The invention relates to a hydraulic temperature compensator. The invention further relates to a hydraulic stroke transmitter having such a hydraulic temperature compensator, in particular an injector.
In order to introduce a desired volume of fuel into arbitrary combustion processes it is generally necessary to use injectors by which it is possible to meter a fuel quantity. Since very many combustion processes execute by the direct injection of fuel that is maintained under high pressure, particularly fast operating actuators are frequently used which drive injectors. This means that an actuator generates a stroke which for example actuates an injector needle which in turn opens a valve and releases a fuel at predetermined time intervals and in adjustable volumetric flow rates for a combustion process. Combustion air is supplied separately in this case.
Injectors for high-pressure direct injection frequently employ high-speed actuators for this purpose, such as “piezoelectric multilayer actuators” (PMA) for example. These are solid-state actuators, the central element of which is composed of a plurality of piezoelectric layers. Also known are so-called magnetostrictive solid-state actuators, which exploit a magnetic-mechanical effect in order to generate a stroke. For generating a stroke it is important that solid-state actuators of the type have a stroke that is too small for opening an injector needle to such an extent that the desired fuel quantity is introduced. This develops into a major problem, particularly in the case of gas injectors, which require a longer stroke than injectors that meter liquid fuel. The consequence is that only designs including a stroke transmitter are given consideration.
An additional aggravating factor when hydrogen is used as the fuel is that the small and lightweight hydrogen molecule easily diffuses through nonmetallic elements such as rubber diaphragms. The choice of a suitable stroke transmitter therefore becomes a central problem in the building of injectors. This also results from the fact that a transmitter determines many characteristics of an injector and in contrast to an actuator can be redesigned in terms of its structure.
In related art solutions to the problem an increase in stroke is achieved by mechanical transmission or by partially nonmetallically sealed hydraulic transmission. Mechanical transmitters, which for example use a mechanical lever, are generally susceptible to wear and tear and to undesirable vibrations. This applies in particular when an idle stroke is required between actuator and transmitter, for example in order to prevent a leakage which could occur in the event of a thermal change in length due to heating. As a result thereof the actuator will for example strike a nozzle needle, thereby unfavorably affecting the injector. Uneven injection and unreliable opening and closing characteristics are the consequence. An idle stroke between actuator and transmitter is also undesirable because the actuator deflection up to the point of contact with the nozzle needle remains unused.
An increase in the stroke of an actuator having a transmission ratio of less than 1:2 is often realized by mechanical levers. With injectors for diesel engines, for example, the mechanical transmission ratio can amount to 1:1.6. Gas injectors typically require greater transmission ratios. Hydraulic transmitters, also referred to as hydraulic levers, are used in gas injectors in most cases. A stroke transmission ratio of 1:6 is used for example in the case of direct injection of CNG (compressed natural gas).
Using a hydraulic transmitter enables the idle stroke to be avoided, with the result that the functional chain between actuator and nozzle needle is permanently present. This is reflected directly in the mechanical engineering design. Considered from a different angle, the deflection of the actuator is utilized and converted to a greater extent by the injector.
A disadvantage in the related art, in the automotive engineering sector for example, is the wide temperature range to be considered, which can range from −40 C.° to +150 C.°. In the consideration of fluid volumes, this can be associated with significant changes in volume. Peak values can lie significantly in excess of a 30% increase in volume. For this reason hydraulic stroke transmitters require a connection to a reservoir in most cases.
The unexamined German patent application publication DE 10 2005 042 786 A1 discloses for example a fuel injector which is equipped with a hermetically sealed hydraulic system. In this publication the use of so-called “guided pistons” is described. Guided pistons of this kind necessitate a high degree of mechanical precision in manufacture and are very susceptible to wear and tear.