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Cyclic variability (CV) in lean homogeneous charge compression ignition (HCCI) combustion at the limits of operation is a known phenomenon, and this work aims at investigating the dominant effects for the cycle evolution at these conditions in a multicylinder engine. Experiments are peiformed in a four-cylinder engine at the operating limits at late phasing of lean HCCI operation with negative valve overlap (nvo). A combtistion analysis method that estimates the utiburned fuel mass on a per-cycle basis is applied on both main combustion and the nvo period revealing and quantifying the dominant effects for the cycle evolution at high CV. The interpretation of the results and comparisons with data from a single-cylinder engine indicate that, at high CV, the evolution of combustion phasing is dominated by low-order deterministic couplings similar to the single-cylinder behavior. Variations, such as air fiow and wall temperature, between cylinders strongly infiuetice the level of CV but the evolution of the combustion phasing is governed by the interactions between engine cycles of the individual cylinders.

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In this work, a physics-based method of estimating the residual mass in a recompression homogeneous charge compression ignition engine is developed and analyzed for real-time implementation. The estimation routine is achieved through in-cylinder pressure and exhaust temperature measurements coupled with energy and mass conservation laws applied during the exhaust period. Experimental results on a multicylinder gasoline homogeneous charge compression ignition engine and dynamic analysis demonstrate the estimation routine’s ability to perform across a wide range of operating conditions as well as on a cycle-by-cycle basis for highly variable combustion phasing data.