Investigations of Diesel Sprays in Optical Engines - Liquid Fuel Penetration and Lift-Off Length

Abstract: Diesel engines are widely used as powertrain solutions for transportation vehicles. Their low fuel consumption and high performance make them attractive, however, their associated pollutant emissions is a major drawback which represent a hazard for public health and the environment. Therefore, the concentrations of the major pollutant species are regulated. Soot, which to a large extent consists of carbon particles emanating from the combustion process, is the most problematic emission for diesel engines. Soot emissions result from two competing processes, the formation and the oxidation. Diesel jets burn as lifted diffusion flames. The lift-off length, which is the distance between the injector and the base of the flame, is an important characteristic of diesel jets since it is strongly connected to the soot formation rate. In this thesis, three studies are devoted to the characterization of the flame stabilization of diesel jets in an engine environment and in a high-pressure combustion vessel. A nonlinear regression model, based on a previous empirical correlation developed in a combustion vessel, is proposed to describe the lift-off length of diesel jets in a particular engine environment. The novelty of this model, apart from being based on a systematic study of lift-off lengths in an engine environment, is the integration of two variables coupled to important engine specific features, the jet-jet and jet-flow interactions. A fourth study compares the respective impacts of soot formation and soot oxidation on soot emissions in an engine. The efficiency of the soot oxidation process is found to have the dominating effect on soot emissions. Upstream of the lift-off region, the penetration length of the liquid-phase fuel is another important characteristic of diesel sprays. This parameter is linked to the spray formation and to the fuel-air mixing and thereby to the entire combustion process. Two studies on the liquid penetration of diesel jets are presented in the second part of this thesis. In the first one, an injection strategy to avoid wall-wetting of late post-injections used for the regeneration of diesel particulate filters is demonstrated. In the second study, the effect of jet-jet interactions on the liquid fuel penetration of diesel jets is investigated. The liquid fuel penetration length is increased with a reduction of the spacing between the jets. In this thesis, optical engines are used to carry out the experimental investigations. In these engines, few metal parts are replaced with a transparent material such as quartz to obtain a visual access into the combustion chamber. High-speed imaging and laser-based diagnostics are combined to capture the time-evolution of the lift-off length and the transient liquid penetration of the fuel.