Large-Eddy Simulation Study of Biofuel Injection in an Optical Direct Injection Engine

The air-fuel mixture formation in an optical direct-injection internal combustion engine is investigated by numerical simulations for the two biofuels Ethanol and 2-Butanone. The gas phase in the internal combustion (IC) engine is predicted by a large-eddy simulation, in which the fuel phase is determined by a spray model based on Lagrangian particle tracking. A hollow-cone injector is used for which the primary breakup is modeled by a series of small full-cone injections, while the Rosin-Rammler initial droplet size distribution is used. The secondary spray break-up is modeled by the Kelvin-Helmholtz-Rayleigh-Taylor (KHRT) model, and the evaporation of the fuel is determined by the Bellan-Harstad model. The gas phase simulation is based on a finite-volume method formulated for hierarchical Cartesian grids, in which the immersed moving boundaries are resolved using a multiple level-set/cut-cell approach. Characteristic boundary conditions are used to avoid artificial reflections at the inlet and outlet ports. Approximately 50 million mesh cells are used to resolve the in-cylinder flow domain. Dynamic load balancing is utilized to redistribute the workload due to grid adaptation, moving valves and piston, as well as fuel droplets. The validation of the spray model is performed by comparison of the numerical findings with pressure chamber measurements.

Good agreement is obtained for the spray penetration and droplet size distribution. Measurements of the in-cylinder flow-field in an optical engine are used to validate the flow field simulation results in the IC engine. The simulation of the mixture formation for the biofuels show that the difference of the evaporation rate between Ethanol and 2-Butanone leads to significant differences in the spray behavior. While wall impingement is a significant factor with Ethanol, this effect is negligible for 2-Butanone. The higher evaporation rate of 2-Butanone results in a less favorable mixture formation due to the lower liquid penetration of the spray.