Patent Description:
High pressure direct injection (HPDI) is a technology for internal combustion engines where gaseous fuel is introduced into a combustion chamber later in the compression stroke and burns in a stratified combustion mode. HPDI technology delivers torque performance comparable to state of the art internal combustion engines that burn diesel fuel, and compared to these diesel engines has reduced emissions and lower fuelling costs. As used herein a gaseous fuel is any fuel that is in a gas state at standard temperature and pressure that in the context of this application is <NUM> degrees Celsius (°C) and <NUM> atmosphere (ATM). A typical gaseous fuel employed in HPDI engines is natural gas. Natural gas is a composition of various gases whose primary constituent is methane, which typically can vary between <NUM> and <NUM>% mole fraction. Besides natural gas and methane fuel, other gaseous fuels include propane, butane, hydrogen, ethane, biogas and mixtures thereof. In conventional diesel fuelled engines, the fuel is ignited by the pressure and temperature established in the combustion chamber during the compression stroke, which is referred to as compression ignition. Methane is a relatively high octane, low cetane fuel (unlike diesel fuel that has a relatively low octane number and high cetane number) that is not easily compression ignitable. Typically, a pilot fuel is employed to ignite the gaseous fuel in HPDI engines. The pilot fuel is introduced later in the compression stroke, before, during and/or after gaseous fuel injection, and is compression ignited; and the combustion of the pilot fuel establishes a pressure and temperature environment suitable for igniting the gaseous fuel. An exemplary pilot fuel is diesel fuel. It is a challenge to directly inject both a gaseous fuel and a pilot fuel into combustion chambers of light duty, medium duty and heavy duty internal combustion engines. For improved ignition and combustion performance, it is advantageous to align gaseous fuel jets with respective pilot fuel jets. However, a number of factors combine to leave little space in the cylinder head to position separate gaseous fuel injectors and pilot fuel injectors. Concentric needle fuel injectors that introduce both a pilot fuel and a gaseous fuel, separately and independently from each other, have a reduced footprint in the cylinder head, compared to separate fuel injectors, and allow an increased amount of symmetry between the gaseous and pilot fuel jets. Integrated side-by-side fuel injectors also have a reduced footprint compared to separate fuel injectors, although there is an increased footprint compared to concentric-needle fuel injectors their footprint is acceptable in some applications. As used herein, dual fuel injectors that introduce a liquid fuel and a gaseous fuel include both concentric needle fuel injectors and side-by-side fuel injectors. A liquid fuel is any fuel in the liquid state at standard temperature and pressure and include diesel, dimethyl ether, biodiesel, kerosene and diesel fuel marine (DFM).

It has been found that an internal combustion engine operating with new and previously unused dual fuel injectors experiences torque loss during an initial break-in period, after which, the rate of torque loss substantially decreases. That is, the torque output of the engine at a particular commanded quantity of fuelling decreases over a break-in period in the absence of any compensating factors. As used herein, break-in period is defined as the amount of time it takes for the rate of torque loss to diminish to a predetermined value, preferably zero, as new fuel injectors are operated with a predefined engine load and speed, or alternatively with a predetermined operational cycle. Characteristically, the break-in period can change depending upon which parts of the engine map the fuel injectors are used. With reference to <FIG>, curve <NUM> illustrates a torque curve for an engine system employing dual fuel injectors that uses no mitigation techniques for deposit accumulation. After time Tl the torque of the engine has reduced from a nominal value to a stable value of torque TR1 as a result of deposit accumulation. As the engine operates, deposits form on nozzle orifices that introduce liquid fuel and gaseous fuel, thereby reducing the cross-sectional flow area of the individual orifices. As deposits accumulate the mass flow rate of fuel through these nozzle orifices decreases, such that over a given injection window at any given injection pressure, the total quantity of injected fuel decreases. This results in a reduction in heat release from combustion and the consequential torque loss. Furthermore, depending upon the particular fuel injector and application, it is possible for deposits to continue to accumulate after the break-in period, leading to further performance degradation over time.

The state of the art, as for example disclosed by <CIT>, is lacking in techniques for mitigating the effects of deposit accumulation in fuel injectors that introduce liquid and/or gaseous fuels. The present method and apparatus provide a technique for deposit mitigation in fuel injectors employed in internal combustion engines.

