Abstract: A housing is mounted on the body of an internal combustion engine to vibrate together therewith. The housing contains a weight that is constructed as a unitary structure together with a vibration unit which has such a rigidity as to resonate at knocking frequencies of the internal combustion engine, and a converter element which converts the pressure received from the weight into electric signals.

Abstract: A suction pipe is provided to suck and introduce air from its one end and is connected at its other end to an internal combustion engine. The suction pipe is provided at its one end with a venturi portion and a throttle valve. A first negative pressure detector is provided to detect the negative pressure at the portion of the suction pipe remote from the internal combustion engine, so as to sense a comparatively gentle pressure rise in the suction pipe. A control valve is provided to be opened and closed in accordance with the output from the first negative pressure detector, thereby to control the flow rate of air supplied into the suction pipe. A second negative pressure detector is provided to detect the negative pressure at a portion of the suction pipe remote from the internal combustion engine, so as to sense an abrupt increase of the negative pressure in the suction pipe.