CLIAMS:We claim:
1. A metering unit (10) for a fuel injection system, said metering unit (10) comprising:
- a housing (12) comprising a bore (14) and a cirumferential groove (15) in fluid communication with said bore (14);
- a piston (16) adapted to reciprocate in said bore, at least a portion of said piston (16) in fluid communication with a fuel inlet (18) in said housing (12);
characterized in that
said piston (16) comprising at least one opening (20) on said portion of said piston (16) in fluid communication with said fuel inlet (18), profile of said at least one opening (20) being such that said metering unit (10) controls the quantity of fuel flowing back to a fuel tank.
2. The metering unit (10) as claimed in claim 1, wherein said piston (16) comprising pluralities of openings (20) on said portion of said piston (16) in fluid communication with said fuel inlet (18).

3. The metering unit (10) as claimed in claim 1, wherein profile of said at least one opening (20) is chosen group of profiles such as triangular, rectangular, square, circular and the like.

4. The metering unit (10) as claimed in claim 3, wherein said profile of at least one opening (20) controls the flow rate of the metering unit (10).

5. The metering unit (10) as claimed in claim 2, wherein each of said pluralities of openings (20) have a constant profile.

6. The metering unit (10) as claimed as claim 2, wherein each of said pluralities of openings (20) have a varying profile.

7. A fuel injection system (22) comprising:
- a fuel tank (24) adapted to store fuel;
- a high pressure pump (26) adapted to receive low pressure fuel from said fuel tank (24) through a feed pump (28) and a filter (30) and supply high pressure fuel to a fuel accumulator (32);
characterized in said fuel injection system (22),
a metering unit (10) located in back-flow path (34) from said high pressure pump (26) back to said fuel tank (24).
8. A fuel injection system (22) comprising:
- a fuel tank (24) adapted to store fuel;
- a high pressure pump (26) adapted to receive low pressure fuel from said fuel tank (24) through a feed pump (28) and a filter (30) and supply high pressure fuel to a fuel accumulator (32);
characterized in said fuel injection system (22),
- a metering unit (10) located in back-flow path (34) from said high pressure pump (26) back to said fuel tank (26), said metering unit (10) comprising:
o a housing (12) comprising a bore (14) and a cirumferential groove (15) in fluid communication with said bore (14);
o a piston (16) adapted to reciprocate in said bore, at least a portion of said piston (16) in fluid communication with a fuel inlet (18) in said housing (12);
said piston (16) comprising at least one opening (20) on said portion of said piston (16) in fluid communication with said fuel inlet (18), profile of said at least one opening (20) being such that said metering unit (10) controls the quantity of fuel flowing back to said fuel tank (24).
9. The fuel injection system (22) as claimed in claim 7 or 8, wherein said metering unit (10) is adpated to control rate of flow of fuel from said high pressure pump (26) to said fuel tank (24).
10. The fuel injection system (22) as claimed in claim 7 or 8, wherein said metering unit (10) is adapted to control rate of flow of fuel from said high pressure pump (26) to said fuel accumulator (32). ,TagSPECI:FIELD OF THE INVENTION
[001] This invention relates to a metering unit and a fuel injection system thereof.
BACKGROUND OF THE INVENTION
[002] Fuel injection systems known in the state of the art comprise a fuel tank, a high pressure pump and fuel accumulator. The function of the fuel injection system in the simplest form can be explained as the high pressure pump receive low pressure fuel from the fuel tank through a feed pump and filter and supplying high pressure fuel to the fuel accumulator. A metering unit is located downstream of the filter and upstream high pressure pump. The metering unit control the flow to the inlet of the high pressure pump. It is the function of this metering unit to accurately meter the amount of fuel supplied to high pressure pump which then is pumped to required pressures by the high pressure pump into the rail. The metering unit has an opening on the piston which leads to high flow rate requirement from fuel feed pump. Also, due to the metering unit being present at the inlet of the high pressure pump, pressure drop across the metering unit is high and this necessitates inclusion of overflow valve at the metering unit to drain the excess amount of fuel from the system. The metering unit are solenoid operated metering units, wherein the piston of the metering unit reciprocates with the metering unit depending on the actuation of the solenoid.

