High-pressure fuel supply system and method of supplying fuel

[0002] The disclosure of Japanese Patent Application No. 2000-191083 filed on Jun. 21, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.

[0003] 1. Field of Invention

[0004] The invention relates to a high-pressure fuel supply system for fuel injection in an internal combustion engine and to a method of supplying fuel.

[0005] 2. Description of Related Art

[0006] In order to inject fuel directly into cylinders of an internal combustion engine, it is necessary to supply high-pressure fuel to fuel injection valves. High-pressure fuel supply systems for this purpose are known.

[0007] In general, a high-pressure fuel supply system has a reservoir leading to fuel injection valves, a high-pressure pump for force-feeding high-pressure fuel to the reservoir, and a low-pressure pump that is connected to the high-pressure pump on its intake side to ensure that the high-pressure pump withdraws fuel from a fuel tank. In general, the low-pressure pump is of an electrically driven type and can force-feed fuel at a rated discharge pressure since the starting of an engine, whereas the high-pressure pump is of an engine driven type. Because the internal combustion engine is driven by a starter motor and is at a low speed when it is started, the high-pressure pump cannot force-feed fuel well when the engine is started.

[0008] Thus, various propositions have been made including a proposition to boost a pressure in the reservoir to a rated discharge pressure (e.g., 0.3 MPa) of the low-pressure pump and to start fuel injection. However, this pressure is much lower than a target high fuel pressure (e.g., 12 MPa) in the reservoir during normal operation, and it is difficult to realize good fuel injection.

[0009] In order to solve this problem, Japanese Patent Application Laid-Open No. 5-321787 employs a pressure-boosting pump having a large-diameter piston and a small-diameter piston that are connected to each other in the axial direction. When starting an engine, a discharge pressure of a low-pressure pump is applied to the large-diameter piston so that the large-diameter piston and the small-diameter piston are displaced in the axial direction. Thus, the pressure of fuel in a small-diameter cylinder is boosted by the small-diameter piston by an amount corresponding to a ratio between pressure-receiving areas of the large-diameter piston and the small-diameter piston. It has been proposed to force-feed this pressure-boosted fuel to a reservoir that is connected to the small-diameter cylinder so as to boost a pressure in the reservoir to a pressure higher than a rated discharge pressure of the low-pressure pump.

[0010] Meanwhile, since fresh fuel is continuously supplied to the reservoir from a fuel tank while the engine is in operation, the temperature of fuel in the reservoir is lower than the temperature of a reservoir housing. However, after the engine has been stopped, fresh fuel is no longer supplied and the temperature of fuel in the reservoir becomes substantially equal to the temperature of the reservoir housing. Thus, immediately after the engine has been stopped, fuel in the reservoir receives heat from the reservoir housing, is heated up, and expands thermally. The reservoir is generally provided with a safety valve to prevent the pressure of fuel in the reservoir from rising above a predetermined level. Thus, the safety valve is operated by thermal expansion of fuel immediately after the engine has been stopped, and the pressure of fuel in the reservoir is maintained at a predetermined value.

[0011] After that, the temperature of the reservoir housing and fuel gradually falls to an outside air temperature. However, since fuel has a greater thermal expansion coefficient than the reservoir that is generally made from a metal, fuel thermally contracts more greatly than the reservoir housing in proportion to a fall in temperature. The pressure of fuel (i.e., the pressure in the reservoir) eventually becomes negative, and fuel vapors are generated in the reservoir.

[0012] In the case where such fuel vapors are generated in the reservoir, even if a pressure booster as described above is operated when starting the engine, some or all of the fuel that is force-fed from the small-diameter cylinder to the reservoir is used to eliminate the fuel vapors. Therefore, the pressure in the reservoir cannot be boosted as desired.

[0013] The invention has been made as a solution to the problem described above. It is thus one object of the invention to provide a high-pressure fuel supply system having a high-pressure pump, a low-pressure pump that can discharge fuel substantially at a rated discharge pressure since the starting of an engine, and a pressure booster for boosting a pressure of fuel in a reservoir to a pressure higher than a discharge pressure of the low-pressure pump when starting the engine, wherein the pressure booster can reliably boost a pressure in the reservoir when starting the engine even if there is a difference in thermal expansion coefficient between the reservoir housing and fuel.

