Superheated Sprays of Alternative Fuels for Direct Injection Engines

Alternative and oxygenated fuels are nowadays being studied in order to increase engine efficiency and reduce exhaust emissions and also to limit the automotive industry's economical dependency from crude oil. These fuels are considered more ecological compared to hydrocarbons because they are obtained using renewable sources. Fuels like anhydrous/hydrous ethanol, methanol or alcohol/gasoline blends which are injected in liquid form must vaporize quickly, especially in direct injection engines, therefore their volatility is a very important factor and strongly depends on thermodynamic conditions and chemical properties. When a multi-component fuel blend is injected into a low pressure environment below its saturation pressure, a rapid boiling of the most volatile component triggers a thermodynamic atomization mechanism. These kinds of sprays show smaller droplets and lower penetration compared to mechanical break up. The prediction of vapor liquid equilibrium is very important when different components are blended to form a fuel, especially dealing with flash boiling applications and non ideal mixtures like alcohol/hydrocarbons. This work presents a combined 1D/3D numerical modeling of fuel injection processes under superheated conditions involving different single and multi-component hydrocarbons and alternative fuels for automotive applications, such as ethanol, methanol and alcohol/gasoline blends. Primary break up through multi-hole injectors is affected by phase change within the nozzle of the fuel blend due to flash boiling. Particular attention must be given to the thermo-physics description of the fuel properties, since the vaporization process is a key in liquid fuel injections and mixture formation. The vapor pressures of the partial components are calculated using the Peng-Robinson equation of state and the non ideal phase equilibrium of alcohol/hydrocarbon mixtures is modeled using activity coefficients. An Homogeneous Relaxation Model (HRM) is adopted to model the thermodynamic instability of the fuel within a 1D nozzle flow model. 1D simulation results, such as droplet size distribution, are used to define initial conditions for 3D Lagrangian spray simulations performed by using a specific evaporation model, designed to reproduce the vaporization rate from superheated droplets.