Reappraising energy conversion in internal combustion engines: the ideal cycle and corresponding efficiency definition from a chemical-energy perspective

Conventional ideal internal combustion engine cycles are grounded in the air-standard assumption, which obscures the compositional evolution of the working fluid and mischaracterizes the true nature of energy input as external heat addition rather than in-cylinder chemical conversion. In this work, the internal combustion engine is reinterpreted as a chemically driven power system, and its ideal cycle is reconstructed accordingly. The efficiency is redefined as the ratio of actual work output to ideal work output, with a limiting value of unity attainable only when the entire cycle proceeds thermodynamically reversibly. In particular, thermodynamic reversibility of the chemical reaction requires the reaction Gibbs free-energy change to vanish. By introducing reaction extent as an independent state variable, a three-dimensional pressure–volume–reaction extent framework is developed to describe thermodynamic state evolution and chemical conversion within a unified representation. Among the representative fuels analyzed, only hydrogen admits a physically feasible solution within the valid temperature range, with the corresponding reaction temperature being approximately 4310.18 K. This fuel-dependent result demonstrates that the efficiency limit of internal combustion engines is governed not only by cycle configuration, but more fundamentally by the irreversibility intrinsic to the chemical reaction path. Comparison with experimentally measured brake thermal efficiencies shows that the proposed efficiency is slightly higher in most cases, while remaining reasonably close to the experimental values. This work provides a unified thermodynamic basis for evaluating ideal work potential, irreversible losses, and deviations of real engine processes from the reversible limit.