DE102012204286B4 - Method for operating a fuel injection system and fuel injection system with an injector with controlled pressure reduction - Google Patents

Abstract

Method for operating a fuel injection system of an internal combustion engine, which has a pressure accumulator (rail), at least one injection valve with a piezo direct drive, in which a piezo actuator is in direct drive connection with a closing element of the injection valve via an actuating element and the actuating element can be moved through a guide opening of a a wall enclosing the space accommodating the closure element and has a control and regulation unit, characterized in that a piezo actuator is used which, in addition to the active piezo area used for actuating the closure element, has a passive piezo area as a force sensor, using this force sensor to the force acting on the passive piezo area is determined, and when a pressure reduction in the system is required, the active piezo area is controlled in such a way that a pressure reduction occurs through switching leakage of the actuating element, without this a force threshold corresponding to the opening of the closure element is reached.

Description

The present invention relates to a method for operating a fuel injection system of an internal combustion engine, which has a pressure accumulator (rail), at least one injection valve with a piezo direct drive, in which a piezo actuator is in direct drive connection with a closing element of the injection valve via an actuating element and the actuating element movably extends through a guide opening of a wall closing off a space accommodating the closure element, and has a control and regulation unit.

Fuel injection systems, with which fuel is injected into a combustion chamber of an internal combustion engine, have been known for a long time. Such injection systems include at least one injection valve (injector) and at least one control and regulating unit connected to the injection valve for controlling the injection process. The injection valve has a space from which fuel can be injected through an injection opening into the combustion chamber. The injection opening is opened and closed by means of a closing element (nozzle needle) that can be actuated (moved) by an actuator. The space is supplied with fuel via a high-pressure accumulator and a fuel line.

The actuator is an element for moving the closure element. This controls an injection process using the actuator. In this case, the actuator is in direct driving connection with the closure element, which means that the actuator and the closure element are in mechanical contact with one another via an interposed actuating element. Normally, several actuating elements are provided here, which are designed, for example, in the form of a pin, a bell and levers. It is essential here that there is no hydraulic or pneumatic coupling between the actuator and the closure element.

The actuator is a piezo actuator which, due to the piezoelectric effect, expands (increases its length) when subjected to electrical energy and in this way moves the closure element directly via the actuating element.

From the patent DE 10 2010 021 169 B4 For example, such an injection valve with a piezo direct drive is known. The piezo actuator allows conclusions to be drawn about force inputs based on its capacitance signal and is also used as a sensor element. For example, the piezo capacitance changes during the opening process of the nozzle depending on the counteracting force acting on the nozzle needle.

From the disclosure document DE 199 60 971 A1 a piezo actuator for converting an electrical control signal into a mechanical stroke movement is also known, which has a stack of piezo elements that are electrically coupled to one another, the piezo actuator having a second stack of piezo elements that interact with one another and lie mechanically in series directly on an end face of the piezo actuator, which acts as a sensor is used and emits an electrical signal proportional to the stroke movement.

In order to carry out a pressure reduction in the pressure accumulator (rail pressure reduction) in such fuel injection systems with a piezo direct drive, special pressure control valves (PCV/PDV) are used in the systems of the prior art.

Unlike the patents DE 10 2005 001 499 B4 and DE 10 2005 012 998 B3 known fuel injection systems, in which a pressure reduction in the high-pressure area is realized in that at least one servo-injection valve, which by design is subject to a switching leakage, is activated for such a short time that a switching leakage flow is generated when the injection nozzle is closed, this is not possible when using conventional direct-operated injection valves, which is why the specified pressure control valves (PCV/PDV) are required.

The use of such additional valves increases the costs of the overall system. Furthermore, a permanent leakage occurs during a stop in start-stop operation, so that the pressure required in the pressure accumulator (rail pressure) for the next start cannot be maintained.

The object of the present invention is to provide a method of the type described at the outset that can be carried out particularly cost-effectively.

