Fuel/air ratio control for internal combustion engine

An engine system, comprising:

• a direct injection type internal combustion engine comprising an engine cylinder;
• an actuating system for operating the engine in one of a stratified charge combustion mode and a homogeneous charge combustion mode, the actuating system comprising a fuel injection system for injecting fuel directly into the engine cylinder and varying an actual fuel injection quantity to the engine cylinder in response to a fuel control signal, an intake system comprising a throttle valve for varying an intake air quantity to the engine cylinder in response to an intake air control signal, and an EGR system for recirculating an exhaust gas mixture of the engine to the intake system as an EGR gas and varying an EGR gas quantity in response to an EGR control signal;
• an input device for collecting input information to determine an engine operating parameter indicative of an engine load;
• and
• a controller for producing a changeover request signal requesting a combustion changeover from the stratified combustion mode to the homogeneous combustion mode in accordance with the engine operating parameter, for responding to the combustion changeover request signal by changing a target fuel air ratio from a first target ratio for the stratified combustion mode to a second target ratio for the homogeneous combustion mode, by changing the intake air control signal from a first target air quantity level to achieve the first target ratio to a second target air quantity level to achieve the second target ratio, by starting to increase a desired fuel air ratio from the first target ratio toward the second target ratio gradually with time, by controlling the fuel control signal in accordance with the desired fuel air ratio signal, by changing the EGR control signal from a stratified mode EGR level for the stratified mode to a homogeneous lean mode EGR level to reduce the EGR quantity, and by starting to decrease an estimated residual EGR quantity representing an actual residual EGR gas quantity gradually from the stratified mode EGR level to the homogeneous lean mode EGR level with time, for producing a combustion changeover command signal to allow the combustion changeover responsive to the combustion changeover request signal when the desired fuel air ratio becomes equal to or greater than a predetermined threshold, and for responding to the combustion changeover command signal by increasing the desired fuel air ratio abruptly from the threshold level to a predetermined enriched level to improve stability of combustion if the residual EGR quantity is still equal to or greater than a predetermined value.