Increased residual levels in Homogeneous Charge Compression Ignition (HCCI) engines employing valve strategies such as recompression or negative valve overlap (NVO) imply that accurate estimation of residual gas fraction (RGF) is critical for cylinder pressure heat release analysis. The objective of the present work was to evaluate three residual estimation methods and assess their suitability under naturally aspirated and boosted HCCI operating conditions: i) the Simple State Equation method employs the Ideal Gas Law at exhaust valve closing (EVC); ii) the Mirsky method assumes isentropic exhaust process; and iii) the Fitzgerald method models in-cylinder temperature from exhaust valve opening (EVO) to EVC by accounting for heat loss during the exhaust process and uses measured exhaust temperature for calibration. Simulations with a calibrated and validated “virtual engine” were performed for representative HCCI operating conditions of engine speed, fuel-air equivalence ratio, NVO and intake pressure (boosting). The State Equation method always overestimated RGF by more than 10%. The Mirsky method was most robust, with average errors between 3–5%. The Fitzgerald method performed consistently better, ranging from no error to 5%, where increased boosting caused the largest discrepancies. A sensitivity study was also performed and determined that the Mirsky method was most robust to possible pressure and temperature measurement errors.

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