Experimental Studies of High-pressure Gasoline Fuel Sprays for Advanced Internal Combustion Engines

Abstract

While much has been learned about gasoline direct injection sprays, there are still large gaps in the fundamental knowledge of the effects of high-pressure (over 500 bar) on gasoline sprays and on the role of fuel spray/nozzle interactions on particulate emissions. Particulate emissions are a critical concern for direct fuel injection spark-ignited engines, as future regulations may be difficult to meet without after-treatment. The objective of this dissertation was to quantitatively and qualitatively characterize fuel spray development for gasoline under engine-relevant conditions using non-intrusive optical techniques including high fuel injection pressures and canonical studies of the effects of internal flow structures on external spray development. Two experimental facilities were used to study high-pressure gasoline spray development and one experimental facility was used to study injector tip wetting and the effects on engine-out particulate emissions. Experiments were conducted at the University of Michigan with a constant volume chamber and diffuse backlit shadowgraph imaging for a range of chamber pressures (1 – 20 bar), injection pressures (300 – 1500 bar), and for several canonical fuel injector nozzle geometries including different nozzle exit diameters, converging and diverging nozzles, and rounded and abrupt inlet nozzles. The images were used to measure penetration distance and rate and spray angle as a function of time for each injection event, which are compared with results of previous experimental studies and simplified physics-based models that have been proposed in the literature. Trends in fuel spray development were similar to those observed previously for diesel sprays, which was unexpected given the significant differences in thermal-physical properties. Some abnormal spray features were identified and quantified, including spray flutter (i.e., asymmetric variation in spray angle). Injector internal flow characterization and spray development measurements were also performed at Centro Motores Termicos, an engine research division at the Universitat Politecnica de Valencia (UPV). Rate of injection, and momentum flux, were measured using two facilities and spray development was imaged using schlieren in another facility for a range of chamber pressures (5 – 30 bar), and injection pressures (600 – 1500 bar), and included vaporizing and non-vaporizing chamber temperatures of 400 – 800 K. The work revealed the important effect of internal flow transitions on injector performance, where nozzles with inlet rounding resulted in 20% higher mass flow rate compared with straight cylindrical nozzles. Spray momentum coefficients showed a negative trend with increased pressure differential indicating all nozzles were cavitating under all conditions tested. Lastly, trends in measured engine-out particulate number (PN) emissions were correlated as a function of a large array of fuel injectors, multiple engine architectures, and a large parametric space of operating conditions. PN was measured directly from sampling engine-out exhaust using a laser-scattering condensation particle counting system. The tests were specifically identified as conditions where fuel injector tip wetting is considered a significant source of PN. Five conceptual mechanisms were developed to describe the tip wetting and drying processes and to interpret the effects of fuel injection pressure, engine load, time scales, injector temperature, and injector geometry on PN. Overall, the data provide a better understanding of the effects of operating and hardware parameters on the development of gasoline fuel sprays and on particulate number emissions. The results directly inform strategies for novel combustion concepts, current engine architectures, and fundamental spray theory.