1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine having a cantilevered valve for opening and closing an intake passage that communicates with an intake port of the engine.
2. Description of Related Art
Conventionally, an intake system of an internal combustion engine that curbs dispersion of combustion by stabilizing generation of a swirling flow (tumble flow) in a combustion chamber of the engine by reducing a passage sectional area of an intake passage, through which intake air is supplied to the combustion chamber, is proposed (see, e.g., JP-A-2008-25363). As shown in FIG. 10A and FIG. 10B, this intake system of the engine includes an intake manifold 101 of the engine, an intake air flow control valve for opening and closing an intake passage 102 formed in the intake manifold 101, and a rotatable shaft 104 that supports a valve 103 which is a valve body of the intake air flow control valve.
A cantilevered valve which offsets a centre position of the rotatable shaft 104 (rotation center position of the valve 103) with respect to a centre position of the valve 103 is used as the valve body of the intake air flow control valve. An actuator that includes, for example, a motor for driving the rotatable shaft 104 of the valve 103 and a return spring for urging the valve 103 to be in a fully open state (fully open position) is connected to the intake air flow control valve. The motor is configured such that opening and closing timing of the valve 103, for example, is controlled by an engine control unit (ECU). To fully close the valve 103 of the intake air flow control valve, the rotatable shaft 104 is driven using driving torque of the motor through the supply of electric power to the motor so that a degree of opening of the valve 103 is in a fully closed state (fully closed position). To fully open the valve 103 of the intake air flow control valve, by stopping the supply of electric power to the motor, the return spring urges the valve 103 using spring force of the return spring such that the degree of opening of the valve 103 is in the fully open state (fully open position).
However, the cantilevered valve which offsets the centre position of the rotatable shaft 104 with respect to the centre position of the valve 103, is employed for the intake air flow control valve described in JP-A-2008-25363. In this case, due to differential pressure between upstream and downstream sides of the valve 103 generated when the valve 103 of the intake air flow control valve is fully closed, rotary torque (bending moment) in a direction of valve opening operation with the rotatable shaft 104 of the intake air flow control valve as its center, is applied to the valve 103. Accordingly, maintenance accuracy of the opening degree in maintaining the valve 103 of the intake air flow control valve in the fully closed state (fully closed position) deteriorates. As a result, the motor requires excessive holding torque to maintain the valve 103 of the intake air flow control valve in the fully closed state (fully closed position). Therefore, defects such as upsizing of the actuator including the motor and increase in power consumption of the motor are caused.
In addition, the valve 103 of the intake air flow control valve is disposed normally in the intake passage 102 to whose interior portion large negative pressure of intake air and small atmospheric pressure are repeatedly applied in accordance with ascent and descent of a piston of the engine and opening and closing of an inlet valve. Thus, pressure of intake air is applied in a pulsatile fashion to the valve 103 disposed in the intake passage 102. When the pressure of intake air is applied in a pulsatile fashion in the above-described manner, the valve 103 flaps in the intake passage 102. Accordingly, it is difficult to maintain the valve 103 of the intake air flow control valve in the fully closed state (fully closed position) or to maintain the valve 103 of the intake air flow control valve in the fully open state (fully open position), and the opening degree maintenance accuracy in maintaining the valve 103 of the intake air flow control valve in the fully open position and in the fully closed position deteriorates. As a result, the generation of a swirling flow (tumble flow) in a combustion chamber of the engine becomes unstable. Consequently, an effect of improving combustion efficiency of the engine and a fuel efficiency improvement effect by the stabilization of combustion are reduced.