Background of the Invention [0002] The benefits of converting diesel-stroke engines so that they operate burning gaseous fuels has spurred on many recent developments in this area of engine technology. Gaseous fuels, such as natural gas or hydrogen, by way of example, are clean burning fuels (relative to diesel), which means that when an engine substitutes such gaseous fuels for diesel fuel, the engine can, depending on a number of variables including the gaseous fuel chosen, operate with reduced emission levels of particulate matter (PM), hydrocarbons and nitrogen oxides (NOx)- [0003] A known method that allows diesel engines to operate using gaseous fuels utilizes a second fuel as well as the gaseous fuel. For the purposes of this discussion natural gas will constitute the gaseous fuel, however, other gaseous fuels such as hydrogen, methane, ethane, propane, lighter flammable hydrocarbon derivatives, etc. will also operate as gaseous fuels.

Generally, natural gas is mixed with the intake air prior to the introduction of the air/natural gas mixture into

the engine cylinder (a process known in the field as fumigation). A homogeneous air/natural gas mixture is thus introduced into the piston cylinder during the intake stroke. During the compression stroke, the pressure and temperature of the homogeneous mixture is increased. Near the end of the compression stroke, a small quantity of pilot diesel fuel is employed to ignite the air/natural gas mixture. The advantage of employing a homogeneous mixture of air and gas is that the combustion fuel to air ratio (F/A ratio) can be controlled so as to burn in a lean homogeneous manner and achieve lower NOx emissions and lower particulate matter, compared to equivalent diesel-fuelled engines.

[0004] Note also that the homogeneous mixture of fuel and air can also be ignited by a spark or hot surface.

Rather than the ignition of pilot fuel, a spark or hot surface can be employed to cause the homogeneous mixture to ignite, optimally near top dead center at the commencement of the power stroke.

[0005] This method of gaseous combustion, namely, employing a fumigated fuel, has a number of disadvantages. The first main disadvantage is encountered at high load engine operating conditions, when the elevated temperature and pressure in the piston cylinder during the compression stroke makes the air/natural gas mixture susceptible to"excessive knocking". Knocking is an uncontrolled combustion process resulting in a very high rate of heat release. Knocking is characterized, in most instances, by relatively rapid fluctuations in combustion chamber pressure. Excessive knocking is associated with conditions where the rapid heat release rate causes excessive combustion chamber pressures that are large enough to damage engine components. Excessive knocking can also cause engine damage through excessive heat release resulting in

thermal damage to engine components, such as, by way of example, the piston crown.

[0006] A few measures for reducing the risk of excessive knocking include lowering the compression ratio of the engine or limiting the power and torque output, but these measures cause a corresponding reduction in the engine's cycle efficiency (that is, not as much power is available from each piston stroke). A"knock limit"is designated and defined as that set of conditions within the cylinder at which excessive knock can occur as described above. Ultimately, fumigated fuel is knock limited and, as such, is unable to meet load demands beyond a certain level dictated by the knock limit.

[0007] For the purposes of this application, "fumigated fuel"or"fumigated gaseous fuel"will be a fuel (generally gaseous) and oxygen mix. Fumigated fuel is a fuel/oxygen mix where the fuel has been mixed with oxygen by the time the piston has reached top dead center immediately prior to both combustion and the piston power stroke. The oxygen would generally be provided as a constituent of intake air, however, it could be provided in some other manner. The fumigated fuel can be mixed with oxygen in the combustion chamber or in the intake manifold (or intake conduit) or partially mixed within both the intake manifold and combustion chamber. For the purposes of this discussion, the fumigated fuel will generally be substantially homogeneous, however, it can also be stratified to some extent.

[0008] The second main disadvantage of a homogeneous mixture of fuel and air is that, under low load engine operating conditions, the mixture of fuel and air becomes too lean to support stable combustion via flame propagation and results in incomplete combustion or misfiring. The intake air flow can be throttled to maintain a F/A ratio above a flammability limit, however, such throttling adversely affects the engine efficiency.

As will be discussed further below, the flammability limit is defined as the condition limits with the engine under which the F/A ratio will begin to support a flame propagation combustion event.

[0009] Third, during start-up it is important that the fumigated fuel, introduced into the cylinder be ignited.

However, as is well known in the field involved here, an initial injection of fumigated fuel and, in some cases, a pilot fuel, into the cylinder does not necessarily cause the engine to start on each attempt. When this occurs, a highly flammable mixture can flood the cylinder as well as the exhaust system. This could cause an uncontrolled combustion event when engine ignition is next attempted.

[0010] Recently, a different type of dual fuel combustion engine, herein referred to as a high pressure direct injection gas engine, has become known in the field. Similar to the conventional dual fuel method described above, high pressure direct injection gas engines burn a large quantity of gaseous fuel, yielding an improvement over diesel-fuelled engines by reducing the emission levels of NO,, and particulate matter. In addition, high pressure direct injection gas engines have been demonstrated to achieve the same combustion efficiency, power and torque output as state-of-the-art diesel-fuelled engines. The operational principle underlying high pressure direct injection gas engines is that two fuels are injected under pressure into the chamber near the end of the compression stroke.

According to one method, a small quantity of"pilot fuel" (typically diesel) is injected into the cylinder immediately followed by a more substantial quantity of gaseous fuel. The pilot fuel readily ignites at the pressure and temperature within the cylinder at the end of the compression stroke, and the combustion of the pilot fuel initiates the combustion of the gaseous fuel that might otherwise be difficult to ignite. Known high

pressure direct injection gas engines have no fumigated fuel. As a result, the directly injected gas operates in a"diffusion combustion"mode, rather than a premixed combustion mode. In a diffusion combustion mode typical of homogeneous fuel combustion, the bulk of the combustion is believed to occur in a local near- stoichiometric reaction zone, where the temperature and resulting NOX formation are relatively high (compared to the temperature and resulting NOX formation caused by a lean burn premixed combustion).

[0011] The present fuel injection technique provides a method for the injection of fuel into the combustion chamber of an internal combustion engine that avoids the problem noted above.

[0012] At the same time, the present fuel injection technique provides a method of mono and dual fuel injection into the combustion chamber of an internal combustion engine which combines the advantages of lean premixed combustion over a range of operational conditions with some of the advantages of diffusion combustion in low and high load operational conditions while avoiding problems associated with such a method.

These problems arise where limited directly injected fuel is employed and where limited fumigated fuel is employed.

[0013] The amount of fumigated fuel is generally controlled by the amount of fuel introduced for fumigation by a variable control valve. During operation, there is some leakage across the variable control valve when the fuel stop valve is open.

