This application is a U.S. National Stage Application of International Application No. PCT/EP2013/055519 filed Mar. 18, 2013, which designates the United States of America, and claims priority to DE Application No. 10 2012 204 252.0 filed Mar. 19, 2012, the contents of which are hereby incorporated by reference in their entirety.
The present invention relates to a method for operating a fuel injection system, which has a pressure reservoir (rail), at least one injection valve, in which a piezoelectric actuator actuates a servo valve arranged in a servo valve space counter to the force of a closing spring so that a closure element opens an injection opening connected to the pressure reservoir by a fuel line, and a feedforward and feedback control unit.
Fuel injection systems with which fuel injection into a combustion chamber of an internal combustion engine is performed have long been known. Injection systems of this kind comprise at least one injection valve (injector) and at least one feedforward and feedback control unit, connected to the injection valve, for controlling the injection process. Here, the injection valve has a space from which fuel can be injected into the combustion chamber through an injection opening. The opening and closing of the injection opening is performed by means of a closure element (nozzle needle), which can be actuated (moved) by an actuator. The space is supplied with fuel via a high-pressure reservoir and a fuel line.
The actuator is an element for moving the closure element. Thus, an injection process is controlled with the aid of the actuator. At the same time, the actuator is not in direct drive connection with the closure element but actuates a servo valve in order to discharge fuel under high pressure from a servo valve space and, in this way, to bring about actuation of the closure element and to open the associated injection opening.
The actuator is a piezoelectric actuator which expands (increases in length) by virtue of the piezoelectric effect when supplied with electrical energy and in this way raises the servo valve from its seat in order thereby to actuate the closure element.
In order to carry out a pressure reduction in the pressure reservoir (rail pressure reduction) in such fuel injection systems with piezoelectric servo injection valves, special pressure control valves (PLV, PCV, PDV) are used in prior art systems. These additional valves increase the costs of the overall system. In another procedure, a pilot-controlled servo valve is used in reducing the pressure in the pressure reservoir. With a pilot-controlled servo valve of this kind, however, it is not always possible to reduce the pressure in the pressure reservoir in the desired manner, and therefore it is nevertheless necessary to provide additional valves for pressure reduction, depending on customer requirements and the injection valve design.
One embodiment provides a method for operating a fuel injection system of an internal combustion engine, which has a pressure reservoir, at least one injection valve, in which a piezoelectric actuator actuates a servo valve arranged in a servo valve space counter to the force of a closing spring so that a closure element opens an injection opening connected to the pressure reservoir by a fuel line, and a feedforward and feedback control unit, wherein the piezoelectric actuator used has a passive piezoelectric region as a force sensor in addition to the active piezoelectric region used to actuate the servo valve; the force acting on the passive piezoelectric region when the servo valve is opened, and, from said force, the pressure in the servo valve space, is determined with the aid of this force sensor, taking into account the closing spring force; and the active piezoelectric region is activated in such a way if a pressure reduction is required in the pressure reservoir that a pressure reduction occurs through the opening of the servo valve without a servo valve space pressure corresponding to opening of the closure element being reached during this process.
In a further embodiment, a pressure reduction is carried out in a phase in which no injection is taking place.
In a further embodiment, the limiting pressure Pst_limit in a control space for the closure element, which the pressure in the control space must not undershoot so as to avoid opening the closure element, is determined from the actual pressure in the pressure reservoir (rail pressure) Prail_ist.
In a further embodiment, the setpoint control space pressure P_st_s is determined in accordance with the setpoint rail pressure Prail_s and with the actual rail pressure Prail_ist and is limited in a downward direction by the limiting pressure Pst_limit in the control space.
In a further embodiment, the setpoint pressure for the valve space P_v_s is determined from the setpoint control space pressure P_st_s and the actual rail pressure Prail_ist.
In a further embodiment, the servo valve is moved by activating the active piezoelectric region until the actual valve space pressure P_v_ist has reached the setpoint pressure for the valve space P_v_s, after which the valve space pressure is adjusted to P_v_s by activating and deactivating the active piezoelectric region.
In a further embodiment, the fuel injection system has a plurality of injection valves, wherein, in the case in which the injection valve currently being used for pressure reduction is supposed shortly afterwards to carry out an injection process, other injection valves, which are currently not injecting, are used for the pressure reduction.
In a further embodiment, the pressure reduction is continued until the rail pressure reaches the setpoint thereof, after which the servo valve or the servo valves are closed again by discharging the piezoelectric actuator or piezoelectric actuators.
Another embodiment provides a fuel injection system for an internal combustion engine, which has a pressure reservoir, at least one injection valve, in which a piezoelectric actuator actuates a servo valve arranged in a servo valve space counter to the force of a closing spring so that a closure element opens an injection opening connected to the pressure reservoir by a fuel line, and a feedforward and feedback control unit, wherein it is configured to perform a method as described above.
In a further embodiment, the passive piezoelectric region is formed by an additional, serially arranged, passive piezoelectric layer.
Example embodiments of the invention are explained in detail below with reference to the drawings, in which:
Embodiments of the present invention provide a method for operating a fuel injection system at particularly low cost.
In some embodiments, the piezoelectric actuator used has a passive piezoelectric region as a force sensor in addition to the active piezoelectric region used to actuate the servo valve; the force acting on the passive piezoelectric region when the servo valve is opened, and, from said force, the pressure in the servo valve space, is determined with the aid of this force sensor, taking into account the closing spring force; and the active piezoelectric region is activated in such a way if a pressure reduction is required in the pressure reservoir that a pressure reduction occurs through the opening of the servo valve without a servo valve space pressure corresponding to opening of the closure element being reached during this process.
