Influences of molecular profiles of biodiesels on atomization, combustion and emission characteristics

Abstract

In this thesis, aspects of atomization, combustion and emission were investigated for a range of biodiesels. The selected fuels have different carbon chain lengths and unsaturation degrees and consequently different fuel oxygen contents (FO). Fundamental studies of secondary atomization were conducted using an air cross flow stream. Investigation of spray flames was performed using a hot co-flow burner. Fuel utilization was examined using a well-instrumented common-rail engine. Secondary atomization was characterized in terms of three fluid elements namely small drops, large objects and ligaments. It was found that at a low We number, a significant change in shape probabilities occurs when moving from a bag break-up regime to higher We numbers and some small differences in liquid shapes are found amongst the tested fuels. However, up to a certain Weber number (such as We = 200), the probability of detection of different shapes is almost independent of the breakup regimes as well as the fuel properties. Comparing performances of flame structures in the hot co-flow burner indicated that the auto-ignition characteristics (e.g. the change in chemiluminescence emission and the growth of reaction zone width) are affected by the fuel-air ratio and also by the fuel molecular structure. Engine studies confirmed that differences in biofuel molecular profiles significantly affect engine combustion and emission characteristics. The study on engine cycle variability established a link between the cyclic variability and the oxygen ratio, which is a good indicator of stoichiometry. The current research also revealed that a critical key to reducing the total particle mass, particle size, particle number, and black carbon concentration is to increase the FO. However, an increase in the FO leads to a substantial increase in the total particle number per unit of particle mass, the amount of black carbon per unit of particle mass, and the reactive oxygen species concentration.