In-Nozzle Flow, Spray and Combustion Investigations of Two-Stroke Marine Diesel Engine Fuel Injectors

Doctoral thesis, 2020

This thesis focuses on the experimental studies of the cavitating in-nozzle flow of large two-stroke marine Diesel engine fuel injector nozzles and how it affects the spray morphology and combustion behaviour. 
To further increase the efficiency of large two-stroke marine Diesel engines, a better understanding of the fuel spray atomization process is necessary. Previous work has shown that the cavitating in-nozzle flow of those fuel injectors significantly influences the spray breakup. Spray deflections have been observed which can lead to cylinder wall wetting resulting in increased fuel consumption, emissions, component temperatures and loss of lubrication film.
Large two-stroke marine Diesel engines have significantly different injector nozzle designs compared to light- and heavy-duty internal combustion engines and the research is not fully congruent. Additionally, there are very limited reference experiments for large two-stroke marine Diesel engines.
To gather experimental data for validation of CFD codes and predictive spray models, and to further deepen the understanding, this work presents the development of experimental equipment and the acquisition of optical measurements to investigate the influence of cavitating in-nozzle flow on the spray morphology and combustion behaviour. 
A newly developed transparent nozzle holder in combination with transparent nozzles made from polymethyl methacrylate allowed to visualize the cavitating in-nozzle flow under engine-like fuel pressures and with identical geometrical nozzle properties. Although only single-orifice nozzles were investigated, an extensive experimental data set has been acquired and evaluated showing significant differences between the various nozzle geometries investigated.
Hydro-erosive grinding has been applied to reduce cavitation patterns by creating differently pronounced inlet radii between nozzle main bore and nozzle orifice. The investigated cavitating in-nozzle flow shows different cavitation patterns for the nozzle types investigated and a unique swirl cavitation pattern for the eccentrically arranged orifice nozzle design. Simultaneous in-nozzle flow and spray morphology optical measurements revealed a strong influence of the cavitation within the orifice on the spray morphology.
Using metal nozzles matching the investigated and characterised transparent nozzles, combustion experiments have been performed at a constant-volume spray and combustion chamber to link the cavitating in-nozzle flow to combustion characteristics.