Gaseous fuel injector using liquid fuel as lubricant and pressure-transmitting medium

This application is based upon and claims benefit of priority of Japanese Patent Application No. 2007-185582 filed on Jul. 17, 2007, the content of which is incorporated herein by reference.

1. Field of the Invention

The present invention relates to an injector that injects high-pressure gaseous fuel directly into a combustion chamber of an internal combustion engine.

2. Description of Related Art

It is difficult to drastically reduce pollutant gas such as nitrogen oxides, hydrocarbons or carbon oxides included in exhaust gas of an internal combustion engine, as long as liquid petroleum is used as fuel for the internal combustion engine. It has been proposed to use gaseous fuel, such as liquid natural gas (LNG, CNG), liquid petroleum gas (LPG) or hydrogen gas, in place of liquid fuel to improve exhaust gas quality of internal combustion engines. The gaseous fuel is already used in certain engines.

However, certain problems are involved in using the gaseous fuel for engines to which fuel is directly injected by injectors. That is, the sufficient durability is not realized in the injectors because lubrication ability of the gaseous fuel is low. Further, the engines are not stably operated if gases having a low cetane number, such as hydrogen gas or pressurized natural gas, are used for diesel engines because ignitability of those gases is not high. Some proposals have been made to cope with those problems, for example, by JP-A-2003-232234 and JP-A-2006-118470.

JP-A-2003-232234 proposes to use liquid fuel (such as gasoline) and gaseous fuel (such as LPG) selectively or simultaneously according to operating conditions of an engine. In this proposal, a first system for supplying liquid fuel and a second system for supplying gaseous fuel are provided. When the fuel supplied to the engine is switched to the gaseous fuel, a liquid fuel is added to the gaseous fuel. In this manner, it is avoided that an air-fuel ratio in an intake mixture becomes too lean, which results in misfiring and bad driveability. In this system, however, the engine operating region where the liquid fuel is used is relatively large, and therefore an advantage of using the gaseous fuel is not sufficiently attained. In addition, since two systems including injectors and fuel tanks for supplying the liquid fuel and the gaseous fuel are required, an entire fuel supply system becomes large and expensive.

JP-A-2006-118470 proposes to improve lubrication in an injector by partially using the liquid fuel together with the gaseous fuel. A valve seat of the fuel injector is lubricated by the liquid fuel, and ignitability of mixture is improved by the liquid fuel injected together with the gaseous fuel injected from injection holes at a high speed. If liquid fuel having a high cetane number, such as light oil or DME (dimethyl ether) is used as the liquid fuel to be mixed with the gaseous fuel, self-ignition is possible by compression without using a spark plug. In this proposal, however, the high-pressure liquid fuel is directly supplied to a sliding portion of an injector needle. Therefore, an amount of liquid fuel injected together with the gaseous fuel tends to fluctuate, causing deviation in combustion pressures among cylinders. To reduce the fluctuation in the amount of the liquid fuel supplied, it is required to manufacture the injectors with a very high precision, resulting in a high manufacturing cost.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved gaseous fuel injector, in which liquid fuel is utilized as a medium for transmitting a pressure for operating an injection nozzle and for lubricating sliding portions in the injector.

The fuel injector of the present invention injects fuel directly into a combustion chamber of an internal combustion engine. High-pressure gaseous fuel is mainly injected into a cylinder, and liquid fuel is used as a medium for transmitting a driving force and as a lubricant. The liquid fuel is also injected into the cylinder together with the high-pressure gaseous fuel.

The fuel injector includes a nozzle member having injection holes at its tip, a needle slidably disposed in an elongated center hole formed in the nozzle member, and a control chamber for driving the needle. The high-pressure gaseous fuel is introduced into a high-pressure gaseous fuel chamber formed between the nozzle member and the needle. Liquid fuel is introduced into the control chamber, and pressure of the liquid fuel in the control chamber is controlled by a switching valve driven by an actuator such as an electromagnetic actuator. The needle slidably coupled to the nozzle member is driven to open or close the injection holes.

The liquid fuel in the control chamber is supplied to a small sliding space between the nozzle member and the needle to lubricate the small sliding space through a liquid fuel leading passage. The liquid fuel is further supplied to a connecting passage formed as an elongated center hole in the needle after lubricating the small sliding space. The connecting passage leads the liquid fuel to the injection holes when the injection holes are open. High-pressure gaseous fuel is injected into the cylinder when the injection holes are opened by lifting the needle. The liquid fuel is also injected from the injection holes together with the high-pressure gaseous fuel. The amount of liquid fuel injected is controlled to a certain level by controlling the pressure in the control chamber.

A first one-way valve that prevents liquid fuel flow from the connecting passage to the liquid fuel leading passage at a first predetermined pressure, and a second one-way valve that prevents the liquid fuel from entering into the liquid fuel leading passage at a predetermined second pressure may be provided in the liquid fuel leading passage. By providing these one-way valves, an amount of liquid fuel injected per one injection is more precisely controlled. The needle may be composed of two needles, a first needle slidably coupled to the nozzle member and a second needle slidably coupled to the first needle, to inject first a small amount of high-pressure gaseous fuel and then mixture of the high-pressure gaseous fuel and the liquid fuel. In this manner, ignitability can be further improved. A piezoelectric actuator may be used to control the control chamber pressure at a higher response.

