A thermodynamic principle for engine design: Exergy optimization of spark-ignition engines fueled by hydrogen-enriched biogas based on otto cycle foundations

Gómez Montoya J.P. · Applied Thermal Engineering (2025) · DOI: 10.1016/j.applthermaleng.2025.128376

Resumen

The energy transition requires spark-ignition (SI) engine design to evolve from empirical adjustment toward thermodynamically grounded principles. This study proposes and validates a generalized exergy-based framework, rooted in the ideal Otto cycle, as the foundation of a thermodynamic design principle for SI engines fueled by renewable gaseous blends. A comprehensive methodology—combining thermodynamic modeling, sensitivity analysis, chemical exergy assessment, and experimental validation—was applied to twenty gaseous blends: twelve measured in a CFR engine and eight simulated using AVL Methane 3.0 software to estimate methane number (MN) and critical compression ratio (CCR). The analysis establishes the influence of MN, adiabatic coefficient (γ), and hydrogen-to-carbon ratio (H/C) on Otto and exergetic efficiency. Hydrogen enrichment raises γ but reduces MN, while CO2 dilution enhances MN and knock tolerance but depresses γ and H/C. Sensitivity analysis of ηotto with respect to MN, γ, and H/C reveals γ as the dominant driver at low MN (<90), H/C as the key differentiator at high MN (>90), and MN as an enabling but secondary parameter. Four fuels with identical MN = 100 highlight how γ amplifies efficiency and H/C gates exergy quality, with pure methane achieving the highest combined performance. Additional analysis within the 97 < MN < 120 range confirms that balanced MN–γ–H/C configurations, rather than maximizing any single variable, yield the most robust results. Optimal conditions are obtained with ∼ 5–10 % hydrogen in CH4–CO2 mixtures, maintaining MN > 100 while sustaining γ ≈ 1.30, enabling theoretical Otto efficiencies above 53 %, peaking at 54.1 % for methane. Overall, the proposed principle provides a systematic thermodynamic framework for SI engine design, enhancing renewable fuel integration, guiding early-stage decisions, and supporting chemical engineering strategies for decarbonization and circular economy goals.

Instituciones: Universidad Tecnológica del Perú