JP2001263200A - Delivery valve of fuel injection pump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a two-way delivery valve of a fuel injection pump, to improve injection pressure at the time of injecting fuel, to improve a retraction effect at the time of finishing injection and to improve combustion efficiency. SOLUTION: A compression part to compress fuel in a space (hole upper part 3d) on the downstream side from a delivery valve 11 is formed on the delivery valve 11 by dividing a hole 3a in a valve casing 3 vertically by the delivery valve 11 on a two-way delivery valve 10 of a fuel injection pump 1 constituted by fitting the delivery valve 11 and a return valve 12 in the hole 3a in the valve casing 3. Additionally, a retraction part to retract fuel in the inside of the space (hole upper part 3d) on the downstream side of the delivery valve 11 is formed on an outer peripheral part of the delivery valve 11 by vertically dividing the space (hole 3a) in the valve casing 3 by the delivery valve 11 on the two-way delivery valve 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology of a pressure equalizing valve in a fuel injection pump of a fuel injection type internal combustion engine, which performs pumping of fuel from the start to the end of fuel pumping of the pump and prevents backflow and suckback of the fuel.

[0002]

2. Description of the Related Art In the prior art, a technique relating to a constant pressure valve of a fuel injection pump constituted by a delivery valve and a return valve is known. 8 and 9
As shown in the figure, the cross section of the delivery valve 19 is square, and the four corners of the delivery valve 19 in a sectional view are in sliding contact with the inner peripheral surface of the hole 3a of the valve casing 3, and the front, rear, left and right four sides of the delivery valve 19 and the hole 3
are formed with the inner peripheral surface a. Then, when fuel is fed from below, the return valve 12 and the delivery valve 19 are pushed up,
The upper and lower portions of the delivery valve 19 communicate with each other through the gaps 3b, 3b,..., So that the fuel is pressure-fed by the plunger, passes through the gaps 3b, 3b,. It is.

[0003]

However, in such a configuration of the prior art, as soon as the fuel is fed under pressure, the upper and lower portions of the delivery valve 19 communicate with each other. There is almost no compression, and the rise of the pressure rise of the injected fuel is small. As a result, the ejection pressure at the start of fuel injection has been low. Further, in such a configuration, at the end of the fuel injection, the suction effect by the delivery valve 19 is small, and the plunger is lowered, and the pressure drop in the plunger barrel is directly reduced to the pressure drop in the fuel injection passage, and the fuel injection is performed. The pressure drop at the end is slowed down, which also causes the fuel to drip from the tip of the injection nozzle. For the above reasons, the mixing of the injected fuel with the air in the fuel chamber becomes worse, and the Sd (exhaust color) becomes higher.

[0004] In the present invention, the pressure equalizing valve of the fuel injection pump is improved to increase the injection pressure at the start of fuel injection.
An object of the present invention is to improve the suction effect at the end of injection to improve combustion efficiency.

[0005]

The above is the problem to be solved by the present invention. Next, means for solving the problem will be described. That is, in a pressure equalizing valve of a fuel injection pump configured by fitting a delivery valve and a return valve into a space in a valve casing, the space in the valve casing is vertically divided by the delivery valve. Then, a compression section for compressing fuel in the space downstream of the delivery valve is formed.

According to a second aspect of the present invention, in a pressure equalizing valve of a fuel injection pump having a delivery valve and a return valve fitted in a space in a valve casing, the space in the valve casing is vertically divided by the delivery valve. A suction portion is formed at an outer peripheral portion of the delivery valve to suck back fuel in a space downstream of the delivery valve.

According to a third aspect of the present invention, in the fuel injection pump, an outer peripheral portion of the delivery valve is perforated to communicate a space above and below the delivery valve.

