Fuel supply control method and ultrasonic atomizer

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., when the engine is to be started in low-temperature conditions, no fuel is injected until a predetermined time has elapsed, and the fuel injection is started after the combustion chamber temperature has been raised by means of compression heat by driving the starter, thereby improving the cold startability even in the case of a fuel with a relatively high flash point. When the engine is in a transient operating condition, fuel injection from the ultrasonic atomizer is executed immediately before the velocity of an air stream in the vicinity of the ultrasonic atomizer rises, whereby the fuel that is atomized with a sufficient spread in the intake pipe can be carried in this state by the air stream to the combustion chamber where it is burned. An ultrasonic atomizer comprises a vibrator horn (3) having at the distal end a slant portion and a reduced-diameter portion of a smaller diameter than the slant portion, and a sleeve (4) disposed around the outer periphery of the vibrator horn to feed fuel over the entire circumference of the horn, the sleeve having an opening (4a) which faces the slant portion, whereby the fuel can be smoothly atomized by ultrasonic vibrations and sprayed with a predetermined spray angle.