US20040101418A1

US20040101418A1 - Fuel pump

Info

Publication number: US20040101418A1
Application number: US10/305,248
Authority: US
Inventors: David Roth; David Fiddes; Christopher Glaspie
Original assignee: DaimlerChrysler Co LLC
Current assignee: Old Carco LLC
Priority date: 2002-11-27
Filing date: 2002-11-27
Publication date: 2004-05-27

Abstract

A fuel pump for an engine may include a cylinder, a piston displaceably disposed in the cylinder, and a cam driven by the engine. The cam engages the piston to form a cam-follower mechanism so that the rotation of the cam generates a reciprocal movement of the piston inside the cylinder. The cam preferably includes a variable cam lobe having a maximum radius that is a function of engine speed so that the displacement of the piston generated by the variable cam lobe is a function of the engine speed. The cam may also include a weight that is subject to a centrifugal force when the ram is rotating. Alternatively, the maximum radius of the variable cam lobe may be a function of pump outlet pressure so that the displacement of the piston generated by the variable cam lobe is a function of the pump outlet pressure.

Description

FIELD OF THE INVENTION

The present invention relates to a fuel pump, in particular to a fuel pump having a variable displacement.

BACKGROUND OF THE INVENTION

One of the problems associated with conventional fuel pumps is that they cannot rapidly pressurize the fuel system during both very cold (−20 to −40° F.) and hot-soak restart conditions, unless they are drastically oversized with respect to all other operating conditions. The cause of this phenomenon is that the fuel pumps are inefficient at a low pump speeds, such as below 90 RPM. The efficiency of the fuel pumps is about 30% at low speed but is more than 90% at a high speed. Since most conventional fuel pumps are driven by engine cam-shafts and are operating at one half of the engine speed, they frequently operate in the low-efficiency range. Typically, the worst condition is encountered at cold startup temperatures, which can cause the engine idle speed to drop as low as 80 RPM and the pump speed to 40 RPM.

The problem is especially profound with respect to direct injection spark ignition engines. The advantages of the direct injection spark ignition are lower cold start hydrocarbons and better engine start times resulting from better fuel preparation. Thus the fuel preparation system must operate properly during startup conditions to achieve these advantages. The high pressure fuel injectors, which are better than today's PFI fuel injectors from a fuel atomization standpoint, depend on higher fuel pressures to provide the level of atomization needed to produce better cold start results.

SUMMARY OF THE INVENTION

The present invention solves this problem of the conventional fuel pump by providing a fuel pump with a displacement that varies with one or more engine parameters.

In accordance with one aspect of the invention, a fuel pump for an engine may include a cylinder, a piston displaceably disposed in the cylinder, and a cam driven by the engine. The cam engages the piston to form a cam-follower mechanism so that the rotation of the cam generates a reciprocal movement of the piston inside the cylinder. The cam preferably includes a variable cam lobe having a maximum radius that is a function of engine speed. As a result, the displacement of the piston generated by the variable cam lobe is also a function of engine speed.

A pivotable arm pivotably connects a weight to the cam and is connected to the variable cam lobe. When the cam is rotating, the weight extends radially outwards under the centrifugal force to vary the maximum radius of the variable cam lobe. The cam may further include spring biasing the weight radially inwards to counter the centrifugal force.

When the variable cam lobe is at the extended position, a stop, by engaging at least one of the arm, the weight and the variable cam lobe, prevents the variable cam lobe from being pushed beyond the extended position by the centrifugal force.

The variable cam lobe may have only an extended position and a retracted position. When the engine speed is below a given engine speed value, the variable cam lobe is at the extended position, and when the engine speed is above a given engine speed value, the variable cam lobe is at the retracted position. Alternatively, the maximum radius of the variable cam lobe may be adjusted continuously between the fully retracted and fully extended positions.

In accordance with another aspect of the invention, a fuel pump for an engine may include a cylinder, a piston displaceably disposed in the cylinder, and a cam driven by the engine. The cam engages the piston to form a cam-follower mechanism so that the rotation of the cam generates a reciprocal movement of the piston inside the cylinder. The cam preferably includes a variable cam lobe having a maximum radius that is a function of pump outlet pressure so that the displacement of the piston generated by the variable cam lobe is a function of the pump outlet pressure. Preferably, the maximum radius of the variable cam lobe decreases as the pump outlet pressure increases.

When the pump outlet pressure is below a given value, the variable cam lobe is at the extended position, and when the pump outlet pressure is above the given value, the cam lobe is at the retracted position.