[0014] A high-pressure fuel supply system according to one aspect of the invention comprises a reservoir for supplying fuel injection valves with high-pressure fuel, a high-pressure pump for force-feeding high-pressure fuel to the reservoir, a low-pressure pump that can discharge fuel substantially at a rated discharge pressure since the starting of an engine, a pressure booster that boosts a pressure of fuel in the reservoir when starting the engine, and a fuel passage that allows fuel to flow only from the fuel tank to the reservoir so as to prevent fuel vapors from being generated in the reservoir while the engine is out of operation.

[0015] Even if there is a difference in thermal expansion coefficient between a reservoir housing and fuel, the fuel passage prevents fuel vapors from being generated due to a negative pressure in the reservoir while the engine is out of operation. Thus, the pressure booster can reliably boost a pressure in the reservoir when starting the engine.

[0016] A high-pressure fuel supply system according to another aspect of the invention comprises a reservoir for supplying fuel injection valves with high-pressure fuel, a high-pressure pump for force-feeding high-pressure fuel to the reservoir, a low-pressure pump that can discharge fuel substantially at a rated discharge pressure since the starting of an engine, a pressure booster that boosts a pressure of fuel in the reservoir when starting the engine, and a delay device that delays operation of the pressure booster at least until fuel vapors in the reservoir are eliminated.

[0017] Other aspects of the invention involve methods of supplying high-pressure fuel.

[0018] The above-mentioned embodiment and other embodiments, objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:

[0019] FIG. 1A is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a first embodiment of the invention;

[0020] FIG. 1B is an enlarged view showing a part of the high-pressure fuel supply system shown in FIG. 1A in detail;

[0021] FIG. 2 is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a second embodiment of the invention;

[0022] FIG. 3 is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a third embodiment of the invention;

[0023] FIG. 4 is a cross-sectional view of a pressure booster employed in a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a fourth embodiment of the invention; and

[0024] FIG. 5 is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a fifth embodiment of the invention.

[0025] In the following description and the accompanying drawings, the present invention will be described in more detail with reference to exemplary, preferred embodiments.

[0026] FIG. 1A is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a first embodiment of the invention. The internal combustion engine will be described hereinafter as an engine having four cylinders. However, the invention is not limited thereto but is also applicable to an internal combustion engine having six cylinders, eight cylinders, more than eight cylinders, less than four cylinders, etc. In FIG. 1A, a reservoir 2 supplies high-pressure fuel to four fuel injection valves that are disposed in cylinders of the internal combustion engine respectively. The reservoir 2 is provided with a pressure sensor 5 that detects a pressure of fuel in the reservoir 2. Disposed in each of the fuel injection valves 1 is a valve body for opening and closing an injection hole and a solenoid for attracting the valve body in its opening direction. A spring force and a pressure of fuel in the reservoir 2 are applied to the valve body in its closing direction. If the solenoid has been demagnetized, reliable closing of the valve body is guaranteed, and fuel injection is stopped. If the solenoid has been excited, it attracts the valve body S in its opening direction against the spring force and the pressure of fuel, and fuel injection is carried out.

[0027] A low-pressure pump 4 is disposed in a fuel tank 3. The low-pressure pump 4 is a battery-driven electric pump and has a rated discharge pressure of, e.g., 0.3 MPa. The low-pressure pump 4 is operated in response to an ON-signal from a starter switch. Disposed in the low-pressure pump 4 on its intake side is a filter (not shown) that prevents admission of foreign matters when fuel is withdrawn from the fuel tank 3.

[0028] A high-pressure pump 7 maintains the pressure of fuel in the reservoir 2 close to a target high fuel pressure of, e.g., 12 MPa. The high-pressure pump 7 is of an engine-driven type wherein fuel is force-fed by a plunger that is driven by a cam connected to a crank shaft. In this embodiment, a discharge stroke of the high-pressure pump 7 occurs every time fuel injection is carried out in two cylinders.

