THE IMPORTANCE OF PYROLYSIS IN DIESEL IGNITION OF RESIDUAL FUEL OILS

Recent attempts to increase yields of distillate fuels from crude oil have resulted in a decline in the ignition quality of residual fuel oils and claims of ignition problems in large marine Diesel engines. This has revived an interest in fundamental studies of the ignition of liquid fuels and in new ways of defining and measuring ignition quality.

As part of an extensive study of the ignition of a range of residual fuels oils and during the development of novel teats of ignition quality it became obvious that thermal cracking of the fuel could play an important part in the pre-ignition chemistry. This paper reports kinetic data from simple experiments performed on the pyrolyses of residual fuel oils. Small samples and high heating rates were used to try to match the conditions inside a Blow-speed Diesel engine. This data has been used to assess the contribution that thermal cracking makes to ignition processes.

Pyrolyses were carried out on a small silica-coated platinum coil in the inlet of a gas chromatograph. Light hydrocarbon pyrolysis products were formed, and the extent and rate of cracking determined at four temperatures.

Assuming that reaction took place on a surface surrounding the oil and that a constant supply of reactants were available, pseudo zero order rate constants for the initial part of the reaction were evaluated. They showed an Arrhenius relationship with temperature giving an overall apparent activation energy of 110 kJ mol^-1. Extrapolating rate constants to temperatures expected in Diesel engines, and assuming typical droplet sizes end ignition delays, it was shown that thermal cracking of fuels was possible before ignition and that it is likely that these reactions will have a strong influence on the processes leading to ignition. It was also shown that the presence of oxygen greatly increases the rate of thermal cracking.