The present invention has a number of advantages over conventional fuel pumps. For example, since the cam of the pump has a variable cam lobe whose maximum radius varies with engine speed or fuel pressure, the pump does not have to be oversize for high-speed operations. As a result, energy is not wasted pumping unneeded fuel at high-speed operations, and engine efficiency is improved. In addition, the size of the fuel pump can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an embodiment of the present invention having a fixed cam lobe and a variable cam lobe with the variable cam lobe in an extended position. [0012]
FIG. 2 is a schematic drawing of the embodiment of FIG. 1 with the variable cam lobe in a retracted position. [0013]
FIG. 3 is a schematic drawing of another embodiment of the present invention having two fixed cam lobe and two variable cam lobes with the variable cam lobes in an extended position. [0014]
FIG. 4 is a schematic drawing of the embodiment of FIG. 3 with the variable cam lobes in a retracted position. [0015]
FIG. 5 is a schematic drawing of a further embodiment of the present invention having a variable cam lobe, the position of which is controlled by fuel pressure.[0016]

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate an embodiment of the fuel pump of the present invention. The [0017] fuel pump 10 shown in FIGS. 1 and 2, which may be used with an engine, such as an automotive engine, includes a cylinder 20, a piston 30 disposed in the cylinder 20, and a cam 40 driven by the engine.
The [0018] cylinder 20 shown in FIGS. 1 and 2 may be connected to an inlet fuel line 22 and an outlet fuel line 24. The inlet fuel line 20 includes a one-way valve 26 that allows only inflow, and the outlet fuel line 24 includes a one-way valve 28 that allows only outflow. As the piston 30 moves upwards, the one-way valve 26 in the inlet fuel line 22 opens to allow fuel to flow into the cylinder 20 from a fuel tank or from a primary fuel supply pump, and the one-way valve 28 in the outlet fuel line 24 closes to prevent the fuel from flowing back into the cylinder 20. As the piston 30 moves downwards, the one-way valve 28 in the outlet fuel line 24 opens to allow the fuel to be pumped to, for example, a fuel injector, and the one-way valve 26 in the inlet fuel line 22 closes to prevent the fuel from being pumped back into the fuel tank or the primary fuel supply pump.
In the embodiment shown in FIGS. 1 and 2, the [0019] piston 30 is connected to a cam follower 32 via a piston rod 34. The cam follower 32 preferably is pushed against the cam 40 by a spring (not shown) or the like to ensure the cam follower 32 and the cam 40 are in contact. As the cam 40 rotates, the cam 40 and spring push the cam follower 32 and piston 30 back and forth to pump fuel.
The [0020] cam 40 shown in FIGS. 1 and 2 has a fixed cam lobe 42 and a variable cam lobe 44, although a cam of the present invention may include more than one variable cam lobe and may have no fixed cam lobe. Each cam lobe has a maximum radius, which is defined as the point on the cam lobe which is farthest from the cam's center of rotation. In certain cases, the maximum radius of the cam lobe corresponds to the position of the piston at an end of its stroke. The maximum radius of a fixed cam lobe is constant, while the variable cam lobe can extend and retract to change its maximum radius. Preferably, the variable cam lobe 44 shown in FIGS. 1 and 2 has only two positions, i.e., fully retracted and fully extended positions, although in some other embodiments the maximum radius of the variable cam lobe may be adjusted continuously between the fully retracted and fully extended positions.
The [0021] cam 40 shown in FIGS. 1 and 2 also includes a weight 46 that is connected to the variable cam lobe 44 by an arm 48 that is pivotably connected to the cam 40. As the arm 48 pivots, the variable cam lobe 44 moves between the retracted position, as shown in FIG. 2, and the extended position, as shown in FIG. 1. The weight 46 generates a centrifugal force when it rotates with the cam 40, and the centrifugal force tends to pivot the arm 48 to push the variable cam lobe 44 towards the retracted position.
The [0022] cam 40 shown in FIGS. 1 and 2 also includes a spring 50, and the spring 50 biases the variable cam lope 44 towards the extended position. In other words, the spring force tends to push the variable cam lobe 44 towards the fully extended position. In the embodiment shown in FIGS. 1 and 2, this means that the spring 50 is in tension, although the spring 50 may be placed in a position so that it is compressed. Preferably, the spring 50 has a pretension when the variable cam lobe 44 is at the fully extended position. The pretension of the spring 50 may be selected so that when the centrifugal force is below a given value (or when the engine speed is below a given value), the pretension of the spring 50 is able to overcome the centrifugal force and keep the variable cam lobe 44 in the extended position. When the centrifugal force exceeds the given value (or when the engine speed exceeds the given value), the centrifugal force is able to overcome the spring force and push the variable cam lope 44 into the fully retract position. Alternatively, the weight 46 and the characteristics of the spring 50 can be selected so that the position of the variable cam lobe 44 can be adjusted continuously between the fully retracted and fully extended positions.
In the embodiment shown in FIGS. 1 and 2, the [0023] cam 40 may also have a stop 52. Preferably, when the variable cam lope 44 is at the extended position, one of the variable cam lope 44, arm 48 and weight 46 rests against the stop 52 to prevent the variable cam lope 44 from being pulled beyond the extended position by the spring 50. Additionally, the cam 40 may have another stop 54 that prevents the variable cam lope 44 from being pushed beyond the fully retracted position by the centrifugal force.