[0029] The high-pressure pump 7 is connected on its discharge side to the reservoir 2 via a high-pressure line 8, and is connected on its intake side to a discharge side of the low-pressure pump 4 via a low-pressure line 9. Thus, since fuel that is sucked (withdrawn) from the low-pressure line 9 during a suction stroke of the high-pressure pump 7 has been pressurized to 0.3 MPa by the low-pressure pump 4 as described above, fuel vapors resulting from a negative pressure in the low-pressure line 9 are unlikely to be generated. A check valve 8athat opens at a set pressure is disposed in the high-pressure line 8 so as to prevent fuel from flowing backwards due to pressure pulsations generated by the high-pressure pump 7.

[0030] The high-pressure pump 7 adjusts a required amount of fuel so that the pressure of fuel in the reservoir 2 becomes equal to a target high fuel pressure, and force-feeds the fuel. Out of all the fuel discharged by the plunger, an unnecessary amount of fuel is returned to the fuel tank 3 via the low-pressure line 9. At this moment, it is undesirable that high-pressure fuel flow backwards in the low-pressure pump 4. Therefore, the low-pressure line 9 may communicate with the fuel tank 3 via a safety valve that opens at a pressure slightly exceeding the rated discharge pressure of the low-pressure pump 4. In order to prevent a pressure of fuel in the reservoir 2 from rising abnormally for some reason, the reservoir 2 and the fuel tank 3 communicate with each other via a return line 12 having a safety valve 12athat opens at a fuel pressure slightly exceeding a target high fuel pressure.

[0031] If a return line such as the return line 12 is provided, the high-pressure pump 7 may be designed to always force-feed all the fuel discharged by the plunger to the reservoir 2 without adjusting an amount of fuel.

[0032] Thus, in either case, if the high-pressure pump 7 operates well after the starting of the engine, the pressure in the reservoir 2 can be maintained at a pressure close to the target high fuel pressure, and fuel injection is carried out well via the fuel injection valves 1. Although the pressure of fuel in the reservoir 2 needs to be boosted quickly when starting the engine, the high-pressure pump 7 is of an engine-driven type and thus does not operate well at a low engine speed realized by a starter motor. Therefore, the pressure in the reservoir 2 cannot be boosted at the time of engine start-up.

[0033] On the other hand, the low-pressure pump 4 is of an electrically driven type and thus can operate well even when starting the engine and force-feed fuel at the rated discharge pressure. Thus, the pressure in the reservoir 2 can be quickly made equal to the rated discharge pressure of the low-pressure pump 4. However, as described above, the rated discharge pressure of the low-pressure pump 4 is much lower than the target high fuel pressure. This makes it difficult not only to perform fuel injection in a desired fuel spray mode but also to perform fuel injection at desired timings because injection of a required amount of fuel necessitates prolonging an opening period of the fuel injection valves 1.

[0034] The high-pressure fuel supply system of this embodiment has a pressure booster 10 in order to boost the pressure in the reservoir 2 to a pressure higher than the rated discharge pressure of the low-pressure pump 4. The pressure booster 10 has a small-area piston 10athat penetrates a hole portion 2bformed in one wall portion 2adefining the reservoir 2 and that has a variable length of protrusion into the reservoir 2. The small-area piston 10ahas a uniform circular cross-section slightly smaller in diameter than the hole portion 2b, and slides with respect to the hole portion 2b. Furthermore, the pressure booster 10 is located outside the reservoir 2 in order to press the small-area piston 10aso that its protrusion amount into the reservoir 2 increases. The pressure booster 10 also has a large-area piston 10bthat has a uniform cross-section larger than the uniform circular cross-section of the small-area piston 10a.

[0035] A cylinder 10cfor slidably holding the large-area piston 10bis integrated with the one wall portion 2a. The small-area piston 10a, the hole portion 2bin which the small-area piston 10aslides, the large-area piston 10b, and the cylinder 10cin which the large-area piston 10bslides have circular cross-sections. However, as long as these sliding movements are possible, the small-area piston 10a, the hole portion 2b, the large-area piston 10b, and the cylinder 10cmay have a cross-section of an arbitrary shape. For reason of a reduction in weight, the large-area piston 10bis bored on the side of the small-area piston 10ain the shape of a circular cylinder that is concentric with the small-area piston 10a. The small-area piston 10aabuts at its end face on a bottom portion that has been formed by boring the large-area piston 10b. Although detailed description will be made later, the large-area piston 10bneed not be integrally connected to the small-area piston 10aso as to exclusively perform the function of pressing the small-area piston 10a. In a construction in which the large-area piston 10bis not connected to the small-area piston 10a, a center axis of the cylinder 10calong which the large-area piston 10bslides and a center axis of the hole portion 2balong which the small-area piston 10aslides need not coincide with each other as long as they are parallel to each other. Also, the cylinder 10cand the hole portion 2bcan be machined easily.