Therefore, where a flow rate at or below the leakage flow rate is required, control over the fumigation flow can be compromised where that flow is determined by the leakage rate across the variable control valve. While this problem flows from the apparatus generally employed to control fumigation and, therefore, fumigated fuel combustion, it does not generally arise where fumigated

fuels alone are employed to drive a piston. That is, stable combustion is difficult to achieve at a fumigation flow rate at or below the leakage flow rate. As will be discussed below, however, this problem is an important consideration if fumigated fuel is employed in conjunction with other combustion techniques such as high pressure direct injection of gaseous fuels.

[0014] The present fuel injection technique addresses or manages these concerns associated with combined directly injected fuel and fumigated fuel.

[0015] The present fuel injection technique also provides a method of fuel injection into the combustion chamber of an internal combustion engine that compensates for problems known to engines that employ conventional fumigation during start-up low load conditions when undesirable flammable mixtures are injected into the cylinder and, initially, fail to ignite.

[0016] The present fuel injection technique further provides a set of operating parameters for fuel injection into the combustion chamber of an internal combustion engine which avoids excessive knock associated with high load conditions as seen in prior art noted above.

Summary of the Invention [0017] The present method and apparatus overcome the shortcomings noted above by introducing fuel into a combustion chamber of an internal combustion engine having at least one cylinder with a piston. The cylinder and piston partially defining the combustion chamber where the piston oscillates within the cylinder between top dead center and bottom dead center when the internal combustion engine is operating. The method comprises selecting either a low load operating mode or a high load operating mode. The operating modes are distinguished by the combustion characteristics of a fumigated gaseous fuel within the combustion chamber. The low load

operating mode is defined where the fumigated gaseous fuel is unable to support stable premixed combustion.

The high load operating mode is defined where the fumigated gaseous fuel is able to support stable premixed combustion. In the high load operating mode, the fumigated gaseous fuel is introduced into the combustion chamber before the piston is at top dead center. In the low load operating mode, if fumigated gaseous fuel is introduced, realizing, by way of example, that some may not be introduced during start-up, providing that fumigated gaseous fuel into the combustion chamber before the piston is at top dead center. In the low load operating mode, a quantity of a second gaseous fuel is directly injecting into the combustion chamber when the piston is at or near top dead center. The fumigated gaseous fuel and the second gaseous fuel are ignited within the combustion chamber when the piston is at or near top dead center.

[0018] A further embodiment of the present method and apparatus includes, in the high load operating mode, directly injecting a quantity of the second gaseous fuel into the combustion chamber when the piston is at or near top dead center. The second gaseous fuel and the fuel in the fumigated fuel can be the same.

[0019] In the high load operating mode, the fumigated gaseous fuel can be ignited by way of homogeneous charge compression ignition.

[0020] Also, the second gaseous fuel can be ignited as a result of ignition of a pilot fuel that is more auto- ignitable than the second gaseous fuel. In the high load operating mode, ignition of the fumigated gaseous fuel can be as a result of a change in the auto-ignition properties of the fumigated gaseous fuel resulting from the addition of a pilot fuel mixed with the fumigated gaseous fuel where the pilot fuel is more auto-ignitable than the fumigated gaseous fuel. In either case the

pilot fuel can be directly injected into the combustion chamber.

[0021] In the low load operating mode, the pilot fuel is directly injected into the combustion chamber when the piston is at or near top dead center. In the high load operating mode, the pilot fuel can be directly injected into the combustion chamber during an intake stroke or compression stroke of the piston or when the piston is at or near bottom dead center. A preferred embodiment has injection of pilot fuel effected between 120 and 60 degrees prior to top dead center as measured in degrees of crankshaft rotation. In either high or low load modes, the pilot fuel can also be injected in two stages, a first stage during an intake stroke or compression stroke of the piston or when the piston is near bottom dead center or when the piston is between 120 and 60 degrees prior to top dead center as measured in degrees of crankshaft rotation.

A second stage injection of pilot fuel is included when the piston is at or near top dead center prior to or at the commencement of the power stroke.

[0022] A further embodiment the method includes igniting the fumigated gaseous fuel and the second gaseous fuel by way of a hot surface, including a glow plug, or a spark.

[0023] A further embodiment includes in the low load operating mode, employing a start-up low load operating mode and a transition low load operating mode. The start- up low load operating mode defined in part by a load range below a pre-defined lower load. The transition low load operating mode is defined by a load above at or above the pre-defined lower load. In the start-up low load operating mode, barring the fuel in the fumigated gaseous fuel from the combustion chamber thereby eliminating fumigated fuel from the combustion chamber. In the transition low load operating mode, a flow of the fuel in

the fumigated gaseous fuel is provided in excess of a pre- defined leakage flow rate.

[0024] In the above embodiment, the flow of the fuel in the fumigated gaseous fuel is regulated by at least one of a shut-off valve and a variable control valve.

Each of these components is disposed within a fuel passage defined by a fuel conduit. The fuel passage is in communication with an intake passage defined by an intake conduit such that the fuel passage and the intake passage direct the fuel in the fumigated gaseous fuel into the combustion chamber during an intake stroke.

[0025] A further embodiment includes the method where the shut-off valve is closed in the start-up low load operating mode prohibiting the fuel in the fumigated gaseous fuel from entering the combustion chamber.

[0026] The fuel in the fumigated gaseous fuel can also be directly injected into the combustion chamber and, therefore, substantially mixed with oxygen in the combustion chamber.

[0027] The method includes a fumigated gaseous fuel at a quantity at or below a pre-defined knock limit when the high load operating mode is employed. The quantity of the second gaseous fuel can be zero when combustion of the fumigated gaseous fuel meets engine load requirements. If load is not met by fumigated gaseous fuel combustion, the quantity of the second gaseous fuel is determined by engine load requirements beyond that met by combustion of the fumigated gaseous fuel.

[0028] An apparatus is also disclosed that introduces fuel into a combustion chamber of an internal combustion engine having at least one cylinder with a piston, the cylinder and the piston partially defining the combustion chamber. The apparatus includes measuring devices for collecting operational data from the engine such as engine speed, engine load demand and excessive knock information.

A controller is also included that is capable of

processing the operational data to create an engine profile, and, with that information, directing the engine to operate in one of a low load operating mode and a high load operating mode based on the engine profile. The low load operating mode and the high load operating mode are distinguished by combustion characteristics of a fumigated gaseous fuel within the combustion chamber. In the low load operating mode the fumigated gaseous fuel is unable to support stable premixed combustion. In the high load operating mode the fumigated gaseous fuel is able to support stable premixed combustion. The apparatus further includes a main fuel injector capable of directly injecting a second gaseous fuel into the combustion chamber. A pilot fuel injector is provided that is capable of injecting a pilot fuel into the combustion chamber. The pilot fuel is more auto-ignitable than the fumigated gaseous fuel or the second gaseous fuel. Finally, an intake conduit is provided for directing the fumigated gaseous fuel into the combustion chamber.

