Field of the Invention
Embodiments of the present invention relate to variable nozzle turbochargers.
Description of the Related Art
Japanese Utility Model Publication No. 63-115532 and Japanese Utility Model Publication No. 63-92036 disclose a variable nozzle turbocharger equipped with a variable nozzle mechanism. The variable nozzle mechanism adjusts the degree of opening of variable nozzles having nozzle vanes through rotation of a unison ring. Through this adjustment, the flow velocity of exhaust gas to a turbine wheel is controlled. In Japanese Utility Model Publication No. 63-115532, plate springs respectively biasing the variable nozzles in an axial direction of the unison ring are fixed to the unison ring. As a result, it is possible to prevent rattling caused by a gap between a shaft portion of each variable nozzle and a housing supporting the shaft portion. In Japanese Utility Model Publication No. 63-92036, a tension coil spring biasing each variable nozzle in the opening direction is provided between a link member of each variable nozzle and the unison ring. As a result, it is possible to prevent rattling caused by a gap between each link member and the unison ring.
In a general variable nozzle mechanism, exhaust gas passing between the nozzle vanes of adjacent variable nozzles imparts a force so as to open the nozzle vanes. The force is transmitted to the unison ring and an actuator for driving the unison ring. Each member of the power transmission route within the actuator as well as each variable nozzle receives a force with which it is pressed in one direction. As a result, it is possible to suppress rattling caused by a gap or backlash or the like between the members connected together in the power transmission route.
As the nozzle vanes open, the amount of exhaust gas acting on the nozzle vanes decreases. In some cases, it is impossible to obtain the force required to press each member in one direction, e.g., the rotational force acting on the nozzle vanes so as to open the nozzle vanes with exhaust gas. Conventionally, to prevent this, the pivot ratio (or the rotary shaft position) of the variable nozzle, or the configuration of the nozzle vanes, has been changed. This, however, results in an increase in the pressure loss of the exhaust gas, thereby resulting in performance deterioration.
Each of the springs as disclosed in Japanese Utility Model Publication No. 63-115532 and Japanese Utility Model Publication No. 63-92036 is provided between each variable nozzle and the unison ring. The springs do not bias the unison ring so as to open the variable nozzles, but rather the springs bias the variable nozzle. Thus, the springs cannot cope with the above problem.
Therefore, there is a need in the art for a variable nozzle turbocharger capable of preventing rattling caused by a gap, backlash or the like between members that are coupled together in the power transmission route of a variable nozzle mechanism.