Internal combustion engine with auxiliary combustion chamber

An internal combustion engine (1) includes a main combustion chamber (2) and a small auxiliary combustion chamber (15). The main chamber (2) is formed by a cylinder, the undersurface of a cylinder head (3), and the crown (5a) of a piston (5). A main ignition unit (13) is disposed in the cylinder head (3), including a main body (16) and a spark plug (17). The auxiliary chamber (15) is formed within the main ignition unit (17). The main (2) and auxiliary chambers (15) are connected via a communication passage (18). The piston crown (5a) includes a cavity (14). When a fuel injection valve (12) sprays fuel into the cavity (14) on compression stroke, a tumble flow of fuel gas is generated to locally form a relatively rich air-fuel mixture near the communication passage (18). The air-fuel mixture partly flows into the auxiliary chamber (15) as the piston (5) travels upwardly. The air-fuel mixture in the auxiliary chamber (15) is ignited by the spark plug (17) so that a combustion flame propagates into the main chamber (2).