According to the present invention, the liquid fuel is effectively utilized as a lubricant besides utilizing it as a medium for transmitting a driving force to the needle. An amount of liquid fuel injected together with the high-pressure gaseous fuel is controlled to a certain level to thereby improve ignition of fuel in the cylinder. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.

FIG. 1 is a cross-sectional view showing a fuel injector as a first embodiment of the present invention, taken along line I-I shown in FIG. 3 ;

FIG. 2 is a cross-sectional view showing the fuel injector, taken along line II-II shown in FIG. 3 ;

FIG. 3 is a plan view showing the fuel injector, viewed in direction III shown in FIG. 1 ;

FIGS. 4A-4C are cross-sectional views, in an enlarged scale, explaining operation of the fuel injector when injection holes are closed;

FIGS. 5A-5C are cross-sectional views, in an enlarged scale, explaining operation of the fuel injector when injection holes are opened;

FIG. 6 is a cross-sectional view showing a fuel injector as a second embodiment of the present invention;

FIG. 7 is a time chart showing pressure of liquid fuel and gaseous fuel, injection quantity and amounts of valve lifts in the second embodiment;

FIG. 8 is a cross-sectional view showing a fuel injector as a third embodiment of the present invention in a state where injection holes are closed;

FIG. 9 is a cross-sectional view showing a nozzle portion of the fuel injector shown in FIG. 8 , in an enlarged scale;

FIG. 10 is a cross-sectional view showing the same injector as shown in FIG. 8 in a state where the injection holes are opened;

FIGS. 11A-11C are cross-sectional views, in an enlarged scale, explaining operation of the fuel injector shown in FIG. 8 ; and

FIG. 12 is a time chart showing pressure of liquid fuel and gaseous fuel, quantity of injected fuel and amounts of valve lifts in the third embodiment.

The fuel injector according to the present invention supplies high-pressure gaseous fuel into a combustion chamber of an internal combustion engine. Opening and closing injections holes are controlled pressure of liquid fuel supplied to the fuel injector separately from the gaseous fuel. The liquid fuel is also utilized as lubricant for lubricating sliding portions between a nozzle and a needle. The liquid fuel is not directly supplied to the sliding portions, but the liquid fuel led out of a control chamber for reducing a pressure therein is supplied to the sliding portions. A predetermined amount of the liquid fuel is injected into the combustion chamber together with the high-pressure gaseous fuel.

Thus, the liquid fuel is effectively used for improving lubrication in the fuel injector injecting gaseous fuel and for attaining good ignitability in the internal combustion engine. Gases such as natural gas (LNG, CNG), petroleum gas (LPG) or hydrogen gas are used as the high-pressure gaseous fuel, and fuel having a high cetane number, such as light oil or dimethyl ether (DME) is used as the liquid fuel.

A first embodiment of the present invention will be described with reference to FIGS. 1-5 . The fuel injector of the present invention is used in a compression-ignition engine (Diesel engine) having multi-cylinders, and one injector is installed in each cylinder.

First, referring to FIGS. 1-3 , an entire structure of a fuel injector 1 will be described. The fuel injector 1 includes an injector body 300 , a nozzle 10 disposed below the injector body, an actuator 60 disposed above the injector body via a switching valve 50 , and a needle 20 slidably disposed in the injector body 300 . The nozzle 10 is connected to the injector body 300 with a nozzle holder 40 , and the actuator 60 is connected to the injector body 300 with an actuator holder 70 .

The nozzle 10 is formed in an elongated cylindrical shape having a nozzle body 101 that includes a closed bottom end. In the nozzle 10 , a sliding hole 106 , in which a sliding surface 201 of the needle 20 is slidably disposed, is formed. Injection holes 103 that are opened or closed by a valve portion 200 of the needle 20 are formed in the closed bottom end of the nozzle body 101 . A high-pressure gaseous fuel chamber 102 is formed between a needle shaft 205 and an inner wall of the nozzle body 101 . The high-pressure gaseous fuel chamber 102 is formed in a circular shape surrounding the needle shaft 205 . A sack chamber 107 is formed below the high-pressure gaseous fuel chamber 102 . The injection holes 103 are formed through a wall of the sack chamber 107 .

A feeding passage 105 (refer to FIG. 2 ) for feeding liquid fuel is formed in the nozzle body 101 and connected to the high-pressure gaseous fuel chamber 102 . An upper end of the passage 105 is connected to gaseous fuel passages 330 , 332 , 333 (described later). A liquid fuel leading passage 320 for leading liquid fuel from a control chamber 510 (explained later) is formed in the injector body 300 , and a passage 108 connected to the liquid fuel leading passage 320 is formed in the nozzle body 101 . An outer diameter of the needle shaft 205 is smaller than a diameter of the sliding surface 201 , and a diameter of an inner hole 209 is larger than a diameter of the sliding hole 106 , thereby making a space of the high-pressure gaseous fuel chamber 102 sufficiently large.

A needle portion 200 in a conical-shape is formed at the bottom end of the needle shaft 205 (refer to FIG. 4C ). A valve surface 208 of the needle portion is made to be able to closely contact a valve surface 104 of the nozzle body 101 . On the sliding surface 201 of the needle 20 slidably contacting the sliding hole 106 of the nozzle body 101 , circular grooves 202 are formed. A connecting passage 206 connected to the liquid fuel leading passage 108 through the liquid fuel feeding passage 109 is formed along a center axis of the needle 20 . One end of the connecting passage 206 is open at an opening 204 of the circular groove 202 , and the other end of the connecting passage 206 is open to an opening 207 on the valve surface 208 (refer to FIG. 4C ).