[003] DE10205811 discloses a fuel injection system wherein the metering unit is located between feed pump and high pressure pump. The flow of fuel from the metering unit to the high pressure pump is controlled by the signal received from the electronic control unit.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[004] Different modes of the invention are disclosed in detail in the description and illustrated in the accompanying drawing:
[005] Figure 1 illustrates a sectional view of the metering unit with a first embodiment of the piston in accordance with this disclosure;
[006] Figure 2 illustrates a sectional view of the metering unit with a second embodiment of the piston in accordance with this disclosure;
[007] Figure 3 illustrates a sectional view of the metering unit with a third embodiment of the piston in accordance with this disclosure; and
[008] Figure 4 illustrates the fuel injection system in accordance with this disclosure.
DETAILED DESCRIPTION
[009] Figure 1 illustrates a sectional view of the metering unit with a first embodiment of the piston in accordance with this disclosure. The metering unit 10 for a fuel injection system comprises a housing 12 comprising a bore 14 and a circumferential groove 15 in fluid communication with said bore 14 and a piston 16 adapted to reciprocate in the bore 14. At least a portion of the piston 16 is in fluid communication with a fuel inlet 18 in the housing 12. The piston 16 comprises at least one opening 20 on the portion of the piston 16 in fluid communication with fuel inlet 18. The profile of at least one opening 20 being such that the metering unit 10 controls the quantity of fuel flowing back to a fuel tank.
[0010] As seen in figure 1, the opening 20 has a triangular profile. The profile of at least one opening 20 may be chosen group of profiles such as triangular, rectangular, square, circular and the like. The profile of the opening controls the flow rate of the metering unit. Depending on the flow rate required from the metering unit the dimensions of the triangular profile may be designed. With the change in triangle base, area of triangle and hence, flow area changes and this helps meet flow requirements for metering unit can be met
[0011] In accordance with the second and third embodiment of the piston 16 of the metering unit as seen in figure 2 and figure 3, the piston 16 of the metering unit 10 comprises pluralities of openings 20 on the portion of the piston 16 in fluid communication with the fuel inlet 18. Each of the pluralities of openings 20 may have a constant profile. Each of the pluralities of openings 20 may have a varying profile.
[0012] The pluralities of openings as seen in figure 2 are a series of holes provided on the portion of the piston 16 which is in fluid communication with the fuel inlet 18 when the piston 16 reciprocates in the piston bore 14. The series of holes shown in figure 2 are provided in a helical formation.
[0013] The pluralities of openings as seen in figure 3 are a series of holes provided on the portion of the piston 16 which is in fluid communication with the fuel inlet 18 when the piston 16 reciprocates in the piston bore 14. The series of holes shown in figure 3 are provided in the form of triangle.
[0014] In accordance with other embodiments of the piston depending on the flow rate required it is possible that the dimensions of the holes 20 provided on the piston 16 may remain constant or the dimensions of the holes 20 provided on the piston 16 may vary.
[0015] In accordance with another aspect of this disclosure a fuel injection system 22 comprises a fuel tank 24 adapted to store fuel and a high pressure pump 26 adapted to receive low pressure fuel from the fuel tank 24 through a feed pump 28 and a filter 30 and supply high pressure fuel to a fuel accumulator 32. The fuel injection system 22 is characterized by a metering unit 10 located in the back-flow path 34 from the high pressure pump back to the fuel tank 24.
[0016] In accordance with another aspect of this disclosure the fuel injection system not only comprises the metering unit in the back-flow path but the metering unit comprises a piston comprising a openings 20 with different profiles. The fuel injection system 22 comprises a fuel tank 24 adapted to store fuel and a high pressure pump 26 adapted to receive low pressure fuel from the fuel tank 24 through a feed pump 28 and a filter 30 and supply high pressure fuel to a fuel accumulator 32. The fuel injection system is characterized by a metering unit 10 located in back-flow path 34 from the high pressure pump 26 back to the fuel tank 24. The metering unit 10 comprises a housing 12 comprising a bore 14 and a circumferential groove 15) in fluid communication with the bore 14 and a piston 16 adapted to reciprocate in the bore 14. At least a portion of the piston 16 in fluid communication with a fuel inlet 18 in the housing 12. The piston 16 comprises at least one opening 20 on the portion of the piston 16 in fluid communication with the fuel inlet 18. The profile of the at least one opening 20 being such that the metering unit 10 controls the quantity of fuel flowing into the fuel accumulator and fuel flowing back to the fuel tank.
[0017] In accordance with the two fuel injection systems explained above since the metering unit 10 is located in the back-flow path which takes the fuel from the high pressure pump 26 to the fuel tank 24, the metering unit is not required to meter as much fuel as it would have to if the metering unit 10 was located between the feed pump 28 and high pressure pump 26. Thus in comparison with the metering unit known the state of the art the dimensions of the opening 20 on the piston will be lower in accordance with this disclosure.
[0018] The working of the fuel injection system 22 in accordance with this invention can be explained as follows. For the purposes of the this explanation we consider embodiment shown in figure 2, wherein the openings 20 are in the forms of a series of holes 20 which are helically arranged on the portion of the piston 16 which is in fluid communication with the fuel inlet 18 and the circumferential groove 15 in the housing 12 of the metering unit 10. For the ease of understanding we consider only two position of the piston 16 when it reciprocates in the bore 14. A first position is when the solenoid of the metering unit 10 is not actuated in this position the piston is not moving against a spring force. In the second position when the solenoid is actuated the piston 16 moves, it is pushed against the spring force. During this movement of the piston 16 at least one of the holes 20 on the piston 16 comes in contact with the circumferential groove 15 on the housing 12. Fuel enters the metering unit 10 through the fuel inlet 18 and based on area of the opening 20 open with respect to the reference edge 19 on circumferential groove 15, passes through the opening 20 into the central bore (not shown in figures) in piston 16 and then down through an outlet to tank 24 in the direction of arrow 21.
[0019] The size of the opening 20 with respect to the circumferential groove on the housing helps to achieve the required flow rates through the metering unit10. The metering unit in accordance with this disclosure is not only able to control the fuel flowing from the high pressure pump 26 back to the fuel tank 24 but also is able to control the fuel flowing from the high pressure pump 26 to the fuel accumulator 32. Further since the metering unit 10 is located in the back-flow path of the fuel injection system 22, the size of the metering unit can be reduces since the metering unit in accordance with this disclosure will meter less fuel than if the metering unit was placed between the feed pump and high pressure pump.
[0020] It must be understood that the embodiments explained in the above paragraphs are only illustrative and do not limit the scope of the disclosure in terms of the profiles of the opening 20 on the piston 16 of the metering unit 10. The scope of this disclosure is also not limited to the dimension or the profile of the openings 20 or a combination of profiles provided on the piston 10. The scope of this disclosure in only limited by the scope of the claims.