Alternatively, in the pressure equalizing valve of the fuel injection pump, a groove is formed in an inner peripheral surface of a space in which the pressure equalizing valve is fitted, and a space below and above the delivery valve is communicated.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The problems to be solved and the configuration of the present invention are as described above, and an embodiment of the present invention shown in the accompanying drawings will be described below. FIG. 1 is a side sectional view of a fuel injection pump provided with an equal pressure valve according to the present invention, and FIG.
FIG. 3 is a side view showing the structure of the delivery valve according to the embodiment.
2 is a cross-sectional view taken along line AA in FIG. 2, FIG. 4 is a side cross-sectional view showing a mechanism of the equal pressure valve according to the first embodiment of the present invention, and FIG. 5 shows a structure of a delivery valve according to a second embodiment of the present invention. FIG. 6 is a cross-sectional view taken along the line BB in FIG. 5, FIG. 7 is a cross-sectional side view showing the mechanism of the constant pressure valve according to the second embodiment of the present invention,
9 is a side view showing the structure of a conventional delivery valve, FIG. 9 is a cross-sectional view taken along the line CC in FIG. 8, FIG. 10 is a side cross-sectional view showing the mechanism configuration of the conventional equal pressure valve, and FIG. FIG. 12 is a diagram showing the pressure change curve, and FIG. 12 shows the amount of rise of the injection nozzle valve.

First, the overall configuration of the fuel injection pump 1 will be described. As shown in FIG. 1, the equal pressure valve 10 of the fuel injection pump 1 is constituted by a delivery valve 11, a return valve spring receiver 12, a ball valve 13, and the like. The housing 2 and the valve casing 3 are disposed with the holes 2a and 3a dug in a shape, and the holes 2a and 3a are opposed to each other. The constant pressure valve 10 is fitted into the holes 2a and 3a. And the housing 2 by bolts 9
And the valve casing 3 are fastened and fixed.

The inside diameter of the hole 3a of the valve casing 3 is larger than the inside diameter of the hole 2a of the housing 2,
On the upper surface of the housing 2, the periphery of the opening of the hole 2 a is formed on the seat surface of the delivery valve 11, and the delivery valve 11 is fitted into the hole 3 a of the valve casing 3. The delivery valve 11 is configured to be vertically slidable along the inner peripheral surface of the hole 3a.
A delivery valve lift stopper 14 for restricting sliding of the delivery valve 11 is fitted into the upper part of the inside. The lower end of a delivery valve spring 15 is in contact with the upper surface of the delivery valve 11, and the delivery valve lift stopper 14 also serves as a spring receiver, and receives the upper end of the delivery valve spring 15. ing. A hole is formed from the bottom surface of the hole 3a of the valve casing 3 (the upper surface in the hole 3a in FIG. 1) to the upper surface of the valve casing 3 to form a fuel injection passage 16.

A guide surface of the return valve spring 17 is formed on the inner peripheral surface of the hole 2a of the housing 2, and the lower end of the return valve spring 17 is received with the bottom surface of the hole 2a as a spring receiver. A return valve spring receiver 1 is provided at the upper end of the return valve spring 17.
2 is in contact with the lower surface. As shown in FIG. 2, the delivery valve 11 is cylindrically dug and drilled upward from a lower surface to form a depression 11a, a recess 11d is formed upward from the upper surface of the depression 11a, and a return valve spring is formed. A recess 12 d is formed downward from the upper surface of the receiver 12, and the recesses 11 d and 12 d and the upper and lower portions of the ball valve 13 are fitted and sandwiched. Further, at the position of the axis of the delivery valve 11, the delivery valve 1 is
The through hole 11b is formed by piercing the upper surface of the substrate 1.

As shown in FIG. 1, a plunger barrel 21 is formed in the housing 2 immediately below the return valve spring receiver 12, and a plunger 22 is fitted to the plunger barrel 21 so as to be vertically movable. ing. Also,
A hole is formed from the upper surface of the plunger barrel 21 to the bottom surface of the hole 2a of the housing 2 to form a fuel pressure supply passage 24.

In such a configuration, the fuel is pressure-fed into the plunger barrel 21 by an oil feed pump or the like, and the plunger 22 is raised by a cam device (not shown). 2a
Is pumped into. As a result, the pressure in the hole 2a increases, and the return valve spring receiver 12, the ball valve 1
3. The delivery valve 11 is pushed up so that the hole 2a of the housing 2 communicates with the hole 3a of the valve casing 3, and the fuel is further pumped upward, passes through the fuel ejection passage 16, and is injected from an injection nozzle (not shown). It is done.

Next, the structure of the conventional equal-pressure valve 10 of the fuel injection pump 1 and the structure of the equal-pressure valve 10 of the fuel injection pump 1 according to the present invention will be described. There are two embodiments according to the present invention (first embodiment and second embodiment).

