Abstract: Fuel supply in an ultrasonic atomizer is conducted according to a fuel increment ratio pattern in which the increment of fuel in fuel increment control for starting and warming up is 70% or less of that in a typical conventional pressure injection valve system, thereby improving startability, accelerability and fuel consumption rate and further enabling a reduction in exhaust emissions. When the engine is started in low-temperature conditions, the fuel is supplied by continuous injection to make uniform and reduce the mean diameter of droplets of atomized fuel, thereby improving the ignitability and startability. The fuel injection start timing is varied in accordance with the combustion chamber temperature at the time of starting the engine, i.e.

Abstract: In an ultrasonic oscillator which comprises an ultrasonic vibrator for driving an ultrasonic vibrator horn of an ultrasonic atomizer, a balanced circuit in which the ultrasonic vibrator is connected so that a balanced condition is established with respect to the damping capacity of the ultrasonic vibrator, and amplifier means for sending a signal to the balanced circuit and which includes a feedback circuit for positively feeding back the signal output from the balanced circuit, the feedback circuit is connected to means for adjusting the feedback quantity so that, when the ultrasonic oscillator starts the oscillation, the signal feedback quantity output from the balanced circuit is adjusted to make the gain of the amplifier means much greater than 1, and when the oscillator is in the steadily oscillating condition, the signal feedback quantity output from the balanced circuit is adjusted to broaden the allowable frequency band width of the ultrasonic vibrator.