In the low load operating mode, the control unit then directs, if fumigated gaseous fuel is to be provided, the fumigated gaseous fuel through the intake conduit making it available within the combustion chamber before the piston is at top dead center. The main fuel injector is directed, in the same operating mode, to introduce the second gaseous fuel into the combustion chamber when the piston is at or near top dead center. Finally, in the low load operating mode, the pilot fuel injector introduces the pilot fuel into the combustion chamber when the piston is at or near top dead center. In the high load operating mode, the control unit directs the fumigated gaseous fuel through the intake conduit to be available in the combustion chamber when the piston is at or near top dead center. The fumigated fuel in this mode falls below a pre-defined knock limit. The pilot fuel injector introduces pilot fuel into the combustion chamber during

the compression stoke of the engine when operating. In low load, some early diesel pilot may also be introduced along with late diesel pilot. Here, a base amount of late diesel pilot injected near top dead center may be supplemented with early diesel pilot injected, for example, early in the compression stroke of the piston.

[0029] The apparatus control unit can further direct the main fuel injector to introduce the second gaseous fuel into the combustion chamber when the piston is at or near top dead center.

[0030] Finally, the control unit in the apparatus, in the low load operating mode, can direct the engine to operate in either a start-up low load operating mode or a transition low load operating mode based on the engine profile. The low load operating modes are distinguished by engine load and by the operating strategy itself. In start-up low load operating mode the engine load range is from and including zero up to a pre-defined load. In the transition low load operating mode, the engine load is at or above the pre-defined load. In start-up low load operating mode, the control unit prohibits the fumigated gaseous fuel from the combustion chamber. In the transition low load operating mode, the control unit directs the fuel in the fumigated gaseous fuel through to the intake conduit in a pre-determined quantity. In general, this quantity is in excess of a pre-defined leakage flow rate. This transition low load operating mode may also begin introducing an earlier pilot injection of diesel fuel in addition to the late pilot injection used during start-up low load operating mode.

[0031] The present fuel injection technique provides a method of mono-and dual-fuel injection into the combustion chamber of an internal combustion engine, which combines the advantages of lean fumigated fuel combustion over a range of operational conditions with

some of the advantages of diffusion combustion in low and high load operational conditions.

[0032] The present fuel injection technique provides for a fuel injection strategy that provides more reliable start-up and, at the same time, is able to operate taking advantage of fumigated fuel combustion characteristics once the engine is started and operating under load.

[0033] The present fuel injection technique provides a set of operating parameters for fuel injection into the combustion chamber of an internal combustion engine, which utilizes fumigated fuel to benefit from lean burn homogeneous combustion under higher loads of operation while avoiding excessive knock associated with high load conditions.

[0034] The present fuel injection technique provides a method of fuel injection into the combustion chamber of an internal combustion engine, which retains the high efficiency and high cycle output of high pressure direct injection, retains the advantage of lower No,, and particulate matter emissions normally associated with lean burn combustion of fumigated fuel, and helps maximize power density while avoiding excessive knocking.

[0035] The present fuel injection technique finally provides a method of introducing fuel into a combustion chamber of an operating internal combustion engine having at least one cylinder with a piston, said engine having a low load mode of operation and a high load mode of operation, and wherein said fuel comprises a main fuel and a pilot fuel that is more auto-ignitable than said main fuel, said method comprising: (a) detecting a set of load conditions on said engine ; and (b) employing the low load operating mode when a first predetermined set of load conditions is detected, the first predetermined set of load conditions corresponding to load conditions

that exist when the desired ratio of the main fuel to air is less than a calibrated premixed combustion stability limit of a homogeneous mixture of the main fuel and intake air, and employing the high load operating mode when a second predetermined set of load conditions is detected, wherein at an operating engine speed as measured in the form of crankshaft revolutions per minute, the second set of load conditions corresponds to an engine load that is greater than the engine load corresponding to the first predetermined set of load conditions.

Brief Description of the Drawings [0036] FIG. 1 shows an illustrative plot of how load conditions can be employed to define a plurality of operating modes and the interaction between these operating modes. In FIG. 1, the horizontal axis represents engine speed, the vertical axis represents load conditions (which can be equated to the fuel to air ratio, a variable that directly correlates with load conditions) and line C represents the premixed combustion stability limit, which separates the two different operating modes. Line M represents the maximum load limit for the engine. Area P represents the initial start-up low load conditions utilizing high pressure direct injection while area L shows a low load condition for a preferable mode of operation above a pre-determined engine load where line 0 defines where the engine transition between these modes occurs. This second low load operating mode is transition low load mode. Region H represents high load operating mode.

[0037] FIG. 2 is a partial cross section view of an engine combustion chamber that shows pilot fuel and a

gaseous fuel, both being injected into the combustion chamber when the piston is at or near top dead center [0038] FIG. 3, which comprises FIGS. 3a and 3b, depicts partial cross section views of an engine combustion chamber, showing a second mode of operation under low load conditions, preferably employed after engine load exceeds a pre-determined value but more particularly where fumigated fuel with directly injected fuel are introduced. This mode of operation shows the sequential injection of fuel into the combustion chamber during three stages or events. FIG. 3a depicts a gaseous fuel being injected into the combustion chamber via the intake inlet introduced during the intake stroke along with a quantity of air. FIG. 3b depicts the injection of a quantity of pilot fuel and a directly injected quantity of gaseous fuel into the cylinder when the piston is at or near top dead center, prior to or upon commencement of the power stroke. l0039] FIG. 4, which comprises FIGS. 4a, 4b and 4c, depicts partial cross section views of an engine combustion chamber, showing the sequential injection of fuel into the combustion chamber under a preferred set of high load conditions. FIG. 4a depicts a fumigated gaseous fuel being injected into the combustion chamber through the intake valve during the intake stroke, and FIG. 4b depicts the injection of a pilot fuel during the early stages of the compression stroke, and FIG. 4c shows a final direct injection of gaseous fuel near top dead center just prior to or at the commencement of the power stroke.

[0040] FIG. 5, which comprises FIGS. 5a and 5b, depicts partial cross sectional views of an engine combustion chamber, showing the sequential events of provision or injection of fuel into the combustion chamber under a second set of high load conditions. FIG.

5a depicts a fumigated gaseous fuel being injected into

the combustion chamber through the intake valve during the intake stroke, and FIG. 5b depicts the injection of a pilot fuel and gaseous fuel when the piston is at or near top dead center.

