Speed responsive systems

A speed-responsive system for a throttle controlled internal combustion engine, comprising means for generating a first voltage which changes with the rotational speed of a rotatable member driven from said internal combustion engine, means for generating a second voltage which is proportional to the rate of change of said first voltage, means for generating a reference voltage, first comparator means for combining said first and second voltages with the reference voltage, said first comparator means being arranged to change its output signal dependent upon the relative magnitude of said first voltage, said second voltage and said reference voltage, second comparator means fed with said output signal from said first comparator means and with a periodic voltage of constant frequency so as to produce at the output of said second comparator means a train of pulses of constant frequency and of a width varying with the relative magnitude of said first voltage, said second voltage and said reference voltage, an actuator device comprising a chamber partly bounded by a movable wall and connected to atmosphere through a permanently open orifice and an electromagnetic valve which is maintained in a closed position when de-energised and which controls the admission to said chamber of reduced gas pressure from the inlet manifold of said engine when said valve is open and means for feeding said train of pulses from said second comparator to energise said electromagnetic valve and thereby admit pulses of said reduced gas pressure from said inlet manifold of said engine to said chamber, and the resultant movement of said movable wall of said chamber is employed to control the engine throttle.