A first aspect of the present invention provides a method for deposit mitigation in a gaseous fuel injector that introduces a gaseous fuel through a gaseous fuel orifice directly into a combustion chamber of an internal combustion engine as recited in claim <NUM>.

An improved gaseous fuel injector for directly introducing a gaseous fuel into a combustion chamber of an internal combustion engine as recited in claim <NUM>.

Referring to <FIG>, fuel system apparatus <NUM> is illustrated according to one embodiment for supplying fuel to internal combustion engine <NUM>. Pumping apparatus <NUM> pressurizes liquid fuel from liquid fuel storage vessel <NUM> and delivers the pressurized liquid fuel to fuel pressure bias apparatus <NUM>. Pumping apparatus <NUM> can include a transfer pump located in liquid fuel storage vessel <NUM>, an inlet metering valve and a common rail pump, in addition to other fuel system components known to those skilled in the technology. Pumping apparatus <NUM> pressurizes gaseous fuel from cryogenic storage vessel <NUM> and delivers it to fuel pressure bias apparatus <NUM> through heat exchange apparatus <NUM> where the enthalpy of the gaseous fuel is increased. Gaseous fuel is stored in liquefied form in cryogenic storage vessel <NUM> such that pumping apparatus <NUM><NUM> is a cryogenic pumping apparatus, and the gaseous fuel changes from the liquid state to either gas or supercritical state between vessel <NUM> and downstream of heat exchange apparatus <NUM>. Although pumping apparatus <NUM> and heat exchange apparatus <NUM> are illustrated external to cryogenic storage vessel <NUM>, the pumping apparatus alone or in combination with the heat exchange apparatus can be located inside the cryogenic storage vessel, in whole or in part, as would be known by those familiar with the technology. Heat exchange apparatus <NUM> can employ engine coolant, fluidly communicated from and to engine <NUM> through conduits <NUM> and <NUM> respectively, as a heat exchange medium. Shut-off valve <NUM> is employed to separate heat exchange apparatus <NUM> from the downstream fuel system when the internal combustion engine is shut down. Accumulator <NUM> stores a predetermined volume of pressurized and vaporized gaseous fuel as a buffer against fuel demand from the internal combustion engine, which may be a vessel or appropriately sized piping. Fuel pressure bias apparatus <NUM> is employed to keep liquid fuel pressure in liquid fuel rail <NUM> within a predetermined range of gaseous fuel pressure in gaseous fuel rail <NUM>, which is needed for proper operation of dual fuel injector <NUM>. <CIT>, and co-owned by the Applicant, discloses various embodiments of fuel pressure bias apparatus <NUM> that can be employed herein, although other techniques for maintaining a pressure bias between two fuels can also be employed. Dual fuel injector <NUM> is fluidly connected with liquid fuel rail <NUM> and gaseous fuel rail <NUM> and is operative to separately and independently inject liquid fuel and gaseous fuel into a combustion chamber. In a typical embodiment, fuel injector <NUM> employs the liquid fuel as a hydraulic fluid for actuating the injector, and accordingly the pressure bias between the liquid fuel and the gaseous fuel is maintained within a predetermined range to operate the fuel injector. Although only one such fuel injector is illustrated, there can be a plurality of fuel injectors in other embodiments. Fuel injector <NUM> can be like the dual fuel injector found in the Applicant's co-owned <CIT>, although other types of dual fuel injectors can be employed. Electronic controller <NUM> is operatively connected with pumping apparatuses <NUM> and <NUM>, shut-off valve <NUM> and fuel injector <NUM> to command their operation to reduce deposit accumulation in gaseous and liquid orifices as will be described in more detail below.

