at step 123 the ecu 30 executes throttle angle control for stratified charge combustion the throttle valve is substantially completely opened and temporarily terminates subsequent processing if it is determined that the engine 1 is performing uniform charge combustion the ecu 30 proceeds to step 103 and determines whether or not the present air fuel ratio a f is greater than the stoichiometric air fuel ratio when the present air fuel ratio a f is greater than the stoichiometric air fuel ratio that is when the air fuel mixture is lean the ecu 30 proceeds to step 104 and determines whether or not the rich flag xrich is presently set at zero the rich flag xrich indicates whether the air fuel mixture was rich or lean in the previous cycle of the routine if the air fuel mixture was rich the flag xrich was set at one if the air fuel mixture is lean the flag xrich is set at zero when it is determined that the rich flag xrich is set at zero this indicates that the air fuel mixture was lean in the previous cycle in addition to the present cycle in this case the ecu 30 proceeds to step 105 and sets the.