In a throttle valve device provided in an intake system of an internal combustion engine, icing or deposition of ice on an inner wall of a throttle bore owing to the freezing of moisture that condenses in the throttle bore is required to be avoided so that the throttle valve member may be allowed to be opened and closed without fail. For this purpose, it was proposed to provide a concentric annular ridge around the entire circumference of the throttle bore and install a heater buried within the annular ridge over the entire circumference thereof (see Japanese patent laid open publication No. 2002-206434).
In such a throttle valve device which is disposed in a lateral orientation with the throttle bore extending in the horizontal direction, because the inner wall of the throttle bore adjoining the peripheral edge of the throttle valve member is elevated with respect to the adjoining parts, accumulation or deposition of moisture between the peripheral edge of the throttle valve member at its fully closed position and the inner wall of the throttle bore can be avoided. Moreover, under an operating condition where freezing of moisture could occur, the heater is energized so that ice deposition that may exist can be melted, and any freezing or seizing between the outer peripheral part of the throttle valve and the ridge can be avoided. In particular, because the ridge is given with a small width, the dissipation of heat from the heater to the surrounding part of the throttle body by conduction can be minimized, and the consumption of electric power for deicing the throttle valve device can be minimized.
However, in such a conventional throttle valve device, because the ridge extends over the entire circumference of the inner wall of the throttle bore adjoining the outer edge of the throttle valve member at its fully closed position, and the width of the ridge is relatively small, the effective cross sectional area of the throttle bore abruptly increases when the throttle valve member has turned by a small angle from its fully closed position. Therefore, the air flow that is metered by the throttle valve member or the intake flow rate abruptly increases as the throttle valve member opens from the fully closed position, and this makes the intake flow rate control highly difficult.
Also, in the conventional arrangement, the heater extends over the entire circumference of the throttle bore, and heats the entire circumference of the throttle bore including the upper part thereof and parts adjoining the valve shaft. Therefore, the heater is employed to heat not only the necessary part but also unnecessary parts, and this causes a significant part of the heating energy to be wasted.