A novel methodology is proposed to estimate the exergy of liquid and gaseous fuels for spark ignition (SI) engines, based on the thermodynamics laws and two scientific methodologies, one for measurements of fuels methane number (MN), and other for estimation of gaseous fuels energy quality. Two engines were used 1) A CFR engine to measure fuel's critical compression ratio (CCR) and MN. 2) A hybrid diesel engine with a high compression ratio (CR) converted to SI using custom pistons for high turbulence intensity designed by CFD simulations, for biogas SI (BSI) combustion. In both engines, fuels with MNs ranging from 37 to 140 were used, biogases, methane, propane, and blends of biogas with methane/propane and hydrogen. The BSI engine combines diesel engines characteristics (high airflow inlet, and high combustion pressures) with SI engines characteristics (premixed combustion using SI for combustion phasing at the knocking threshold (KT)). The BSI engine was used to measure the fuel´s maximum electrical energy generation and thermal efficiency at the KT. Correlations between MN and research octane number (RON) were used to develop the methodology. Fuel´s exergy efficiency is estimated with fuel´s γ, hydrogen/carbon (H/C) ratio, and limited by fuel´s MN/RON. The methodology proposed correlates fuel´s exergy, fuel´s entropy, adiabatic flame temperature (Tad), and fuel´s exergy efficiency forOtto cycles limited by the knocking.