Modeling the Effects of Combustion Variability for Application to Idle Speed Control in SI Engines

Combustion in the cylinder of a spark ignition engine, particularly under low load conditions, is subject to cycle-by-cycle variations due to factors such as mixture quality and quantity and internal exhaust gas recirculation. The major result of this phenomenon is an increase in the variability of indicated engine torque at a given engine operating point. Automotive control problems dealing with torque production at low engine loads, particularly the control of idle speed, rely on accurate information about the transfer functions of different engine subsystems, however combustion variability and the effect it has on torque production is often overlooked. In this paper we illustrate the effects that combustion variability at idle has on different transfer functions related to indicated torque, and propose new models for torque production at constant operating points. We also present a model of an in-line, six-cylinder, 4.0 litre Ford engine for use in idle speed simulations and control applications. The integration of combustion variability into an idle speed control framework is also discussed.