Experimental and theoretical study of the direct diesel injection process at low temperatures

Resum

The fuel injection system has been one of the most important parts of the diesel engine in the past decades. This is because the air-fuel mixing process strongly depends on the injection condition, affecting the engine performance, efficiency and the pollutant formation. Along the diesel engine history, the injection system studies have been focused on standard operating conditions, at common pressure and temperature values. However, the diesel engine could operate in other conditions, less frequent but more critical, such as the very low temperature conditions. At these particular conditions the information related to the injection system characterisation is very limited, almost nonexistent. Previous studies focused on cold start combustion have demonstrated that at some operating points, the combustion is deficient. Since the injection process is one of the factors that influence the spray development and the combustion performance, it is necessary to understand the injection process at such critical cold conditions. The global objective of this thesis is the theoretical and experimental study of the injection process at very low temperature conditions, focused on the quantification and evaluation of the diesel injection characteristics at this cold temperature condition (between 255 and 273 K). This study is carried out testing three axi-symmetric convergent nozzles, which have different diameters. The experimental devices employed for the injection process characterisation are: the injection rate test rig, the momentum flux device and the spray visualisation test rig for non-evaporative conditions. During this work the experimental techniques and processing tools are improved and adapted for the low temperature condition tests. Moreover, the fuel thermophysic properties are determined for a wide range of temperatures and pressures (up to 180 MPa). The used fuel is a special diesel developed for winter temperatures. On the other hand, some mathematical models are extended and employed for predicting the internal flow of the diesel nozzles. The computational tool used is a novel 3D technique, which predicts in a suitable manner the turbulent pattern of a flow, known as Large Eddy Simulation (LES). Such theoretical analysis allows the understanding of some phenomena that has not been described yet experimentally due to current technological limitations.