Coordinated control of lean burn gasoline engines with continuously variable transmissions

We explore the use of a continuously variable transmission (CVT) for torque management during mode transitions in lean burn gasoline engines. These mode transitions are between fuel efficient lean operation and rich operation required for purging an essential exhaust after treatment component of the lean burn engines, the so-called lean NOx trap (LNT). We demonstrate that an intuitively sound CVT gear ratio control strategy which attempts to completely cancel the engine torque disturbance may result in unstable zero dynamics. Several control schemes are then described that coordinate the gear ratio control and engine torque production to achieve stable operation and prevent the torque disturbance caused by the mode transition from affecting the vehicle operation.