Abstract:
A fuel injection system for internal combustion engines with a pressure step-up means is proposed, in which the fuel is pumped out of the low-pressure supplier into the injection nozzle via a feed line and is measured correctly in terms of time and quantity via a control line by the high-pressure part of the injection pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 00/03242 filed on Sep. 19, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fuel injection system for internal combustion engines and to a method for injecting fuel into the combustion chamber of an internal combustion engine. 
     2. Description of the Prior Art 
     As exhaust gas standards become more and more stringent, ever higher injection pressures are demanded to improve mixture formation and combustion. The result is greater mechanical and thermal stresses on the fuel injection system. In addition, the demand for drive capacity increases disproportionately, since with the pressure, the losses in the fuel injection system rise as well. 
     In a fuel injection system known from German Published, Nonexamined Patent Application DE-OS 197 38 804, a pressure step-up means is connected between the injection pump and the injection nozzle. As a result, the full injection pressure is applied only in the region around the injection nozzle. Fuel supply is effected through a bypass, directly from the highpressure region of the injection pump. When the pressure is increased in the injection pump, but also upon flow through the bypass, the fuel heats up greatly, which has negative impacts on the compressibility of the fuel and on its density. 
     The object of the invention is to furnish a fuel injection system in which the thermal stress on the injection pump is reduced and the possible pressure increase rates in the fuel injection system are improved. In addition, higher injection pressures should be made possible, and at the same time the stress on the injection pump and demand for drive power of the injection pump should be reduced. 
     SUMMARY OF THE INVENTION 
     According to the invention, the above objects are attained by a fuel injection system for internal combustion engines, having an injection nozzle and having an injection pump that has a high-pressure part, the high-pressure part of the injection pump being operatively connected to the injection nozzle via a control line, communicating with a low-pressure side of a pressure step-up means, and via a highpressure path communicating with a high-pressure side of the pressure step-up means, and a feed line is present which feeds fuel to the injection nozzle. A first check valve disposed in the feed line prevents the reverse flow of fuel from the injection nozzle into the feed line, and the feed line communicates with a low-pressure supplier. 
     This fuel injection system has the advantage that the injection pressure is applied only between the high-pressure side of the pressure step-up means and the injection nozzle. At the same time, the pressure forces acting on the injection pump are reduced. As a result, the leakage and throttling losses are reduced as well, which leads to a reduction in the demand for drive capacity and improves the hydraulic efficiency of the fuel injection system. Furthermore, the fuel in the high-pressure region of the fuel injection system remains comparatively cold, since it is delivered directly from the low-pressure part of the injection pump. As a result, the compressibility of the fuel is less, which results in an improved pressure increase rate in the fuel injection system, and a greater mass flow can be pumped through the injection nozzle. In addition, the thermal and hydraulic improvement of the fuel injection system makes smaller injection port diameters of the injection nozzle possible, which improves the mixture formation at all operating points. 
     In one embodiment of the invention, the pressure step-up means has a step-up piston, which is displaceable in a bore and whose end faces each define one pressure chamber, whose first, larger end face of the step-up piston defines a first pressure chamber communicating with the control line, and whose second, opposed, smaller end face of the step-up piston defines a second pressure chamber, communicating with the high-pressure path. 
     In a further embodiment of the invention, it is provided that the feed line communicates with the second pressure chamber, so that the fuel is introduced in the part of the high-pressure region farthest away from the injection nozzle and is pumped from there as far as the injection nozzle. This has the advantage that in the highpressure region of the fuel injection system, the fuel is continuously replaced with relatively cold fuel. 
     Another embodiment provides that a first check valve is disposed in the feed line and prevents the reverse flow of fuel from the injection nozzle into the feed line, so that the low-pressure supplier of the injection pump is not acted upon by the injection pressure. 
     In one embodiment of the invention, the first check valve is spring-loaded, so that with maximum reliability under all operating conditions, the reverse flow of fuel from the injection nozzle into the feed line is prevented. 
     A further embodiment provides that the change in cross section of the step-up piston and a shoulder in a housing of the pressure step-up means define a relief chamber, so that possible leakage losses of the pressure step-up means can be collected and carried away. 
     In another embodiment of the invention, it is provided that the relief chamber communicates through a connecting line with the part of the feed line that is located between the low-pressure supplier and the first check valve, so that the leakage from the pressure step-up means is returned to the fuel injection system. 
     In one embodiment of the invention, a restoring spring is fastened in the relief chamber, is braced on a stationary support and in the process acts on the step-up piston at the change in cross section toward the relief chamber and as a function of the standing pressure in the control line, the end faces of the step-up piston, and the opening pressure of the first check valve presses the step-up piston against its stop toward the pump between injections, so that when the control line is pressure-relieved, the step-up piston is brought quickly, and independently of the pressure in the feed line, to its outset position. Moreover, the restoring spring requires only little installation space. 