First, a description will be given of the conventional equal pressure valve 10 of the fuel injection pump 1. As shown in FIGS. 8 and 9, the cross section of the conventional delivery valve 19 is a square shape, and four corners of the delivery valve 19 in a sectional view are in sliding contact with the inner peripheral surface of the hole 3 a of the valve casing 3. Left and right four sides and the hole 3a
The gaps 3b are formed between the inner peripheral surfaces.

As shown in FIGS. 10A and 10B, when fuel is fed into the hole 2a of the housing 2, the pressure in the hole 2a increases, and the return valve spring receiver 12 and the ball The valve 13 and the delivery valve 19 are pushed up, but in such a conventional configuration, the gap 3 is set as soon as the delivery valve 19 is raised.
(FIG. 9) communicates with the hole 2a of the housing 2 and the hole 3a of the valve casing 3 through b. The fuel is pressure-fed by the plunger 22 (FIG. 1), passes through the gaps 3b, 3b,... Without difficulty, and is pressure-fed to the upper fuel ejection passage 16 as it is, so that in the hole 3a of the valve casing 3, The fuel is not compressed and delivered by the delivery valve 19. In this way, the change in the oil pressure of the fuel due to the rise of the plunger 22 is transmitted to the fuel as it is and is fed upward.

FIG. 11 and FIG. 12 are graphs showing this state. The pressure rise in the fuel ejection passage 16 is a rising curve from point a to point c in FIG.
However, it is an inflection point at the point b ′, and once becomes a gentle rise. As shown in FIG. 10B, when the fuel injection is started, the hole 2a of the housing 2 and the valve casing 3 are immediately inserted through the gaps 3b.
The holes 3a communicate with each other, the volume of the space occupied by the fuel suddenly increases, and the pressure temporarily rises gradually. In this way, the degree of increase in the injection pressure during fuel injection once decreases, so that the fuel cannot be sufficiently atomized, thereby affecting the mixing of the fuel and air in the combustion chamber, This also affects the Sd (exhaust color) of the combustion gas.

Further, as shown in FIG. 10C, after the fuel injection is completed, the plunger 22 (FIG. 1) is lowered, the pressure in the hole 2a of the housing 2 is reduced, and the delivery valve 19 is immediately returned to its original state. In this state, the pressure in the hole 2a of the housing 2 is lower than the pressure in the hole 3a of the valve casing 3 and the ball valve 1
3 and the return valve spring receiver 12 are in a depressed state. The hole 3a and the fuel ejection passage 16
As shown from the point c to the point d 'in FIG. 11, the ball valve 13 and the return valve 12 gradually rise, and the pressure in the fuel ejection passage 16 rises as shown in FIG. When the set pressure in the second hole 2a is balanced, the ball valve 13 closes the concave portion 19d (FIG. 8) of the depressed portion 19a of the delivery valve 19 and returns to the original position. In this way, the pressure in the fuel ejection passage 16 is kept constant at the end of the injection by the equal pressure valve 10.

The lower the curve from the point c to the point d ', the steeper the descending curve, the better the combustion characteristics. That is, the fuel is prevented from dripping from the tip of the injection nozzle. Unburned fuel is eliminated until the end of the injection.

Next, the constant pressure valve 10 of the fuel injection pump 1 according to the first embodiment of the present invention will be described. In the first embodiment,
As shown in FIGS. 2 and 3, the delivery valve 11 constituting the equal-pressure valve 10 has a lower portion in which the outer peripheral surface of the delivery valve 11 and the inner peripheral surface of the hole 3 a of the valve casing 3 are loosely fitted. At its upper part, the outer peripheral surface of the delivery valve 11 and the inner peripheral surface of the hole 3a are slidably fitted tightly, that is, the upper part 3 located downstream of the delivery valve 11 in the hole 3a.
d and a lower portion 3 e located upstream of the delivery valve 11.
Is divided by the delivery valve 11. A plurality of communication holes 11c are provided in the upper part of the delivery valve 11, and the communication holes 11c
11c ... upper part 3d and lower part 3 of the hole 3a
e.

