Equivalence Ratio and Proportional Encoding

This paper examines the traditional use of the fuel-air equivalence ratio as a metric in combustion analysis, highlighting limitations arising from its semi-infinite, asymmetric scale and lack of physical directness. The authors demonstrate through statistical modeling and combustion simulations that using equivalence ratio can misrepresent the distribution and comparative behavior of fuel-lean and fuel-rich mixtures, leading to ambiguity and misleading interpretations. To address these shortcomings, two alternative metrics are considered: the stoichiometry metric and the equivalence metric. Both compress the infinite range of the equivalence ratio into a finite, symmetric interval with the stoichiometric condition at the origin, enhancing the interpretability and fairness when comparing mixtures. The equivalence metric, a simple transformation of Law’s normalized equivalence ratio, enables a smoother and more balanced representation of combustion properties, while the stoichiometry metric more directly relates to available chemical energy but exhibits undesirable nonlinearities. The study advocates for these new metrics to improve both physical insight and proportional encoding in the visualization and analysis of combustion processes, especially in complex or stratified systems.