[0041] FIG. 6, which comprises FIGS. 6a, 6b and 6c, depicts partial cross sectional views of an engine combustion chamber, showing the sequential events of provision or injection of fuel into the combustion chamber under a third set of high load conditions. FIG.

6a depicts a fumigated gaseous fuel being injected into the combustion chamber through the intake valve during the intake stroke. FIG. 6b depicts the injection of a first amount of pilot fuel during the early stages of the compression stroke. FIG. 6c shows the injection of a second amount of pilot fuel and a direct injection of gaseous fuel introduced when the piston is at or near top dead center.

[0042] FIG. 7 is a control logic diagram that provides an example of the logic that might be employed by an electronic control unit to implement the fuel injection timing under the pre-defined set of operating parameters.

In this embodiment of the method, the fuel to air ratio, which correlates to engine load, is determined and employed to select the desired operating mode.

Detailed Description of Preferred Embodiment (s) [0043] In the present method, fuel is injected into the combustion chamber of an internal combustion engine having at least one reciprocating piston and a crankshaft associated with the piston. The method generally involves two fuels, one of which is a pilot fuel, which auto-ignites more readily than the gaseous fuel.

However, the method, generally, can also utilize ignition sources other than pilot fuel such as a hot surface ignition source such as a glow plug. The focus of the method, most generally, is on the premixed combustion

stability limit and the engine demands under low load conditions. Nevertheless, the gaseous fuel is preferably a gaseous fuel such as, for example, natural gas, other gaseous hydrocarbons, or hydrogen gas. Where used, the pilot fuel can be a liquid hydrocarbon fuel or a blend of liquid fuels, but in preferred embodiments, the pilot fuel is conventional diesel fuel or dimethylether.

[0044] According to the present method, the load conditions of the engine are monitored and certain conditions are defined where different operating modes are triggered. For example, the introduction of fuel into the combustion chamber of an operating engine can be controlled differently in a low load operating mode compared to a high load operating mode.

[0045] The present method contemplates two operating modes, each of which can employ at least one operating parameter. FIG. 1 is a plot of engine load against engine speed. Engine speed can be measured, for example, by measuring the speed of crankshaft rotation in rotations per minute (RPM). Engine load, measured on the vertical axis correlates to overall fuel to air ratio.

FIG. 1 graphically illustrates two regions divided by line C determined by the"premixed combustion stability limit"of the fuel at a given fuel to air ratio. The premixed combustion stability limit is defined as the quantity of fuel mixed with oxygen, that is, fumigated fuel, resulting in a fuel to air ratio (F/A) at which the quantity of fumigated gaseous fuel within the piston cylinder will not support stable premixed combustion.

For example, premixed combustion is defined by either flame propagation or homogeneous charge compression ignition combustion, whereby combustion of the fumigated fuel occurs at many points substantially simultaneously throughout the combustion chamber with substantially no flame propagation. Homogeneous charge compression ignition is thought to be controlled primarily by

chemical kinetics, with turbulence inside the combustion chamber having no significant influence on this mode of combustion. Therefore, the premixed combustion stability limit is distinguished from a flammability limit, in that the flammability limit is defined as the quantity of gaseous fuel within the piston cylinder resulting in a F/A ratio that will not support a flame propagation mode of combustion. Accordingly, since premixed combustion can operate with a fuel mixture that is too lean to support flame propagation, the load or F/A ratio associated with the premixed combustion stability limit is lower than the load or F/A ratio associated with the flammability limit. That is, by using the premixed combustion stability limit as the upper limit for low load operating region L, the extent of this region is reduced compared to the case where the flammability limit is employed as the upper limit for region L, namely, because such stable combustion incorporates more than just flame propagation combustion, namely homogeneous charge combustion. Therefore, an advantage of using the premixed combustion stability limit as the upper limit for low load operating region L, is that the extent of high load operating region H is increased.

[0046] In a preferred embodiment, there are three operating modes that correspond to predefined low and high load conditions. These operating modes are illustrated graphically in FIG. 1. The horizontal axis represents engine speed (that is, crankshaft rotation), measured in revolutions per minute (RPM), and the vertical axis, indicates fuel to air ratio, which is a parameter that correlates to the load on the engine. The fuel to air ratio increases with engine load for a fixed engine speed. The low load region is represented by two operating regions, a start-up low load operating region P and a transition low load operating region L. Region H represents an area corresponding to high load conditions.

Focusing on the low load mode, that is where the engine load requires a fuel to air ratio that is below the premixed combustion stability limit (line C), this operating area corresponds to a set of load conditions under which a low load operating mode is employed. FIG.

1 shows that the low load operating mode is split into two operating modes according to engine load and demands on the engine. Therefore, the current technique optimizes a set of operational modes triggered both by engine load and to a lesser extent engine speed.

[0047] Region P is referred to herein as a start-up low load operating mode. This operating mode includes start-up conditions of the engine. It can also be desirable to operate in this mode for a stable low engine load operating mode should such a mode be desirable. For example an idle mode necessary for engine operation over an extended period of time might benefit from operating in the start-up low load operating mode. As suggested by region P, the operating mode can extend through all engine operating speeds below a pre-defined load. This mode is distinguished, as will be discussed below, by the fact that it is driven by directly injected fuel that burns in a diffusion mode.

[0048] Likewise, region L will be referred to herein as a transition low load operating mode. This mode of operation includes a range of engine speeds above a low load condition. It is dictated, as will be discussed below, by a minimum amount of fumigated fuel within the cylinder just prior to start of combustion. This amount can be set by the valve leakage rate or a rate chosen above the leakage flow rate. However, its range is determined by an amount of fumigated fuel that falls below a fuel to air ratio, also known as the equivalence ratio for the fumigated fuel, that would support independent combustion of this fumigated fuel, or, put another way, homogeneous combustion or propagation

combustion of this fumigated fuel. Region L can be seen as an operating mode wherein the engine operates under idle conditions. However, for reasons that will be mentioned below, it is preferable to transition back down into region P if there is a desire to idle for an extended period of time.

[0049] Region P as a start-up mode need not be restricted to direct injection of fuel under start-up conditions. Where time is needed to pressurize gas for direct injection, fumigation with a late diesel pilot may also be used until the engine high pressure gas is available for direct injection. As such, under this limited condition, region P would support combustion of a fumigated charge that is ignited by the diffusion burn of a relatively large amount of pilot fuel. As this is not an ideal start-up mode, transition back to direct injection of gaseous fuel near top dead center would generally be initiated as soon as pressure levels for direct injection are secured.

