Effect of injection pressure on ethanol and gasoline sprays in a spark-ignition direct-injection engine

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Copyright: Bao, Yongming
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Abstract
This study aims to clarify the spray development of ethanol, gasoline and iso-octane fuel, delivered by a multi-hole injector and spark-ignition direct-injection (SIDI) fuelling system. The focus is on how fuel properties and injection pressure impact temporal and spatial evolution of sprays at various 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, Mie-scattering imaging is performed to measure penetrations of spray plumes at various injection pressures of 4, 7, 11 and 15 MPa. Experiments were first performed in the spray chamber to measure the spray tip penetration and penetration rate of ethanol, gasoline and iso-octane. 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. In the same warmed optical engine, the effect of injection pressure on the structural transformation of flash-boiling sprays of gasoline and ethanol is investigated for two fuel injection timings of 90 and 300 crank angle degrees after top dead centre, corresponding to low and high ambient pressure conditions, respectively. The macroscopic spray structure was quantified using spray tip penetrations, spray spreading angles and spray areas. From the measurements, it is found that fuel sprays injected at the earlier injection timing, when the vapour pressure of the fuel is higher than the ambient pressure, show the convergence of the spray plumes towards the injector axis evidencing the flash-boiling phenomenon. By contrast, fuel injected at the later timing, and hence, higher ambient pressure than the fuel vapour pressure, show typical spray structures with distinct plumes for each nozzle hole. The flash-boiling sprays appear to be influenced by the injection pressure significantly. It is observed that the convergence of spray plumes associated with the flash-boiling is less evident with increasing injection pressure, as higher injection momentum outperforms the plume-to-plume interaction resulting in increased directionality of the spray plumes. It is also found that the enhanced fuel vaporisation due to increased injection pressure is more prevalent in flash-boiling sprays than non-flash-boiling sprays. These findings suggest a great benefit of high pressure injection and flash-boiling sprays in achieving well-mixed charge in SIDI engines. Between gasoline and ethanol, the degree of flash-boiling appears to be different. It is observed that the gasoline sprays undergo flash-boiling and spray collapsing at the low ambient pressure condition while the transitional flash-boiling occurs at the high pressure condition. In comparison, the ethanol sprays experience the transitional flash-boiling at the low pressure condition and no flash-boiling at the high pressure condition. Also, the effect of injection pressure on the spray convergence and fuel vaporisation is more pronounced for the gasoline sprays, all because gasoline fuel has a greater degree of superheating than that of ethanol. Findings from this study may contribute to the development of flex-fuel SIDI engines with better charge preparation and potentially higher fuel economy.
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Author(s)
Bao, Yongming
Supervisor(s)
Kook, Sanghoon
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Publication Year
2013
Resource Type
Thesis
Degree Type
Masters Thesis
UNSW Faculty
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