Patent Publication Number: US-6655355-B2

Title: Fuel injection system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fuel injection system for use in internal combustion engines. 
     2. Description of the Prior Art 
     For the sake of better comprehension of the description and claims, several terms will first be explained: The fuel injection system of the invention is embodied as pressure-controlled. Within the scope of the invention, a pressure-controlled fuel injection system is understood to mean that by means of the fuel pressure prevailing in the nozzle chamber of an injection nozzle, a nozzle needle is moved counter to the action of a closing force (spring), so that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the injection pressure, while the term system pressure is understood to mean the pressure at which fuel is available or is kept on hand inside the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term leakage is understood to be a quantity of fuel that occurs in operation of the fuel injection (such as a reference leakage or diversion quantity) that is not used for the injection and is returned to the fuel tank. The pressure level of this leakage can have a standing pressure, and the fuel is then depressurized to the pressure level of the fuel tank. 
     In common rail systems, the injection pressure can be adapted to both load and rpm. To reduce noise, a preinjection is often performed. To reduce emissions, a pressure-controlled injection is known to be favorable. 
     In pressure-controlled systems, a triangular injection course results in the main injection. The nozzle needle closes in response to the drop in pressure in the nozzle chamber. It has been demonstrated that a fast closure (rapid spill) of the nozzle needle is advantageous. This rapid closure can be attained in pressure-controlled fuel injection systems by means of a fast relief of the nozzle chamber. However, the pressure reduction should not proceed so fast that the injection pressure is already reduced while the nozzle needle is still open because of its inertia. That would cause a blowback of combustion gases into the nozzle chamber. By the reinforcement of the needle closure, the relief of the nozzle chamber can proceed more slowly, so that cavitation damage caused by overly rapid relief of the nozzle chamber is avoided. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The hydraulic reinforcement of the closing performance causes a fast pressure reduction in the nozzle chamber and thus faster closure of the nozzle needle. The closure, hydraulically reinforced according to the invention, of the pressure-controlled nozzle needle can also be employed for fuel injection systems with a pressure booster, for the sake of improved pressure reduction and refilling. It is advantageous to place the relief valve as close as possible to the nozzle chamber. Another advantage in terms of the closing performance is attained by having the diversion valve communicate not directly with the leakage line but rather via the spring chamber of the injection nozzle. To optimize the relief performance, a throttle can additionally be disposed at the outlet of the nozzle chamber. One additional valve for performing the hydraulically reinforced closure of the nozzle needle can be dispensed with, if for that purpose the diversion flow from the metering valve is used for the fuel injection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which: 
     FIG. 1 schematically illustrates a first fuel injection system according to the teaching of the invention; 
     FIG. 2 schematically illustrates a second fuel injection system according to the teaching of the invention; 
     FIG. 3 schematically illustrates a third fuel injection system according to the teaching of the invention; 
     FIG. 4 schematically illustrates a fourth fuel injection system according to the teaching of the invention; 
     FIG. 5 schematically illustrates a fifth fuel injection system according to the teaching of the invention; and 
     FIG. 6 illustrates the principle of a pressure-controlled fuel injection system in accordance with the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the prior art pressure-controlled fuel injection system  1  shown in FIG. 6, a quantity-controlled fuel pump  2  pumps fuel  3  from a tank  4  via a supply line  5  into a central pressure reservoir  6  (or common rail), from which a plurality of pressure lines  7 , corresponding to the number of individual cylinders, lead away to the individual injection nozzles  8 , protruding into the combustion chamber of the internal combustion engine to be supplied. Only one of the injection nozzles  8  is shown in detail in FIG.  6 . With the aid of the fuel pump  2 , a system pressure is generated and stored in the pressure reservoir  6 , at a pressure of from 300 to approximately 1800 bar. 
     Located in the region of the pressure reservoir  6  are metering valves  9 , which are embodied as 3/2-way magnet valves. With the aid of the metering valve  9 , the injection for each cylinder is achieved under pressure control. A pressure line  10  connects the pressure reservoir  6  to a nozzle chamber  11 . The injection takes place with the aid of a nozzle needle  12 , which is axially displaceable in a guide bore, and which has a conical valve sealing face  13  on one end with which it cooperates with a valve seat face on the housing of the injection nozzle  8 . Injection openings are provided on the valve seat face of the housing. Inside the nozzle chamber  11 , a pressure face  14  pointing in the opening direction of the nozzle needle  12  is subjected to the pressure prevailing there, which is delivered to the nozzle chamber  11  via the pressure line  10 . 
     After the opening of the metering valve  9 , a high-pressure fuel wave travels in the pressure line  10  to the nozzle chamber  11 . The nozzle needle  12  is lifted from the valve seat face counter to a restoring force, and the injection event can begin. 
     Upon termination of the injection and a closed communication between the nozzle chamber and pressure reservoir  6 , the pressure in the nozzle chamber  11  drops, because the pressure line  10  is connected to a leakage line  15 . The nozzle needle  12  begins its closing process. 
