JPH02233864A - Unit injector - Google Patents

Abstract

PURPOSE:To make any fuel gas in a fuel chamber not flow in the circumference of a needle via a nozzle by connecting a fuel overflow passage at the downstream of an overflow valve to a needle back pressure chamber and adding high pressure to the inside of this needle back pressure chamber at time of opening the overflow valve. CONSTITUTION:When an electric charge once charged to a piezoelectric element 67 to stop any fuel injection is discharged, this piezoelectric element 67 contracts, a piston 63 goes up, fuel pressure in a variable displacement chamber 68 and a pressure control chamber 55 goes down, and an overflow valve 31 moves to the right at once by dint of spring force of a compression spring 46, so that this valve 31 is separated from a valve seat part 40, thus the valve 31 is opened at once. If so, high pressure fuel in a fuel pressure chamber 15 is sprayed to the inside of a first fuel overflow chamber 43 via a fuel overflow passage 49 and a pressurized fuel intake chamber 42, and it is temporarily brought to high pressure owing to a throttle 48, and since it is added to the inside of a needle back pressure chamber 24 via a fuel passage 25, fuel pressure in this back pressure chamber 24 also comes high temporarily. In consequence, since a needle 7 is quickly closed, such a possibility that fuel gas flows into a ring fuel passage 19 via a nozzle 3 is checked.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a unit injector.

[Conventional technology]

A plunger driven by an engine, a fuel pressurization chamber filled with fuel and pressurized by the plunger, and a fuel pressurization chamber that responds to the fuel pressure in the fuel pressurization chamber when the fuel pressure exceeds a predetermined pressure. The valve includes a needle for opening the valve, a fuel overflow passage for causing fuel in the fuel pressurizing chamber to overflow, and an overflow valve driven by an actuator and disposed within the fuel overflow passage. A unit injector that injects fuel when a flow path is blocked is known (see Japanese Utility Model Application No. 187965/1983). In a unit injector in which the injected fuel is controlled by an actuator in this way, when the overflow valve opens, the fuel in the fuel pressure chamber is caused to overflow and the fuel pressure in the fuel pressure chamber is reduced as quickly as possible. The flow resistance in the fuel overflow passage is usually kept as low as possible so that the needle closes as quickly as possible. Furthermore, when a highly responsive piezoelectric element is used as an actuator like this unit injector, the overflow valve opens rapidly, causing the fuel pressure in the fuel pressurizing chamber to drop dramatically.

[Problems to be Solved by the Invention] In such a unit injector, the needle is normally held in a closed state by a spring force, and high pressure is applied to the conical needle pressure receiving surface formed on the outer peripheral surface of the needle. Fuel injection is performed by lifting the needle by applying fuel pressure. Next, when the overflow valve is opened and the fuel pressure in the fuel pressurizing chamber is suddenly reduced, at the same time, the fuel pressure acting on the needle pressure receiving surface is also rapidly reduced, causing the needle to move in the valve closing direction due to the spring force. Close the nozzle. However, the responsiveness of the needle is not very good due to its own inertia and friction with its surroundings. Therefore, even if the fuel pressure acting on the pressure receiving surface of the needle suddenly decreases, the needle does not immediately close the nozzle opening, and the needle remains closed for a while. Then the needle closes the nozzle opening. Therefore, if the fuel pressure acting on the pressure receiving surface of the needle suddenly decreases and becomes lower than the pressure inside the combustion chamber, the problem arises that combustion gas in the combustion chamber flows into the area around the needle via the nozzle before the needle closes. be.

[Means to solve the problem]

In order to solve the above problems, the present invention includes a plunger driven by an engine, a fuel pressurizing chamber filled with fuel and pressurized by the plunger, and a fuel pressurizing chamber filled with fuel and pressurized by the plunger. A needle that opens when the fuel pressure exceeds a predetermined pressure, a fuel overflow passage that causes fuel in the fuel pressurization chamber to overflow, and an overflow that is driven by an actuator and located within the fuel overflow passage. It is equipped with a flow valve,
In a unit injector in which fuel is injected when the overflow valve blocks the fuel overflow passage, the fuel overflow passage downstream of the overflow valve is connected to the needle back pressure chamber, and high pressure is applied to the needle back when the overflow valve is opened. I try to add it inside the pressure chamber.

