PURPOSE: To supplement an assist air to an engine while keeping an optimal diameter of a knife-edge nozzle capable of atomizing fuel efficiently, by providing a by-pass passage 81 connected at one end thereof to a downstream side of a flow control valve 29 and at the other end to a portion of the intake system located on the downstream side of a throttle valve.
CONSTITUTION: At the time of idling operation of an engine, a control circuit 31 opens a flow control valve 29. which functions to pass air at a rate corresponding to the engine temperature. The air thus passed through the valve 29 serves as "assist air" and applies shearing force to fuel flow at the position of an injector 11, so that atomization of fuel is promoted. Here, the diameter D of a knife-edge nozzle 51 is selected at a value D1 capable of atomizing fuel most efficiently. On the other hand, when a cold-start injector 18 is set into operation, the control circuit 31 produces a signal for opening a valve 83 in addition to the flow control valve 29. Resultantly, air is supplied to a surge tank 7 from the by-pass passage 81.
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