According to the invention, this object is achieved in a method of the specified type in that
that a piezo actuator is used which, in addition to the active piezo area used for actuating the closure element, has a passive piezo area as a force sensor, using this force sensor to determine the force acting on the passive piezo area and, if the pressure in the system needs to be reduced, the active piezo area so is controlled that a pressure reduction by switching leakage of the actuating element without a force threshold corresponding to the opening of the closure element being reached.

With the solution according to the invention, the signal of a force sensor arranged and integrated in series with the piezo actuator is used to carry out a pressure reduction with the injection valve itself without an additional pressure control valve (PCT/PDV), without the leakage during normal operation compared to the injection valves of the prior art technology is increased. The force acting on the passive piezo area is determined with the aid of the force sensor provided according to the invention. If a pressure reduction of the system is required, the active piezo area is controlled in such a way that the pressure is reduced by switching leakage of the actuating element, this being carried out in such a way that the force determined using the force sensor does not reach the corresponding force threshold for opening the closure element.

In particular, a piezo direct drive is used in the method according to the invention, in which, in the closed state of the closure element, the actuating element does not allow any leakage through the guide opening without actuating the piezo actuator. In order to achieve this, an actuating element is used in particular, which has a shaft guided in the guide opening and a head that covers and thus closes the guide opening. In this case, such an actuating element is preferably used, so that a relatively large leakage is already achieved by a small stroke of the actuating element.

In the injection systems of the prior art, a so-called pin is used as the actuating element, which is a pin-shaped element that is movably guided in a guide opening in a wall that closes a space accommodating the closure element (the nozzle needle). According to the invention, this pin is now replaced by a pin with a head, which can be completely closed by contacting the head against the wall having the guide opening and thus covering the guide opening, so that no leakage through the guide opening occurs without actuation of the piezo actuator. The actuating element (the pin) is therefore used according to the invention as a control element in order to use the guide opening accommodating the actuating element for discharging fuel under high pressure from the space accommodating the closure element. The corresponding guide opening can be completely closed and/or opened with a small stroke of the actuating element (pin) for a relatively large leakage.

The headed pin used according to the invention is in contact with the piezoelectric actuator with its shaft end and is moved by stretching or elongating the same. Above its head, the pin is in mechanical contact with the closure element via further actuating elements, in particular a bell and levers that interact with it, in order to actuate it and thus open and close an injection opening for carrying out an injection process.

In order to enable a pressure reduction in an injection valve with a piezo direct drive without using a special pressure control valve, the method according to the invention preferably uses a special construction of the actuating element in addition to using the signal from the force sensor (passive piezo area) arranged in series with the piezo actuator made, which enables a corresponding leakage control.

In the method according to the invention, the force threshold of the actuator corresponding to the opening of the closure element is preferably calculated as a function of the actual rail pressure. In particular, the stroke of the piezo actuator and thus the stroke of the actuating element (pin) is adjusted via the electrical energy supplied to the piezo actuator as a function of the pressure gradient. In particular, the ACTUAL stroke of the actuating element and thus the stretching of the piezo actuator is determined, compared with a TARGET stroke and the corresponding deviation is used to correct the electrical energy supplied to the piezo actuator in order to implement the TARGET stroke.

wobei bedeuten

d_l

= Dehnung Piezo-Aktuator

l

= Länge des Aktuators

s_33

= Kehrwert Aktuatorsteifigkeit

F_s

= Kraft Piezo-Aktuator

A_a

= Aktuatorfläche

d_33

= Piezokonstante

U

= Spannung am Piezo-Aktuator

d

= Schichtdicke Piezo-Aktuator

The stroke of the actuating element (pins) and thus the stretching of the piezo actuator is determined in particular from the following equation: $$\text{i.e}\_1/1=-\text{s}\_33*\text{f}\_\text{s}/\text{A}\_\text{a}+\text{i.e}\_33*\text{u}/\text{i.e}$$

where mean

d_l

= strain piezo actuator

l

= length of the actuator

s_33

= reciprocal of actuator stiffness

F_s

= force piezo actuator

A_a

= actuator area

d_33

= piezo constant

u

= voltage at the piezo actuator

i.e

= layer thickness of piezo actuator

In a special embodiment of the method according to the invention, a pin with a head is used as the actuating element for controlling a further actuating element, which is designed as a bell. The pin with the head and the guide opening for this are designed in such a way that a defined amount of leakage can flow through. Furthermore, the pin head is designed in such a way that it can close the guide opening without leakage flow. In the closed state without actuation of the piezo actuator, there is therefore no leakage through the guide opening for the pin. On the other hand, even a slight lift of the pin head from its seat can result in dethrottling.