[0036] The inside of the cylinder 10cis divided into two spaces by the large-area piston 10b. One of the spaces on the side of the small-area piston 10ais an atmospheric chamber 10d, and the other space is a pressure chamber 10e. The atmospheric chamber 10dcommunicates with the fuel tank 3 via a return line 11. On the other hand, the pressure chamber 10ecommunicates with the low-pressure line 9 via a branch pipe 13.

[0037] When starting the engine, the high-pressure fuel supply system thus constructed applies the rated discharge pressure of the low-pressure pump 4 to the pressure chamber 10evia the branch pipe 13. The large-area piston 10bpresses and displaces the small-area piston 10ainstantaneously. Thereby the length of the small-area piston 10aprotruding into the reservoir 2 is increased. Because the volume of the reservoir 2 is reduced accordingly, fuel in the reservoir 2 is compressed. The pressure of fuel can be boosted to a predetermined pressure (e.g., 4 MPa) that is obtained by multiplying a discharge pressure of the low-pressure pump 4 by an area ratio S_L/S_Sbetween a cross-sectional area S_Lof the large-area piston 10band a cross-sectional area S_Sof the small-area piston 10a, i.e., to a pressure far above the rated discharge pressure of the low-pressure pump 4. Thus, it becomes possible to perform fuel injection well when starting the engine.

[0038] In this embodiment, no sealing member for applying a great frictional force during sliding movements is disposed between the small-area piston 10aand the hole portion 2bor between the large-area piston 10band the cylinder 10c. Thus, if a pressure is applied in the pressure chamber 10ewhen starting the engine, the small-area piston 10ais pressed and displaced instantaneously and the pressure of fuel in the reservoir 2 is boosted to the above predetermined pressure. Therefore, fuel injection can be started at an early stage.

[0039] However, since no sealing member is disposed as described above, it is possible that fuel in the pressure chamber 10emay leak to the atmospheric chamber 10dfrom a gap between the large-area piston 10band the cylinder 10c. However, since the pressure chamber 10eis at the rated discharge pressure of the low-pressure pump 4, i.e., at a low pressure, such leakage of fuel is substantially prevented by suitably selecting a width of the gap. It is also possible that fuel in the reservoir 2 may leak to the atmospheric chamber 10dfrom a gap between the small-area piston 10aand the hole portion 2bdue to a rise in pressure. However, the predetermined pressure at this moment is lower than the target high fuel pressure of the reservoir 2, and fuel leakage can be substantially prevented by suitably selecting a width of the gap.

[0040] Even in the case where a small amount of fuel has leaked to the atmospheric chamber 10dfrom the pressure chamber 10eand/or the reservoir 2, since the atmospheric chamber 10dcommunicates with the fuel tank 3 via the return line 11, the fuel is returned to the fuel tank 3 by gravity. Therefore, no problem is caused.

[0041] However, if the high-pressure pump 7 has operated normally after the starting of the engine and if the pressure of fuel in the reservoir 2 has reached a very high pressure close to the target high fuel pressure, fuel surely leaks from the gap between the small-area piston 10aand the hole portion 2bunless a sealing member is provided. Therefore, fuel leakage must be prevented. In this embodiment, as shown in FIG. 1B, the small-area piston 10alocated in the reservoir 2 is provided at its end with an enlarged portion 10fthat is concentric with the small-area piston 10aand that is in the shape of a truncated cone. An O-ring 10gas a sealing member is fitted into a groove that is formed in the enlarged portion 10fin such a manner as to extend around an axis thereof.

[0042] If the pressure of fuel in the reservoir 2 has reached the target high fuel pressure, the small-area piston 10ais pushed back against a pressure applied to the large-area piston 10b. At this moment, the O-ring 10gis compressed and comes into close contact with an inner wall surface 2cof the one wall portion 2aas well as the entire groove in the enlarged portion 10f. Thus, the hole portion 2bis sealed, and fuel leakage as described above can be prevented.