Referring now to <FIG>, fuel injector <NUM> is described in further detail, and in particular the portion of the fuel injector disposed in the combustion chamber referred to as nozzle <NUM>. Fuel injector <NUM> has a concentric needle arrangement in the illustrated embodiment, including valve body <NUM>, first valve member <NUM> and second valve member <NUM>. Valve body <NUM> further includes known structures for housing respective actuator assemblies (not shown) for first and second valve members <NUM> and <NUM>, and inlets for receiving respective fuels and delivering same to the illustrated nozzle portion of valve body <NUM>. Valve body <NUM> at the nozzle is generally tubular in shape and comprises gaseous fuel orifices <NUM> formed in wall <NUM>. Although only two gaseous fuel orifices <NUM> are representatively illustrated in the cross section shown in <FIG> it is understood by those familiar with the technology that there are typically a plurality of gaseous fuel orifices spaced around the nozzle perimeter. First valve member <NUM>, in the form of a hollow needle or sleeve, is actuatable for reciprocating movement within valve body <NUM>. First valve member <NUM> is made to reciprocate to open and close first injection valve <NUM>. When first injection valve <NUM> is open, gaseous fuel stored in plenum <NUM> is introduced into combustion chamber <NUM> through gaseous fuel orifice <NUM>. In the illustrated fuel injector, plenum <NUM> is an annular cavity formed between valve body <NUM> and first valve member <NUM>. Second valve member <NUM>, also known as a needle, is actuatable for reciprocating movement within the hollow interior of first valve member <NUM>, and is made to reciprocate to open and close second injection valve <NUM> to introduce liquid fuel into combustion chamber <NUM> through liquid fuel orifices <NUM> formed in tip wall <NUM> of the first valve member. Although only two liquid fuel orifices <NUM> are representatively illustrated in the cross section shown in <FIG>, it is understood by those familiar with the technology that there can be a plurality of liquid fuel orifices spaced around the perimeter of the nozzle tip. While second injection valve <NUM> is open, liquid fuel flows from annular plenum <NUM> formed between first and second valve members <NUM> and <NUM>, through second injection valve <NUM> and exits nozzle <NUM> through liquid fuel orifices <NUM>, into combustion chamber <NUM>. First and second valve members <NUM> and <NUM> can be electrically actuated and made to move directly by magnetic forces or hydraulically actuated using apparatus known by people skilled in fuel injector technology. Match-fit <NUM> at distal ends of valve body <NUM> and first valve member <NUM>, comprises opposite facing surfaces that allow the first valve member to move relative to the valve body when actuated between open and closed positions. Match-fit <NUM> can alternatively be called nose clearance, referring to the clearance between the injector body and the valve member.

Deposits can accumulate in orifices <NUM> and <NUM> as engine <NUM> operates, and these deposits lead to gaseous fuel flow loss and liquid fuel flow loss through the respective nozzle holes, resulting in torque loss in the engine. Several techniques were developed to mitigate the effects of deposit formation, including orifice geometries inhibiting deposit formation, and adaptation of gaseous and liquid fuel pressure and temperature to remove deposits and/or reduce the formation of deposits altogether, as will now be described. As used herein, deposit (including coking) mitigation refers to the removal of deposits in orifices <NUM> and/or orifices <NUM>, and/or the reduction in the rate of accumulation of deposits in these orifices.

As injector <NUM> is operated for the first time it experiences gaseous fuel and liquid fuel flow loss through respective orifices <NUM> and <NUM> over an initial break- in period, after which the flow stabilizes. As used herein, flow loss refers to the reduction in gaseous fuel and liquid fuel mass flow rates for given injection pressures through respective orifices <NUM> and <NUM>. Injection pressure is defined herein to be the difference between fuel pressure upstream of the injection valve and the pressure in the combustion chamber. Typically, the pressure upstream of the injection valve is substantially equal to the pressure in the fuel rail. Liquid fuel injection pressure is the difference between liquid fuel pressure in liquid fuel rail <NUM> and the pressure in combustion chamber <NUM>, and gaseous fuel injection pressure is the difference between gaseous fuel pressure in gaseous fuel rail <NUM> and the pressure in the combustion chamber. In some applications the reduction in mass flow rate after the initial break-in period is acceptable. However, when the flow loss is reduced, and particularly for gaseous fuel flow loss when the engine primarily derives power from the gaseous fuel, the torque performance of engine <NUM> is improved. It was discovered that by shortening the length of orifices <NUM> and <NUM> the flow loss in these orifices is reduced, A reduction in a previous length of a previous gaseous fuel orifice showing deposit accumulation above a predetermined threshold of either orifices <NUM> and <NUM>, or both, by substantially between a range of <NUM>% and <NUM>% shows a statistically significant reduction in flow loss and improvement in torque performance. There is a limit to how much the length of these orifices can be reduced without impacting the structural stability and thermal integrity of nozzle <NUM>. In those applications where fuel injectors like fuel injector <NUM> are currently being employed, break-in flow loss can be reduced and torque performance improved when the nozzle orifice lengths are decreased.

