On the effect of ambient turbulence and thermodynamic conditions on fuel spray development for direct-injection spark-ignition engines

摘要

High-pressure multi-hole injectors for direct-injection spark-ignition engines offer certain flexibility in spray directionality by selecting the number and angle of the nozzle’s holes to suit the design of a particular combustion chamber. However, the spray’s pattern can change significantly for injector-body temperatures representative of real engine operation at low-load conditions with injection strategies in the early intake stroke. This is due to rapid phase change effects from flash boiling of the high-volatility components of gasoline. This work presents results from an optical investigation into the effects of injector-body temperature and back pressure on the pattern of spray formation, especially when coupled to different levels of ambient turbulence. Specifically, gasoline and iso-octane fuels were tested in the range of 20–120 °C injector-body temperatures and for ambient pressures of 0.5–5.0 bar. Additionally, the ambient turbulence was varied in the range 0–4 m/s to observe its effect on flash-boiling and non-flash-boiling sprays. Results from a combination of high-speed shadowgraphy and simultaneous Schlieren and Mie scattering optical techniques are presented in terms of imaged spray areas and plume penetration. Calculations of the Stokes number are also discussed with respect to turbulence and fuel properties. The results demonstrate a marginal effect of the degree of turbulence intensity on non-flash-boiling sprays that maintained their nominal plume directionality throughout the injection event. However, a significant effect on the spray’s penetration and mixing at conditions of fuel flash-boiling was observed with increasing levels of turbulence intensity; the collapsed pattern of the spray’s formation exhibited much faster dispersion and mixing.