A method of producing electrical energy from a cyclic combustion process which employ a device comprising two oppositely disposed aligned spark ignited internal combustion engine cylinders axially spaced from each other, a piston in each of the cylinders, means rigidly connecting the pistons so that they reciprocate in unison in their respective cylinders, means for alternately firing the two cylinders so as to reciprocate the piston system composed of the two pistons and the connecting means, a reciprocatory electrical generator-motor means driven by said reciprocatory piston system, said generator-motor means comprising a coil rigidly attached to and reciprocating with the said piston system, and means creating a constant strength magnetic field within which the coil reciprocates, said method comprising beginning the combustion process in each combustion chamber at a low compression ratio sufficient for the initial ignition of the fuel-air mixture in transmitting an electric spark, causing said combustion to proceed at a diminishing volume under the influence of the opposite combustion chamber forces, and terminating the combustion process at an expanding volume during the reverse piston stroke, whereby there is induced an electromotive voltage in the coil, the device having a source of electrical current, and means selectively to energize the coil, the method comprising switching-on the last named means, in the initial reciprocation of the piston system to energize the coil so that it drivingly reciprocates the piston system.