Closed loop air-fuel ratio control system

Claim 1

A closed loop air-fuel ratio control system for an engine having an air induction passage for receiving a flow of air, a main fuel passage for receiving a flow of liquid fuel, and a purge passage for receiving fuel vapor and air from a fuel vapor storage region, said engine being effective to combine the fluids received through said passages into an air-fuel mixture, said air-fuel ratio control system comprising the combination of means responsive to the air-fuel ratio of said mixture for producing a signal indicative of the fuel flow required to provide a desired air-fuel ratio, a metering valve in said main fuel passage, said metering valve being responsive to said signal for varying fuel flow through said fuel passage generally linearly with said signal, and a purge valve in said purge passage, said purge valve being responsive to said signal for minimizing flow through said purge passage when said signal approaches its maximum and its minimum values and for maximizing flow through said purge passage when said signal is intermediate its maximum and minimum values, whereby fuel vapor purged from said region is used to provide the desired air-fuel ratio only when fuel vapor purged from said region cannot unduly enrich said mixture and air purged from said region cannot unduly lean said mixture.