Spray Penetrations of Ethanol, Gasoline and Iso-Octane in an Optically Accessible Spark-Ignition Direct-Injection Engine

The spray development of ethanol, gasoline and iso-octane has been studied in an optically accessible, spark-ignition direct-injection (SIDI) engine. The focus is on how fuel properties impact temporal and spatial evolution of sprays at realistic ambient conditions. Two optical facilities were used: (1) a constant-flow spray chamber simulating cold-start conditions and (2) a single-cylinder SIDI engine running at normal, warmed-up operating conditions. In these optical facilities, high-speed Mie-scattering imaging is performed to measure penetrations of spray plumes at various injection pressures of 4, 7, 11 and 15 MPa. The results show that the effect of fuel type on the tip penetration length of the sprays depends on the injection conditions and the level of fuel jet atomisation and droplet breakup. It is observed that at 4 MPa injection pressure, the tip penetration length of ethanol sprays is shorter than that of gasoline sprays, likely due to lower injection velocity and increased nozzle loss associated with higher density and increased viscosity of ethanol, respectively. This assertion is further supported by the longest penetration length of iso-octane that has the lowest density among tested fuels and similar viscosity to gasoline. At higher injection pressure of 7 and 11 MPa, the penetration length difference between ethanol and gasoline sprays decreases and eventually ethanol sprays show a longer penetration length than that of gasoline sprays at the highest injection pressure of 15 MPa. This reversed trend is possibly because the penetration regime is changed such that the tip penetration is limited by aerodynamic drag force applied to fuel droplets, instead of the injection velocity or nozzle loss of the liquid jet. It is suggested that with increasing injection pressure, the fuel jet atomisation and droplet breakup enhance and therefore the lower aerodynamic drag associated with higher droplet size of ethanol sprays than that of gasoline sprays leads to a longer penetration length. The same trends of spray penetrations of ethanol, gasoline, and iso-octane are observed in the warmed optical engine with overall higher tip penetration length than that in the cold spray chamber primarily due to decreased air density and increased fuel temperature.