[For production]

When the overflow valve opens, high pressure is applied to the needle back pressure chamber, so that as soon as the overflow valve opens, the needle closes immediately due to the high pressure in the needle back pressure chamber. As a result, combustion gas within the combustion chamber is prevented from flowing around the needle.

〔Example〕

Referring to FIGS. 1, 2, 4 and 5, 1
2 is the housing body of the unit injector, 2 is the nozzle with the nozzle hole 3 formed at its tip, 4 is the spacer, and 5 is the nozzle.
1 denotes a sleeve, and 6 denotes a nozzle holder for fixing the nozzle 2, spacer 4, and sleeve 5 to the housing body l, respectively.

Inside the nozzle 2 is a needle 7 that controls the opening and closing of the nozzle roller 3.
is slidably inserted, and the top of the needle 7 is connected to a pressure pin 8.
It is connected to the spring retainer 9 via. This spring retainer 9 is constantly pressed downward by a compression spring 10, and this pressing force is transmitted to the needle 7 via the pressure pin 8. Therefore, the needle 7 is always urged in the valve closing direction by the compression spring 10.

On the other hand, a plunger hole 11 is formed coaxially with the needle 7 in the housing body 1, and a plunger 12 is slidably inserted into the plunger hole 11. Plunger 1
2 is connected to a tappet 13, and the upper end of the tappet 1
3 is constantly urged upward by a compression spring 14.

This tappet 13 is moved up and down by an engine-driven cam (not shown), thereby causing the plunger 12 to move up and down within the plunger hole 11.

On the other hand, a fuel pressurizing chamber 15 defined by the lower end portion 12a of the plunger 12 is formed in the plunger hole 11 below the plunger 12. This fuel pressurizing chamber 15 is connected to a needle pressurizing chamber 18 via a rod-shaped filter 16 and a fuel passage 17 (FIG. 5). It is connected to the nozzle row 3.

Further, a fuel supply port 20 is formed on the inner wall surface of the plunger hole 11, as shown in FIG. 4, and opens into the fuel pressurizing chamber 15 when the plunger 12 is in the upper position. 2 to 3 k in fuel pressurization chamber l5
Fuel is supplied at a feed pressure of about g/ci. This fuel supply port 20 has a fuel discharge passage 20a extending perpendicularly from the fuel supply port 20 and a valve opening pressure of 2 to 3.
It is connected to, for example, a fuel tank (not shown) via a relief valve (not shown) of approximately kg/cffl. Further, as shown in FIG. 4, a fuel boat 21 is formed on the opposite side of the plunger hole 11 from the fuel supply port 20, which is inevitably formed during the drilling work of the fuel supply boat 20. The outer end of 21 is closed by a blind plug 22. The fuel boat 21 extends coaxially with the fuel supply port 20 and opens into the plunger hole 11 . A circumferential groove 23 extending from the fuel supply port 20 to the fuel port 21 is formed on the inner wall surface of the plunger hole 11 . Therefore, when the plunger 12 is lowered and the plunger 12 closes the fuel supply port 20 and the fuel port 21, the fuel supply port 20 and the fuel port 21 are communicated with each other via the circumferential groove 23, and therefore, inside the fuel boat 21. The fuel pressure is maintained at the same feed pressure as in the fuel supply boat 20. On the other hand, a circumferential groove 26 is formed on the outer circumferential surface of the plunger 12 slightly above the plunger lower end surface 12a, and this circumferential groove 26 allows fuel to flow through a fuel relief hole 27 bored in the plunger 12. It is communicated with the inside of the pressurizing chamber 15.

