The invention relates to a piston engine comprising one or more air/gas-compressing cylinder-piston units, a crank mechanism (10) connected to the piston (5) in the said cylinder (4) or cylinders for movement of the piston in the cylinder, which crank mechanism (10) is drivingly connected to a rotor (14) in an electric motor (2). The invention also relates to a coupling and a simple connection for a counterweight.
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