Dissertation

Diesel Spray Studies in Modern Diesel Engines

Hulkkonen, Tuomo

2020

Overview

In this doctoral thesis, diesel injection and spray formation in modern diesel engines were studied. The goal of this thesis is to answer some fundamental questions and hypotheses about injection and spray formation in modern diesel engines. First, the fundamental spray characteristics of renewable diesel were studied under non-evaporative conditions. Second, these spray characteristics were studied under extremely high cylinder pressure. Third, the spray characteristics of conical nozzle orifice geometry were studied. Finally, the spray characteristics of biofuel blends were studied in an optical engine during late-post-injection, which is relevant to exhaust gas after-treatment.Studies showed that the spray tip penetrations with renewable diesel and petroleum diesel were similar under non-evaporative conditions. The spray angle was slightly wider, spray tip velocities were higher, and the inner delay of the injector was shorter with renewable diesel. The conclusion of the study was that there is no need to redesign the combustion chamber or readjust the injection parameters due to wall impact or spray collision.Very high in-cylinder pressure and density have a significant effect on spray penetration. Higher gas phase mixing was observed with higher in-cylinder density. No negative aspects were found for extremely high gas density.When the spray tip penetration was compared between different conical geometries and a cylindrical nozzle orifice geometry, a clear difference was not found under non-evaporative conditions. This result is inconsistent with earlier studies. The main reasons for this inconsistency may be the different approach and high injection pressure. The spray angle was smaller, and the mass flow rate higher, with conical nozzle orifice geometry. Standard hydraulic flow measurement with an injection pressure of 100 bar underestimates the flow rate of conical orifices due to lack of cavitation.Different hypotheses about higher spray tip penetration and cylinder wall-wetting during late post-injection were studied. A clear difference in the spray tip penetration was not observed when three different fuel blends were compared. The conditions that would be needed for droplets to evaporate before reaching the cylinder wall are not attained with very late injection. Hence, hypotheses that the amount of fuel ending up on the cylinder walls is higher with biofuels are unlikely. The main reason for oil dilution rate differences between fuel blends is probably related to the volatility of the fuel fraction, or because the control unit increases the volume of the post-injections due to the lower volumetric heat value of renewable diesel.

• Publisher: ProQuest Dissertations & Theses
• Subject: Petrology