On the other hand, a sliding hole 30 extending laterally is formed in the housing body 1 in the vicinity of the side of the plunger hole 11 . Therefore, the sliding hole 30 is formed so that its axis is parallel to a straight line substantially perpendicular to the common axis of the plunger 12 and the needle 7, with a distance therebetween. An overflow valve 3l is slidably inserted into this sliding hole 30. As shown in FIGS. 1 and 2, the sliding hole 30 consists of a small diameter hole 32 and a large diameter hole 33 that are arranged coaxially with each other, and there is a step between the small diameter hole 32 and the large diameter hole 33. A section 34 is formed. On the other hand, a conical shape 35 is formed coaxially with the small diameter hole 32 in the housing body 1 on the opposite side of the large diameter hole 33 with respect to the small diameter hole 32, and a valve seat having a conical outer circumferential surface is provided within the conical shape 35. Member 36 is fitted. This valve seat member 36 is fixed within the conical hole 35 by a nut 37 screwed onto the housing body 1. A groove 39 is formed on the end surface 38 of the valve seat member 36 facing the small diameter hole 32.
A substantially conical valve seat portion 40 is formed on the inner peripheral surface of the valve seat portion 40 . On the other hand, one end of the overflow valve 31 protrudes into the groove 39, and an outer circumference 41 of the one end of the overflow valve 31 is formed into a conical shape. The apex angle of the cone forming the conical valve seat portion 40 is smaller than the apex angle of the cone forming the conical outer circumferential portion 41 of the overflow valve 31, so that the outer circumferential edge of the conical outer circumferential portion 4l of the overflow valve 3l is It is seated on a conical valve seat portion 40 . Therefore, at this time, the overflow valve 31 comes into line contact with the valve seat portion 40.

When the overflow valve 31 is seated on the valve seat member 36, the outer peripheral surface of the overflow valve 31, the inner wall surface of the groove 39, and the housing main body 1
An annular pressurized fuel introduction chamber 42 is formed between the overflow valve 3 and the annular pressurized fuel introduction chamber 42 .
A first fuel overflow chamber 43 is formed between the tip end surface of the groove 1 and the inner peripheral surface of the groove 39. On the other hand, a second fuel overflow chamber 44 is formed in the large diameter hole 33 of the sliding hole 30, and a first fuel overflow chamber 44 is formed inside the large diameter hole 33 of the sliding hole 30.
3 and the second fuel overflow chamber 44 are communicated with each other via a fuel passage 45 formed within the overflow valve 31.

Inside the second fuel overflow chamber 44 is an enlarged end 31a of the overflow valve 31.
A compression spring 46 is inserted between the step portion 34 and the step portion 34 to bias the overflow valve 3l toward the right. This second fuel overflow chamber 44 is located in the eighth
As shown in the figure, it is connected to, for example, a fuel tank (not shown) via a fuel discharge passage 47. On the other hand, as shown in FIGS. 1 and 2, a throttle 48 is fitted and fixed in the fuel passage 45 of the overflow valve 31. In addition, the first fuel overflow chamber 43
is connected via a fuel passage 25 to a needle back pressure chamber 24 that accommodates the compression spring 10.

As shown in FIG. 5, a fuel overflow passage 49 is formed in the housing body 1, extending upward from the fuel passage 17 and opening into the pressurized fuel introduction chamber 42 at all times. This fuel overflow path 4
9 is always in communication with the fuel pressurization chamber 15, and therefore the pressurized fuel introduction chamber 42 is always in communication with the fuel pressurization chamber 15.

As shown in FIGS. 1 and 2, a rod guide 51 that guides and supports a rod 50 is fitted into the outer end of the large diameter hole 33 of the sliding hole 30, and this rod guide 51 has a rod inside. A hole 52 is provided. The rod 50 has a rod hole 52
a hollow cylindrical small diameter portion 53 slidably inserted therein;
The large diameter portion 54 is located within the large diameter hole 33.
4 is brought into contact with the end surface of the overflow valve 31. A pressure control chamber 55 defined by the end surface of the small diameter portion 53 is formed at the end of the rod 50 opposite to the large diameter portion 54 . An actuator 60 is arranged above this pressure control chamber 55. Rod 50 as shown in FIGS.
has a hollow cylindrical shape, so the mass of the iron 50 is considerably small.

Actuator 6 as shown in FIGS.
0 is an actuator housing 62 that is integrally formed with the housing body 1 and has a piston hole 61 formed therein.
a piston 63 slidably inserted into the piston hole 61; an end plate 64 covering the top of the actuator housing 62; and an end plate holder 65 for fixing the end plate 64 to the top of the actuator housing 62. A synthetic resin cap 66 that covers the upper end of the end plate 64 is provided.

