Claims:We claim:
1. A fuel injection system (100) comprising a fuel tank (102), a fuel filter (104) to filter fuel from said tank (102), a pump (105) to pressurize fuel received from said filter (104), an injector (106) to receive pressurized fuel from said pump (105), said fuel injection system (100) characterized in that said pump (105) is a mechanical pump and said injector (106) is an electronic injector.
2. A fuel injection system (100) according to claim 1 wherein said pump (105) comprises :
- a plunger (204) and
- a cam, driving said plunger (204) in said pump (105), said cam having a profile to hold plunger (204) at a constant lift position for a pre-defined angle of rotation of cam.
3. A fuel injection system (100) according to claim wherein said injector (106) is a common rail injector.
4. A fuel injection system (100) according to claim wherein said injector (106) is controlled by an engine control unit (108.
5. A fuel injection system (100) according to claim 2 wherein said plunger (204) has a helix (216) at top part of said plunger (204).
6. A fuel injection system (100) according to claim 2 wherein said plunger (204) has a helix (216 at bottom part of said plunger (204).
7. A fuel injection system (100) according to claim 2 wherein said plunger (204) is cylindrical.
8. A fuel injection system (100) according to claim 2 wherein said plunger (204) is rotated using a mechanical linkage.
9. A fuel injection system (100) according to claim 2 wherein said plunger (204) is rotated using a motor.
10. A fuel injection system (100) according to claim 2 wherein said plunger (204) is rotated in dependence of accelerator pedal position.
11. A fuel injection system (100) according to claim 1 wherein said pump (105) has a constant pressure valve to limit pressure at the high pressure line.
, Description:Field of the invention:
[0001] This invention relates to the field of fuel injection systems in general. The invention relates to a hybrid fuel injection system where a mechanical pump is used along with an electronically controlled fuel injector.

Background of the invention:
[0002] Fuel injection systems having electronic injectors is known in prior arts. The US patent 5394851 discloses an electronic fuel injection system for providing variable injection timing for a compression ignition engine having individual fuel delivery mechanisms, such as a fuel pump having a constant displacement fuel pumping chamber, and fuel injector for each corresponding cylinder, and has a timing signal generator, such as a multi-track optical encoder coupled to a camshaft, for generating a timing signal and a cylinder index signal.

Brief description of the accompanying drawings:
[0003] An embodiment of the disclosure is described with reference to the following accompanying drawing,
[0004] Fig. 1 illustrates a fuel injection system
[0005] Fig. 2 illustrates a pump used in the fuel injection system
[0006] Fig. 3 illustrates the curve representing the lift of the cam
[0007] Fig. 4 illustrates a pressure curve

Detailed description of the embodiments:
[0008] Shown in fig. 1 is a fuel injection system 100 comprising a fuel tank 102, a fuel filter 104, a pump 105, an injector 106 and an engine control unit (ECU) 108, apart from all the required sensors which are not shown. Here the pump 105 is a mechanical pump whose cut section is illustrated in Fig. 2. The injector 106 is an electronically controlled injector which may be a common rail injector controlled by the ECU 108. This system 100 is a hybrid fuel injection system where the pump 105 and the pump control logic are mechanical whereas the injector 106 and the injector controls are electronically operated. Here injector control refers to the ECU and the other sensors and actuators. The pump control logic refers to the mechanism to operate the pump 105. The injector 106 typically has a solenoid which is controlled by the ECU 108. The power supplied to the injector 106 is controlled by the ECU to inject the fuel.
[0009] FIG. 2 shows a cross section of a pump 105 used in any typical fuel injection system 100. The pump 105 consists of a barrel 202, a plunger 204, an inlet port 208, a spill port 212 and a delivery valve 214. The plunger 204 is driven by a cam which is not shown in fig. 2. The plunger 204 has a helix 216 cut on its top part. This helix may be also referred as top helix.
[0010] In another embodiment the helix may be at the bottom part of the plunger. In another embodiment the helix may not be present on the plunger or the plunger may not have any grooves on its surface.
[0011] The cam is driven by an engine which is not shown. The rotational orientation of the cam is synchronized with the engine firing point. This phase relationship is typically stored in the ECU 108 which controls the engine operations. The phase relationship is used for controlling injection timing. The cam moves the plunger 204 in the delivery direction (pointed upwards in the figure) and the plunger spring not shown, ensures the plunger 204 returns to the starting position by the end of each cycle. The top dead centre TDC is shown as upwards and the bottom dead centre BDC is shown as downwards in fig. 2. The bore 202 has the inlet port 208 for drawing the fuel from the fuel tank into the pump chamber 210. The pump 105 has the spill port 212 through which the excess fuel returns to the tank 102. The pressurized fuel is delivered through the delivery valve 214.It is also possible that the pump does not have the spill port 212. In such cases, the pump manages the excess fuel through the inlet port 208 itself.
[0012] In addition to the spill port, the chamber NNN is connected to a constant pressure valve (CPV) which is not shown in fig. The CPV is a mechanical valve which opens when the pressure in the high pressure line reaches a predefined threshold. The CPV guides the excess fuel back to the chamber, thereby maintaining the pressure in the high pressure line below the predefined pressure. The high pressure line refers to the pipe connecting pump and the injector.