     In a further embodiment of the invention, in the connecting line between the relief chamber and the feed line, a second check valve is provided, which blocks the communication in the direction from the feed line to the relief chamber, so that the feed line is not excited by the pressure fluctuations in the relief chamber. 
     A further embodiment of the invention provides that a scavenging valve designed as a check valve with a blocking direction from the control line to the feed line is disposed between the control line and the feed line, so that as soon as the pressure in the control line drops below the pressure in the feed line, filling of the control line is achieved through the scavenging valve. This leads to a drop in the temperature level in this region as well and thus improves the hydraulic performance of the fuel injection system and lessens the danger of seizing in the injection pump. 
     Further in the invention, it is provided that the scavenging valve does not open until an adjustable pressure difference between the control line and the feed line is reached, so that the motion of the step-up piston to its outset position is supported in this version as well by the pressure in the feed line, and the filling of the control line in the region between the injection pump and the pressure step-up means, which is difficult above all at high rpm, is assured since at high rpm the pressure in the feed pump is high as well. 
     A further embodiment of the invention provides that the part with the larger end face of the step-up piston upon which the pressure of the control line acts when the step-up piston is resting on its stop toward the pump is larger than the smaller end face of the step-up piston; that in the feed line between the first check valve and the injection pump, a third check valve with the same blocking direction is provided; and that between the control line and the first and third check valves, a connecting line with a fourth check valve with a blocking direction from the feed line to the control line is provided, so that at the beginning of injection, bypassing the pressure step-up means, fuel is pumped from the high-pressure part of the injection pump directly into the injection nozzle. As a result, the pressure increase rate changes at the onset of injection, and as a result the combustion noise can be abated, and furthermore it becomes easier to meter small preinjection quantities by means of provisions taken at the pump. 
     In a further feature of the invention, it is provided that the third and fourth check valves are combined into a bypass valve, so that the number of components is reduced, thus reducing expenses. 
     In another variant of the invention, the low-pressure supplier is part of the injection pump, so that the number of component groups is reduced, and a drive is required only for the high-pressure part of the injection pump and for the low-pressure supplier. 
     Further in the invention, a step-up piston embodied in two parts is provided, thus improving production, assembly and the hydraulic properties of the injection system. 
     In a further embodiment of the invention, it is provided that at least two injection nozzles are present; that one control line and one pressure step-up means each are disposed between each injection nozzle and the injection pump; and that all the injection nozzles communicate with the low- pressure supplier via feed lines. 
     The object stated at the outset is also attained by a method for injecting fuel into the combustion chamber of an internal combustion engine, by means of the fuel injection system in which 
     a pressure relief of the control line takes place between injections; 
     fuel is pumped out of the low-pressure supplier via the feed line to the injection nozzle; 
     the step-up piston is moved to its stop toward the pump; and 
     the fuel injection is controlled by the high-pressure part of the injection pump. 
     In this method, the full injection pressure is applied only directly in front of the injection nozzle; the maximum injection pressure is increased, and at the same time the load on the injection pump from pressure forces and temperatures is reduced. Furthermore, because of the reduced leakage and reduced throttling losses, the hydraulic efficiency of the system is improved and thus the requisite drive capacity is reduced further. The low temperature makes steeper pressure increases possible, because of the reduced elasticity of the fuel, and for the same pumping quantity, a higher flow rate through the nozzle is made possible. The thermal and hydraulic improvements in the fuel injection system allow smaller injection port diameters of the injection nozzles and thus better mixture formation at all operating points. 
     In a modification of the method of the invention, it is provided that until an adjustable pressure difference between the control line and the high-pressure side of the pressure step-up means is reached, the fuel injection is controlled, bypassing the pressure step-up means, by the high-pressure part of the injection pump; and that above the adjustable pressure difference between the control line and the high-pressure of the pressure step-up means, the fuel injection is controlled by the high-pressure part of the injection pump with the aid of the pressure step-up means. This method has the advantage that because of the different injection rate at the onset of injection, combustion noise is abated, and the metering of small preinjection quantities becomes easier through provisions made in the pump. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and advantageous features of the invention can be learned from the ensuing description, taken with the drawings, in which: 
     FIG. 1 is a schematic illustration of a first embodiment of the fuel injection system of the invention; 
     FIG. 2 is a schematic illustration of a second embodiment of the fuel injection system of the invention; 
     FIG. 3 is a schematic illustration of a third embodiment of the fuel injection system of the invention; 
     FIG. 4 is a schematic illustration of a fourth embodiment of the fuel injection system of the invention; and 
     FIG. 5 is a schematic illustration of a combination of various embodiments of a fuel injection system of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail, FIG. 1 shows a fuel injection system with an injection nozzle  1  and an injection pump  3 ; the pump has a high- pressure part  5  and a low-pressure supplier  7 . The low-pressure supplier  7  can also be embodied as a separate pump from the injection pump  3 . In the exemplary embodiments described below, the low-pressure supplier  7  and high- pressure part  5  of the injection pump  3  are always shown as a unit. However, a version is also always conceivable in which the aforementioned separation of the low-pressure supplier  7  and injection pump  3  exists. 