With reference to <FIG>, gaseous fuel orifice <NUM>' and liquid fuel orifice <NUM>' are illustrated with a geometry that inhibits the formation of deposits according to a first embodiment. Orifice <NUM>' includes inlet opening <NUM>, into which gaseous fuel enters from chamber <NUM>, aind outlet opening <NUM>, from which gaseous fuel exits the orifice into combustion chamber <NUM>. The surfaces of openings <NUM> and <NUM> are at right angles to longitudinal axis <NUM> of orifice <NUM>'. When the inlet and outlet openings of orifice <NUM>' are not at right angles to longitudinal axis <NUM>, openings <NUM> and <NUM> are defined to be the projection of these surfaces onto the plane that is at a right angle to longitudinal axis <NUM>. Orifice <NUM>' includes inlet opening <NUM>, into which liquid fuel enters from chamber <NUM>, and outlet opening <NUM>, from which liquid fuel exits the orifice into combustion chamber <NUM>. The surfaces of openings <NUM> and <NUM> are at right angles to longitudinal axis <NUM> of orifice <NUM>'. When the inlet and outlet openings of orifice <NUM>' are not at right angles to longitudinal axis <NUM>, openings <NUM> and <NUM> are defined to be the projection of these surfaces onto the plane that is at a right angle to longitudinal axis <NUM>. Diameter dl is the diameter of inlet opening <NUM>, and diameter d2 is the diameter of outlet opening <NUM>, and diameters dl and d2 are selected such that orifice <NUM>' has an inwardly tapering profile. Diameter dl is less than diameter d2 such that lines <NUM> and <NUM> extending between openings <NUM> and <NUM> are substantially linear and outwardly diverging with respect to chamber <NUM> and the cross-sectional area between chamber <NUM> and combustion chamber <NUM> is outwardly diverging. Diameter d3 is the diameter of inlet opening <NUM>, and diameter d4 is the diameter of outlet opening <NUM>, and diameters d3 and d4 are selected such that orifice <NUM>' has an inwardly tapering profile. Diameter d3 is less than diameter d4 such that lines <NUM> and <NUM> extending between openings <NUM> and <NUM> are substantially linear and outwardly diverging with respect to chamber <NUM> and the cross-sectional area between chamber <NUM> and combustion chamber <NUM> is outwardly diverging. The diverging nature of orifices <NUM>' and <NUM>' protects the hydraulic diameter from external deposits, which may enter orifices <NUM>' and <NUM>' through respective outlet openings <NUM> and <NUM> during and after combustion of fuel in combustion chamber <NUM>. Surprisingly, deposit formation is inhibited in orifice <NUM> of <FIG> when orifi ce <NUM>' is employed in combination with orifice <NUM>. Further reduction in deposit formation is experienced when orifices <NUM>' and <NUM>' are used in combination. A firs), difference in diameter between d2 and dl is in the range of substantially between three (<NUM>) micrometers and fifty (<NUM>) micrometers, and more preferably in the range of substantially between fifteen (<NUM>) micrometers and thirty (<NUM>) micrometers. A second difference in diameter between d4 and d3 is in the range of substantially between three (<NUM>) micrometers and fifty (<NUM>) micrometers, and more preferably in the range of substantially between fifteen (<NUM>) micrometers and thirty (<NUM>) micrometers.

