Claims:We Claim:

1. A cam (108) to drive a plunger, said cam having a profile on cam surface, said profile comprising
- a first part (200) to assist the lift of said plunger with a first slope (206) of lift versus angular rotation of said cam (108)
- a second part (202) to assist the lift of said plunger with a second slope (208) of lift versus angular rotation of said cam (108)

2. A cam (108) to drive a plunger according to claim 1 wherein there is a constant lift maintained for said plunger between said first slope and said second slope
3. A cam (108) to drive a plunger according to claim 1 wherein said second slope starts immediately where first slope ends
4. A fuel pump (102) comprising
- a housing,
- a plunger reciprocally moving in said housing
- a cam (108) driving said plunger; said cam (108) having a profile on cam surface, said profile comprising a first part (200) to assist the lift of said plunger with a first slope (206) of lift of plunger versus angular rotation of cam; a second part (202) to assist the lift of said plunger with a second slope 208 of lift of plunger versus angular rotation of cam;

5. A fuel injection system (100) comprising:
- a pump (102) comprising a housing; a plunger reciprocally moving in said housing ; a cam (108) driving said plunger; said cam having a profile on cam surface, said profile comprising a first part (200) to assist the lift of said plunger with a first slope (206) of lift of plunger versus angular rotation of cam; a second part (202) to assist the lift of said plunger with a second slope (208) of lift of plunger versus angular rotation of cam;
- an injector (104) to receive pressurized fuel from said pump (102); said injector injecting pressurized fuel into an engine
6. A fuel injection system (100) according to claim1 where said pump (102) is a mechanical pump
7. A fuel injection system according to claim1 where said injector (104) is an electronic injector controlled by an electronic control unit (106)
, Description:Field of the invention
[001] This invention relates to the field of fuel injection systems in general. The invention relates to a fuel pump with a varying cam surface to effectively control the injection of fuel into an engine.

Background of the invention

[002] Typical diesel fuel injection systems in the prior arts use a common rail and an electronically controlled injector to inject fuel into an engine. The common rail is maintained at a required pressure and depending upon the engine operating conditions, the injector is operated to inject required quantity of fuel into the engine.

[003] The pumps supplying fuel to the common rail are driven by cams. Us patent application 20020096145 discloses a fuel injection system comprising a high pressure fuel pump. The high pressure fuel pump is driven by a cam having varying surface.

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 schematic of a fuel injection system.

[006] FIG. 2A and 2B illustrate a schematic of a cam with varying cam surface according to one embodiment of the invention.

[007] FIG. 3A and 3B illustrate a schematic of a cam with varying cam surface according to another embodiment of the invention.

Detailed description of the embodiments

[008] Shown in fig. 1 is a fuel injection system 100. The main components of the fuel injection system are a fuel pump102, a fuel injector 104 and an electronic control unit ECU 106. The fuel pump 102 is a high pressure pump. The fuel pump is just referred as pump. This pump 102 may be preceded by a low pressure pump. The pump 102 receives fuel from the fuel tank 110 and pressurizes the fuel. The pressurized fuel is delivered to the injector 104 to inject the fuel into an engine. Also shown in fig. 1 are a pressure sensor 110 to read fuel pressure, a temperature sensor 112 to read coolant temperature, engine sped sensor 114, accelerator load sensor 116 and a fuel filter 118.

[009]Here the pump 102 is a mechanical pump driven by a cam 108 which is driven by the engine. The injector is an electronic injector and is controlled by an electronic control unit ECU. The ECU controls the injector by opening or closing the injector for fuel injection. The ECU determines the quantity of fuel to be injected into the engine, based on various engine operating parameters like engine speed, engine load, fuel pressure, coolant temperature etc. Based on the quantity of fuel to be injected into the engine the ECU determines the energizing time for the injector. The ECU provides required power to the injector in terms of pulse width modulation for the duration of calculated energizing time. The injector valve opens and the fuel injection takes place. After the energizing time is over, the ECU power to the injector, the injector valve comes back to closed position thereby stopping the fuel injection.