A Viezo piezoelectric element 67, which is a stack of multiple piezoelectric element plates, is inserted between the piston 63 and the end plate 64, and a variable volume chamber 68 defined by the lower end surface of the piston 63 is formed in the piston hole 61 below the piston 63. be done. This variable volume chamber 68 communicates with the pressure control chamber 55 via a fuel passage 69. An annular cooling chamber 70 is formed between the piston 63 and the actuator housing 62, and a compression spring 71 that constantly urges the piston 63 upward is provided in the cooling chamber 70.
is inserted. When the piezoelectric element 67 is charged, the piezoelectric element 67 expands in the axial direction, and as a result, the volume of the variable volume chamber 68 decreases. On the other hand, when the electric charge stored in the piezoelectric element 67 is discharged, the piezoelectric element 67 contracts in the axial direction, and as a result, the volume of the variable volume chamber 68 increases.

As shown in FIG. 6, the housing body 1 has a check valve 7.
2 is inserted. This check valve 72 includes a ball 74 that controls opening and closing of a valve boat 73, a rod 75 that regulates the lift amount of the ball 74, and a compression spring 76 that constantly presses the ball 74 and rod 75 downward. , so the valve port 73 is normally closed by the ball 74. The valve port 73 of the check valve 72 is connected to, for example, a low pressure fuel pump (not shown) via a fuel inflow passage 77, and is connected to a low pressure fuel pump (not shown).
kg/c{low pressure fuel is supplied from the fuel inlet passage 77. The check valve 72 can flow only toward the variable volume chamber 68, so that the fuel pressure within the variable volume chamber 68 is between 2 and 3.
When the pressure falls below 3 kg/cJ, fuel is replenished into the variable volume chamber 68 via the check valve 72. Therefore, variable volume chamber 6
8 is always filled with fuel. On the other hand, as shown in FIG.
8 to a low pressure fuel pump (not shown), and low pressure fuel of 2 to 3 kg/d flows into the fuel inlet passage 78.
is supplied into the cooling chamber 70 from. The piezoelectric element 67 is cooled by this fuel. Further, as shown in FIG. 4, the lower end of the cooling chamber 70 is connected to the fuel supply port 20 via a fuel outflow passage 79.
A check valve 80 that allows flow only from the cooling chamber 70 to the fuel supply boat 20 is disposed within the fuel supply boat 20 . This check valve 80 has a ball 82 that controls opening and closing of a valve boat 81, and a ball 82 that controls opening and closing of a valve boat 81.
2, and a compression spring 84 that constantly presses the ball 82 and rod 83 upward.
Consisting of After the fuel in the cooling chamber 70 cools the piezoelectric element 67, it is supplied to the fuel supply port 20 via the fuel outflow passage 79.

As mentioned above, fuel is supplied into the cooling chamber 70 via the fuel inlet passage 78, and this fuel is then supplied to the piezoelectric element 6.
After cooling the fuel 7, it is supplied into the fuel supply port 20 via the fuel outflow passage 79 and the check valve 80. As shown in FIG. 4, when the plunger l2 is in the upper position, fuel is supplied from the fuel supply boat 20 into the fuel pressurizing chamber 15, and therefore, at this time, the amount in the fuel pressurizing chamber 15 is 2 to 3 kg/kg.
The pressure is as low as ci. On the other hand, at this time, the Viezo piezoelectric element 67 is at the maximum contraction position, and at this time, the fuel pressure in the variable volume chamber 68 and the pressure control chamber 55 is 2 to 3 k.
The pressure is as low as g/ci. Therefore, at this time, the overflow valve 31 is moved to the right in FIGS. 1 and 2 by the spring force of the compression spring 46 and is away from the valve seat portion 40, that is, the overflow valve 31 is open. Therefore, the low-pressure fuel in the fuel pressurization chamber 15 flows into the second fuel overflow through the fuel overflow passage 49, the pressurized fuel introduction chamber 42, the first fuel overflow chamber 43, and the fuel passage 45 in the overflow valve 31. supplied into the chamber 44,
The fuel supplied into the second fuel overflow chamber 44 is discharged from the fuel outflow passage 47. Therefore, at this time, the pressurized fuel introduction chamber 4
2. The inside of the first fuel overflow chamber 43 and the second fuel overflow chamber 44 is also 2 to 3 kg/cn! is filled with low-pressure fuel.

Next, when the plunger l2 descends, the fuel supply boat 20
Since the fuel boat 21 is closed by the plunger 12 but the overflow valve 31 is open, the fuel pressurization chamber 15
The fuel inside flows out into the second fuel overflow chamber 44 via the fuel overflow path 49 and the pressurized fuel introduction chamber 42 . Therefore, at this time as well, the fuel pressure in the fuel pressurizing chamber 15 is at a low pressure of about 2 to 3 kg/ctA.