[0050] Further, region L is used to begin transitioning the timing and quantity of the pilot fuel to an early diesel fuel injection. That is the diesel fuel injection would fall earlier in the compression stroke allowing this fuel to mix out with the intake charge. Both an early and late diesel pilot would likely be used at points in this region to stabilize combustion.

The idea of introducing the earlier diesel pilot is to change the auto-ignition characteristics of the intake charge, preparing for a stable premixed combustion condition within the combustion chamber when the piston is at or near top dead center.

[0051] The plots depicted in FIG. 1 show only the general shape of predefined regions that correspond to different operating modes. Persons skilled in the technology involved here will understand that the actual numerical values for RPM and load or fuel to air ratio

depend upon the individual characteristics of a particular engine design (for example, the size of the cylinder bore, the length of the stroke, the compression ratio, the shape of the combustion chamber, or the type of fuel). That is, engines of different design can have different regional boundaries between operating modes.

The numerical values and the shape of the boundary line for a specific engine supplied with specific fuels can be determined empirically or theoretically.

[0052] FIGs. 2,3, 4 and 6 illustrate the preferred three modes of operating using the disclosed method.

Throughout the description, the preferred method of operation contemplates dual fuel operation. However, the dual-fuel injection strategy can be adapted to mono-fuel operation. That is, the disclosure can be adapted to a strategy utilizing hot surface or spark ignited combustion. Where reference is made to late pilot fuel injection (an injection of pilot fuel when the piston is near top dead center), this can be substituted with glow plug or spark ignition timing that is adjusted to provide an ignition source with the same effect as that provided by the pilot fuel combustion timing. Also, combustion can be driven by homogeneous charge compression ignition of the fumigated fuel or self-ignition of the directly injected quantity of fuel, in either case, when the piston is at or near top dead center.

[0053] FIG. 2 shows the preferred initial mode of operation under low load conditions, namely, during start-up low load.

[0054] Initially, during start-up low load, intake valve 11 and exhaust valve 12 are closed as shown in FIG.

2. Piston 13 is at or near top dead center in piston cylinder 14 completing or having just completed the compression stroke, the direction of which is indicated by arrow 18. Also, fuel shut-off valve 22 is closed preventing fuel from entering the cylinder through the

intake valve during the intake stroke, thus preventing fumigated fuel from entering into the combustion chamber in this embodiment. At this time, the power stroke is initiated by the introduction of pilot fuel 16 through injector 15. Since there is an"ignition delay"between the introduction of pilot fuel 16 and actual ignition and combustion, pilot fuel 16 is preferably introduced before piston 13 reaches top dead center such that the actual ignition of the pilot fuel will occur when the piston is near top dead center. Pilot fuel 16 can be introduced between fifty degrees before top dead center and twenty degrees after top dead center (measured by degrees of crankshaft rotation). The actual timing within this range can be set with consideration to measured operating parameters such as engine speed. For example, as engine speed increases, pilot fuel 16 can be introduced into combustion chamber 10 earlier in the engine cycle so that combustion after the ignition delay will substantially coincide with when piston 13 is at or near top dead center.

[0055] Gaseous fuel 17 is preferably introduced into combustion chamber 10 sequentially, that is, after the introduction of pilot fuel 16. However, as shown in FIG.

2, pilot fuel 16 and gaseous fuel 17 can also be introduced simultaneously, for example, by commencing pilot and gaseous fuel introduction simultaneously, or by overlapping fuel introduction (that is, introducing pilot fuel 16 first, but then introducing gaseous fuel 17 while pilot fuel 16 is still being injected). Whether or not there is an overlap can depend, for example, upon engine speed and/or the quantity of fuel to satisfy the engine load. For example, as engine speed and the quantity of fuel increase, there can be more of an overlap.

[0056] According to this low load method, gaseous fuel 17 is not well mixed with the intake air and gaseous fuel 17 burns in a diffusion mode. In the low load operating

mode, less mixing is desirable to improve combustibility since the overall fuel to air ratio is below the premixed combustion stability limit. Accordingly, in this mode, gaseous fuel 17 is more concentrated and ignitable near the injector where pilot fuel is burning. Unlike conventional methods that introduce gaseous fuel with the intake air, there is no need to restrict the quantity of air that is drawn into the combustion chamber to ensure the combustibility of the fuel. Consequently, in this embodiment when this disclosed low load operating mode is employed, there are little or no parasitic pressure losses caused by restricting the flow through the air intake.

[0057] Moreover, this mode of operation provides other advantages during start-up low load. Start-up low load is more consistent and better controlled when gaseous fuel is directly injected at high pressures in comparison to fumigated fuel start-up low load. Fumigated fuel where the fuel is natural gas is relatively difficult to ignite. Many of the variables associated with a fumigated gaseous fuel under start-up low load mode, such as a reduction in the level of air introduced into the cylinder, are also better managed.

[0058] Like a conventional direct injection engine, compression of the intake air raises its temperature such that at the end of the compression stroke, the pressure and temperature within combustion chamber 10 is sufficient to ignite pilot fuel 16 after it is introduced (that is, after the ignition delay). The ignition of pilot fuel 16 ensures the ignition of gaseous fuel 17.

The quantities of pilot fuel 16 and gaseous fuel 17 introduced in this region of the low load operating mode can be adjusted depending upon the calculated total fuel requirement which is determined from the, detected load conditions on the engine operating at a particular speed.

[0059] Referring to FIG. 3a, as start-up low load stabilizes fuel shut off valve 22 is opened. At this point a fuel at least partially fumigated in the intake valve is introduced into the cylinder along with the directly injected gaseous fuel and pilot fuel.

[0060] In FIG. 3a, piston 13 is moving away from top dead center in the direction of arrow 23. Fuel 24 is introduced into intake manifold 26 through open fuel shutoff valve 22. The quantity of fuel is controlled by variable control valve 30. During this stage of the intake stroke, gaseous fuel 24 mixes with air within intake manifold 26 whereby fumigated fuel 25 is introduced into cylinder 10 through intake valve 11.

[0061] Referring to FIG. 3b piston 13 is completing a compression stroke moving in the direction of arrow 27.

The piston is positioned near top dead center. Intake valve 11 and exhaust valve 12 are closed. At this point, pilot fuel 16a is introduced and gaseous fuel 17a is introduced with much the same timing as noted above with reference to FIG. 2 and the start-up low load mode of operation. Therefore, as noted above,, pilot fuel 16a can be introduced followed sequentially by an introduction of directly injected gaseous fuel 17a or the introduction of the two fuels can overlap.

[0062] The equivalence ratio of fumigated gaseous fuel 24 introduced during the intake stroke is generally set by the leakage flow rate of the variable control valve.

