This disclosure concerns a method for determining the force conditions at the nozzle needle of a directly driven piezo injector.
Fuel injection systems of the latest generation usually work on the common rail principle and often contain injectors driven piezo-electrically. Here one or more such piezo injectors, which can be opened and closed in a targeted manner, are provided at each combustion chamber of the internal combustion engine. When the injectors are open, fuel enters the interior of the combustion chamber and combusts there. To ensure good combustion and exhaust emissions, and for comfort reasons, the injected fuel quantity should be determined as precisely as possible.
WO 2009/010374 A1 discloses a method and a device for forming an electrical control signal for an injection pulse of a fuel injector. This electrical control signal activates a piezo-electric actuator to inject a predefined fuel quantity into a cylinder of an internal combustion engine. Using the curve of the electrical control signal, an injection rate of the fuel injector is regulated as a function in particular of the rail pressure, the stroke travel and/or the opening duration of the fuel injector. For at least a partial fuel quantity to be injected, the curve of the electrical control signal can be freely formed in relation to at least one pulse flank and/or amplitude. The form of the injection pulse is structured such that the predefined fuel quantity for injection is held constant irrespective of the curve of the electrical control signal.
When forming the rate curve for the fuel, it is important to maintain the injection quantities required by the internal combustion engine for mixture formation within tight tolerances in order to influence the emissions and fuel consumption of the respective motor vehicle in the desired manner.
One essential aspect in the forming of the rate curve is the so-called part-stroke operation. Here the nozzle needle is held in a middle position between the nozzle seat (injector closed) and the end stroke position (injector opened to the maximum) of the nozzle needle in order to influence the fuel flow through the nozzle and hence the mixture formation.
In practice there is a problem in setting and achieving the said part stroke precisely, in that the injection quantity required by the internal combustion engine can be guaranteed as an integral of the fuel flow through the nozzle, which is dependent on the injection nozzle needle stroke. This problem arises because in part-stroke operation, component tolerances of the injector under different ambient conditions (pressure, temperature) in operation of the injector in an internal combustion engine, because of the steepness of the flow curve of the nozzle, over the needle stroke, have a tendentially greater effect than is the case in full-stroke operation of the injector.
In internal combustion engines, the benefits of forming the rate curve and its influence on emissions have been primarily studied on internal combustion engines in which the cylinder pressure, various temperatures and sometimes also the needle stroke are monitored by means of external sensors. Use of such sensors is costly and is not therefore applied in motor vehicles for cost reasons.
One embodiment provides a method for determining a force acting on the nozzle needle of a directly driven piezo injector, wherein by means of a charging process an electrical voltage is built up at the piezo actuator which drives the nozzle needle, wherein at the end of the charging process, the voltage present at the piezo actuator is measured again, a voltage gradient is determined from consecutive voltage values, and conclusions about the force acting on the nozzle needle can be drawn from the voltage gradients.
In a further embodiment, using the voltage gradient determined, a database is addressed in which a force value is allocated to each of a plurality of voltage gradients.
In a further embodiment, conclusions about the stroke of the nozzle needle can be drawn from the force value determined.
In a further embodiment, using the force value determined, a database is addressed in which a stroke value is allocated to each of a plurality of force values.
In a further embodiment, conclusions about the fuel flow can be drawn from the nozzle needle stroke.
In a further embodiment, using the stroke value determined, a database is addressed in which a fuel flow value is allocated to each of a plurality of stroke values.
In a further embodiment, conclusions about the fuel quantity injected can be drawn from the fuel flow value.
In a further embodiment, the conclusions about the fuel quantity injected can be drawn by forming an integral of the fuel flow value.
Example embodiments are discussed below with reference to the figures, in which:
Embodiment of the present invention provide a method for determining the force acting on the nozzle needle of a directly driven piezo injector.
According to one embodiment, a method is provided for determining the force acting on the nozzle needle of a directly driven piezo injector, wherein during the opening process and in a part-stroke operation, by means of a charging process an electrical voltage is built up at the piezo actuator which drives the nozzle needle, and wherein after the end of the charging process, the voltage present at the piezo actuator is measured again, a voltage gradient is determined from consecutive voltage values, and conclusions about the force acting on the nozzle needle can be drawn from the voltage gradients.
The information determined about the force acting on the nozzle needle can advantageously be used to draw conclusions about the stroke of the nozzle needle. Knowledge of the stroke of the nozzle needle in turn allows the fuel flow through the piezo injector to be determined. Finally the fuel quantity injected can be determined from the fuel flow by integral formation. This in turn allows a precise setting of a part-stroke operation, in order at the end of the work cycle, to guarantee the injection quantity required by the internal combustion engine as an integral of the fuel flow through the nozzle, which is dependent on the injection nozzle needle stroke, although in this operating mode the component tolerances in the injector and different ambient conditions in operation of the injector in the internal combustion engine, because of the steepness of the flow curve of the nozzle, over the needle stroke, have a tendentially greater effect than in a full-stroke operation.
