Optical investigation of flash boiling and its effect on in-cylinder combustion for butanol isomers and iso-octane

Performance of a gasoline direct injection engine significantly depends on the fuel injection phenomenon. It has been shown that variation in the fuel thermo-physical properties and in-cylinder thermodynamic conditions can adversely affect engine performance. Spray collapse due to flash boiling is the consequence of such varying in-cylinder thermodynamics. It has also been observed that gasoline direct injection engines have higher particulate matter emissions compared to port fuel injection engines. One of the possible reasons for this observation may be fuel impingement on the piston head and spray collapse at high fuel injector temperature. In the present work, experiments have been performed to understand spray characteristics under flash boiling conditions and ultimately its effect on in-cylinder combustion quality for the three different fuels: n-butanol, iso-butanol and iso-octane. To mimic in-cylinder conditions, hot fuel was introduced inside an optically accessible engine. It was observed that fuel temperature and their thermo-physical properties have a significant effect on piston head wetting and pool fire on the piston top. Butanol isomers showed significant reduction in sooty combustion with increase in fuel temperature. However, iso-octane showed higher wall wetting at elevated fuel temperature due to spray collapse.