By way of example, a leakage flow rate can be 10 kg/hr per valve but can vary depending on the variable control valve, fumigation system and other factors. A flow of fuel to be fumigated below a valve's minimum rated flow may be dictated by the leakage flow rate. As the leakage flow rate behavior is variable, control over fumigated gaseous fuel is compromised. Therefore, variable control valve 30 is generally set slightly above the leakage flow rate when fumigated fuel 24 is introduced.

[0063] It may also be desirable to introduce the fumigated fuel well above the leakage flow rate. That is, the initial introduction of fumigated fuel could be based on an amount well in excess of the leakage flow rate.

[0064] In operation, during this second mode of operation, the power stroke is, for the most part, driven almost exclusively by the directly injected gaseous fuel.

The engine is operating below the premixed combustion stability limit. Therefore, the nominal fumigated fuel introduced during the intake stroke cannot support a homogeneous or propagation combustion event. It is thought that only that portion of the fumigated fuel that approaches the diffusion flame, via advection or otherwise, will actually burn.

[0065] The introduction of an early diesel pilot also helps with combustion of the fumigated charge and prepare control of the engine to transition into high load operating mode. Therefore, while initial operation in this region of the engine map is controlled by the injection of pilot fuel and gaseous fuel near top dead center, a transition with earlier pilot fuel and late pilot fuel is preferred before the fumigated charge has the properties to take it over the premixed combustion stability limit. Moreover, combustion of any fumigated charge in the low load operating mode is assisted even where the properties of the charge falls below the premix combustion stability limit. The introduction of the early diesel pilot effectively changes the auto-ignition properties of the charge supporting combustion when initiated by the diffusion combustion of the main directly injected quantity of fuel.

[0066] Note, as would be understood by persons skilled in the technology involved, both the shut-off valve and the variable control valve can be replaced by an injector that directly injects gaseous fuel into the intake path.

[0067] FIGs. 4,5 and 6 illustrate modes of operation where engine load conditions have exceeded premixed combustion stability limit noted as line C on FIG. 1.

That is, these figures demonstrate an engine operating mode under high load conditions where the power stroke of the piston is driven by at least two distinct combustion events, namely, diffusion and propagation or homogeneous compression charge combustion. Each mode of operation generally considers injection of pilot and gaseous fuel in three separate events that includes some fumigated fuel into the combustion cylinder.

[0068] FIG. 4, which comprises FIGS. 4a, 4b and 4c, shows a preferred method of operating above the premixed combustion stability limit. Generally, in the preferred method of operating under a high load condition (s), the first of the three events, shown in FIG. 4a, introduces a fumigated fuel into the cylinder during the intake stroke when the piston is moving in the direction of arrow 31.

That is, variable control valve 30 is regulated to allow for a desired amount of gaseous fuel 24a into intake manifold 26. The gaseous fuel mixes with the intake air and fumigated fuel 25a is drawn into the cylinder by the action of the intake stroke. Further mixing of fumigated fuel 25a can take place within the cylinder.

[0069] Next, referring to FIG. 4b, a quantity of pilot fuel is introduced. This quantity can be introduced during the intake or compression stroke. In this preferential mode, pilot fuel 16b can be introduced at the completion of the intake stroke, however, it is preferably introduced during the compression stroke.

Most preferably, pilot fuel 16b is introduced between 120 and 60 degrees prior to top dead center, measured by crankshaft rotation angle, thus balancing: (a) control of the start of combustion of the fumigated charge-a charge whose auto-ignition properties are altered to the extent that an

early pilot fuel injection is introduced, where this is best achieved with an injection near top dead center ; and, (b) greater opportunity for pilot fuel homogeneity with intake charge found within the cylinder, which is best achieved with an injection well prior to the piston reaching top dead center.

Introducing the pilot fuel well prior to top dead center allows the pilot fuel to mix with intake air, at least partially, and potentially substantially, within the cylinder and, as such, burn cleaner with a reduction in NOX emissions and particulate emissions once it does ignite. Preferably, to the extent that combustion stability and ignition timing is adequately controlled by earlier pilot injection alone, late pilot fuel injection can be eliminated in this high load operating mode. See also discussion of Fig. 6 below.

[0070] Referring to FIG. 4c, following the provision of fumigated fuel and the pilot fuel injection, a direct injection of gaseous fuel 17b into the cylinder commences when the piston is near top dead center, during or just prior to the commencement of the power stroke.

[0071] In the aggregate, the energy available from fumigated gaseous fuel 25a is dictated by the knock limit of the fumigated fuel within cylinder 10 under the load condition experienced at a particular point in time.

That is, for a given set of high load conditions, the equivalence ratio of the initial injection of gaseous fuel during the intake stroke is increased as required by the operator until it, preferably, nears the knock limit for that amount of fumigated fuel thereby maximizing the amount of power provided by combustion of fumigated fuel 25a without excessive knocking of the fumigated fuel.

Sensors, such as start of combustion and knock sensors can be used to monitor excessive knocking. At the same time, the directly injected quantity of fuel is reduced

or eliminated to the extent that all power is provided by the fumigated charge.

[0072] Under high load conditions, further load requirements beyond that provided by the knock limited premixed combustion can be met by an increased supply of directly injected gaseous fuel 17b: see FIG. 4c. This gaseous fuel, as discussed, is injected near top dead center and burns in a diffusion mode. Emission penalties result from employing a diffusion mode of combustion.

However, these penalties are offset by the fact that direct injection allows the engine to meet much higher load demands than would otherwise be the case. An important strategy for this method is to provide as much fumigated fuel 25a as possible without introducing excessive knocking thus limiting emissions, namely, NO, and particulate emissions, over a broad range of operating conditions.

[0073] Put another way, because gaseous fuel 17b is injected near top dead center at the end of the compression stroke or during the power stroke, it does not have an opportunity to thoroughly mix with the air in combustion chamber 10. Accordingly, this gaseous fuel 17b burns, substantially, in a diffusion mode of combustion. Since the quantity of fumigated fuel that can be introduced when the piston is near top dead center is knock limited, engine power output beyond a certain point, usually determined by this knock limit, is preferentially controlled by adjusting the quantity of gaseous fuel 17b introduced directly as shown in FIG. 4c.

That is, premixed combustion will serve high load conditions only up to the knock limit of fumigated fuel 25a.

[0074] When gaseous fuel 17b and 24a is a fuel such as natural gas or hydrogen, and pilot fuel 16a is diesel fuel, introducing an additional charge of gaseous fuel 17b when the piston is near top dead center increases the

proportion of gaseous fuel that can be employed to satisfy engine requirements under high load conditions.

In a preferred embodiment of the high load operating mode, the quantity of pilot fuel 16b represents about 1- 15% of the total quantity of fuel on an energy basis, with gaseous fuel 17b and 24a providing the balance.