[0013] The operation of the pump is explained below with the help of fig. 1 and fig. 2 together.
[0014] As the plunger travels towards BDC, the helix 216 uncovers the inlet port 208 and fuel from the inlet port 208 begins to flow into the pump chamber 210. Once the plunger 204 reaches BDC, it starts travelling towards TDC. The point at which bottom edge of the helix 216 on the plunger 204 closes the inlet port 208 on its upward motion, marks the beginning of the pressure generation phase. As the plunger 204 continues to move upwards towards TDC, the fuel pressure increases, the delivery valve 214 opens, thereby delivering the pressurized fuel through delivery valve 214. As the plunger 204 moves towards TDC, the fuel pressure keeps increasing and reaches a maximum value when the plunger reaches TDC. After this point is the dwell period where the pressure is held at a peak pressure because of cam profile. Hence, the end of pressurization is dependent on the plunger stroke, thus, delivering required fuel to the fuel injection system and raising the injection pressure. On the return stroke, the delivery valve 214 closes because pressure inside the pump chamber 210 is lesser than the fuel pressure on the injector side. As the plunger 204 travels downwards, the helix 216 uncovers the inlet port 208 again and fuel from the pump inlet begins to flow into the pump chamber 210. The fuel pressure in pump chamber 210 stabilizes at the pump inlet pressure. The plunger travel between the points at which the inlet port 208 is closed and TDC position of plunger is called the effective stroke of the pump 105. The position of the helix 216 along the circumference can be altered by rotating the plunger 204 using a regulating sleeve which is not shown in fig. This alters the effective stroke and the start of delivery and therefore the pressure developed by the pump 105 during a given cycle. The regulating sleeve is mechanically linked to the load linkage of the engine or the accelerator pedal of the engine. Thus by operating the accelerator pedal, the plunger 204 is rotated. It is also possible that the plunger 204 is rotated using a motor. The motor may be controlled by the ECU 08 based on the accelerator pedal position..

[0015] The top helix 216 of the plunger 204 ensures that there is the possibility of a variable pressure build up based on the load requirement. Furthermore, the top helix 216 ensures that the pressure build up in the system always ends at the same pump phase irrespective of the position of the helix 216. The cam design is such that there is provided a dwell, C-D as shown in fig. 3, once the plunger reaches TDC. The cam design, top helix 216 on the plunger 204 and constant pressure at the pump outlet work together and result in a characteristic pressure fluctuation inside the system. The advantages of the system are: It is possible to vary injection quantity and injection timing independent of engine operating point. Start and duration of injection can be controlled through a purely mechanical system.
[0016] Fig. 3 shows an indicative lift vs cam angle variation of the system. The X axis represents cam angle and Y axis represents lift ‘h’ of the plunger. As the cam begins to rotate, the lift of the plunger 204 initially increases from the zero lift position to the maximum lift position A. The cam is designed such that the lift of the plunger is maintained at the maximum lift level A for a short range of cam angle positions, represented by line between A and B. This region is called the dwell. On the completion of the dwell, the return stroke starts where the plunger returns to the zero lift position.
[0017] Shown in fig. 4 are 2 pressure curves. The X axis represents the time t or the angle of rotation of cam. The Y axis represents pressure P at the delivery valve 214. The two curves E and F correspond to two different accelerator pedal positions which correspond to two different loads of the engine. The curve E is for higher accelerator pedal position whereas the curve F is for relatively lower accelerator pedal position. At higher accelerator pedal position, the plunger 204 rotates to a position with no helix which leads to higher amount of fuel being trapped in element chamber. Hence the higher amount of fuel compression at higher accelerator pedal position leads to higher injection pressures. Hence the pressure at the delivery valve 214 will be higher at higher accelerator pedal position compared to the pressure at lower position.

