The invention provides a cylinder-cover-free engine with a novel crankshaft-connecting-rod mechanism. A power output unit of the engine comprises the novel crankshaft-connecting-rod mechanism, a power output gear and an air cylinder opposition device. The novel crankshaft-connecting-rod mechanism comprises a straight shaft, round wheels, connecting rods and fixed track plates. The two fixed track plates are driven by the air cylinder opposition device to do opening and closing reciprocating linear movement, and then the two connecting rods are driven to simultaneously conduct pushing drive and pulling drive on the round wheels at the two ends of the straight shaft correspondingly in the opposite directions, so that the two round wheels drive the straight shaft and the power output gear to rotate continuously. According to the engine, the novel crankshaft-connecting-rod mechanism which is simple in structure and reliable in operation is used for replacing conventional crankshaft-connecting-rod mechanisms with many defects, and the advanced air cylinder opposition device is adopted for driving; and an acting air cylinder of the air cylinder opposition device is not provided with a cylinder cover, automatic supercharging can be achieved while the air cylinder acts, the displacement can be flexibly changed, the fuel conversion efficiency is remarkably improved, and the two problems of lubrication and complete air exchange of conventional engines are effectively solved.