Understanding GDI Spray Characteristics of Isooctane and Alcohols and its Effect on In-cylinder Combustion

Kale, Rakesh and Banerjee, Raja (2019) Understanding GDI Spray Characteristics of Isooctane and Alcohols and its Effect on In-cylinder Combustion. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Successful implementation of GDI technology needs precise knowledge of fuel injection process and proper understanding of spray characteristics under engine relevant conditions. It may be noted that fuel injection happens at sub atmospheric cylinder pressure conditions for homogeneous charge mode, whereas in-cylinder pressure and temperature are high for stratified charge mode. Varying in-cylinder conditions can significantly alter the spray plume morphology, which eventually can affect the combustion quality. During engine running condition, heat transfer from the hot in-cylinder gases heats up the fuel injector and increases the fuel temperature before its injection in the cylinder. These situations give rise to the flash boiling phenomenon. When hot fuel is injected into the sub-saturation pressure conditions, vapor bubbles are formed inside the liquid. Subsequent expansion and explosion of these vapor bubbles result in a catastrophic breakup of the liquid jet. Therefore, proper understanding of fuel injection phenomenon plays an important role in GDI engine operation. Present work is basically divided in to three different parts: A) Spray characteristics under elevated ambient pressure and temperature conditions. B) Effect of fuel temperature on GDI spray behavior. C) Effect of flash boiling on in-cylinder spray and combustion behavior. In the first part of the study experiments were performed in a constant volume spray chamber. Four different fuels namely isooctane, ethanol, n-butanol and isobutanol were tested under engine like elevated pressure and temperature conditions. Results showed that thermo-physical properties of the fuel such as boiling point, surface tension, density, viscosity and latent heat of vaporization have significant effect on the spray characteristics. Alcohols showed longer liquid penetration and droplet size due to their higher latent heat of evaporation. In the second part of the study effect of higher fuel temperature was tested in the constant volume spray chamber. The selected temperature conditions were kept between 298 K to 523 K. Results showed that with increase in fuel temperature, spray collapsed towards the injector axis due to flash boiling of the fuel. The liquid and the vapor penetration lengths were observed to be higher under elevated fuel temperature conditions. PDPA data showed flash boiling can significantly improve the droplet atomization even for the poor atomizing liquid such as butanol. The last part of the experiments showed the effect of flash boiling sprays on in-cylinder combustion behavior. It was observed that higher fuel temperature can be helpful in reducing sooty combustion due to wall wetting. However, spray collapse can increase the susceptibility of pool fire due to localize wall wetting

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