Field of the invention
The present disclosure relates to a low pressure circuit for common rail fuel injection systems.
Background of the invention
All components of the common rail hydraulic injection system are fuel lubricated. In a common rail fuel injection system a high pressure pump is used to pressurize fuel before the fuel is delivered to the injector for injection into the engine cylinders. During first start of the high pressure pump, the air that is located in the high pressure pump needs to be flushed out, this is usually done by circulating fuel at a pressure that is higher than that required for normal working of the high pressure pump. This operation is known is first filing. Before a high pressure fuel pump in mounted on the vehicle, a first filling is recommended. In the test bench external lift pump systems are used to accompli sh the first filling operation. When the high pressure fuel pump is mounted inside a vehicle the first filling operation is accomplished with an additional electric feed pump that sucks fuel from the fuel tank. However, an overflow valve that is located upstream to the high pressure pump prevents fuel from entering the high pressure pump as this overflow valve opens a path for the fuel to flow back to the fuel tank, leading to improper flushing of air from the high pressure pump.

Brief description of the accompanying drawings
[0003] An embodiment of the invention is described with
reference to the following accompanying drawings:
[0004] Fig. 1 depicts a common rail fuel injection system.
Detailed description of the drawings
[0005] Figure 1 depicts a common rail fuel injection system (100).
The common rail fuel injection system (100) comprises common rail (108), injectors (109) and at least a low pressure circuit. The low pressure circuit of a common rail fuel injection system comprises a high pressure fuel pump (101), a fuel filter (102), an electric feed pump (103) and at least a fuel tank (104). The low pressure circuit further comprises a first flow path (107) connecting the fuel filter (102) and the electric feed pump (103), an electromagnetic solenoid valve (105) and at least a second flow path (110). The electromagnetic solenoid valve (105) further comprises an inlet and an outlet port (111). The electromagnetic solenoid valve (105) is energized by an Electronic Control Unit (ECU (106)) of the vehicle. The second flow path (110) connects the inlet port of the electromagnetic solenoid valve (105) to the first flow path (107). The outlet port (111) of the electromagnetic solenoid valve (105) is connected to the fuel tank (104).

[0006] In an embodiment the common rail fuel injection system
(100) also comprises a pre-filter and a first flow path (107) connecting the pre-filter and the fuel filter (102).
[0007] The electric feed pump (103) draws fuel from the fuel tank
(104) and supplies it upstream to the fuel filter (102). When a first filling operation is performed in a common rail fuel injection system (100) the electromagnetic solenoid valve (105) is not energized. No current passes through the solenoid valve, hence there is no flow of fuel in the inlet or outlet of the electromagnetic solenoid valve. The fuel flows through the first flow path (107) from the electric fuel to the fuel filter (102). There is no flow of fuel through the output port of the electromagnetic solenoid valve (105) to the fuel tank (104). This helps build up the requisite pressure in the high pressure fuel pump (101) required for first filling.
[0008] Consequently when the first filling operation of the fuel
pump is achieved, the electromagnetic solenoid valve (105) is energized. A current is passed through the electromagnetic solenoid valve (105) by the ECU (106). Depending upon the pressure requirement in the first flow path (107) the amount of current passed through the electromagnetic solenoid valve (105) is varied to open it partially or completely. When the electromagnetic solenoid valve (105) is completely or partially opened, there is a flow of excess fuel from the second flow path (110) through the inlet port into the outlet port (111) of the electromagnetic solenoid valve (105). The excess fuel flows through the output port into the fuel tank

(104) to ensure that the required pressure is maintained in the low pressure circuit.
[0009] This idea to develop a low pressure circuit for common rail
fuel injection systems (100) fulfills pressure requirements in the common rail fuel injection systems, both during the first filling and after the first filling operation of the high pressure fuel pump (101). The use of the electromagnetic solenoid valve (105) instead of an over-flow valve ensures that the high pressure required in the high pressure fuel pump (101) is maintained and that there are no startability issues in the common rail fuel injection system (100). Further the fuel flowing into the electromagnetic solenoid valve (105) through the second flow path (110) is unfiltered fuel. Hence the fuel flowing into the fuel tank (104) from the outlet is also unfiltered. In the present system having the overflow valve, the fuel flowing from the backflow into the fuel tank (104) is filtered fuel. This fuel has to filtered again and this reduces the efficiency of common rail fuel injection system (100). This invention overcomes this deficiency, thereby also improving the efficiency of the common rail fuel injection systems.
[0010] It must be understood that the embodiments explained in the
above detailed description are only illustrative and do not limit the scope of this invention. Any modification to a low pressure circuit for common

rail fuel injection systems are envisaged and form a part of this invention. The scope of this invention is limited only by the claims.

We Claim: 1. A low pressure circuit of a common rail fuel injection system (100) comprising a high pressure fuel pump (101), a fuel filter (102) , an electric feed pump (103) and at least a fuel tank (104), characterized in that low pressure circuit: a first flow path (107) connecting the fuel filter (102) and the electric feed pump (103);
an electromagnetic solenoid valve (105) comprising an inlet and an outlet, where the electromagnetic solenoid valve (105) is energized by an Electronic Control Unit ;
a second flow path (110) connecting the inlet port of the electromagnetic solenoid valve (105) to the first flow path (107);
the outlet port (111) of the electromagnetic solenoid valve (105) connected to the fuel tank (104).