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    Energy and exergy analysis of a cycle‑skipping strategy in an HCCI engine fueled with natural gas
    (Springer Nature Link, 2025) Atak, Nisa Nur; Tunçer, Erdal; Doğan, Battal; Ünal, Kenan
    Homogeneous charge compression ignition (HCCI) engines have attracted considerable interest due to its incorporation of features from both gasoline and diesel engines. In this study, the effects of cycle-skipping strategies on engine perfor mance, efficiency, and emissions in a natural gas-fired HCCI engine were investigated experimentally. Experiments were conducted under constant engine speed, at 25, 50, and 75% load levels, in the Normal, 3 Normal-1 Skip (3N1S), 2 Normal-1 Skip (2N1S), and 1 Normal-1 Skip (1N1S) cycle modes. Emissions, fuel consumption, energy–exergy flow, thermal losses, irreversibilities, entropy generation, thermal, and exergy efficiencies were calculated based on the experimental data. Under 50% engine load, the NOx emission in the 3N1S operating mode was measured at 1594 ppm, whereas it increased by 52.5% to 2431 ppm in the 2N1S mode. The correlation between cycle-skipping tactics and thermal efficiency was determined to be contingent upon engine load. In Normal and 3N1S modes, thermal efficiency generally improves with elevated engine loads, while the 2N1S and 1N1S modes provide superior performance at low to medium loads. At a constant 50% load, heat effi ciency in the 2N1S mode increased by 7.84% to reach 28.34% compared to the Normal mode. Additionally, thermodynamic analyses revealed that the 1N1S mode has the lowest entropy generation and the least irreversibility, at 0.021 kW/K. These results demonstrate that cycle-skipping strategies can be an effective tool for optimizing engine performance based on load.