The piezo actuator 1 includes a plurality of individual thin layers which expand on application of an electrical voltage, i.e. they translate an applied electrical voltage into mechanical work or energy. Conversely, mechanical influences on the piezo actuator provoke electrical signals which can be measured. The achievable expansion of a piezo actuator is dependent on parameters which include its nominal length, the number of layers, the quality of polarization achieved and the ratio of its active area to its total area. When a piezo actuator is charged, it remains in its achieved expansion for the duration of the injection concerned.
The exemplary embodiment shown in
The nozzle body 9 expands temperature-dependently. The purpose of the nozzle needle spring 7 is to hold the nozzle needle 8 in its seat. Said expansion of the nozzle body 9 in the direction of its longitudinal axis, the so-called nozzle elongation, influences the maximum needle stroke. The rail pressure predominating in the rail (not shown) also causes an elongation of the nozzle body and a compression of the nozzle needle.
In a needle opening process, first the piezo actuator 1 is charged by the application of current. After overcoming the idle stroke, the expansion of the piezo actuator 1 is transmitted via the pin 2 to the bell 4, wherein the pin 2 is guided in the lever housing 3. The bell 4 presses symmetrically on both sides on the lever 5 which forms a lever pair. These levers roll on the intermediate disc 6 in the manner of a rocker. The respective contact point of the two levers lies in a notch in the nozzle needle 8.
Due to the mechanism described above, the axial compression force of the piezo actuator 1 is transmitted to the nozzle needle 8. The nozzle needle is lifted from its seat as soon as the lever force is greater than the sum of the spring force and the hydraulic force, and the elasticity of the nozzle body 9 no longer ensures that the nozzle seat follows the nozzle needle.
After a defined travel, the needle stop hits the intermediate disc. A contact force is built up which acts back on the piezo actuator 1.
With such piezo actuators 1 it is possible to raise the nozzle needle 8 only partially from its seat and hold it in a so-called part stroke. The opened flow cross section between the nozzle needle and the nozzle body is here smaller than the sum of the cross sections of all nozzle bores.
a shows the curve of the voltage U present at the piezo injector during the part-stroke operation as a function of the time t, for several different voltages present at the piezo injector. The considerations below relate to the voltages U1 and U2 shown in
It is clear from
If a voltage U2 is present at the piezo actuator during the charging process, then from time t0=0 the voltage at the piezo actuator rises up to a maximum value M2 which is lower than the maximum value M1. After reaching the maximum M2, the voltage value of voltage U2 remains at the same voltage value which corresponds to the maximum value M2.
In some embodiments, the curves shown in
To this end, the voltage is measured after the end of the charging process i.e. when maximum value M1 or M2 is reached. A voltage gradient (see G1 and G2 in
b shows the curve of the force acting on the piezo actuator during part-stroke operation over time t, again for the multiplicity of different voltages present at the piezo injector. The force curve K1 shown in
A previously stored database contains data records which, for a predefined pressure value, allocate a force value to each of a plurality of voltage gradients. By means of the voltage gradients determined, consequently this database can be addressed to determine the associated force value.
The force values determined are preferably in turn used to address a further previously stored database. This further database in turn, for a predefined rail pressure value, allocates a value for the needle stroke to each of a plurality of force values.
This is illustrated in
Also in relation to the force-needle stroke pair, a database is provided in which, for a predefined value of the rail pressure, a stroke value is allocated to each of a plurality of force values. This database can then be addressed by means of a force value in order to determine an associated stroke value.
From this stroke value again conclusions can be drawn about an associated fuel flow or fuel flow rate. Thus
This association between needle stroke and flow rate is again found in a previously stored database in which, for a predefined value of rail pressure, an associated flow rate value is stored for each of a plurality of stroke values. By means of a rail pressure value, said database can be addressed to determine an associated flow rate value.
Finally from the flow rate value, by integral formation, conclusions can be drawn about the fuel quantity injected. Using these values for the injected fuel quantity, the part-stroke operation can be regulated to ensure that the desired fuel quantity is always injected. This in turn has the advantage that said part-stroke control with its emissions benefits can be used over the entire load and rotation speed range.
It should be understood that the method steps disclosed above may be performed by a controller including a processor and computer-readable logic stored in non-transitory memory and executable by the processor for performing any of the disclosed functionality.
Number | Date | Country | Kind |
---|---|---|---|
10 2011 005 934.2 | Mar 2011 | DE | national |
This application is a U.S. National Stage Application of International Application No. PCT/EP2012/053960 filed Mar. 8, 2012, which designates the United States of America, and claims priority to DE Application No. 10 2011 005 934.2 filed Mar. 23, 2011, the contents of which are hereby incorporated by reference in their entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP2012/053960 | 3/8/2012 | WO | 00 | 9/23/2013 |