CLIAMS:We Claim:
1. A fuel filter (200) in a fuel injection system,
said fuel filter (200) located downstream from a fuel tank 100 and upstream from a high pressure pump (104), said fuel filter comprising at least a dirt side (201) and a clean side 203; characterized in that
said fuel filter (200) has a vent hole (202) in fluid communication with said dirt side (201), said vent hole (202) establishes a path for flow of air and fuel from said dirt side (201) of said fuel filter back to said fuel tank (100).

2. The fuel filter (200) of claim 1, wherein said vent hole (202) has a cross-sectional profile which is at least one profile chosen from a group of hole profiles such as conical, cylindrical , square, rectangular, triangle, quadrilateral, trapezoidal and the like.

3. The fuel filter (200) of claim 1, wherein said hole has a profile such that different portions of the hole have varying diameters.

4. The fuel filter (200) of claim 1, wherein said vent hole (202) is provided with a cap which has an opening.

5. A low pressure circuit in a fuel injection system; comprising
a fuel tank (100) with an electric feed pump;
a fuel filter (200) with a vent hole (202) and a return line (103) back to said fuel tank (100); and
a high pressure pump (104).
said fuel filter has a vent hole (202) in fluid communication with said dirt side 201, said vent hole (202) establishes a path for flow of air and fuel from said dirt side 201 of said fuel filter back to said fuel tank (100). ,TagSPECI:The following specification particularly describes the invention and the manner in which it is to be performed.

Field of the invention
[001] This invention relates to the field of fuel filter in a fuel injection system.

Background of the invention
[002] For effective working of a fuel injection system, it is required that the rail that receives the high pressure fuel from the high pressure pump is free from air. Presence of air in fuel leads to pressure oscillations and in turn affecting the combustion of fuel. In the present day fuel injection system, air present in the fuel is vented out through the backflow valve present in the high pressure pump. Since the high pressure pump is oil lubricated, there is a chance for oil to flow along with air, back to the tank. This leads to “blackening” of fuel. Moreover, when the volume of air present in the fuel injection system is very high, especially when a vehicle is started for the first time in a plant, the load experienced by the backflow valve will be very high and it becomes difficult to completely eliminate air from the fuel injection system. Hence there is a need to vent out the air present in fuel and air mix before the fuel reaches the inlet of the high pressure pump in order to avoid pressure oscillations in the common rail.

[003] Prior art patent application WO-1990011444 discloses an air vent apparatus for a fuel supply circuit for a diesel engine, which aims at removing air in the fuel supply circuit without loosening an air bleed plug. The air vent apparatus includes a fuel filter disposed higher than a fuel injecting pump, a filter cap provided at the upper portion of fuel filter and having an air vent bore communicating with a fuel outlet, a pipe joint screwable with the air vent bore at the filter cap and having an orifice of a diameter so small as not to hinder a fuel injection quantity of the fuel injection pump, and an air vent pipe connected at one end to the pipe joint and communicating at the other end with a fuel tank.

Brief description of the accompanying drawing
[004] Different modes of the invention are disclosed in detail in the description and illustrated in the accompanying drawing:
[005] FIG. 1 illustrates a fuel injection system; and
[006] FIG. 2 illustrates a fuel filter in the fuel injection system.

Detailed description of the embodiments
[007] FIG. 1 illustrates a fuel injection system. The fuel injection system comprises a fuel tank 100 with an electric feed pump 102, a fuel filter 200, a high pressure pump 104, a common rail 106 and the injector 108. A series of supply lines connect each of the components of the fuel injection system for efficient transport of fuel. Fuel from the fuel tank 100 passes through the fuel filter 200 and reaches the inlet of the high pressure pump 104 where the fuel is pressurized. Pressurized fuel from the high pressure pump 104 is sent to the common rail 106 for injection through the injector. The components of the fuel injection system are controlled through an Electronic control unit (ECU). The fuel injection system comprises a low pressure circuit and the high pressure circuit. The part of the circuit downstream from the high pressure pump 104 is the high pressure circuit, and the part of the circuit upstream of the high pressure pump 104 is the low pressure circuit. The low pressure circuit comprising the proposed fuel filter 200 is now described in further detail.