There are also conditions under which the quantity of pilot fuel may fall below 1%, especially where higher loads are being demanded of the engine.

[0075] Accordingly, the disclosed method of introducing gaseous fuel and pilot fuel in three separate events during high load conditions reduces the likelihood of excessive knocking, provides efficient operation by utilizing both a lean premixed combustion and a diffusion mode of combustion that increases the range of engine operation compared to methods employing premixed combustion alone. An advantage of using the premixed combustion stability limit as the lower limit for high load operation (see FIG. 1, line C) is that this allows the engine to benefit from more lean burn homogeneous combustion (and reduced emissions) compared to the case when the flammability limit is employed as this lower limit. This method also maintains high engine power output while reducing NOX and particulate matter emissions compared to conventional methods of operation.

[0076] Note also, as mentioned above, this high load method of operation can, under less demanding high load conditions, meet some power requirements by significantly reducing or, in some cases, completely eliminating the directly injected gaseous fuel. That is, if power demands can be met with fumigated gaseous fuel 25a alone, emissions can be further reduced by this reduction in the quantity of fuel directly injected. Such a reduced high load operating environment would be bounded, for the most part, by the knock limit such that fumigated gaseous fuel could meet power requirements at an equivalence ratio

below the knock limit. Within this operating mode, the quantity of gaseous fuel 24a and fumigated fuel 25a introduced into combustion chamber 10 can be adjusted in response to engine load. However, once the knock limit is reached, the equivalence ratio of the fumigated gaseous fuel is generally constant and power requirements are then met by variations in the quantity of directly injected gaseous fuel 17b.

[0077] Care should be taken when operating with reduced or eliminated directly injected gaseous fuel, however. A direct injection of gaseous fuel may help to manage overheating of the injector tip, depending on injector design. So long as an injector tip is provided within the cylinder to service higher loads of operation, heating of the tip, depending on tip design, can advantageously be monitored. It may be desirable to maintain some direct injection of gaseous fuel at or beyond a pre-defined injector tip maintenance limit.

This should help to cool the injector from cycle to cycle.

[0078] A second mode of two-stage gaseous fuel operation (with, in the mode described, pilot fuel) under high load conditions, as seen in FIGS. 5a and 5b, begins with injection of fumigated gaseous fuel 25b during the intake stroke as described above: see FIG. 4. Pilot fuel 16c is injected near top dead center followed by gaseous fuel 17c directly injected into the combustion chamber near top dead center. This gaseous fuel injection commences during or shortly after the injection of pilot fuel 16c. Again, the fumigated gaseous fuel equivalence ratio is preferably maximized without exceeding the knock limit at the load conditions in question assuming those load conditions exceed that which can be met by a knock limited fumigated fuel. Remaining power requirements are accommodated by the resulting diffusion combustion of gaseous fuel 17c, which is directly injected.

[0079] Referring to FIGS. 6a, 6b and 6c, an additional embodiment is shown for operating under high load conditions. This method was referred to above as it illustrates the injection of pilot fuel at two different points during each cycle. Referring to FIG. 6b, early injection pilot fuel 16d is introduced preferably at or near the commencement of the compression stroke.

Referring to FIG. 6c, late injection pilot fuel 16e is then introduced when the piston is near top dead center.

This allows more control over start of combustion as the required amount of pilot fuel is introduced just prior to the desired start of combustion when the piston is near top dead center. However, it allows a quantity of pilot fuel to also mix with intake air or oxygen resulting in a cleaner burn of the resulting fuel, a mixture of pilot fuel and intake fuel. In this embodiment, injection of fumigated fuel 25c (FIG. 6a) and directly injected fuel 17d (FIG. 6c) corresponds to the embodiment depicted in FIGS. 4 and 5 for high load operation.

[0080] Note that there are other ways of operating with the same general high-load multi-event gaseous fuel introduction arrangement noted above. These include the introduction of gaseous fuel directly injected into the cylinder under high pressure conditions during the intake stroke or early in the compression stroke. This arrangement does introduce a gaseous fuel that will be mixed with the intake air drawn in during the intake stroke thus delivering a fumigated gaseous fuel by the time the power stroke commences. The gaseous fuel injected directly during the intake stroke or early in the compression stroke is injected at pressure. That is, such a fumigated fuel will first pass through the compressor consuming energy to compress this gas prior to injection. No real advantage is realized by injecting this gaseous fuel under pressure at this point in the engine cycle. The energy required to compress the

gaseous fuel in the compressor is wasted, as no significant benefit is derived. This is not as significant a consideration where the fuel is stored as liquefied natural gas. As the fuel is easier to pressurize as a liquid, the compressor demands become less significant assuming energy would be factored into a comparison with compressed natural gas, the other means of storing fuel.

[0081] Referring to the operating arrangement set out above, the entire cycle, utilizing a pilot fuel, preferably operates in the following manner. During start-up low load, the engine is powered utilizing combustion of pilot fuel 16 and gaseous fuel 17 each of which is injected when piston 13 is near top dead center.

Both fuel quantities are directly injected under pressure. Shortly after stable low load operation has been achieved after start-up or where the engine load is negligible and running for an extended period under idle conditions, by way of example see region P of FIG. 1, the engine operating mode is moved into a transition low load operating mode L. Here shut-off valve 22 is opened and gaseous fuel 24 is introduced into intake manifold 26 via variable control valve 30 which is initially set to provide a low equivalence ratio fumigated fuel 25 that exceeds the fumigated fuel quantity that would result from the minimum flow rate established by the control valve leakage flow rate. Once the operator requires the engine to meet higher load demands beyond the premixed combustion stability limit, by way of example above line C into region H of FIG. 1, in region L, pilot fuel 16b is introduced directly into the cylinder earlier in the compression stroke or in the intake stroke. This early injection is supplemented with a late pilot injection below the pre-mixed combustion stability limit.

Gradually, as load increases, the quantity of early pilot is increased.

[0082] Eventually, the quantity of pilot fuel with the increasing fumigated quantity, as dictated by the load demand for the most part, reaches a pre-mixed combustion stability limit such that it ignites when required when the piston is near top dead center. Late diesel pilot can also be eliminated where combustion is satisfactorily controlled by the auto-ignition properties of the fumigated charge mixed with an early pilot fuel injection. Transitioning at line C into region H, late diesel pilot can be shut off and the quantity of directly injected fuel as a measure of total energy content can be reduced. Beyond this limit, additional power can be provided through the addition of fumigated fuel until a knock limit is reached. Power demands beyond the knock limit can be met by increasing and/or reintroducing gaseous fuel 17b directly injected near top dead center.

