Analysis Methods for the Effects of Injection Rate Control in Direct Injection Diesel Engines

摘要

Fuel injection characteristics play a decisive role in modern direct injection diesel engines for the formation of nitrogen oxide and soot. The paper reports on investigations carried out to evaluate the various methods of injection rate shaping. The main emphasis is on split injection, i.e. a second opening phase of the nozzle needle immediately after a first opening phase, which offers the largest potential for soot reduction. This phenomenon is investigated using a variety of numerical and optical methods. Among others, thermodynamic cycle analysis was carried out including considerations regarding the generation of turbulence by the injection jet. Analysis is extended using two optical methods, namely the measurement of flame radiation and video imaging. Both methods only need access to the cylinder through the cylinder head for an endoscope-like probe and are based on the measurement of radiation at two or more wavelengths in the visible range emitted by glowing soot particles. The difference between the two methods is that with the video system the desired information is plane resolved whereas the flame radiation probe yields a single information valid for the whole cone of sight. The measurement results in terms of radiation intensity are converted to temperature and soot concentration using the well known two-color method. Three-dimensional CFD calculations are used to verify the findings from the other analysis methods. As one of the results it became clear that the main effect of post injection is the increase of the temperature level in the burning gas which enhances soot oxidation. Another mechanism of the enhanced soot burn-off with split injection may be the higher local level of turbulent kinetic energy generated by the retarded second injection phase.