Patent Publication Number: US-6712047-B2

Title: Method for determining the rail pressure of an injector having a piezoelectrical actuator

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for determining the rail pressure of an injector including a voltage-controlled piezoelectrical actuator. 
     BACKGROUND INFORMATION 
     Conventionally, in an injector including a piezoelectrical actuator, the motion of the nozzle needle is not driven directly but via a hydraulic coupler. One task of the coupler is to reinforce the stroke of a control valve. For correct functioning, however, the hydraulic coupler must be completely charged, especially since in every driving of the piezoelectrical actuator a portion of the fluid is squeezed out of the hydraulic coupler through leakage gaps. In this context, the recharging occurs in the pause between two injections. In order to release a predetermined quantity of fluid in the high-pressure channel, it is necessary to know the pressure in the high-pressure channel. This pressure may be measured by an appropriate sensor, which is arranged in the high-pressure line system (common rail system) at an appropriate location. In this context, the problem may arise that an erroneous rail pressure measurement may result from the failure of the pressure sensor. Due to the incorrect rail pressure measurement, it is then no longer assured that the predetermined injection quantity will actually be released. This may be critical especially in a motor-vehicle including an internal combustion engine, if the predetermined quantity of fuel is not injected. The result may be abrupt disruptions in functioning and potentially the shutdown of the internal combustion engine. Furthermore, undesirable, large injection quantities may also occur. 
     SUMMARY OF THE INVENTION 
     In contrast, the method according to the present invention for determining the rail pressure of an injector including a voltage-controlled piezoelectrical actuator may provide the advantage that the pressure in the high-pressure channel of the injector is measured by measuring the induced piezovoltage. The result is a redundant pressure measurement, which makes it possible to monitor the measured value of the pressure sensor. 
     It may be advantageous that, using an algorithm, for example, in the form of a linear equation or a table, it is possible to reach conclusions regarding the prevailing rail pressure on the basis of the measured piezovoltage. In this manner, it is possible to obtain an electrical characteristic quantity that is assigned to the rail pressure and that may easily be further processed by the electronics. 
     By comparing the calculated rail pressure with the measured value of the pressure sensor, it is possible, in a manner, to monitor the normal functioning of the pressure sensor. If the pressure sensor fails, for example, as a result of a line break or a fault, then the redundant measured value may be retrieved for emergency operation in maintaining the functioning of the internal combustion engine. 
     In the case of a fault, it may be advantageous to store the measured voltage values or the pressure value, so that the event may be reconstructed at a later time point. This may be especially important for an internal combustion engine that includes a common rail injection system, to assure operating reliability. 
     An example embodiment of the present invention is illustrated in the drawings and is discussed in greater detail in the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic representation of an injector including a piezoelectrical actuator. 
     FIG. 2 illustrates an allocation diagram. 
     FIG. 3 illustrates a voltage diagram. 
     FIG. 4 illustrates a block diagram. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1, in a schematic representation, shows an injector  1  including a central bore. In the upper part of the bore, a piezoelectrical actuator  2  is introduced, at whose lower end an operating piston  3  is mounted. Operating piston  3  stops a hydraulic coupler  4  towards the top, the coupler including an opening towards the bottom including a connecting channel to a first seat and a control valve  5  including a sealing member  12  arranged in the coupler. In this context, sealing member  12  is configured so that it seals first seat  6 , if actuator  2  is in the resting phase, i.e., if no drive voltage U a  is applied to it. When actuator  2  is actuated by the application of drive voltage U a  at clamps +, −, actuator  2  actuates operating piston  3  and, via hydraulic coupler  4 , pushes control valve  5  including sealing member  12  in the direction of second seat  7 . Arranged below second seat  7  in a corresponding channel is a nozzle needle  11 , which closes or opens the outlet for high-pressure channel  13 , for example, a common rail system, depending on the level of drive voltage U a  and pressure P 1  that are applied in the high-pressure area. The high pressure is conveyed via a supply line  9  by the medium to be injected, for example, fuel for an internal combustion engine. Via a supply-line throttle  8  and an outlet throttle  10 , the inflow quantity of the medium is controlled in the direction of nozzle needle  11  and hydraulic coupler  4 . In this context, hydraulic coupler  4  is configured, on the one hand, to intensify the stroke of piston  5  and, on the other hand, to decouple control valve  5  from the static temperature expansion of actuator  2 . 
     The dimensioning of hydraulic coupler  4  is such that the latter is refilled by a pressure derived from the rail pressure, specifically when sealing member  12  is positioned on first seat  6 . This may be realized, for example, as a constant transmission ratio. If this transmission ratio is, for example, 1:10, then the pressure in hydraulic coupler  4  is only {fraction (1/10)} of the rail pressure. 
     In what follows, the mode of functioning of injector  1  is discussed in greater detail. In response to each driving of actuator  2 , operating piston  3  moves in the direction of hydraulic coupler  4 . In this context, control valve  5  including sealing member  12  also moves in the direction of second seat  7 . In this context, a portion of the medium in hydraulic coupler  4 , for example, the fuel, is squeezed out through a leakage gap. Thus, between two injections, hydraulic coupler  4  must be refilled, to maintain its functional reliability. A coupler  4  that is empty or only partially filled has the effect that nozzle needle  11  may not release high-pressure channel  13  for the injection of the preestablished quantity of fluid, so that injection misfires may arise. 
     As described above, a high pressure predominates in supply line channel  9  amounting, in the common rail system, for example, to between 200 and 1600 bar. This pressure pushes against nozzle needle  11  and holds it closed against the pressure of an undepicted spring, so that no fuel may escape. If, as a consequence of drive voltage U a , actuator  2  is actuated and therefore sealing member  12  moves in the direction of the second seat, then the pressure in the high-pressure area declines and nozzle needle  11  releases the injection channel. After drive voltage U a  is withdrawn, hydraulic coupler  4  is once again refilled. 
     For the injection of fuel into an internal combustion engine, especially in direct injection, the fuel quantity to be injected should be determined as a function of the engine conditions and driving conditions of the vehicle. Determining the injection quantity should be accomplished as precisely as possible for each actuation of nozzle needle  11 , in order to achieve an optimal combustion in the cylinder of the internal combustion engine with respect to exhaust gas emission requirements, fuel economy, and performance spectrum. Therefore, the instantaneous pressure may be measured using a pressure sensor that is arranged at an appropriate location in the high-pressure system of the common rail lines, and the instantaneous pressure is made available to an appropriate control unit as a measured value. Because this pressure sensor should operate very reliably, the present invention provides that a further pressure measurement be performed, which is redundant with respect to the measurement of the pressure sensor. This second pressure measurement is performed using the piezovoltage that is induced in piezoelectrical actuator  2 , the piezovoltage arising as a result of the pressure in hydraulic coupler  4  and is measurable at actuator  2 . On account of the fact that the coupler pressure, assuming complete charging, is a function of the rail pressure, the instantaneous rail pressure may be derived from the induced voltage. In this context, this induced voltage U i  functions as a further (redundant) measuring signal for the pressure prevailing in high-pressure channel  13 . For the pressure measurement, the control unit now receives two measured values, which make it possible, on the one hand, to monitor the measuring signal of the pressure sensor. On the other hand, in the event of the failure of the pressure sensor, induced voltage U i  may be used to assure emergency operation of the internal combustion engine. 
     FIG. 2 illustrates an allocation diagram, in which voltage U i , induced in actuator  2 , is plotted on the y-axis and pressure P 1 , measured by pressure sensor D for the high-pressure line system, is plotted on the x-axis. The curve U i =f(P 1 ) indicates the relationship between the two cited variables. Illustrated is a linear equation 
     