The liquid fuel led out from the control chamber 510 flows through the liquid fuel leading passage 320 , the liquid fuel leading passage 108 in the needle, the liquid fuel feeding passage 109 and the connecting passage 206 . The liquid fuel is thus supplied to the circular grooves 202 and to the opening 207 on the valve surface 208 . The liquid fuel supplied to the circular grooves 202 lubricates the sliding surface 201 of the needle 20 , and the liquid fuel supplied to the opening 207 lubricates the valve surface 208 . In the lower-most circular groove 202 , a seal member 203 for interrupting a flow of the liquid fuel to the high-pressure gaseous fuel chamber 102 and for interrupting a flow of the gaseous fuel to the circular grooves is disposed. Since the liquid fuel is not supplied to the sliding surface below the seal member 203 , a clearance C (refer to FIG. 4B ) is made between the sliding surface 201 and the sliding hole 106 by reducing the diameter of the sliding surface 201 .

A control piston 21 connected to the needle 20 is slidably disposed in a sliding hole 301 of the injector body 300 . A sliding surface 211 of the control piston 21 is lubricated by the liquid fuel supplied to circular grooves 212 formed on the sliding surface 211 from the control chamber 510 . The switching valve 50 forming the control chamber 510 is disposed at an upper portion of the control piston 21 . In the control chamber 510 , a ball valve 520 is disposed to close an upper opening of the control chamber 510 . The control chamber 510 is formed by a space confined by the sliding hole 301 and the ball valve 520 .

The liquid fuel pressure in the control chamber 510 is applied to the control piston 21 connected to the needle 20 in a direction to close the injection holes 103 , i.e., in the direction pushing down the control piston 21 . A return spring 220 disposed in a spring chamber 302 applies a force downwardly to the needle 20 . The pressure of the high-pressure gaseous fuel in the chamber 102 is applied to the needle in a direction to open the injection holes 103 . As a result, the needle 20 is pushed down by the pressure in the control chamber 510 and the return spring 220 , while it is pushed up by the pressure of the gaseous fuel in the chamber 102 . Therefore, the injection holes 103 are opened or closed by changing the pressure in the control chamber 510 . In a space between a piston shaft 213 and an inner wall of the injector body 300 , the liquid fuel for generating a counter balance pressure is introduced.

The switching valve 50 that is driven by the actuator 60 includes a ball valve 520 and a ball valve chamber 530 (refer to FIG. 4A ). In the ball valve chamber 530 , a first port Po 1 , a second port Po 2 and a third port Po 3 are formed, i.e., a two-position-three-way valve is formed. The first port Po 1 is opened when the ball valve 520 is lifted from a valve seat 521 and closed when the ball valve 520 sits on the valve seat 521 . The second port Po 2 is opened when the ball valve 520 is lifted from the valve seat 522 and closed when the ball valve 520 sits on the valve seat 522 . The second port Po 2 and the third port Po 3 communicate with each other when the first port Po 1 is closed, while the first port Po 1 and the third port Po 3 communicate with each other when the second port Po 2 is closed.

The control chamber 510 is connected to the liquid fuel supply passages 310 , 311 via an inlet orifice passage 511 , and connected to the first port Po 1 via an outlet orifice passage 512 . A cross-sectional area of the inlet orifice passage 511 is made smaller than that of the outlet orifice passage 512 . High-pressure liquid fuel (at pressure Pd) contained in a common rail 90 (e.g., Pd=70 MPa) is supplied to the liquid fuel supply passages 310 , 311 through a conduit 91 . The second port Po 2 is connected to a release passage 340 that communicates with a liquid fuel tank (not shown), and the third port Po 3 is connected to the liquid fuel leading passage 320 .

When the first port Po 1 is closed, the high-pressure liquid fuel introduced into the control chamber 510 applies a pressure to the needle 20 via the control piston 21 in the direction to close the injection holes 103 . When the first port Po 1 is opened and the second port Po 2 is closed, the control chamber 510 communicates with the liquid fuel leading passage 320 via the first port Po 1 and the third port Po 3 . The liquid fuel in the control chamber 510 is led out to the liquid fuel leading passage 320 , and thereby a force for closing the injection holes 103 applied to the needle 20 through the control piston 21 decreases.

The actuator 60 that controls positions of the ball valve 520 is composed of a cylindrical solenoid 601 , an armature 602 having a T-shaped cross-section and a biasing spring 603 . The actuator 60 is controlled by an electronic control unit (ECU, not shown). When the solenoid 601 is not energized, the first port Po 1 is closed by the biasing force of the biasing spring 603 , while the ball valve 520 leaves the second valve seat 522 to open the second port Po 2 . When the solenoid 601 is energized, the first port Po 1 is opened because the ball valve 520 connected to the armature 602 moves upward against the biasing force of the biasing spring 603 , and the second port Po 2 is closed. The liquid fuel communicating with the releasing passage 340 is introduced into upper and lower spaces of the armature 602 .