[0043] In this embodiment, the area ratio (S_L/S_S) between the large-area piston 10band the small-area piston 10ais set such that a predetermined pressure lower than the target high fuel pressure in the reservoir 2 is applied to the small-area piston 10ain a balancing manner when the rated discharge pressure of the low-pressure piston 4 is applied to the large-area piston 10b. Thus, as soon as the pressure in the reservoir 2 reaches a pressure higher than the predetermined pressure due to the high-pressure pump 7, the small-area piston 10ais pushed back and sealing of the reservoir 2 is guaranteed. Thus, if the pressure in the reservoir 2 has reached a pressure close to the target high fuel pressure, more complete sealing of the reservoir 2 can be guaranteed.

[0044] In order to further improve fuel injection when starting the engine, the area ratio (S_L/S_S) between the large-area piston 10band the small-area piston 10amay be further increased so that the above predetermined pressure becomes close to the target high fuel pressure.

[0045] Because fresh fuel is continuously supplied to the reservoir 2 from the fuel tank 3 while the engine is in operation, the temperature of fuel in the reservoir 2 is lower than the temperature of a reservoir housing. However, since no fresh fuel is supplied after the engine has been stopped, the temperature of fuel in the reservoir 2 becomes substantially equal to the temperature of the reservoir housing. Thus, immediately after the engine has been stopped, fuel in the reservoir 2 receives heat from the reservoir housing, is heated up, and expands thermally. Thereby the safety valve 12ain the return line 12 is operated, and the pressure of fuel in the reservoir 2 is maintained at a pressure close to the target high fuel pressure.

[0046] After that, although the temperature of fuel and of the reservoir housing gradually falls to an outside air temperature, fuel thermally contracts more greatly than the reservoir housing due to a difference in thermal expansion coefficient between the reservoir housing and fuel. Conventionally, at this moment, the pressure of fuel becomes negative and fuel vapors are generated in the reservoir 2. Thus, even if a pressure booster as described above has been operated when starting the engine, operation of the small-area piston 10aserves only to crush fuel vapors in the reservoir 2, and the pressure in the reservoir 2 cannot be boosted to a set pressure.

[0047] In this embodiment, in order to solve this problem, the reservoir 2 communicates with the fuel tank 3 via a communication pipe 14 in which a check valve 14athat allows fuel to flow only from the fuel tank 3 to the reservoir 2 is disposed. The check valve 14ais opened easily by a small differential pressure. Thus, if the pressure of fuel in the reservoir 2 becomes lower than an atmospheric pressure after the engine has been stopped, the check valve 14ais opened so that fuel flows from the fuel tank 3 into the reservoir 2 via the communication pipe 14 and that the pressure in the reservoir 2 is prevented from becoming negative. Therefore, no fuel vapors are generated in the reservoir 2. Thus, the pressure booster can reliably boost a pressure in the reservoir when starting the engine.

[0048] FIG. 2 is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a second embodiment of the invention. In this embodiment, structural components identical with those of the first embodiment are denoted by the same reference numerals. The following description will be focused exclusively on differences between the second and first embodiments. In the second embodiment, although the communication pipe 14 via which the reservoir 2 communicates with the fuel tank 3 is not provided, a set pressure valve 15 that opens when the pressure on the side of the low-pressure pump 4 is equal to or higher than a set pressure is disposed in a branch pipe 13′ for applying the rated discharge pressure of the low-pressure pump 4 to the pressure booster 10.

[0049] In this embodiment, when starting the engine, fuel vapors may be present in the reservoir 2. However, since the set pressure valve 15 is closed immediately after operation of the low-pressure pump has been started, the pressure booster 10 is out of operation. At this moment, the high-pressure pump 7 operates slowly due to cranking, and fuel from the low-pressure pump 4 flows into the reservoir 2 via the high-pressure pump 7. With the rated discharge pressure of the low-pressure pump 4 being equal to 0.3 MPa, the check valve 8adisposed in the high-pressure line 8 opens at a set pressure of, e.g., 0.1 MPa. On the other hand, the set pressure valve 15 opens at a pressure of, e.g., 0.2 MPa.