Next, when the piezoelectric element 67 is charged with an electric charge to start fuel injection, the piezoelectric element 67 expands in the axial direction, and as a result, the piston 63 descends, so that the fuel in the variable volume chamber 68 and the pressure control chamber 55 is reduced. Pressure increases rapidly. When the fuel pressure in the pressure control chamber 55 increases, the rond 50
moves to the left in FIG. 1 and FIG. is forced to close. When the overflow valve 31 closes, the fuel pressure in the fuel pressurizing chamber 15 rises rapidly due to the downward movement of the plunger 12.
When the fuel pressure in the fuel pressurizing chamber 15 exceeds a predetermined pressure, for example, a constant pressure of 1500 kg/c++1 or more, the needle 7 opens and fuel is injected from the nozzle row 3. At this time, the pressurized fuel introduction chamber 42
Although high pressure is applied inside the valve, no driving force is applied to the overflow valve 31 due to this high pressure.

Next, when the electric charge charged in the piezoelectric element 67 is discharged in order to stop fuel injection, the piezoelectric element 67 contracts. As a result, the piston 63 is raised by the spring force of the compression spring 7l, so that the fuel pressure in the variable volume chamber 68 and the pressure control chamber 55 decreases. As mentioned above, the mass of the rod 50 is small, so when the fuel pressure in the pressure control chamber 55 decreases, the rod 50 and the overflow valve 31 are immediately moved to the right in FIGS. 1 and 2 by the spring force of the compression spring 46. The overflow valve 31 moves away from the valve seat part 40 and the three overflow valves immediately open. When the overflow valve 31 opens, the high-pressure fuel in the fuel pressurizing chamber 15 is ejected into the first fuel overflow chamber 43 via the fuel overflow path 49 and the pressurized fuel introduction chamber 42 . However, since a throttle 48 is provided in the fuel passage 45 of the overflow valve 31, when the overflow valve 31 opens, the fuel in the first fuel overflow chamber 43 temporarily becomes high pressure, and then gradually decreases. . Therefore, when the overflow valve 3l is opened, the high-pressure fuel in the first fuel overflow chamber 43 is added to the needle back pressure chamber 24 through the fuel passage 25, so that the fuel pressure in the needle back pressure chamber 24 also temporarily decreases. become higher. As a result, the needle 7 is rapidly closed due to the increase in fuel pressure within the needle back pressure chamber 24. In this way, even if the overflow valve 31 is opened and the fuel pressure in the fuel pressurizing chamber 15 is reduced, and at the same time the fuel pressure in the annular fuel passage 19 is reduced, the needle 7 is rapidly closed. The combustion gases in the combustion chamber (not shown) are prevented from flowing into the annular fuel passage l9 via the nozzle row 3.

In addition, since a throttle 48 is disposed in the fuel passage 45 of the overflow valve 31, the fuel in the first fuel overflow chamber 43 flows through the throttle 48.
The fuel is allowed to overflow into the second overflow chamber 44 relatively slowly. As a result, the fuel pressure in the fuel pressurizing chamber 15 also decreases relatively slowly, so that the entire fuel in the annular fuel passage 19 does not move toward the fuel pressurizing chamber 15. As a result, the pressure within the annular fuel passage 19 does not temporarily become low, for example, below atmospheric pressure, and the generation of minute bubbles within the annular fuel passage 19 is thus prevented. Next, when the fuel pressure in the first fuel overflow chamber 43 decreases to the feed pressure, the fuel pressure in the needle back pressure chamber 24 also decreases to the feed pressure. On the other hand, when the piezoelectric element 67 is contracted to open the overflow valve 31 and the fuel pressure in the variable volume chamber 68 is lowered, the fuel pressure in the variable volume chamber 68 is lowered to the fuel inlet passage 77 (the sixth If the fuel pressure becomes lower than the fuel pressure in FIG.

Next, when the plunger 12 further descends, the plunger 12
A circumferential groove 26 formed on the outer peripheral surface of the fuel supply port 2
0 and the fuel port 21. At this time, when the fuel pressure in the fuel pressurizing chamber 15 is higher than the fuel pressure in the fuel supply boat 20 and the fuel port 21, the fuel in the fuel pressurizing chamber 15 flows through the fuel relief hole 27 formed in the plunger 12 and It is discharged via circumferential groove 26 into fuel supply boat 20 and fuel port 21 . The plunger l2 then rises, returns to the upper end position, and begins to descend again.

