Fluid sprinkling – spraying – and diffusing – Processes – Vibratory or magneto-strictive projecting
Reexamination Certificate
2001-05-10
2004-06-15
Evans, Robin O. (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Processes
Vibratory or magneto-strictive projecting
C239S005000, C239S102100, C239S102200, C239S585100, C239S584000, C239S533900
Reexamination Certificate
active
06749126
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fuel injector, and to a method for operating a fuel injector.
BACKGROUND INFORMATION
A fuel injector for fuel-injection systems of internal combustion engines is known from German Patent Application No. 195 38 791, where a valve-closure member that interacts with a valve-seat surface to form a sealing seat is controlled by an actuator, via a valve needle.
The principal problem in using piezoelectric actuators is their thermal expansion. In contrast to customary materials such as steel or plastics, the piezoelectric materials have a negative coefficient of thermal expansion. This causes the piezoelectric actuator to contract with increasing temperature, while the surrounding housing expands. The different thermal expansion coefficients of the piezoelectric actuator, on one hand, and the housing, on the other hand, produces a temperature-dependent valve lift when this is not compensated for by appropriate measures.
Temperature compensation for a first piezoelectric actuator by a second piezoelectric actuator is known from the dissertation of Niko Herakovic, “Die Untersuchung der Nutzung des Piezoeffektes zur Ansteuerung fluidtechnischer Ventile” [“Analyzing the Use of the Piezoelectric Effect for Controlling Fluid Valves”], TH Aachen 1996, pp. 75-77, Wissenschaftsverlag [Scientific Publishing House] Aachen, ISBN 3-89653-041-0. In this case, the two piezoelectric actuators are each accommodated in one housing. To compensate for temperature, the second piezoelectric actuator counteracts the first piezoelectric actuator on a cylinder disposed between the two piezoelectric actuators. The cylinder is raised as a function of the operating voltage of the first actuator. When the temperature of the two actuators is increased, then the thermal expansions of the two actuators compensate for each other.
A disadvantage of the temperature compensation known from this printed publication is that, in order to actuate a valve needle of the fuel injector, the valve needle must be connected, via a suitable connecting device, to the cylinder supported between the two actuators. Additional parts encompassing at least one of the actuators are necessary for this, which means that the width of the fuel injector increases. In addition, the actuators are spaced far apart from each other so that, in response to the first piezoelectric actuator warming up sharply as a result of operating conditions, the second actuator is not able to compensate for the thermal expansion of the first actuator. Even in long-term operation, the temperature gradient formed between the first piezoelectric actuator and the second piezoelectric actuator results in insufficient temperature compensation. In the exemplary embodiment of the dissertation, the temperature of the two actuators is actively adjusted by cooling or heating elements. In summary, this temperature compensation is costly and not suitable for practical use.
To compensate for temperature, German Patent Application No. 195 38 791 proposes designing the valve housing as two pieces made of different materials. For example, it is proposed that one housing part be manufactured from steel, and the other housing part be manufactured from Invar. By choosing a suitable length for the first housing part made of steel and the second housing part made of Invar, it is intended that the total, resulting thermal expansion of the housing be adapted to the thermal expansion of the piezoelectric actuator, and therefore, that the piezoelectric actuator and the housing encompassing the piezoelectric actuator expand in the same manner, as a function of temperature.
A disadvantage of this design approach is that the valve housing is difficult to manufacture, and the material for the second housing part, preferably Invar, is expensive. Furthermore, it must be taken into consideration, that the valve housing and the actuator can have different temperatures. Thus, the waste heat of the piezoelectric actuator, which especially results from frequent operation of the fuel injector, can heat up the piezoelectric actuator, and it can only transfer its temperature slowly to the valve housing. On the other hand, the temperature of the valve housing is affected by the waste heat of the internal combustion engine on which the fuel injector is mounted. Therefore, this type of temperature compensation is not satisfactory.
A fuel injector for fuel-injection systems of internal combustion engines is known from German Patent No. 195 19 192, where an actuator acts on a valve needle, via a hydraulic transmission system. The transmission device includes a primary piston having an inner opening, in which a secondary piston is moveably guided. The secondary piston is connected to a valve needle, which is sealingly and movably guided in the valve housing. In the valve housing is a working chamber, which is filled with fuel and delimited by the primary piston and the secondary piston. The piezoelectric actuator contacts the primary piston on the side of the primary piston opposite to the working chamber. Since the volume of the fuel-filled working chamber must be maintained, a movement of the primary piston due to the action of the piezoelectric actuator causes the secondary piston to move in the primary piston, a suitable stroke transmission ratio being given by appropriately dimensioning the surfaces on the primary piston and the secondary piston, on the side of the working chamber. The temperature compensation is attained through a defined slot between the primary piston and the secondary piston. To that end, a portion of the fuel can be expelled from the working chamber in response to a temperature-dependent, quasistatic expansion of the fuel in the working chamber.
A disadvantage of this design approach is that the hydraulic temperature compensation causes the action of the actuator to be transmitted to the valve needle in a damped manner, which increases the response time of the valve needle, and does not allow the fuel injector to be used as a rapid-actuation fuel injector.
SUMMARY OF THE INVENTION
In contrast, the fuel injector of the present invention has the advantage that the temperature compensation of the actuator is considerably better. In addition, the fuel injector according to the present invention can also be used as a rapid-actuation fuel injector. Further advantages lie in the precise adjustability of the course of injection, whereby the injection operation can be adjusted to the specific operating condition and the operating requirements of the internal combustion engine; and in the small number of mechanically movable components, so that the fuel injector is designed to have a low rate of wear, and is easy to construct.
It is advantageous when the supporting element lies against a shoulder formed in the valve housing. This can reduce the number of additional parts. In this case, the supporting element can rest against a shoulder formed in the valve housing, by means of an elastically deformable seat element. This allows the valve needle to sit centered in the sealing seat. In addition, abrupt pressure pulses acting on the valve needle can be absorbed, which means that stress on the valve needle is reduced.
It is also advantageous when a large, initial stress is applied to at least one of the actuators, which means that, in the case of non-actuated actuators, the valve needle is supported against the sealing seat, in the closed position, by a force given by the difference in initial stress. This can eliminate the need for an additional compression spring for pressing the valve needle into the sealing seat.
The supporting element is advantageously secured in the valve housing by a screw element, which allows the initial stress acting on at least one of the actuators to be adjusted. This allows the contact force of the valve needle in the sealing seat, and the opening force acting on the valve needle in the case of non-actuated actuators, to be discreetly adjusted. This is especially useful in connection with t
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