The [0024] cam 40 may be driven by the engine crankshaft or, in most cases, by the engine camshaft, which rotates at one half of the crankshaft speed.
In operation, during the startup phase or when engine speed is low, the centrifugal force generated by the [0025] weight 46 is not able to overcome the pretension of the spring, 50, and the variable cam lope 44 is in the extended position, as shown in FIG. 1. Therefore, for every rotation of the cam 40, the fixed and variable cam lopes 42, 44 displace the piston 30 twice to pump fuel. When the engine speed exceeds a preset threshold, the centrifugal force generated by the weight 46 is able to overcome the pretension of the spring 50, and the variable cam lope 44 is placed in the retracted position, as shown in FIG. 2. Therefore, for every rotation of the cam 40, only the fixed cam lope 42 displaces the piston 30 to pump fuel. In other words, the displacement (i.e. the capacity) of the fuel pump 10 is reduced at high engine speed.
The difference between the different pump displacements at high and low engine speeds is determined by the difference between the maximum radii of the fixed and [0026] variable cam lobes 42, 44. Therefore, this difference between the different pump displacements can be adjusted by selecting the maximum radii of the fixed and variable cam lobes 42, 44.
FIGS. 3 and 4 illustrate another embodiment of the fuel pump of the present invention. The [0027] fuel pump 110 shown in FIGS. 3 and 4 also includes a cylinder 20, a piston 30 disposed in the cylinder 20, and a cam 140.
The [0028] cylinder 20 and piston 30 are identical to those shown in FIGS. 1 and 2 and therefore will be not described in connection with this embodiment.
The [0029] cam 140 shown in FIGS. 3 and 4 has two fixed cam lobes 142 and two variable cam lobes 144. The variable cam lobes 144 may have only two positions, i.e., fully retracted and fully extended positions, or they may be adjusted continuously between the fully retracted and fully extended positions.
The positions of the [0030] variable cam lobes 144 are adjusted by rotating a second cam 156. The second cam 156 has a slotted arm 158 and a pin 160, which is attached to a pivotable arm 148 and is slideably disposed in the slot 162 of the slotted arm 158. The pivotable arm 148 is attached to a weight 146. As the pivotable arm 148 pivots, the pin 160 slides in the slot 162 of the slotted arm 158 and rotates the second cam 156. As the cam 140 rotates, the weight 146 generates a centrifugal force, and the centrifugal force tends to pivot the pivotable arm 148 to retract the variable cam lobes 144.
The [0031] cam 140 shown in FIGS. 3 and 4 also includes a spring 150, and the spring force acts against the centrifugal force of the weight 146 to pivot the pivotable arm 148 to extend the variable cam lobes 144. Preferably, the spring 150 has a pretension when the variable cam lobes 144 are at the fully extended position. The pretension of the spring 150 may be selected so that when the centrifugal force is below a given value (or when the engine speed is below a given value), the pretension of the spring 150 is able to overcome the centrifugal force and keep the variable cam lobes 144 in the extended position. When the centrifugal force exceeds the given value (or when the engine speed exceeds the given value), the centrifugal force is able to overcome the spring force and push the variable cam lopes 144 into the fully retract position. Alternatively, the weight 146 and the characteristics of the spring 150 can be selected so that the positions of the variable cam lobes 144 can be adjusted continuously between the fully retracted and fully extended positions.
In the embodiment shown in FIGS. 3 and 4, the [0032] cam 140 may also have a stop 152 to prevent the variable cam lope 144 from being pulled beyond the extended position by the spring 150. Additionally, the cam 140 may have another stop 154 that prevents the variable cam lopes 144 from being pushed beyond the fully retracted position by the centrifugal force.
FIG. 5 illustrates a further embodiment of the fuel pump of the present invention. The [0033] fuel pump 210 shown in FIG. 5 includes a cylinder 20, a piston 30 disposed in the cylinder 20, and a cam 240.
The [0034] cylinder 20 and piston 30 are identical to those shown in FIGS. 1 and 2 and therefore will be not described in connection with this embodiment.
The [0035] cam 240 shown in FIG. 5 has a fixed cam lobe 242 and a variable cam lobe 244, although the cam may include more than one variable cam lobe and may have no fixed cam lobe. The variable cam lobe 244 shown in FIG. 5 may only two positions, i.e., fully retracted and fully extended positions, although the maximum radius of the variable cam lobe 244 may be adjusted continuously between the fully retracted and fully extended positions.
The [0036] cam 240 shown in FIG. 5 includes a piston 264 connected to the variable cam lobe 244 by a piston rod 266. The piston 264 is slidably disposed in the cam 240. One side of the piston 264, such as the radially outward side 268 of the piston 264 as shown in FIG. 5, is in fluid communication with the pressurized fuel output from the pump 210. This can be carried out by, for example, connecting this side of the piston 264 with the fuel outlet 24 of the pump 210. On the other side of the piston, such as the radially inward side 270 of the piston 264, a spring 272 pushes the piston 264 radially outwards against the force generated by the pressurized fuel.
In operation, when the fuel pressure is too low, for example when the engine is in the startup phase or when the engine speed is low, the spring force is able to overcome the force generated by the pressurized fuel to push the [0037] variable cam lobe 244 radially outwards to the extended position. On the other hand, when the fuel pressure is within the operating range, the force generated by the pressurized fuel is able to overcome the spring force to push the variable cam lobe 244 radially inwards to the retracted position.

