Abstract: The invention discloses a differential speed reciprocating internal combustion engine. It consists of one or more cylinders, a pair of pistons in each cylinder, and a crank connecting rod mechanism for each piston. The piston pair consists of a power piston and an auxiliary piston that are positioned oppositely in the same cylinder. The pistons keep a differential angle of 35°-75° CA and make differential speed movement under the control of a coordination mechanism. Since combustion takes place at a position close to the middle of the travel of the power piston, the crank connecting rod mechanism has a large lever arm coefficient when it is under the maximum combustion pressure. Thus the combustion thermal energy can be more efficiently utilized.