a piston for use in an internal combustion engine and including a central portion ; a side wall portion adapted for sliding engagement with a cylinder ; a connector end adapted for connection with a connecting rod ; and a driven end adapted to receive combustion generated forces . the driven end defines a primary chamber adapted to receive fuel and accommodate combustion thereof ; and the central body portion defines an auxiliary chamber , an inlet port providing a fuel injection path between the primary and auxiliary chambers , and outlet port means providing between the auxiliary and primary chambers a discharge path for combustion products generated by combustion in the auxiliary chamber .