If a pressure reduction is now required, the pin head for the injection valve in the phase where no injection takes place (pressure reduction phase) is controlled by charging the piezo actuator via a charging ramp. At the same time, the force exerted by the pin on the passive piezo area (force sensor) and the resulting voltage are measured. The force at the force sensor initially increases until a maximum is reached. After that, the power signal falls. The charging of the piezo actuator is controlled in such a way that the desired pressure reduction occurs through switching leakage of the pin head without opening the closure element.

In concrete terms, a force threshold of the actuator for opening the closure element is calculated as a function of the actual rail pressure. The stroke of the actuator and thus the stroke of the pin head is adjusted via the charge depending on the pressure reduction gradient. The ACTUAL stroke of the pin head is determined from the equation given above. The ACTUAL lift is compared to the TARGET lift and the deviation is used to correct the charge height in order to achieve the TARGET lift. The force is monitored via the force sensor and the TARGET stroke is limited so that the force threshold is not exceeded.

In the method according to the invention, the pressure reduction process is preferably continued until the rail pressure reaches the desired value. The actuating element (pin head) is then closed again by discharging the piezo actuator.

In a further special embodiment of the method according to the invention, in the case in which the injection valve currently in the pressure reduction phase is about to enter the injection phase, the piezo actuator is further charged in order to actuate the closure element to carry out the injection process, after which the pressure reduction continues injection is continued until the TARGET pressure is reached.

In the case of multiple injections, particularly when the injection distance between the individual injections is small, the piezo actuator is preferably discharged to end the first injection in such a way that the closure element is closed, but the actuating element remains open with a defined force and stroke, after which the piezo Actuator is further charged to perform the second injection. This means that a small spray distance can be achieved.

The present invention also relates to a fuel injection system of an internal combustion engine with a pressure accumulator (rail), at least one injection valve with a piezo direct drive, in which a piezo actuator is in direct drive connection with a closing element of the injection valve via an actuating element and the actuating element is movable a guide opening of a wall enclosing a space accommodating the closure element, and a control and regulation unit. This system is set up to carry out a method of the type described above.

The passive piezo area, which acts as a force sensor, is formed in particular by an additional serially arranged passive piezo layer. As already described, the piezo direct drive preferably has an actuating element that has a shaft guided in the guide opening and a head that covers and thus closes the guide opening. The piezo direct drive comprises, in particular, an actuating element designed as a pin with a head and a further actuating element designed as a bell that interacts with it.

• 1 einen schematischen Teillängsschnitt durch ein Einspritzventil mit Druckabbaufunktion, wobei Teile des Ventils weggelassen worden sind;
• 2 einen schematischen Teillängsschnitt durch einen Piezo-Aktuator mit Kraftsensor;
• 3 das Prinzip zur Ansteuerung und Regelung des Druckabbaus in einem Zeit-Diagramm; und
• 4 ein Ablaufdiagramm, das die Regelung des Druckabbaus zeigt.

The invention is explained in detail below using an exemplary embodiment in conjunction with the drawing. Show it:

• 1 a schematic partial longitudinal section through an injection valve with pressure reduction function, wherein parts of the valve have been omitted;
• 2 a schematic partial longitudinal section through a piezo actuator with force sensor;
• 3 the principle for controlling and regulating the pressure reduction in a time diagram; and
• 4 a flow chart showing the regulation of the pressure reduction.