Aspects of the invention are based on the concept of adding a passive piezoelectric region to the active piezoelectric region of the actuator and using this passive piezoelectric region as a sensor for force measurement. When a pressure reduction is required, the servo valve is opened by activating the active piezoelectric region while simultaneously measuring the force on the piezoelectric sensor. From the force measurement, the pressure in the servo valve space is determined, taking into account the closing spring force. The opening of the servo valve brings about the desired pressure reduction, and the pressure reduction is controlled in such a way that a servo valve space pressure corresponding to opening of the closure element is not reached. The closure element therefore remains closed in the pressure reduction phase.
Thus, the rail pressure reduction is performed by the injection valve itself without the need for an additional pressure control valve or pressure reduction valve. It is thereby possible to carry out the disclosed method at particularly low cost.
In some embodiments, a pressure reduction is performed in a phase in which no injection is taking place. A pressure reduction phase therefore takes place before or after an injection phase, and, in the case of a plurality of injection valves, the pressure reduction can be distributed between different injection valves. Thus, in the case in which the injection valve currently being used for pressure reduction is supposed shortly afterwards to carry out an injection process, for example, other injection valves, which are currently not injecting, are used for the pressure reduction. There is therefore no need for the full pressure reduction to be carried out with a single injection valve; instead, this valve can be used to carry out just a part of the pressure reduction, while the remainder of the pressure reduction is taken over by the other injection valves.
In particular, in some embodiments of the method, the limiting pressure Pst_limit in a control space for the closure element, which the pressure in the control space must not undershoot so as to avoid opening the closure element, is determined from the actual pressure in the pressure reservoir (rail pressure) Prail_ist. It is sufficient here if the ratio of Pst_limit to Prail_ist is greater than a threshold.
The setpoint control space pressure P_st_s is determined in accordance with the setpoint rail pressure P_rail_s and with the actual rail pressure Prail_ist and is limited in a downward direction by the limiting pressure Pst_limit in the control space. If the pressure reduction gradient is supposed to be greater, a lower setpoint control space pressure is chosen.
The setpoint pressure for the valve space P_v_s is then determined from the setpoint control space pressure P_st_s and the actual rail pressure Prail_ist, the setpoint pressure for the valve space corresponding to a pressure which produces opening or a switching leakage of the servo valve without opening the closure element.
In this process, the servo valve is moved by activating the active piezoelectric region until the actual valve space pressure P_v_ist has reached the setpoint pressure for the valve space P_v_s, after which the valve space pressure is adjusted to P_v_s by activating and deactivating (charging and discharging) the active piezoelectric region.
The pressure reduction (rail pressure reduction) carried out by the disclosed method using one or more injection valves is continued until the rail pressure reaches the setpoint thereof, after which the servo valve or the servo valves are closed again by discharging the piezoelectric actuator or piezoelectric actuators.
Other embodiments provide a fuel injection system for an internal combustion engine, which has a pressure reservoir (rail), at least one injection valve, in which a piezoelectric actuator actuates a servo valve arranged in a servo valve space counter to the force of a closing spring so that a closure element opens an injection opening connected to the pressure reservoir by a fuel line, and a feedforward and feedback control unit. This fuel injection system is configured to perform a method of the type described above.
In particular, the passive piezoelectric region acting as a force sensor is formed by an additional, serially arranged, passive piezoelectric layer.
The drive connection between the piezoelectric actuator and the closure element is preferably designed in such a way that the piezoelectric actuator is connected by a multiplication lever to a control piston which opens and closes the servo valve and thus brings about the desired pressure reduction.
Here, the servo valve is opened counter to the force of a closing spring and is situated in a servo valve space which is connected via a restrictor to a control space which is connected to the fuel line and accommodates the closure element or a piston for the closure element. When the servo valve is opened, the pressure in the servo valve space and hence the rail pressure is thus reduced in a controlled manner.
The nozzle needle 7 is controlled by means of a piezoelectric actuator 1. Depending on activation, the piezoelectric actuator 1 can change in length and exert a force via a multiplication lever 17 on a control piston 9, the latter making contact with a servo valve 4, which is pressed against a valve seat by way of a closing spring. The servo valve 4 is arranged in a valve space 16 which is connected via a restrictor to a control space 8 for the closure element. The control space 8 accommodates a piston 5, which actuates the nozzle needle 7.
When the piezoelectric actuator 1 is supplied with electrical energy (charged), it increases in length and thereby causes the control piston 9 to raise the servo valve 4 from the seat thereof, with the result that the pressure prevailing in the servo valve space 16 is reduced. Owing to this pressure reduction, the needle piston 5 and the nozzle needle 7 move upward in the figure and, in the process, expose the injection opening to enable an injection process to be carried out.
In addition, the opening of the servo valve 4 brings about a pressure reduction process without opening the nozzle needle 7 in order to achieve a rail pressure reduction. During this process, the servo valve 4 is opened only to the extent that, although a controlled pressure reduction takes place, the closure element or nozzle needle 7 does not open.
In addition to the active piezoelectric region 12 used to actuate the nozzle needle 7, the piezoelectric actuator 1, which is illustrated only schematically in
Number | Date | Country | Kind |
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10 2012 204 252 | Mar 2012 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2013/055519 | 3/18/2013 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2013/139723 | 9/26/2013 | WO | A |
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