[010]In typical prior art systems for diesel fuel injection, the fuel injection systems comprise a common rail where fuel is stored. The pump supplies the fuel to the common rail where it is stored at more or less a constant pressure. Common rail supplies the fuel to the injectors. As the pressure in the common rail is at much higher level than the operating pressure of the injectors there may not be any fluctuations while injecting.

[011] In the present invention, the fuel injection system is for low cost vehicles and hence the common rail is eliminated. The pump supplies the pressurized fuel to the injector directly. The pumps running with typical cam profile as in prior arts will have a lift period for the plunger, then a dwell period where the plunger is held at constant position and then a drop period of the plunger. During the lift period, the cam pushes the plunger thereby increasing the pressure. During the dwell period the cam is rotating but the plunger is held at a constant position because of the profile of the cam. This is to maintain the fuel pressure at constant level in the fuel supply path. During the drop period the cam rotates further and the plunger retards. Retarding of the plunger refers to the movement of the plunger towards the cam. The pressure in the fuel supply path decreases.

[012] This type of pump operation in the prior arts has a disadvantage that during the dwell period where the pressure in the fuel supply path is supposed to be constant, the pressure in actual scenario drops. This is because of the leakage in the injectors. This causes fluctuations in the injector operation.

[013] To overcome this the invention proposes a cam profile which eliminates the drop in the pressure during the dwell period.

[014] Fig 2A shows a cam profile according to one embodiment of the invention. The Fig. 2B shows a graph of lift of plunger versus the angular rotation of cam. The X axis represents angular rotation of cam and Y axis represents lift of plunger. The cam profile has a first part 200 to lift the plunger at a first rate 206 of lift, a second part 202 to lift the plunger with a second rate of lift 208 and then part to retard the plunger. The first rate of lift is also referred as a first slope 206 of lift of plunger versus angular rotation of cam. The second rate of lift is also referred as a second slope 208 of lift of plunger versus angular rotation of cam

[015] When the cam 108 rotates, it either pushes the plunger or allows the plunger to retard. When the cam pushes the plunger, the pressure of the fuel in the fuel supply path increases. When the plunger retards, the pressure of the fuel in the fuel supply path decreases. Fuel supply path is the path connecting pump to the injector.

[016] When the first part 200 of the cam profile is in contact with the plunger, the cam lifts the plunger with a first rate of lift 206, thereby pressurizing the fuel. The lift of the plunger in this period has a first slope 206 corresponding to a first angle. When the plunger is in contact with first part 200 of the cam profile, it is referred as first lift period. When the second part 202 of the cam profile is in contact with the plunger, the cam lifts the plunger with a second rate of lift 208, thereby pressurizing the fuel further. The lift of the plunger in this period has a second slope 208 corresponding to a second angle. The first angle is steeper than the second angle. When the plunger is in contact with second part 202 of the cam profile, it is referred as second lift period.

[017] The injector may start injecting when the plunger reaches end of first lift period. As the lift of the plunger continues further till the end of the second lift period, the pressure keeps increasing. This compensates for any leakage in the injectors and the injection of fuel continues smoothly.

[018] Fig 3A shows cam profile according to another embodiment of the invention. The Fig. 3B shows a graph of lift of plunger versus the angular rotation of cam. The X axis represents angular rotation of cam and Y axis represents lift of plunger. The cam profile shown in fig. 3A has a first part 200, a first dwell part D1, a second part 202, a second dwell part D2 and a drop part. This type of cam profile is useful for multiple injections of fuel in one cycle. It may be a pre-injection, main injection and a post injection. Pre-injection may occur just before the main injection and post injection may occur just after main injection. The drop part refers to the period when the pressure starts decreasing. This is caused when the plunger starts retarding. In this embodiment, the cam pressurizes fuel, holds it constant for D1 period, then again pressurizes the fuel, holds it constant for D2 period and then retards. This is shown in fig. 3B.

[019] The invention has advantage that the common rail is eliminated from the fuel injection system thereby reducing drastically the cost of the fuel injection system. The variations in the pressure during the injection are eliminated by having a second lift period where pressure in the fuel supply path increases.