1 FIG. 1 shows diagrammatically an injector used, for example, in a diesel engine of a car is used. It is used to inject fuel into a combustion chamber of an internal combustion engine. It has a space 7 which is connected via a fuel line (high-pressure line) 3 to a pressure accumulator (high-pressure accumulator) (rail). The injection valve shown here is one of a large number of injection valves which in a common rail system are each connected to the same pressure accumulator via fuel lines. At the lower end of the injection valve (not shown), this has an injection opening through which fuel can be injected from chamber 7 into the combustion chamber. A nozzle needle serving as a closing element (not shown) is arranged in space 7 and can be used to open and close the injection opening. If the nozzle needle is in an open position, in which it releases the injection opening, fuel that is under high pressure is injected from space 7 into the combustion chamber. In a closed position of the nozzle needle, in which the nozzle needle closes the injection opening, the injection of fuel into the combustion chamber is prevented.

The nozzle needle is controlled by means of a piezo actuator 1 that actuates it directly. Depending on the control, the piezo actuator 1 can change its length and exert a force on the nozzle needle, the force being able to be transmitted to the nozzle needle via a pin 4, a bell 5 and a lever (not shown). The piezo actuator 1 and the nozzle needle are thus directly mechanically coupled via the pin 4, the bell 5 and the lever. A force exerted by the piezo actuator 1 is therefore transmitted directly to the nozzle needle. Conversely, a mechanical force exerted by the nozzle needle is transmitted directly to the piezo actuator 1 . If the piezo actuator 1 is not supplied with electrical energy, a closing spring (not shown) presses the nozzle needle in 1 down so that it closes the injection opening against the pressure in space 7 and prevents injection. If electrical energy is applied to the piezo actuator 1, it increases in length and exerts a force on the nozzle needle, as a result of which the injection opening is opened by means of the nozzle needle.

the inside 1 The piezo actuator 1 shown only schematically has a passive piezo area as a force sensor 2 in addition to the active piezo area used to actuate the nozzle needle. This force sensor 2 is used to determine the force acting on the piezo actuator via pin 4.

The pin 4 is designed as a pin with a head, which is movably guided in a guide opening within a wall 6 . The wall 6 closes off the high-pressure chamber 7 accommodating the closure element. The desired pressure reduction takes place via this guide opening by means of a corresponding switching leakage, in that fuel from the space 7 through the wall 6 or plate in 1 is discharged upwards into the low-pressure space.

The pin 4 with a head is designed in such a way that the head covers the guide opening and closes it when it is in contact with the wall 6 . In this state, therefore, no leakage occurs. If electrical energy is applied to the piezo actuator 1, it increases in length so that the head of the pin 4 is lifted from its seat on the wall and a corresponding leakage can occur through the guide opening. This effect is used in a pressure reduction phase outside of the actual injection phase.

In this pressure reduction phase, electrical energy is applied to the piezo actuator 1 in such a way that the pin 4 is only moved far enough for a leak to reduce the pressure to occur, without opening the closure element as a result. At the same time, the force exerted by pin 4 on piezo actuator 1 is determined by force sensor 2 . The force exerted is continuously monitored and the TARGET stroke of pin 4 is limited in such a way that a force threshold is not reached that would lead to the closure element opening.

2 shows schematically the structure of the piezo actuator 1, which forms a unit that has the active piezo area 12 for actuating the nozzle needle and the passive piezo area 13, which serves as a force sensor. The active piezo region 12 is composed of a multiplicity of active piezo layers which are arranged one above the other and each have a corresponding connection electrode 10 on the left and right. A passive piezo layer, which forms the piezo region 13 acting as a force sensor, is arranged separately on the uppermost active piezo layer by suitable insulation 14 . The passive piezo layer is provided with corresponding connection electrodes 15 on both sides.

The corresponding principle for controlling and regulating the pressure reduction is shown in the time diagram 3 shown. It can be seen that the sensor force increases until it reaches a maximum, at which point the pin or pin head is opened and the leakage is generated. In the subsequent regulation, the force is kept below the force threshold for opening the nozzle needle.

Furthermore, in 3 the charge or voltage on the actuator, the actuator stretch and the pressure in the injection valve (injector), which corresponds to the rail pressure, are shown over time.

The corresponding method for controlling and regulating the pressure reduction runs in the manner described in the general part of the description.