With reference to <FIG>, a method of reducing deposit accumulation in orifice holes <NUM>, <NUM>', <NUM> and <NUM>' is now discussed according to a second embodiment. As used herein, reducing deposit accumulation can refer to cleaning the orifices (removing deposits) that already have deposits built up in them, and/or limiting deposit accumulation in the orifices. Limiting deposit accumulation can refer to reducing the rate at which deposits accumulate and/or maintaining deposit accumulation below a predetermined level, which can refer to a thickness of a deposit layer, such as a coking layer, in the orifices. In step <NUM> it is determined that deposit mitigation is needed. This determination may involve employing a model to estimate a level of deposit accumulation and when the estimated level reaches a predetermined value it is determined that deposit mitigation is needed. Such a model may accept engine speed and engine load as inputs and generate an amount of deposit formation that occurs per engine cycle, which can be integrated from engine cycle to engine cycle to determine a total level of deposit accumulation. Alternatively, a counter or timer can be employed to count fuel injection cycles or elapsed engine operation time respectively, and when a predetermined number of cycles or amount of time has occurred it is determined that deposit mitigation is needed. In step <NUM> injection pressure of gaseous fuel and/or liquid fuel is increased such that deposit formation in respective fuel orifices (<NUM>, <NUM>', <NUM>, <NUM>') is reduced, limited and/or removed. The injection pressures of the two fuels can be increased together when a predetermined differential pressure between gaseous fuel and liquid fuel is to be maintained, or when the dual fuel injector is operating in a single fuel mode the injection pressure of the single fuel can be increased alone. By increasing injection pressure across the respective orifices, deposits can be blown out and the orifices cleaned. Additionally, combustion products from the liquid fuel can be further inhibited from entering into orifices <NUM>, <NUM>' when gaseous fuel injection pressure increases, thereby reducing the further formation of deposits therein.

There are a variety of ways in which gaseous and liquid fuel injection pressure can be increased. Electronic controller <NUM> can command liquid fuel pumping apparatus <NUM> and gaseous fuel pumping apparatus <NUM> to increase liquid and gaseous fuel pressure respectively; while fuel pressure bias apparatus <NUM> maintains the differential pressure bias between these two fuels. Liquid fuels are incompressible fluids so the pressure of liquid fuel can be increased relatively efficiently compared to gaseous fuels, which are compressible fluids. It takes substantially more energy and time to compress gaseous fuels due to their compressible nature. As a result there is a practical limit to how much the pressure of liquid fuel and gaseous fuel can be increased in rails <NUM> and <NUM> before the fuel economy of engine <NUM> begins to be significantly impacted. Nevertheless, the technology for compressing compressible fluids is continuously developing and improving, and increasing the pressure in liquid and gaseous fuel rails <NUM> and <NUM> is a preferred technique for increasing liquid and gaseous fuel injection pressures.

Fuel system <NUM> and engine <NUM> are a high pressure direct injection system, where fuel injection timing for both liquid and gaseous fuels typically begins later in the compression stroke. As an example, liquid and gaseous fuel injection timing can begin after <NUM> degrees (°) before top dead center (BTDC) in the compression stroke. As a reference point, <NUM>° BTDC in the compression stroke is when the piston (not shown) is at bottom dead center position (BDC) and <NUM>° BTDC in the compression stroke is when the piston is at top dead center position (TDC), as would be known by those skilled in the technology. As the piston travels from <NUM>° BTDC to <NUM>° BTDC the pressure in combustion chamber <NUM> increases since the volume therein decreases. Injection pressure can be increased by advancing the timing of liquid and/or gaseous fuel injection, when combustion chamber pressure is less compared to normal fuel injection timing. In an exemplary embodiment liquid fuel injection timing can be advanced by at least <NUM>° and gaseous fuel injection timing can be advanced by at least <NUM>°, although any amount of advance in timing may have a beneficial effect over time. In those embodiments where engine <NUM> comprises a turbocharger or supercharger (not shown), injection pressure can also be increased when the engine is operating without boost, such that the charge of air inhaled into combustion chamber <NUM> during the intake stroke is substantially at atmospheric pressure. Further, advancing fuel injection timing can be used in combination with operation in those parts of the engine map where engine <NUM> is operating without boost to further increase fuel injection pressure.

When engine <NUM> comprises a plurality of dual fuel injectors associated with respective combustion chambers, skip-firing can be employed in combination with increasing fuel injection pressure to increase the fuel injection window in any particular combustion chamber such that the deposits in respective orifices <NUM>, <NUM>', <NUM>, <NUM>' are exposed to higher than normal fuel mass flow for a longer period of time to increase the likelihood that the deposits are removed. Skip-firing is the technique of skipping the introduction and subsequent combustion of fuel in one or more combustion chambers, and instead introducing a larger amount of fuel into another combustion chamber, such that the total amount of fuel consumed by engine <NUM> remains the same. Instead of employing fuel to remove deposits in the fuel orifices of fuel injector <NUM>, compressed air can be employed to blow-out the orifices at shutdown after liquid and/or gaseous fuel has been removed from the respective fuel rails <NUM> and <NUM>. Compressed air can be obtained by bleeding off a portion of compression air from combustion chamber <NUM> during each engine cycle, or by employing engine brake air.