     The high-pressure part  5  is operatively connected to the injection nozzle  1  via a control line  9  and a high- pressure path  10 . A pressure step-up means  11  is disposed between the control line  9  and the high-pressure path  10 . The pressure step-up means  11  has a first pressure chamber  13 , a second pressure chamber  15 , a step-up piston  17  is guided in a bore  18 , and a relief chamber  19 . The step-up piston  17  can be embodied in one or two parts. Two-part step-up pistons  17  comprise a first piston, which has the diameter of the first pressure chamber  13  of the pressure step-up means  11 , and a further piston, which has the diameter of the second pressure chamber  15  of the pressure step-up means  11 . The hydraulic force acting on the first piston is transmitted directly or indirectly to the second piston. 
     Two-part step-up pistons  17  can have advantages over one-part step-up pistons  17  in terms of production, assembly and hydraulic properties. 
     The first pressure chamber  13  and the end face, protruding into the first pressure chamber  13 , of the step-up piston  17  form the low-pressure side of the pressure step-up means  11 . The second pressure chamber  15  and the end face, protruding into the second pressure chamber  15 , of the step-up piston  17  form the high-pressure side of the pressure step-up means  11 . 
     Since the end face of the step-up piston  17  that is in hydraulic communication with the high-pressure part  5  of the injection pump  3  is larger than the end face of the step-up piston  17  protruding into the second pressure chamber  15 , the pressure in the second pressure chamber  15  is higher than that of the high-pressure part  5  of the injection pump  3 , in accordance with the proportion of the two end faces of the step-up piston  17 . 
     The relief chamber  19  is defined by a change in cross section  20  of the step-up piston  17  and by a shoulder in a housing of the pressure step-up means  11 . 
     Via a feed line  21 , the second pressure chamber  15  is filled between injections with fuel from the low-pressure supplier  7  of the injection pump  3 . Once both the second pressure chamber  15  and the high-pressure path  10  have been filled with fuel, the injection event can take place, in that the high-pressure part  5  of the injection pump  3  begins to pump fuel. In the pressure step-up means  11 , the pressure is increased, and with this increased pressure the injection of the fuel into the combustion chamber takes place through the injection nozzle  1 . 
     So that the feed line  21  and the low-pressure supplier  7  of the injection pump  3  will not be acted upon by the pressure of the second pressure chamber  15 , a first check valve  23  is disposed in the feed line  21 . The first check valve  23  can be spring-loaded, as shown in FIG. 1, or can be embodied without a spring, as suggested for instance in FIG.  2 . 
     The high-pressure region of the fuel injection system of the invention is accordingly limited, in FIG. 1, to the region to the right of the step-up piston  17  and above the first check valve  23 . This subject matter has been indicated by the dashed lines. 
     The leakages that occur between the step-up piston  17  and the housing of the pressure step-up means  11  accumulate in the relief chamber  19  and are transferred upon each injection event into the feed line  21  via a connecting line  25 . 
     Once the injection has taken place, the step-up piston  17  moves back into its outset position. This occurs because the control line  9  is pressure-relieved, for instance via the high-pressure part  5  of the injection pump  3 , and the step-up piston  17  is acted upon in the second pressure chamber  15  and the relief chamber  19 , via the feed line  21 , by the pressure of the low-pressure supplier  7  of the injection pump  3 . Since the pressure in the feed line  21  is higher than the pressure in the pressure-relieved control line  9 , the step-up piston  17  in FIG. 1 moves to the left, against its stop toward the pump. The pressure relief need not proceed as far as a lowering of the pressure to ambient pressure; instead, it can be provided that a standing pressure, which is above ambient pressure, is maintained during the pressure relief as well. A restoring spring can also be provided in addition in the relief chamber  19 . 
     In FIG. 2, a second embodiment of the fuel injection system of the invention is shown. For component groups or components of the fuel injection system that are the same, the same reference numerals as in FIG. 1 have been used. The embodiment of FIG. 2 has a restoring spring  27  in the relief chamber  19  that acts on the step-up piston  17  counter to the injection motion. The restoring spring is fastened between a change in cross section  20  of the step-up piston  17  and a shoulder of the bore  18  or of the housing. By way of example, the restoring spring  27  can coaxially surround the step-up piston  17 . 
