Quantitative DISI Spray Vapor Temperature Study for Different Biofuels by Two-Line Excitation Laser-Induced Fluorescence 2012-01-1658

Biofuels and alternative fuels are increasingly being blended with conventional gasoline fuel to decrease overall CO₂ emissions. A promising way to achieve this is the use of DISI (direct-injection spark-ignition) technology. However, depending on temperature, pressure, chemical composition and the spark timing, unwanted pre-ignition may occur. Despite higher compression ratios, this engine knock can be decreased by lowering the mixing temperature. This results from the larger fuel evaporation enthalpy of certain biofuels which provides a non-homogeneous mixture throughout the combustion chamber. This work focuses on estimating the biofuel evaporation rate from absolute local vapor temperature and concentration. Measurements conducted in a high temperature/pressure cell using a multi-hole injector are carried out by applying planar, 2-line, laser-induced fluorescence and phase doppler interferometry. The temperature, vapor mass fraction and the droplet size distribution of the specific fuel can then be calculated. The pure biofuels n-butanol and ethanol are examined and compared with isooctane as a standard model fuel and with a 3-component biofuel mixture. The resulting local temperature distributions are found to be a function of spray momentum, mixture of vapor with entrained gas, as well as a function of fuel evaporation enthalpy and boiling temperature. In different engine conditions, certain physiochemical properties are found to dominate. For moderate operating conditions the high-boiling point components like n-butanol show a strong cooling effect. The 3-component mixture is found to be strongly influenced by a small amount of the high-boiling point n-butanol. At elevated conditions the cooling behavior changes completely. The fuels show a strong dependence on the evaporation enthalpy rather than to their boiling point. Thus, the temperature reduction in the center of the spray plume is a maximum for ethanol. For late injection timing mode, the evaporation enthalpy of the fuels is the dominating thermophysical property for the evaporation rate.