Closed-Loop Ignition Timing Control for SI Engines Using Ionization Current Feedback

Minimal advance for best torque (MBT) timing for an internal combustion (IC) spark ignition (SI) engine is the minimum advance of spark timing for the best torque or, in other words, for the best fuel economy. But MBT timing is often limited by engine knock in the advanced direction and spark timing is also constrained by partial burn and misfire in the retard direction. It is preferred to operate IC engines at MBT timing when it is not knock limited and at borderline knock limit when it is knock limited. During cold start conditions it is desired to operate IC engines at its maximum retard limit subject to combustion stability constraints to reduce catalyst light-off time. Traditionally, both MBT timing and retard spark limit are open-loop feedforward controls whose values are experimentally determined by conducting spark sweeps at different speed and load points, and at different environmental conditions. The borderline knock limit is controlled by a dual-rate count-up/count-down closed-loop control utilizing information from engine knock sensor signals. A closed-loop control architecture for spark timing is proposed in this paper. Using in-cylinder ionization signals both borderline knock and retard spark limits are regulated using closed-loop stochastic limit controls. MBT timing is also controlled closed-loop using an MBT criterion derived from in-cylinder ionization signals. The proposed control strategy and architecture was experimentally validated on a 3.0-L V6 engine for steady state and slow transient conditions