PURPOSE: To reduce the throttle loss under producing effective swirl by a method wherein a combustion chamber is composed of a main chamber partitioned by the surface of piston and the inner wall of cylinder head, the first sub-chamber connected to the main chamber and the second sub-chamber connected to the first sub-chamber while a protrusion is provided on the surface of piston.

CONSTITUTION: When a piston 4 is lifted up, a protrusion 4b throttles the first communicating cannel 5 to produce swirl in the first sub-chamber 1B and the second sub-chamber 1C synchronously jetting fuel subject to specified characteristics out of the first fuel jetting nozzle 7 and the second fuel jetting nozzle 8 while in case of low and intermediate loading, a dilute mixture and another mixture with air-fuel ratio optimum for ignition are respectively produced in the first sub-chamber 1B and the second sub- chamber 1C. On the other hand, a flame ignited by an ignition plug 9 flows from the second communicating channel 6 to the first sub-chamber 1B to ignite the fuel jetted from the first jetting nozzle 7 spreading the flame from the first communicating channel 5 to the main combustion chamber 1A to start the layer combustion. At this time, the protrusion 4b leaves the first communicating channel 5 in case the piston 4 is lowered expanding the channel space thus reducing the channel resistance resultantly to reduce the throttle loss of combustion energy improving the transmitting efficiency.