An Examination of the Relationship between the Physical Processes of Mixture Preparation and Ignition for CIDI-Relevant Fuel Sprays

Alternative diesel fuels derived from sources other than conventional petroleum refining can exhibit markedly different physical and thermal properties compared to conventional diesel fuels. The effects of variation in fuel physical and thermal properties on CIDI autoignition performance were investigated using a variety of experimental facilities and computer simulations. Measurements of ignition performance were found to be generally uncorrelated with fuel boiling characteristics, indicating that boiling temperature plays a limited role in the CIDI ignition process. Direct observations of fuel injection and jet physical development were also made using an optically-accessible engine, and appropriate use of optically-accessible engines for quality data collection is discussed. While ignition performance was not significantly impacted by fuel boiling characteristics, overall combustion performance was. Fuels with elevated boiling temperatures were observed to have greater liquid fuel impingement in the optical engine, which led to reduced combustion efficiency and load. Differences in fuel viscosity and surface tension were not found to strongly influence the physical development of the fuel jet, which suggests that CIDI-relevant fuel sprays are inertially-dominated flows. Based on this observation, a mixing-limitedmodel of fuel thermal development is presented, which conceptualizes the thermal development of the fuel-air mixture by considering the adiabatic mixing of injected fuel and entrained ambient gas. A new fuel property, the enthalpy demand for thermal development Δh_D is introduced to quantify fuel thermal development under mixing-limited conditions, and techniques for assessing this property in real fuels are developed and presented. The range of fuel Δh_D among commonly used distillate fuels is analyzed and found to be limited, potentially indicating why significant differences in ignition performance due to thermal development effects have not yet been observed in the literature. The sensitivity of fuelautoignition performance to ambient gas conditions is qualitatively investigated as a function of Δh_D using a 0-D thermo-kinetic model of the ignition quality tester (IQT) apparatus, and a potential link between temperature sensitivity and fuel enthalpy demand is demonstrated. This sensitivity dependence has implications for the interpretation of current fuel reactivity metrics (e.g., the DCN), which rely on generalizing fuel ignition performance observed at a reference condition to all other conditions regardless of fuel Δh_D value. Future experimental work utilizing fuels with wide ranges of Δh_D is recommended to ascertain whether a quantitatively significant relationship between fuel thermal development andautoignition performance exists, and if such a relationship is satisfactorily captured by existing fuel reactivity metrics.