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  
         [0001]    The present invention relates to a fuel pump, in particular to a fuel pump having a variable displacement.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    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&#39;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  
         [0004]    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.  
           [0005]    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.  
           [0006]    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.  
           [0007]    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.  
           [0008]    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.  
           [0009]    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.  
           [0010]    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.  
           [0011]    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  
       [0012]    [0012]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.  
         [0013]    [0013]FIG. 2 is a schematic drawing of the embodiment of FIG. 1 with the variable cam lobe in a retracted position.  
         [0014]    [0014]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.  
         [0015]    [0015]FIG. 4 is a schematic drawing of the embodiment of FIG. 3 with the variable cam lobes in a retracted position.  
         [0016]    [0016]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. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]    [0017]FIGS. 1 and 2 illustrate an embodiment of the fuel pump of the present invention. The 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.  
         [0018]    The 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.  
         [0019]    In the embodiment shown in FIGS. 1 and 2, the 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.  
         [0020]    The 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&#39;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.  
         [0021]    The 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.  
         [0022]    The 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.  
         [0023]    In the embodiment shown in FIGS. 1 and 2, the 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.  
         [0024]    The 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.  
         [0025]    In operation, during the startup phase or when engine speed is low, the centrifugal force generated by the 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.  
         [0026]    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 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 .  
         [0027]    [0027]FIGS. 3 and 4 illustrate another embodiment of the fuel pump of the present invention. The 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 .  
         [0028]    The 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.  
         [0029]    The 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.  
         [0030]    The positions of the 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 .  
         [0031]    The 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.  
         [0032]    In the embodiment shown in FIGS. 3 and 4, the 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.  
         [0033]    [0033]FIG. 5 illustrates a further embodiment of the fuel pump of the present invention. The fuel pump  210  shown in FIG. 5 includes a cylinder  20 , a piston  30  disposed in the cylinder  20 , and a cam  240 .  
         [0034]    The 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.  
         [0035]    The 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.  
         [0036]    The 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.  
         [0037]    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 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.