PURPOSE: To prevent injection of quantities of compressed air due to deterioration of atomization of injection fuel and reduction in a pressure in a combustion chamber by a method wherein when fuel is injected in a combustion chamber from an air blast valve, the pressure of compressed air is increased with the increase in a pressure in the combustion chamber.

CONSTITUTION: When a feed valve 5 is opened, the interior of a surge tank 12 is communicated with the interior of a combustion chamber 4, and a scavenging pressure in the surge tank 12 attains a value approximately equal to a pressure in the combustion chamber 4. In this period, a compressed air pressure fed to an air blast valve 21 is regulated to a value approximately 3kg/cm2 higher than a pressure in the combustion chamber 4 by means of a pressure regulator 22. Thus, when, during this period, the nozzle of the air blast valve 21 is opened to inject fuel in the combustion chamber 4, since a compressed air pressure is always regulated to a value higher by a proper pressure than that in the combustion chamber 4, atomization of injection fuel is prevented from worsening due to an increase in a pressure in the combustion chamber 4, and quantities of compressed air is prevented from being injected in the combustion chamber 4 from the air blast valve 21 as a result of a pressure in the combustion chamber 4 being reduced.