4 Fig. 12 is a flow chart showing the control scheme in depressurization.

Claims (15)

Method for operating a fuel injection system of an internal combustion engine, which has a pressure accumulator (rail), at least one injection valve with a piezo direct drive, in which a piezo actuator is in direct drive connection with a closing element of the injection valve via an actuating element and the actuating element can be moved through a guide opening of a extends a wall enclosing the space accommodating the closure element and has a control and regulation unit, characterized in that a piezo actuator is used which, in addition to the active piezo area used for actuating the closure element, has a passive piezo area as a force sensor, with the aid of this force sensor the the force acting on the passive piezo area is determined and, if the pressure in the system needs to be reduced, the active piezo area is controlled in such a way that the pressure is reduced by switching leakage of the actuating element, without d awhere a force threshold corresponding to the opening of the closure element is reached. procedure after claim 1 , characterized in that a piezo direct drive is used, in which the actuating element does not allow any leakage through the guide opening in the closed state of the closure element without actuating the piezo actuator. procedure after claim 1 , characterized in that an actuating element is used which has a shaft guided in the guide opening and a head covering and thus closing the guide opening. Method according to one of the preceding claims, characterized in that an actuating element is used such that a relatively large leakage is already achieved by a small stroke of the actuating element. procedure after claim 3 or 4 , characterized in that an actuating element designed as a pin with a head is used. Method according to one of the preceding claims, characterized in that the force threshold of the actuator corresponding to the opening of the closure element is calculated as a function of the ACTUAL rail pressure. Method according to one of the preceding claims, characterized in that the stroke of the piezo actuator and thus the stroke of the actuating element is adjusted as a function of the pressure gradient via the electrical energy supplied to the piezo actuator. Method according to one of the preceding claims, characterized in that the ACTUAL stroke of the actuating element and thus the stretching of the piezo actuator is determined, compared with a SET stroke and the corresponding deviation is used to correct the electrical energy supplied to the piezo actuator, to realize the TARGET stroke. Method according to one of the preceding claims, characterized in that the stroke of the actuating element and thus the expansion of the piezo actuator is determined from the following equation: $$\text{i.e}\_1/1=-\text{s}\_33*\text{f}\_\text{s}/\text{A}\_\text{a}+\text{i.e}\_33*\text{u}/\text{i.e}$$

where d_l = elongation of the piezo actuator l = length of the actuator s_33 = reciprocal value of the actuator stiffness F_s = force of the piezo actuator A_a = actuator area d_33 = piezo constant U = voltage on the piezo actuator d = layer thickness of the piezo actuator. Method according to one of the preceding claims, characterized in that the pressure reduction process is continued until the pressure accumulator pressure (rail pressure) reaches a target value, after which the guide opening is closed again by discharging the piezo actuator. Method according to one of the preceding claims, characterized in that in the case in which the injector currently in the pressure reduction phase is about to enter the injection phase, the piezo actuator is charged further in order to actuate the closure element for carrying out the injection process, after which the pressure reduction after the injection is continued until the TARGET pressure is reached. Method according to one of the preceding claims, characterized in that at Multiple injections gene to end the first injection, the piezo actuator is discharged in such a way that the closure element is closed, but the actuating element remains open with a defined force and stroke, after which the piezo actuator is further charged in order to carry out the second injection. Fuel injection system of an internal combustion engine with a pressure accumulator (rail), at least one injection valve with a piezo direct drive, in which a piezo actuator is in direct drive connection with a closing element of the injection valve via an actuating element, the piezo actuator in addition to the active one for actuating the The piezo area used in the closure element has a passive piezo area as a force sensor and the passive piezo area (13) is formed by an additional serially arranged passive piezo layer and the actuating element extends movably through a guide opening in a wall enclosing a space accommodating the closure element, and with a control and Control unit, characterized in that the system is set up to carry out a method according to one of the preceding claims. fuel injection system Claim 13 , characterized in that the actuating element has a shaft guided in the guide opening and a head covering and thus closing the guide opening, the guide opening being designed in such a way that switching leakage occurs through this when the head is lifted from its seat on the wall. fuel injection system Claim 14 , characterized in that the piezo direct drive has a further actuating element which interacts with said actuating element and is designed as a bell (5).

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