[008] The low pressure circuit comprises a fuel tank 100 with an electric feed pump 102, a fuel filter 200 with a vent hole 202 and a return line 103 back to the fuel tank 100, and a high pressure pump 104. The fuel filter 200 receives fuel from the fuel tank 100. The fuel filter 200 has a vent hole 202 in fluid communication with the dirt side 201 and establishes a path for flow of air and fuel from the dirt side 201 of the fuel filter 200 back to the fuel tank. The fuel filter 200 may contain air and this need to be vented out of the fuel filter 200 through the vent hole 202 in order to prevent the air to flow along with the fuel and reach the high pressure pump 104. The air is usually present when vehicle is started after filling an empty fuel tank 100, or started for the first time in the plant. The fuel filter 200 comprising the proposed vent hole 202 is now described in further detail.

[009] FIG. 2 illustrates a fuel filter 200 in the fuel injection system.The fuel filter (200) is located downstream from a fuel tank 100 and upstream from a high pressure pump 104.The fuel filter 200 comprises at least a dirt side 201 and a clean side 203, and has a vent hole 202 in fluid communication with the dirt side 201. The vent hole 202 establishes a path for flow of air and fuel from the dirt side 201 of the fuel filter 200 back to the fuel tank 100. The vent hole 202 has a cross-sectional which is at least one profile chosen from a group of hole profiles such as conical, cylindrical, square, rectangular, triangle, quadrilateral, trapezoidal and the like. In an embodiment the hole may have profile such that different portions of the hole have varying diameters. The main purpose of having varying diameters is to ensure throttling functionality of air and fuel by creating a restricted passage for the flow of fuel and air.

[0010] The dirt side 201 of the fuel filter 200 has an inlet 204 that receives un-filtered fuel from the fuel tank 100. The un-filtered fuel entering the fuel tank 100 can take the following flow paths in the fuel injection system. Firstly, the fuel can pass through the filter elements, get filtered and the filtered fuel can pass through clean side 203 to the outlet 206 of the fuel filter 200 to reach the high pressure pump 104. The air that is trapped in the fuel filter 200, or in the flow lines leading to the fuel filter 200, can now flow through the vent hole 202 that is provided on the dirt side 201, back to the fuel tank 100. With this arrangement it is now possible to eliminate the air present in the fuel injection system, before the fuel reaches the high pressure pump 104. Since the fuel reaching the high pressure pump 104 is devoid of any air, the backflow valve, from the high pressure pump 104 back to the fuel tank 100 can now be eliminated.

[0011] The fuel flows through the second flow path, when the metering unit that supplies fuel to the high pressure pump 104 does not require any additional quantity of fuel to reach the high pressure pump 104. Any excess quantity of fuel that is now available has to be sent back to the fuel tank 100. This is achieved by means of a zero delivery throttle. The vent hole 202 that is provided on the fuel filter 200 can now function like a throttle to remove excess fuel from the fuel filter 200 itself. In an embodiment the vent hole 202 is provided with a cap which has an opening. The opening provided on the cap aligns with the vent hole. In an embodiment the cap may have different geometries while the opening is of a standard cross section. In another embodiment the opening provided on the cap may also have different geometrical cross sections.

[0012] The above mentioned configuration of introducing a vent hole 202 the fuel filter 200 enables the fuel filter 200 to perform the following functions in addition to filtration of fuel namely a)Venting of air from the fuel filter 200 and from the delivery line leading to the fuel filter 200 and b)Function as a zero delivery throttle in order to prevent excess fuel reaching the high pressure pump 104. The above mentioned fuel filter 200 can also be used in fuel lubricated pumps. In case of fuel lubricated pumps a throttle at the inlet to the high pressure pump will control the flow of fuel for lubrication of components of the high pressure pump.

[0013] It should be understood that embodiments explained in the description above are only illustrative and do not limit the scope of this invention in terms of type and profile of the vent hole provided on the fuel filter and position of the vent hole on the fuel filter. Many such embodiments and other modifications and changes in the embodiment explained in the description are envisaged. The scope of the invention is only limited by the scope of the claims.