Effect of Gasoline–Ethanol–Water Mixtures on Auto-Ignition in a Spark Ignition Engine

Today ethanol is already blended by 10vol-% in gasoline and a further increase of the ethanol content to 20vol-% is discussed. During the ethanol production process, distillation and molecular sieving are required to remove the water concentration to achieve high-purity ethanol. However, hydrous ethanol can be beneficial to suppress the knock of spark ignition engines. The hygroscopic nature of ethanol can allow to increase the water content in gasoline–water emulsions even more, without adding additional surfactants, and improve the thermal efficiency by optimized combustion phasing, while keeping the system complexity low. Hence, the effect of gasoline – ethanol-water mixtures on the auto-ignition in a single-cylinder spark ignition engine is investigated by using multi-dimensional simulation and detailed chemistry. The gasoline-ethanol mixtures are defined to keep the Research Octane Number constant, while the Motored Octane Number is decreasing. In total five surrogates are defined and investigated: E10 (10vol-% ethanol-in-gasoline), E20, E30, E70 and E100. The water content is determined according to experimentally defined ternary diagrams that evaluated stable gasoline – ethanol-water emulsion at different gasoline-ethanol blending ratios. The auto-ignition modes of the surrogates are analyzed using the ξ-ε diagram, which determines if hotspots are within harmless deflagration or harmful developing detonation regime. The strongest auto-ignition is observed for the E10 surrogate while increasing ethanol content reduces the surrogate reactivity and increases the resonance parameter ξ. No auto-ignition of the unburnt mixture is observed for the E70 and E100 surrogates. The addition of hydrous ethanol decreased the excitation time of the surrogates, especially at low ethanol content, wherefor the reactivity parameter ε is significantly increased. The hotspots for E10, E20, and E30 surrogates with hydrous ethanol are found within the developing detonation regime, while hotspots of the E70 surrogate with hydrous ethanol are found in the transition regime. For the hydrous E100 surrogate, no auto-ignition is predicted because of the reduced temperature of the unburnt mixture due to water vaporization, which outweighs the increased reactivity due to water vapor addition.