Another embodiment is shown in FIG. In this embodiment, the overflow valve 31
An IJ-F valve 90 is provided in the fuel passage 45 of the fuel passage 45 . This relief valve 90 has a valve port 91 and a valve port 91.
It consists of a ball 92 that controls the opening and closing of the valve, and a compression spring 93 that biases the ball 92 toward the valve boat 91.

In this embodiment, when the overflow valve 31 is opened and the high-pressure fuel in the fuel pressurizing chamber 15 flows into the first fuel overflow chamber 43, the relief valve 90 is immediately opened, so that the pressure in the fuel pressurizing chamber 15 is The fuel pressure suddenly decreases to the opening pressure of the relief valve go.

Then, while the plunger 12 is descending, the fuel pressurization chamber 1
5 and the fuel pressure in the first fuel overflow chamber 43 is IJ IJ-
The opening pressure of the valve 90 is maintained. Therefore, in this embodiment, when the overflow valve 31 is opened, a relatively high pressure determined by the opening pressure of the relief valve 90 is generated in the first fuel overflow chamber 43, and this high pressure is passed through the fuel passage 25 to the needle. The back pressure is added to the inside of the back pressure chamber 24. As a result, the needle 7 is rapidly closed, so that the combustion gas in the combustion chamber (not shown) is prevented from flowing into the annular fuel passage 19 via the nozzle row 3.

Furthermore, when the overflow valve 3l is opened, the fuel pressure in the fuel pressurizing chamber 15 drops rapidly, so that the entire fuel in the annular fuel passage 19 moves toward the fuel pressurizing chamber 15, but the fuel pressurizes. Room 15
Since the fuel pressure within the annular fuel passage 19 decreases only to the opening pressure of the relief valve 90, the pressure within the annular fuel passage 19 does not decrease much.
In this way, generation of minute air bubbles is prevented.

Next, when the plunger 12 starts to rise, the fuel pressure in the needle back pressure chamber 24 decreases to the feed pressure.

〔Effect of the invention〕

When the overflow valve opens, the fuel pressure in the needle back pressure chamber increases, causing the needle to close rapidly. As a result, combustion gas in the combustion chamber can be prevented from flowing around the needle.

[Brief explanation of the drawing]

Fig. 1 is a side sectional view of the unit injector taken along line I-1 in Fig. 5, Fig. 2 is an enlarged side sectional view of a part of Fig. 1, and Fig. 3 shows another embodiment. An enlarged side sectional view, FIG. 4 is a side sectional view taken along the TV-IV line in FIG. 5,
5 is a side sectional view taken along line V-V in FIG. 1, FIG. 6 is a side sectional view taken along line VI-Vl in FIGS. 1 and 8, and FIG. The plan view in the figure, and FIG. 8, is a plan sectional view taken along the line ■-■ in FIG. 3... Nozzle mouth, 7... Niddle, 11.
...Plunger hole, l2...Plunger, 15...
Fuel pressurization chamber, 20...Fuel supply port, 30...
Sliding hole, 31... Overflow valve, 35. 35
a...conical hole, 36. 36'... Valve seat member, 39... Concave groove, 40... Valve seat portion, 42... Pressurized fuel introduction chamber, 43... First fuel overflow chamber, 44... No. 2 Fuel overflow chamber, 48... Throttle, 55... Pressure control chamber, 6
7... Piezo piezoelectric element, 68... Variable volume chamber, 90... IJ leaf valve.

Claims (1)

[Claims] a plunger driven by an engine; a fuel pressurization chamber filled with fuel and pressurized by the plunger; The valve includes a needle that opens a valve, a fuel overflow passage that causes fuel in the fuel pressurizing chamber to overflow, and an overflow valve that is driven by an actuator and is disposed within the fuel overflow passage, and the overflow valve is configured to cause fuel to overflow. In a unit injector that performs fuel injection when the overflow passage is blocked, the fuel overflow passage downstream of the overflow valve is connected to the needle back pressure chamber so that high pressure is applied to the needle back pressure chamber when the overflow valve is opened. unit injector.