     In the connecting line  25 , there is a second check valve  29  in this embodiment, which prevents fuel from feed line  21  from reaching the relief chamber  19 . After the end of injection, the step-up piston  17  is moved to its stop toward the pump by the pressure of the feed line  21  in the region of the second pressure chamber  15  and by the restoring spring  27 . Because of the blocking action of the second check valve  29 , vapor pressure prevails in the relief chamber  19  upon this motion of the step-up piston  17 . Leakage flows that reach the relief chamber  19  from the first or second pressure chamber  13  or  15  are expelled via the second check valve  29  upon the injection. 
     An advantage of this embodiment is that the feed line  21  is not subjected to pressure fluctuations that are due to the oscillating motion of the step-up piston  17 . Furthermore, because of the supporting effect of the pressure in the feed line  21 , the restoring spring  27  can be designed with reduced prestressing and a lower spring rate and thus in a space-saving way. 
     In FIG. 3, a further embodiment of the fuel injection system of the invention is shown. In addition to the elements and component groups of the fuel injection system explained in conjunction with FIGS. 1 and 2, in this embodiment a scavenging valve  31  is disposed between the control line  9  and the feed line  21 . The scavenging valve  31  is spring-loaded, so that it opens when a pressure difference determined by the spring of the scavenging valve  31 , such as  15  bar, between the feed line  21  and the control line  9  is reached. Once this pressure difference is reached, fuel is pumped from the feed line  21  into the control line  9 . The thus-improved filling and scavenging of the control line  9  has the advantage over the embodiment of FIGS. 1 and 2 that in this region of the fuel injection system, the temperature level is lowered by the delivery of relatively cold fuel, thus improving the hydraulic performance. Furthermore, the risk of seizing in the high-pressure part  5  of the injection pump  3  is reduced, since this part of the injection system can be better scavenged as well. In addition, filling of the control line, which is especially difficult at high engine rpm, is assured. 
     FIG. 4 shows a further embodiment of the fuel injection system of the invention. The pressure step-up means  11 , in its first pressure chamber  13 , has a pressure stage component  33 . This pressure stage component  33  has the task of assuring that the step-up piston  17  leaves the stop toward the pump only when a certain pressure difference between the control line  9  and the second pressure chamber  15  is reached. This function can be achieved for instance by providing that the pressure stage component  33  covers part of the end face of the step-up piston  17 , protruding into the first pressure chamber  13 , once the step-up piston  17  is in its outset position, the remaining area of the step-up piston  17  being larger than the end face of the step-up piston  17  that protrudes into the second pressure chamber  15 . By the choice of the proportions of these areas and the prestressing of the restoring spring  27 , the pressure difference up to which the high-pressure side of the step-up piston  17 , acted upon by the pressure of the feed line  21 , and the restoring spring  27  keep the step-up piston  17  in its outset position counter to the pressure of the control line  9  is defined. 
     Also shown in FIG. 4 are a third and a fourth check valve  35  and  37 . The third check valve  35  is disposed in the feed line  21  between the first check valve  23  and the injection pump  3 , and it has the same blocking direction as the first check valve  23 . The fourth check valve  37  is disposed in a connecting line  39  between the control line  9  and the feed line  21 . The blocking direction of the fourth check valve  37  is selected such that no fuel can be fed from the feed line  21  into the control line  9  through the connecting line  39 . The connecting line  25  branches off from the feed line  21  between the low-pressure supplier  7  and the third check valve  35 . 
     The cooperation of the pressure stage component  33  and of the third and fourth check valves  35  and  37  has the effect that at the onset of injection, when the pressure in the control line  9  rises, first the fuel, under pressure, in the control line  9  is fed, bypassing the pressure step-up means  11 , through the connecting line  39  and part of the feed line  21  into the second pressure chamber  15  and from there is fed to the injection nozzle  1 . As soon as the force, resulting from the difference in the effective areas of the pressure stage component  33  and the end face of the step-up piston  17  protruding into the second pressure chamber  15 , and from the prestressing of the restoring spring  27 , suffices to overcome the prestressing of the restoring spring  27 , the step-up piston  17  moves out of its outset position. Thus the full end face of the step-up piston  17  protruding into the first pressure chamber  13  is acted upon by the pressure of the control line  9 . As a consequence, the pressure step-up means becomes operative, since the first check valve  23  prevents the further inflow of fuel via the feed line  21  into the second pressure chamber  15 . The bypassing of the pressure step-up means  11  at the onset of the injection cycle changes the rate of pressure increase of the fuel in the second pressure chamber  15  and thus in the injection nozzle  1  as well. As a result, the metering of small preinjection quantities through provisions taken in the pump is made easier, and the combustion noise can be abated. 
     The embodiment of the fuel injection system of the invention shown in FIG. 5 represents the combination of the embodiments shown in FIGS. 2,  3  and  4 . The intent is to show clearly that these embodiments can be combined freely with one another. This is equally true to the embodiment shown in FIG.  1 . 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.