US Patent 7401600 Purge flow control to reduce air/fuel ratio imbalance

A fuel control system that regulates a purge flow from a fueling system to an engine includes a sensor that monitors an engine speed and a first module that determines a PWM frequency of a purge valve based on the engine speed. The PWM frequency includes a first period that is based on a second period that corresponds to two engine cycles. A second module regulates the purge valve based on the PWM frequency during engine operation.