Gaseous fuel pressurized to a high pressure Ph (e.g., Ph=70 MPa) in a common rail 80 (or an accumulator) is introduced into a gaseous fuel passage 330 . A reservoir 331 having a large capacity for reserving the high-pressure gaseous fuel is provided in the gaseous fuel passage 330 in order to inject a large amount of gaseous fuel in a short period of time. The high-pressure gaseous fuel is led to the high-pressure gaseous fuel chamber 102 through gaseous fuel passages 332 , 333 formed in the injector body 300 and a feeding passage 105 formed in the nozzle body 101 . It is also possible to form plural feed passages 105 to quickly introduce the gaseous fuel into the high-pressure gaseous fuel chamber 102 . By providing a sufficient capacity for the high-pressure gaseous fuel chamber 102 , it is possible to secure a sufficient amount and pressure of the gaseous fuel at a beginning of injection. By making a capacity of the reservoir 331 sufficiently large, an amount of fuel injected from the injection holes 103 can be maintained without decreasing its pressure. The sufficient capacity of the reservoir 331 is, e.g., several cubic centimeters (cm 3 ). It is also possible to provide an enlarged space for containing the gaseous fuel in the injector body 300 and/or the nozzle body 101 .

Referring to FIGS. 4A-4C , operation of the injector, when the injection holes 103 are closed, will be explained. As shown in FIG. 4A , the ball valve 520 closes the first port Po 1 by the biasing force of the biasing spring 603 because the actuator 60 is not energized. The second port Po 2 communicates with the third port Po 3 . The high-pressure liquid fuel having a pressure Pd introduced into the control chamber 510 pushes down the needle 20 via the control piston 21 to thereby close the injection holes 103 . The liquid fuel in the control chamber 520 permeates into a small space between the sliding surface 211 and the sliding hole 301 of the injector body 300 to thereby lubricate the sliding surface 211 and the sliding hole 301 . On the other hand, the second port Po 2 communicates with the third port Po 3 , and the liquid fuel leading passage 320 is connected to the releasing passage 340 . Accordingly, a pressure of the liquid fuel in the liquid fuel leading passage 320 drops to a level that is substantially equal to the atmospheric pressure.

As shown in FIGS. 4B and 4C , the needle 20 is pushed down to close the injection holes 103 by the pressure of the liquid fuel in the control chamber 510 . The needle 20 is also pushed down by a return spring 220 interposed between the control piston 21 and the needle 20 ( FIG. 1 ). The pressure of the high-pressure gaseous fuel in the chamber 102 is applied to the needle 20 to open the injection hole 103 . When the first port Po 1 is closed, and the pressure in the control chamber 510 is applied to the needle 20 , the injection holes 103 are closed. As shown in FIG. 4B , circular grooves 202 are formed on the sliding surface 201 of the needle 20 . A seal member 203 is disposed in the lowermost circular groove 202 to prevent the gaseous fuel in the high-pressure gaseous fuel chamber 102 from entering into a sliding space between the sliding surface 201 and the sliding hole 106 . Since lubrication is not properly performed by the gaseous fuel, a portion of the needle 20 , which is lower than the seal member 203 and exposed to the gaseous fuel, is made smaller than the upper portion of the needle 20 to thereby make a clearance C larger ( FIG. 4B ). As shown in FIG. C, a valve surface 208 of the valve portion 200 closely contacts and is seated on a valve seat 104 of the nozzle 10 , thereby closing the injection holes 103 . At the same time, a valve surface opening 207 communicating with the connecting passage 206 is closed, thereby preventing the liquid fuel flow through the opening 207 .

Operation of the fuel injector when the injection holes 103 are opened will be explained with reference to FIGS. 5A-5C . Upon energizing the solenoid 601 of the actuator 60 , the armature 602 moves upward by magnetic attraction of the solenoid 601 . The ball valve 520 connected to the armature 602 moves upwards, thereby opening the first port Po 1 and closing the second port Po 2 , as shown in FIG. 5A . The high-pressure liquid fluid flows out of the control chamber 510 into the liquid fuel leading passage 320 through the outlet orifice passage 512 , the first port Po 1 and the third port Po 3 . Though the liquid fuel is continued to be supplied to the control chamber 510 through the liquid fuel supply passage 311 , the pressure Pd in the control chamber decreases because the cross-sectional area of the inlet orifice passage 511 is made smaller than that of the outlet orifice passage 512 . Accordingly, the needle 20 is pushed up by the pressure of the high-pressure liquid fuel in the chamber 102 , thereby opening the injection holes 103 .

As shown in FIG. 5B , the liquid fuel led to the liquid fuel leading passage 320 quickly flows into the connecting passage 206 formed in the needle 20 through a liquid fuel leading passage 108 formed in the nozzle body 101 , a fuel feeding passage 109 formed in the nozzle body 101 . The liquid fuel lubricates the sliding surface 201 in the sliding hole 106 . As shown in FIG. 5C , when the valve surface 208 is lifted from the valve seat 104 , the high-pressure gaseous fuel in the chamber 102 flows at a very high speed through a small passage between the valve surface 208 and the valve seat 104 . The liquid fuel supplied to the connecting passage 206 is drawn by the high-speed flow of the gaseous fuel (spray effect) through the valve surface opening 207 . The high-pressure gaseous fuel and the liquid fuel enter a sack chamber 107 and injected into the combustion chamber of the engine through the injection holes 103 . A certain amount of the liquid fuel is injected together with the gaseous fuel.

