while the second stream of air flows through polymeric enrichment device 16 which contains a bundle of tubes 17 through which oxygen passes preferentially with the oxygen depleted or nitrogen enriched air being pushed by fan 19 from the retentate side of tube bundle 17 because the oxygen content of the air stream entering the engine is greater than would otherwise be the case it is necessary for controller 34 which is shown in fig2 to calculate a new lower numerical air fuel ratio for the engine controller 34 calculates a modified air fuel ratio as shown in fig3 when it is desired to operate the present system for example during a cold start of the engine the routine begins with the start command at block 42 of fig3 controller 34 which may comprise a conventional engine microprocessor controller having a bank of sensors operatively connected therewith will ask the question at block 44 as to how long the engine has operated if the engine has operated greater than time constant t c then the oxygen enrichment system will be deactivated at block 52 if however the engine is not operated for a time equal to time t c controller 34 will pass to block 46 where it will inquire as to the engine temperature if the engine temperature at block 46 specifically the engine coolant temperature is less than a constant value e ct then the routine will continue at block 48 with the first step of implementing the oxygen enrichment process if however at block 46 engine coolant temperature exceeds e ct the oxygen enrichment system will once again be deactivated at block 52 at block 48 controller 34 calculates a modified.