Abstract: An engine intake system including an intake passage opening to the combustion chamber through an intake port having an intake valve. A control valve is provided in the intake valve for cyclically opening the intake passage. Upstream the control valve, the intake passage is provided with a tank which functions to reverse the negative pressure propagated from the control valve into a positive pressure which is then transmitted to the combustion chamber to increase the intake charge. A timing control mechanism is provided for retarding the opening timing of the control valve in a low speed, high load engine operation.

Abstract: A part of intake passages is made into an arcuate form extending around a surge tank. A rotatable drum is provided in the surge tank to change the length of the arcuate part of the passages. The rotatable drum is controlled by a control mechanism which is operated in accordance with the engine speed for example, so that the effective length of the intake passages may be changed to keep synchronization between the natural frequency of the intake system and the frequency of the pressure wave caused by the reciprocal motion of the piston of the engine.