An amount of liquid fuel supplied to the connecting passage 206 does not exceed an amount of the liquid fuel led out of the control chamber 510 . Since the liquid fuel pressure in the connecting passage 206 gradually decreases according to injection together with the gaseous fuel, an amount of the liquid fuel injected is limited to a certain amount. The spray effect for drawing the liquid fuel is terminated when the pressure in the connecting passage 206 becomes a vapor pressure of the liquid fuel or lower. This means that the amount of liquid fuel does not exceed a certain amount even if the gaseous fuel continues to be injected after supply of the liquid fuel is terminated.

The amount of liquid fuel supplied from the control chamber 510 is controlled according to operating conditions of the engine, e.g., several mm 3 at an idling operation and several tens mm 3 at a full load operation. Further, the amount of the liquid fuel injected can be adjusted by the number of injection holes 103 (that may be changed according to engine capacities), a diameter of the injection holes 103 , cross-sectional areas of the inlet orifice passage 511 and the outlet orifice passage 512 , an inner diameter of the connecting passage 206 , and so on. Therefore, the amount of liquid fuel injected per one injection can be set to a predetermined amount, and ignitability and lubrication can be improved at the same time in the first embodiment of the present invention.

A second embodiment of the present invention will be described with reference to FIGS. 6 and 7 . In this embodiment, a first one-way valve ch 1 is disposed between the liquid fuel leading passage 320 and the connecting passage 206 , and a second one-way valve ch 2 is disposed between the liquid fuel leading passage 320 and the releasing passage 340 ( FIG. 1 ) connected to the fuel tank. Other structures are the same as those of the first embodiment described above. The first one-way valve ch 1 does not allow the liquid fuel to flow from the connecting passage 206 to the liquid fuel leading passage 320 , while it allows the liquid fuel to flow from the liquid fuel leading passage 320 to the connecting passage 206 at a predetermined pressure difference Pch 1 (MPa) between both sides of the first one-way valve ch 1 . The second one-way valve ch 2 does not allow the liquid fuel to flow from the releasing passage 340 to the liquid fuel leading passage 320 , while it allows the liquid fuel to flow from the liquid fuel leading passage 320 to the releasing passage 340 at a predetermined pressure difference Pch 2 (MPa) between both sides of the second one-way valve ch 2 .

Operation of the fuel injector 1 a as the second embodiment of the present invention will be described with reference to FIG. 7 . In FIG. 7 , reference marks represent the followings: Ph: a pressure of the high-pressure gaseous fuel in the chamber 102 ; Pd: a pressure of the liquid fuel in the control chamber 510 ; Pcyl: a pressure in a cylinder of the engine; Pch 1 : a pressure for opening the first one-way valve ch 1 ; Pch 2 : a pressure for opening the second one-way valve ch 2 ; Qh: an amount of the high-pressure gaseous fuel injected; and Qd: an amount of the liquid fuel injected. Time lapsed is shown in the abscissa, and respective amounts marked are shown in the ordinate in FIG. 7 .

Upon energizing the actuator 60 at time t 1 , the port Po 1 is opened and the liquid fuel in the control chamber 510 flows out into the liquid fuel leading passage 320 . The pressure Pd in the control chamber 510 is gradually decreases. The needle 20 is lifted at time t 2 a short time after the first port Po 1 is opened, and the injection holes 103 are opened. The high-pressure gaseous fuel is injected together with the liquid fuel. Since the high-pressure gaseous fuel is always supplied from the common rail or an accumulator 80 , the amount Qh of gaseous fuel injected is maintained at a substantially constant level, as shown in the graph with a dotted line Qh. Accordingly the pressure Ph in the high-pressure gaseous fuel chamber 102 is maintained substantially constant with a slight decrease as shown Ph in the graph.

The pressure Pcyl in the cylinder is increased to a several-MPa level by a compression stroke in the cylinder before the fuel is injected at time t 2 . The fuel (both the gaseous fuel and the liquid fuel) injected into the cylinder is ignited by further compression of mixture in the cylinder, and the pressure in the cylinder rapidly increases as shown with a chained line Pcyl. Pressure in the connecting passage 206 increases, and the first one-way valve ch 1 is closed at time t 3 when the pressure Pcyl plus Pch 1 becomes higher than the pressure Pd in the control chamber 510 , thereby preventing a reverse flow of the liquid fuel in the connecting passage 206 . The liquid fuel is not supplied to the connecting passage 206 , and injection of the liquid fuel is terminated at time t 3 , as shown with line Qd in the graph.

On the other hand, the high-pressure gaseous fuel continues to be injected during a period in which the injection holes 103 are open because the gaseous fuel in the chamber 102 remains at a pressure Ph that is sufficiently higher than the pressure Pcyl in the cylinder. After the first one-way valve ch 1 is closed at time t 3 , the pressure Pd in the control chamber 510 increases although the first port Po 1 is opened because the liquid fuel continues to be supplied to the control chamber 510 from the liquid fuel supply passage 311 ( FIG. 1 ).

When the pressure in the liquid fuel leading passage 320 , which increases according to increase in the pressure Pd in the control chamber 510 , becomes higher than the opening pressure Pch 2 of the second one-way valve ch 2 at time t 4 , the second one-way valve ch 2 is opened, and the liquid fuel is released into the releasing passage 340 . Accordingly the pressure Pd in the control chamber 510 is maintained at the pressure which is equal to the opening pressure Pch 2 of the second one-way valve ch 2 (during a period from time t 4 to time t 5 ).

