The invention relates to a constant-volume combustion engine (10; 110; 210), in particular a reciprocating engine for generating mechanical energy by the expansion of a gas or a hot gas from the combustion of a gas mixture or gas-fuel mixture, having at least one piston/cylinder unit, the piston (14; 114; 214) of which is connected to a piston rod (20; 120; 220), wherein said piston rod (20; 120; 220) is drivingly connected to at least two crankshafts (30, 40; 130, 140; 230a, 230b, 240), the first crankshaft (40; 140; 240) being mounted, such that it can rotate eccentrically, on the second crankshaft (30; 130; 230a, 230b), which is parallel thereto and is rotationally coupled thereto.