Access Type

Open Access Dissertation

Date of Award

January 2012

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Ming-Chia Lai

Abstract

The objective of this study is to understand the characteristics of GDI sprays and behavior of in-cylinder charge motion in a GDI engine which is equipped with variable valve actuation. High speed spray visualization in a spray chamber was conducted for a study of spray development. By Schlieren visualization technique, the effect of different conditions including ambient conditions (temperature, pressure) and injection conditions (fuel type, fuel temperature, nozzle design) on spray formation was discussed in both qualitative and quantitative methods. High-speed visualization of the in-cylinder charge motion was carried out as well using an optical accessible engine. The testing was conducted for the conventional valve strategy with the production cam, and for the advanced valve strategies with the prototype cams. Multi-dimensional CFD was carried out to support the optical engine work.

By the experiments, Schlieren visualization was found to be very effective method to observe the vapor phase of the fuel spray, and the 2-stage thresholding image processing method was developed to process Schlieren images. Evaluation of the spray images revealed that the effect of flash boiling could change the spray shape drastically by plume collapse. Significance of flash boiling depended not only on ambient condition, but on injection condition and a combination of other effects including fuel properties and nozzle design. The optical engine testing revealed that the interaction of spray and in-cylinder flow, especially contradiction of the direction of spray momentum and tumble rotation, was a key factor for mixture formation. Stronger inlet air flow by low-lift cam resulted in higher turbulent energy and enhanced vaporization rate which was proven in both experiment and simulation. Deactivation of one of the intake valves produced swirl charge motion in the cylinder which was mixed with tumble. Soot emission caused by piston impingement of the early injection could be reduced by valve deactivation. The use of high-lift long-duration cam promoted more vigorous charge motion, but dynamic ratios of low-lift short-duration cam developed faster.