At time t 5 , the actuator 60 is de-energized, and the first port Po 1 is closed by the biasing force of the biasing spring 603 . The pressure Pd in the control chamber 510 gradually increases from time t 5 because the first port Po 1 is closed. According to increase in the pressure Pd, the needle 20 is pushed down, and a short time thereafter the injection holes 103 are closed (at time t 6 ). The pressure Pcyl in the cylinder gradually decreases after completion of combustion of the injected fuel.

With reference to FIGS. 8-12 , a third embodiment of the present invention will be described. In this embodiment, the electromagnetic actuator 60 used in the foregoing embodiments is replaced with a piezoelectric actuator 60 b having a piezoelectric element 610 . The piezoelectric actuator 60 b has a higher response than the electromagnetic actuator 60 . The needle 20 used in the foregoing embodiments are divided into two portions, a first needle (an outer needle) 20 b and a second needle (an inner needle) 23 . Injection of the gaseous fuel is controlled by the first needle 20 b , while injection of the liquid fuel is controlled by the second needle 23 . A valve body 520 b in this embodiment is pushed down by the piezoelectric actuator 60 b when it is energized, while the ball valve 520 of the first and the second embodiment is lifted up by the actuator 60 when it is energized. Other structures are the same as those of the foregoing embodiments.

In this embodiment, a fuel injector 1 b injects gaseous fuel from a first injection holes 103 a by lifting the first needle 20 b , and the injected gaseous fuel is mixed with air in a cylinder. Then, the liquid fuel is injected together with the gaseous fuel from the second injection holes 103 b by lifting the second needle 23 . In this manner, energy generated by igniting the liquid fuel is increased, and more stable ignition is realized.

As shown in FIG. 8 , a switching valve 50 b forming a two-position three-way valve is operated by the piezoelectric actuator 60 b . The liquid fuel in a first control chamber 510 b is led to a liquid fuel leading passage 320 b through an outlet orifice passage 512 b . An opening pressure Pch 1 of the first one-way valve ch 1 is set lower than an opening pressure Pch 2 of the second one-way valve ch 2 , as done in the second embodiment.

The liquid fuel supply passage 310 is connected to the first control chamber 510 b through the inlet orifice passage 511 b and to the first port Po 1 b of the switching valve 50 b . The second port Po 2 b of the switching valve 50 b is connected to the liquid fuel leading passage 320 b , and its third port Po 3 b is connected to the first control chamber 510 b through the outlet orifice passage 512 b . One end of the liquid fuel leading passage 320 b is connected to the liquid fuel leading passage 108 b formed in the injector body 300 b through the first one-way valve ch 1 b , and the other end of the liquid fuel leading passage 320 b is connected to the release passage 340 through the second one-way valve ch 2 .

A sliding hole 106 b for slidably disposing the first needle 20 b is formed along the center axis of the nozzle body 101 b . The second needle 23 is slidably disposed in a sliding hole 206 b of the first needle 20 b . A high-pressure gaseous fuel chamber 102 b is formed in a space between the first needle 20 b and the nozzle body 101 b . A first valve seat 104 a , a second valve seat 104 b and a third valve seat 104 c are formed at a bottom portion of the nozzle body 101 b . A sack chamber 107 is formed at bottom tip portion of the nozzle body 101 b . First injection holes 103 a which are opened or closed by the first needle 20 b and second injection holes 103 b which are opened or closed by the second needle 23 are formed through a wall of the sack chamber 107 . A high-pressure gaseous fuel feeding passage 105 b is formed in the nozzle body 101 b . The high-pressure gaseous fuel is fed to the high-pressure gaseous fuel chamber 102 b through the gaseous fuel feeding passage 105 b and the gaseous fuel passage 332 b.

The liquid fuel leading passage 108 b is formed in the injector body 300 b and is connected to passages 320 b , 321 b through the first one-way valve ch 1 to supply the liquid fuel into the sliding hole 106 b . The sliding surface 201 b of the first needle 20 b is slidably disposed in the sliding hole 106 b . The first needle 20 b has a circular flange 209 extending from the sliding surface 201 b , and a first spring 202 b disposed in a spring chamber 110 is engaged with the circular flange 209 so that a biasing force is applied to the first needle 20 b in a direction to close the injection holes 103 a . The first control chamber 510 b is formed at an upper end of the first needle 20 b , and pressure of the liquid fuel is applied to the first needle 20 b . At the bottom end of the first needle 20 b , a first valve surface 208 a contacting a first valve seat 103 a and a second valve surface 208 b contacting a second valve seat 103 b are formed.

The second needle 23 has a flange 232 formed at its upper portion, and the flange 232 slides in a sliding hole 207 formed in the first needle 20 b . A needle axis 233 is formed below the flange 232 , and a needle shaft 235 having a diameter smaller than the axis 233 is formed below the axis 233 . At a bottom end of the second needle 23 , a second needle valve portion 230 having the same diameter as the shaft 235 is formed. A second spring 221 b disposed in the first control chamber 510 b applies a biasing force to the flange 232 of the second needle 23 in a direction to close the injection holes 103 b . The pressure of the liquid fuel in the first control chamber 510 b is also applied to the flange 232 of the second needle 23 . The first spring 202 b may be disposed in the first control chamber 510 b , and in this case the spring chamber 110 is not needed. Around the second needle shaft 235 in the sliding hole 206 b of the first needle 20 b , a connecting passage 236 for supplying liquid fuel is formed. A cut-out 234 is formed in the valve portion 230 so that the liquid fuel in the connecting passage 236 is able to enter sliding positions of the valve portion 230 (refer to FIG. 9 ).