Referring now to <FIG> and <FIG> an apparatus and method of reducing deposit accumulation in orifice holes <NUM>, <NUM>', <NUM> and <NUM>' are now discussed according to a third embodiment. Deposit formation in the nozzle orifices of fuel injector <NUM> can be reduced by adjusting gaseous fuel and/or liquid fuel temperature. During those parts of the engine map of engine <NUM> where combustion chamber temperature and/or fuel injector nozzle temperature is known to be above a predetermined temperature, liquid and/or gaseous fuel temperature can be decreased to reduce the temperature of nozzle <NUM>, thereby inhibiting the formation of deposits in the orifices. Liquids are better conductors of heat and preferably the liquid fuel temperature is adjusted to reduce and/or control nozzle temperature. In dual fuel injectors where the liquid fuel is employed both as a fuel and as a hydraulic fluid, it is possible that liquid fuel can flow through match fits into the gaseous fuel plenum such that it can also cool the gaseous fuel flow passages and injector body. However, there is an advantage to controlling the gaseous fuel temperature apart from the liquid fuel temperature. By using the liquefied gaseous fuel in cryogenic storage vessel <NUM> as a low temperature reservoir, the temperature of both the gaseous fuel and the liquid fuel can be controlled.

With reference to <FIG>, in step <NUM> it is determined that deposit mitigation is needed. This determination may involve transitioning into and operating in a portion of the engine map where it is known that combustion chamber temperature increases above a predetermined value, or may involve an estimation of the nozzle temperature of fuel injector <NUM> based on engine parameters and/or a nozzle (tip) temperature model. An exemplary tip temperature model for a gaseous fuel injector nozzle is disclosed in the Applicant's co-owned International Patent Publication No. <CIT>. In step <NUM> the temperature of gaseous fuel and/or liquid fuel is adjusted (decreased) such that the temperature of nozzle <NUM> is reduced thereby inhibiting the formation of deposits.

With reference to <FIG>, fuel system <NUM> illustrates a technique to adjust gaseous and/or liquid fuel temperature. Variable-flow valve apparatus <NUM> selectively adjusts the flow of engine coolant to heat exchange apparatus <NUM>', by controlling how much engine coolant bypasses the heat exchange apparatus, such that the temperature of gaseous fuel downstream from the heat exchange apparatus can be controlled. Similarly, variable-flow valve apparatus <NUM> selectively adjusts the flow of liquid fuel to heat exchange apparatus <NUM>', by controlling how much liquid fuel bypasses the heat exchange apparatus, such that the temperature of liquid fuel downstream of the variable-flow valve apparatus can be controlled. The portion of liquid fuel fluidly communicated to heat exchange apparatus <NUM>' is cooled by using the gaseous fuel as a low temperature reservoir and then returned to conduit <NUM> where it is recombined with any liquid fuel fluidly communicating directly from variable-flow valve apparatus <NUM>. Heat exchange apparatus <NUM>' can comprise one or more heat exchangers, and can employ one or more heat exchange fluids.

An apparatus and method of reducing deposit formation is now discussed according to a third embodiment. Reductions in deposit formation have been observed when certain types of coatings are applied to nozzle <NUM>, first valve member <NUM> and/or second valve member <NUM>. Hydrophobic and oleophobic coatings when applied provide a non-stick type of protection for dual fuel injector <NUM> that inhibits the ability of deposits to stick to the nozzle and valve members of the injector. A particularly effective category of this type of coating is fluorosilane based coatings, since they have excellent hydrophobic and oleophobic qualities, in addition to a relatively high resistance to solvents, acids and bases, which is advantageous in gaseous and liquid fuel applications.

Catalytic coatings that encourage a beneficial effect can be employed to mitigate deposit formation. A first type of catalytic coating can facilitate the burning of deposits once they are formed thereby reducing and preferably preventing the accumulation of deposits. Catalytic coatings that include cerium oxide and/or oxides of other lanthanide series elements, oxides of transition metals and/or transition metals are particularly suitable for encouraging the chemical reaction of deposits with combustion chamber gases, and in particular oxygen from the intake charge. This type of coating is effective when applied to the outer surface of nozzle <NUM> in the vicinity of orifice <NUM> and to the outer surface of first valve member <NUM> in the vicinity of orifice <NUM> to reduce the formation of deposits around the openings of these orifices. The burning of deposits increases the temperature in the local vicinity, which is generally not preferable within fuel orifices <NUM> and <NUM> and on the valve members within valve body <NUM>.

