art, such as within estimation or modeling error, direct measurement error, or conventional rounding. Applying conventional rounding, “about” 40 means from 35 to 44, “about” 35 means from 34.5 to 35.4, and so on. The range of MeOH proportion by mass of about 40% or greater that produces a relatively unimodal HRR curve may be generally consistent across a range of engine operating conditions including ranges of engine loads, engine speeds, cylinder temperatures, and combinations of these.

Referring now toFIG.4, there is shown a graph300illustrating HRR curves for another plurality of different fuel blends. Numeral310shows a HRR curve for a first fuel of MeOH:DME:H2O=0.00:1.00:0.00 or “neat” DME. Numeral320shows a HRR curve for a fuel blend of MeOH:DME:H2O=0.12:0.85:0.03. Numeral330shows a HRR curve for a fuel blend of MeOH:DME:H2O=0.20:0.75:0.05. Numeral340shows a HRR curve for a fuel blend of MeOH:DME:H2O=0.30:0.625:0.075. Numeral350shows a HRR curve for a fuel blend of MeOH:DME:H2O=0.40:0.50:0.10.FIG.4demonstrates retarding of heat release and decreased multimodality with an increased relative proportion of MeOH.

Referring now toFIG.5, there is shown another graph400illustrating HRR curves for different fuel blends, and including where temperature is varied. Numeral410shows a HRR curve for a fuel blend of MeOH:DME:H2O=0.30:0.625:0.075. Numeral430shows a HRR curve for a fuel blend of MeOH:DME:H2O=0.40:0.50:0.10. Numeral420shows a heat release rate for a fuel blend the same as in numeral430where temperature is increased approximately 10 to 20 Kelvins relative to the cylinder temperature used in connection with430. Put differently, HRR curves420and430are for the same fuel blend, but where cylinder temperature is higher for 420 relative to430. The relatively minor difference between HRR curves420and430shows consistency in desirable HRR properties of the relatively increased MeOH across varied cylinder temperatures.

INDUSTRIAL APPLICABILITY

Referring to the drawings generally, but focusing now onFIG.6, there is shown a flowchart500illustrating example methodology and logic flow in operating an engine system according to the present disclosure. In flowchart500, at a block510a first fuel is injected into a stream of intake air fed through an intake port to a cylinder in an engine. The first fuel may have a first fuel composition, including for example a blend of DME, MeOH, and water, where the mass percent of MeOH is less than approximately 40%.

From block510, flowchart500advances to a block520to autoignite the first fuel in the cylinder to trigger ignition of a direct-injected main fuel in a first engine cycle. The main fuel may be a blend of MeOH and water as discussed herein. From block520flowchart500advances to a block530to receive data indicative of an undesired HRR modality of combustion of the first fuel in the first engine cycle. From block530flowchart500advances to a block540to output a fuel composition command.

As discussed herein a fuel composition command may include an electrical current control signal outputted by fueling control unit70to a fuel composition valve such as fuel composition valve92,192to vary a relative amount of a first one of the plurality of liquid fuels forming the first fuel. As also discussed herein, the varied fuel composition may include an increased relative amount of MeOH. It will also be appreciated that in some instances, a fuel composition command could be outputted to reduce a relative amount of MeOH. Scenarios are contemplated where supplemental MeOH might be provided to limit or eliminate undesired HRR that results from reactor64producing a fuel blend that does not have sufficient MeOH justifying supplementing with additional MeOH. The output of reactor64might change over time, however, such that the supplemental MeOH becomes unnecessary. The increased relative amount of MeOH may result in approximately 40% by mass or greater of MeOH being admitted to an engine cylinder in an engine cycle.

From block540, flowchart500advances to a block550to inject the first fuel into a stream of intake air fed through the intake port. It will be recalled that in the case of the embodiment ofFIG.1an increased relative amount of the one of the plurality of fuels can be obtained by admitting MeOH into the feed of the first fuel at fuel-fuel mixing location96. In the case of the embodiment ofFIG.2an increased relative amount of the one of the plurality of liquid fuels can be obtained by injecting MeOH into the feed of intake air fed through intake port138. From block550flowchart500advances to a block560to autoignite the first fuel having the varied relative amounts of liquid fuels into the cylinder to trigger ignition of the direct-injected main fuel in a second engine cycle.