From Otto-Cycle Exergy to Engine Design: A Thermodynamic Principle for Renewable and Alternative Fuels in Spark-Ignition Engines

Abstract

The design of spark-ignition (SI) engines for renewable and alternative fuels demands strategies that overcome knock constraints and the limited exergetic efficiency of conventional Otto-cycle operation. This research proposes a theoretical framework grounded in a thermodynamic principle that formalizes exergy in Otto cycles by integrating efficiency, entropy generation, fuel physicochemical properties, and combustion conditions as coupled design variables. The work consolidates more than a decade of theoretical and experimental studies, culminating in an integral exergetic balance equation that captures the dynamic evolution of exergetic efficiency across the cycle and yields a general principle applicable to real engines. The framework provides a rational basis for selecting high compression ratios, lean operation, and EGR-based dilution to enable stable operation near the knock threshold while preserving efficiency. Simulation results and experimental validations using biogas, methane, hydrogen, and representative blends are presented, showing measurable gains in thermal and exergetic performance relative to traditional configurations. Finally, the proposed “Fifth Law of Thermodynamics for engines” is introduced as a unifying foundation that links efficiency, irreversibilities, and fuel-adapted design, offering a new scientific pathway for high-efficiency, sustainable engine development—particularly relevant to Latin America. “True efficiency does not lie in the most energetic fuel or the fastest combustion, but in designing the system around the fuel that best resists knock; only then can exergy be maximized and the traditional limits of the Otto cycle be surpassed.”