[0050] Thus, the pressure in the reservoir 2 first of all becomes equal to 0.1 MPa due to fuel discharged from the low-pressure pump 4, so that fuel vapors are eliminated completely. At this moment, the set pressure valve 15 is opened and the pressure booster 10 operates. Therefore, the pressure booster 10 can reliably boost a pressure in the reservoir when starting the engine.

[0051] The set pressure valve 15 can be a valve that is not a check valve and remains open when the pressure on the side of the low-pressure pump 4 is equal to or higher than a set pressure. Therefore, if the pressure of fuel in the reservoir 2 is increased while the engine is in operation, fuel is discharged from a pressure chamber of the pressure booster 10 toward the low-pressure pump 4 due to returning movements of the large-area and small-area pistons. This fuel is sucked by the high-pressure pump 7 via the branch pipe 13′ or returned to the fuel tank 3. Further, in the case where a check valve that opens at a pressure of, e.g., 0.2 MPa is disposed in the branch pipe 13′, instead of a set pressure valve, fuel in the pressure chamber of the pressure booster 10 flows around the large-area piston and into the atmospheric chamber and is returned to the fuel tank 3 via the return line 11.

[0052] In this embodiment, instead of a set pressure valve, a valve mechanism that opens in response to an operation signal from an electromagnetic valve or the like may be disposed in the branch pipe 13. In this case, in order to ensure that operation of the pressure booster is delayed at least until fuel vapors in the reservoir 2 are eliminated by fuel that is supplied to the reservoir 2 via the low-pressure pump 4, the valve mechanism may be opened after the lapse of a set time period or upon detection of a pressure of fuel in the reservoir 2 being at least equal to or higher than an atmospheric pressure, when starting the engine.

[0053] FIG. 3 is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a third embodiment of the invention. In the third embodiment, structural components identical with those of the second embodiment are denoted by the same reference numerals. The following description will be focused exclusively on differences between the third and second embodiments. In the second embodiment, the set pressure valve 15 disposed in the branch line 13′ opens if the pressure on the side of the low-pressure pump 4 becomes equal to a set pressure. In this embodiment, a set pressure valve 16 disposed in a branch pipe 13″ opens if the pressure in the reservoir 2 becomes equal to a set pressure of, e.g., 0.2 MPa.

[0054] To be more specific, the set pressure valve 16 is provided with a piston 16bthat is disposed in a cylinder 16a, a pressure chamber 16cthat is formed in the cylinder 16aby the piston 16b, and a spring 16dthat presses the piston 16btoward the pressure chamber 16c. The pressure chamber 16ccommunicates with the reservoir 2. A space 16eis formed around a central portion of the piston 16b. Thus, if the pressure chamber 16cthat is equal in pressure to the reservoir 2 assumes a pressure of 0.2 MPa, the piston 16bmoves while compressing the spring 16d, and the branch pipe 13″ is opened via the space 16eformed in the piston 16b. The discharge pressure of the low-pressure pump 4 is then applied to the pressure chamber of the pressure booster 10, so that the pressure booster 10 is operated. Thus, as is the case with the second embodiment, operation of the pressure booster 10 is delayed at least until fuel vapors in the reservoir 2 are eliminated by fuel that is supplied to the reservoir 2 via the low-pressure pump 4, and the pressure booster 10 can reliably boost a pressure in the reservoir when starting the engine.

[0055] In the above second and third embodiments, prior to operation of the pressure booster 10 when starting the engine, fuel is caused to flow into the reservoir 2 via the high-pressure pump 7. As a matter of course, however, if a pipe branching off from the low-pressure line 9 is directly connected to the reservoir 2 and if the pipe is provided with a check valve that opens at a small differential pressure and that allows fuel to flow only from the low-pressure pump 4 to the reservoir 2, fuel can flow into the reservoir 2 from the low-pressure pump 4 via the pipe even while the check valve 8ain the high-pressure line 8 is being opened. Thus, fuel vapors in the reservoir 2 can be eliminated at an early stage.

