and the output from the controller 15 is maintained at a constant level while leaving the output from the proportional amplifier 14 to be varied in accordance with the input voltage the system is thus changed to operate in the proportional control mode in this mode of operation the output from the pi controller 8 will be held within a range determined by the output of proportional controller 14 even when the sensed voltage from oxygen sensor 6 rises above the reference voltage vo i e when the engine 39 s air fuel ratio is below the stoichiometric value therefore the engine 39 s air fuel ratio is maintained at a value lower than stoichiometry when the vehicle comes out of the heavy load condition the throttle switch 11 will be released to open the relay contact t 1 to allow charge to be stored on capacitor c1 again to resume integral operation since the charge will build up from the minimum the voltage output from the integral controller 15 will rise gradually this is an advantageous feature of the embodiment of fig2 and 4 since abrupt change in voltage to the actuator 10 is not desirable when the system resumes its feedback control alternatively rely contact contact u 1 is connected across resistor r1 as shown in fig5 to bring the reference voltage vo to the same level as the source voltage vcc when relay u is operated by the signal from the throttle position switch 11 with vo at the source voltage the voltage on the inverting input terminal of the amplifier 13 is always below the.