[0018] Fuel pressure increase at the delivery valve 214 depends on the lift period after the closure of the inlet port 208 of the pump 105. Once the plunger 204 reaches the maximum lift position, there is no further fuel compression by the plunger 204. As a result, the pressure increase also stops at the instant the plunger reaches the maximum lift position as shown by the line between the points C and D. For different helix positions, the effective stroke of plunger varies. As a result, the final pressure reached by the time the plunger 204 reaches the maximum lift position also varies. However, the cam angle position at which pressure increase stops is independent of helix position and is characterized by the cam profile design.
[0019] The point where the pressure is held constant (C-D) is referred as dwell. There is no pressure variation during the dwell period as there is no plunger movement relative to the cam. The duration of the dwell may vary from 10 degrees to 200 degrees of the cam rotation. In one case, when the dwell is designed for 10 degrees, the pressure is held constant for 10 degrees of cam rotation. When the dwell is designed for 200 degrees, the pressure is held constant for 200 degrees of cam rotation. The dwell is achieved by having cam circumference being at equidistant from rotational axis of cam for the dwell duration angle. To achieve a dwell duration of 200 degrees, the cam profile will have 200 degrees out of 360 degrees of its circumference at equidistance from cam rotational axis. This duration of the angle at which the pressure is held constant is predefined and cam is designed accordingly.
[0020] It can be seen that the pressure inside the injector is constant for a short period in every cycle. This happens during the dwell period of the plunger motion. The constant high pressure region provides a time window in every cycle within which the injection can be varied irrespective of the engine operating points. The cam drive of the system is designed to be synchronized with engine such that the constant high pressure zone occurs when the engine piston reaches the firing top dead center.
[0021] In a conventional common rail system without this invention, the ECU 108 takes into account the engine speed, engine load, cam phasing, fuel temperature and rail pressure to control the injected quantity precisely. In a full-fledged common rail system, the logic for corrections on injected quantity are based on the output of these sensors.
[0022] The present invention makes use of pre-defined maps and hence eliminates the need to use a pressure sensor. A map converts the control rod position on the pump to the corresponding accelerator pedal position. Based on this accelerator position, a second map between injection pressure and accelerator position, computes the injection pressure which finally leads to quantity being picked up from a third map of injection pressure with energizing time (ET). Based on this, the electronic injector opens and closes for this pre-defined ET and hence injects a specified quantity which also depends on the through flow of the nozzle and no. of holes. Such maps are pre-defined for certain fuel temperatures and for any other fuel temperature, the algorithm interpolates the available data. These smart functions, in totality, allow the system to be priced competitively, without losing out on the flexibility of a common rail system.
[0023] The driver presses the accelerator pedal which indicates the torque request and hence amount of fuel to inject into the engine. The accelerator pedal position will change the helix position, thereby varying the pressure achieved at the delivery valve. The ECU 108 can determine the pressure depending upon the accelerator pedal position. Depending upon the pressure, the ECU 108 will compute the injector energizing time to inject required amount of fuel.
[0024] By having the hybrid configuration, the fuel injection system 400 provides a low cost system which meets requirements of varying the fuel injection pressure in dependence of the engine speed. By having a mechanical pump and an electronic injector, we are getting the advantage of simple low cost fuel injection system with pressure control for varying engine speed.