Injection valves are typically used in conjunction with a highly pressurized so-called common-rail system. They may include a piezo element used as an actuator. The injection quantity of such common-rail injection valves may be controlled directly but predominantly indirectly by means of a servo valve. This means that the nozzle needle is not directly coupled to the movement of the piezo actuator, but rather the piezo actuator actuates a servo valve.
The fuel is typically supplied at very high pressure via a high-pressure port and a high-pressure line in the injection valve body through a valve plate to a throttle plate. A control chamber is connected via an inflow throttle to the high-pressure line. Furthermore, the control chamber is connected via an outflow throttle to a valve chamber. From the front or lower region pointing toward the combustion chamber, the nozzle needle is preloaded by means of a nozzle spring such that the latter exerts a closing force. Since the control chamber is connected via the high-pressure port to the rail system, in the non-actuated state that, in the control chamber, a high pressure prevails which corresponds to the pressure in the rail system (rail pressure). This results in an additional hydraulic action force which holds the nozzle needle in the closed position, and thus the openings of the injection valve are closed.
If the piezo actuator is actuated, it actuates the servo valve. Fuel then exits the control chamber via the outflow throttle. In this way, the pressure in the control chamber is lowered, and the nozzle needle is opened after a certain opening pressure threshold is reached. If the piezo actuator is subsequently discharged, the servo valve closes, the control chamber is charged again and the pressure in the control chamber builds up to the rail pressure level again, and the nozzle needle closes. Here, the dynamics of the pressure drop and/or pressure build-up in the control chamber, and the needle speed during the needle opening and needle closing movements, are substantially dependent on the dimensions of the inflow and outflow throttles.
To ensure stable operation of a common-rail injector with piezo actuator, a virtually clearance-free coupling between piezo actuator and the valve body of the servo valve is used. Precise steady-state temperature compensation of the thermal change in length is required in the entire drive chain to keep the change in the idle travel of the piezo actuator within narrow limits. For this purpose, the piezo actuator is normally surrounded by an Invar sleeve, which exhibits similar thermal expansion characteristics to the piezo actuator.
A small defined idle travel of the servo valve must be provided, e.g., a small intermediate space between the servo valve body and the base plate of the actuator, however, because it is necessary to prevent a situation in which the servo valve is open when the piezo actuator is in a non-activated state. Conversely, an idle travel set too large causes the required piezo actuator travel to be increased to the same extent, and this in turn correspondingly increases the activation energy required for this purpose. Altogether, this increases the demands on the precision of the system even over relatively long time periods. The use of injection valves in the engine involves thermally highly complex boundary conditions with different heat sources and heat sinks. In the region of the piezo actuator, the inherent heating from electrical losses may play a significant role. In the region of the servo valve, the temperature increase resulting from the expansion of the fuel from rail pressure to ambient pressure constitutes a significant heat source. As a result of the installation of the injector in the cylinder head of an engine, corresponding heat flows arise via various contact points, for example the combustion chamber seal and the contact between the nozzle tip and the combustion gases. An influential variable which must likewise be taken into consideration with regard to the idle travel is the clamping force in the cylinder head. This is also subject to large tolerances.
During steady-state injector operation, the resultant thermal expansions can be substantially compensated through suitable material selection and geometry. During dynamic engine operation, transient, inhomogeneous temperature distributions in the components yield an additional influential variable with regard to the idle travel of the piezo actuator. Furthermore, the idle travel varies during injector operation owing to changes in length of the piezo actuator resulting from polarization changes and wear.
Thermally induced changes in length can be substantially compensated through suitable use of different materials. One examples the use, already mentioned above, of actuator housings composed of Invar, because Invar exhibits substantially the same temperature expansion behavior as the piezo ceramic. Ultimately, however, this constitutes merely basic compensation. Changes in idle travel resulting from wear and/or changes in the polarization state are not addressed. To solve this problem, some piezo common-rail injection nozzles use a hydraulic coupler composed of a cylinder with a drive-input piston on the actuator side and a drive-output piston on the valve side. A disadvantage of this arrangement is that said hydraulic coupler is situated in the low-pressure region. To keep a coupler of said type functional, however, it is necessary to ensure a certain pressure level, normally approximately 10 bar. In the prior art, this is achieved by means of a pressure-maintaining valve.
The increasing use of fuel components with low boiling point, e.g., adding alcohol to the fuel, jeopardizes the functionality of corresponding hydraulic coupling elements in the low-pressure region, and thus constitutes a significant functional risk for such concepts.
EP 1 389 274 describes a directly actuated injection valve with a hydraulic coupler. This however has the disadvantage that the actuator is not adequately decoupled from the nozzle needle, which likewise makes the compensation of wear more difficult.