As shown in FIG. 9 , a first valve surface 208 a and a second valve surface 208 b are formed on a bottom end of the first needle 20 b . A depressed portion is formed between the first valve surface 208 a and the second valve surface 208 b . In the bottom end of the nozzle body 101 b , first injection holes 103 a and second injection holes 103 b are formed. A first valve seat 104 a is formed upstream of the first injection holes 103 a , a second valve seat 104 b is formed downstream of the first injection holes 103 a , and a third valve seat 104 c is formed downstream of the second valve seat 104 b . The first valve surface 208 a of the first needle 20 b sits on the first valve seat 104 a , the second valve surface 208 b sits on the second valve seat 104 b , and the valve surface 238 of the second needle 23 sits on the third valve seat 104 c.

Now, referring FIGS. 8 and 10 , operation of the third embodiment will be explained. The switching valve 50 b is driven by the piezoelectric actuator 60 b according to signals from the electronic control unit (not shown), and the pressure Pd of the liquid fuel in the first control chamber 510 b is controlled, thereby opening or closing the injections holes 103 a , 103 b.

When the piezoelectric actuator 60 b is not energized, the expander 620 is pushed up against the piezoelectric element 610 by the pre-loading spring 621 , and the valve body 520 b in the second control chamber 530 b is pushed up by the spring 531 . Therefore, the valve body 520 b closes the second port Po 2 b , as shown in FIG. 8 . The second port Po 2 b communicates with the third port Po 3 b , thereby leading the liquid fuel supplied from the liquid fuel supply passage 310 to the first control chamber 510 b through the outlet orifice passage 512 b . Accordingly, the pressure Pd in the first control chamber 510 b is maintained at a high pressure. The first needle 20 b and the second needle 23 are pushed down by the pressure Pd in the first control chamber 510 b and the biasing force of the first spring 202 b and the second spring 221 b , thereby closing the injection holes 103 a , 103 b.

When the piezoelectric actuator 60 b is energized, as shown in FIG. 10 , the valve body 520 b is pushed down by the piezoelectric actuator 60 b against the biasing force of the spring 531 . The first port Po 1 b is closed while the second port Po 2 b is opened, and the second port Po 2 b communicates with the third port Po 3 b . The pressure Pd in the first control chamber 510 b gradually decreases because the liquid fuel in the first control chamber 510 b flows out to the liquid fuel leading passage 320 b through the outlet orifice passage 512 b , the second port Po 2 b and the third port Po 3 b . According to pressure decrease in the first control chamber 510 b , the first and the second needles are lifted up and the injection holes 103 a , 103 b are opened.

On the other hand, the pressure in the liquid fuel leading passage 320 b increases by the liquid fuel supplied thereto. According to increase in the pressure in the liquid fuel leading passage 320 b , the first one-way valve opens. The opening pressure Pch 1 of the first one-way valve ch 1 is set to a low level such as 10 MPa, while the opening pressure Pch 2 of the second one-way valve ch 2 is set to a high level such as 70 MPa. Therefore, the first one-way valve ch 1 opens before the second one-way valve ch 2 opens. The liquid fuel in the liquid fuel leading passage 320 b is led to the spring chamber 110 through the first one-way valve ch 1 and the liquid fuel leading passage 108 b formed in the injector body 300 b . The liquid fuel is supplied to the sliding hole 106 b to lubricate the sliding surface 201 b of the first needle 20 b , and then to the connecting passage 236 formed in the first needle 20 b.

Referring to FIGS. 11A-11C , operation of the fuel injector 1 b , during a period in which the closed injection holes are opened, will be explained. As shown in FIG. 11A , the high-pressure gaseous fuel in the chamber 102 b and the liquid fuel in the connecting passage 236 are prevented from being injected from the injection holes 103 a , 103 b , because the first valve seat 104 a , the second valve seat 104 b and the third valve seat 104 c are contacted by the first and second valve surfaces 208 a , 208 b of the first needle 20 b and the valve surface 238 of the second needle 23 , respectively, thereby to close the injection holes 103 a , 103 b . The pressure in the first control chamber 51 b gradually decreases because a cross-sectional area of the inlet orifice passage 511 b is made smaller than that of the outlet orifice passage 512 b.

As shown in FIG. 11C , when the piezoelectric actuator 60 b is energized, the first port Po 1 is closed because the valve body 520 b is pushed down. The pressure in the first control chamber 510 b decreases as the liquid fuel in the first control chamber 510 b is released to the release passage 340 . When the pressure in the high-pressure gaseous fuel in the chamber 102 b becomes higher than a pushing-down force of the pressure in the first control chamber 510 b and the first spring 202 b , the first needle 20 b is lifted and the high-pressure gaseous fuel is injected from the first injection holes 103 a.

As shown in FIG. 11C , when the pressure in the high-pressure gaseous fuel chamber 102 b overcomes the pressure pushing down the second needle 23 in the first control chamber 510 b and the second spring 221 b , the second needle 23 is lifted. The valve surface 238 of the second needle leaves the third valve seat 104 c of the nozzle body 101 b . The liquid fuel in the connecting passage 236 is drawn by spray effect of the high-pressure gaseous fuel that flows at a very high speed through the a small space between the valve surface 208 a , 208 b of the first needle 20 b and the valve seat 104 a , 104 b of the nozzle body 101 b . The liquid fuel thus drawn into the sack chamber 107 is injected from the second injection holes 103 b together with the high-pressure gaseous fuel.