[0056] FIG. 4 is a cross-sectional view of a pressure booster 10′ applied to a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a fourth embodiment of the invention. Structural components other than the pressure booster 10′ are identical with those of the first embodiment. The following description will be focused exclusively on differences between the pressure booster 10′ of this embodiment and the pressure booster 10 of the first embodiment. In the pressure booster 10′, pressure boosting operation can be delayed at least until fuel vapors in the reservoir 2 are eliminated by fuel that is supplied to the reservoir 2 via the low-pressure pump 4. In the pressure booster 10′, a large-area piston 10b′ that abuts on a small-area piston 10a′ is in the shape of a circular cylinder, and a plurality of hole portions 10b1′ are formed radially around the large-area piston 10b′. Disposed in each of the hole portions 10b1′ are a spherical member 10bthat is partially fitted into the hole portion 10b1′ and a spring 10b3′ that urges the spherical member 10b2′ outwards. A cylinder 10c′ is provided with recesses into which the spherical members 10b2′ are partially fitted in a state where the large-area piston 10b′ is located at such a position that a pressure chamber 10e′ assumes its minimum volume. Although the spherical members 10b2 shown in FIG. 4 are employed in this embodiment, there is no need to impose such limitation. For example, roller members with semicircular apices may be employed.

[0057] Thus, until the pressure in the pressure chamber 10e′ reaches a pressure of, e.g., 0.2 MPa, the spherical members 10b2′ that are urged outwards by the springs 10b3′ are fitted in the recesses in the cylinder 10c′ and stabilize the large-area piston 10b′ against a pressing force applied to the large-area piston 10b′. Thus, operation of the pressure booster 10 is delayed at least until fuel vapors in the reservoir 2 are eliminated by fuel that is supplied to the reservoir 2 via the low-pressure pump 4. Thus, the pressure booster 10 can reliably boost a pressure in the reservoir when starting the engine.

[0058] FIG. 5 is a schematic view of a high-pressure fuel supply system for fuel injection in an internal combustion engine according to a fifth embodiment of the invention. In the fifth embodiment, structural components identical with those of the first embodiment are denoted by the same reference numerals. The following description will be focused exclusively on differences between the fifth and first embodiments. A pressure booster 20 of this embodiment is not of a piston type but of an accumulator type. To be more specific, the pressure booster 20 has a control chamber 20aleading to an opening 2b′ of the reservoir 2 and an accumulator 20bleading to the control chamber 20a. Disposed in the control chamber 20aare a valve body 20cthat allows the opening 2b′ to be closed and a spring 20dthat urges the valve body 20cin its closing direction. The valve body 20chas a rod 20hthat extends outside the control chamber 20ain an oil-sealing manner, and a solenoid 20eis disposed around the rod 20h. The accumulator 20bhas a piston 20f, and gases such as nitrogen are encapsulated in a pressure chamber 20gthat is closed by the piston 20f.

[0059] If the pressure in the reservoir 2 has reached a high pressure during engine operation due to such a construction, the valve body 20cis opened easily and the control chamber 20abecomes equal in pressure to the reservoir 2. This pressure is applied to the piston 20fso that nitrogen in the pressure chamber 20gis compressed to the same pressure. Merely by a slight fall in pressure in the reservoir 2, the valve body 20ccloses the opening 2b′. Thus, when the engine is stopped, the pressure chamber 20gof the accumulator 20bis maintained at a maximum pressure in the reservoir 2 during engine operation. When starting the engine, if the valve body 20cis opened by the solenoid 20e, the pressure accumulated in the pressure chamber 20gof the accumulator 20bpresses fuel in the control chamber 20ainto the reservoir 2. Thus, it becomes possible to boost a pressure in the reservoir 2.

[0060] Also in this embodiment in which a pressure booster such as the pressure booster 20 is provided, since the reservoir 2 communicates with the fuel tank 3 via the communication pipe 14 as is the case with the first embodiment, no fuel vapors are generated in the reservoir 2 when starting the engine. Even in the case where the communication pipe 14 is not provided, if the opening of the valve body 20cby the solenoid 20eis delayed at least until fuel vapors in the reservoir 2 are eliminated by fuel that is supplied to the reservoir 2 via the low-pressure pump 4 when starting the engine based on the same idea as in the second and third embodiments, the pressure booster 20 can reliably boost a pressure in the reservoir when starting the engine.

[0061] While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the preferred embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.