Abstract

A dual mode control system for operating an electronic fuel injection system for an internal combustion engine so as to achieve an optimal compromise between engine emissions, fuel economy and driveability. A closed loop control circuit is provided which senses the amount of oxygen in the engine exhaust and normally drives an integrator in a closed loop mode of operation to operate the electronic fuel injection system at the stoichiometric air/fuel ratio at which the best conversion efficiency of hydrocarbons, carbon monoxide and nitrous oxides occur. An open loop control circuit senses high speed operation where hydrocarbons and carbon monoxide conversion are normally high and where nitrous oxide emission is not critical and clamps the output of the integrator to a predetermined value which operates the electronic fuel injection system at a nonstoichiometric, relatively lean, air/fuel ratio for improved fuel economy. Further open loop control circuitry may be provided to sense very low speed, low engine load operation where nitrous oxide emission is negligible for similarly switching to an open loop mode of operation with the integrator output clamped. An override circuit may be provided which senses engine acceleration which normally requires a relatively rich air/fuel ratio for good driveability and overrides the clamped integrator output to restore the closed loop mode of operation regardless of the engine speed thereby achieving an optimal compromise between engine emissions, fuel economy and drivability.