As shown in FIG. 4A, a gap T1 is provided between the upper surface of the delivery valve 11 and the lower surface of the delivery valve lift stopper 14.

As described above, in the first embodiment, the delivery valve 11 compresses the fuel on the downstream side of the upper part 3d of the hole. As shown in FIGS. 2 and 4B, when fuel is fed into the hole 2a of the housing 2, the delivery valve 11 is raised. The fuel fed from the lower part 3e to the upper part 3d of the hole 3a is supplied to the communication hole 11a.
c · 11c · · · and its flow rate is restricted,
Therefore, the change in the oil pressure of the fuel due to the rise of the plunger 22 (FIG. 1) is transmitted to the delivery valve 11 almost as it is. Then, the delivery valve 11 is actuated by the hydraulic pressure.
Is pushed up, contracting the gap T1, compressing the volume of the space above the hole 3d, and compressing the fuel on the downstream side.

In this state, the pressure changes from point a to point b in FIG. 11, and the pressure of the fuel downstream of the upper part 3d (fuel ejection passage 16) of the hole 3a shown in FIG. 2 rises smoothly. The fuel can be effectively atomized. Thus, the mixing of the fuel and the air in the combustion chamber is improved, and the Sd (exhaust color) of the combustion gas is reduced.

At the end of the fuel injection shown in FIG. 4 (c), the plunger 22 is lowered to lower the pressure in the space below the delivery valve 11, and the pressure in the hole 2a of the housing 2 is increased by the pressure in the upper portion 3d of the valve casing 3. It becomes negative pressure with respect to pressure. Therefore, the delivery valve 11
Descends and closes, inside the hole 2a and the lower part 3e of the hole 3a.
Disconnect communication with Further, since the ball valve 13 moves downward and opens, the fuel on the downstream side of the upper hole portion 3 d is sucked back to the upstream side of the lower hole portion 3 e through the through hole 11 b of the delivery valve 11.

At the beginning of the fuel injection, the delivery valve 11 immediately lowers and returns to the gap T again.
In other words, the space in the upper part 3d of the valve casing 3 is expanded by the volume of the gap T1, and the pressure in the upper part 3d of the hole is reduced by further spur. In this way, as shown from the point c to the point d in FIG. 11, the hole 3a after the end of the combustion injection (the fuel ejection passage 16)
The rate of pressure drop in the inside is faster than before, and the pressure drops more rapidly. As a result, the fuel is effectively prevented from dripping from the tip of the injection nozzle, so that there is no unburned fuel at the end of fuel injection.

Next, the constant pressure valve 10 of the fuel injection pump 1 according to a second embodiment of the present invention will be described. In the second embodiment,
As shown in FIGS. 5 and 6, the delivery valve 18 constituting the equal-pressure valve 10 has, at its upper part, a loose fit between the outer peripheral surface of the delivery valve 18 and the inner peripheral surface of the hole 3 a of the valve casing 3. In the lower part, the outer peripheral surface of the delivery valve 18 and the inner peripheral surface of the hole 3a are tightly fitted to each other, that is, the upper part 3d located downstream of the delivery valve 18 in the hole 3a, The delivery valve 18 separates the space from the lower part 3e located on the upstream side.

On the inner peripheral surface of the hole 3a of the valve casing 3, one or a plurality of grooves 3c, 3c,.
Is formed, and the lower end of the delivery valve 18 is formed in the groove 3c.
3c... Ascending to the position, the upper part 3 of the hole 3a
It is configured such that d communicates with the lower part 3e. The grooves 3c may be annular grooves.

As shown in FIG. 7A, a gap T2 is provided between the upper surface of the delivery valve 18 and the lower surface of the delivery valve lift stopper 14.

As described above, in the second embodiment, the delivery valve 18 compresses the fuel on the downstream side of the upper part 3d of the hole. As shown in FIGS. 5 and 7 (b), when the fuel is pressure-fed into the hole 2a of the housing 2, the delivery valve 18 rises, and the grooves 3c, 3c,.
Thereby, the upper hole 3d and the lower hole 3e communicate with each other.
The fuel pumped from the lower part 3e to the upper part 3d of the hole 3a is throttled by the grooves 3c, 3c,... And its flow rate is restricted. Is transmitted to the delivery valve 18 almost as it is. Then, the delivery valve 18 is pushed up by the hydraulic pressure, thereby reducing the gap T2, compressing the volume of the space above the hole 3d, and compressing the fuel on the downstream side.