       
         
           P 
           1 
           =a*U 
           i 
           +b, 
         
       
     
     a is the slope as a proportionality factor and b is an offset value. This curve may be used as an algorithm, alternatively to a table, which may be advantageously determined empirically. 
     FIG. 3 illustrates a segment of a typical voltage diagram in which voltage U i , applied at actuator clamps +, −, is plotted as a function of time. Initially, coupler  4  is filled by time point t 1 , and the measured voltage corresponds to voltage U i  that is induced by the coupler pressure. 
     After time point t 1 , a driving occurs, in which the actuator is initially charged and, at a later time point, is once again completely discharged. In this context, coupler  4  is also emptied accordingly. However, due to the coupler pressure, a voltage U i  is induced. The latter rises at a given gradient, because in this time period coupler  4  is once again filled, until it has reached its setpoint filling, i.e., until the static coupler pressure is built up. 
     To determine the high pressure, it may be advantageous to measure induced voltage U i  at time point t 1 . Derived from this measured value, in accordance with the aforementioned algorithm, is corresponding high-pressure P 1 , which is compared to the measured value of pressure sensor D. In event of a deviation between measured high-pressure P 1  and comparison value U i  beyond a preestablished threshold value, a check is performed as to whether a fault exists in the high-pressure system itself, or whether there is a fault in pressure sensor D. In the event of a fault in pressure sensor D, the pressure value from induced voltage U i  is used for generating drive voltage U a . Using this redundant measurement, it is therefore possible to maintain emergency operation for the fuel injection in an internal combustion engine. 
     FIG. 4 illustrates a block diagram for generating the pressure value from piezovoltage U i , measured at time point t 1 . The algorithm for the conversion is stored in a transformation unit  40 . This algorithm may contain the function P 1 =f(U i (t 1 )) according to FIG. 2 or an appropriate table. The output signal for pressure P 1  then functions as a plausibility check for the measured rail pressure, or as a replacement value for the rail pressure in the event of a fault.