Phenomenological Analysis of the Combustion of Gaseous Fuels to Measure the Fuel's Energy Quality and Availability to Produce Work in Spark Ignition Engines, Part 2

A novel methodology is proposed to evaluate fuel’s performance in spark ignition (SI) engines based on the fuel’s energy quality and availability to produce work. Experiments used a diesel engine with a high compression ratio (CR), modified by SI operation, and using interchangeable pistons. The interchangeable pistons allowed for the generation of varying degrees of turbulence during combustion, ranging from middle to high turbulence. The generating efficiency $(ηq)$⁠, and the maximum electrical energy (E_Emax) were measured at the knocking threshold (KT). A cooperative fuel research (CFR) engine operating at the KT was also used to measure the methane number (MN), and critical compression ratio (CCR) for gaseous fuels. Fuels with MNs ranging from 37 to 140 were used: two biogases, methane, propane, and five fuel blends of biogas with methane/propane and hydrogen. Results from both engines are linked at the KT to determine correlations between fuel’s physicochemical properties and the knocking phenomenon. Certain correlations between knocking and fuel properties were experimentally determined: energy density (E_D), laminar flame speed (S_L), adiabatic flame temperature (T_ad), heat capacity ratio (⁠$γ$⁠), and hydrogen/carbon (H/C) ratio. Based on the results, a mathematical methodology for estimating E_Emax and $ηq$ in terms of E_D, S_L, T_ad, $γ$⁠, H/C, and MN is presented. These equations were derived from the classical maximum thermal efficiency for SI engines given by the Otto cycle efficiency (⁠$ηOtto)$⁠. Fuels with MN > 97 got higher E_Emax, and $ηq$than propane, and diesel fuels.