Stability analysis in homogeneous charge compression ignition (HCCI) engines with high dilution

This paper analyzes the stability of the autoignition process of homogeneous charge compression ignition (HCCI) engines with exhaust dilution. We find conditions under which steady-state multiplicity exists with stable and unstable equilibria. This analysis is conducted taking into account the internal feedback structure of the thermal dynamics. Specifically, HCCI combustion timing determines the combustion heat produced and is determined by the heat provided through high internal exhaust gas recirculation from the previous combustion cycle. It is shown that the thermal equilibria are characterized by a simple returning map consisting of two curves, namely the breathing temperature curve and the combustion temperature curve. The influence of heat transfer and the cooling system in the system stability is also analyzed. The returning map and the stability of the multiple steady-state equilibria are confirmed with a high-order dynamic nonlinear model. The high-order dynamic model includes manifold filling and composition dynamics and has been validated both at steady state and during transient. It is shown that a static feedforward controller can cause instability during switching from a high to a low load. A dynamic feedforward controller, on the other hand, is able to stabilize the transition by reducing temperature excursions and, hence, keeping the temperature trajectories within stable regions.