In this state, the pressure changes from point a to point b in FIG. 11, and the pressure of the fuel downstream of the upper part 3d (fuel ejection passage 16) of the hole 3a shown in FIG. 5 rises smoothly. The fuel can be effectively atomized. Thus, the mixing of the fuel and the air in the combustion chamber is improved, and the Sd (exhaust color) of the combustion gas is reduced.

At the end of the fuel injection shown in FIG. 7 (c), the plunger 22 is lowered to lower the pressure in the space below the delivery valve 18, and the pressure in the hole 2a of the housing 2 is increased by the pressure in the upper part 3d of the valve casing 3. It becomes negative pressure with respect to pressure. Therefore, the delivery valve 18
Descends and closes, inside the hole 2a and the lower part 3e of the hole 3a.
Disconnect communication with Further, since the ball valve 13 moves downward and opens, the fuel on the downstream side of the upper hole 3d is sucked back to the upstream side of the lower hole 3e through the through hole 18b of the delivery valve 18.

At the beginning of the fuel injection, the delivery valve 18 immediately drops and returns to the gap T.
2, the space in the upper part 3d of the valve casing 3 expands by the volume of the gap T2, and further spurs to lower the pressure in the upper part 3d of the hole. In this way, as shown from the point c to the point d in FIG. 11, the hole 3a after the end of the combustion injection (the fuel ejection passage 16)
The rate of pressure drop in the inside is faster than before, and the pressure drops more rapidly. As a result, the fuel is effectively prevented from dripping from the tip of the injection nozzle, so that there is no unburned fuel at the end of fuel injection.

[0034]

The present invention is configured as described above.
The following effects are obtained. That is, in a pressure equalizing valve of a fuel injection pump configured by fitting a delivery valve and a return valve into a space in a valve casing, the space in the valve casing is vertically divided by the delivery valve. By forming a compression section for compressing fuel in the space downstream of the delivery valve, the fuel in the space downstream of the delivery valve is compressed to a high pressure at the start of fuel injection, and as a result, the injection pressure increases. The mixing of the injected fuel in the combustion chamber becomes better, and the Sd
(Exhaust color) will be lower.

According to a second aspect of the present invention, in the equal-pressure valve of the fuel injection pump, the space in the valve casing is vertically divided by a delivery valve, and an outer peripheral portion of the delivery valve is provided in a space downstream of the delivery valve. By forming a suction part for sucking back the fuel, after the end of the combustion injection, the plunger descends and the space upstream of the delivery valve becomes a negative pressure with respect to the space downstream, and the fuel on the downstream side is reduced by the suction part. The pressure is sucked back to the upstream side, and the rate of pressure drop in the space on the downstream side is faster than in the conventional case, and the pressure drops more rapidly. As a result, the fuel is effectively prevented from dripping from the tip of the injection nozzle, and the unburned fuel at the end of the fuel injection is eliminated.

According to a third aspect of the present invention, in the fuel injection pump, an outer peripheral portion of the delivery valve is perforated to communicate a space above and below the delivery valve.
At the time of fuel injection, the pressure-fed fuel is throttled by a communication hole formed in the outer periphery of the delivery valve, the flow rate is limited, and the pressure in the space downstream of the delivery valve is increased. As a result, the injection pressure increases, the mixing of the injected fuel in the combustion chamber becomes good, and the Sd (exhaust color) of the combustion gas decreases. Further, at the end of the fuel injection, the plunger descends and the space upstream of the delivery valve becomes a negative pressure with respect to the space downstream, and the downstream fuel is sucked back to the upstream by opening the ball valve. In the process, the volume of the space downstream of the delivery valve increases, so that the rate of pressure decrease in the downstream space becomes faster than in the past, and the pressure drops more rapidly. As a result, the fuel is effectively prevented from dripping from the tip of the injection nozzle, and the unburned fuel at the end of the fuel injection is eliminated.

