Application of disturbance observers to automotive engine idle speed control for fuel economy improvement

Gasoline engine fuel economy can be improved by minimizing the base spark retard, the spark reserve, during idle. Without additional compensation this approach can also lead to a significant degradation in idle speed control, ISC, disturbance rejection response. In this paper an analysis of ISC lead compensation, feed-forward and disturbance observer design techniques is presented, for ISC systems with minimal spark reserve levels. The integration of the disturbance observer within the ISC system is analyzed, including the impact of observer torque reconstruction techniques and unknown delays on the idle speed disturbance rejection response. Simulation results are used to show that the best combination of disturbance rejection response and delay variation robustness can be achieved by setting the observer torque input to zero. A high fidelity cylinder-by-cylinder nonlinear engine model and ISC simulation are used to validate the linear simulation results. The nonlinear simulation results confirm that setting the disturbance observer torque input to zero produces the best overall result with a 30 percent reduction in the maximum drop in engine speed when compared to an ISC with no lead or disturbance observer compensation. Further the disturbance rejection response compares favorably with responses from conventional ISC with high levels of spark reserve