Homogeneous charge compression ignition (HCCI) combustion timing depends on properties of gas in the cylinder such as temperature and composition. However, these properties are extremely difficult to measure. In this work, we present a method to estimate the thermodynamic state in order to map ignition timing for exhaust-recompression HCCI. Viability is demonstrated on four experiments. To capture the effect of fuel that is injected into recompressed combustion products, the method begins with a hypothesis of recompression reactions. From an initial guess, the method deterministically updates the chemical composition estimate in the cylinder over combustion, exhaust, recompression reactions and induction. The update function is based on incylinder pressure measurements, the ideal gas law and the first law of thermodynamics. The estimates are compared to measurements of inducted air and exhaust oxygen concentration to check the original hypothesis. Finally, the estimated thermodynamic state is correlated to combustion timing to produce a rich ignition map.

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