Impacts of low-temperature heat release on unstretched laminar burning velocity in advanced flex-fuel gasoline-ethanol engines

Improving brake thermal efficiency (BTE) of flex-fuel engines can contribute to decarbonizing conventional and hybridized vehicles running on spark-ignited (SI) engines. High compression ratio (CR) and burning velocity under exhaust gas recirculation (EGR) conditions can improve the engine BTE. Ethanol fuel, which has been attracting increasing attention recently, can improve brake thermal efficiency due to its high-octane number and fast laminar burning velocity (LBV). With higher CRs, low-temperature heat release (LTHR) occurs in the unburned mixture, which affects the compositions, oxidation, and in-cylinder temperature. Such thermodynamic changes on unstretched LBV have not been thoroughly investigated, especially in production-intent next-generation flex-fuel engines. This study aims to clarify the LTHR impacts on LBV under EGR conditions using ethanol-blended gasoline surrogate fuel SI engines. The testbed used a single-cylinder SI engine fueled by E0 (gasoline surrogate), E20 (20 % ethanol + 80 % E0 by volume), E40, E60, E85, and E100. Detailed chemical reaction calculations were conducted using the boundary conditions obtained from production-intent, strong-tumble flow, and flex-fuel engine experiments. This work predicted LTHR in an unburned mixture using a zero-dimensional detailed chemical reaction calculation. LBV is predicted by a 1D kinetic laminar flame code. As a result, LTHR proceeds before the main combustion. The results show that the temperature increase associated with LTHR increases the burning velocity, while the partial oxidants decrease the burning velocity. Moreover, this work examined higher CRs using detailed chemical reaction calculations, and the results show that fuels with higher ethanol blending ratios can increase LBV in the late combustion phase.