Bio- and mineral-fuel spray evolution in non-evaporating and evaporating conditions by image processing techniques

Fuels from renewable resources have obtained an increasing interest for transport application in the last decade because of their biodegradability, potential improvements on exhaust emission and benefits on virtuous CO2 cycle of the earth. Within this framework, the paper presents the influence of biodiesel fuels on the injection process and their impact on the air-fuel mixture preparation. The results of an experimental investigation on the fuel spray from a multi-jet common rail injection system both under non evaporative and evaporative conditions are illustrated. The characteristics of the investigated spray include the fuel delivery and instantaneous flow rate and the jet visualization, both within a high pressure vessel at ambient temperature and in an optically accessible single cylinder diesel engine for the evaporative conditions. Images of the spray evolution, under non evaporative conditions, have been captured, in single-shot mode, by a high spatial resolution CCD camera. They have been processed off-line to estimate the spray tip penetration, cone angle and density distribution. Analogous characterization of the injection images have been performed on the data acquired in the combustion chamber of the optical diesel. The spray tip penetration and liquid fuel distribution have been measured for several engine operating conditions and different biodiesels. The injection profiles appear similar for all the used fuels in terms of pulse slopes and timing distributions. Images recorded in the evaporating system show a negligible variation of the tip penetration for the different fuels dependent on engine condition rather than on fuels properties. Nevertheless a comparison of the tip penetration in evaporative and non evaporative systems has been made. Finally a different spatial distribution of the liquid fuel has been noted analysing the luminous intensity spatial distribution along the jet direction.