Studies on the reduction of nitrogen oxides emission in a large-bore diesel engine

This experimental research studied different technologies for reducing nitrogen oxides (NOx) in the exhaust gases, running with a large-bore medium-speed diesel research engine. NOx mainly form during combustion in local high temperature zones. This study considered primary methods, avoiding high combustion temperatures. In particular, the Miller cycle, which is a proven concept for NOx reduction, was deeply studied. This technology was applied by closing early the intake valves to create a first gas expansion before the compression stroke, thus reducing the effective compression ratio. At high load, a detailed analysis of the mixing-controlled combustion, which constitutes the most influent part of the combustion process, was carried out, reaching in-cylinder pressure of 300 bar. Same NOx level and no soot were achieved increasing the output power, while keeping the same fuel injection pressure. At partial load, a more extensive study of different Miller rates was performed. An advanced Miller rate resulted in NOx reduction up to 55%. Since the Miller cycle presented some limitations, other techniques were implemented together with the Miller cycle. First, a split injection strategy was tested with low fuel injection pressure. Then, a paraffinic fuel, hydrotreated vegetable oil (HVO), was tested, while keeping lower oxygen content in the combustion chamber, obtained by retaining a part of the exhaust gases. The results of the experiments with split injection showed that it was difficult to decrease further NOx emissions compared to the values obtained with the Miller cycle alone, but specific fuel consumption decreased when a small pilot injection was used. However, a later injection timing could be used to obtain lower NOx values with a small drawback in fuel consumption. HVO's ignitability allowed running with very advanced Miller cycle and low oxygen content. It was possible to obtain low NOx figures, but a certain increase of specific fuel consumption took place.This thesis showed that a unique tool for obtaining a significant NOx reduction without drawbacks in the whole engine load spectrum could not be found. Due to lower combustion temperatures, a higher specific fuel consumption was a common downside. Optimization of injection strategy, use of alternative fuels and dilution with inert mass are valuable tools to implement for achieving low engine-out NOx in large-bore engines. However, the investment costs the possible drawbacks must be accurately evaluated in each case.