Abstract: A valve for controlling the admission of gas to a cylinder of a piston-type machine, or for the discharge of gas therefrom, comprises a magnetic armature shiftable between a pair of magnetic poles formed to receive the armature with minimal radial play and with a predetermined air gap from the attracting pole in the energized condition of the coil. The energizing circuit includes an inductive proximity switch which is triggered without engagement by a metal formation on the crankshaft and applies its signal to a pulse former generating a steep-flank pulse triggering a timer to produce two rectangular voltage pulses of different durations. These pulses are applied to a gating circuit which produces a stepped signal with a high initial amplitude which is amplified and applied to a power converter energizing the coil with a corresponding current waveform. A monitoring circuit responds to deenergizing the coil to drain residual magnetism from the pole piece to enable a spring to rapidly close the valve.