Representing recompression HCCI dynamics with a switching linear model

Homogeneous charge compression ignition (HCCI) promises efficient combustion and less NO_x emissions over conventional modes. However, the lack of direct ignition trigger and the cycle-to-cycle coupling in recompression HCCI makes the combustion phasing control problem difficult. To further complicate the matter, we show in this paper that the natural dynamics of HCCI can change drastically from one operating point to another. Our analyses show that the operating range of recompression HCCI can be partitioned into three linear regions such that a single linearized model can reasonably capture the system behavior within each region. As a result, a switching linear model that switches between three linearized systems shows better agreement with the physical testbed compared to using a single linear model. The switching linear model also gives insights on what the appropriate feedback control actions should be in each of the region and reveals that a controller that works well in one region may have a directional error when blindly applied to a different operating region.