A system and method for controlling a direct injection stratified charge
engine during transitions between homogeneous and stratified operating
modes position the throttle to a position corresponding to the desired
operating mode to begin the transition and use valve timing control to
provide a rapid change in air flow to traverse the gap between acceptable
air/fuel ratios associated with the homogeneous and stratified modes. The
invention reduces torque variations while improving engine performance by
coordinating control of an electronic throttle and valve timing to reduce
or eliminate step changes in fuel previously required to traverse the
air/fuel ratio gap between modes. Primary reliance on air flow and fuel
flow to reduce torque variations and improve driveability reduces reliance
on spark timing and associated performance penalties.