Ignition quality of residual fuel oils

Abstract

The relevance of residual fuel oil aromaticity for its ignition performance in diesel engines has been demonstrated previously and led to the concept of calculating aromaticity from known specification properties. Thus the Calculated Carbon Aromaticity Index (CCAI) can be calculated from density and viscosity, and provides a useful tool to rank the ignition quality of different residual fuel oils: the lower the number, the better the ignition characteristics.

Potential improvements to the CCAI concept have been investigated. The CCAI represents the aromaticity of the entire fuel. However, at low load engine operations ignition occurs at relatively low temperatures, when only part of the injected fuel may have evaporated. Under these conditions the high molecular weight, highly aromatic (asphaltene) components in all probability are not all vaporised. Thus the aromaticity of the (lighter) part of the fuel might be different from the bulk and possibly more relevant to ignition quality. A programme to investigate the relationship between ignition delay and the aromaticity of the fuel vapour under certain engine conditions has been carried out.

The relationship between ignition delay and CCAI was demonstrated to be valid at all engine conditions employed and comparable to the one found previously. No improvement could be realised by taking into account the micro carbon residue (MCR) content as a measure for the heavy fraction of the fuel, nor any other of the available fuel parameters. The results of Pyrolysis Combustion Mass Spectrometric Element (PCME) analysis of the fuels, providing detailed compositional information of the vapour at different temperatures, indeed confirm that the aromaticity of the lighter fuel fractions does not dominate the aromaticity of the fuel vapour and therewith does not dominate the ignition performance of the fuel. However, in view of the still rather limited predictive power of the CCAI, other, not yet identified fuel parameters must play a role.