Combustion at the knocking threshold (KT) was tested using fuels with different methane numbers (MN) in a modified spark ignition (SI) engine, with high compression ratio (CR) with high turbulence intensity to the combustion process; also, fuels were tested in a cooperative fuel research (CFR) engine to measure MN and critical compression ratio (CCR); in both engines, tests were performed just into the KT. It is proposed that MN to gaseous fuels will be considered similarly to octane number (ON) to liquid fuels, to indicate the energy quality and the capacity to produce work. According to the tests, biogas has better combustion properties than the others fuels; biogas is the fuel with the highest knocking resistance; biogas is the cleanest fuel with the best energy quality measured with the energy density (ED) and adiabatic flame temperature (Tad); biogas has the highest capacity to produce work in SI engines, because of its high MN, low ED, low laminar flame speed (SL), and low Tad. Fuel combustion phenomenological characteristics were compared using CCR versus: output power, generating efficiency, ED, SL, and Tad. It is suggested that the strategies to suppress knocking are the key to improve the performance of SI engines; the knocking phenomenon is the engine limit to electrical energy generation in SI engines; two equations are proposed to define quantum generating efficiency and maximum electrical energy generated; knocking was defined as a quantum phenomenon using the entropy concepts as filter of the second law of thermodynamics.