Ignition delays of biodiesel-diesel blends: Investigations into the role of physical and chemical processes

Overall ignition delay (IDover) recorded in Compression-Ignition engines (CIEs) is caused by physical and chemical processes. Those processes lead to physical and chemical ignition delay times (IDphys and IDchem, respectively). IDover could be understood as the sum of IDphys and IDchem, although some overlap could exist between them. Knowledge on diesel and biodiesel ignition delay times is quite mature, however, the role of physical and chemical processes on the times is not quite clear and this requires additional studies to further examine these important processes. This paper investigates the contribution of physical and chemical processes into IDphys, IDchem, and IDover of a wide-range of biodiesels and their blends with fossil-diesel in CIEs. IDover is measured here using a cooperative-fuel-research engine and a 6-cylinder engine. A shocktube is used to measure IDchem while physical processes including atomization and evaporation are experimentally examined. It is found that size and distribution of drops derived from the atomization process of the fuels are somewhat similar while the evaporation rate of biodiesels is much slower compared to that of diesel and this could cause their longer IDphys. IDchem is characterised by chemical reactions that could be initiated at high temperature coefficient - HTC, negative temperature coefficient - NTC or low temperature coefficient - LTC regimes. IDchem of fuels tested under HTC is identical while IDchem under NTC and LTC regimes is determined by the fuel molecular structure. Compared to diesel, IDchem of common biodiesels is shorter and this could be attributed to their faster rates of reactions under NTC and LTC; their IDphys is longer and this is mainly due to their longer evaporation rate; but in overall, their IDover is shorter.

Automated summary

The slower evaporation rate of biodiesels leads to longer physical ignition delay, while the faster chemical reaction rates result in shorter chemical ignition delay, thus overall ignition delay is shorter.