A second type of catalytic coating can promote the formation of a porous deposit structure on the surfaces of nozzle <NUM>, first valve member <NUM> and/or second valve member <NUM>. The porous deposit structure can be broken apart by the flow of gaseous fuel and liquid fuel through fuel orifices <NUM> and <NUM> respectively, unlike deposit formations formed without this type of coating. The second type of catalytic coating is a multi-phase microstructure in which one or more phases act as deposit nucleation sites that have a higher tendency to form carbon deposits, and promote the formation of the porous deposit structure. This type of coating is also preferably employed on the outer surface of nozzle <NUM> in the vicinity of orifice <NUM> and to the outer surface of first valve member <NUM> in the vicinity of orifice <NUM> to reduce deposit formation around the openings of these orifices. It is possible that this coating can be applied on the surfaces in these orifices as well, providing they do not substantially interfere with the flow of fuel therethrough. The same type of coatings used for the first type of catalytic coating can be used for the second type of catalytic coating in different combinations to promote the formation of a porous structure as opposed to consuming the deposits by burning. The coatings disclosed herein can be applied to dual fuel injector <NUM> by way of physical vapor deposition or chemical vapor deposition. The coatings discussed heretofore can be applied to any type of fuel injector including monofuel injectors as well as dual fuel injectors, and such injectors can be hydraulically or electro-mechanically actuated.

An apparatus and method of reducing deposit formation is now discussed according to a fourth embodiment. As an alternative to a fluorosilane coating, or in addition to, the surfaces of nozzle <NUM> and valve members <NUM> and <NUM> can be formed with a surface pattern that is characterized by an array of features small enough to reduce the ability of deposits to adhere to the underlying surface, such that the deposits are swept away by the flow of fuel through orifices <NUM> and <NUM>. The surface pattern can be formed by lasers and lithography (including electron-beam lithography) and chemical patterning and chemical etching. An exemplary technique of forming the surface pattern is by way of femtosecond laser nanomachining that allows the surface features to be on the order of <NUM> to <NUM> nanometers. Surface patterns with features of this scale exhibit excellent hydrophobic and oleophobic characteristics. Surface patterns can include an array of elevated spires, an array of elevated polygons, and preferably regular polygons, in addition to other patterns. Surface nanomachining can be employed to remove surface irregularities that promote the adhesion of deposits. The surface patterning techniques disclosed herein can be applied to any type of fuel injector.

As an alternative to the above techniques, or in addition to, deposit control additives, for example detergents, can be mixed with the pilot fuel and/or the gaseous fuel. Surprisingly, when a deposit control additive was mixed with the pilot fuel only, it was discovered that deposit formation in gaseous fuel orifices <NUM> was reduced, in addition to a reduction in deposits in liquid fuel orifice <NUM>. The additives act to reduce the formation of and/or remove existing deposit formations.

Claim 1:
A method for deposit mitigation in a gaseous fuel injector (<NUM>) that introduces a gaseous fuel through a gaseous fuel orifice (<NUM>) directly into a combustion chamber of an internal combustion engine (<NUM>) comprising:
providing the gaseous fuel orifice (<NUM>') with an inlet opening (<NUM>) into which the gaseous fuel enters from the gaseous fuel injector, an outlet opening (<NUM>) from which the gaseous fuel exits into the combustion chamber (<NUM>), an inwardly and substantially linearly tapering profile and with a difference between an outlet-opening diameter (d2) of the outlet opening (<NUM>) and an inlet-opening diameter (d1) of the inlet opening (<NUM>) in a range between <NUM> micrometers and <NUM> micrometers;
wherein the gaseous fuel is in a gas state and a mass flow rate of gaseous fuel through the gaseous fuel orifice (<NUM>) at a predetermined injection pressure is above a predetermined value such that torque loss in the internal combustion engine (<NUM>) due to deposit accumulation in the gaseous fuel orifice (<NUM>) is reduced after a break-in period, wherein the break-in period is the amount of time it takes for the rate of torque loss to diminish to a predetermined value, preferably zero.