With reference to FIG. 12 , operation of the fuel injector 1 b as the third embodiment of the present invention will be described. In FIG. 12 , reference marks represent the followings: Ph: a pressure of the high-pressure gaseous fuel in the chamber 102 b ; Pd: a pressure of the liquid fuel in the first control chamber 510 b ; Pcyl: a pressure in a cylinder of the engine; Pch 1 : a pressure for opening the first one-way valve ch 1 ; Pch 2 : a pressure for opening the second one-way valve ch 2 ; Qh: an amount of the high-pressure gaseous fuel injected; and Qd: an amount of the liquid fuel injected. Time lapsed is shown in the abscissa, and respective amounts marked are shown in the ordinate in FIG. 12 .

Upon energizing the piezoelectric actuator 60 b at time t 1 , the port Po 1 is closed and the liquid fuel in the first control chamber 510 b flows out through the second port Po 2 and the third port Po 3 (both opened). The pressure Pd in the first control chamber 510 b is gradually decreases. The first needle 20 b is lifted at time t 2 a short time after the first port Po 1 is closed, and the injection holes 103 a are opened. The high-pressure gaseous fuel is injected from the opened injection holes 103 a . Since the high-pressure gaseous fuel is always supplied from the common rail or an accumulator 80 , the amount Qh of gaseous fuel injected is maintained at a substantially constant level, as shown in the graph with a dotted line Qh. Accordingly, the pressure Ph in the high-pressure gaseous fuel chamber 102 b is maintained substantially constant with a slight decrease as shown Ph in the graph.

As the pressure Pd in the first control chamber 510 b further decreases, the second needle 23 is lifted at time t 2 ′ and the second injection holes 103 b are opened. The liquid fuel in the connecting passage 236 is injected from the second injection holes 103 b together with the high-pressure gaseous fuel. As shown by the graphs Qh and Qd, a small amount of gaseous fuel is first injected from the first injection holes 103 a and is mixed with compressed air in the cylinder. Then, the liquid fuel is injected from the second injection holes 103 b together with the gaseous fuel. Therefore, a high energy is generated when the liquid fuel is ignited, and stable ignition is realized.

The pressure Pcyl in the cylinder is increased to a several-MPa level by a compression stroke in the cylinder before the fuel is injected at time t 2 . The fuel (both the gaseous fuel and the liquid fuel) injected into the cylinder is ignited by further compression of mixture in the cylinder, and the pressure in the cylinder rapidly increases as shown with a chained line Pcyl. Pressure in the connecting passage 236 increases due to a high pressure Pcyl in the cylinder, and the first one-way valve ch 1 is closed at time t 3 when the pressure Pcyl plus Pch 1 becomes higher than the pressure Pd in the first control chamber 510 b , thereby preventing a reverse flow of the liquid fuel in the connecting passage 236 . The liquid fuel is not supplied to the connecting passage 236 , and injection of the liquid fuel is terminated at time t 3 , as shown with line Qd in the graph.

On the other hand, the high-pressure gaseous fuel continues to be injected during a period in which the first injection holes 103 a are open because the gaseous fuel in the chamber 102 b remains at a pressure Ph that is sufficiently higher than the pressure Pcyl in the cylinder. After the first one-way valve ch 1 is closed at time t 3 , the pressure Pd in the first control chamber 510 b increases although the second port Po 2 is opened because the liquid fuel continues to be supplied to the first control chamber 510 b from the liquid fuel supply passage 310 through the inlet orifice passage 511 b.

When the pressure in the liquid fuel leading passage 320 b , which increases according to increase in the pressure Pd in the first control chamber 510 b , becomes higher than the opening pressure Pch 2 of the second one-way valve ch 2 , the second one-way valve ch 2 is opened at time t 4 , and the liquid fuel is released into the releasing passage 340 . Accordingly, the pressure Pd in the first control chamber 510 b is maintained at the pressure which is equal to the opening pressure Pch 2 of the second one-way valve ch 2 (during a period from time t 4 to time t 5 ).

At time t 5 , the actuator 60 b is de-energized, and the first port Po 1 is opened. The pressure Pd in the first control chamber 510 b gradually increases from time t 5 because leading out of the liquid fuel from the first control chamber 510 b is stopped. According to increase in the pressure Pd, the first needle 20 b and the second needle 23 are pushed down, and a short time thereafter the injection holes 103 a , 103 b are closed (at time t 6 ). The pressure Pcyl in the cylinder gradually decreases after completion of combustion of the injected fuel.

The amount Qd of the injected liquid fuel and timing for opening the first needle 20 b and the second needle 23 can be adjusted by various ways. For example, biasing loads of the first spring 202 b and the second spring 221 b may be changed. Areas of the first needle 20 b and the second needle 23 , to which the pressure in the first control chamber 510 b is applied, may be changed, or a size of the valve surfaces 208 a , 208 b may be changed.

The present invention is not limited to the embodiments described above, but it may be variously modified. For example, the actuator 60 used in the second embodiment may be replaced with the piezoelectric actuator. Though the gaseous fuel passage 330 and the liquid fuel passage 310 are formed in the same cross-sectional plane in the third embodiment, it is possible to form them in planes perpendicular to each other as is in the first and the second embodiments. While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.