     In accordance with the invention, and in contrast to FIG. 6, FIG. 1 shows that instead of the 3/2-way valve  8 , two 2/2-way valves  16  and  17  are used in a fuel injection system  18 . The 2/2-way valve  16  takes on the metering of the high pressure from the pressure reservoir, while the 2/2-way valve  17  takes on the relief or diversion task. It is advantageous to place the relief valve  17  near the nozzle chamber  11 . The metering valve  16  can likewise be mounted in the nozzle holder. Both valves  16  and  17  can also be controlled by an actuator, for the sake of reducing effort and expense. Disposing the metering valve on the pressure reservoir  6  additionally enables an elevation in the injection pressure by utilizing the line oscillations. A decisive advantage with regard to the closing performance of the nozzle needle is now achieved because the relief valve  17  does not connect the pressure line  10  directly with a leakage line  19  but rather via a pressure chamber  20  of the injection nozzle  8 . This pressure chamber  20  communicates with the leakage line  19  via a throttled connection. Thus upon diversion of fuel from the pressure line  10 , a hydraulic overpressure occurs in the pressure chamber  20 , which hydraulically reinforces a nozzle spring  21  in the closing process. The result is a combination of stroke- and pressure-controlled closure. The closing time is shortened. A blowback of combustion gases into the injection nozzle is prevented. The spring chamber of the nozzle spring  21  can also be used as the pressure chamber  20 . The relief of the system after the injection is effected via the pressure chamber  20  and the leakage line  19 . 
     FIG. 2 shows the hydraulically reinforced closing process for a pressure-controlled fuel injection system  22 , which additionally has a pressure booster  23 . The use of the relief valve  17  in the pressure line  10  has an especially favorable effect here, because the pressure reduction on the high-pressure side of the pressure booster  23  takes place directly at the injection nozzle. To optimize the relief operation, a throttle  24 , which limits the pressure drop, is additionally disposed at the outlet of the nozzle chamber. The refilling of the pressure booster is accomplished on the basis of the pressure decrease on the high-pressure side. After the closure of the metering valve  16 , the pressure booster  23 , with the pressure line  10  relieved, fills again because of the compression spring in the idle volume and returns to its outset position. 
     From FIG. 3, it can be seen that in a fuel injection system  25 , a 3/2-way valve  26  is used as the metering valve. Once again, the closure of the nozzle needle  12  is effected with hydraulic reinforcement. The injection takes place under pressure control. For filling a pressure booster  27 , a check valve  28  is provided, which can be connected either to a pressure line  29  or to the fuel pump (the latter indicated by dashed lines). To achieve a hydraulically reinforced closure of the nozzle needle  12 , a closing piston  30 , which defines a pressure chamber  31 , is provided on the injection nozzle. The pressure chamber  31  can be subjected to pressure via a 2/2-way valve  32 . Via a throttle  33 , the pressure chamber  31  is pressure-relieved, with the valve  32  closed. A pressure face  34  is designed such that with the valve  32  open, a hydraulic force is generated, which forces a closure of the nozzle needle. The injection pressure in the nozzle chamber  11  is applied unchanged. By the closure of the valve  32 , the pressure chamber  31  can be relieved again, and the nozzle needle  12  opens again. A postinjection at high pressure then takes place. 
     In FIG. 3, the elevated pressure from the high-pressure chamber of the pressure booster is used to close the nozzle needle  12 . It is equally possible, given a suitable design of the pressure face  34 , also to use the pressure prevailing in the pressure reservoir  6  to close the nozzle needle  12 , as shown in FIG.  4 . In this fuel injection system  35 , a supply line  36  is provided between the valves  26  and  32 . Additional leakage through the valve  32  is prevented. 
     The exemplary embodiment of FIG. 5 avoids the disadvantage of using an additional valve  32 , by using the diversion flow from the metering valve  26  to close the nozzle needle  12 . FIG. 5 shows the fuel injection system  37 , with control of the metering by means of the 3/2-way valve  26 , and with an integrated, hydraulically reinforced closure of the nozzle needle  12  with the aid of the diversion flow. In this fuel injection system  37 , the relief flow from the pressure booster  27  is carried through the valve  26  into the pressure chamber  31  at the end of injection. This subjects the closing piston  30  to pressure. A hydraulically reinforced closure of the nozzle needle  12  is forced to happen. A new injection can then be effected by re-triggering of the metering valve  26 . A slow pressure reduction in the pressure booster and injection region can be achieved by means of a small flow cross section of a throttle  38 . Thus given a suitable design, without an additional valve  32  (see FIG.  4 ), a fast closure of the nozzle needle  12  and a postinjection at high pressure can be attained. The overlap of the opening cross section and the relief cross section, which often occurs in a 3/2-way valve, is no disadvantage in this fuel injection system  37 . A desired additional pressure buildup in the pressure chamber  31  is briefly achieved. 
     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.