Claims

1. A fuel pump for an engine, the fuel pump comprising:

a cylinder;

a piston displaceably disposed in the cylinder; and

a cam driven by the engine, the cam engaging the piston to form a cam-follower mechanism so that the rotation of the cam generates a reciprocal movement of the piston inside the cylinder, the cam including a variable cam lobe having a maximum radius that is a function of engine speed so that the displacement of the piston generated by the variable cam lobe is a function of the engine speed.

2. The fuel pump of claim 1, further comprising:

a weight that is subject to a centrifugal force when the ram is rotating;

a pivotable arm that pivotably connects the weight to the cam and is connected to the variable cam lobe, so that the weight extends radially outwards under the centrifugal force to vary the maximum radius of the variable cam lobe.

3. The fuel pump of claim 2, further comprising:

a spring biasing the weight radially inwards to counter the centrifugal force.

4. The fuel pump of claim 2, further comprising:

a stop for engaging at least one of the arm, the weight and the variable cam lobe when the variable cam lobe is at the extended position, wherein the stop maintains the variable cam lobe at the extended position.

5. The fuel pump of claim 2, wherein the variable cam lobe has only an extended position and a retracted position, wherein when the engine speed is below a given engine speed value the variable cam lobe is at the extended position, and wherein when the engine speed is above a given engine speed value the variable cam lobe is at the retracted position.

6. The fuel pump of claim 1, wherein the variable cam lobe has only an extended position and a retracted position, wherein when the engine speed is below a given engine speed value the variable cam lobe is at the extended position, and wherein when the engine speed is above the given engine speed value the variable cam lobe is at the retracted position.

7. The fuel pump of claim 2, wherein the variable cam lobe is a first cam lobe, and the pump further comprising a second cam lope, wherein the second cam lope has a fixed maximum radius.

8. The fuel pump of claim 1, wherein the variable cam lobe is a first cam lobe, and the pump further comprising a second cam lope, wherein the second cam lope has a fixed maximum radius.

9. A fuel pump for an engine, the fuel pump comprising:

a cylinder having an inlet and an outlet;

a piston displaceably disposed in the cylinder; and

a cam driven by the engine, the cam engaging the piston to form a cam-follower mechanism so that the rotation of the cam generates a reciprocal movement of the piston inside the cylinder, the cam including a variable cam lobe having a maximum radius that is a function of pump outlet pressure so that the displacement of the piston generated by the variable cam lobe is a function of the pump outlet pressure.

10. The fuel pump of claim 9, wherein the maximum radius of the variable cam lobe decreases as the pump outlet pressure increases.

11. The fuel pump of claim 10, further comprising:

a spring biasing the variable cam lobe against the pump outlet pressure.

12. The fuel pump of claim 9, wherein the variable cam lobe has only an extended position and a retracted position, wherein when the pump outlet pressure is below a given value the variable cam lobe is at the extended position, and wherein when the pump outlet pressure is above the given value the variable cam lobe is at the retracted position.

13. The fuel pump of claim 9, wherein the variable cam lobe is a first cam lobe, and the pump further comprising a second cam lope, wherein the second cam lope has a fixed maximum radius.