Flash Boiling Effects on the Development of Gasoline Direct-Injection Engine Sprays

The influence of the injector temperature on the spray distribution and fuel volatility of a high-pressure swirl injector of the type used in direct-injection gasoline engines and thus of flash boiling effect was investigated in a pressure chamber with optical access. Laser-induced (exciplex) fluorescence was used to visualize the liquid phase and the vapour phase of the spray. The experiments were conducted at a chamber pressure of 50 kPa and a chamber temperature of 323 K by varying the injector temperature (323 K, 343 K, 363 K and 381 K) at a constant rail pressure of 8 MPa. Three single-component fuels with different boiling points (n-hexane: T_b = 339 K, iso-octane: T_b = 372 K and n-octane: T_b = 398 K) and a non-aromatic multi-component fuel (mcf) (T_b = 303 K - 473 K) were chosen for the investigations. The dopant was a combination of 2% by mass TEA (triethylamine) and 3.4% by mass benzene in the non-fluorescing substitutional fuels. At low chamber pressure and an injector temperature of 343 K fuel with a low boiling point change the spray structure from a hollow-cone to a solid-cone distribution. By increasing the injector temperature the solid-cone structure can also be seen for fuels with higher boiling points. This change was simultaneously detected on the camera for the liquid phase and on the camera for the vapour phase. The observed spray behaviour is caused by a flash boiling effect where a superheated liquid above its boiling temperature will disintegrate abruptly into a spray of small droplets.