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The potential for simultaneous reduction of soot and NOx emissions and higher fuel conversion efficiency has already been demonstrated for reactivity-controlled compression ignition (RCCI) engines. The RCCI engine has a relatively higher peak pressure rise rate (PPRR) and cyclic variations compared to the conventional diesel engine. The upper and lower operating load boundaries of the RCCI engine are restricted by higher PPRR and cyclic variations, respectively. The cyclic variations in the air-fuel ratio are one of the main factors which govern the variations in combustion parameters. The cyclic variations in combustion need to be controlled for stable engine operation. The present study estimates the cyclic air-fuel ratio from the measured in-cylinder pressure data for the RCCI engine. The RCCI experiments are performed on a modified single-cylinder compression ignition (CI) engine equipped with a development ECU.

Cyclic variations are inherent in the combustion of internal combustion engines. However, extreme cyclic combustion variations limit the operation of spark-ignition (SI) engines, particularly at highly lean and diluted charge operation. Lean charge operation is desired due to its expected benefits in fuel efficiency and engine-out NOx and HC emissions. Studies suggested the existence of the low-dimensional deterministic nature of cyclic variations, which is essential from the perspective of designing a high-frequency controller. The lean limit of a SI engine operation may be extended by controlling the deterministic component of cyclic variations to meet the future strict emissions and fuel economy regulations. This paper presents a review of the evolution of the experimental and analytical understanding of cyclic combustion variations of spark-ignition engines.