[0083] Heating of the injector tip can be managed with the flow of pilot fuel or gaseous fuel. Also, with vented needles, the directly injection quantity of fuel could be reduced through region L until it was shut off with late diesel pilot once the premixed combustion stability limit was reached. There would then be a region beyond line C that was powered with combustion of the fumigation charge alone.

[0084] Note that while the introduction of extremely lean fumigated fuel 25 into cylinder 13 during transition low load operation L is driven initially by the desire to provide a controlled leakage flow into the cylinder, this mode of operation is beneficial in the low load operating mode at equivalence ratios above the leakage rate of the variable control valve. In this arrangement, the focus is on the amount of fuel introduced into the cylinder in the fumigation stage and whether that results in the introduction of a second combustion event during the power stroke beyond diffusion combustion created by high pressure direct injection gaseous fuel 17a alone. There

is a desire to move in this operational mode from high pressure direct injection gaseous fuel 17a to fumigating after stable start-up low load has been secured.

Therefore, one advantage of fumigating and early diesel pilotunder low load conditions is that it introduces an operating mode with a controlled amount of fumigation that does provide only nominal power to the engine. The system is then prepared for transition to the dual burn lower emission high load operation while still allowing start-up low load under high pressure direct injection 17 alone. Likewise, there is also a way to move back into an idle mode under diffusion combustion alone in preparation for further load requirements as required by an operator.

[0085] As noted above, there are other operational strategies as is apparent to persons skilled in the technology involved here. However, the method disclosed represents the preferred method to: (a) meet the load requirements of an engine across a broad range of operational demands; (b) minimize emissions, most significantly, NOx and particulate emissions; and, (c) provide modes of operating that facilitate transition from start-up low load through transition low load and to high load operation beyond the premixed combustion stability limit.

[0086] For engines that comprise a plurality of pistons, a plurality of auxiliary injectors can be employed.

[0087] In a preferred embodiment, the proportion of gaseous fuel 24,24a, 24b, 24c and directly injected gaseous fuel 17,17a, 17b, 17c, 17d (as the case may be) to pilot fuel 16,16a, 16b, 16c, 16d and 16e and the timing for introducing fuel into combustion chamber 13 is determined by an electronic control unit (ECU). FIG. 7

is a control logic diagram that depicts the logic that such an ECU can be programmed to follow.

[0088] With reference to FIG. 7, a measurement subsystem 201 can be employed to collect data relating to current operating conditions. In a preferred embodiment, measurement subsystem 201 collects data relating to the throttle position and the engine speed. Measurement subsystem 201 can optionally provide additional data, such as intake manifold charge temperature, engine coolant temperature, intake manifold charge pressure, air flow, information that indicates the occurrence of excessive knocking and information that indicates the start of combustion.

[0089] For example, the ECU preferably receives data indicating the measured throttle position and employs this data to calculate, on an energy basis, the total amount of fuel (Ft) required to satisfy the current engine load. The ECU then determines the desired quantity of pilot fuel (Fp), which can be obtained, for example, from a look-up table that has been calibrated for the engine. The quantity of gaseous fuel (Fg) is then easily calculated (that is, Fg = Ft-Fp).

[0090] Meanwhile, the ECU can also receive data from measurement subsystem 201 that indicates the current air flow (A), or data that the ECU can employ to calculate A.

With Fg and A known, the ECU can calculate the gaseous fuel to air ratio (that is, Fg/A).

[0091] In the preferred embodiment, measurement subsystem 201 also sends data to the ECU indicating the current engine speed. Accordingly, with Fg/A and other operating information from that delivered to system 201, the ECU can determine the desired operating mode, by referring, for example, to a look-up table that stores information similar to that depicted in FIG. 1. In the preferred embodiment, a low load operating mode is selected if Fg/A is less than the premixed combustion

stability limit (C), a high load operating mode is selected if Fg/A is greater than C. Boost as calibrated onto an engine map, can be used as well to determine pre- mixed combustion stability limit. The ECU will also determine, based, for the most part, on operator demand (i. e. , load demand) at start-up low load, when the fuel shut-off valve is to be opened commencing low load fumigated gaseous fuel supply from purely high pressure direct injection gaseous and pilot fuel supply.

[0092] After the desired operating mode is selected, the timing is set for introducing the fuel into the combustion chamber. As described above, timing and the sequence of gaseous fuel (fumigated and directly injected) and pilot fuel injection depends upon the selected operating mode. In addition, the specific timing within an operating mode can also be dictated by engine speed and the quantity of fuel to be injected.

Accordingly, after the ECU determines the desired operating mode, gaseous fuel and pilot fuel injection timing is set. Finally, the gaseous fuel and pilot fuel are injected into the combustion chamber at the appropriately set times.

[0093] In an embodiment where the measurement subsystem detects the occurrence of excessive knocking, the ECU can recalibrate its stored values to re-calibrate the value for knock limit K to prevent the re-occurrence of excessive knocking. That is, if excessive knocking is detected during a high load operating mode, the ECU can re-calibrate itself to reduce the quantity of fumigated gaseous fuel (Ffg) [0094] Also, the ECU can vary such parameters as the intake manifold temperature, quantity of early diesel pilot and the equivalence ratio of the fuel to bring the knock back below the excessive knock limit level.

[0095] The ECU can also recalibrate the premixed combustion stability limit to compensate for changes in

intake manifold pressure and temperature. Alternatively, the intake manifold temperature and pressure can be monitored and adjusted to maintain substantially constant values for predetermined operating conditions.

[0096] As used herein, the term"throttle"or "throttle position"has been used in a general sense to convey the load request on the engine. Typically, such a load request is set by the user and can be a foot pedal placement (in the case of a vehicular engine) or a predetermined load request (in the case of a stationary energy generating engine). In general, there are many ways in which a user can set the load request and the term"throttle"should be understood in this general sense.

[00971 The above disclosure was derived for stationary power applications. The method and associated apparatus and controls are adaptable to vehicular applications.

The main difference between the two applications is the fact that stationary applications tend to operate under constant load conditions for much longer periods than is the case for vehicular applications where there is typically more load variability. The method disclosed, however, does provide a way of stepping between different load conditions in either the stationary power application or the vehicular application. A vehicular application would simply move more frequently from region P through to region H via region L.

[0098] Further, although the present description is directed to four-stroke engines, persons skilled in the technology involved here will understand that the present fuel injection method can also be applied to other engine types such as, for example, two-stroke combustion engines.

[0099] While particular elements, embodiments and applications of the present method and apparatus have been shown and described herein, it will be understood,

of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.

It is therefore contemplated by the appended claims to cover such modifications as incorporate those features that come within the scope of the invention.