Alternatively, in the pressure equalizing valve of the fuel injection pump, a groove may be formed in an inner peripheral surface of a space in which the pressure equalizing valve is fitted, and a space below and above the delivery valve may be communicated. Thus, at the time of fuel injection, the pressure-fed fuel is throttled by the groove, the flow rate is restricted, and the pressure in the space downstream of the delivery valve is increased. As a result, the injection pressure increases, the mixing of the injected fuel in the combustion chamber becomes good, and the Sd of the combustion gas increases.
(Exhaust color) will be lower. Further, at the end of the fuel injection, the plunger descends and the space upstream of the delivery valve becomes a negative pressure with respect to the space downstream, and the downstream fuel is sucked back to the upstream by opening the ball valve. In the process, the volume of the space downstream of the delivery valve increases, so that the rate of pressure decrease in the downstream space becomes faster than in the past, and the pressure drops more rapidly.

[Brief description of the drawings]

FIG. 1 is a side sectional view of a fuel injection pump provided with a constant pressure valve according to the present invention.

FIG. 2 is a side view showing the structure of the delivery valve according to the first embodiment of the present invention.

FIG. 3 is a sectional view taken along line AA in FIG. 2;

FIG. 4 is a side sectional view showing the mechanism of the equal pressure valve according to the first embodiment of the present invention. (A) A state in which both the delivery valve and the return valve are closed during no fuel injection. (B) A state in which the delivery valve is opened and the return valve is closed during fuel injection. (C) At the end of fuel injection, the delivery valve is closed and the return valve is open.

FIG. 5 is a side view showing a structure of a delivery valve according to a second embodiment of the present invention.

FIG. 6 is a sectional view taken along line BB in FIG. 5;

FIG. 7 is a side sectional view showing a mechanism of a constant pressure valve according to a second embodiment of the present invention. (A) A state in which both the delivery valve and the return valve are closed during no fuel injection. (B) A state in which the delivery valve is opened and the return valve is closed during fuel injection. (C) At the end of fuel injection, the delivery valve is closed and the return valve is open.

FIG. 8 is a side view showing the structure of a conventional delivery valve.

FIG. 9 is a sectional view taken along the line CC in FIG. 8;

FIG. 10 is a side sectional view showing a mechanism configuration of a conventional type equal pressure valve. (A) A state in which both the delivery valve and the return valve are closed during no fuel injection. (B) A state in which the delivery valve is opened and the return valve is closed during fuel injection. (C) At the end of fuel injection, the delivery valve is closed and the return valve is open.

FIG. 11 is a pressure change curve diagram in a fuel ejection passage.

FIG. 12 is a diagram showing a rising amount of an injection nozzle valve.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 fuel injection pump 3 valve casing 3 a hole 3 c groove portion 3 d upper hole 3 e lower hole 10 constant pressure valve 11 delivery valve (first embodiment) 11 c communication hole 12 return valve spring receiver 13 ball valve 16 fuel ejection passage 18 delivery valve (second embodiment) 19) Delivery valve (conventional type)

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshihiro Yome 1-32 Chayamachi, Kita-ku, Osaka City, Osaka Prefecture F-term in Yanmar Diesel Co., Ltd. 3G066 AA07 AB02 AD12 BA03 BA05 BA06 BA09 BA11 CA08 CA09 CA21 CA22T

Claims (4)

[Claims] In a constant pressure valve of a fuel injection pump, wherein a delivery valve and a return valve are fitted in a space in a valve casing, the space in the valve casing is vertically divided by a delivery valve. A constant pressure valve for a fuel injection pump, wherein a compression portion for compressing fuel in a downstream space is formed. 2. A constant pressure valve of a fuel injection pump, wherein a delivery valve and a return valve are fitted in a space in a valve casing. The space in the valve casing is vertically divided by a delivery valve, and the delivery valve is provided on an outer peripheral portion of the delivery valve. And a suction portion for sucking back fuel in a space downstream of the delivery valve. 3. The constant pressure valve for a fuel injection pump according to claim 1, wherein an outer peripheral portion of the delivery valve is perforated to communicate a space above and below the delivery valve. 4. The constant pressure valve of the fuel injection pump,
3. The pressure equalizing valve for a fuel injection pump according to claim 1, wherein a groove is formed in an inner peripheral surface of a space in which the pressure equalizing valve is fitted, and communicates with a space above and below the delivery valve.