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Effects of fuel ethanol content and injector temperature on the spray characteristics of gasoline direct-injection atomizers Kowsari, Reza

Abstract

Recently, the Energy Policy Act of 2005 was passed into U.S. Law. This comprehensive energy legislation includes, for the first time in U.S. history, a national renewable fuels standard (RFS). The RFS legislates, with immediate effect, a significant increase in renewable fuel usage over a six-year period starting at 4 billion gallons in 2006 and increasing to 7.5 billion gallons in 2012. In practice, these legal requirements are most likely to be met by the increased use Biodiesel fuels and, most significantly, of ethanol as an additive to conventional gasoline. It is well known that the addition of ethanol (C₂H₅OH) to gasoline, which is itself a multi-component hydrocarbon blend, can significantly change the distillation characteristics of the fuel. In a fully warm, port-fuel-injected (PFI) engine, the change in fuel properties may not be significant. However, it is to be expected that the next generation of gasoline engines will use direct-injection fuelling strategies. The potential advantages of direct-injection spark-ignition (DISI) engine systems are substantial. However, the limited time available for fuel preparation in DlSl systems suggests that the technology may be more sensitive to fuel properties than previous generation injection technologies. This study investigates the changes in DISI fuel spray structure induced by the addition of ethanol, in varying amounts, to a base fuel of fixed composition, a process typically known as ’splash blending’. Five fuels of different ethanol content were injected into atmosphere at an injection pressure of 10.0 MPa through two prototype DISI fuel injectors (one single hole and one multi-hole design). The fuels were: a 5-component model fuel, comprised of isopentane, iso-octane, n-octane, n-decane and dodecane, simulated E5 (5% ethanol 95% model fuel), simulated E10 (10% ethanol 90% model fuel), simulated E22 (22% ethanol, 78% model fuel), E80 (80% ethanol, 20% model fuel) and finally pure ethanol. The effect of each fuel on the global characteristics of the spray was examined at four different temperatures (20, 40, 60 and 80 degrees Celsius) using Mie-Scattering, Schlieren and Shadowgraphy imaging techniques. Different stages of the spray development were identified. In accordance with the literature, increasing the fuel temperature was seen to increase the spray penetration while narrowing the spray structure for all fuels. At low fuel temperatures, the impact of fuel composition was negligible. The results showed the effect of fuel composition on the spray structure to be most evident in intermediate temperatures. Further increasing the fuel temperature caused spray collapse for all fuel compositions. Evidence of flash boiling was observed at high fuel temperatures. The ethanol content of fuel blends was not found to be proportional to the changes in spray structure. Finally, observations showed the sprays exhibited the highest level of shot-to-shot variability in the case of intermediate fuel temperatures. Sprays from the swirl-type injector were found to be more consistent than those of multi-hole injectors.

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